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Improved Solution Thermodynamics Modeling in Pervaporation
The separation performance of commercial crosslinked poly (vinyl alcohol) (PVA) membranes (i.e., the new commercial membrane PERVAP™ 4100 H F and standard membrane PERVAP™ 4100) used for the dehydration of two alcohol-water systems (i.e., ethanol-water and isopropanol-water mixtures, with an azeotropic point) were studied based on pervaporation process (PV) experimental data and mathematical modeling. Pervaporation process experiments proved that these two membranes have excellent applicability for the dehydration of alcohol. A semi-empirical solution-diffusion transport model was developed to describe the mass transport in the PVA membranes, which showed a good agreement with the experimental values. The universal quasi-chemical (UNIQUAC) model was utilized to predict the activity coefficient of nonideal alcohol-water systems in PVA membranes. In addition to the UNIQUAC model, the transport of alcohol-water across the commercial polymeric membrane was modeled using the conventional driving force model. The PV process experimental data with the mathematical model were used to develop the diffusivity correlations for water and alcohol (i.e., ethanol and isopropanol) through the PVA membranes. It was found that for swollen membranes (PVA), the developed correlations of water and alcohol diffusivity were strongly influenced by the feed water activity and feed temperature. Based on the mass transport model and developed diffusivity correlations, the permeation flux of water and alcohol through the PVA membranes was predicted, and the results showed a good agreement between the experimental data and the predictive model. The mean relative errors estimated for the permeate mass fluxes of water were 8.4%, and 3.8%, and for the permeate mass fluxes of ethanol were 18%, and 13.6% for the PERVAP™ 4100 and 4100 H F, respectively, as well as for the IPA-water-PVA system are as follows: 5% and 2.8% for the permeate mass fluxes of water and 14.4%, and 12.6% for the permeate mass fluxes of IPA for the PERVAP™ 4100 and 4100 H F, respectively.
Advanced processes, which are alternatives to ordinary distillation, are essential to dehydrate azeotropic alcoholic mixtures for biofuel production. In that regard, this work focuses on the analysis of heterogeneous azeotropic distillation for the separation of a 2-propanol + water mixture in order to recover the alcohol with a sufficiently low water content. By comparing the performances of various entrainers on the basis of ternary maps, isooctane was selected for further process analysis. An advantage it poses is related to the fact that traces of it within the recovered dehydrated alcohol are highly welcome with a view to its subsequent use as a fuel. Aspen Plus® V11 software was employed for the simulation of the process, thus filling the gap existing in the literature due to the lack of studies on the process analysis of the heterogeneous azeotropic distillation of the 2-propanol + water system using isooctane as an entrainer.
By using binary ethanol/water mixtures, the separation performance of two improved commercial pervaporation membranes (PERVAPTM) are presented. Separation performance data are analyzed and compared as a function of operating conditions. The effect of initial feed concentration on the separation performance and the effect of feed concentration on apparent activation energy are presented. For membranes that swell, we show that permeance values and selectivity are also dependent on operating conditions. In addition, the results show the importance of initial feed concentration in pervaporation tests. Arbitrary initial feed concentrations lead to different separation performance. Nevertheless, this membrane feature can be used for tuning the final separation performance.
Accurate and reliable mathematical models play a key role in membrane system design. Process simulators have been proven to be successful in modeling, simulate, and optimize various industrial processes. Most of the commercial simulators can be used to solve mathematical models based on membrane applications. The present review is discussing the various process simulators such as Aspen Plus®, CHEMCAD®, gPROMS®, Hysys®, PRO/II®, ROSA®, SuperPro Designer® with relevance to the membrane-based processes and system design. The benefits of these tools, customization using ACM®, MATLAB® for rigorous modeling, simulation, and optimization, were discussed. Interfacing among simulators using CAPE-OPEN was briefed. State-of-the-art simulation for pervaporation, membrane distillation, membrane filtration, membrane reactors, and membrane-based gas separations with a special focus on CO2 capture was reviewed. The review is also discussed open-source and commercially developed applications, along with challenges and prospects. Finally, the review covered various mathematical tools usage in membrane system design.
Developing a reliable and adequate model for designing and simulation of pervaporation processes can be very useful for industrial purposes. This work explains particular design criteria on which a pervaporation system may be based. Initially, a semi-empirical trans-membrane flux model was developed considering the recognized characteristics of multicomponent permeation, flux coupling, and thermodynamic interaction. Later, the developed model was implemented to simulate a single unit of pervaporation as well as multi-stage arrangements, integrated with external interstage heaters, for dehydration of isopropanol under adiabatic and ideal isothermal conditions. Two multi-stage arrangements, including modules with either equal temperature drop or surface area per each stage, were purposed for this aim. Regarding a specific separation case study, the ideal isothermal condition underestimated the membrane area required even more than 47 times. While the implementation of multi-stage arrangements resulted in more than 97% reduction of the required membrane surface area. The results revealed that the multi-stage arrangement with equal temperature drop per stage was more efficient due to the utilization of not only less membrane area but also less number of sequential stages. To draw a technical comparison, the profiles of critical parameters were depicted along with the modules. The results obtained in this work proved that the validated flux model along with the design procedure can be considered as a reasonable tools to be used in the process simulation software packages.
The application of pervaporation for the separation of multicomponent mixtures may involve coupling effects among components and the membrane, which could increase or decrease the permeance of the target compound. In order to study and describe this phenomenon, mixtures of components present in two model reactions have been considered: the transesterification reaction between methyl acetate and butanol to produce methanol and butyl acetate, and the transesterification reaction between methanol and ethyl acetate to produce ethanol and methyl acetate. Both reactions are of utmost interest in the chemical industry and present high cost of separation due to the presence of azeotropic mixtures (i.e., methanol/methyl acetate; butanol/butyl acetate; ethanol/ethyl acetate).
The separation performance of four commercial membranes (i.e., PERVAP 1255-30, PERVAP 4155-40, PERVAP 1255-50, PERVAP 4155-80) from Sulzer Chemtech, Switzerland, is evaluated. The effect of the feed concentration and the temperature on the separation performance was studied in terms of permeance and selectivity. Coupling effects were observed when the permeance of pure solvents was compared with that of the components in the mixture. The coupling effects were analyzed by the Hansen solubility approach and by the variation of activities within the membrane.
In the past decades, it has been proven that pervaporation is an effective and energy-efficient membrane process for the separation of liquids that are difficult to separate in classical processes. The demand for new process applications has increased the need for mass transfer simulation methods, considering the interactions between the system components and the influence of process parameters on the permeation fluxes and at the same time requiring as few experiments as possible.
The aim of the study was to find out whether the calculation of mass fluxes of multicomponent fluids based on the system of generalized Maxwell-Stefan equations (GMSE), using Maxwell-Stefan (M-S) diffusion coefficients, performed according to the method recently proposed by Kubaczka [J. Membr. Sci. 470, 2014, 389–398] could meet these expectations. For this purpose, computational experiments were conducted where the predicted molar fluxes, concerning pervaporative dehydration of ethanol and isopropanol with a poly(vinyl alcohol) (PVA) membrane, were compared with the experimental results. Free volume parameters, necessary for determining the self-diffusion coefficients, were estimated according to Vrentas and Vrentas’ method [Eur. Polym. J. 34, 1998, 797–803] using the PRODE database correlations for viscosity and density of the components of the analyzed systems.
Moreover, the study examined the importance and influence of the controversial parameter ξip, being the ratio of the molar volume of the solvent jumping unit to the molar volume of the polymer jumping unit, on the achieved permeation fluxes. The correlations derived from the experimental data necessary for their correlation are also given.
The equilibrium concentrations in PVA were calculated by using the modified UNIFAC-FV method.
The results obtained from the computational experiments have confirmed that the proposed method provides accurate predictions of mass fluxes in pervaporation systems.
Sizing of large scale pervaporation units from experimental data requires mathematical models which can predict the pervaporation process performance adequately. In this work, a modeling approach is reported with a semi-empirical flux model regressed over experimental flux data measured at lab scale. The regression approach generates a temperature and concentration dependent flux function that can be solved simultaneously with material and energy balance over the length of a pervaporation module. For a binary mixture only four parameters are required for model fitting which can be obtained using lab-scale data: temperature and concentration of one component on feed side, permeate side pressure, and component flux. The semi-empirical flux model was verified using two separate experimental dataset for 1-butanol/water dehydration system using PERVAP 2510 membrane. The flux model was then used to solve a one-dimensional (1-D) pervaporation model to generate flux, concentration, temperature and PSI profiles over the length of the membrane modules. The 1-D model also calculates the size and arrangement of the pervaporation modules. Using the experimental data reported in literature, the proposed approach was used to size pervaporation modules for three dehydration systems: 1-butanol over PERVAP 2510, isobutanol over PERVAP 2510, and isobutanol over PERVAP 1510. Sensitivity analysis was also conducted.
Pumps are devices for supplying energy or head to a flowing liquid in order to overcome head losses due to friction and also, if necessary, to raise the liquid to a higher level. The important heads to consider in a pumping system are the suction, discharge, total and available net positive suction heads. In centrifugal pumps, energy or head is imparted to a flowing liquid by centrifugal action. The most common type of centrifugal pump is the volute pump. In volute pumps, liquid enters near the axis of a high speed impeller and is thrown radially outward into a progressively widening spiral casing. The performance of a centrifugal pump for a particular rotational speed of the impeller and liquid viscosity is represented by plots of total head against capacity, power against capacity, and required Net Positive Suction Head (NPSH) against capacity. These are known as characteristic curves of the pump. Characteristic curves have a variety of shapes depending on the geometry of the impeller and pump casing. Pump manufacturers normally supply these curves only for operation with water. However, methods are available for plotting curves for other viscosities from the water curves.