Joint optimization method of mechanical vapor recompression system and reverse osmosis system

Abstract

The invention discloses a joint optimization method for a mechanical vapor recompression system and a reverse osmosis system. The joint optimization method comprises the following steps: establishing a reverse osmosis superstructure network model, a reverse osmosis unit model, a mechanism model of the mechanical vapor recompression system and economic models of the reverse osmosis system and the mechanical vapor recompression system; establishing an optimal design model of the reverse osmosis system combined with the mechanical vapor recompression system by taking the lowest total annual cost as an objective function and relating to related design variables; and solving the established optimization model, processing a differential equation therein, converting a problem into a mixed integer nonlinear programming problem, and solving by adopting a mixed integer nonlinear programming solver to obtain a joint system design structure under the lowest total annual cost. According to the invention, the actual level of design and operation of the combined system is improved, the utilization rate of water resources is improved, and the initial investment and operation maintenance cost required by the system are reduced, so that the total annual cost is reduced, and the economic benefit is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of seawater and saline wastewater treatment, and particularly relates to a design structure of a combined optimization reverse osmosis system and a mechanical vapor recompression system, so that the total annualized cost of the combined system is minimized. Background Art

[0002] Nowadays, the scarcity of fresh water resources has become a global environmental problem. With the development of social economy and the gradual advancement of industrialization, the discharge of industrial wastewater has caused varying degrees of pollution of water resources. With the growth of the world's population today, people's demand for fresh water is increasing, so that many areas in the world have become or are becoming short of fresh water. For industrial high-salt wastewater, seawater desalination technology is urgently needed to achieve wastewater desalination treatment and minimize the pollution of discharged industrial wastewater. On the one hand, it can reduce the pollution of water resources, and on the other hand, the treated fresh water can be used for people's daily life, alleviating the contradiction between supply and demand of water resources. At present, most coal chemical projects use the "dual membrane" reuse technology of "ultrafiltration + reverse osmosis" to treat industrial wastewater. Among them, reverse osmosis technology is currently the main method of use due to its simple structure, easy operation, high fresh water quality and short engineering construction period.

[0003] Reverse osmosis technology uses pressure as a driving force to separate the fresh water and salt water of coal-fired power plant wastewater through a reverse osmosis membrane. The main components of the high-concentration brine after separation are Cl - 、SO4 2- 、Na + , Ca2 + If inorganic salt ions such as chlorinated salt are directly discharged, it will not only cause environmental pollution but also waste resources. Therefore, the concentrated brine produced by the reverse osmosis system is concentrated and crystallized through a mechanical vapor compression system to obtain a saturated salt solution that can be crystallized and more fresh water, further improving the utilization rate of coal-fired power wastewater. The saturated salt solution can be further purified and separated to obtain high-purity salt products that meet market demand, such as sodium chloride, potassium chloride, sodium sulfate, etc.

[0004] The concentrated brine produced by the reverse osmosis system can be used as the feed of the mechanical vapor compression system for concentration and crystallization. Since the flow rate and concentration of the concentrated brine produced by the reverse osmosis system are determined by the quality of its feed water and the design structure, these factors not only affect the reverse osmosis system itself, but also directly affect the feed conditions of the mechanical vapor compression system. The heat exchange area of ​​the evaporator and heat exchanger of the mechanical vapor compression system will also affect the design and operating costs of the system. It can be seen that the design structure of the reverse osmosis system and the design structure of the mechanical vapor compression system will affect the annualized total cost of the entire system. Therefore, it is very meaningful to conduct a joint optimization analysis of the two to determine the optimal design structure.

[0005] At present, reverse osmosis systems and mechanical vapor recompression systems are usually optimized independently, and the design of reverse osmosis systems usually relies on special design software provided by domestic and foreign membrane manufacturers. These software help designers design systems by simulating and calculating the operating parameters under a given scheme, such as fresh water quality and system feed water pressure. However, due to the arbitrariness of setting the scheme, the design process may undergo repeated revisions from infeasible to feasible, and the target optimality of the final scheme cannot be guaranteed. On the other hand, the design of mechanical vapor recompression systems usually aims at minimum energy consumption and optimizes the heat transfer area to reduce investment costs. However, this single optimization method does not take into account the mutual influence between the two. For example, different reverse osmosis system design structures will produce different brine salt concentrations and flow rates, which will affect the operation of the mechanical vapor recompression system, and then affect the water production and economic costs of the entire system.

[0006] In contrast, joint optimization can optimize at the overall system level, including all feasible design solutions through the superstructure network model, taking into account the design structure, equipment heat exchange area and operating conditions of the reverse osmosis system and mechanical vapor recompression system. By combining investment costs, operating costs and design variables, joint optimization can obtain the optimal system structure, thereby avoiding local optimal or even unfeasible design solutions that may appear in separate optimization. Joint optimization can improve the rationality and economy of the design.

[0007] In view of the shortcomings of the current design modes of reverse osmosis systems and mechanical vapor recompression systems, the present invention proposes a joint design optimization method for mechanical vapor recompression systems and reverse osmosis systems. The method establishes a strict mechanism model based on the treatment requirements of coal-fired power wastewater and the principle of reverse osmosis. By combining the superstructure network model, the reverse osmosis process model, and the mechanical vapor recompression system model, the joint system is optimized with the lowest total annualized cost as the optimization goal, and the joint system design structure with the lowest total annualized cost is obtained. In the design of the reverse osmosis system, the superstructure network model covers design variables such as the number of pressure vessels, the number and type of membrane elements in the pressure vessels, and the amount of concentrated water reflux. The reverse osmosis system can select and optimize between multiple design structures. In the design of the mechanical vapor recompression system, two design variables, the heat transfer area of ​​the evaporator and the heat exchanger, are covered. The total annualized cost includes the investment cost and operating cost of the joint system. Summary of the invention

[0008] The purpose of the present invention is to provide a joint design optimization method for a mechanical vapor recompression system and a reverse osmosis system.

[0009] The present invention specifically comprises the following steps:

[0010] Step 1: Establish a reverse osmosis superstructure network model:

[0011] The reverse osmosis superstructure network includes N ro RO nodes and N ps There are N boost nodes in total ps +2 logistics nodes, the added 2 logistics nodes refer to the concentrated water outlet and the fresh water outlet that finally flow out of the network. ro RO nodes and N ps The boosting nodes correspond one to one on the same circuit; each reverse osmosis node is the entrance of a reverse osmosis section, and each reverse osmosis section consists of N pv Pressure vessels are connected in parallel, each of which consists of N mem reverse osmosis membrane elements are connected in series.

[0012] The first boosting node is used as the feed water inlet. A part of the feed water is transported to the first reverse osmosis node by a high-pressure pump and then enters the first reverse osmosis stage 1. The concentrated water / fresh water stream produced by the first reverse osmosis stage can enter any boosting node. The concentrated water / fresh water entering the i-th boosting node enters the j-th reverse osmosis stage after passing through the corresponding j-th reverse osmosis node; the concentrated water or fresh water stream produced by any reverse osmosis stage can enter any boosting node. The stream entering the boosting node is pressurized by a vacuum pump or directly enters the corresponding reverse osmosis node.

[0013] The feed water is mixed with part of the concentrated water / fresh water produced by any reverse osmosis membrane.

[0014] The constraints that the entire superstructure network needs to meet are:

[0015] Among them, Q pout represents the freshwater flow leaving the reverse osmosis network, C pout Represents the salt concentration of fresh water leaving the reverse osmosis network; represents the minimum freshwater flow requirement for the design, Indicates the maximum freshwater salt concentration requirement for the design;

[0016] Inlet feed water flow rate Q of the reverse osmosis superstructure network fraw =Q bout +Q pout ; According to the law of conservation of materials in the reverse osmosis process, the feed water flow rate Q at the feed port of the reverse osmosis superstructure network f and the feed water salt concentration C at the feed port of the reverse osmosis superstructure network fraw Satisfaction: Q fraw C fraw =Q bout C bout +Q pout C pout ;Q bout represents the concentrate flow rate leaving the reverse osmosis network, C bout Indicates the salt concentration of the concentrate leaving the reverse osmosis network;

[0017] Inlet flow rate of the first boosting node Among them, Q fraw represents the inlet feed water flow rate of the reverse osmosis superstructure network, Q b,1,j represents the concentrated water flow rate of the jth reverse osmosis entering the first boosting node, Q p,1,j represents the freshwater flow rate from the jth reverse osmosis section into the first boosting node;

[0018] The water flow rate Q of the first boosting node ps,1 and the inlet water concentration C at the first boosting node ps,1 satisfy: Among them, C fraw represents the feed water salt concentration of the reverse osmosis superstructure network, C b,1,j represents the salt concentration of the concentrated water entering the first boosting node of the jth reverse osmosis section, C p,1,j represents the salt concentration of freshwater entering the first boosting node of the jth reverse osmosis section; the pressure of the first boosting node P ps,1 Equal to the inlet pressure P of the reverse osmosis superstructure network fraw .

[0019] The water inlet flow rate of the first boosting node also satisfies: Q ps,1 =Q hpp +Q pxlin , where Q hpp Indicates the flow rate of feed water entering the high-pressure pump; the inlet flow rate Q of the first boosting node ps,1 and the inlet concentration C of the first boost node ps,1 Satisfaction: Q ps, 1C ps,1 =Q hpp C hpp +Q pxlin C pxlin , where C hpp Indicates the salt concentration of the feed water entering the high-pressure pump, C pxlin represents the salt concentration of the feed water at the low-pressure water inlet of the energy recovery device; at this time, the feed flow rate Q of the first reverse osmosis node ro,1 =Q hpp +Q pxhout ; Feed water flow rate Q of the first reverse osmosis node ro,1 and the feed water concentration C of the first reverse osmosis node ro,1 Satisfaction: Q ro,1 C ro,1 =Q hpp C hpp +Q pxhout C pxhout ; where C pxhoutIt indicates the salt concentration of the high-pressure feed water flowing out of the high-pressure water inlet outlet after pressure exchange of the energy recovery device;

[0020] Inlet flow of the i-th booster i=2,3,...,Nps; where Q b,i,j represents the concentrated water flow rate from the jth reverse osmosis section to the i-th boosting node, Q p,i,j represents the freshwater flow leaving the jth reverse osmosis section and entering the i-th boosting node;

[0021] The water inlet flow rate Q of the i-th boosting node ps,i and the inlet water concentration C of the i-th boost node ps,i satisfy: i=2,3,...,Nps; where C b,i,j represents the salt concentration of the concentrated water entering the i-th boosting node of the j-th reverse osmosis section, C p,i,j It represents the salt concentration of fresh water entering the i-th boosting node through the j-th reverse osmosis section.

[0022] The concentrated water flow rate produced by the jth reverse osmosis stage Freshwater flow rate generated by the jth reverse osmosis stage The feed water flow rate Q of the jth reverse osmosis node RO,j =Q ps,i , j=i, j=1,2,3...,Nro; the feed water concentration C of the jth reverse osmosis node RO,j =C ps,i , j=i, j=1,2,3...,Mro; the pressure P of the jth reverse osmosis node RO,j =P ps,i +P_boost j j=i,j=1,2,3...,Nro;P ps,i represents the feed water pressure entering the i-th boost node, P_boost j Indicates the boost pressure of the pump, j=1 indicates HPP, j>1 indicates BP, P RO,j Represents the outlet pressure of the jth booster stage.

[0023] The feed water flow, concentration and pressure of the jth reverse osmosis node are transmitted to the reverse osmosis process model established in step 2 to calculate the concentrate flow, concentrate concentration, concentrate pressure and fresh water flow, fresh water concentration and fresh water pressure generated by the jth reverse osmosis section.

[0024] Step 2: Establish the reverse osmosis unit model:

[0025] Step 2.1, establish the connection model between the reverse osmosis superstructure network and the reverse osmosis process model:

[0026] Q f =Q RO,j / n pv,j ; C f =C RO,j ;p f =p RO,j ; where Q f , C f , P f Respectively represent the feed water flow, salt concentration and pressure of the reverse osmosis section, N pv,j Represents the number of pressure vessels in the jth reverse osmosis stage.

[0027] Step 2.2: According to the law of conservation of materials in the reverse osmosis process of each reverse osmosis section, establish the solution conservation and flow conservation equations: Q f =Q p -Q r ;Q f C f =Q p C p +Q r C r ; where C p represents the salt concentration of fresh water produced by the reverse osmosis stage, C r Indicates the salt concentration of the concentrate produced by the reverse osmosis stage.

[0028] The fresh water flow rate generated by the reverse osmosis section can also be expressed as:

[0029] Among them J v(z) is the reverse osmosis solvent flux, n l , W and L represent the number of blades, width and length of the reverse osmosis membrane respectively.

[0030] According to the dissolution-diffusion model: The solvent flux J through the reverse osmosis membrane v(z) =A w(z) (P f -P d(z) -P p(z) -Δπ (z) ); solute flux through the reverse osmosis membrane J s(z) =B s (C m(z) -C p(z) );

[0031] Among them, Aw (z) is the solvent transport parameter, Pf is the feed water pressure of the reverse osmosis section, Pd (z) is the pressure drop at position z in the reverse osmosis membrane channel, P p(z) is the osmotic pressure at position z in the membrane channel, Δπ (z) is the osmotic pressure difference at position z in the membrane channel. Bs is the solute transport parameter, C m(z)is the solute concentration measured on the surface of the reverse osmosis membrane at position z in the membrane channel, C p(z) It is the solute concentration measured on the surface of the reverse osmosis membrane at position z in the membrane channel.

[0032] Feed water pressure P at position z in the reverse osmosis membrane channel b(z) =P f -P d(z) ; Transmembrane pressure difference ΔP at position z in the reverse osmosis membrane channel (z) =(P b(z) -P p(z) );Osmotic pressure difference Δπ at position z in the reverse osmosis membrane channel (z) =RT(C m(z) -C p(z) );According to the above formula: J v(z) =A w (ΔP (z) -Δπ (z) ).

[0033] Solvent transport parameter A w and solute transport parameter B s is a function of temperature: Where N memtype Indicates the number of membrane elements, A j is the intrinsic solvent transport parameter of the membrane element used in the jth reverse osmosis stage, B j is the intrinsic solute transport parameter A of the membrane element used in the jth reverse osmosis k , B k is the inherent transport parameter of the kth reverse osmosis membrane element model under standard conditions, x j,k It is a 0-1 variable. When the j-th reverse osmosis pressure vessel uses the k-th type of membrane element, it takes the value of 1, otherwise it takes the value of 0. α1, α2 and β1 represent constants, and T represents the Kelvin temperature at which the system operates.

[0034] Considering the influence of concentration polarization in the reverse osmosis process, the concentration polarization phenomenon is that the salt concentration on the surface of the reverse osmosis membrane increases and the osmotic pressure increases. In order to maintain the original water flux, the external pressure must be increased: concentration polarization value Among them C b(z) represents the salt concentration of concentrated water at position z in the reverse osmosis membrane channel, k c(z) It represents the membrane mass transfer coefficient at the position z in the reverse osmosis membrane channel.

[0035] Solute mass transfer coefficient k c(z) Obtained by the following equation: Re (z) =ρ (z) V (z) d e / μ(z) Sc (z) =μ (z) / (ρ (z) D AB(z) );

[0036] Among them Sh (z) Re represents the Sherwood number at position z in the reverse osmosis membrane channel, Re (z) Represents the Reynolds number at position z in the reverse osmosis membrane channel, Sc (z) represents the Schmidt number at position z in the reverse osmosis membrane channel, d e represents the hydraulic diameter of the feed spacing channel, V (z) is the axial velocity of seawater at position z in the reverse osmosis channel;

[0037] ρ (z) represents the density of the permeate water at the position z in the reverse osmosis channel, μ (z) Indicates dynamic viscosity, D AB(z) represents the kinematic viscosity, and the above parameters are calculated by the regression equation:

[0038] Among them, k1, k2, b1, b2, a1, a2, and a3 are all regression equation coefficients, and M is a variable related to the Kelvin temperature T.

[0039] Reverse osmosis membrane solvent flux J v(z) and solute flux J s(z) Satisfaction: J s(z) =J v(z) ×C p(z) ;

[0040] Without considering the influence of pollutants on the system during the reverse osmosis process, following the law of conservation of materials and the mechanism shown above, the change of the pressure loss Pd(z) in the reverse osmosis membrane channel along the channel is expressed as: λ (z) =6.23K λ Re (z) -0.3 ; K λ is the empirical parameter related to the membrane element, λ (z) is the friction coefficient at position z in the reverse osmosis membrane channel.

[0041] Due to the pressure in the reverse osmosis membrane channel So there is

[0042] And when z=0, P b(0) =P f ; When z = L, P b(L) =P r L is the length of the membrane channel, z = L refers to the end of the membrane channel, Pr It is the concentrate pressure at the outlet of the reverse osmosis section.

[0043] Axial flow velocity of seawater in the reverse osmosis channel V (z) The changes along the channel are expressed as:

[0044] When z = 0, When z = L, h sp is the height of the feed interval channel. Qr is the concentrated water flow rate at the outlet of the reverse osmosis section.

[0045] Solute concentration (salt concentration) C in the reverse osmosis membrane channel b(z) The changes along the channel are expressed as:

[0046] And when z=0, C b(0) =C f , when z=L, C b(L) =C r . C r It is the salt concentration of the concentrate at the outlet of the reverse osmosis section.

[0047] According to the above formula, when the fresh water pressure P p When it is 0, given the flow rate, pressure, and salt concentration of the feed water of the jth reverse osmosis node, the final fresh water flow rate Q at the outlet of the corresponding reverse osmosis section can be obtained. p and freshwater salt concentration C p .

[0048] Step 2.3: Establish the connection model between the reverse osmosis process model and the reverse osmosis superstructure network:

[0049] Q b,j =Q r *N pv,j ; C b,j =C r ;P b,j =P r ; where Q r , C r , P r They represent the concentrated water flow, salt concentration and pressure at the outlet of the reverse osmosis section respectively. p,j =Q p ; C p,j =C p ;P p,j =P p ; where Q p , C p , P p They represent the fresh water flow, salt concentration and pressure at the outlet of the reverse osmosis section respectively.

[0050] Step 3: Establish the mechanism model of the mechanical vapor recompression system:

[0051] The concentrated water finally produced by the reverse osmosis system flows into the mechanical vapor recompression system for evaporation and concentration treatment; the mechanical vapor recompression system includes two heat exchangers, a circulating pump, an evaporator, a demister and a steam compressor. The inlets of the two heat exchangers are connected to the concentrated water flowing out of the concentrated water outlet of the reverse osmosis superstructure network, and the outlets of the two heat exchangers are connected to the circulating pump through pipelines. After being heated by the heat exchanger, the feed water is pressurized by the circulating pump and enters the evaporator, and is evenly sprayed on the outer wall of the hot pipe of the evaporator through the built-in nozzle of the evaporator; the demister is arranged in the evaporator, and is connected to the air inlet of the steam compressor through a pipeline, the air outlet of the steam compressor is connected to the hot pipe of the evaporator, and the condensed water outlet of the hot pipe of the evaporator is connected to the hot end inlet of heat exchanger 1; the concentrated water outlet of the evaporator is connected to the hot end inlet of heat exchanger 2.

[0052] When in use, the concentrated water flowing out of the reverse osmosis superstructure network concentrated water outlet is fully heat exchanged with the concentrated water and condensed water produced by the evaporator through two heat exchangers, and then reaches the evaporator through a circulating pump, and is evenly sprayed on the outer wall of the hot pipe of the evaporator through the built-in nozzle of the evaporator. The water vapor in the concentrated water flowing out of the reverse osmosis superstructure network concentrated water outlet flows into the steam compressor through the demister, and the steam compressor compresses the water vapor into high-temperature and high-pressure superheated gas; the compressed superheated gas enters the pipeline of the evaporator, exchanges heat with the feed water sprayed on the outer wall of the pipeline, and then condenses, flows into heat exchanger 1 through the condensed water outlet, and exchanges heat with part of the concentrated water flowing out of the reverse osmosis superstructure network concentrated water outlet, and the generated fresh water flows out from the hot end outlet of heat exchanger 1. The higher concentration concentrated water that has not been evaporated in the evaporator flows out through the concentrated water outlet of the evaporator and flows into heat exchanger 2 to exchange heat with the remaining concentrated water flowing out of the reverse osmosis superstructure network concentrated water outlet, and the generated concentrated brine flows out from the hot end outlet of heat exchanger 2.

[0053] Assuming that the salt concentration of steam is 0%, there is no energy loss in the evaporator, preheater, pipeline and pump system; according to the principle of mechanical vapor compression evaporation, the law of conservation of energy and the law of conservation of heat, mechanical vapor compression evaporation is described by the following equation:

[0054] Mechanical vapor recompression system feed water mass flow rate M f =M b +M d ; where M d represents the mass flow rate, M b represents the mass flow rate of concentrated brine; the mass flow rate of feed water in the mechanical vapor recompression system M f And the mass percentage concentration of feed water X f Satisfaction: M f X f =M b X b +Md X d ; Among them, X d Indicates the mass percentage concentration of fresh water, X b Indicates the mass percentage concentration of concentrated brine.

[0055] Mechanical vapor recompression system feed water mass flow rate M f =Q bout ×1000, mass percentage concentration of feed water X f =C bout ÷10;Q bout , C bout They represent the brine flow rate and brine concentration ultimately produced by the reverse osmosis superstructure network, respectively.

[0056] The heat generated in a heat exchanger Q HEX1 =M d C pd (T d -T d0 )=M f1 C pf (t i1 -T i ), the heat generated in the other heat exchanger Q HEX2 =M b C pb (T b -T b0 )=M f2 C pf (T i2 -T i );C pd represents the specific heat capacity of the condensed water in the recompression system, C pf represents the specific heat capacity of the feed water in the recompression system, C pb represents the specific heat capacity of the concentrated brine in the recompression system, M f1 represents the feed mass flow rate through heat exchanger 1, M f2 represents the feed mass flow rate through heat exchanger 2, T i Indicates the temperature of the feed water, T b Indicates the concentrated water temperature at the evaporator outlet (i.e. the boiling point temperature of the evaporator feed water), T d Indicates the temperature of the condensed steam at the evaporator outlet, T bo represents the temperature of the concentrated brine after passing through heat exchanger 2, T do represents the temperature of fresh water after passing through heat exchanger 1, T i1 represents the temperature of the feed water after heat exchange in heat exchanger 1, T i2 Indicates the temperature of the feed water after heat exchange in heat exchanger 2.

[0057] It can also be expressed as: Q HEX1 =UHEX A HEX1 (ΔT LM1 CF), Q HEX2 =U HEX A HEX2 (ΔT LM2 CF); where U HEX represents the heat transfer of the heat exchanger, A HEX1 represents the heat transfer area of ​​heat exchanger 1, A HEX2 represents the heat transfer area of ​​heat exchanger 2, ΔT LM1 and ΔT LM2 They represent the logarithmic mean temperature difference at both ends of the heat exchanger of heat exchanger 1 and heat exchanger 2 respectively, and CF represents the plate combination correction coefficient.

[0058] Heat generated outside the tube in the evaporator Q e =M d λ vp +M f C pf (T b -T f ) ; The heat generated in the tube of the evaporator Q c =M d λ d +M d C pv (T s -T d );Q e =Q c ; vp , d They represent the latent heat below the demister and the latent heat of the steam in the tube, T s Indicates the temperature of superheated steam. f Indicates the temperature after passing through the heat exchanger, C pv It represents the specific heat capacity of the steam above the demister, and Td represents the temperature of the condensed water discharged from the evaporator.

[0059] Temperature below the demister T vp =T b -BPE;T b Indicates the boiling point temperature of feed water, BPE indicates the boiling point elevation coefficient of evaporator feed water, BPE = f·Δ0′; f indicates the correction coefficient, T VP represents the temperature below the demister, and r represents the latent heat of vaporization of water. The boiling point elevation coefficient of the solution at normal pressure is Δ0′=69.8×X b 3 -8.15×X b 2 +5.19×X b ; T vp=T v ; T v Indicates the temperature above the demister. b Indicates the mass percentage concentration of concentrated brine, ΔT = T d -T b ; ΔT represents the difference between the condensing steam temperature at the evaporator outlet and the concentrated water temperature at the evaporator outlet.

[0060] The heat transfer coefficient of the evaporator can also be expressed as: Q e =U e A e ΔT; U e represents the overall heat transfer coefficient, A e Indicates the heat transfer area of ​​the evaporator.

[0061] H s =2499.15+1.955×T s -1.927×10 -3 ×T s 2 ;

[0062] Hs and Hv represent the enthalpy of the superheated steam at the compressor outlet and the enthalpy of the steam above the demister, respectively.

[0063] Power of inverter compressor W fre =(H s -H v )·0.000277·ε / η com ; ε represents the compression ratio, η com Indicates the efficiency of the compressor.

[0064] Step 4: Establish economic model of reverse osmosis system and mechanical vapor recompression system:

[0065] Step 4.1: Establish an economic model for the reverse osmosis system:

[0066] Investment cost:

[0067] Membrane element cost Among them, Pric k represents the unit price of the k-th reverse osmosis membrane element, N mem Indicates the number of reverse osmosis membrane elements in the pressure vessel, N pv,j represents the number of pressure vessels in the jth reverse osmosis section;

[0068] Pressure vessel cost Pric pv Indicates the unit price of the pressure vessel;

[0069] Feed water intake pump CC in =1417.3×'Qfraw ) 0.8 ;Q fraw Indicates the flow rate of feed water to the reverse osmosis system.

[0070] High pressure pump cost Indicates the pressure of the high-pressure pump of the reverse osmosis system.

[0071] Vacuum pump cost P_boost j Represents the pressure of the pump corresponding to the jth boosting node in the reverse osmosis system.

[0072] Cost of energy recovery devices Q pxhin It indicates the high-pressure brine flow rate of the last stage (stageNro) of reverse osmosis entering PX.

[0073] Total investment cost of reverse osmosis system CC = (CC mem +CC pv +CC in +CC hpp +CC bp +CC px )×1.411;

[0074] Operating costs:

[0075] Chemical addition costs for feed water pretreatment OC ch =kQ fraw ; k represents the conversion coefficient between chemical additive cost and feed flow rate, Q fraw Indicates the flow rate of feed water to the reverse osmosis system.

[0076] Energy consumption cost of feed water intake pump P0 represents the outlet pressure of the water intake pump, P elc represents the electricity price, η IP It represents the efficiency of the seawater intake pump and PLF represents the load factor.

[0077] Operating electricity costs ε p Indicates the high-pressure pump efficiency, ε VFD Indicates the mechanical efficiency of the variable frequency drive, ε bp Indicates the booster pump efficiency.

[0078] Reverse osmosis membrane module replacement cost OC me =CC mem ·ζ re ;ζ re Indicates the replacement rate of membrane components.

[0079] Reverse osmosis membrane module maintenance cost OC mn =Qpout ×24×365×PLF×0.01;

[0080] Labor cost OC lb =Pri LB ·N LB ; N LB Represents the number of laborers.

[0081] OC px =0.027Q pxhin P pxhin η px P elc ;OC px Indicates the operating energy consumption of the energy recovery device, Q pxhin Indicates the flow rate of high-pressure concentrated brine entering the energy recovery device in the last section of reverse osmosis, P pxhin Indicates the pressure of high-pressure brine entering the energy recovery device in the last section of reverse osmosis, η px Indicates the efficiency of the energy recovery device.

[0082] Total operating cost OC = OC ip +OC en +OC me +OC mn +OC ch +OC lb -OC px .

[0083] Step 4.2: Establish an economic model for the mechanical vapor recompression system:

[0084] Investment cost of heat exchange equipment: CC area =(A e +A HEX1 +A HEX2 )Pric area ;Pric area It represents the unit heat transfer area cost of the heat exchanger and evaporator of the mechanical vapor recompression system.

[0085] Operating cost: including the energy cost of the compressor and the energy cost of the circulation pump;

[0086] Energy cost of compressor: PC com =W fre ×P elc ×ρ×24×365;P elc represents the electricity price, ρ represents the density of the solution, W fre is the power of the variable frequency compressor.

[0087] Energy cost of circulation pump: OC cir =M f ×24×365×Pelc ×ρ×β bemp β bemp Indicates the efficiency of the circulation pump.

[0088] Step 4.3: Establish a joint economic model of the reverse osmosis system combined with the mechanical vapor recompression system:

[0089] TAC=OC+CC+CC area +OC com +OC cir ;

[0090] Step 5: Establish an optimal design model for the reverse osmosis system combined with the mechanical vapor recompression system:

[0091] The reverse osmosis system combined with the mechanical vapor recompression system is expressed as a mixed integer nonlinear program:

[0092] Objective function:

[0093] Constraints: Model constructed in steps 1 to 4;

[0094] Step 6: Solve the established optimization model:

[0095] The mean value of the parameters at the beginning and end of the membrane element is used to characterize the element, and the finite difference method is used to discretize the differential equation. This method approximates the derivative of the function by the difference between the value of the function at a certain point and its value at the adjacent point, and uses h(i) to represent the length of the i-th grid after the membrane element is meshed. At the grid point, the derivative term is replaced by the difference quotient of the function at two adjacent grid points, and the differential equation is replaced by the difference quotient of the function at two adjacent grid points. Converted to the following algebraic equation:

[0096]

[0097] The converted mixed integer nonlinear programming problem is obtained and solved by using a mixed integer programming solver. The results obtained by the solution are analyzed and the optimal design scheme of the joint system is obtained.

[0098] Furthermore, in the reverse osmosis superstructure network model established in step 1:

[0099] The concentrated water produced by the last reverse osmosis stageNro is pressure-recovered by the energy recovery device and then enters the concentrated water outlet; the high-pressure concentrated water inlet of the energy recovery device is connected to the incoming concentrated water, and the low-pressure water inlet of the energy recovery device is connected to the first boosting node to flow in the remaining feed water; after the high-pressure concentrated water and the low-pressure remaining feed water to be treated are pressure-exchanged, the high-pressure concentrated water is reduced in pressure and flows out of the low-pressure concentrated water outlet of the energy recovery device and flows out through the concentrated water outlet, and the low-pressure remaining feed water to be treated is pressure-exchanged with the high-pressure concentrated water and then boosted in pressure and flows out of the high-pressure water inlet of the energy recovery device, and then is pressurized again by the vacuum pump and flows into the corresponding reverse osmosis node and then enters the corresponding reverse osmosis stage.

[0100] The material balance of the high-pressure pump and the energy recovery device, ignoring the leakage of the energy recovery device, is the feed water flow rate Q at the high-pressure water inlet outlet of the energy recovery device. pxhout Equal to the low pressure water inlet feed water flow rate Q pxlin , the low-pressure concentrated water outlet concentrated water flow rate Q of the energy recovery device pxlout Equal to the concentrated water flow rate Q at the high-pressure concentrated water inlet pxhin , the concentrated water flow rate Q at the high-pressure concentrated water inlet of the energy recovery device pxhin Equal to the high-pressure concentrated water flow Q of the last reverse osmosis stageNro entering the energy recovery device bpx,Nro ;

[0101] Concentrate flow rate Q at the high-pressure concentrate inlet of the energy recovery device pxhin and the brine salt concentration C at the high-pressure brine inlet of the energy recovery device hin Satisfaction: Q pxhin C hin =Q bpx,Nro C bpx,Nro ; where C bpx,Nro It indicates the salt concentration of the high-pressure concentrated water entering the energy recovery device from the last reverse osmosis stageNro.

[0102] represents the brine flow rate from the jth reverse osmosis stage to the brine outlet; represents the concentration of concentrated water flowing to the concentrated water outlet of the jth reverse osmosis section, C pxlout Indicates the salt concentration of the concentrate at the low-pressure concentrate outlet of the energy recovery device. in, represents the fresh water flow from the jth reverse osmosis section to the fresh water outlet, It represents the salt concentration of fresh water flowing to the fresh water outlet of the jth reverse osmosis stage.

[0103] As a preferred embodiment, the optimization process described in step 6 is as follows: firstly, a larger number of boosting nodes N is given. psand the number of RO nodes N ro As the initial value to solve; after obtaining the optimal solution, the Q b,i,j and Q p,i,j , if greater than 0, there are concentrated brine and fresh water streams in the reverse osmosis system from the jth reverse osmosis flow to the i-th boosting node; if equal to 0, there are no concentrated brine and fresh water streams in the reverse osmosis system from the jth reverse osmosis flow to the i-th boosting node; N mem,j Indicates the number of membrane elements to be placed in each pressure vessel in the jth reverse osmosis section, N pv,j represents the number of pressure vessels to be placed in the jth reverse osmosis section, x j,k Equal to 1 means that the jth reverse osmosis uses the kth membrane element type, A e Represents the heat transfer area of ​​the evaporator, A HEX1 and A HEX2 Represents the heat transfer area of ​​the two heat exchangers. The presence of the equipment is indirectly determined by analyzing the operating variables, and finally the optimal design structure of the mechanical vapor recompression system and the reverse osmosis system is obtained to minimize the total annualized cost under given feed conditions.

[0104] The present invention establishes a strict reverse osmosis unit model based on the reverse osmosis mechanism and the structure of the entire process, and combines the reverse osmosis superstructure network model to include all feasible design structures. The reverse osmosis system can select between multiple design structures and perform simulation calculations to obtain the flow rate and salt concentration of brine and fresh water under the design scheme. A mechanical vapor recompression system mechanism model is established, and the brine produced by the reverse osmosis system flows into the mechanical vapor recompression system for evaporation and concentration, and the recovery rate is improved by recovering condensed water. The evaporated and concentrated brine can be used for subsequent evaporation and crystallization to obtain salt substances. An economic model of the reverse osmosis system and the mechanical vapor recompression system is established, and the lowest total annualized cost is taken as the objective function. The optimization design model of the reverse osmosis system combined with the mechanical vapor recompression system is established in connection with relevant design variables, and the established optimization model is solved, and the differential equations therein are processed, and the problem is converted into a mixed integer nonlinear programming problem, and a mixed integer nonlinear programming solver is used to solve it, and the joint system design structure under the lowest total annualized cost is obtained. Through the optimization design method of the present invention, the actual level of design and operation of the combined system can be significantly improved, the utilization rate of water resources can be further improved, the initial investment and operation and maintenance costs required for the system can be reduced, thereby reducing the total annualized cost, improving economic benefits, and increasing the return on investment for end users. At the same time, after being treated by a mechanical vapor recompression device, the negative impact of direct wastewater discharge on the environment can be reduced, thereby promoting sustainable development. BRIEF DESCRIPTION OF THE DRAWINGS

[0105] Figure 1 It is a schematic diagram of the reverse osmosis superstructure network;

[0106] Figure 2 It is a basic flow diagram of the reverse osmosis system;

[0107] Figure 3 It is a schematic diagram of a mechanical vapor recompression system;

[0108] Figure 4 It is a schematic diagram of the combination of a mechanical vapor recompression system and a reverse osmosis system;

[0109] Figure 5 It is the combined structure of the mechanical vapor recompression system and the reverse osmosis system before optimization in the embodiment;

[0110] Figure 6 This is the optimized combined structure of the mechanical vapor recompression system and the reverse osmosis system in the embodiment. DETAILED DESCRIPTION

[0111] The present invention is further described below in conjunction with the accompanying drawings.

[0112] A joint optimization method of a mechanical vapor recompression system and a reverse osmosis system comprises the following steps:

[0113] Step 1: Establish a reverse osmosis superstructure network model:

[0114] like Figure 1 As shown, the reverse osmosis superstructure network includes N ro RO nodes and N ps There are N boost nodes in total ps +2 logistics nodes, the added 2 logistics nodes refer to the concentrated water outlet and the fresh water outlet that finally flow out of the network. ro RO nodes and N ps The boosting nodes correspond one to one on the same circuit; each reverse osmosis node is the entrance of a reverse osmosis section, and each reverse osmosis section consists of N pv Pressure vessels are connected in parallel, each of which consists of N mem reverse osmosis membrane elements are connected in series.

[0115] The first boosting node is used as the feed water inlet. A portion of the feed water (mixed with a portion of the concentrated water / fresh water produced by any reverse osmosis membrane) is transported to the first reverse osmosis node through a high pressure pump (HPP) and then enters the first reverse osmosis stage (stage 1). The concentrated water / fresh water stream produced by the first reverse osmosis stage can enter any boosting node. The concentrated water / fresh water that enters the i-th boosting node enters the j-th reverse osmosis stage after passing through the corresponding j-th reverse osmosis node; the concentrated water or fresh water stream produced by any reverse osmosis stage can enter any boosting node. The stream that enters the boosting node enters the corresponding reverse osmosis node after being pressurized by a vacuum pump (BP), or can directly enter a reverse osmosis node without being pressurized by BP.

[0116] In this embodiment, the fresh water produced by all reverse osmosis stages flows out directly through the fresh water outlet. Except for the last reverse osmosis stage (stageNro), the concentrated water produced by the other reverse osmosis stages enters any boosting node, enters the corresponding reverse osmosis node and enters the corresponding reverse osmosis stage for further treatment. The remaining part enters the next boosting node or flows out directly through the concentrated water outlet. The concentrated water produced by the last reverse osmosis stage (stageNro) flows out directly through the concentrated water outlet.

[0117] The concentrated water produced by the last reverse osmosis stage (stageNro) can also enter the concentrated water outlet after pressure recovery through the energy recovery device (PX); the high-pressure concentrated water inlet of the energy recovery device (PX) is connected to the incoming concentrated water (high pressure), and the low-pressure water inlet of the energy recovery device (PX) is connected to the remaining feed water (mixed with part of the concentrated water / fresh water produced by any reverse osmosis stage, low pressure) flowing into the first boosting node; after the high-pressure concentrated water and the low-pressure remaining feed water to be treated are pressure-exchanged, the high-pressure concentrated water is reduced in pressure and flows out of the low-pressure concentrated water outlet of the energy recovery device (PX) and flows out through the concentrated water outlet, and the low-pressure remaining feed water to be treated is pressure-exchanged with the high-pressure concentrated water and then boosted out of the high-pressure water inlet of the energy recovery device (PX) and then pressurized again by the vacuum pump and flows into the corresponding reverse osmosis node and then enters the corresponding reverse osmosis stage (the boosting is to ensure that the pressure of the remaining feed water to be treated is consistent with that of the feed water entering the reverse osmosis node through the HPP).

[0118] The constraints that the entire superstructure network needs to meet are:

[0119] Among them, Q pout represents the freshwater flow leaving the reverse osmosis network, C pout Represents the salt concentration of fresh water leaving the reverse osmosis network; represents the minimum freshwater flow requirement for the design, Indicates the maximum freshwater salt concentration requirement for the design;

[0120] Inlet feed water flow rate Q of the reverse osmosis superstructure network fraw =Q bout +Q pout ; According to the law of conservation of materials in the reverse osmosis process, the feed water flow rate Q at the feed port of the reverse osmosis superstructure network f and the feed water salt concentration C at the feed port of the reverse osmosis superstructure network fraw Satisfaction: Q fraw C fraw =Q bout C bout +Q pout C pout ;Q bout represents the concentrate flow rate leaving the reverse osmosis network, C bout Indicates the salt concentration of the concentrate leaving the reverse osmosis network;

[0121] In this embodiment, the material balance of the high-pressure pump and the energy recovery device does not consider the leakage of the energy recovery device. The feed water flow rate Q at the high-pressure water inlet outlet of the energy recovery device is pxhout Equal to the low pressure water inlet feed water flow rate Q pxlin , the low-pressure concentrated water outlet concentrated water flow rate Q of the energy recovery device pxlout Equal to the concentrated water flow rate Q at the high-pressure concentrated water inlet pxhin , the concentrated water flow rate Q at the high-pressure concentrated water inlet of the energy recovery device pxhin Equal to the high-pressure concentrated water flow Q of the last reverse osmosis stage (stageNro) entering the energy recovery device (PX) bpx,Nro ;

[0122] Concentrate flow rate Q at the high-pressure concentrate inlet of the energy recovery device pxhin and the brine salt concentration C at the high-pressure brine inlet of the energy recovery device hin Satisfaction: Q pxhin C hin =Q bpx,Nro C bpx,Nro ; where C bpx,Nro It indicates the salt concentration of the high-pressure concentrated water entering the energy recovery device (PX) from the last reverse osmosis stage (stageNro).

[0123] represents the brine flow rate from the jth reverse osmosis stage to the brine outlet; represents the concentration of concentrated water flowing to the concentrated water outlet of the jth reverse osmosis section, C pxlout Indicates the salt concentration of the concentrate at the low-pressure concentrate outlet of the energy recovery device (PX). in, represents the fresh water flow from the jth reverse osmosis section to the fresh water outlet, It represents the salt concentration of fresh water flowing to the fresh water outlet of the jth reverse osmosis stage.

[0124] Inlet flow rate of the first boosting node Among them, Q fraw represents the inlet feed water flow rate of the reverse osmosis superstructure network, Q b,1,j represents the concentrated water flow rate of the jth reverse osmosis entering the first boosting node, Q p,1,j represents the fresh water flow rate of the jth reverse osmosis section entering the first boosting node (the fresh water flow rate is zero in this embodiment);

[0125] The water flow rate Q of the first boosting node ps,1 and the inlet water concentration C at the first boosting node ps,1 satisfy: Among them, C fraw represents the feed water salt concentration of the reverse osmosis superstructure network, C b,1,j represents the salt concentration of the concentrated water entering the first boosting node of the jth reverse osmosis section, C p,1,j represents the salt concentration of freshwater entering the first boosting node of the jth reverse osmosis section; the pressure of the first boosting node P ps,1 Equal to the inlet pressure P of the reverse osmosis superstructure network fraw .

[0126] The water inlet flow rate of the first boosting node also satisfies: Q ps,1 =Q hpp +Q pxlin , where Q hpp Indicates the flow rate of feed water entering the high-pressure pump; the inlet flow rate Q of the first boosting node ps,1 and the inlet concentration C of the first boost node ps,1 Satisfaction: Q ps, 1C ps,1 =Q hpp C hpp +Q pxlin C pxlin , where C hpp Indicates the salt concentration of the feed water entering the high-pressure pump, C pxlin represents the salt concentration of the feed water at the low-pressure water inlet of the energy recovery device; at this time, the feed flow rate Q of the first reverse osmosis node ro,1 =Q hpp +Q pxhout ; Feed water flow rate Q of the first reverse osmosis node ro,1 and the feed water concentration C of the first reverse osmosis node ro,1 Satisfaction: Q ro,1 C ro,1 =Q hpp C hpp +Q pxhout C pxhout ; where C pxhout It indicates the salt concentration of the high-pressure feed water flowing out of the high-pressure water inlet outlet after pressure exchange of the energy recovery device;

[0127] Inlet flow of the i-th booster i=2,3,...,Nps; where Q b,i,j represents the concentrated water flow rate from the jth reverse osmosis section to the i-th boosting node, Q p,i,j represents the fresh water flow rate leaving the j-th reverse osmosis section and entering the i-th boosting node (the fresh water flow rate is zero in this embodiment);

[0128] The water inlet flow rate Q of the i-th boosting node ps,i and the inlet water concentration C of the i-th boost nodeps,i satisfy: i=2,3,...,Nps; where C b,i,j represents the salt concentration of the concentrated water entering the i-th boosting node of the j-th reverse osmosis section, C p,i,j It represents the salt concentration of fresh water entering the i-th boosting node through the j-th reverse osmosis section.

[0129] The concentrated water flow rate produced by the jth reverse osmosis stage Freshwater flow rate generated by the jth reverse osmosis stage In actual use, the concentrated water produced by the reverse osmosis section generally does not enter the fresh water outlet, and the fresh water produced by the reverse osmosis section generally does not enter the concentrated water outlet.

[0130] The feed water flow rate Q of the jth reverse osmosis node RO,j =Q ps,i , j=i, j=1,2,3...,Nro; the feed water concentration C of the jth reverse osmosis node RO,j =C ps,i , j=i,j=1,2,3...,Nro;The pressure P of the jth reverse osmosis node RO,j =P ps,i +P_boost j j=i,j=1,2,3...,Nro;P ps,i represents the feed water pressure entering the i-th boost node, P_boost j represents the boost pressure of the pump (j=1 represents HPP, j>1 represents BP), P RO,j It represents the outlet pressure of the j-th boosting stage (after HPP or BP boosting).

[0131] The feed water flow, concentration and pressure of the jth reverse osmosis node are transmitted to the reverse osmosis process model established in step 2 to calculate the concentrate flow, concentrate concentration, concentrate pressure and fresh water flow, fresh water concentration and fresh water pressure generated by the jth reverse osmosis section.

[0132] Step 2: Establish the reverse osmosis unit model:

[0133] Step 2.1, establish the connection model between the reverse osmosis superstructure network and the reverse osmosis process model:

[0134] Q f =Q RO,j / N pv,j ; C f =C RO,j ;P f =P RO,j ;like Figure 2 As shown, Q f , C f , Pf Respectively represent the feed water flow, salt concentration and pressure of the reverse osmosis section, N pv,j Represents the number of pressure vessels in the jth reverse osmosis stage.

[0135] Step 2.2: According to the law of conservation of materials in the reverse osmosis process of each reverse osmosis section, establish the solution conservation and flow conservation equations: Q f =Q p -Q r ;Q f C f =Q p C p +Q r C r ; where C p represents the salt concentration of fresh water produced by the reverse osmosis stage, C r Indicates the salt concentration of the concentrate produced by the reverse osmosis stage.

[0136] The fresh water flow rate generated by the reverse osmosis section can also be expressed as:

[0137] Among them J v(z) is the reverse osmosis water flux, n l , W and L represent the number of blades, width and length of the reverse osmosis membrane respectively.

[0138] According to the dissolution-diffusion model: Solvent flux (water flux) through the reverse osmosis membrane J v(z) =A w(z) (P f -P d(z) -P p(z) -Δπ (z) ); solute flux (salt flux) through the reverse osmosis membrane J s(z) =B s (C m(z) -C p(z) );

[0139] Among them, Aw (z) is the solvent transport parameter, Pf is the feed water pressure of the reverse osmosis section, Pd (z) is the pressure drop at position z in the reverse osmosis membrane channel, P p(z) is the osmotic pressure at position z in the membrane channel, Δπ (z) is the osmotic pressure difference at position z in the membrane channel. Bs is the solute transport parameter, C m(z) is the solute concentration measured on the surface of the reverse osmosis membrane at position z in the membrane channel, C p(z) It is the solute concentration measured on the surface of the reverse osmosis membrane at position z in the membrane channel.

[0140] Feed water pressure P at position z in the reverse osmosis membrane channel b(z) =P f -Pd(z) ; Transmembrane pressure difference ΔP at position z in the reverse osmosis membrane channel (z) =(P b(z) -P p(z) );Osmotic pressure difference Δπ at position z in the reverse osmosis membrane channel (z) =RT(C m(z) -C p(z) );According to the above formula: J v(z) =A w (ΔP (z) -Δπ (z) ).

[0141] Solvent transport parameter A w and solute transport parameter B s is a function of temperature: Where N memtype Indicates the number of membrane elements, A j is the intrinsic solvent transport parameter of the membrane element used in the jth reverse osmosis stage, B j is the intrinsic solute transport parameter A of the membrane element used in the jth reverse osmosis k , B k is the inherent transport parameter of the kth reverse osmosis membrane element model under standard conditions, x j,k It is a 0-1 variable. When the j-th reverse osmosis pressure vessel uses the k-th type of membrane element, it takes the value of 1, otherwise it takes the value of 0. α1, α2 and β1 represent constants, and T represents the Kelvin temperature at which the system operates.

[0142] Considering the influence of concentration polarization in the reverse osmosis process, the concentration polarization phenomenon is that the salt concentration on the surface of the reverse osmosis membrane increases and the osmotic pressure increases. In order to maintain the original water flux, the external pressure must be increased: concentration polarization value Among them C b(z) represents the salt concentration of concentrated water at position z in the reverse osmosis membrane channel, k c(z) It represents the membrane mass transfer coefficient at the position z in the reverse osmosis membrane channel.

[0143] Solute mass transfer coefficient k c(z) Obtained by the following equation: Re (z) =ρ (z) V (z) d e / μ (z) Sc (z) =μ (z) / (ρ (z) D AB(z) );

[0144] Among them Sh (z)Re represents the Sherwood number at position z in the reverse osmosis membrane channel, Re (z) Represents the Reynolds number at position z in the reverse osmosis membrane channel, Sc (z) represents the Schmidt number at position z in the reverse osmosis membrane channel, d e represents the hydraulic diameter of the feed spacing channel, V (z) is the axial velocity of seawater at position z in the reverse osmosis channel;

[0145] ρ (z) represents the density of the permeate water at the position z in the reverse osmosis channel, μ (z) Indicates dynamic viscosity, D AB(z) represents the kinematic viscosity, and the above parameters are calculated by the regression equation:

[0146] Among them, k1, k2, b1, b2, a1, a2, and a3 are all regression equation coefficients, and M is a variable related to the Kelvin temperature T.

[0147] Reverse osmosis membrane solvent flux J v(z) and solute flux J s(z) Satisfaction: J s(z) =J v(z) ×C p(z) ;

[0148] Without considering the influence of pollutants on the system during the reverse osmosis process, following the law of conservation of materials and the mechanism shown above, the change of the pressure loss Pd(z) in the reverse osmosis membrane channel along the channel is expressed as: λ (z) =6.23K λ Re (z) -0.3 ; K λ is the empirical parameter related to the membrane element, λ (z) is the friction coefficient at position z in the reverse osmosis membrane channel.

[0149] Due to the pressure in the reverse osmosis membrane channel So there is

[0150] And when z=0, P b(0) =P f ; When z = L, P b(L) =P r L is the length of the membrane channel, z = L refers to the end of the membrane channel, P r It is the concentrate pressure at the outlet of the reverse osmosis section.

[0151] Axial flow velocity of seawater in the reverse osmosis channel V (z) The changes along the channel can be expressed as:

[0152] When z = 0, When z = L, h sp is the height of the feed interval channel. Qr is the concentrated water flow rate at the outlet of the reverse osmosis section.

[0153] Solute concentration (salt concentration) C in the reverse osmosis membrane channel b(z) The changes along the channel can be expressed as:

[0154] And when z=0, C b(0) =C f , when z=L, C b(L) =C r . C r It is the salt concentration of the concentrate at the outlet of the reverse osmosis section.

[0155] According to the above formula, when the fresh water pressure P p When is 0, given the feed water flow, pressure, and salt concentration of the jth reverse osmosis node, the final fresh water flow Q at the outlet of the corresponding reverse osmosis section can be obtained. p and freshwater salt concentration C p .

[0156] Step 2.3: Establish the connection model between the reverse osmosis process model and the reverse osmosis superstructure network:

[0157] Q b,j =Q r *N pv,j ; C b,j =C r ;P b,j =P r ; where Q r , C r , P r They represent the concentrated water flow, salt concentration and pressure at the outlet of the reverse osmosis section respectively. p,j =W p ; C p,j =C p ;P p,j =P p ; where Q p , C p , P p They represent the fresh water flow, salt concentration and pressure at the outlet of the reverse osmosis section respectively.

[0158] Step 3: Establish the mechanism model of the mechanical vapor recompression system:

[0159] like Figure 3 and 4As shown, the concentrated water finally produced by the reverse osmosis system flows into the mechanical vapor recompression system for evaporation and concentration treatment. The mechanical vapor recompression system includes two heat exchangers, a circulation pump, an evaporator, a demister and a steam compressor. The inlets of the two heat exchangers are connected to the feed water of the mechanical vapor recompression system (the concentrated water flowing out of the concentrated water outlet of the reverse osmosis superstructure network), and the outlets of the two heat exchangers are connected to the circulation pump through pipelines. After being heated by the heat exchanger, the feed water is pressurized by the circulation pump and enters the evaporator, and is evenly sprayed on the outer wall of the hot pipe of the evaporator through the built-in nozzle of the evaporator; the demister is arranged in the evaporator, connected to the air inlet of the steam compressor through a pipeline, the air outlet of the steam compressor is connected to the hot pipe of the evaporator, and the condensed water outlet of the hot pipe of the evaporator is connected to the hot end inlet of heat exchanger 1; the concentrated water outlet of the evaporator is connected to the hot end inlet of heat exchanger 2.

[0160] When in use, the feed water of the mechanical vapor recompression system is fully heat-exchanged with the concentrated water and condensed water produced by the evaporator through two heat exchangers to increase the temperature of the feed water (heating), and then the feed water reaches the evaporator through the circulation pump; the built-in nozzle of the evaporator is evenly sprayed on the outer wall of the hot pipe of the evaporator, and the water vapor in the feed water flows into the steam compressor through the demister, and the steam compressor compresses the water vapor into high-temperature and high-pressure superheated gas; the compressed superheated gas enters the pipe of the evaporator, exchanges heat with the feed water sprayed on the outer wall of the pipe, and then condenses, flows into heat exchanger 1 through the condensed water outlet, exchanges heat with part of the feed water, and the generated fresh water flows out from the hot end outlet of heat exchanger 1. The feed water (higher concentration concentrated water) that is not evaporated in the evaporator flows out through the concentrated water outlet of the evaporator and then flows into heat exchanger 2, and the generated concentrated brine flows out from the hot end outlet of heat exchanger 2.

[0161] Assuming that the salt concentration of steam is 0%, there is no energy loss in the evaporator, preheater, pipeline and pump system; according to the principle of mechanical vapor compression evaporation, the law of conservation of energy and the law of conservation of heat, mechanical vapor compression evaporation is described by the following equation:

[0162] Mechanical vapor recompression system feed water mass flow rate M f =M b +M d ; where M d represents the mass flow rate, M b represents the mass flow rate of concentrated brine; the mass flow rate of feed water in the mechanical vapor recompression system M f And the mass percentage concentration of feed water X f Satisfaction: M f X f =M b X b +M d X d ; Among them, X d Indicates the mass percentage concentration of fresh water, Xb Indicates the mass percentage concentration of concentrated brine.

[0163] Mechanical vapor recompression system feed water mass flow rate M f =Q bout ×1000, mass percentage concentration of feed water X f =C bout ÷10;Q bout , C bout They represent the brine flow rate and brine concentration ultimately produced by the reverse osmosis superstructure network, respectively.

[0164] The heat generated in a heat exchanger Q HEX1 =M d C pd (T d -T d< )=M f1 C pf (T i1 -T i ), the heat generated in the other heat exchanger Q HEX2 =M b C pb (T b -T b0 )=M f2 C pf (T i2 -t i );C pd represents the specific heat capacity of the condensed water in the recompression system, C pf represents the specific heat capacity of the feed water in the recompression system, C pb represents the specific heat capacity of the concentrated brine in the recompression system, M f1 represents the feed mass flow rate through heat exchanger 1, M f2 represents the feed mass flow rate through heat exchanger 2, T i Indicates the temperature of the feed water, T b Indicates the brine temperature at the evaporator outlet (i.e. the boiling point of the evaporator feed water), T d Indicates the temperature of the condensed steam at the evaporator outlet, T bo represents the temperature of the concentrated brine after passing through heat exchanger 2, T do represents the temperature of fresh water after passing through heat exchanger 1, T i1 represents the temperature of the feed water after heat exchange in heat exchanger 1, T i2 Indicates the temperature of the feed water after heat exchange in heat exchanger 2.

[0165] It can also be expressed as: Q HEX1 =U HEX A HEX1 (ΔT LM1 CF), Q HEX2=U HEX A HEX2 (ΔT LM2 CF); where U HEX represents the heat transfer of the heat exchanger, A HEX1 represents the heat transfer area of ​​heat exchanger 1, A HEX2 represents the heat transfer area of ​​heat exchanger 2, ΔT LM1 and ΔT LM2 They represent the logarithmic mean temperature difference at both ends of the heat exchanger of heat exchanger 1 and heat exchanger 2 respectively, and CF represents the plate combination correction coefficient.

[0166] Heat generated outside the tube in the evaporator Q e =M d λ vp +M f C pf (T b -T f ) ; The heat generated in the tube of the evaporator Q c =M d λ d +M d C pv (T s -T d );Q e =Q c ; vp , d They represent the latent heat below the demister and the latent heat of the steam in the tube, T s Indicates the temperature of superheated steam. f Indicates the temperature after passing through the heat exchanger, C pv It represents the specific heat capacity of the steam above the demister, and Td represents the temperature of the condensed water discharged from the evaporator.

[0167] Temperature below the demister T vp =T b -BPE;T b Indicates the boiling point temperature of feed water, BPE indicates the boiling point elevation coefficient of evaporator feed water, BPE = f·Δ0′; f indicates the correction coefficient, T VP represents the temperature below the demister, and r represents the latent heat of vaporization of water. The boiling point elevation coefficient of the solution at normal pressure is Δ0′=69.8×X b 3 -8.15×X b 2 +5.19×X b ; T vp =T v ; T v Indicates the temperature above the demister. bIndicates the mass percentage concentration of concentrated brine, ΔT = T d -T b ; ΔT represents the difference between the condensing steam temperature at the evaporator outlet and the concentrated water temperature at the evaporator outlet.

[0168] The heat transfer coefficient of the evaporator can also be expressed as: Q e =U e A e ΔT; U e represents the overall heat transfer coefficient, A e Indicates the heat transfer area of ​​the evaporator.

[0169]

[0170] H s =2499.15+1.955×T s -1.927×10 IK ×T s 2 ;

[0171] Hs and Hv represent the enthalpy of the superheated steam at the compressor outlet and the enthalpy of the steam above the demister, respectively.

[0172] Power of inverter compressor W fre =(H s -H v )·0.000277·ε / η com ; ε represents the compression ratio, η com Indicates the efficiency of the compressor.

[0173] Step 4: Establish economic model of reverse osmosis system and mechanical vapor recompression system:

[0174] Step 4.1: Establish an economic model for the reverse osmosis system:

[0175] Investment cost:

[0176] Membrane element cost Among them, Pric k represents the unit price of the k-th reverse osmosis membrane element, N mem Indicates the number of reverse osmosis membrane elements in the pressure vessel, N pv,j represents the number of pressure vessels in the jth reverse osmosis section;

[0177] Pressure vessel cost Pric pv Indicates the unit price of the pressure vessel;

[0178] Feed water intake pump CC in =1417.3×(Q fraw ) 0.8 ;Qfraw Indicates the flow rate of feed water to the reverse osmosis system.

[0179] High pressure pump cost Indicates the pressure of the high-pressure pump of the reverse osmosis system.

[0180] Vacuum pump cost P_boost j Represents the pressure of the pump corresponding to the jth boosting node in the reverse osmosis system.

[0181] Cost of energy recovery devices Q pxhin It indicates the high-pressure brine flow rate of the last stage (stageNro) of reverse osmosis entering PX.

[0182] Total investment cost of reverse osmosis system CC = (CC mem +CC pv +CC in +CC hpp +CC bp +CC px )×1.411;

[0183] Operating costs:

[0184] Chemical addition costs for feed water pretreatment OC ch =kQ fraw ; k represents the conversion coefficient between chemical additive cost and feed flow rate, Q fraw Indicates the flow rate of feed water to the reverse osmosis system.

[0185] Energy consumption cost of feed water intake pump ; P0 represents the outlet pressure of the water pump, P elc represents the electricity price, η IP It indicates the efficiency of the seawater intake pump, and PLF indicates the load factor, which can generally be 0.7-0.9.

[0186] Operating electricity costs ε p Indicates the high-pressure pump efficiency, ε VFD Indicates the mechanical efficiency of the variable frequency drive, ε bp Indicates the booster pump efficiency.

[0187] Reverse osmosis membrane module replacement cost OC me =CC mem ·ζ re ;ζ re Indicates the replacement rate of membrane components.

[0188] Reverse osmosis membrane module maintenance cost OC mn =Q pout×24×365×PLF×0.01;

[0189] Labor cost OC lb =Pri LB ·N LB ; N LB Represents the number of laborers.

[0190] OC px =0.027Q pxhin P pxhin η px P elc ;OC px Indicates the operating energy consumption of the energy recovery device, Q pxhin Indicates the flow rate of high-pressure concentrated brine entering the energy recovery device in the last section of reverse osmosis, P pxhin Indicates the pressure of high-pressure brine entering the energy recovery device in the last section of reverse osmosis, η px Indicates the efficiency of the energy recovery device.

[0191] Total operating cost OC = OC ip +OC en +OC me +OC mn +OC ch +OC lb -OC px .

[0192] Step 4.2: Establish an economic model for the mechanical vapor recompression system:

[0193] Investment cost of heat exchange equipment: CC area =(A e +A HEX1 +A HEX2 )Pric area ;Pric area It represents the unit heat transfer area cost of the heat exchanger and evaporator of the mechanical vapor recompression system.

[0194] Operating cost: including the energy cost of the compressor and the energy cost of the circulation pump;

[0195] Energy cost of compressor: OC com =W fre ×P elc ×ρ×24×365;P elc represents the electricity price, ρ represents the density of the solution, W fre is the power of the variable frequency compressor.

[0196] Energy cost of circulation pump: OC cir =M f ×24×365×P elc ×ρ×βbemp β bemp Indicates the efficiency of the circulation pump.

[0197] Step 4.3: Establish a joint economic model of the reverse osmosis system combined with the mechanical vapor recompression system:

[0198] TAC=OC+CC+CC area +OC com +OC cir ;

[0199] Step 5: Establish an optimization design model for the reverse osmosis system combined with the mechanical vapor recompression system:

[0200] The reverse osmosis system combined with the mechanical vapor recompression system is expressed as a mixed integer nonlinear program:

[0201] Objective function:

[0202] Constraints: Model constructed in steps 1 to 4;

[0203] Step 6: Solve the established optimization model:

[0204] The mean value of the parameters at the beginning and end of the membrane element is used to characterize the element, and the finite difference method is used to discretize the differential equation. This method approximates the derivative of the function by the difference between the value of the function at a certain point and its value at the adjacent point, and uses h(i) to represent the length of the i-th grid after the membrane element is meshed. At the grid point, the derivative term is replaced by the difference quotient of the function at two adjacent grid points, and the differential equation is replaced by the difference quotient of the function at two adjacent grid points. Converted to the following algebraic equation:

[0205]

[0206] The converted mixed integer nonlinear programming problem is solved by using a mixed integer programming solver. The optimal design scheme of the joint system can be obtained by analyzing the results obtained by solving the problem.

[0207] Specifically: First, a larger number of boost nodes N is given ps and the number of RO nodes N ro As the initial value, in this embodiment, because the industry generally has three sections of reverse osmosis projects, 3 is taken; after obtaining the optimal solution, the Q b,i,j and Q p,i,j If it is greater than 0, it means that there is a brine stream and a fresh water stream in the reverse osmosis system flowing from the jth reverse osmosis section to the ith boosting node. If it is equal to 0, it means that there is no brine stream and a fresh water stream in the reverse osmosis system flowing from the jth reverse osmosis section to the ith boosting node. N mem,jIndicates the number of membrane elements to be placed in each pressure vessel in the jth reverse osmosis section, N pv,j represents the number of pressure vessels to be placed in the jth reverse osmosis section, x j,k Equal to 1 means that the jth reverse osmosis uses the kth membrane element type, A e Represents the heat transfer area of ​​the evaporator, A HEX1 and A HEX2 Represents the heat transfer area of ​​the two heat exchangers. Analyzing the operating variables can indirectly determine whether the equipment exists or not. For example, if the pressure of the booster pump P_boost,j is greater than 0, there is a booster pump. If it is equal to 0, there is no booster pump. For example, the flow rate Q entering the jth reverse osmosis section ro,j If it is greater than 0, it means there is a j-th reverse osmosis stage, and if it is equal to 0, it means there is no j-th reverse osmosis stage. (Add what the final result is), and finally the optimal design structure of the mechanical vapor recompression system and the reverse osmosis system with the lowest total annualized cost under given feed conditions is obtained.

[0208] When the feed concentration is 24000 mg / L, the required water production Q pout More than 50m 3 / h, produced water salt concentration C pout Less than 500mg / L. The combined structure of the mechanical vapor recompression system and the reverse osmosis system before optimization is as follows Figure 5 As shown, the optimized structure is Figure 6 The results before and after optimization are shown in the following table:

[0209]

[0210] As shown in the table, the reverse osmosis system process used before optimization is one stage, and the required feed flow rate is 167.86m 3 / h, the salt content of the produced water is 0.197mg / L, the type of membrane element used is Hydraulic Proc10, the number of pressure vessels required is 11, the number of membrane elements inside each pressure vessel is 8, the pressure of the high-pressure pump required is 21.49bar, there is no inter-stage pressurization, the high-pressure water inlet outlet of PX also needs a booster pump to reach the pressure entering the reverse osmosis section after the high-pressure pump, the total heat exchange area required is 2.4㎡, and the total annualized cost is 2.3689 million yuan.

[0211] After optimization, the reverse osmosis system process used is two-stage, and the required feed flow rate is 120.15m 3 / h, the salt content of the produced water is 0.209 mg / L, the type of membrane element used is HydraNergy Proc10, the number of pressure vessels required for the first stage is 7, the number of membrane elements inside each pressure vessel is 5, the number of pressure vessels required for the second stage is 6, the number of membrane elements inside each pressure vessel is 6, the pressure of the required high-pressure pump is 32.1 bar, the inter-stage pressurization is 10.21 bar, the total heat exchange area of ​​the heat exchanger and evaporator is 2.3 m2, and the total annualized cost is 2.2156 million yuan.

[0212] Taking the lowest total annual cost as the optimization goal, the cost was reduced by 153,300 yuan compared with before optimization, and the reverse osmosis system structure when the total annual cost was the lowest and the heat exchange area required for the heat exchanger and evaporator of the mechanical vapor recompression system were determined.

[0213] The above contents are further explanations of the present invention by combining specific examples, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art of the present invention, simple deductions or substitutions can be made to a certain extent without departing from the inventive concept, which should be regarded as falling within the protection scope of the present invention.

Claims

1. A joint optimization method of a mechanical vapor recompression system and a reverse osmosis system, characterized in that: The specific steps include: Step 1: Establish a reverse osmosis superstructure network model: The reverse osmosis superstructure network includes N ro RO nodes and N ps There are N boost nodes in total ps +2 logistics nodes, the added 2 logistics nodes refer to the concentrated water outlet and the fresh water outlet that finally flow out of the network; N ro RO nodes and N ps The boosting nodes correspond one to one on the same circuit; each reverse osmosis node is the entrance of a reverse osmosis section, and each reverse osmosis section consists of N pv Pressure vessels are connected in parallel, each of which consists of N mem Reverse osmosis membrane elements are connected in series; The first boosting node is used as the feed water inlet. A part of the feed water is transported to the first reverse osmosis node through a high-pressure pump and then enters the first reverse osmosis stage 1. The concentrated water / fresh water stream produced by the first reverse osmosis section can enter any boosting node. The concentrated water / fresh water entering the i-th boosting node enters the j-th reverse osmosis section after passing through the corresponding j-th reverse osmosis node. The concentrated water or fresh water stream produced by any reverse osmosis section can enter any boosting node. The stream entering the boosting node is boosted by a vacuum pump or directly enters the corresponding reverse osmosis node. The feed water is mixed with part of concentrated water / fresh water produced by any reverse osmosis membrane; The constraints that the entire superstructure network needs to meet are: Among them, Q pout represents the freshwater flow leaving the reverse osmosis network, C pout Represents the salt concentration of fresh water leaving the reverse osmosis network; represents the minimum freshwater flow requirement for the design, Indicates the maximum freshwater salt concentration requirement for the design; Inlet feed water flow rate Q of the reverse osmosis superstructure network fraw =Q bout +Q pout According to the law of conservation of materials in the reverse osmosis process, the feed water flow rate Q at the feed port of the reverse osmosis superstructure network is f and the feed water salt concentration C at the feed port of the reverse osmosis superstructure network fraw Satisfaction: Q fraw C fraw =Q bout C bout +Q pout C pout ;Q bout represents the concentrate flow rate leaving the reverse osmosis network, C bout Indicates the salt concentration of the concentrate leaving the reverse osmosis network; Inlet flow rate of the first boosting node Among them, Q fraw represents the inlet feed water flow rate of the reverse osmosis superstructure network, Q b，1，j represents the concentrated water flow rate of the jth reverse osmosis entering the first boosting node, Q p，1，j represents the freshwater flow rate from the jth reverse osmosis section into the first boosting node; The water flow rate Q of the first boosting node ps，1 and the inlet water concentration C at the first boosting node ps，1 satisfy: Among them, C fraw represents the feed water salt concentration of the reverse osmosis superstructure network, C b，1，j represents the salt concentration of the concentrated water entering the first boosting node of the jth reverse osmosis section, C p，1，j represents the salt concentration of freshwater entering the first boosting node of the jth reverse osmosis section; the pressure of the first boosting node P ps，1 Equal to the inlet pressure P of the reverse osmosis superstructure network fraw ; The water inlet flow rate of the first boosting node also satisfies: Q ps，1 =Q hpp +Q pxlin , where Q hpp Indicates the flow rate of feed water entering the high-pressure pump; the inlet flow rate Q of the first boosting node ps，1 and the inlet concentration C of the first boost node ps，1 Satisfaction: Q ps，1 C ps，1 =Q hpp C hpp +Q pxlin C pxlin , where C hhp Indicates the salt concentration of the feed water entering the high-pressure pump, C pxlin represents the salt concentration of the feed water at the low-pressure water inlet of the energy recovery device; at this time, the feed flow rate Q of the first reverse osmosis node ro，1 =Q hpp +Q pxhout ; Feed water flow rate Q of the first reverse osmosis node ro，1 and the feed water concentration C of the first reverse osmosis node ro，1 Satisfaction: Q ro, 1C ro，1 =Q hpp C hpp +Q pxhout C pxhout ; where C pxhout It indicates the salt concentration of the high-pressure feed water flowing out of the high-pressure water inlet outlet after pressure exchange of the energy recovery device; Inlet flow rate of the i-th booster Among them, Q b，i，j represents the concentrated water flow rate from the jth reverse osmosis section to the i-th boosting node, Q p,i,j represents the freshwater flow leaving the jth reverse osmosis section and entering the i-th boosting node; The water inlet flow rate Q of the i-th boosting node ps,i and the inlet water concentration C of the i-th boost node ps，i satisfy: Among them, C b，i,j represents the salt concentration of the concentrated water entering the i-th boosting node of the j-th reverse osmosis section, C p，i,j represents the salt concentration of fresh water entering the i-th boosting node through the j-th reverse osmosis section; The concentrated water flow rate produced by the jth reverse osmosis stage Freshwater flow rate generated by the jth reverse osmosis stage The feed water flow rate Q of the jth reverse osmosis node R o,j=Qps,i,j=i,j=1,2,3...,Nro;The feed water concentration C of the jth reverse osmosis node RO，j =C ps，i , j=i, j=1,2,3...,Nro; the pressure P of the jth reverse osmosis node RO，j =P ps，i +P-_oost j j=i,j=1,2,3...,Nro;P ps，i represents the feed water pressure entering the i-th boost node, P_boost j Indicates the boost pressure of the pump, j=1 indicates HPP, j>1 indicates BP, P RO，j represents the outlet pressure of the jth booster stage; The feed water flow rate, concentration and pressure of the th reverse osmosis node are transmitted to the reverse osmosis process model established in step 2 to calculate the concentrated water flow rate, concentrated water concentration, concentrated water pressure and the fresh water flow rate, fresh water concentration and fresh water pressure generated by the th reverse osmosis section; Step 2: Establish the reverse osmosis unit model: Step 2.1, establish the connection model between the reverse osmosis superstructure network and the reverse osmosis process model: Q f =Q RO,j / N pv，j ; C f =C RO，j ;P f =P RO，j ; where Q f , C f , P f Respectively represent the feed water flow, salt concentration and pressure of the reverse osmosis section, N pv，j represents the number of pressure vessels in the jth reverse osmosis stage; Step 2.2: According to the law of conservation of materials in the reverse osmosis process of each reverse osmosis section, establish the solution conservation and flow conservation equations: Q f =Q p -Q r ;Q f C f =Q p C p +Q r C r ; where C p represents the salt concentration of fresh water produced by the reverse osmosis stage, C r Indicates the salt concentration of the concentrated water produced by the reverse osmosis stage; The fresh water flow rate generated by the reverse osmosis section can also be expressed as: Among them J v(z) is the reverse osmosis solvent flux, n l , W and L represent the number of blades, width and length of the reverse osmosis membrane, respectively; According to the dissolution-diffusion model: the solvent flux J through the reverse osmosis membrane v(z) =A w(z) (P f -P d(z) -P p(z) -Δπ (z) ); solute flux through the reverse osmosis membrane J s(z) =B s (C m(z) -C p(z) ); Among them, Aw (z) is the solvent transport parameter, Pf is the feed water pressure of the reverse osmosis section, Pd (z) is the pressure drop at position z in the reverse osmosis membrane channel, P p(z) is the osmotic pressure at position z in the membrane channel, Δπ (z) is the osmotic pressure difference at position z in the membrane channel; Bs is the solute transport parameter, C m(z) is the solute concentration measured on the surface of the reverse osmosis membrane at position z in the membrane channel, C p(z) is the solute concentration measured on the surface of the reverse osmosis membrane at position z in the membrane channel; Feed water pressure P at position z in the reverse osmosis membrane channel b(z) =P f -P d(z) ; Transmembrane pressure difference AP at position z in the reverse osmosis membrane channel (z) =(P b(z) -P p(z) );Osmotic pressure difference Δπ at position z in the reverse osmosis membrane channel (z) =RT(C m(z) -C p(z) );According to the above formula: J v(z) =A w (ΔP (z) -Δπ (z) ); Solvent transport parameter A w and solute transport parameter B s is a function of temperature: Where N memtype Indicates the number of membrane elements, A j is the intrinsic solvent transport parameter of the membrane element used in the jth reverse osmosis stage, B j is the intrinsic solute transport parameter A of the membrane element used in the jth reverse osmosis k , B k is the inherent transport parameter of the kth reverse osmosis membrane element model under standard conditions, x j,k is a 0-1 variable. When the j-th reverse osmosis pressure vessel uses the k-th type of membrane element, it takes the value of 1, otherwise it takes the value of 0; α1, α2 and β1 represent constants, and T represents the Kelvin temperature at which the system operates; Considering the influence of concentration polarization in the reverse osmosis process, the concentration polarization phenomenon is that the salt concentration on the surface of the reverse osmosis membrane increases and the osmotic pressure increases. In order to maintain the original water flux, the external pressure must be increased: concentration polarization value Among them C b(z) represents the salt concentration of concentrated water at position z in the reverse osmosis membrane channel, k c(z) It represents the membrane mass transfer coefficient at the position z in the reverse osmosis membrane channel; Solute mass transfer coefficient k c(z) Obtained by the following equation: Re (z) =ρ (z) V (z) d e / μ (z) Sc (z) =μ (z) / (ρ (z) D AB(z) ); Among them Sh (z) Re represents the Sherwood number at position z in the reverse osmosis membrane channel, Re (z) Represents the Reynolds number at position z in the reverse osmosis membrane channel, Sc (z) represents the Schmidt number at position z in the reverse osmosis membrane channel, d e represents the hydraulic diameter of the feed spacing channel, V (z) is the axial velocity of seawater at position z in the reverse osmosis channel; ρ (z) represents the density of the permeate water at the position z in the reverse osmosis channel, μ (z) Indicates dynamic viscosity, D AB(z) represents the kinematic viscosity, and the above parameters are calculated by the regression equation: Among them, k1, k2, b1, b2, a1, a2, and a3 are regression equation coefficients, and M is a variable related to Kelvin temperature T; Reverse osmosis membrane solvent flux J v(z) and solute flux J s(z) Satisfaction: J s(z) =J v(z) ×C p(z) ; Without considering the influence of pollutants on the system during the reverse osmosis process, following the law of conservation of materials and the mechanism shown above, the change of the pressure loss Pd(z) in the reverse osmosis membrane channel along the channel is expressed as: λ (z) =6.23K λ Re (z) -0.3 ; K λ is the empirical parameter related to the membrane element, λ (z) is the friction coefficient at position z in the reverse osmosis membrane channel; Due to the pressure in the reverse osmosis membrane channel So there is And when z=0, P b(0) =P f ; When z = L, P b(L) =P r ; L is the length of the membrane channel, z = L refers to the end of the membrane channel, P r is the concentrate pressure at the outlet of the reverse osmosis section; Axial flow velocity of seawater in the reverse osmosis channel V (z) The changes along the channel are expressed as: When z = 0, When z = L, h sp is the height of the feed interval channel; Qr is the concentrated water flow rate at the outlet of the reverse osmosis section; Solute concentration (salt concentration) C in the reverse osmosis membrane channel b(z) The changes along the channel are expressed as: And when z=0, C b(0) =C f , when z=L, C b(L) =C r ; C r is the salt concentration of the concentrate at the outlet of the reverse osmosis section; According to the above formula, when the fresh water pressure P p When is 0, given the feed water flow, pressure, and salt concentration of the RO node, the final fresh water flow Q at the outlet of the corresponding RO section can be obtained. p and freshwater salt concentration C p ; Step 2.3: Establish the connection model between the reverse osmosis process model and the reverse osmosis superstructure network: Q b,j =Q r *N pv，j ; C b，j =C r ;P b，j =P r ; where Q r , C r , P r Respectively represent the concentrated water flow, salt concentration and pressure at the outlet of the reverse osmosis section; Q p,j =Q p ; C p，j =C p ;P p，j =P p ; where Q p , C p , P p They represent the fresh water flow, salt concentration and pressure at the outlet of the reverse osmosis section respectively; Step 3: Establish the mechanism model of the mechanical vapor recompression system: The concentrated water finally produced by the reverse osmosis system flows into the mechanical vapor recompression system for evaporation and concentration treatment; the mechanical vapor recompression system includes two heat exchangers, a circulation pump, an evaporator, a demister and a steam compressor, the two heat exchanger inlets are connected to the concentrated water flowing out of the concentrated water outlet of the reverse osmosis superstructure network, the two heat exchanger outlets are connected to the circulation pump through pipelines, the feed water heated by the heat exchanger is pressurized by the circulation pump and enters the evaporator, and is evenly sprayed on the outer wall of the hot pipe of the evaporator through the built-in nozzle of the evaporator; the demister is arranged in the evaporator, connected to the steam compressor inlet through a pipeline, the steam compressor outlet is connected to the hot pipe of the evaporator, and the condensed water outlet of the hot pipe of the evaporator is connected to the hot end inlet of heat exchanger 1; the concentrated water outlet of the evaporator is connected to the hot end inlet of heat exchanger 2; When in use, the concentrated water flowing out of the concentrated water outlet of the reverse osmosis superstructure network is fully heat-exchanged with the concentrated water and condensed water generated by the evaporator through two heat exchangers, and then reaches the evaporator through a circulating pump, and is evenly sprayed on the outer wall of the hot pipe of the evaporator through the built-in nozzle of the evaporator. The water vapor in the concentrated water flowing out of the concentrated water outlet of the reverse osmosis superstructure network flows into the steam compressor through the demister, and the steam compressor compresses the water vapor into a high-temperature and high-pressure superheated gas; the compressed superheated gas enters the pipeline of the evaporator, exchanges heat with the feed water sprayed on the outer wall of the pipeline, and then condenses, flows into the heat exchanger 1 through the condensed water outlet, and exchanges heat with part of the concentrated water flowing out of the concentrated water outlet of the reverse osmosis superstructure network, and the generated fresh water flows out from the hot end outlet of the heat exchanger 1; the higher concentration concentrated water that has not been evaporated in the evaporator flows out through the concentrated water outlet of the evaporator and flows into the heat exchanger 2 to exchange heat with the remaining concentrated water flowing out of the concentrated water outlet of the reverse osmosis superstructure network, and the generated concentrated brine flows out from the hot end outlet of the heat exchanger 2; Assuming that the salt concentration of steam is 0%, there is no energy loss in the evaporator, preheater, pipeline and pump system; according to the principle of mechanical vapor compression evaporation, the law of conservation of energy and the law of conservation of heat, mechanical vapor compression evaporation is described by the following equation: Mechanical vapor recompression system feed water mass flow rate M f =M b +M d ; where M d represents the mass flow rate, M b represents the mass flow rate of concentrated brine; the mass flow rate of feed water in the mechanical vapor recompression system M f And the mass percentage concentration of feed water X f Satisfaction: M f X f =M b X b +M d X d ; Among them, X d Indicates the mass percentage concentration of fresh water, X b Indicates the mass percentage concentration of concentrated brine; Mechanical vapor recompression system feed water mass flow rate M f =Q bout ×1000, mass percentage concentration of feed water X f =C bout ÷10;Q bout , C bout They represent the brine flow rate and brine concentration ultimately produced by the reverse osmosis superstructure network respectively; The heat generated in a heat exchanger Q HEX1 =M d C pd (T d -T d0 )=M f1 C pf (T i1 -T i ), the heat generated in the other heat exchanger Q HEX2 =M b C pb (T b -T b0 )=M f2 C pf (T i2 -T i );C pd represents the specific heat capacity of the condensed water in the recompression system, C pf represents the specific heat capacity of the feed water in the recompression system, C pb represents the specific heat capacity of the concentrated brine in the recompression system, M f1 represents the feed mass flow rate through heat exchanger 1, M f2 represents the feed mass flow rate through heat exchanger 2, T i Indicates the temperature of the feed water, T b Indicates the concentrated water temperature at the evaporator outlet (i.e. the boiling point temperature of the evaporator feed water), T d Indicates the temperature of the condensed steam at the evaporator outlet, T bo represents the temperature of the concentrated brine after passing through heat exchanger 2, T do represents the temperature of fresh water after passing through heat exchanger 1, T i1 represents the temperature of the feed water after heat exchange in heat exchanger 1, T i2 Indicates the temperature of the feed water after heat exchange in heat exchanger 2; It can also be expressed as: Q HEX1 =U HEX A HEX1 (ΔT LM1 CF), Q HEX2 =U HEX A HEX2 (ΔT LM2 CF); where U HEX represents the heat transfer of the heat exchanger, A HEXl represents the heat transfer area of ​​heat exchanger 1, A HEX2 represents the heat transfer area of ​​heat exchanger 2, ΔT LM1 and ΔT LM2 represents the logarithmic mean temperature difference between the two ends of the heat exchanger of heat exchanger 1 and heat exchanger 2 respectively, and CF represents the plate combination correction factor; Heat generated outside the tube in the evaporator Q e =M d λ vp +M f C pf (T b -T f ) ; The heat generated in the tube of the evaporator Q c =M d λ d +M d C pv (T s -T d );Q e =Q c ; vp , d They represent the latent heat below the demister and the latent heat of the steam in the tube, T s Indicates the temperature of superheated steam; T f Indicates the temperature after passing through the heat exchanger, C pv represents the specific heat capacity of the steam above the demister, and Td represents the temperature of the condensed water discharged from the evaporator; Temperature below the demister T vp =T b -BPE;T b Indicates the boiling point temperature of feed water, BPE indicates the boiling point elevation coefficient of evaporator feed water, BPE = f·Δ0′; f indicates the correction coefficient, T VP represents the temperature below the demister, r represents the latent heat of vaporization of water; the boiling point elevation coefficient of the solution at normal pressure is Δ0′=69.8×X b 3 -8.15×X b 2 +5.19×X b ; T vp =T v ; T v Indicates the temperature above the defogger; X b Indicates the mass percentage concentration of concentrated brine, ΔT = T d -T b ; ΔT represents the difference between the condensing steam temperature at the evaporator outlet and the concentrated water temperature at the evaporator outlet; The heat transfer coefficient of the evaporator can also be expressed as: Q e =U e A e ΔT; U e represents the overall heat transfer coefficient, A e Indicates the heat transfer area of ​​the evaporator; H s =2499.15+1.955×T s -1.927×10 -3 ×T s 2 ; Hs and Hv represent the enthalpy of the superheated steam at the compressor outlet and the enthalpy of the steam above the demister, respectively. Power of inverter compressor W fre =(H s -H v )·0.000277·ε / η com ; ε represents the compression ratio, η com Indicates the efficiency of the compressor; Step 4: Establish economic model of reverse osmosis system and mechanical vapor recompression system: Step 4.1: Establish an economic model for the reverse osmosis system: Investment cost: Membrane element cost Among them, Pric k represents the unit price of the k-th reverse osmosis membrane element, N mem Indicates the number of reverse osmosis membrane elements in the pressure vessel, N pv，j represents the number of pressure vessels in the jth reverse osmosis section; Pressure vessel cost Pric pv Indicates the unit price of the pressure vessel; Feed water intake pump CC in =1417.3×(Q fraw ) 0.8 ;Q fraw Indicates the flow rate of feed water to the reverse osmosis system; High pressure pump cost Indicates the pressure of the high-pressure pump of the reverse osmosis system; Vacuum pump cost P_boost j represents the pressure of the pump corresponding to the j-th boosting node in the reverse osmosis system; Cost of energy recovery devices Q pxhin It represents the high-pressure brine flow rate of the last stage (stageNro) of reverse osmosis entering PX; Total investment cost of reverse osmosis system CC = (CC mem +CC pv +CC in +CC hpp +CC bp +CC px )×1.411; Operating costs: Chemical addition costs for feed water pretreatment OC ch =kQ fraw ; k represents the conversion coefficient between chemical additive cost and feed flow rate, Q fraw Indicates the flow rate of feed water to the reverse osmosis system; Energy consumption cost of feed water intake pump P0 represents the outlet pressure of the water intake pump, P elc represents the electricity price, η IP represents the efficiency of the seawater intake pump, and PLF represents the load factor; Operating electricity costs ε p Indicates the high-pressure pump efficiency, ε VFD Indicates the mechanical efficiency of variable frequency drive, ε bp Indicates the booster pump efficiency; Reverse osmosis membrane module replacement cost OC me =CC mem ·ζ re ;ζ re Indicates the replacement rate of membrane components; Reverse osmosis membrane module maintenance cost OC mn =Q pout ×24×365×PLF×0.01; Labor cost OC lb =Pri LB ·N LB ; N LB Indicates the number of laborers; OC px =0.027Q pxhin P pxhin η px P elc ;OC px Indicates the operating energy consumption of the energy recovery device, Q pxhin Indicates the flow rate of high-pressure concentrated brine entering the energy recovery device in the last section of reverse osmosis, P pxhin Indicates the pressure of high-pressure brine entering the energy recovery device in the last section of reverse osmosis, η px Indicates the efficiency of the energy recovery device; Total operating cost OC = OC ip +OC en +OC me +OC mn +OC ch +OC lb -OC px ; Step 4.2: Establish an economic model for the mechanical vapor recompression system: Investment cost of heat exchange equipment: CC area =(A e +A HEx1 +A HEX2 )Pric area ;Pric area It represents the unit heat exchange area cost of the heat exchanger and evaporator of the mechanical vapor recompression system; Operating cost: including the energy cost of the compressor and the energy cost of the circulation pump; Energy cost of compressor: OC com =W fre ×P elc ×ρ×24×365;P elc represents the electricity price, ρ represents the density of the solution, W fre is the power of the variable frequency compressor; Energy cost of circulation pump: OC cir =M f ×24×365×P elc ×ρ×β bemp β bemp Indicates the efficiency of the circulation pump; Step 4.3: Establish a joint economic model of the reverse osmosis system combined with the mechanical vapor recompression system: TAC=OC+CC+CC area +OC com +OC cir ; Step 5: Establish an optimal design model for the reverse osmosis system combined with the mechanical vapor recompression system: The reverse osmosis system combined with the mechanical vapor recompression system is expressed as a mixed integer nonlinear program: Objective function: Constraints: Model constructed in steps 1 to 4; Step 6: Solve the established optimization model: The mean value of the parameters at the beginning and end of the membrane element is used to characterize the element, and the finite difference method is used to discretize the differential equation. This method approximates the derivative of the function by the difference between the value of the function at a certain point and its value at the adjacent point. h(i) is used to represent the length of the i-th grid after the membrane element is meshed. The derivative term is replaced by the difference quotient of the function at two adjacent grid points at the grid point, and the differential equation is replaced by the difference quotient of the function at two adjacent grid points. Converted to the following algebraic equation: The converted mixed integer nonlinear programming problem is obtained and solved by a mixed integer programming solver; the results obtained by the solution are analyzed and the optimal design scheme of the joint system is obtained.

2. The combined optimization method of the mechanical vapor recompression system and the reverse osmosis system according to claim 1, characterized in that: In the reverse osmosis superstructure network model established in step 1: The concentrated water produced by the last reverse osmosis stageNro is pressure-recovered by the energy recovery device and then enters the concentrated water outlet; the high-pressure concentrated water inlet of the energy recovery device is connected to the incoming concentrated water, and the low-pressure water inlet of the energy recovery device is connected to the first boosting node to flow in the remaining feed water; after the high-pressure concentrated water and the low-pressure remaining feed water to be treated are pressure-exchanged, the high-pressure concentrated water is depressurized and flows out of the low-pressure concentrated water outlet of the energy recovery device and flows out through the concentrated water outlet, and the low-pressure remaining feed water to be treated is pressure-exchanged with the high-pressure concentrated water and then boosted and flows out of the high-pressure water inlet of the energy recovery device, and then is pressurized again by the vacuum pump and flows into the corresponding reverse osmosis node and then enters the corresponding reverse osmosis stage; The material balance of the high-pressure pump and the energy recovery device, ignoring the leakage of the energy recovery device, is the feed water flow rate Q at the high-pressure water inlet outlet of the energy recovery device. pxhout Equal to the low pressure water inlet feed water flow rate Q pxlin , the low-pressure concentrated water outlet concentrated water flow rate Q of the energy recovery device pxlout Equal to the concentrated water flow rate Q at the high-pressure concentrated water inlet pxhin , the concentrated water flow rate Q at the high-pressure concentrated water inlet of the energy recovery device pxhin Equal to the high-pressure concentrated water flow Q of the last reverse osmosis stageNro entering the energy recovery device bpx，Nro ; Concentrate flow rate Q at the high-pressure concentrate inlet of the energy recovery device pxhin and the brine salt concentration C at the high-pressure brine inlet of the energy recovery device hin Satisfaction: Q pxhin C hin =Q bpx，Nro C bpx，Nro ; Among them Cb px，Nro It indicates the salt concentration of the high-pressure concentrated water entering the energy recovery device in the last reverse osmosis stageNro; represents the brine flow rate from the jth reverse osmosis stage to the brine outlet; represents the concentration of concentrated water flowing to the concentrated water outlet of the jth reverse osmosis section, C pxlout Indicates the salt concentration of the concentrated water at the low-pressure concentrated water outlet of the energy recovery device; in, represents the fresh water flow from the jth reverse osmosis section to the fresh water outlet, It represents the salt concentration of fresh water flowing to the fresh water outlet of the jth reverse osmosis stage.

3. The combined optimization method of the mechanical vapor recompression system and the reverse osmosis system according to claim 1, characterized in that: The optimization process described in step 6 is as follows: first, a larger number of boost nodes N is given. ps and the number of RO nodes N ro Solve as initial value; After obtaining the optimal solution, the Q b,i,j and Q p，i，j , if greater than 0, there are concentrated brine and fresh water streams in the reverse osmosis system from the jth reverse osmosis flow to the i-th boosting node; if equal to 0, there are no concentrated brine and fresh water streams in the reverse osmosis system from the jth reverse osmosis flow to the i-th boosting node; N mem，j Indicates the number of membrane elements to be placed in each pressure vessel in the jth reverse osmosis section, N pv，j represents the number of pressure vessels to be placed in the jth reverse osmosis section, x j,k Equal to 1 means that the jth reverse osmosis uses the kth membrane element type, A e Represents the heat transfer area of ​​the evaporator, A HEX1 and A HEX2 represents the heat transfer area of ​​the two heat exchangers; The existence of equipment is indirectly determined by analyzing the operating variables, and finally the optimal design structure of the combination of mechanical vapor recompression system and reverse osmosis system with the lowest total annualized cost under given feed conditions is obtained.

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