Prediction Method and System for Heat and Mass Transfer Behavior during Condensation Process of Vacuum Condensing Fractionator

Abstract

The present invention discloses a method and system for predicting the heat and mass transfer behavior during the condensation process of a vacuum condensation fractionator. The method includes: establishing a heat and mass transfer model in the vacuum condensation fractionator according to the material balance and heat balance relationships of the ethanol water vapor condensation heat and mass transfer process under vacuum conditions; predicting the condensate flow rate, condensate ethanol concentration, outlet steam flow rate, and outlet steam ethanol concentration of the vacuum condensation fractionator through the heat and mass transfer model under given operating conditions and structural parameters, so as to calculate the total heat transfer amount of the entire condensation process, and thereby calculate the calculated structural parameters of the vacuum condensation fractionator. The present invention is beneficial to solving the problems that in the vacuum condensation fractionator of the existing ethanol fermentation-membrane separation coupling process, the outlet parameters of the vacuum condensation fractionator cannot be predicted, and a basis for the design of the structural parameters of the vacuum condensation fractionator cannot be provided.

Description

Technical Field

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

[0002] Fuel ethanol is a bulk basic chemical, and approximately 90% of ethanol is produced by fermentation. By adopting a fermentation-membrane separation coupling process for ethanol production, the separation membrane can selectively and in-situ remove ethanol in real time, solve the product inhibition phenomenon existing in traditional fermentation industries, and thus overcome the problems such as a large amount of fermentation waste liquid and high energy consumption in subsequent ethanol refining in the current production process.

[0003] In the coupling system, a vacuum is often pumped downstream of the membrane to timely remove the permeated steam to ensure the mass transfer driving force required for the membrane separation process. However, under vacuum conditions, the thermal conductivity of the dilute permeated steam is poor and the dew point is low, and a low-temperature (-10°C) strong cooling device or liquid nitrogen is often used to condense and recover the permeated steam. However, due to high energy consumption and non-continuous operation, the above recovery methods cannot be industrially applied, which is also the main reason why the pervaporation technology has not been industrially applied.

[0004] To solve the above problems, in related technologies, a hierarchical condensation process of vacuum fractionation-mechanical vapor compression-atmospheric pressure condensation has been developed for the steam condensation process downstream of the membrane. On the one hand, this process utilizes the dew point difference between ethanol vapor and water vapor to achieve gradient recovery of the permeated steam. On the other hand, it improves the dew point and thermal conductivity of the permeated steam through compression, realizes the recovery of the permeated steam using normal-temperature water as the refrigerant, eliminates low-temperature strong cooling and liquid nitrogen condensation, so as to achieve the purpose of reducing energy consumption and facilitating scale-up. In this process, the permeated steam first enters a vacuum condenser fractionator, where the water vapor with a high dew point is preferentially condensed to form a condensate with a low ethanol concentration; the permeated steam with a relatively high ethanol concentration reaches the normal level of the dew point after compression and enters a secondary atmospheric condenser to form a condensate with a relatively high ethanol concentration. Both levels of the vacuum condenser fractionator use normal-temperature water as the refrigerant. When the ethanol concentration of the primary condensate is at a relatively low level, it is expected to be directly reused in the process without entering the subsequent ethanol refining section, and only the secondary condensate with a relatively high ethanol concentration is used for ethanol recovery, significantly reducing the raw material processing volume in the ethanol refining section and reducing the refining energy consumption.

[0005] The condensation heat transfer behavior of ethanol vapor under vacuum conditions is affected by the operating parameters and structural parameters of the equipment, making the condensation effect of the vacuum condensation fractionator on the permeating vapor 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 vapor ethanol concentration, condensate, and outlet vapor flow rate, etc.) will all change, thus affecting the final wastewater generation amount of the process and the energy consumption in the subsequent ethanol refining section.

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

[0007] Related technologies have applied the pervaporation - fractional condensation process to recover valuable aromas from fermentation media, wine fermentation broth, and seafood industrial wastewater for food processing. The results show that this process can achieve high - selectivity recovery of the target product and has certain application potential in the food industry. However, the above - mentioned research only considered the influence of the vacuum degree downstream of the membrane and the refrigerant temperature on the separation effect, and did not correlate the structural parameters of the vacuum condensation fractionator with the separation effect. So far, the vacuum fractional condensation concept has been rarely applied in the ethanol industry, and there is even less research on the vacuum condensation fractionator in the ethanol fermentation - membrane separation coupling process.

[0008] Therefore, the defect of the existing technology is that in the vacuum condensation fractionator of the existing ethanol fermentation - membrane separation coupling process, the outlet parameters of the vacuum condensation fractionator cannot be predicted, nor can a basis be provided for the design of the structural parameters 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 during the condensation process of a vacuum condensation fractionator, aiming to solve the problem that in the vacuum condensation fractionator of the existing ethanol fermentation - membrane separation coupling process, the outlet parameters of the vacuum condensation fractionator cannot be predicted, nor can a basis be provided for the design of the structural parameters of the vacuum condensation fractionator.

[0010] To achieve the above purpose, a method for predicting the heat and mass transfer behavior during the condensation process of a vacuum condensation fractionator provided by the present invention includes the following steps:

[0011] Establish a heat and mass transfer model in the vacuum condensation fractionator according to the material balance and heat balance relationships of the condensation heat and mass transfer process of ethanol vapor under vacuum conditions;

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

[0013] Furthermore, the steps of establishing the heat and mass transfer model in the vacuum condensation fractionator according to the material balance and heat balance relationships of the ethanol-water vapor condensation heat and mass transfer process under vacuum conditions include:

[0014] Establish a heat transfer formula according to the rule that the heat transfer loads of the vacuum condensation fractionator, the hot fluid, and the cold fluid are equal;

[0015] Divide the heat transfer process into heat transfer sections according to the ethanol concentration gradient in the ethanol-water vapor in the vacuum condensation fractionator to obtain multiple heat transfer sections for calculating the heat transfer amount and logarithmic mean heat transfer temperature difference of each heat transfer section, and then calculate the total heat transfer amount and total logarithmic mean heat transfer temperature difference of the vacuum condensation fractionator;

[0016] Divide the mass transfer section for each heat transfer section to obtain multiple mass transfer units;

[0017] Establish a permeating steam flow rate relationship and an ethanol concentration relationship through the mass conservation equation, where the permeating steam flow rate relationship is the relationship between the inlet steam flow rate, condensate flow rate, and outlet steam flow rate in each heat transfer section, and the ethanol concentration relationship is the relationship between the inlet steam ethanol concentration, condensate ethanol concentration, and outlet steam ethanol concentration in each heat transfer section;

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

[0019] Furthermore, the steps of establishing a heat transfer formula according to the rule that the heat transfer loads of the vacuum condensation fractionator, the hot fluid, and the cold fluid are equal include:

[0020] Establish a calculation formula for the heat transfer load Q t in the vacuum condensation fractionator;

[0021] Establish a calculation formula for the heat transfer load Q r of the hot fluid in the vacuum condensation fractionator;

[0022] Establish a calculation formula for the heat transfer load Q a of the cold fluid in the vacuum condensation fractionator;

[0023] Establish a calculation formula for the logarithmic mean heat transfer temperature difference Δt m,t in the condensation heat transfer process;

[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] Wherein, K is the overall heat transfer coefficient, with the unit of W / (m 2 ·°C); A is the actual heat transfer area of the vacuum condensation fractionator, with the unit of m 2 ; Δt m,t is the logarithmic mean temperature difference in the condensation heat transfer process, with the unit of °C; M c is the condensate flow rate at the lower outlet of the vacuum condensation fractionator, with the unit of kg / h; x c is the ethanol concentration in the condensate at the lower outlet of the vacuum condensation fractionator, with the unit of wt%; q m is the flow rate of the circulating cooling water, with the unit of kg / h; r e is the latent heat of vaporization of ethanol, r w is the latent heat of vaporization of water, both with the unit of kJ / kg; c p is the specific heat capacity of the circulating cooling water, with the unit of kJ / (kg·°C); 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 the circulating cooling water, wherein the fluids in the tube side and the shell side are in complete countercurrent flow.

[0029] Furthermore, the step of dividing the heat transfer process into heat transfer sections according to the ethanol concentration gradient in the ethanol water vapor in the vacuum condensation fractionator to obtain multiple heat transfer sections for calculating the heat transfer amount and the logarithmic mean temperature difference of each heat transfer section, and then calculating the total heat transfer amount and the total logarithmic mean temperature difference of the vacuum condensation fractionator includes:

[0030] In the step-by-step calculation process, the heat transfer process is evenly divided into N sections according to the concentration difference between the inlet and outlet of the vacuum condensation fractionator, the steam side temperature is determined by the dew point temperature curve graph, and the specific calculation process of the heat transfer temperature difference of each heat transfer section is as follows:

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

[0032] y j = y0 + jΔy (5);

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

[0034]

[0035] According to y j and the dew point temperature curve, obtain the outlet steam temperature T j of the j-th heat transfer section, the outlet steam flow rate M j of each heat transfer section, the ethanol concentration x c,j of the condensate, the condensate flow rate M c,j and the heat transfer amount Q j obey the mass conservation equation, and finally obtain the temperature difference between the inlet and outlet of the cooling water and the heat transfer temperature difference of each heat transfer section as follows:

[0036] Temperature difference between the inlet and outlet of the cooling water of each heat transfer section:

[0037]

[0038] where t N-j is the inlet temperature of the cooling water of the j-th heat transfer section, and t N-(j-1) are the outlet temperatures of the cooling water of the j-th heat transfer section, and the unit is °C;

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

[0040]

[0041] The total heat transfer amount Q t of the vacuum condensation fractionator is:

[0042]

[0043] The total logarithmic mean heat transfer temperature difference Δt m,t of the vacuum condensation fractionator is:

[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] wherein, Δt m,j is the logarithmic mean temperature difference of heat transfer in the j-th heat transfer section, T j is the outlet steam temperature of the j-th heat transfer section, T j-1 is the inlet steam temperature of the j-th heat transfer section, and the unit of all is °C.

[0048] Further, the steps of establishing the permeate steam flow relationship and the ethanol concentration relationship through the mass conservation equation include:

[0049] The mass conservation equation of the permeate steam flow in the j-th 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 of the ethanol flow in the j-th 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] wherein, M j-1 and M j,0 are both the inlet steam flow rates of the j-th heat transfer section, and M j,n is also the outlet steam flow rate of the j-th heat transfer section; y j-1 and y j,0 are both the ethanol concentrations of the inlet steam of the j-th heat transfer section, and y j,n is also the ethanol concentration of the outlet steam of the j-th heat transfer section.

[0054] Further, the steps of determining the ethanol mass conservation equation of each mass transfer unit in each heat transfer section and determining the calculation formula of the outlet steam flow rate in each heat transfer section include:

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

[0058] In Equation (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] Substitute Equations (15) and (16) into Equation (14), and neglect high-order infinitesimals, to obtain:

[0062]

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

[0064] The boundary conditions of Equation (17) are:

[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] Solve the integral in Equation (19), and the result is shown in Equation (20):

[0069]

[0070] Where:

[0071]

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

[0073] Furthermore:

[0074] Calculate the condensate flow rate of the j-th heat transfer section according to the mass conservation equation (12) of the pervaporation steam flow rate as follows:

[0075] M c,j = M j,n - M j,0 (23);

[0076] Calculate the condensate ethanol concentration x c,j of the j-th heat transfer section according to the mass conservation equation (13) of the ethanol flow rate:

[0077]

[0078] Calculate the heat transfer amount of the j-th heat transfer section according to the heat fluid heat exchange load calculation formula (2) 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 given operating conditions and structural parameters, through a heat and mass transfer model, predicting the condensate flow rate, condensate ethanol concentration, outlet steam flow rate, and outlet steam ethanol concentration of the vacuum condensation fractionator to calculate the total heat transfer amount of the entire condensation process, and thus calculating the calculated structural parameters of the vacuum condensation fractionator, the steps include:

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

[0082] Setting the initial outlet steam ethanol concentration of the vacuum condensation fractionator as the average value of the initial inlet steam ethanol concentration and the maximum outlet steam ethanol concentration;

[0083] Dividing the vacuum condensation fractionator into multiple heat transfer sections with equal concentration gradients according to the set number of heat transfer sections, and using the inlet steam ethanol concentration and the outlet steam ethanol concentration to calculate the ethanol concentration difference in each heat transfer section of the vacuum condensation fractionator, the outlet steam ethanol concentration in each heat transfer section, and the outlet steam temperature in each heat transfer section;

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

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

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

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

[0088] Judging whether the calculated heat transfer area of the vacuum condensation fractionator is less than the set error value compared with the actual heat transfer area;

[0089] If so, outputting the prediction result, where the prediction result includes: the outlet steam ethanol concentration, the outlet steam flow rate in each heat transfer section, the condensate flow rate, and the condensate ethanol concentration.

[0090] Further, after the step of judging whether the calculated heat transfer area of the vacuum condensation fractionator is less than the set error value compared with the actual heat transfer area, it further includes:

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

[0092] When the calculated heat transfer area is greater 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 less than the actual heat transfer area, the inlet steam ethanol concentration is set equal to the outlet steam ethanol concentration, and the step of setting the initial outlet steam ethanol concentration of the vacuum condensation fractionator to the average value of the initial inlet steam ethanol concentration and the maximum outlet steam ethanol concentration is repeated until the steps of calculating the logarithmic mean temperature difference of each heat transfer section, the total heat transfer of the vacuum condensation fractionator, and the total logarithmic mean temperature difference in the entire condensation heat transfer process of the vacuum condensation fractionator are performed until the difference between the calculated heat transfer area and the actual heat transfer area of the vacuum condensation fractionator is less than the set error value, and then the prediction result is output.

[0094] To achieve the above object, the present invention also provides a pervaporation membrane separation system, including a fermentation tank, a circulation pump, a membrane separator, a vacuum condensation fractionator, a vacuum compression pump, and an atmospheric condenser connected in sequence; applying the prediction method to predict the condensate flow rate, condensate ethanol concentration, outlet steam flow rate, and outlet steam ethanol concentration of the vacuum condensation fractionator, so as to calculate the total heat transfer in the entire condensation process, and thus calculate the calculated structural parameters of the vacuum condensation fractionator.

[0095] In the technical solution of the present invention, a heat and mass transfer model for the condensation process in the vacuum condensation fractionator is established to predict the outlet parameters of the material. Under the given operating conditions and structural parameters, through the heat and mass transfer model, the condensate flow rate, condensate ethanol concentration, outlet steam flow rate, and outlet steam ethanol concentration of the vacuum condensation fractionator are predicted, and then the total heat transfer in the entire condensation process is calculated through 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. Therefore, the present invention is beneficial to solving the problem that in the vacuum condensation fractionator of the existing ethanol fermentation-membrane separation coupling process, the outlet parameters of the vacuum condensation fractionator cannot be predicted, and no basis can be provided for the structural parameter design of the vacuum condensation fractionator. Description of the Drawings

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

[0097] Figure 2 is a schematic diagram of the condensation process of the permeating steam in the vacuum condensation fractionator of the present invention;

[0098] Figure 3 is a 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 of the present invention;

[0100] Figure 5 It is a calculation flow chart of the heat and mass transfer model.

[0101] Explanation of the reference numerals in the attached drawings:

[0102] 1 - Fermenter; 2 - Circulation pump; 3 - Membrane separator; 4 - Vacuum condensation fractionator; 5 - Vacuum compression pump; 6 - Atmospheric condenser;

[0103] a - Ethanol aqueous solution; b - Heat cycle 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 realization, functional features and advantages of the present invention will be further described with reference to the embodiments and the accompanying drawings. Specific embodiments

[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 for the convenience of explaining the present invention and have no specific meaning in themselves. Therefore, "unit", "component" or "unit" can be used interchangeably.

[0107] Please refer to Figures 1 to 5 , to achieve the above object, in the first embodiment of the present invention, a method for predicting the heat and mass transfer behavior during the condensation process of a vacuum condensation fractionator is proposed, including the following steps:

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

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

[0110] In the technical solution of the present invention, a heat and mass transfer model for 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, through the heat and mass transfer model, the condensate flow rate, ethanol concentration in the condensate, outlet steam flow rate, and ethanol concentration in the outlet steam of the vacuum condensation fractionator are predicted. Then, the total heat transfer amount of the entire condensation process is calculated through the condensate flow rate and ethanol concentration, and thus the calculated structural parameters of the vacuum condensation fractionator can be obtained to guide the structural design of vacuum condensation fractionation. Therefore, the present invention is beneficial to solving the problems that in the vacuum condensation fractionator of the existing ethanol fermentation-membrane separation coupling process, the outlet parameters of the vacuum condensation fractionator cannot be predicted and no basis can be provided for the design of the structural parameters 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 Figure 2 shown ( Figure 2 in which G represents the direction of gravity and Q represents heat transfer). The steam in the tube side releases heat under the cooling of the circulating cooling water in the shell side. At the same time, part of the steam condenses on the tube wall to form a liquid film, which flows into the liquid collector along the inner wall of the tube under the action of gravity. 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 the dew point and thermal conductivity of water vapor and ethanol vapor, the heat release and condensation behaviors of the two are different, and there are mass transfer behaviors and heat transfer behaviors that affect each other in this condensation process.

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

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

[0115] Step S11, establish a heat transfer formula according to the rule that the heat transfer loads of the vacuum condensation fractionator, the hot fluid, and the cold fluid are equal;

[0116] In step S12, 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, obtaining multiple heat transfer sections, which are used to calculate the heat transfer amount and logarithmic mean temperature difference of each heat transfer section, and then the total heat transfer amount and total logarithmic mean temperature difference of the vacuum condensation fractionator are calculated;

[0117] In step S13, each heat transfer section is divided into mass transfer intervals to obtain multiple mass transfer units;

[0118] In step S14, an osmotic vapor flow rate relationship and an ethanol concentration relationship are established through the mass conservation equation, where the osmotic vapor flow rate relationship is the relationship among the inlet vapor flow rate, condensate flow rate and outlet vapor flow rate in each heat transfer section, and the ethanol concentration relationship is the relationship among the inlet vapor ethanol concentration, condensate ethanol concentration and outlet vapor ethanol concentration in each heat transfer section;

[0119] In step S15, the ethanol mass conservation equation of each mass transfer unit in each heat transfer section is determined, and the calculation formula for the outlet vapor flow rate in each heat transfer section is determined.

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

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

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

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

[0124] c) The system is in a steady state and the condensate film and vapor contact interface on each heat transfer section satisfy vapor-liquid equilibrium;

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

[0126] e) The product of the total average overall 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 and the corresponding heat transfer areas of each heat transfer section.

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

[0128] Establish the calculation formula for the heat transfer load Q in the vacuum condensation fractionator t ;

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

[0130] Establish the calculation formula for the heat transfer load Q of the cold fluid in the vacuum condensation fractionator a ;

[0131] Establish the calculation formula for the logarithmic mean temperature difference Δt in the condensation heat transfer process m,t ;

[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] Among them, K is the total heat transfer coefficient, with the unit of W / (m 2 ·℃); A is the actual heat transfer area of the vacuum condensation fractionator, with the unit of m 2 ; Δt m,t is the logarithmic mean temperature difference in the condensation heat transfer process, with the unit of ℃; M c is the condensate flow rate at the lower end outlet of the vacuum condensation fractionator, with the unit of kg / h; x c is the ethanol concentration of the condensate at the lower end outlet of the vacuum condensation fractionator, with the unit of wt%; q m is the flow rate of the circulating cooling water, with the unit of kg / h; r e is the latent heat of vaporization of ethanol, r w is the latent heat of vaporization of water, and the units are all kJ / kg; c p is the specific heat capacity of the circulating cooling water, with the unit of 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 the circulating cooling water, where the fluids in the tube side and the shell side are in complete countercurrent flow.

[0137] Based on the principle that the heat transfer amount of the hot and cold fluids is equal to the heat transfer load of the vacuum condensation fractionator, the above heat transfer formula is established. Since the latent heat released by the condensation of ethanol 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 the heat and mass transfer behavior during the condensation process of the vacuum condensation fractionator of the present invention, in the fourth embodiment of the method for predicting the heat and mass transfer behavior during the condensation process of the vacuum condensation fractionator of the present invention, the step S12 includes:

[0139] During the step-by-step calculation process, the heat transfer process is evenly divided into N segments according to the concentration difference between the inlet and outlet of the vacuum condensation fractionator (as shown on the left side), the steam side temperature is determined by the dew point temperature curve graph ( Figure 3 ), and the specific calculation process of the heat transfer temperature difference of each heat transfer segment is as follows: Figure 4 )

[0140] Calculate the outlet steam ethanol concentration of each heat transfer segment:

[0141] y j = y0 + jΔy (5);

[0142] Where, y0 is the inlet steam ethanol concentration of the vacuum condensation fractionator, y j represents the outlet steam ethanol concentration of the jth heat transfer segment, Δy is the ethanol concentration difference between the inlet and outlet steam of each heat transfer segment, 1 ≤ j ≤ N, and the calculation formula of Δy is as follows:

[0143]

[0144] According to y j and the dew point temperature curve graph, obtain the outlet steam temperature T j of the jth heat transfer segment. The outlet steam flow rate M j , the ethanol concentration x c,j of the condensate, the condensate flow rate M c,j and the heat transfer amount Q j of each heat transfer segment follow the mass conservation equation, and finally the temperature difference between the inlet and outlet of the cooling water and the heat transfer temperature difference of each heat transfer segment are obtained as follows:

[0145] The temperature difference between the inlet and outlet of the cooling water of each heat transfer segment:

[0146]

[0147] Where, t N-j is the inlet temperature of the cooling water of the jth heat transfer segment, and t N-(j-1) are both the outlet temperatures of the cooling water of the jth heat transfer segment, and the units are all °C;

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

[0149]

[0150] Total heat transfer quantity Q of the vacuum condensation fractionator t is:

[0151]

[0152] Total logarithmic mean temperature difference Δt of the 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] where, Δt m,j is the logarithmic mean 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, and the units are all °C.

[0157] In the case where the flow rate and temperature of the steam in the tube side of the vacuum condensation fractionator change greatly during the condensation process, the sectional calculation method is used to divide the heat transfer process into intervals to improve the calculation accuracy. By calculating the heat transfer quantity and logarithmic mean temperature difference of each heat transfer section respectively, the total heat transfer quantity and total logarithmic mean temperature difference can be calculated. Assuming that the total heat transfer coefficient is constant, the total heat transfer area required for this heat transfer load can be calculated.

[0158] The steam side temperature can be determined by the dew point temperature curve fitted by Aspen data (as Figure 4 shown).

[0159] Outlet steam flow rate M j of each section, ethanol concentration x c,j of the condensate, condensate flow rate M c,j and heat transfer quantity Q j follow the mass conservation equation, and the specific expression forms are shown in Equations (19) and (23)-(25).

[0160] Heat transfer quantity Q j is the amount of latent heat released by the steam.

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

[0162] The mass conservation equation of the permeating steam flow rate in the j-th 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 of the ethanol flow rate in the j-th 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] Wherein, M j-1 and M j,0 are both the inlet steam flow rates of the j-th heat transfer section, M c,j is the condensate flow rate of the j-th heat transfer section, M j and M j,n are both the outlet steam flow rates of the j-th heat transfer section; y j-1 and y j,0 are both the ethanol concentrations of the inlet steam of the j-th heat transfer section, x c,j is the ethanol concentration of the condensate of the j-th heat transfer section, y j and y j,n are both the ethanol concentrations of the outlet steam of the j-th heat transfer section.

[0167] In order to obtain the condensation fractionation parameters of ethanol water vapor under the inlet and outlet conditions of the j-th heat transfer section, when establishing the mass transfer model of permeating steam under vacuum conditions, first perform microelement division, establish the relationship between the outlet steam flow rate of the j-th section, the inlet steam flow rate of the j-th section, and the ethanol concentrations of the inlet and outlet steam through the mass conservation equation, and then use the composite Simpson numerical integration formula and the fitting equation of the ethanol water-liquid equilibrium curve under constant pressure conditions to solve for the corresponding outlet steam flow rate when the ethanol concentrations of the given inlet and outlet steam and the inlet steam flow rate are known. Finally, the corresponding condensate flow rate and its concentration can be calculated through the mass conservation equation.

[0168] In the fifth embodiment of the prediction method for the heat and mass transfer behavior during the condensation process of the vacuum condensation fractionator according to the present invention, in the sixth embodiment of the prediction method for the heat and mass transfer behavior during the condensation process of the vacuum condensation fractionator of the present invention, step S15 includes:

[0169] The ethanol mass conservation equation for 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] Wherein, M j,i-1 is the inlet steam flow rate of the i-th mass transfer unit in the j-th heat transfer section, y j,i-1 is the ethanol concentration of the inlet steam of the i-th mass transfer unit in the j-th heat transfer section; ΔM j,i is the condensate mass of the i-th mass transfer unit in the j-th heat transfer section, x j,i is the ethanol concentration of the condensate of the i-th mass transfer unit in the j-th heat transfer section; M j,i is the outlet steam flow rate of the i-th mass transfer unit in the j-th heat transfer section, y j,i is the ethanol concentration of the outlet steam of the i-th mass transfer unit in the j-th heat transfer section, 0 < i ≤ n, and n is the number of mass transfer units in each heat transfer section;

[0172] In Equation (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] Substitute Equations (15) and (16) into Equation (14), and ignoring higher-order infinitesimals, we get:

[0176]

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

[0178] The boundary conditions of Equation (17) are as follows:

[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 obtain:

[0181]

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

[0183]

[0184] Where:

[0185]

[0186]

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

[0188] During the model calculation process, n can be taken as 10.

[0189] In the present invention, the mass transfer process in each heat transfer section of the vacuum condensation fractionator is divided into n mass transfer units (as shown in Figure 3 ). Taking the ith mass transfer unit for analysis, where i takes values from 1 to n in sequence, the analysis of all mass transfer units in each heat transfer section can be completed.

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

[0191] In the sixth embodiment of the prediction method for the heat and mass transfer behavior during the condensation process of the vacuum condensation fractionator according to the present invention, in the seventh embodiment of the prediction method for the heat and mass transfer behavior during the condensation process of the vacuum condensation fractionator of the present invention:

[0192] Calculate the condensate flow rate of the j-th heat transfer section according to the mass conservation equation (12) of the permeating steam flow rate as follows:

[0193] M c,j = M j,n - M j,0 (23);

[0194] Calculate the ethanol concentration x of the condensate in the j-th heat transfer section according to the mass conservation equation (13) of the ethanol flow rate c,j :

[0195]

[0196] Calculate the heat transfer amount of the j-th heat transfer section according to the heat transfer load calculation formula (2) of the hot fluid as follows:

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

[0198] In the seventh embodiment of the prediction method for the heat and mass transfer behavior during the condensation process of the vacuum condensation fractionator according to the present invention, in the eighth embodiment of the prediction method for the heat and mass transfer behavior during the condensation process of the vacuum condensation fractionator of the present invention, the step S20 includes:

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

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

[0201] Step S23, divide the vacuum condensation fractionator into multiple heat transfer sections with equal concentration gradients according to the set number of heat transfer sections, and use the inlet steam ethanol concentration and the outlet steam ethanol concentration to calculate the ethanol concentration difference, the outlet steam ethanol concentration of each heat transfer section, and the outlet steam temperature of each heat transfer section in the vacuum condensation fractionator;

[0202] Step S24: Calculate the outlet steam flow rate and condensate flow rate in each heat transfer section according to the outlet steam flow rate calculation formula and permeate steam flow rate relationship in each heat transfer section; calculate the ethanol concentration in the condensate in each heat transfer section according to the ethanol concentration relationship; calculate the heat transfer amount in each heat transfer section according to the calculation formula of the heat transfer load of the hot fluid.

[0203] Step S25: Calculate the outlet temperature of the cooling water in each heat transfer section.

[0204] Step S26: Calculate the logarithmic mean temperature difference in each heat transfer section, the total heat transfer amount of the vacuum condensation fractionator, and the total logarithmic mean temperature difference in the entire condensation heat transfer process of the vacuum condensation fractionator.

[0205] Step S27: Calculate the calculated heat transfer area of the vacuum condensation fractionator under the set total heat transfer coefficient.

[0206] Step S28: Determine whether the difference between the calculated heat transfer area and the actual heat transfer area of the vacuum condensation fractionator is less than the set error value δ.

[0207] Step S29: If so, output the prediction result, where the prediction result includes: the ethanol concentration in the outlet steam, the outlet steam flow rate, the condensate flow rate, and the ethanol concentration in the condensate in each heat transfer section.

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

[0209] During the condensation heat transfer process of ethanol water vapor in a certain 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 and the inlet temperature is t0. Calculate the ethanol concentration y of the outlet steam of the vacuum condensation fractionator under this operating condition N . The detailed steps of calculating this process using the heat and mass transfer model are as Figure 5 shown, and the main process is as follows:

[0210] 1) Input the known initial process parameters;

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

[0212] 3) After dividing the vacuum condensation fractionator 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 NSubstitute into Equation (6) to obtain the ethanol concentration difference Δy (i.e., ethanol concentration increment) in each heat transfer section of the vacuum condensation fractionator, and then obtain the ethanol concentration y of the outlet steam in each heat transfer section according to Equation (5) and the dew point temperature curve graph j and the corresponding outlet steam temperature T j ;

[0213] 4) For 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 , the outlet steam ethanol concentration y j , substitute them into Equation (19) to calculate the outlet steam flow rate of this section, which is the inlet steam flow rate M of the next section j , substitute into Equation (23) to obtain the condensate flow rate M of each heat transfer section c,j , then calculate the ethanol concentration x of the condensate in each heat transfer section through Equation (24) c,j , and then calculate the heat transfer quantity Q of each heat transfer section through Equation (25) j ;

[0214] 5) Substitute the calculated ethanol concentration x of the condensate in each heat transfer section c,j and the condensate flow rate M c,j into Equation (25) to obtain the heat transfer quantity Q of each heat transfer section j , substitute the heat transfer quantity Q of each heat transfer section j and the cooling water flow rate q m and its inlet temperature t0 into Equation (7), and cyclically calculate to obtain the cooling water outlet temperature t of each heat transfer section N-(j-1) ;

[0215] 6) According to the calculated T j and t N-(j-1) and T0 and t0 in the known conditions, combined with Equation (8), calculate the logarithmic mean temperature difference Δt of each heat transfer section m,j ; then calculate the total heat transfer quantity Q obtained from Equation (9) t and the heat transfer quantity Q of each heat transfer section j and the logarithmic mean temperature difference Δt m,j , substitute into Equation (10) to obtain the total logarithmic mean temperature difference Δt in the entire condensation heat transfer process m,t ;

[0216] 7) Substitute the total heat transfer quantity Q t and the total logarithmic mean temperature difference Δt m,t into Equation (11), and then the calculated heat transfer area A' under this condition 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, output the outlet steam ethanol concentration y at this time. N ; Otherwise, judge the magnitudes 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 outlet process parameters.

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

[0219] Step S210, if not, compare the calculated heat transfer area and the actual heat transfer area of the vacuum condensation and fractionation device;

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

[0221] Step S212, when the calculated heat transfer area is less than the actual heat transfer area, let the inlet steam ethanol concentration be equal to the outlet steam ethanol concentration, and repeat steps S22 to S26 until the calculated heat transfer area and the actual heat transfer area of the vacuum condensation and fractionation device are less than the set error value, and then output the prediction result.

[0222] To achieve the above object, the present invention also proposes a pervaporation membrane separation system, including a fermentation tank, a circulation pump, a membrane separator, a vacuum condensation and fractionation device, a vacuum compression pump, and an atmospheric condenser connected in sequence; applying the prediction method to predict the condensate flow rate, condensate ethanol concentration, outlet steam flow rate, and outlet steam ethanol concentration of the vacuum condensation and fractionation device, so as to calculate the total heat transfer amount during the entire condensation process, and thus calculate the calculated structural parameters of the vacuum condensation and fractionation device.

[0223] Adopt the following scheme to verify the effectiveness of the present invention.

[0224] 1. Experimental materials

[0225] The feed liquid upstream of the membrane is a model solution with an ethanol concentration of 5 wt%, and the ethanol used is of analytical grade and purchased from Chengdu Sailesi Technology Co., Ltd. The PDMS membrane used is 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 is 0.024 m 2 .

[0226] 2. Experimental Procedures

[0227] The experimental setup is as Figure 1 shown. During the experiment, the ethanol - water model solution upstream of the membrane circulates between the fermenter 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 vapor on the other side of the membrane under the action of the vacuum compression pump 5 downstream of the membrane. When the permeate vapor 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 condensate with a lower ethanol concentration, and then flows along the tube wall into the liquid collector. The uncondensed vapor is transported to the secondary atmospheric condenser 6 under the action of the vacuum compression pump 5 and is condensed and recovered. During the experiment, three PDMS membranes with different permeation fluxes were selected to study the overall heat transfer coefficient of the vacuum condensation fractionator 4 under different vapor flow rates, and then analyze the separation behavior of the vacuum condensation fractionator 4 under different overall heat transfer coefficients. For each vapor flow rate condition, 2 - 3 groups of parallel experiments were set up, and the averaged results were taken for data processing.

[0228] 3. Testing Methods

[0229] The inlet and outlet steam temperatures T1, T N and the outlet condensate temperature T c of the primary vacuum condensation fractionator are measured using a thermocouple thermometer; the shell - side circulating cooling water is provided by a low - temperature constant - temperature bath, and the inlet temperature t0 is its digital display, and the outlet temperature t N is calculated by heat balance. The ethanol concentration x c of the condensate collected at the lower ends of the primary vacuum condensation fractionator and the secondary atmospheric condenser, and the ethanol concentration y N of the outlet steam are measured using an alcohol meter. The vacuum degree downstream of the membrane is measured using a vacuum gauge. The mass of the collected condensate is measured using an electronic balance.

[0230] 4. Result Analysis

[0231] ① Verification of the Accuracy of the Heat and Mass Transfer Model

[0232] To verify the accuracy of the calculation results of the heat and mass transfer model, the experimental test results and the model calculation results of parameters such as the outlet steam flow rate, the ethanol concentration of the outlet steam, the condensate flow rate, and the ethanol concentration of the condensate of the vacuum condensation fractionator were compared in this invention.

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

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

[0235] The present invention uses the mean absolute percentage error (MAPE) calculation formula to comprehensively evaluate 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 ethanol concentration in the outlet steam is 4.80%. It can be seen from this 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 conducts design calculations on the vacuum condensation fractionator in the fermentation - membrane separation coupling pilot plant with an annual output of 300 tons of ethanol, which is a cooperation between this research group and Guangxi COFCO Biomass Energy Co., Ltd. The circulating cooling water in the shell side of the vacuum condensation fractionator uses the on - site utilities of the company, 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 designed is 5.79 m 2 , compared with the heat transfer area of 0.01256 m in the laboratory 2 , the scale is enlarged by about 460 times.

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

[0239]

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

[0241] When this model is applied to the pilot plant, it can still accurately predict the ethanol concentration in the outlet steam and the mass flow rates of the condensate and outlet steam. Generally speaking, the model established in the present invention can be used to predict the condensation behavior of vacuum condensation fractionators of different scales, and the applicable range is relatively large.

[0242] Based on the heat and mass transfer characteristics of steam condensation in the vacuum condensation fractionator, the present invention establishes a heat and mass co - transfer model that correlates the equipment structure parameters and operation parameters 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 end of the vacuum condensation fractionator, and the mass flow rates of the condensate and steam. The MAPE between the prediction results and the experimental values is less than 5%; the analysis results show that in terms of structural parameters, as the overall heat transfer coefficient and heat transfer area increase, the ethanol concentrations in the condensate and steam at the outlet of the vacuum condensation fractionator both show an upward trend, while the secondary steam recovery rate and secondary ethanol recovery rate both show a downward trend, and the reduction amplitude of the secondary steam recovery rate is greater; in terms of operation parameters, as the circulating cooling water temperature and inlet steam flow rate increase, the ethanol concentrations in the condensate and steam both show a downward trend, and the secondary steam recovery rate and secondary ethanol recovery rate both show an upward trend. As the inlet steam ethanol concentration increases, the ethanol concentrations in the condensate and steam at the outlet both show an upward trend, while the secondary steam recovery rate and secondary ethanol recovery rate basically remain unchanged. When this model is applied to the pilot plant, it can still accurately predict the ethanol concentration in the outlet steam, and the mass flow rates of the condensate and outlet steam, and the MAPE is less than 6%.

[0243] 5. Model calculation example

[0244] Taking the experimental data of the first day in a week of the pilot plant as an example, the outlet parameters of the vacuum condensation fractionator are predicted through the heat and mass co - transfer model established in 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 : 3500 kg / h; cooling water inlet temperature t0: 18 °C; actual heat transfer area A: 5.79 m 2; Heat transfer coefficient K: 8.55 W / (m 2 ·°C); 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) Calculate step by step

[0251] Divide the entire vacuum condensation fractionator into N segments according to the concentration gradient (N = 5 in this calculation process), and calculate the outlet steam ethanol concentration y of each segment j :

[0252] y j = y0 + jΔy (0 ≤ j ≤ 5) (S12);

[0253] where

[0254]

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

[0256] Table 2. Outlet steam ethanol concentration and corresponding temperature in each heat transfer segment

[0257]

[0258] Calculate the outlet condensate flow rate M c,j 、outlet steam flow rate M j 、condensate ethanol concentration x c,j and heat transfer quantity Q j , taking the first heat transfer segment as an example, the calculation process is as follows:

[0259] Outlet steam flow rate 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 = 0.952 kg / h (S15);

[0263] The ethanol concentration x of the condensate c,1 :

[0264]

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

[0266]

[0267] Calculate the outlet temperature t of the cooling water in each heat transfer section in turn through equations (7)-(10) N-(j-1) , the logarithmic mean temperature difference Δt in each heat transfer section m,j and the total heat transfer quantity Q t , the total 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 outlet temperature t of the cooling water in each heat transfer section N-(j-1) should be calculated starting from the last section. The specific process is as follows:

[0268]

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

[0270] Table 4. Inlet and outlet temperatures of the shell-side cooling water and the heat transfer temperature difference in each heat transfer interval

[0271]

[0272] The total heat transfer quantity Q t :

[0273]

[0274] The total logarithmic mean temperature difference Δt m,t :

[0275]

[0276] 4) Heat transfer area calculation

[0277] Calculate the required heat transfer area A' through equation (11):

[0278]

[0279] Compare the 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 = 65.36 wt%, repeat steps S22 to S26, continue the loop calculation until the condition that the difference is less than δ is satisfied, and then output y at this time N .

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

[0283] It should be noted that in the present invention, the term "comprising", "including", or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article, or system including a series of elements not only includes those elements but also includes other elements not explicitly listed, or further includes elements inherent to such a process, method, article, or system. Without further limitation, an element defined by the statement "including one..." does not exclude the existence of another identical element 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 the preferred embodiments of the present invention, and do not limit the patent scope of the present invention accordingly. Any equivalent structure or equivalent process transformation made by using the content of the specification and drawings of the present invention, or directly or indirectly applied in other related technical fields, shall be equally included in the patent protection scope of the present invention.

Claims

1. A method for predicting the heat and mass transfer behavior during the condensation process of a vacuum condensation fractionator, characterized in that The method includes the following steps: According to the material balance and heat balance relationships of the ethanol vapor condensation heat and mass transfer process under vacuum conditions, establish a heat and mass transfer model in the vacuum condensation fractionator, including: establish a heat transfer formula according to the rule that the heat transfer loads of the vacuum condensation fractionator, the hot fluid, and the cold fluid are equal; divide the heat transfer process into heat transfer sections according to the ethanol concentration gradient in the ethanol vapor in the vacuum condensation fractionator to obtain multiple heat transfer sections for calculating the heat transfer amount and logarithmic mean temperature difference of each heat transfer section, and then calculate the total heat transfer amount and total logarithmic mean temperature difference of the vacuum condensation fractionator; divide the mass transfer section for each heat transfer section to obtain multiple mass transfer units; establish a permeate steam flow relationship and an ethanol concentration relationship through the mass conservation equation, where the permeate steam flow relationship is the relationship between the inlet steam flow, condensate flow, and outlet steam flow in each heat transfer section, and the ethanol concentration relationship is the relationship between the inlet steam ethanol concentration, condensate ethanol concentration, and outlet steam ethanol concentration in each heat transfer section; determine the ethanol mass conservation equation of each mass transfer unit in each heat transfer section and determine the calculation formula for the outlet steam flow in each heat transfer section. Under the given operating conditions and structural parameters, predict the condensate flow rate, condensate ethanol concentration, outlet steam flow rate, and outlet steam ethanol concentration of the vacuum condensation fractionator through the heat and mass transfer model to calculate the total heat transfer amount of the entire condensation process, thereby calculating the calculated structural parameters of the vacuum condensation fractionator.

2. The method for predicting the heat and mass transfer behavior during the condensation process of the vacuum condensation fractionator according to claim 1, wherein The step of establishing a heat transfer formula according to the rule that the heat transfer loads of the vacuum condensation fractionator, the hot fluid, and the cold fluid are equal includes: Establish the calculation formula for the heat transfer load in the vacuum condensation fractionator ; Establish the calculation formula for the heat transfer load of the hot fluid in the vacuum condensation fractionator ; Establish the calculation formula for the heat transfer load of the cold fluid in the vacuum condensation fractionator ; Establish the calculation formula of the logarithmic mean temperature difference in the condensation heat transfer process ; (1)； (2)； (3)； (4)； Among them, K is the overall heat transfer coefficient, with the unit of W / (m 2 ·℃); A is the actual heat transfer area of the vacuum condensation fractionator, with the unit of m 2 ; Δ t m,t is the logarithmic mean temperature difference in the condensation heat transfer process, with the unit of ℃; M c is the condensate flow rate at the lower outlet of the vacuum condensation fractionator, with the unit of kg / h; x c is the ethanol concentration of the condensate at the lower outlet of the vacuum condensation fractionator, with the unit of wt%; q m is the flow rate of the circulating cooling water, with the unit of kg / h; r e is the latent heat of vaporization of ethanol, r w is the latent heat of vaporization of water, and the units are both kJ / kg; c p is the specific heat capacity of the circulating cooling water, with the unit of kJ / (kg·℃); T 0 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, t 0 is the inlet temperature of the circulating cooling water, t N is the outlet temperature of the circulating cooling water. Among them, the fluids in the tube side and the shell side are in complete countercurrent flow.

3. The method for predicting the heat and mass transfer behavior during the condensation process of the vacuum condensation fractionator according to claim 2, wherein The step of dividing the heat transfer process into heat transfer sections according to the ethanol concentration gradient in the ethanol vapor in the vacuum condensation fractionator to obtain multiple heat transfer sections for calculating the heat transfer amount and logarithmic mean temperature difference of each heat transfer section, and then calculating the total heat transfer amount and total logarithmic mean temperature difference of the vacuum condensation fractionator includes: During the step-by-step calculation process, the heat transfer process is evenly divided into N sections according to the concentration difference between the inlet and outlet of the vacuum condensation fractionator. The temperature of the vapor phase is determined by the dew point temperature curve. The specific calculation process of the heat transfer temperature difference for each heat transfer section is as follows: Calculate the outlet steam ethanol concentration of each heat transfer section: (5)； Among them, y 0 is the inlet steam ethanol concentration of the vacuum condensation fractionator, y j represents the outlet steam ethanol concentration of the j th heat transfer section, and Δ y is the ethanol concentration difference of the inlet and outlet steam of each heat transfer section, , and Δ y The calculation formula is as follows: (6)； According to y j and the dew point temperature curve, the outlet steam temperature of the j th heat transfer section is obtained. T j The outlet steam flow rate of each heat transfer section, M j the ethanol concentration in the condensate, x c, j the condensate flow rate, M c, j and the heat transfer amount Q j follow the mass conservation equation. Finally, the temperature differences between the inlet and outlet of the cooling water and the heat transfer temperature differences of each heat transfer section are obtained as follows: The temperature differences between the inlet and outlet of the cooling water in each heat transfer section: (7)； Among them, t N-j is the inlet temperature of the cooling water for the j th heat transfer section, t N-(j-1) is the outlet temperature of the cooling water for the j th heat transfer section, and the units are both °C; The logarithmic mean temperature difference of each heat transfer section: (8)； Total heat transfer of the vacuum condensation fractionator is as follows: (9)； The total logarithmic mean temperature difference Δ of the vacuum condensation fractionator t m, t : (10)； The overall heat transfer coefficient is K When calculating the heat transfer area of the vacuum condensation fractionator A´ is as follows: (11)； where, Δ t m, j is the logarithmic mean temperature difference of the j th heat transfer section, T j is the outlet steam temperature of the j th heat transfer section, T j-1 is the inlet steam temperature of the j th heat transfer section, and the units are all °C.

4. The method for predicting the heat and mass transfer behavior during the condensation process of the vacuum condensation fractionator according to claim 3, wherein The step of establishing a permeate steam flow relationship and an ethanol concentration relationship through the mass conservation equation includes: The j mass conservation equation for the permeate steam flow rate in the first heat transfer section is as follows: (12)； The j mass conservation equation for the ethanol flow rate in the first heat transfer section is as follows: (13)； Among them, M j-1 and M j,0 are both the inlet steam flow rates of the j th heat transfer section, M j,n and also the outlet steam flow rates of the j th heat transfer section; y j-1 and y j,0 are both the inlet steam ethanol concentrations of the j th heat transfer section, y j,n and also the outlet steam ethanol concentrations of the j th heat transfer section.

5. The method for predicting the heat and mass transfer behavior during the condensation process of the vacuum condensation fractionator according to claim 4, 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 calculation formula for the outlet steam flow in each heat transfer section includes: The j ethanol mass conservation equation for the i th mass transfer unit in the th heat transfer section is as follows: (14)； Among them, M j,i-1 is the inlet steam flow rate of the j th mass transfer unit in the i th heat transfer section, y j,i-1 is the ethanol concentration of the inlet steam of the j th mass transfer unit in the i th heat transfer section; Δ M j,i is the condensate mass of the j th mass transfer unit in the i th heat transfer section, x j,i is the ethanol concentration of the condensate of the j th mass transfer unit in the i th heat transfer section; M j,i is the outlet steam flow rate of the j th mass transfer unit in the i th heat transfer section, y j,i is the ethanol concentration of the outlet steam of the j th mass transfer unit in the i th heat transfer section, , n is the number of mass transfer units in each heat transfer section; In Equation (14), let: (15)； (16)； Substitute Equations (15) and (16) into Equation (14) and ignore the higher-order infinitesimals to obtain: (17)； Among them, M is the mass flow rate on the mass transfer unit, and M includes the condensate flow rate and the outlet steam flow rate on the mass transfer unit; y is the ethanol concentration in the steam on the mass transfer unit, x is the ethanol concentration in the condensate on the mass transfer unit; The boundary conditions of Equation (17) are: (18)； Therefore, after integrating Equation (17), we get: (19)； Solve the integral in Equation (19), and the result is shown in Equation (20): (20)； Where: (21)； (22)； h is the step size; I is the function f ( y ) the integral result on the interval ( y j-1 , y j ); y j,2i is the outlet steam ethanol concentration corresponding to the mass transfer unit of the even terms in the j th heat transfer section, y j,2i-1 is the outlet steam ethanol concentration corresponding to the mass transfer unit of the odd terms in the j th heat transfer section; the inlet steam ethanol concentration through the vacuum condensation fractionator y j-1 , and the outlet steam ethanol concentration y j , calculate the integral of the function f ( y ) on the interval ( y j-1 , y j ), substitute the integral result into Equation (19), and then calculate the steam flow rate at the outlet of the vacuum condensation fractionator under the given conditions.

6. The method for predicting the heat and mass transfer behavior of the condensation process of the vacuum condensation fractionator according to claim 5, characterized in that: The condensate flow rate of the j th heat transfer section is calculated according to the mass conservation equation (12) of the pervaporation flux as follows: (23)； The ethanol concentration of the condensate in the j th heat transfer section is calculated according to the mass conservation equation (13) of the ethanol flow rate x c, j : (24)； The heat transfer amount of the j th heat transfer section calculated according to the heat transfer load calculation formula (2) of the hot fluid is as follows: (25)。 7. The method for predicting the heat and mass transfer behavior in the condensation process of the vacuum condensation fractionator according to claim 6, characterized in that, Under the given operating conditions and structural parameters, through the heat and mass transfer model, predicting the condensate flow rate, condensate ethanol concentration, outlet steam flow rate, and outlet steam ethanol concentration of the vacuum condensation fractionator to calculate the total heat transfer amount of the entire condensation process, and thus calculating the calculated structural parameters of the vacuum condensation fractionator, including: Obtaining the known structural parameters and operating parameters of the vacuum condensation fractionator; Setting the initial outlet steam ethanol concentration of the vacuum condensation fractionator as the average value of the initial inlet steam ethanol concentration and the maximum outlet steam ethanol concentration; Dividing the vacuum condensation fractionator into multiple heat transfer sections with equal concentration gradients according to the set number of heat transfer sections, and using the inlet steam ethanol concentration and the outlet steam ethanol concentration to calculate the ethanol concentration difference in each heat transfer section of the vacuum condensation fractionator, the outlet steam ethanol concentration in each heat transfer section, and the outlet steam temperature in each heat transfer section; Calculating the outlet steam flow rate and condensate flow rate in each heat transfer section according to the outlet steam flow rate calculation formula and the permeate steam flow rate relationship in each heat transfer section; calculating the condensate ethanol concentration in each heat transfer section according to the ethanol concentration relationship; calculating the heat transfer amount in each heat transfer section according to the calculation formula of the heat transfer load of the hot fluid; Calculating the cooling water outlet temperature of each heat transfer section; Calculating the logarithmic mean temperature difference of each heat transfer section, the total heat transfer amount of the vacuum condensation fractionator, and the total logarithmic mean temperature difference in the entire condensation heat transfer process of the vacuum condensation fractionator; Calculating the calculated heat transfer area of the vacuum condensation fractionator under the set total heat transfer coefficient; Judging whether the calculated heat transfer area of the vacuum condensation fractionator is less than the set error value compared with the actual heat transfer area; If so, outputting the prediction results, where the prediction results include: the outlet steam ethanol concentration, the outlet steam flow rate in each heat transfer section, the condensate flow rate, and the condensate ethanol concentration.

8. The method for predicting the heat and mass transfer behavior during the condensation process of the vacuum condensation fractionator according to claim 7, characterized in that After the step of judging whether the calculated heat transfer area of the vacuum condensation fractionator is less than the set error value compared with the actual heat transfer area, it further includes: If not, comparing the size of the calculated heat transfer area of the vacuum condensation fractionator with the actual heat transfer area; When the calculated heat transfer area is greater than the actual heat transfer area, making the maximum outlet steam ethanol concentration equal to the outlet steam ethanol concentration; When the calculated heat transfer area is less than the actual heat transfer area, making the inlet steam ethanol concentration equal to the outlet steam ethanol concentration, and repeating the step of setting the initial outlet steam ethanol concentration of the vacuum condensation fractionator as the average value of the initial inlet steam ethanol concentration and the maximum outlet steam ethanol concentration to the step of calculating the logarithmic mean temperature difference of each heat transfer section, the total heat transfer amount of the vacuum condensation fractionator, and the total logarithmic mean temperature difference in the entire condensation heat transfer process of the vacuum condensation fractionator until the calculated heat transfer area of the vacuum condensation fractionator is less than the set error value compared with the actual heat transfer area, and then outputting the prediction results.

9. A pervaporation membrane separation system, characterized in that, It includes a fermentation tank, a circulation pump, a membrane separator, a vacuum condensation fractionator, a vacuum compression pump and an atmospheric condenser connected in sequence; by applying the prediction method described in any one of claims 1 to 8, the condensate flow rate, condensate ethanol concentration, outlet steam flow rate and outlet steam ethanol concentration of the vacuum condensation fractionator are predicted to calculate the total heat transfer amount of the entire condensation process, so as to calculate the calculated structural parameters of the vacuum condensation fractionator.

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