Modelling of multicomponent gas separation with asymmetric hollow fibre membranes—methane enrichment from biogas

A mathematical model for the dynamic performance of gas separation with high flux, asymmetric hollow fibre membranes was developed considering the permeate pressure build‐up inside the fibre bore and cross flow pattern with respect to the membrane skin. The solution technique provides reliable examination of pressure and concentration profiles along the permeator length (both residue/permeate streams) with minimal effort. The proposed simulation model and scheme were validated with experimental data of gas separation from literature. The model and solution technique were applied to investigate dynamic performance of several membrane module configurations for methane recovery from biogas (landfill gas or digester gas), considering biogas as a mixture of CO₂, N₂ and CH₄. Recycle ratio plays a crucial role, and optimum recycle ratio vital for the retentate recycle to permeate and permeate recycle to feed operation was found. From the concept of two recycle operations, complexities involved in the design and operation of continuous membrane column were simplified. Membrane permselectivity required for a targeted separation to produce pipeline quality natural gas by methane‐selective or nitrogen‐selective membranes was calculated. © 2012 Canadian Society for Chemical Engineering     

A generalized methodology for optimal configurations of hybrid distillation-pervaporation processes

The aim of this work is to illustrate the structural and parametric optimization of continuous hybrid distillation-pervaporation process with different configurations such as series, parallel, and series-parallel arrangement of pervaporation modules in the pervaporation network, and to propose a generalized methodology for difficult separations. A superstructure representation of hybrid process is considered and the process is modeled and optimized using an MINLP approach. The optimization strategy is to obtain the desired degree of either the retentate or the distillate purity without violating the composition constraints of products and heat exchange policy which minimizes the required membrane area by increasing the flux through the membrane. The structural and operating parameters such as number of trays required, feed tray location, reflux ratio, retentate recycle location, permeate recycle location, membrane feed location, number of pervaporation modules required, target composition (which is directly related to membrane area), and membrane selectivity are optimized for each configuration by minimizing the total annual cost (TAC) for the separation system. The optimization studies have been carried out with General Algebraic Modeling System software (GAMS/SBB/CONOPT) and the results of different configurations have been compared on the basis of TAC required for the separation. A total of three industrial case studies have been dealt with. The separation of isopropanol-water as an azeotropic mixture, propylene-propane as a close boiling mixture (system with a low relative volatility) and acetone-water as a tangent pinch mixture have been studied as representative examples.     

Optimization of membrane gas separation systems using genetic algorithm

Genetic algorithm is applied for the optimization of the membrane gas separation systems. Air separation for enriched oxygen production is the selected system for investigation. Optimizations for single and triple objective functions are studied. The optimization problem involves the selection of the optimal system configurations from three alternatives, including continuous membrane column (CMC), single stripper permeator (SSP), and two stripper in series permeator (TSSP), as well as the optimal operating conditions. Models of the three configurations and the genetic algorithm procedure are computerized. The objective functions discussed are the Rony separation index, power consumption per unit equivalent pure oxygen, and the membrane area. Both high-pressure and low-pressure (vacuum) operation modes are optimized and the effects of different oxygen product purity and feed rate are analyzed. For single objective function optimization, the solutions obtained using genetic algorithm are slightly inferior in one case but superior in other cases compared to those by pure mathematical optimization methods. For triple objective function optimization, the Pareto plots presenting multiple trade-off solutions are generated. In general, compared to high-pressure operation mode, the product recovery and power consumption for low-pressure operation mode are lower. For almost all the cases studied, CMC configuration with its high flexibility appears in the optimal solutions.     

Economic comparison of several membrane configurations for H2/N2 separation

Several one- and two-stage membrane systems were compared for use in separating H₂/N₂ mixtures from coal gasification processes. Computer models of cross flow membrane modules were used in the evaluations. The processing cost, determined by a discounted cash flow analysis of capital and operating expenses, was used as the basis for the comparison. Membrane properties were those of a poly[etherimide] composite membrane. Hydrogen mole fractions from 34% to 97% in the feed and from 80% to 99.9% in the product (permeate) were examined; in all cases it was required that 95% of the feed H₂ be recovered. Four configurations were evaluated: single module (SM), single module with recycle (SMR), two-module series (SER), and two-stage cascade (CAS). The results showed that for conditions where SM was capable of performing the separation the optimum recycle rate in SMR was zero, and thus SMR and SM were identical. In some conditions, SER also reduced to SM. In general, SM is best for easy separations (where the feed and product compositions are similar), CAS is best for difficult separations (where feed and product compositions are very different), and SER is best for separations of moderate difficulty; in this example, where the H₂ recovery is fixed, SMR is never the best configuration. In some circumstances, it is economically better to treat only a portion of the feed in the membrane system, but to a higher purity than is required, and then to mix the overconcentrated stream with the untreated feed to make a product stream of the desired purity.     

Membrane system design for multicomponent gas mixtures via mixed-integer nonlinear programming

An optimal design strategy for membrane networks separating multicomponent gas mixtures based on an approximate permeator model and mixed-integer nonlinear programming (MINLP) is proposed. A permeator system superstructure is used to embed a very large number of possible network configurations and allows the permeator feed-side pressure to be fixed or a design variable. A MINLP design model is developed to minimize the total annual process cost by simultaneous optimization of the permeator configuration and operating conditions. Case studies for the separation of acid gases (CO₂ and H₂S) from crude natural gas mixtures with spiral-wound permeators are presented. Permeator configurations are derived for different number of separation stages for both continuous and discrete membrane areas. The method is sufficiently robust to handle product compositions that vary five orders of magnitude. The proposed approach provides an efficient methodology for preliminary screening of multi-stage membrane separation systems for multicomponent gas mixtures.     

Nonsymmetrical Separations in Thermal Diffusion Columns

Abstract

The influence of the feed composition upon the actual degrees of separation attained at the top and bottom sections of a thermogravitational column is discussed using the classical phenomenological theory of Furry, Jones, and Onsager. It is shown that, except for a feed composition of C ₀ = 0.5 (mass fraction), the separation profile is nonsymmetric, i.e., the separations in the top and bottom sections of the column are nonsymmetric with respect to the feed composition, the asymmetry increasing as the feed composition moves away from C ₀ = 0.5. An equation for the determination of the optimum feed location as a function of the feed composition is derived.     

Techno-economic evaluation of top,middle, and bottom divided wall column configurations for benzene,toluene, and xylene processing in aromatic production plant

Divided wall column (DWC) configurations were simulated and optimized for five different feed’s compositions to demonstrate the effect of feed composition and location of dividing wall in DWC in optimum operating conditions, energy requirement, operating, capital and total annualized cost (TAC) to produce market-grade benzene, toluene, and xylene. Operating conditions and cost for each DWC strongly depend on feed composition. Middle DWC (M-DWC) requires significantly lower energy compared with top and bottom DWCs except for feed dominated with xylene where energy requirement is comparable in studied configurations and M-DWC seems the best for ternary mixture separation.     

Asymmetric Permeators—A Conceptual Study

Abstract

The asymmetric permeator concept of Ohno et. al. utilizing two different membranes for rare gas separation has been explored in general. Various geometrical arrangements and possible applications to gas separations other than rare gas-nitrogen mixtures have been discussed. The utility of an asymmetric permeator for multicomponent gas separations has been investigated. The separation factor of a ternary system in a perfectly mixed asymmetric permeator has been obtained. The amount of separation obtained with a ternary feed in a perfect crossflow stage having no axial mixing has been analytically determined for some limiting cases with an asymmetric permeator. The asymmetric permeator concept has been extended also to a high separation factor liquid solution separation process like reverse osmosis desalination. Preliminary calculations have been carried out to show that an asymmetric desalinator with reverse osmosis (RO) and piezodialysis (PD) membranes has a lower increase in brine concentration along the module length for a given water recovery resulting in a lower operating pressure. With hollow fiber asymmetric desalinators having RO and PD membranes, the concentration polarization, if any, may be significantly reduced. Practical applications of asymmetric permeator's for phenol-water separation etc. have been discussed.     

Hybrid Membrane-Cryogenic Distillation Air Separation Process for Oxygen Production

A hybrid air separation process for oxygen production uses a membrane gas permeator with bypass to increase the oxygen concentration of the feed to 23.5% before the cryogenic distillation plant. A 23.5% limit on oxygen is required to avoid use of more expensive construction materials. By pre-concentrating the oxygen, the feed flowrate to the compressor, heat exchangers and the distillation column can be reduced by 11%, which reduces power requirements and sizes of the downstream equipment. Achieving this concentration with very low energy requirements is surprisingly difficult with commercially available membranes. New membrane materials with higher fluxes and selectivities could make the process more economical.     

Cycling Zone Adsorption: Variable-Feed Mode of Operation

Abstract

Relatively large separations are possible using a single cycling zone adsorption column when operated with product recycle. Such a recycle may cause a large cyclic step-change in feed concentration, resulting in significant differences in alternating product stream concentrations. It is concluded that such a “variable-feed mode of operation” may allow the separation of compounds whose solid—fluid equilibrium distribution coefficients are only slightly affected by changes in a thermodynamic potential, or compounds which are difficult to separate by classical techniques such as distillation or extraction. It will also be apparent that the “variable-feed mode of operation” of a single zone (column) produces separations that could only be previously developed by a number of constant feed concentration zones operating in a series. Theoretical and experimental studies show that longitudinal diffusion is the primary cause of effluent concentration profile distortion, limiting the duration of peak product concentration flow.     

Some Liquid Thermal Diffusion Separations in a Continuous Thermogravitational Column

Abstract

The separation of three binary mixtures on n-heptane-1,1,2,2-tetrachloroethylene, n-propanol-1,1,2,2-tetrachloroethylene, and n-heptane-methylcyclohexane was studied in a continuously operated thermogravitational column. The effects of feed rate, the ratio of top and bottom product rates, and temperature difference on separation were determined. The separations were found to be dependent on the feed rate with the maximum separation occurring at flow rates approaching zero. Over the range studied, the effect of top to bottom product flow rate ratio was found to be negligible. Temperature difference appeared to influence separation when finite flow rates were used.     

Stepwise periodic distillation—II: Separation of a binary mixture

Stepwise periodic operation of a distillation column in which a binary feed is separated in top and bottoms product is described. A simple but realistic model, both for stepwise periodic and controlled cycling operation and a simulation algorithm are proposed. The results of an extensive parametric study of stepwise periodic operation are presented along those for ideal controlled cycling (no axial mixing).Although in some cases inferior to controlled cycling, it is shown that, for reasonable tray efficiencies and difficult separations, stepwise periodic operation can compare favorably or be superior to ideal controlled cycling.     

Gas Separation by a Continuous Membrane Column

Abstract

The continuous membrane column provides a revolutionary new separation technique. In gaseous diffusion the continuous membrane column is used to separate as highly concentrated products both the most permeable and least permeable gases from a feed mixture of any composition. The main features of the column are countercurrent enrichment, high reflux and minimal backmixing. The new method eliminates the need for numerous interstage compressors and extensive product stream recycling found in conventional gaseous diffusion cascades.

Experiments are carried out for systems of C0₂-O₂, O₂2-N₂ (air), and CO₂-N₂ mixtures using continuous membrane columns made out of silicone rubber membrane. Also, a theoretical model is developed to interpret the experimental data. The agreement between theory and experiment is excellent. The maximum degree of separation can be achieved at total reflux. It is experimentally verified that the maximum degree of enrichment attainable by a conventional method can easily be exceeded without limit when a continuous membrane column is employed.

Finally, a comparative study has been conducted for a conventional gas permeator and a continuous membrane column.     

The Role of Recycle in Countercurrent Recycle Distillation Cascades. I. Constant Reflux,Ideal, and Squared-Off Cascades

Abstract

In most textbooks concerned with countercurrent multistage separations, minimum reflux ratio for continuous distillation is usually defined only in terms of a graphical construction on a McCabe—Thiele diagram: it is the recycle ratio (liquid flow rate) associated with the operating line that touches the equilibrium curve at the feed point. However, it is easily shown that minimum recycle ratio depends on local α and composition, as well as product compositions, and thus, it is a stage-composition phenomenon. As a result, for a specified separation, each ideal stage in a continuous distillation cascade has a specific minimum recycle ratio associated with it. For constant α, the minimum recycle ratio increases as the stage compositions depart more from product (distillate or bottoms) compositions. As a result, the textbooks only consider the maximum minimum recycle ratio. This paper presents the results of some theoretical calculations which illustrate how minimum recycle ratio varies with stage α, stage and product compositions, and presents an example of distillation cascade behavior when minimum recycle ratio is approached at a composition other than the feed point. An example is also presented which shows how the separation is effected when the reflux ratio is reduced below the design value in a distillation column containing a fixed number of ideal stages. A brief comparison is also made between constant reflux, ideal, and squared-off cascades in terms of number of stages, total interstage flow, and relative energy requirements for the different designs to illustrate and emphasize the consequences of the stagewise behavior of minimum recycle ratio.     

Feed concentration and column temperature in production and preparative chromatography

For large scale chromatographic separations, economic operation will require high concentrations of the feed material in the mobile phase entering the column. The different types of ultimate concentration limit encountered in liquid and gas chromatography are described. When separating thermally stable materials with a gas—liquid column, an optimum column temperature exists where the throughput at the ultimate concentration limit goes through a maximum. Equations are derived for calculating the optimum temperature and throughput. For heat-sensitive materials the column should be operated at the optimum temperature or the degradation temperature, whichever is the lower. The effect of temperature on the length of column required for separation is described.The treatment is made possible by taking the width and concentration of the feed band as roughly independent parameters, a satisfactory approximation when wide bands are used.     

A synthesis method for multicomponent distillation sequences with fewer columns

An easy‐to‐use matrix‐based method for the systematic synthesis of distillation configurations using less than n‐1 columns to separate any zeotropic n‐component feed into n product streams is described. The method is easily extended to obtain additional thermally coupled configurations. The only information needed to generate the configurations is the number of components in the feed, or equivalently, the number of distinct composition final product streams. We have successfully enumerated configurations for feeds containing up to eight components. This has resulted in a large number of hitherto unknown configurations even for four‐component separations. Some of the novel configurations generated using the method have substantially lower heat duty than the previously known fewer column configurations for a four‐component feed separation. Therefore, it is essential to include these novel configurations in the search space to find the optimal distillation configuration with fewer columns for a given application. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2479–2494, 2012     

Transport and Permeation Properties of a Ternary Gas Mixture in a Medium-Size Polysulfone Hollow Fiber Permeator

Abstract

Permeation properties were analyzed for a mixture of CO₂, O₂, and N₂ in a medium-size polysulfone hollow fiber permeator with a net permeation area of 4.22 m². Measurements were conducted as a function of feed composition, reject flow rate, and feed pressure. Results included variations in species permeability, separation factor, permeate enrichment, reject depletion, and stage cut as a function of system parameters. Variations in permeation properties show strong dependence on feed composition, reject flow rate, and feed pressure. Permeability of carbon dioxide was higher at larger feed pressures and higher carbon dioxide content in the feed stream. Effect of increasing the reject flow rates on the permeability of carbon dioxide was affected by the system pressure and the carbon dioxide content in the feed stream. At low pressures, increase of the reject flow rate resulted in a decrease of carbon dioxide permeability. The opposite behavior was obtained at higher feed pressures. Increase of the reject flow rate reduced the gas residence time within the permeator. Increase of reject flow rate reduced species residence within the permeator and in turn increased resistance to species transport within the permeator. However, higher system pressures and carbon dioxide content in the feed stream resulted in larger levels of membrane plasticization, which increased the permeation rates of all species. The combined efféct of reducing the species residence time within the permeator and the level of membrane plasticization favored the permeation of carbon dioxide versus the other two species. Variations in other permeation properties, which include oxygen and nitrogen permeabilities, stage cut, permeate enrichment in carbon dioxide, and reject depletion in carbon dioxide, were also explained in terms of resistances encountered within the permeator and the membrane.     

萃取精馏分离苯乙酮与α-苯乙醇的模拟研究

苯乙酮与α-苯乙醇属近沸点物系,用普通精馏方法很难将二者进行有效地分离。在对已报道的分离方法进行比较分析的基础上,提出了采用萃取精馏方法来分离苯乙酮和α-苯乙醇。文中首先通过定性判断和基团贡献法定量地估算选择了该二元物系合适的萃取剂为丙三醇。然后采用Aspen Plus化工模拟软件中的RadFrac模块进行了萃取精馏塔的模拟,分别考察了溶剂与原料进料位置、回流比、溶剂比对分离效果的影响。结果表明:对于处理量为1 000 kg/h的待分离物系,操作压力为5 kPa,在塔板数为30的条件下萃取精馏塔在原料进料位置为第19块塔板,溶剂进料位置为第6块塔板,回流比为3∶1(质量比),溶剂流率为800 kg/h的优化条件下,可以使塔顶苯乙酮质量分数达到99.8%,且塔釜几乎不含苯乙酮。模拟结果对进一步的实验研究和工业生产具有一定的指导意义。     

连续精馏分离乙酸乙酯与丙酮体系的模拟研究

对制备超细高氯酸铵产生的废液即乙酸乙酯-丙酮混合液的分离过程进行了模拟研究.采用Aspen Plus化工模拟软件中的RadFrac模块进行连续精馏模拟,分别考察了塔板数、进料位置、回流比及塔顶出料量对分离效果的影响.结果表明:对于处理流量3 kg/h乙酸乙酯-丙酮的混合液,精馏塔在塔板数40,进料位置第16块塔板,回流...     

Experimental and theoretical studies of countercurrent solvent-gas extraction—II: The construction and operation of a laboratory differential vapour-absorption column

The construction and operation of a simple 30 mm differential vapour-absorption column are described, the column being packed with a knitted wire packing. The results of separations of binary volatile mixtures are reported and analysed by means of a multistage computation procedure. It has been confirmed that in accordance with theoretical prediction, the separation efficiency can be improved in some cases by imposing longitudinal temperature differences in the column. Computed values of HETP are reported which show a linear increase with solvent rate in the range 0·2-1·01/hr. For the same loadings, HETP values are 50–300 per cent greater than for continuous gas chromatography and are about four times greater than those measured for a normal distillation process using the same packing material.