Method and system for predicting simultaneous heat and mass transfer behavior in condensation process of vacuum condensation fractionator

Abstract

The invention discloses a method and a system for predicting simultaneous heat and mass transfer behaviors in a condensation process of a vacuum condensation fractionator, and the method comprises the steps: building a simultaneous heat and mass transfer model in the vacuum condensation fractionator according to a material balance and heat balance relational expression in an ethanol vapor condensation heat and mass transfer process under a vacuum condition; under the condition of given operation conditions and structure parameters, the condensate flow, the condensate ethanol concentration, the outlet steam flow and the outlet steam ethanol concentration of the vacuum condensation fractionator are predicted through a heat and mass simultaneous transfer model so as to calculate the total heat transfer capacity of the whole condensation process, and therefore the calculation structure parameters of the vacuum condensation fractionator are obtained through calculation. The method is beneficial for solving the problems that in a vacuum condensation fractionator of an existing ethanol fermentation-membrane separation coupling process, outlet parameters of the vacuum condensation fractionator cannot be predicted, and a basis cannot be provided for structural parameter design of the vacuum condensation fractionator.

Description

Technical Field

[0001] The present invention relates to the technical field of vacuum fractionation, and in particular to a method and system for predicting heat and mass transfer behavior in a condensation process of a vacuum condensation fractionator. Background Art

[0002] Fuel ethanol is a major basic chemical, and about 90% of ethanol is produced by fermentation. The fermentation-membrane separation coupling process can be used to produce ethanol. The separation membrane can selectively remove ethanol in situ and in real time, solving the product inhibition phenomenon in the traditional fermentation industry, thereby overcoming the problems of large amounts of fermentation waste liquid and high energy consumption of subsequent ethanol refining in the current production process.

[0003] In the coupled system, the downstream of the membrane is often vacuumed to promptly remove the steam that permeates the membrane to ensure the mass transfer driving force required for the membrane separation process. However, under vacuum conditions, the thermal conductivity of the dilute permeate steam is poor and the dew point is low. Low-temperature (-10°C) strong cooling devices or liquid nitrogen are often used to condense and recover the permeate steam. However, due to high energy consumption and inability to operate continuously, the above recovery method cannot be applied industrially, which is also the main reason why pervaporation technology has not yet been applied industrially.

[0004] To solve the above problems, in the related art, a graded condensation process of vacuum fractionation-mechanical vapor compression-normal pressure condensation is developed for the steam condensation process downstream of the membrane. On the one hand, the process uses the dew point difference between ethanol vapor and water vapor to realize the gradient recovery of permeate vapor. On the other hand, the dew point and thermal conductivity of the permeate vapor are improved through compression, so as to realize the recovery of permeate vapor with normal temperature water as the refrigerant, and eliminate the low-temperature strong cooling and liquid nitrogen condensation, so as to achieve the purpose of reducing energy consumption and facilitating amplification. In this process, the permeate vapor first enters the vacuum condensation fractionator, and the water vapor with high dew point in the vacuum condensation fractionator is preferentially condensed to form a condensate with low ethanol concentration; the permeate vapor with high ethanol concentration reaches a normal level after compression, and enters the secondary normal pressure condenser to form a condensate with high ethanol concentration. Both the two-stage vacuum condensation fractionators use normal temperature water as the refrigerant. When the ethanol concentration of the primary condensate is at a low level, it is expected to be directly reused in the process without entering the subsequent ethanol refining section. Only the secondary condensate with relatively high ethanol concentration is used for ethanol recovery, which greatly reduces the raw material processing volume of the ethanol refining section and reduces the refining energy consumption.

[0005] The condensation heat transfer behavior of ethanol water vapor under vacuum conditions will be affected by the operating parameters and structural parameters of the equipment, making the condensation effect of the vacuum condensation fractionator on the permeate steam unstable. When the operating conditions (such as the inlet working fluid flow rate, working fluid temperature, working fluid ethanol concentration, refrigerant temperature and refrigerant flow rate) and the structural parameters of the vacuum condensation fractionator (such as the total heat transfer area, total heat transfer coefficient, etc.) change, its condensation behavior (such as the outlet steam ethanol concentration, condensate and outlet steam flow rate, etc.) will change, thereby affecting the final wastewater generation of the process and the energy consumption of the subsequent ethanol refining section.

[0006] It can be seen that a systematic study of the condensation behavior of the vacuum condensation fractionator is helpful to further optimize the entire process. However, due to the unknown thermodynamic properties of the working fluid under vacuum conditions and the particularity of the application scenario, there is currently little research on the vacuum condensation fractionator.

[0007] Related technologies have applied the pervaporation-fractionation condensation process to recover valuable aromas from fermentation medium, red wine fermentation broth, and seafood industrial wastewater for food processing. The results show that the process can achieve highly selective recovery of target products and has certain application potential in the food industry. However, the above research currently only considers the effects of the vacuum degree and refrigerant temperature downstream of the membrane on the separation effect, and does not associate the structural parameters of the vacuum condensation fractionator with the separation effect. So far, the concept of vacuum fractionation condensation has been rarely used in the ethanol industry, and there is even less research on vacuum condensation fractionators in the ethanol fermentation-membrane separation coupling process.

[0008] Therefore, the defects of the prior art are: in the existing vacuum condensation fractionator of the ethanol fermentation-membrane separation coupling process, it is impossible to predict the outlet parameters of the vacuum condensation fractionator, and it is also impossible to provide a basis for the structural parameter design of the vacuum condensation fractionator. Summary of the invention

[0009] The main purpose of the present invention is to provide a method and system for predicting the heat and mass transfer behavior of the condensation process of a vacuum condensation fractionator, aiming to solve the problem that the outlet parameters of the vacuum condensation fractionator in the existing ethanol fermentation-membrane separation coupling process cannot be predicted, and a basis cannot be provided for the structural parameter design of the vacuum condensation fractionator.

[0010] To achieve the above object, the present invention provides a method for predicting heat and mass transfer behavior in a condensation process of a vacuum condensation fractionator, comprising the following steps:

[0011] According to the material balance and heat balance relationship of the heat and mass transfer process of ethanol water vapor condensation under vacuum conditions, a heat and mass transfer model in the vacuum condensation fractionator is established.

[0012] Under given operating conditions and structural parameters, the condensate flow rate, condensate ethanol concentration, outlet steam flow rate and outlet steam ethanol concentration of the vacuum condensation fractionator are predicted by the heat and mass transfer model to calculate the total heat transfer of the entire condensation process, thereby obtaining the calculated structural parameters of the vacuum condensation fractionator.

[0013] Furthermore, the step of establishing a heat and mass transfer model in a vacuum condensation fractionator according to the material balance and heat balance relationship of the ethanol water vapor condensation heat and mass transfer process under vacuum conditions includes:

[0014] According to the rule that the heat transfer load of the vacuum condensing fractionator, the heat transfer load of the hot fluid and the heat transfer load of the cold fluid are all equal, a heat transfer formula is established;

[0015] The heat transfer process is divided into heat transfer intervals according to the ethanol concentration gradient in the ethanol water vapor in the vacuum condensation fractionator to obtain a plurality of heat transfer sections, so as to calculate the heat transfer amount and the logarithmic mean heat transfer temperature difference of each heat transfer section, and then the total heat transfer amount and the total logarithmic mean heat transfer temperature difference of the vacuum condensation fractionator are calculated;

[0016] Divide each heat transfer section into mass transfer intervals to obtain multiple mass transfer units;

[0017] The permeate steam flow relationship and the ethanol concentration relationship are established by the mass conservation equation, wherein the permeate steam flow relationship is the relationship between the inlet steam flow, the condensate flow and the outlet steam flow in each heat transfer section, and the ethanol concentration relationship is the relationship between the inlet steam ethanol concentration, the condensate ethanol concentration and the outlet steam ethanol concentration in each heat transfer section;

[0018] Determine the ethanol mass conservation equation for each mass transfer unit in each heat transfer section, and determine the outlet steam flow calculation formula in each heat transfer section.

[0019] Furthermore, the step of establishing a heat transfer formula according to the rule that the heat exchange load of the vacuum condensing fractionator, the heat exchange load of the hot fluid and the heat exchange load of the cold fluid are all equal includes:

[0020] Establish the heat transfer load Q in the vacuum condensation fractionator t Calculation formula of

[0021] Establish the heat transfer load Q of the hot fluid in the vacuum condensation fractionator r Calculation formula of

[0022] Establish the heat transfer load Q of the cooling fluid in the vacuum condensation fractionator a Calculation formula of

[0023] Establish the logarithmic mean heat transfer temperature difference Δt in the condensation heat transfer process m,t Calculation formula of

[0024] Q t =KAΔt m,t (1);

[0025] Q r =M c x c r e +M c (1-x c )r w (2);

[0026] Q a =q m c p (t N -t0) (3);

[0027]

[0028] Where K is the total heat transfer coefficient, unit is W / (m 2 ·℃); A is the actual heat transfer area of ​​the vacuum condensation fractionator, in m 2 ; Δt m,t is the logarithmic mean heat transfer temperature difference during the condensation heat transfer process, in °C; M c is the condensate flow rate at the lower outlet of the vacuum condensation fractionator, in kg / h; x c is the ethanol concentration of the condensate at the lower outlet of the vacuum condensation fractionator, in wt%; q m is the flow rate of circulating cooling water, in kg / h; r e is the latent heat of vaporization of ethanol, r w is the latent heat of vaporization of water, in kJ / kg; c p is the specific heat capacity of circulating cooling water, in kJ / (kg·℃); T0 is the steam temperature at the inlet of the vacuum condensation fractionator, T N is the steam temperature at the outlet of the vacuum condensation fractionator, t0 is the inlet temperature of the circulating cooling water, t N is the outlet temperature of circulating cooling water, where the fluids in the tube side and shell side are completely countercurrent.

[0029] Furthermore, the heat transfer process is divided into heat transfer intervals according to the ethanol concentration gradient in the ethanol water vapor in the vacuum condensation fractionator to obtain a plurality of heat transfer sections for calculating the heat transfer amount and the logarithmic mean heat transfer temperature difference of each heat transfer section, and then the total heat transfer amount and the total logarithmic mean heat transfer temperature difference of the vacuum condensation fractionator are calculated, comprising:

[0030] In the process of stage-by-stage calculation, the heat transfer process is divided into N stages according to the difference in the inlet and outlet concentrations of the vacuum condensation fractionator. The temperature on the steam side is determined by the dew point temperature curve. The specific calculation process of the heat transfer temperature difference in each heat transfer stage is as follows:

[0031] Calculate the outlet vapor ethanol concentration of each heat transfer section:

[0032] y j =y0+jΔy (5);

[0033] Among them, y0 is the ethanol concentration of the inlet steam of the vacuum condensation fractionator, y j represents the ethanol concentration of the outlet steam of the jth heat transfer section. Δy is the difference in ethanol concentration of the inlet and outlet steam of each heat transfer section. 1≤j≤N. The calculation formula of Δy is as follows:

[0034]

[0035] According to y j The outlet steam temperature T of the jth heat transfer section can be obtained by using the dew point temperature curve. j , steam flow rate at the outlet of each heat transfer section M j , condensate ethanol concentration x c,j , condensate flow rate M c,j and heat transfer Q j Following the mass conservation equation, the cooling water inlet and outlet temperature differences and heat transfer temperature differences of each heat transfer section are finally obtained as follows:

[0036] The difference in cooling water inlet and outlet temperature of each heat transfer section:

[0037]

[0038] Among them, t N-j is the inlet temperature of cooling water in the jth heat transfer section, t N-(j-1) are the outlet temperatures of cooling water in the jth heat transfer section, in °C;

[0039] Logarithmic mean heat transfer temperature difference of each heat transfer section:

[0040]

[0041] Total heat transfer Q of vacuum condensation fractionator t for:

[0042]

[0043] Total logarithmic mean heat transfer temperature difference Δt of vacuum condensation fractionator m,t :

[0044]

[0045] When the total heat transfer coefficient is K, the calculated heat transfer area A' of the vacuum condensation fractionator is as follows:

[0046]

[0047] Among them, Δt m,j is the logarithmic mean heat transfer temperature difference of the jth heat transfer section, T j is the outlet steam temperature of the jth heat transfer section, T j-1 is the inlet steam temperature of the jth heat transfer section, in degrees Celsius.

[0048] Furthermore, the step of establishing the permeate steam flow relationship and the ethanol concentration relationship through the mass conservation equation includes:

[0049] The mass conservation equation of the permeate steam flow rate in the jth heat transfer section is as follows:

[0050] M j,0 =M j-1 =M c,j +M j =M c,j +M j,n (12);

[0051] The mass conservation equation for the ethanol flow rate in the jth heat transfer section is as follows:

[0052] M j，0 y j,0 =M j-1 y j-1 =M c,j x c,j +M j y j =M c,j x c,j +M j,n y j,n (13);

[0053] Among them, M j-1 and M j,0 are the inlet steam flow rate of the jth heat transfer section, M j,n It is also the outlet steam flow rate of the jth heat transfer section; y j-1 and j,0 are the inlet steam ethanol concentration of the jth heat transfer section, y j,n It is also the outlet steam ethanol concentration of the jth heat transfer section.

[0054] Furthermore, the step of determining the ethanol mass conservation equation of each mass transfer unit in each heat transfer section and determining the outlet steam flow calculation formula in each heat transfer section includes:

[0055] The ethanol mass conservation equation on the i-th mass transfer unit in the j-th heat transfer section is as follows:

[0056] M j, i-1 y j, i-1=x j, i ΔM j,i +M j, i y j, i (14);

[0057] Among them, M j,i-1 is the steam flow rate at the inlet of the i-th mass transfer unit in the j-th heat transfer section, y j,i-1 is the steam ethanol concentration at the inlet of the i-th mass transfer unit in the j-th heat transfer section; ΔM j,i is the mass of condensate in the i-th mass transfer unit in the j-th heat transfer section, x j,i is the ethanol concentration of the condensate in the i-th mass transfer unit in the j-th heat transfer section; M j,i is the outlet steam flow rate of the ith mass transfer unit in the jth heat transfer section, y j,i is the outlet steam ethanol concentration of the i-th mass transfer unit in the j-th heat transfer section, 0<i≤n, n is the number of mass transfer units in each heat transfer section;

[0058] In formula (14), let:

[0059] M j,i =M j,i-1 -ΔM j,i (15);

[0060] y j,i =y j,i-1 +Δy j,i (16);

[0061] Substituting equations (15) and (16) into equation (14) and ignoring higher-order infinitesimals, we obtain:

[0062]

[0063] Wherein, M is the mass flow rate on the mass transfer unit, M includes the condensate flow rate and the outlet steam flow rate on the mass transfer unit; y is the steam ethanol concentration on the mass transfer unit, and x is the condensate ethanol concentration on the mass transfer unit;

[0064] The boundary condition of formula (17) is:

[0065] M=M j,0 :y=y j,0 , M=M j,n :y=y j,n (18);

[0066] Therefore, after integrating equation (17), we get:

[0067]

[0068] Solving the integral in equation (19), the result is shown in equation (20):

[0069]

[0070] in:

[0071]

[0072] h is the step size; I is the function f(y) in the interval (y j-1 ,y j ) on the integral result; y j,2i is the outlet steam ethanol concentration corresponding to the even-numbered mass transfer unit in the jth heat transfer section, y j,2i-1 is the outlet steam ethanol concentration corresponding to the odd-numbered mass transfer unit in the jth heat transfer section; the inlet steam ethanol concentration y j-1 , and outlet steam ethanol concentration y j , the function f(y) is calculated in the interval (y j-1 ,y j ) and put the integral result into equation (19) to calculate the steam flow rate at the outlet of the vacuum condensation fractionator under given conditions.

[0073] Further:

[0074] According to the mass conservation equation (12) of the permeate steam flow rate, the condensate flow rate of the jth heat transfer section is calculated as follows:

[0075] M c,j =M j,n -M j,0 (twenty three);

[0076] According to the mass conservation equation of ethanol flow (13), the condensate ethanol concentration x of the jth heat transfer section is calculated. c,j :

[0077]

[0078] According to the heat transfer load calculation formula (2), the heat transfer capacity of the jth heat transfer section is calculated as follows:

[0079] Q j =M c,j x c,j r e +M c,j (1-x c,j )r w (25).

[0080] Further, under the given operating conditions and structural parameters, the condensate flow rate, condensate ethanol concentration, outlet steam flow rate and outlet steam ethanol concentration of the vacuum condensation fractionator are predicted by the heat and mass transfer model to calculate the total heat transfer of the entire condensation process, thereby calculating the calculated structural parameters of the vacuum condensation fractionator, including:

[0081] Obtaining known structural parameters and operating parameters of a vacuum condensation fractionator;

[0082] The initial outlet steam ethanol concentration of the vacuum condensation fractionator is set to be the average of the initial inlet steam ethanol concentration and the maximum outlet steam ethanol concentration;

[0083] According to the set number of heat transfer sections, the vacuum condensation fractionator is divided into a plurality of heat transfer sections with equal concentration gradients, and the ethanol concentration difference of each heat transfer section in the vacuum condensation fractionator, the ethanol concentration of the outlet steam of each heat transfer section and the outlet steam temperature of each heat transfer section are calculated by using the inlet steam ethanol concentration and the outlet steam ethanol concentration;

[0084] According to the calculation formula of the outlet steam flow rate and the relationship formula of the permeate steam flow rate in each heat transfer section, the outlet steam flow rate and the condensate flow rate in each heat transfer section are calculated; according to the relationship formula of the ethanol concentration, the ethanol concentration of the condensate in each heat transfer section is calculated; according to the calculation formula of the heat transfer load of the hot fluid, the heat transfer amount in each heat transfer section is calculated;

[0085] Calculate the cooling water outlet temperature of each heat transfer section;

[0086] Calculate the logarithmic mean heat transfer temperature difference of each heat transfer section, the total heat transfer of the vacuum condensation fractionator, and the total logarithmic mean heat transfer temperature difference of the entire condensation heat transfer process of the vacuum condensation fractionator;

[0087] Calculate the heat transfer area of ​​the vacuum condensation fractionator under the set total heat transfer coefficient;

[0088] Determine whether the calculated heat transfer area and the actual heat transfer area of ​​the vacuum condensation fractionator are less than the set error value;

[0089] If yes, the prediction results are output, wherein the prediction results include: outlet steam ethanol concentration, outlet steam flow rate in each heat transfer section, condensate flow rate and condensate ethanol concentration.

[0090] Furthermore, after the step of determining whether the calculated heat transfer area and the actual heat transfer area of ​​the vacuum condensation fractionator are less than the set error value, the method further includes:

[0091] If not, compare the calculated heat transfer area of ​​the vacuum condensation fractionator with the actual heat transfer area;

[0092] When the calculated heat transfer area is larger than the actual heat transfer area, the maximum outlet steam ethanol concentration is set equal to the outlet steam ethanol concentration;

[0093] When the calculated heat transfer area is smaller than the actual heat transfer area, the inlet steam ethanol concentration is set equal to the outlet steam ethanol concentration, and the steps of setting the initial outlet steam ethanol concentration of the vacuum condensation fractionator to the average of the initial inlet steam ethanol concentration and the maximum outlet steam ethanol concentration, to the steps of calculating the logarithmic mean heat transfer temperature difference of each heat transfer section, the total heat transfer of the vacuum condensation fractionator, and the total logarithmic mean heat transfer temperature difference of the entire condensation heat transfer process of the vacuum condensation fractionator are repeated until the calculated heat transfer area of ​​the vacuum condensation fractionator and the actual heat transfer area are smaller than the set error value, and the prediction result is output.

[0094] To achieve the above-mentioned purpose, the present invention also proposes a pervaporation membrane separation system, comprising a fermentation tank, a circulation pump, a membrane separator, a vacuum condensation fractionator, a vacuum compression pump and a normal pressure condenser connected in sequence; applying the prediction method, the condensate flow rate, the condensate ethanol concentration, the outlet steam flow rate and the outlet steam ethanol concentration of the vacuum condensation fractionator are predicted to calculate the total heat transfer of the entire condensation process, thereby calculating the calculation structural parameters of the vacuum condensation fractionator.

[0095] In the technical solution of the present invention, a heat and mass transfer model of the condensation process in a vacuum condensation fractionator is established to predict the outlet parameters of the material. Under given operating conditions and structural parameters, the condensate flow rate, condensate ethanol concentration, outlet steam flow rate and outlet steam ethanol concentration of the vacuum condensation fractionator are predicted by the heat and mass transfer model, and then the total heat transfer of the entire condensation process is calculated by the condensate flow rate and ethanol concentration, and the calculated structural parameters of the vacuum condensation fractionator can be calculated to guide the structural design of the vacuum condensation fractionation, so that the present invention is conducive to solving the problem that the outlet parameters of the vacuum condensation fractionator in the existing ethanol fermentation-membrane separation coupling process cannot be predicted, and a basis cannot be provided for the structural parameter design of the vacuum condensation fractionator. BRIEF DESCRIPTION OF THE DRAWINGS

[0096] Figure 1 It is a schematic diagram of the pervaporation membrane separation process in the present invention;

[0097] Figure 2 Schematic diagram of the condensation process of permeate steam in the vacuum condensation fractionator of the present invention;

[0098] Figure 3 Schematic diagram of the heat and mass transfer process in the vacuum condensation fractionator of the present invention;

[0099] Figure 4 is a dew point temperature curve diagram in the present invention;

[0100] Figure 5 Flow chart for the calculation of the heat and mass transfer model.

[0101] Description of Figure Numbers:

[0102] 1- fermentation tank; 2- circulation pump; 3- membrane separator; 4- vacuum condensation fractionator; 5- vacuum compression pump; 6- atmospheric condenser;

[0103] a-ethanol aqueous solution; b-hot circulating water cooling water; c-permeate steam; d-condensate with low ethanol concentration; e-concentrated ethanol water vapor; f-circulating cooling water; h-condensate with high ethanol concentration.

[0104] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings in conjunction with the embodiments. DETAILED DESCRIPTION

[0105] It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not used to limit the present invention.

[0106] In the following description, suffixes such as "unit", "component" or "unit" used to represent elements are only used to facilitate the description of the present invention, and have no specific meanings. Therefore, "unit", "component" or "unit" can be used in a mixed manner.

[0107] See also Figures 1 to 5 To achieve the above object, a first embodiment of the present invention provides a method for predicting heat and mass transfer behavior in a condensation process of a vacuum condensation fractionator, comprising the following steps:

[0108] Step S10, establishing a heat and mass transfer model in a vacuum condensation fractionator according to the material balance and heat balance relationship of the heat and mass transfer process of ethanol water vapor condensation under vacuum conditions;

[0109] Step S20, under the given operating conditions and structural parameters, the condensate flow rate, condensate ethanol concentration, outlet steam flow rate and outlet steam ethanol concentration of the vacuum condensation fractionator are predicted through the heat and mass transfer model to calculate the total heat transfer of the entire condensation process, thereby calculating the calculated structural parameters of the vacuum condensation fractionator.

[0110] In the technical solution of the present invention, a heat and mass transfer model of the condensation process in a vacuum condensation fractionator is established to predict the outlet parameters of the material. Under given operating conditions and structural parameters, the condensate flow rate, condensate ethanol concentration, outlet steam flow rate and outlet steam ethanol concentration of the vacuum condensation fractionator are predicted by the heat and mass transfer model, and then the total heat transfer of the entire condensation process is calculated by the condensate flow rate and ethanol concentration, and the calculated structural parameters of the vacuum condensation fractionator can be calculated to guide the structural design of the vacuum condensation fractionation, so that the present invention is conducive to solving the problem that the outlet parameters of the vacuum condensation fractionator in the existing ethanol fermentation-membrane separation coupling process cannot be predicted, and a basis cannot be provided for the structural parameter design of the vacuum condensation fractionator.

[0111] The flow rate in the present invention refers to the mass flow rate.

[0112] Specifically, the condensation process of ethanol-water vapor downstream of the pervaporation membrane in the vacuum condensation fractionator is as follows: Figure 2 As shown ( Figure 2 In the figure, G represents the direction of gravity, and Q represents heat transfer). The steam in the tube releases heat under the cooling of the shell circulating cooling water. At the same time, part of the steam condenses on the tube wall to form a liquid film. Under the action of gravity, it flows into the liquid collector along the inner wall of the tube. The cooling water absorbs the latent heat released by the liquefaction of the steam and is heated, and the temperature rises. However, due to the different thermodynamic parameters such as dew point and thermal conductivity of water vapor and ethanol vapor, the heat release and condensation behaviors of the two are different. In this condensation process, there are mass transfer behaviors and heat transfer behaviors that affect each other.

[0113] According to the material balance and heat balance relationship of the ethanol water vapor condensation heat and mass transfer process under vacuum conditions, a heat and mass transfer model in the vacuum condensation fractionator can be established to accurately describe its heat and mass transfer behavior and predict the outlet parameters of the material. After the condensation heat and mass transfer behavior reaches a steady state, the mass transfer process at the interface between the ethanol water vapor and the condensation liquid film on the tube wall satisfies the gas-liquid balance. Therefore, by dividing the entire heat and mass transfer process into intervals, establishing the relationship between the condensation amount of the vacuum condensation fractionator and the inlet flow rate and the ethanol concentration in the inlet and outlet steam, and then calculating the total heat transfer of the entire condensation process by the condensate flow rate and concentration, some calculation structural parameters of the vacuum condensation fractionator can be calculated.

[0114] Based on the first embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, in the second embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, step S10 includes:

[0115] Step S11, establishing a heat transfer formula according to the rule that the heat exchange load of the vacuum condensing fractionator, the heat exchange load of the hot fluid and the heat exchange load of the cold fluid are all equal;

[0116] Step S12, dividing the heat transfer process into heat transfer intervals according to the ethanol concentration gradient in the ethanol water vapor in the vacuum condensation fractionator to obtain a plurality of heat transfer sections, so as to calculate the heat transfer amount and the logarithmic mean heat transfer temperature difference of each heat transfer section, and then calculating the total heat transfer amount and the total logarithmic mean heat transfer temperature difference of the vacuum condensation fractionator;

[0117] Step S13, dividing each heat transfer section into mass transfer intervals to obtain a plurality of mass transfer units;

[0118] Step S14, establishing a permeate steam flow relationship and an ethanol concentration relationship through a mass conservation equation, wherein the permeate steam flow relationship is a relationship between an inlet steam flow, a condensate flow, and an outlet steam flow in each heat transfer section, and the ethanol concentration relationship is a relationship between an inlet steam ethanol concentration, a condensate ethanol concentration, and an outlet steam ethanol concentration in each heat transfer section;

[0119] Step S15, determining the ethanol mass conservation equation of each mass transfer unit in each heat transfer section, and determining the outlet steam flow calculation formula in each heat transfer section.

[0120] The heat and mass transfer model divides the entire condensation process into heat transfer intervals according to the concentration gradient. The total heat transfer area required under the given total average heat transfer coefficient is calculated by the heat transfer relationship. The heat transfer amount of each heat transfer interval is calculated by the mass conservation relationship and the outlet steam flow calculation formula.

[0121] When establishing the heat and mass transfer model, the following assumptions are made:

[0122] a) The tube side is under negative pressure (vacuum) condition, and the steam components in the tube side can be regarded as ideal gas;

[0123] b) The activity coefficient of ethanol-water solution is set as a constant;

[0124] c) The system is in steady state and the contact interface between the condensate film and the steam on each heat transfer section satisfies the gas-liquid equilibrium;

[0125] d) During the condensation heat transfer process, the temperature of ethanol water vapor is equal to the dew point temperature at the corresponding gas phase ethanol concentration;

[0126] e) The product of the total average heat transfer coefficient and the total heat transfer area during the condensation process is the sum of the products of the total heat transfer coefficients of each heat transfer section and the corresponding heat transfer area.

[0127] Based on the second embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, in the third embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, step S11 includes:

[0128] Establish the heat transfer load Q in the vacuum condensation fractionator t Calculation formula of

[0129] Establish the heat transfer load Q of the hot fluid in the vacuum condensation fractionator r Calculation formula of

[0130] Establish the heat transfer load Q of the cooling fluid in the vacuum condensation fractionator a Calculation formula of

[0131] Establish the logarithmic mean heat transfer temperature difference Δt in the condensation heat transfer process m,t Calculation formula of

[0132] Q t =KAΔt m,t (1);

[0133] Q r =M c x c r e +M c (1-x c )r w (2);

[0134] Q a =q m c p (t N -t0) (3);

[0135]

[0136] Where K is the total heat transfer coefficient, unit is W / (m 2 ·℃); A is the actual heat transfer area of ​​the vacuum condensation fractionator, in m 2 ; Δt m,t is the logarithmic mean heat transfer temperature difference during the condensation heat transfer process, in °C; M c is the condensate flow rate at the lower outlet of the vacuum condensation fractionator, in kg / h; x c is the ethanol concentration of the condensate at the lower outlet of the vacuum condensation fractionator, in wt%; q m is the flow rate of circulating cooling water, in kg / h; r e is the latent heat of vaporization of ethanol, r w is the latent heat of vaporization of water, in kJ / kg; c p is the specific heat capacity of circulating cooling water, in kJ / (kg·℃); T0 is the steam temperature at the inlet of the vacuum condensation fractionator, T N is the steam temperature at the outlet of the vacuum condensation fractionator, t0 is the inlet temperature of the circulating cooling water, t Nis the outlet temperature of circulating cooling water, where the fluids in the tube side and shell side are completely countercurrent.

[0137] The above heat transfer formula is established based on the principle that the heat exchange of cold and hot fluids and the heat exchange load of the vacuum condensation fractionator are equal. Since the latent heat released by the condensation of ethanol water vapor is much greater than the sensible heat of the condensate cooling, only the latent heat is considered in the heat transfer process analysis, and the heat loss is ignored.

[0138] Based on the third embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, in a fourth embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, step S12 includes:

[0139] In the stage-by-stage calculation process, the heat transfer process is divided into N stages according to the difference in the inlet and outlet concentrations of the vacuum condensation fractionator (e.g. Figure 3 The steam side temperature is shown in the left diagram. The steam side temperature is shown in the dew point temperature curve ( Figure 4 ), the specific calculation process of the heat transfer temperature difference of each heat transfer section is as follows:

[0140] Calculate the outlet vapor ethanol concentration of each heat transfer section:

[0141] y j =y0+jΔy (5);

[0142] Among them, y0 is the ethanol concentration of the inlet steam of the vacuum condensation fractionator, y j represents the ethanol concentration of the outlet steam of the jth heat transfer section. Δy is the difference in ethanol concentration of the inlet and outlet steam of each heat transfer section. 1≤j≤N. The calculation formula of Δy is as follows:

[0143]

[0144] According to y j The outlet steam temperature T of the jth heat transfer section can be obtained by using the dew point temperature curve. j , steam flow rate at the outlet of each heat transfer section M j , condensate ethanol concentration x c,j , condensate flow rate M c,j and heat transfer Q j Following the mass conservation equation, the cooling water inlet and outlet temperature differences and heat transfer temperature differences of each heat transfer section are finally obtained as follows:

[0145] The difference in cooling water inlet and outlet temperature of each heat transfer section:

[0146]

[0147] Among them, t N-j is the inlet temperature of cooling water in the jth heat transfer section, t N-(j-1) are the outlet temperatures of cooling water in the jth heat transfer section, in °C;

[0148] Logarithmic mean heat transfer temperature difference of each heat transfer section:

[0149]

[0150] Total heat transfer Q of vacuum condensation fractionator t for:

[0151]

[0152] Total logarithmic mean heat transfer temperature difference Δt of vacuum condensation fractionator m,t :

[0153]

[0154] When the total heat transfer coefficient is K, the calculated heat transfer area A' of the vacuum condensation fractionator is as follows:

[0155]

[0156] Among them, Δt m,j is the logarithmic mean heat transfer temperature difference of the jth heat transfer section, T j is the outlet steam temperature of the jth heat transfer section, T j-1 is the inlet steam temperature of the jth heat transfer section, in degrees Celsius.

[0157] In the case of large changes in flow and temperature of the steam in the tube of the vacuum condensing fractionator during the condensation process, the heat transfer process is divided into intervals using the segment-by-segment calculation method to improve the calculation accuracy. The total heat transfer and the total logarithmic mean heat transfer temperature difference are calculated by calculating the heat transfer and the logarithmic mean heat transfer temperature difference of each heat transfer segment. Assuming that the total heat transfer coefficient is constant, the total heat transfer area required under the heat exchange load can be calculated.

[0158] The steam side temperature can be obtained by fitting the dew point temperature curve of Aspen data (such as Figure 4 shown) to confirm.

[0159] Steam flow rate at each section outlet M j , condensate ethanol concentration x c,j , condensate flow rate M c,j and heat transfer Q j Following the mass conservation equation, the specific expressions are shown in equations (19) and (23)-(25).

[0160] Heat transfer Q j That is, the latent heat released by steam.

[0161] Based on the fourth embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, in the fifth embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, step S14 includes:

[0162] The mass conservation equation of the permeate steam flow rate in the jth heat transfer section is as follows:

[0163] M j,0 =M j-1 =M c,j +M j =M c,j +M j,n (12);

[0164] The mass conservation equation for the ethanol flow rate in the jth heat transfer section is as follows:

[0165] M j，0 y j,0 =M j-1 y j-1 =M c,j x c,j +M j y j =M c,j x c,j +M j,n y j,n (13);

[0166] Among them, M j-1 and M j,0 are the inlet steam flow rate of the jth heat transfer section, M c,j is the condensate flow rate of the jth heat transfer section, M j and M j,n are the outlet steam flow rates of the jth heat transfer section; y j-1 and j,0 are the inlet steam ethanol concentration of the jth heat transfer section, x c,j is the ethanol concentration of the condensate in the jth heat transfer section, y j ,y j,n are the outlet steam ethanol concentrations of the jth heat transfer section.

[0167] In order to obtain the condensation fractionation parameters of ethanol water vapor under the inlet and outlet conditions of the jth heat transfer section, the microelement division is first performed when establishing the mass transfer model of permeation steam under vacuum conditions. The relationship between the outlet steam flow rate of the jth section and the inlet steam flow rate of the jth section and the ethanol concentration of the inlet and outlet steam is established through the mass conservation equation. Then, the complex Simpson numerical integration formula and the ethanol-water vapor-liquid equilibrium curve equation under constant pressure conditions are used to solve the outlet steam flow rate corresponding to the given inlet and outlet steam ethanol concentrations and inlet steam flow rate. Finally, the corresponding condensate flow rate and its concentration can be calculated through the mass conservation equation.

[0168] Based on the fifth embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, in the sixth embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, step S15 includes:

[0169] The ethanol mass conservation equation on the i-th mass transfer unit in the j-th heat transfer section is as follows:

[0170] M j, i-1 y j, i-1 =x j, i ΔM j,i +M j, i y j, i (14);

[0171] Among them, M j,i-1 is the steam flow rate at the inlet of the i-th mass transfer unit in the j-th heat transfer section, y j,i-1 is the steam ethanol concentration at the inlet of the i-th mass transfer unit in the j-th heat transfer section; ΔM j,i is the mass of condensate in the i-th mass transfer unit in the j-th heat transfer section, x j,i is the ethanol concentration of the condensate in the i-th mass transfer unit in the j-th heat transfer section; M j,i is the outlet steam flow rate of the ith mass transfer unit in the jth heat transfer section, y j,i is the outlet steam ethanol concentration of the i-th mass transfer unit in the j-th heat transfer section, 0<i≤n, n is the number of mass transfer units in each heat transfer section;

[0172] In formula (14), let:

[0173] M j,i =M j,i-1 -ΔM j,i (15);

[0174] y j,i =y j,i-1 +Δy j,i (16);

[0175] Substituting equations (15) and (16) into equation (14) and ignoring higher-order infinitesimals, we obtain:

[0176]

[0177] Wherein, M is the mass flow rate on the mass transfer unit, M includes the condensate flow rate and the outlet steam flow rate on the mass transfer unit; y is the steam ethanol concentration on the mass transfer unit, and x is the condensate ethanol concentration on the mass transfer unit;

[0178] The boundary condition of formula (17) is:

[0179] M=M j,0 :y=y j,0 , M=M j,n :y=y j,n (18);

[0180] Therefore, after integrating equation (17), we get:

[0181]

[0182] Solving the integral in equation (19), the result is shown in equation (20):

[0183]

[0184] in:

[0185]

[0186]

[0187] h is the step size; I is the function f(y) in the interval (y j-1 ,y j ) on the integral result; y j,2i is the outlet steam ethanol concentration corresponding to the even-numbered mass transfer unit in the jth heat transfer section, y j,2i-1 is the outlet steam ethanol concentration corresponding to the odd-numbered mass transfer unit in the jth heat transfer section; the inlet steam ethanol concentration y j-1 , and outlet steam ethanol concentration y j , the function f(y) is calculated in the interval (y j-1 ,y j ) and put the integral result into equation (19) to calculate the steam flow rate at the outlet of the vacuum condensation fractionator under given conditions.

[0188] During the model calculation process, n can be set to 10.

[0189] The present invention divides the mass transfer process in the heat transfer process of each heat transfer section of the vacuum condensation fractionator into n mass transfer units (such as Figure 3 As shown in the figure, take the i-th mass transfer unit for analysis, i is successively 1 to n, and the analysis of all the mass transfer units in each heat transfer section can be completed.

[0190] The contact interface between the condensate film and the steam in this unit satisfies the gas-liquid equilibrium, and the ethanol concentration in the gas and liquid phases conforms to the gas-liquid equilibrium curve of ethanol-water solution at 8 kPa obtained by fitting the Aspen data.

[0191] Based on the sixth embodiment of the method for predicting the heat and mass transfer behavior of the condensation process of a vacuum condensation fractionator of the present invention, in the seventh embodiment of the method for predicting the heat and mass transfer behavior of the condensation process of a vacuum condensation fractionator of the present invention:

[0192] According to the mass conservation equation (12) of the permeate steam flow rate, the condensate flow rate of the jth heat transfer section is calculated as follows:

[0193] M c,j =M j,n -M j,0 (twenty three);

[0194] According to the mass conservation equation of ethanol flow (13), the condensate ethanol concentration x of the jth heat transfer section is calculated. c,j :

[0195]

[0196] According to the heat transfer load calculation formula (2), the heat transfer capacity of the jth heat transfer section is calculated as follows:

[0197] Q j =M c,j x c,j r e +M c,j (1-x c,j )r w (25).

[0198] Based on the seventh embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, in the eighth embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, step S20 includes:

[0199] Step S21, obtaining known structural parameters and operating parameters of a vacuum condensation fractionator;

[0200] Step S22, setting the initial outlet steam ethanol concentration of the vacuum condensation fractionator to the average of the initial inlet steam ethanol concentration and the maximum outlet steam ethanol concentration;

[0201] Step S23, dividing the vacuum condensation fractionator into a plurality of heat transfer sections with equal concentration gradients according to the set number of heat transfer sections, and calculating the ethanol concentration difference of each heat transfer section in the vacuum condensation fractionator, the ethanol concentration of the outlet steam of each heat transfer section, and the outlet steam temperature of each heat transfer section by using the inlet steam ethanol concentration and the outlet steam ethanol concentration;

[0202] Step S24, according to the calculation formula of the outlet steam flow rate in each heat transfer section and the relationship formula of the permeate steam flow rate, the outlet steam flow rate and the condensate flow rate in each heat transfer section are calculated; according to the relationship formula of the ethanol concentration, the ethanol concentration of the condensate in each heat transfer section is calculated; according to the calculation formula of the heat exchange load of the hot fluid, the heat transfer amount in each heat transfer section is calculated;

[0203] Step S25, calculating the cooling water outlet temperature of each heat transfer section;

[0204] Step S26, calculating the logarithmic mean heat transfer temperature difference of each heat transfer section, the total heat transfer of the vacuum condensation fractionator, and the total logarithmic mean heat transfer temperature difference during the entire condensation heat transfer process of the vacuum condensation fractionator;

[0205] Step S27, calculating the heat transfer area of ​​the vacuum condensation fractionator under the set total heat transfer coefficient;

[0206] Step S28, determining whether the calculated heat transfer area and the actual heat transfer area of ​​the vacuum condensation fractionator are less than a set error value δ;

[0207] Step S29: If yes, output the prediction results, wherein the prediction results include: outlet steam ethanol concentration, outlet steam flow rate in each heat transfer section, condensate flow rate and condensate ethanol concentration.

[0208] Based on the established heat and mass transfer model, this embodiment will describe in detail the calculation process when the model is applied to a specific process flow.

[0209] In the process of ethanol water vapor condensation heat transfer in a shell and tube heat exchanger, the inlet steam flow rate of the tube side is M0, and the inlet steam ethanol concentration is y0; the actual heat transfer area of ​​the vacuum condensation fractionator is A; the flow rate of the circulating cooling water is q m , the inlet temperature is t0, calculate the outlet steam ethanol concentration y of the vacuum condensation fractionator under this condition N The detailed steps of calculating this process using the heat and mass transfer model are as follows: Figure 5 As shown, the main process is as follows:

[0210] 1) Input known initial process parameters;

[0211] 2) Take the initial outlet steam ethanol concentration value y N is the initial inlet steam ethanol concentration y0 and the maximum outlet steam ethanol concentration y m (100wt%) average value;

[0212] 3) After the vacuum condensation fractionator is divided into multiple heat transfer sections with equal concentration gradients according to the set number of heat transfer sections, the inlet steam ethanol concentration y0 and the outlet steam ethanol concentration y NSubstituting into formula (6) we can get the ethanol concentration difference Δy (i.e., ethanol concentration increment) of each heat transfer section in the vacuum condensation fractionator. Then, according to formula (5) and the dew point temperature curve, we can get the outlet steam ethanol concentration y of each heat transfer section. j And the corresponding outlet steam temperature T j ;

[0213] 4) Set the inlet steam flow rate M of each heat transfer section j-1 (The inlet steam flow rate of the first section is M0), the inlet steam ethanol concentration y j-1 , outlet steam ethanol concentration y j Substitute it into formula (19) to calculate the outlet steam flow rate of this section, which is the inlet steam flow rate of the next section, M j , substituting into equation (23) to obtain the condensate flow rate M of each heat transfer section c,j , and then the condensate ethanol concentration x in each heat transfer section is calculated by formula (24): c,j , and then the heat transfer amount Q of each heat transfer section is calculated by formula (25): j ;

[0214] 5) The calculated ethanol concentration x of the condensate in each heat transfer section c,j and condensate flow M c,j Substituting into equation (25) we can get the heat transfer Q of each heat transfer section: j , the heat transfer of each heat transfer section is Q j and cooling water flow q m Substitute its inlet temperature t0 into equation (7) and calculate the cooling water outlet temperature t of each heat transfer section cyclically. N-(j-1) ;

[0215] 6) According to the calculated T j and t N-(j-1) With the known conditions T0 and t0, combined with formula (8), the logarithmic mean heat transfer temperature difference Δt of each heat transfer section is calculated. m,j ; The total heat transfer Q calculated by formula (9) t And the heat transfer Q of each heat transfer section j and logarithmic mean heat transfer temperature difference Δt m,j Substituting into equation (10) we can get the total logarithmic mean heat transfer temperature difference Δt during the entire condensation heat transfer process: m,t ;

[0216] 7) The total heat transfer Q t and the total logarithmic mean heat transfer temperature difference Δt m,t Substituting into formula (11), the calculated heat transfer area A′ under the conditions and process requirements can be obtained;

[0217] 8) Compare A' with the given actual heat transfer area A. If the absolute value of the difference between A' and A is less than the set error value, the outlet steam ethanol concentration y at this time is output. N ; Otherwise, determine the size of A′ and A. When A′ is greater than A, let y m =y N ; When A′ is less than A, let y0 = y N Then repeat the above steps 2)-6) until the absolute value of the error between A′ and A is less than the set value, and output y at this time N And other export process parameters.

[0218] Based on the eighth embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, in a ninth embodiment of the method for predicting heat and mass transfer behavior of a condensation process of a vacuum condensation fractionator of the present invention, after step S28, the method further includes:

[0219] Step S210, if not, compare the calculated heat transfer area of ​​the vacuum condensation fractionator with the actual heat transfer area;

[0220] Step S211, when the calculated heat transfer area is larger than the actual heat transfer area, the maximum outlet steam ethanol concentration is set equal to the outlet steam ethanol concentration;

[0221] Step S212, when the calculated heat transfer area is smaller than the actual heat transfer area, the inlet steam ethanol concentration is set equal to the outlet steam ethanol concentration, and steps S22 to S26 are repeated until the calculated heat transfer area of ​​the vacuum condensation fractionator and the actual heat transfer area are smaller than the set error value, and the prediction result is output.

[0222] To achieve the above-mentioned purpose, the present invention also proposes a pervaporation membrane separation system, comprising a fermentation tank, a circulation pump, a membrane separator, a vacuum condensation fractionator, a vacuum compression pump and a normal pressure condenser connected in sequence; applying the prediction method, the condensate flow rate, the condensate ethanol concentration, the outlet steam flow rate and the outlet steam ethanol concentration of the vacuum condensation fractionator are predicted to calculate the total heat transfer of the entire condensation process, thereby calculating the calculation structural parameters of the vacuum condensation fractionator.

[0223] The following scheme is adopted to verify the effectiveness of the present invention.

[0224] 1. Experimental Materials

[0225] The upstream feed liquid of the membrane was a model solution with an ethanol concentration of 5 wt%. The ethanol used was analytical grade and purchased from Chengdu Celes Technology Co., Ltd. The PDMS membrane used was purchased from Nanjing Jiusi Membrane Technology Co., Ltd. and Shandong Lanjing Membrane Technology Engineering Co., Ltd. The effective membrane area of ​​the membrane module used was 0.024 m 2 .

[0226] 2. Experimental process

[0227] Experimental setup Figure 1 As shown. During the experiment, the ethanol-water model solution upstream of the membrane circulates between the fermentation tank 1 and the membrane separator 3 through the circulation pump 2. When the solution flows through the membrane surface, part of the ethanol and water molecules are selectively adsorbed by the membrane and desorbed as steam on the other side of the membrane under the action of the vacuum compression pump 5 downstream of the membrane. When the permeate steam passes through the primary vacuum condensation fractionator 4, part of the ethanol-water vapor condenses on the inner wall of the heat exchange tube to form a condensate with a lower ethanol concentration, and then flows into the liquid collector along the tube wall, while the uncondensed steam is transported to the secondary atmospheric condenser 6 under the action of the vacuum compression pump 5 to be condensed and recovered. During the experiment, three PDMS membranes with different permeation fluxes were selected for experiments to study the total heat transfer coefficient of the vacuum condensation fractionator 4 under different steam flow rates, and then analyze the separation behavior of the vacuum condensation fractionator 4 under different total heat transfer coefficients. The experiments under each steam flow rate condition were set up with 2-3 groups of parallel experiments, and the data processing results were averaged.

[0228] 3. Test methods

[0229] The inlet and outlet steam temperatures of the first-stage vacuum condensation fractionator are T1 and T N and outlet condensate temperature T c Thermocouple thermometer is used for measurement; the shell side circulating cooling water is provided by a low temperature constant temperature bath, the inlet temperature t0 is digitally displayed, and the outlet temperature t N Calculated by heat balance. The ethanol concentration of the condensate collected at the lower end of the first-stage vacuum condensation fractionator and the second-stage atmospheric condenser is x c , outlet steam ethanol concentration y N The alcohol content is measured by an alcohol meter. The vacuum degree downstream of the membrane is measured by a vacuum gauge. The mass of the collected condensate is measured by an electronic balance.

[0230] 4. Results Analysis

[0231] ①、Accuracy verification of heat and mass transfer model

[0232] In order to verify the accuracy of the calculation results of the heat and mass transfer model, the present invention compares the experimental test results of parameters such as the outlet steam flow rate and outlet steam ethanol concentration and the condensate flow rate and condensate ethanol concentration of the vacuum condensation fractionator with the model calculation results.

[0233] The model calculation value is obtained by substituting the total heat transfer coefficient (obtained from the experiment) at different inlet steam flow rates, the corresponding inlet steam flow rate and ethanol concentration, and the inlet cooling water parameters (flow rate 261.8 kg / h, inlet temperature 15°C) into the heat and mass transfer model, and the experimental value is obtained through experimental data. The results calculated by the model are not much different from the experimentally measured data, and the relative percentage error is between 0.6-10%, indicating that the model is more accurate in predicting the mass flow rate at the outlet of the vacuum condensation fractionator.

[0234] In the comparison results of experimental data and model data, the condensate concentration calculated by the model is close to the experimental data when the external force is small, and the maximum difference between the two is about 1.5wt%; when the external force is large, the difference is large with the experimental data, and the difference is about 5wt%. The experimental value and simulation value of the outlet steam ethanol concentration are not much different, and the relative error is 1.1%-11.9%, indicating that the model has a high prediction accuracy for outlet steam ethanol.

[0235] The present invention uses the mean absolute percentage error (MAPE) calculation formula to make a comprehensive evaluation of the prediction accuracy of the heat and mass transfer model. Among them, the MAPE of the condensate flow rate is 2.43%, the MAPE of the outlet steam flow rate is 3.88%, and the MAPE of the outlet steam ethanol concentration is 4.80%. It can be seen that the heat and mass transfer model established in the present invention can well predict the mass flow rate of the condensate, the mass flow rate of the outlet steam and the ethanol concentration, and the error between the prediction result and the actual value is less than 5%.

[0236] ② Practical application of the model

[0237] The present invention designs and calculates the vacuum condensation fractionator in the fermentation-membrane separation coupling pilot plant with an annual output of 300 tons of ethanol, which is jointly developed by the research group and Guangxi COFCO Biomass Energy Co., Ltd. The circulating cooling water of the shell side of the vacuum condensation fractionator adopts the company's on-site public engineering, and other process parameters related to the vacuum condensation fractionator are shown in Table 1. The heat transfer area of ​​the pilot-scale vacuum condensation fractionator obtained by design reaches 5.79m 2 , compared with the laboratory heat transfer area of ​​0.01256m 2 , the scale has expanded by about 460 times.

[0238] Table 1 Process parameters of vacuum condensation fractionator in ethanol recovery section of pilot plant

[0239]

[0240] According to the measured experimental data, the ethanol concentration of the condensate of the vacuum condensation fractionator is maintained between 10-15wt%, and the ethanol concentration in the outlet steam is 41-46wt%. After calculation, the secondary permeate steam recovery efficiency and ethanol recovery efficiency reach 59-76% and 82-94wt%, respectively. That is, under the pilot scale, fractional distillation and condensation also achieve the gradient recovery and secondary concentration of permeate steam, and the industrial application potential of the fractional distillation and condensation-steam compression integrated technology has been effectively proved.

[0241] When the model is applied to the pilot plant, it can still accurately predict the ethanol concentration in the outlet steam and the mass flow rate of the condensate and the outlet steam. In general, the model established in the present invention can be used to predict the condensation behavior of vacuum condensation fractionators of different scales, and has a wide range of applications.

[0242] Based on the heat and mass transfer characteristics of steam condensation in a vacuum condensation fractionator, the present invention establishes a heat and mass transfer model that associates equipment structural parameters with operating parameters, which is used to predict and analyze the influence of different parameter changes on its separation performance. The model has relatively accurate prediction results for the ethanol concentration in the steam at the outlet of the vacuum condensation fractionator, the mass flow rate of the condensate and the steam, and the MAPE between the model and the experimental value is less than 5%; the analysis results show that from the structural parameters, with the increase of the total heat transfer coefficient and the heat transfer area, the ethanol concentration in the condensate and the steam at the outlet of the vacuum condensation fractionator both shows an upward trend, and the secondary steam recovery rate and the secondary ethanol recovery rate both show a downward trend, and the reduction range of the secondary steam recovery rate is greater; from the operating parameters, with the increase of the circulating cooling water temperature and the inlet steam flow rate, the ethanol concentration in the condensate and the steam both shows a downward trend, and the secondary steam recovery rate and the secondary ethanol recovery rate both show an upward trend, and with the increase of the inlet steam ethanol concentration, the ethanol concentration in the condensate and the steam at the outlet both shows an upward trend, and the secondary steam recovery rate and the secondary ethanol recovery rate remain basically unchanged. When the model is applied to the pilot plant, it can still accurately predict the outlet steam ethanol concentration, the mass flow rate of condensate and outlet steam, and the MAPE is less than 6%.

[0243] 5. Model calculation examples

[0244] Taking the experimental data of the first day of the week of the pilot plant as an example, the outlet parameters of the vacuum condensation fractionator are predicted by the heat and mass transfer model established by the present invention:

[0245] 1) Input initial parameters

[0246] Inlet steam flow rate M0: 4.53 kg / h; Inlet steam ethanol concentration y0: 30.71 wt%; Circulating cooling water flow rate q m : 3500kg / h; cooling water inlet temperature t0: 18℃; actual heat transfer area A: 5.79m 2;Heat transfer coefficient K: 8.55W / (m 2 ℃); absolute error δ: 0.01;

[0247] 2) Calculate the initial outlet steam ethanol concentration y N

[0248]

[0249] where y m is the maximum outlet steam ethanol concentration, take y m =100%.

[0250] 3) Segment-by-segment calculation process

[0251] The entire vacuum condensation fractionator is divided into N sections according to the concentration gradient (N = 5 in this calculation process), and the ethanol concentration y of the outlet steam of each section is calculated. j :

[0252] y j =y0+jΔy(0≤j≤5)(S12);

[0253] in

[0254]

[0255] After calculating the outlet steam ethanol concentration of each heat transfer section, the temperature value corresponding to the inlet steam ethanol concentration and the outlet steam ethanol concentration of each section can be queried through the dew point temperature curve. The specific results are shown in Table 2.

[0256] Table 2. Ethanol concentration and corresponding temperature of outlet steam in each heat transfer section

[0257]

[0258] Calculate the condensate flow rate M at the outlet of each heat transfer section in turn c,j , outlet steam flow M j , condensate ethanol concentration x c,j And heat transfer Q j , the calculation process taking the first heat transfer section as an example is as follows:

[0259] Outlet steam flow M1:

[0260]

[0261] Condensate flow rate M c,1 :

[0262] M c,1 =M 1,n -M 1,0 =M j-M0=4.53-3.578=0.952kg / h (S15);

[0263] Condensate ethanol concentration x c,1 :

[0264]

[0265] Table 3. Mass transfer calculation results in each heat transfer section

[0266]

[0267] The cooling water outlet temperature t of each heat transfer section is calculated in turn by equations (7)-(10): N-(j-1) , the logarithmic average heat transfer temperature difference Δt of each heat transfer section m,j And the total heat transfer Q t , total heat transfer temperature difference Δt m,t Since the heat exchange between the tube side and the shell side of the vacuum condensation fractionator is countercurrent heat exchange, the cooling water outlet temperature of each heat transfer section is t N-(j-1) The calculation should start from the last paragraph, and the specific process is as follows:

[0268]

[0269] The specific results are shown in Table 4:

[0270] Table 4. Shell cooling water inlet and outlet temperatures and heat transfer temperature difference in each heat transfer zone

[0271]

[0272] Total heat transfer Q t :

[0273]

[0274] Total logarithmic mean heat transfer temperature difference Δt m,t :

[0275]

[0276] 4) Calculation of heat transfer area

[0277] The required heat transfer area A′ is calculated by formula (11):

[0278]

[0279] Comparison of heat transfer area

[0280] A′-A|=|10.90-5.79|=5.11>0.01(S22);

[0281] At the same time, A′>A; as shown in the calculation process Figure 5As shown, y should be taken at this time m =y N =65.36wt%, repeat steps S22 to S26, and continue the cycle calculation until the difference is less than δ, and then output y at this time N .

[0282] In the description of this specification, the description with reference to the terms "an embodiment", "another embodiment", "other embodiments", or "first embodiment to Xth embodiment" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, method steps or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0283] It should be noted that, in the present invention, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or system including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or system. In the absence of further restrictions, an element defined by the sentence "comprises a ..." does not exclude the existence of other identical elements in the process, method, article or system including the element.

[0284] The serial numbers of the above embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0285] The above are only preferred embodiments of the present invention, and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for predicting heat and mass transfer behavior in a vacuum condensation fractionator condensation process, characterized in that: The steps include: According to the material balance and heat balance relationship of the heat and mass transfer process of ethanol water vapor condensation under vacuum conditions, a heat and mass transfer model in the vacuum condensation fractionator is established. Under given operating conditions and structural parameters, the condensate flow rate, condensate ethanol concentration, outlet steam flow rate and outlet steam ethanol concentration of the vacuum condensation fractionator are predicted by the heat and mass transfer model to calculate the total heat transfer of the entire condensation process, thereby obtaining the calculated structural parameters of the vacuum condensation fractionator.

2. The method for predicting heat and mass transfer behavior of a vacuum condensation fractionator condensation process according to claim 1, characterized in that: The step of establishing a heat and mass transfer model in a vacuum condensation fractionator according to the material balance and heat balance relationship of the ethanol water vapor condensation heat and mass transfer process under vacuum conditions comprises: According to the rule that the heat transfer load of the vacuum condensing fractionator, the heat transfer load of the hot fluid and the heat transfer load of the cold fluid are all equal, a heat transfer formula is established; The heat transfer process is divided into heat transfer intervals according to the ethanol concentration gradient in the ethanol water vapor in the vacuum condensation fractionator to obtain a plurality of heat transfer sections, so as to calculate the heat transfer amount and the logarithmic mean heat transfer temperature difference of each heat transfer section, and then the total heat transfer amount and the total logarithmic mean heat transfer temperature difference of the vacuum condensation fractionator are calculated; Divide each heat transfer section into mass transfer intervals to obtain multiple mass transfer units; The permeate steam flow relationship and the ethanol concentration relationship are established by the mass conservation equation, wherein the permeate steam flow relationship is the relationship between the inlet steam flow, the condensate flow and the outlet steam flow in each heat transfer section, and the ethanol concentration relationship is the relationship between the inlet steam ethanol concentration, the condensate ethanol concentration and the outlet steam ethanol concentration in each heat transfer section; Determine the ethanol mass conservation equation for each mass transfer unit in each heat transfer section, and determine the outlet steam flow calculation formula in each heat transfer section.

3. The method for predicting heat and mass transfer behavior of the condensation process of a vacuum condensation fractionator according to claim 2, characterized in that: The step of establishing a heat transfer formula according to the rule that the heat exchange load of the vacuum condensing fractionator, the heat exchange load of the hot fluid and the heat exchange load of the cold fluid are all equal comprises: Establish the heat transfer load Q in the vacuum condensation fractionator t Calculation formula of Establish the heat transfer load Q of the hot fluid in the vacuum condensation fractionator r Calculation formula of Establish the heat transfer load Q of the cooling fluid in the vacuum condensation fractionator a Calculation formula of Establish the logarithmic mean heat transfer temperature difference Δt in the condensation heat transfer process m,t Calculation formula of Q t =KAΔt m,t (1); Q r =M c x c r e +M c (1-x c )r w (2); Q a =q m c p (t N -t0) (3); Where K is the total heat transfer coefficient, unit is W / (m 2 ·℃); A is the actual heat transfer area of ​​the vacuum condensation fractionator, in m 2 ; Δt m,t is the logarithmic mean heat transfer temperature difference during the condensation heat transfer process, in °C; M c is the condensate flow rate at the lower outlet of the vacuum condensation fractionator, in kg / h; x c is the ethanol concentration of the condensate at the lower outlet of the vacuum condensation fractionator, in wt%; q m is the flow rate of circulating cooling water, in kg / h; r e is the latent heat of vaporization of ethanol, r w is the latent heat of vaporization of water, in kJ / kg; c p is the specific heat capacity of circulating cooling water, in kJ / (kg·℃); T0 is the steam temperature at the inlet of the vacuum condensation fractionator, T N is the steam temperature at the outlet of the vacuum condensation fractionator, t0 is the inlet temperature of the circulating cooling water, t N is the outlet temperature of circulating cooling water, where the fluids in the tube side and shell side are completely countercurrent.

4. The method for predicting heat and mass transfer behavior of a vacuum condensation fractionator condensation process according to claim 3, characterized in that: The heat transfer process is divided into heat transfer intervals according to the ethanol concentration gradient in the ethanol water vapor in the vacuum condensation fractionator to obtain a plurality of heat transfer sections for calculating the heat transfer amount and the logarithmic mean heat transfer temperature difference of each heat transfer section, and then the total heat transfer amount and the total logarithmic mean heat transfer temperature difference of the vacuum condensation fractionator are calculated, comprising: In the process of stage-by-stage calculation, the heat transfer process is divided into N stages according to the difference in the inlet and outlet concentrations of the vacuum condensation fractionator. The vapor phase temperature is determined by the dew point temperature curve. The specific calculation process of the heat transfer temperature difference of each heat transfer stage is as follows: Calculate the outlet vapor ethanol concentration of each heat transfer section: y j =y0+jΔy (5); Among them, y0 is the ethanol concentration of the inlet steam of the vacuum condensation fractionator, y j represents the ethanol concentration of the outlet steam of the jth heat transfer section. Δy is the difference in ethanol concentration of the inlet and outlet steam of each heat transfer section. 1≤j≤N. The calculation formula of Δy is as follows: According to y j The outlet steam temperature T of the jth heat transfer section can be obtained by using the dew point temperature curve. j , steam flow rate at the outlet of each heat transfer section M j , condensate ethanol concentration x c,j , condensate flow rate M c,j and heat transfer Q j Following the mass conservation equation, the cooling water inlet and outlet temperature differences and heat transfer temperature differences of each heat transfer section are finally obtained as follows: The difference in cooling water inlet and outlet temperature of each heat transfer section: Among them, t N-j is the inlet temperature of the cooling water in the jth heat transfer section, t N-(j-1) is the outlet temperature of cooling water in the jth heat transfer section, in °C; Logarithmic mean heat transfer temperature difference of each heat transfer section: Total heat transfer Q of vacuum condensation fractionator t for: Total logarithmic mean heat transfer temperature difference Δt of vacuum condensation fractionator m,t : When the total heat transfer coefficient is K, the calculated heat transfer area A' of the vacuum condensation fractionator is as follows: Among them, Δt m,j is the logarithmic mean heat transfer temperature difference of the jth heat transfer section, T j is the outlet steam temperature of the jth heat transfer section, T j-1 is the inlet steam temperature of the jth heat transfer section, in degrees Celsius.

5. The method for predicting heat and mass transfer behavior in the condensation process of a vacuum condensation fractionator according to claim 4, characterized in that: The step of establishing the permeate steam flow relationship and the ethanol concentration relationship through the mass conservation equation comprises: The mass conservation equation of the permeate steam flow rate in the jth heat transfer section is as follows: M j,0 =M j-1 =M c,j +M j =M c,j +M j,n (12); The mass conservation equation for the ethanol flow rate in the jth heat transfer section is as follows: M j，0 y j,0 =M j-1 y j-1 =M c,j x c,j +M j y j =M c,j x c,j +M j,n y j,n (13); Among them, M j-1 and M j,0 are the inlet steam flow rate of the jth heat transfer section, M j,n It is also the outlet steam flow rate of the jth heat transfer section; y j-1 and j,0 are the inlet steam ethanol concentration of the jth heat transfer section, y j,n It is also the outlet steam ethanol concentration of the jth heat transfer section.

6. The method for predicting heat and mass transfer behavior in the condensation process of a vacuum condensation fractionator according to claim 5, characterized in that: The step of determining the ethanol mass conservation equation of each mass transfer unit in each heat transfer section and determining the outlet steam flow calculation formula in each heat transfer section includes: The ethanol mass conservation equation on the i-th mass transfer unit in the j-th heat transfer section is as follows: M j,i-1 y j,i-1 =x j,i ΔM j,i +M j,i y j,i (14); Among them, M j,i-1 is the steam flow rate at the inlet of the i-th mass transfer unit in the j-th heat transfer section, y j,i-1 is the steam ethanol concentration at the inlet of the i-th mass transfer unit in the j-th heat transfer section; ΔM j,i is the mass of condensate in the i-th mass transfer unit in the j-th heat transfer section, x j,i is the ethanol concentration of the condensate in the i-th mass transfer unit in the j-th heat transfer section; M j,i is the outlet steam flow rate of the ith mass transfer unit in the jth heat transfer section, y j,i is the outlet steam ethanol concentration of the i-th mass transfer unit in the j-th heat transfer section, 0<i≤n, n is the number of mass transfer units in each heat transfer section; In formula (14), let: M j,i =M j,i-1 -ΔM j,i (15); y j,i =y j,i-1 +Δy j,i (16); Substituting equations (15) and (16) into equation (14) and ignoring higher-order infinitesimals, we obtain: Wherein, M is the mass flow rate on the mass transfer unit, M includes the condensate flow rate and the outlet steam flow rate on the mass transfer unit; y is the steam ethanol concentration on the mass transfer unit, and x is the condensate ethanol concentration on the mass transfer unit; The boundary condition of formula (17) is: M=M j,0 :y=y j,0 , M=M j,n :y=y j,n (18); Therefore, after integrating equation (17), we get: Solving the integral in equation (19), the result is shown in equation (20): in: h is the step size; I is the function f(y) in the interval (y j-1 ,y j ) on the integral result; y j,2i is the outlet steam ethanol concentration corresponding to the even-numbered mass transfer unit in the jth heat transfer section, y j,2i-1 is the outlet steam ethanol concentration corresponding to the odd-numbered mass transfer unit in the jth heat transfer section; the inlet steam ethanol concentration y j-1 , and outlet steam ethanol concentration y j , the function f(y) is calculated in the interval (y j-1 ,y j ) and put the integral result into equation (19) to calculate the steam flow rate at the outlet of the vacuum condensation fractionator under given conditions.

7. The method for predicting heat and mass transfer behavior in the condensation process of a vacuum condensation fractionator according to claim 6, characterized in that: According to the mass conservation equation (12) of the permeate steam flow rate, the condensate flow rate of the jth heat transfer section is calculated as follows: M c,j =M j,n -M j,0 (23); According to the mass conservation equation of ethanol flow (13), the condensate ethanol concentration x of the jth heat transfer section is calculated. c,j : According to the heat transfer load calculation formula (2), the heat transfer capacity of the jth heat transfer section is calculated as follows: Q j =M c,j x c,j r e +M c,j (1-x c,j )r w (25)。 8. The method for predicting heat and mass transfer behavior in the condensation process of a vacuum condensation fractionator according to claim 7, characterized in that: The step of predicting the condensate flow rate, condensate ethanol concentration, outlet steam flow rate and outlet steam ethanol concentration of the vacuum condensation fractionator by using a heat and mass transfer model under given operating conditions and structural parameters to calculate the total heat transfer of the entire condensation process, thereby calculating the calculated structural parameters of the vacuum condensation fractionator, comprises: Obtaining known structural parameters and operating parameters of a vacuum condensation fractionator; The initial outlet steam ethanol concentration of the vacuum condensation fractionator is set to be the average of the initial inlet steam ethanol concentration and the maximum outlet steam ethanol concentration; According to the set number of heat transfer sections, the vacuum condensation fractionator is divided into a plurality of heat transfer sections with equal concentration gradients, and the ethanol concentration difference of each heat transfer section in the vacuum condensation fractionator, the ethanol concentration of the outlet steam of each heat transfer section and the outlet steam temperature of each heat transfer section are calculated by using the inlet steam ethanol concentration and the outlet steam ethanol concentration; According to the calculation formula of the outlet steam flow rate and the relationship formula of the permeate steam flow rate in each heat transfer section, the outlet steam flow rate and the condensate flow rate in each heat transfer section are calculated; according to the relationship formula of the ethanol concentration, the ethanol concentration of the condensate in each heat transfer section is calculated; according to the calculation formula of the heat transfer load of the hot fluid, the heat transfer amount in each heat transfer section is calculated; Calculate the cooling water outlet temperature of each heat transfer section; Calculate the logarithmic mean heat transfer temperature difference of each heat transfer section, the total heat transfer of the vacuum condensation fractionator, and the total logarithmic mean heat transfer temperature difference of the entire condensation heat transfer process of the vacuum condensation fractionator; Calculate the heat transfer area of ​​the vacuum condensation fractionator under the set total heat transfer coefficient; Determine whether the calculated heat transfer area and the actual heat transfer area of ​​the vacuum condensation fractionator are less than the set error value; If yes, the prediction results are output, wherein the prediction results include: outlet steam ethanol concentration, outlet steam flow rate in each heat transfer section, condensate flow rate and condensate ethanol concentration.

9. The method for predicting heat and mass transfer behavior in the condensation process of a vacuum condensation fractionator according to claim 8, characterized in that: After the step of determining whether the calculated heat transfer area and the actual heat transfer area of ​​the vacuum condensation fractionator are less than the set error value, the method further includes: If not, compare the calculated heat transfer area of ​​the vacuum condensation fractionator with the actual heat transfer area; When the calculated heat transfer area is larger than the actual heat transfer area, the maximum outlet steam ethanol concentration is set equal to the outlet steam ethanol concentration; When the calculated heat transfer area is smaller than the actual heat transfer area, the inlet steam ethanol concentration is set equal to the outlet steam ethanol concentration, and the steps of setting the initial outlet steam ethanol concentration of the vacuum condensation fractionator to the average of the initial inlet steam ethanol concentration and the maximum outlet steam ethanol concentration, to the steps of calculating the logarithmic mean heat transfer temperature difference of each heat transfer section, the total heat transfer of the vacuum condensation fractionator, and the total logarithmic mean heat transfer temperature difference of the entire condensation heat transfer process of the vacuum condensation fractionator are repeated until the calculated heat transfer area of ​​the vacuum condensation fractionator and the actual heat transfer area are smaller than the set error value, and the prediction result is output.

10. A pervaporation membrane separation system, characterized in that: The invention comprises a fermentation tank, a circulation pump, a membrane separator, a vacuum condensation fractionator, a vacuum compression pump and a normal pressure condenser connected in sequence; the prediction method as described in any one of claims 1 to 9 is applied to predict the condensate flow rate, the ethanol concentration of the condensate, the outlet steam flow rate and the outlet steam ethanol concentration of the vacuum condensation fractionator to calculate the total heat transfer of the whole condensation process, thereby calculating the calculation structural parameters of the vacuum condensation fractionator.

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