Showing papers in "Industrial & Engineering Chemistry Research in 1998"

Effective Continuous-Time Formulation for Short-Term Scheduling. 1. Multipurpose Batch Processes

Abstract: During the last decade, the problem of production scheduling has been realized to be one of the most important problems in industrial plant operations especially when multipurpose/multiproduct batch processes are involved. This paper presents a novel mathematical formulation for the short-term scheduling of batch plants. The proposed formulation is based on a continuous time representation and results in a mixed integer linear programming (MILP) problem. The novel elements of the proposed formulation are (i) the decoupling of the task events from the unit events, (ii) the time sequencing constraints, and (iii) its linearity. In contrast to the previously presented continuous-time scheduling formulations, the proposed approach leads to smaller and simpler mathematical models which exhibit fewer binary and continuous variables, have smaller integrality gaps, require fewer constraints, need fewer linear programming relaxations, and can be solved in significantly less CPU time. Several examples are presented ...

Metal-Anion Sorption by Chitosan Beads: Equilibrium and Kinetic Studies

Abstract: Chitosan is a well-known biopolymer, whose high nitrogen content confers remarkable ability for the sorption of metal ions from dilute effluents. However, its sorption performance in both equilibrium and kinetic terms is controlled by diffusion processes. Gel bead formation allows an expansion of the polymer network, which improves access to the internal sorption sites and enhances diffusion mechanisms. Molybdate and vanadate recovery using glutaraldehyde cross-linked chitosan beads reaches uptake capacities as high as 7−8 mmol g-1, depending on the pH. The optimum pH (3−3.5) corresponded to the predominance range of hydrolyzed polynuclear metal forms and optimum electrostatic attraction. While for beads, particle size does not influence equilibrium, for flakes, increasing sorbent radius significantly decreases uptake capacities to 1.5 mmol g-1. Sorption kinetics are mainly controlled by intraparticle diffusion for beads, while for flakes the controlling mechanisms are both external and intraparticle diff...

Catalytic Abatement of Water Pollutants

Abstract: The paper reviews solid-catalyzed oxidation and reduction processes for the treatment of wastewater that contains small concentrations of toxic compounds and for which separation is not economical while biological treatment is not feasible. Specifically, the objectives are (1) to understand the interactions between catalytic materials and various pollutants, (2) to provide a database for catalyst selection, and (3) to assess the potential of these processes for commercialization. The review suggests the following well-investigated solutions: (1) Supported metal (Ru/CeO2, Pt/CeO2, and Ru/C) and metal oxides (CuO−ZnO−CoO, MnO2/CeO2, CoO/Bi2O3, and V2O5/Al2O3) are the most promising catalysts for the destruction of refractory organic compounds with nearly 100% selectivity to CO2; (2) CoO/CeO2 and MnO2/CeO2 are the most active catalysts for ammonia oxidation at temperatures of 263−400 °C; (3) activated carbon, preferably in the presence of copper ions, is an active catalyst for the oxidation of cyanides and ...

Solubility of homopolymers and copolymers in carbon dioxide

Abstract: The cloud points of various amorphous polyether, polyacrylate, and polysiloxane homopolymers, and a variety of commercially available block copolymers, were measured in CO2 at temperatures from 25 to 65 °C and pressures of ca. 1000−6000 psia. Almost without exception, the solubility of amorphous polymers increases with a decrease in the cohesive energy density, or likewise, the surface tension of the polymer. With this decrease in surface tension, the polymer cohesive energy density becomes closer to that of CO2. Consequently, solubility is governed primarily by polymer−polymer interactions, while polymer−CO2 interactions play a secondary role. The solubility is strongly dependent upon molecular weight for the less CO2-philic polymers. The solubilities of high-molecular-weight poly(fluoroalkoxyphosphazenes) in CO2 were comparable to those of poly(1,1-dihydroperfluorooctylacrylate), one of the most CO2-soluble polymers known.

Effective Continuous-Time Formulation for Short-Term Scheduling. 2. Continuous and Semicontinuous Processes

Abstract: Part 1 presented a novel continuous-time mathematical formulation for the short-term scheduling of batch processes. Most production networks, however, involve batch and continuous processes. On the basis of the same principles, this paper extends the proposed formulation to describe continuous processes. Two industrial case studies from fast moving consumer goods manufacturing are presented to illustrate the capability of the proposed formulation to describe plants with both batch and continuous processes, to incorporate cleanup requirements, and to consider storage requirements and limitations. It is demonstrated that the proposed approach outperforms all previously proposed continuous-time models for the short-term scheduling of continuous processes.

Cellulose Pyrolysis Kinetics: Revisited

Abstract: In the same thermogravimetric analyzer (TGA) under identical conditions, samples of pure, ash-free cellulose (i.e., Avicel PH-105, Whatman CF-11, Millipore ash-free filter pulp, and Whatman #42) obtained from different manufacturers undergo pyrolysis at temperatures which differ by as much as 30 °C. Thus, the pyrolysis chemistry of a sample of pure cellulose is not governed by a universal rate law, as is the case with a pure hydrocarbon gas (for example). Nevertheless, the pyrolytic weight loss of all the samples studied in this work is well represented by a high activation energy (228 kJ/mol), first-order rate law at both low and high heating rates. These results do not corroborate the recent findings of Milosavljevic and Suuberg (Ind. Eng. Chem. Res. 1995, 34, 1081−1091). For a particular cellulose sample (for example, Avicel PH-105), variations in the preexponential constant determined at different heating rates reflect uncontrolled, systematic errors in the dynamic sample temperature measurement (ther...

Olefin/Paraffin Separations by Reactive Absorption: A Review

Abstract: Light olefins and paraffins are commonly separated by cryogenic distillation. A process based upon reversible chemical complexation, which employs a mass-separating agent rather than an energy-separating agent, presents an attractive alternative to distillation. Use of such a facilitated-transport-assisted process could substantially reduce the capital costs and energy requirements of olefin/paraffin separations. Copper(I) and silver(I) have long been known to form electron donor/acceptor complexes with olefins. Several chemical systems using these transition metals as the selective separating agent have been studied. A review of copper- and silver-based complexing solutions for olefin/paraffin separations via gas/liquid contacting is presented.

Decomposition of Formic Acid under Hydrothermal Conditions

Abstract: The thermal decomposition of formic acid was studied in dilute aqueous solutions and in the absence of added oxygen at temperatures between 320 and 500 °C and pressures between 178 and 303 atm for residence times between 1.4 and 80 s. Under these conditions, the formic acid conversion ranged from 38% to 100%, and the major products were always CO2 and H2, which indicates that decarboxylation is the preferred reaction path for formic acid decomposition under hydrothermal conditions. CO also appeared as a product, which shows that a dehydration path is available, but the CO yield was always at least an order of magnitude lower than the yields of CO2 and H2. The kinetics of formic acid disappearance and product formation at temperatures above 320 °C are consistent with a reaction rate law that is first order in formic acid. The implications of the present results to the generally accepted molecular decomposition mechanism are discussed, as are the alternative free-radical, ionic, and surface-catalyzed reacti...

A Modified UNIFAC (Dortmund) Model. 3. Revision and Extension

Abstract: The group contribution method Modified UNIFAC (Dortmund) is a well-known model for the reliable prediction of phase equilibria (VLE, LLE, SLE of eutectic systems, azeotropic data and γ∞) and excess...

Using molecular orbital calculations to describe the phase behavior of cross-associating mixtures

Abstract: In previous studies, we have used molecular orbital calculations to determine the thermodynamic changes of dimerization for a number of pure and cross-associated species. We have shown that by using these results in a physical equation of state, the statistical associating fluid theory (SAFT), we are able to accurately model the phase behavior of pure self-associating compounds and binary mixtures of a self-associating compound and a nonassociating compound with a reduction in the number of adjustable parameters. In this study, we consider the phase behavior of binary mixtures in which cross-associated species may occur. To determine the equation of state parameters describing cross association, we introduce a mixing rule based on the results of the molecular orbital calculations. We show that using information derived from the quantum-mechanical calculations results in correlations of mixture vapor-liquid equilibrium data with fewer adjustable parameters and no loss of accuracy, indeed frequently with improved accuracy, compared to the original SAFT model.

Equilibrium Uptake, Sorption Dynamics, Process Development, and Column Operations for the Removal of Copper and Nickel from Aqueous Solution and Wastewater Using Activated Slag, a Low-Cost Adsorbent

Abstract: Activated slag developed from blast furnace waste material has been used for the removal of copper and nickel. The effects of particle size distribution, contact time, and surface loading of these metal ions on the adsorbent for their removal have been studied at the optimum pH (5.0 for Cu2+ and 4.0 for Ni2+). Kinetic studies were performed to decide the mechanistic steps of the process and to obtain the thermodynamic parameters. Sorption data have been correlated with both Langmuir and Freundlich adsorption models. Column operations were also performed in an attempt to simulate industrial conditions. The bed-depth-service-time (BDST) model has successfully been applied to the sorptive removal of nickel and copper. Some feasibility experiments have been performed with a goal to recover adsorbate and chemical regeneration of the spent columns without dismantling the same.

Sunflower Stalks as Adsorbents for the Removal of Metal Ions from Wastewater

Abstract: Sunflower stalks as adsorbents for the removal of metal ions such as copper, cadmium, zinc, and chromium ions in aqueous solutions were studied with equilibrium isotherms and kinetic adsorptions. The maximum adsorptions of four heavy metals are 29.3 mg/g (Cu2+), 30.73 mg/g (Zn2+), 42.18 mg/g (Cd2+), and 25.07 mg/g (Cr3+), respectively. Particle sizes of sunflower stalks affected the adsorption of metal ions; the finer size of particles showed better adsorption to the ions. Temperature also plays an interesting role in the adsorption of different metal ions. Copper, zinc, and cadmium exhibited lower adsorption on sunflower stalks at higher temperature, while chromium showed the opposite phenomenon. The adsorption rates of copper, cadmium, and chromium are quite rapid. Within 60 min of operation about 60−80% of these ions were removed from the solutions.

Prediction of Binary and Ternary Diagrams Using the Statistical Associating Fluid Theory (SAFT) Equation of State

Abstract: A modified statistical associating fluid theory (SAFT) equation of state has been applied to predict the phase equilibria behavior of binary and ternary mixtures. In order to study multicomponent s...

Commercial Steam Reforming Catalysts To Improve Biomass Gasification with Steam−Oxygen Mixtures. 2. Catalytic Tar Removal

Abstract: Eight different commercial catalysts, nickel based, for steam reforming of naphthas and of natural gas are tested in biomass gasification for hot gas cleanup and conditioning. They were manufactured by BASF AG, ICI−Katalco, UCI, and Haldor Topsoe a/s. The catalysts were tested in a slip flow after a biomass gasifier of fluidized bed type at small pilot-plant scale (10−20 kg of biomass/h). The gasifying agent used is steam-oxygen mixtures. A guard bed containing a calcined dolomite is used to decrease the tar content in the gas at the inlet of the catalytic bed. Main variables studied are catalyst type, bed temperature, H2O + O2 to biomass feed ratio, and time-on-stream. All catalysts for reforming of naphthas show to be very active and useful for tar removal and gas conditioning (in biomass gasification). 98% tar removal is easily obtained with space velocities of 14 000 h-1 (n.c.). No catalysts deactivation is found in 48 h-on-stream tests when the catalyst temperature is relatively high (780−830 °C). Us...

Biodiesel Fuel from Animal Fat. Ancillary Studies on Transesterification of Beef Tallow

Abstract: Transesterification of beef tallow was investigated. The solubility of ethanol in beef tallow was much higher than that of methanol. At 100 °C the solubility of methanol was 19% (w/w). The solubility of ethanol in beef tallow reached 100% (w/w) at about 68 °C. For the distribution of methanol between beef tallow methyl esters (BTME) and glycerol, the percentage of total methanol in the glycerol phase was higher than that in the fatty acid methyl ester (FAME) phase in a simulated system at room temperature. At 65-80 °C, however, the percentage of total methanol in FAME (60% (w/w)) was higher than that in glycerol (40% (w/w)) in a 90:10 (w/w) blend of FAME and glycerol. This coincided with the methanol distribution in the transesterified product. The process for making beef tallow methyl esters should recover methanol using vacuum distillation, separate the ester and glycerol phases, and then wash the beef tallow methyl esters with warm water. At neutral pH, the separation of ester and glycerol and water washing was easier because it reduced emulsion formation.

Quantifying hydrate formation and kinetic inhibition

Abstract: In the Prausnitz tradition, molecular and macroscopic evidence of hydrate formation and kinetic inhibition is presented. On the microscopic level, the first Raman spectra are presented for the formation of both uninhibited and inhibited methane hydrates with time. This method has the potential to provide a microscopic-based kinetics model. Three macroscopic aspects of natural gas hydrate kinetic inhibition are also reported: (1) The effect of hydrate dissociation residual structures was measured, which has application in decreasing the time required for subsequent formation. (2) The performance of a kinetic inhibitor (poly(N-vinylcaprolactam) or PVCap) was measured and correlated as a function of PVCap molecular weight and concentrations of PVCap, methanol, and salt in the aqueous phase. (3) Long-duration test results indicated that the use of PVCap can prevent pipeline blockage for a time exceeding the aqueous phase residence time in some gas pipelines.

Optimal Design of Distributed Wastewater Treatment Networks

Abstract: This paper deals with the optimum design of a distributed wastewater network where multicomponent streams are considered that are to be processed by units for reducing the concentration of several contaminants. The proposed model gives rise to a nonconvex nonlinear problem which often exhibits local minima and causes convergence difficulties. A search procedure is proposed in this paper that is based on the successive solution of a relaxed linear model and the original nonconvex nonlinear problem. Several examples are presented to illustrate that the proposed method often yields global or near global optimum solutions. The model is also extended for selecting different treatment technologies and for handling membrane separation modules.

Robust Process Planning under Uncertainty

Abstract: The need to model uncertainty in process design and operations has long been recognized. A frequently taken approach, the two-stage paradigm, involves partitioning the problem variables into two stages: those that have to be decided before and those that can be decided after the uncertain parameters reveal themselves. The resulting two-stage stochastic optimization models minimize the sum of the costs of the first stage and the expected cost of the second stage. A potential limitation of this approach is that it does not account for the variability of the second-stage costs and might lead to solutions where the actual second-stage costs are unacceptably high. In order to resolve this difficulty, we introduce a robustness measure that penalizes second-stage costs that are above the expected cost. Incorporating this measure into stochastic programming formulations does not introduce nonlinearlities, thus making possible the solution of large-scale problems through linear programming techniques. The propose...

Mechanism and Kinetics of Cellobiose Decomposition in Sub- and Supercritical Water

Abstract: Cellobiose decomposition kinetics and products in sub- and supercritical water were studied with a flow apparatus at temperatures from 300 to 400 °C at pressures from 25 to 40 MPa, and at short residence times (0.04−2 s). Cellobiose was found to decompose via hydrolysis of the glycosidic bond and via pyrolysis of the reducing end. Pyrolysis products were glycosylerythrose (GE) and glycosylglycolaldehyde (GG) which were confirmed by FAB-MS. Hydrolysis products were glucose, erythrose, and glycolaldehyde from cellobiose, GE, and GG, respectively, as well as glucose decomposition products. The kinetics from glucose decomposition were used to fit the experimental results and evaluate rate constants of hydrolysis (kH) and pyrolysis rate constants (k1 and k2). The activation energy for the hydrolysis of cellobiose and pyrolysis products GG and GE was found to be 108.6, 110.5, and 106.1 kJ/mol, respectively. In the supercritical region, there was a decrease in the pyrolysis rates k1 and k2 and a corresponding in...

A general method for the conversion of fly ash into zeolites as ion exchangers for cesium

Abstract: A general method of converting various coal ashes into zeolites was investigated. Fly ashes from several utility power plants were fused with sodium hydroxide at 550 °C followed by dissolution in w...

Optimal Planning and Scheduling of Offshore Oil Field Infrastructure Investment and Operations

Abstract: A multiperiod mixed-integer linear programming (MILP) model formulation is presented for the planning and scheduling of investment and operation in offshore oil field facilities. The formulation employs a general objective function that optimizes a selected economic indicator (e.g., net present value). For a given planning horizon, the decision variables in the model are the choice of reservoirs to develop, selection from among candidate well sites, the well drilling and platform installation schedule, capacities of well and production platforms, and the fluid production rates from wells for each time period. The formulation incorporates the nonlinear reservoir performance, surface pressure constraints, and drilling rig resource constraints. The resulting MILP model contains several thousand binary variables and is intractable using a full space branch and bound technique. A sequential decomposition strategy using aggregation of time periods and wells, followed by successive disaggregation, is proposed. Two examples are presented to illustrate the performance of the algorithm.

Biomass gasification with air in fluidized bed : Reforming of the gas composition with commercial steam reforming Catalysts

Abstract: Four commercial catalysts for steam reforming of higher hydrocarbons (naphthas) and three for steam reforming of light hydrocarbons are tested for hot gas clean up and upgrading in biomass gasification with air in fluidized bed. The catalysts used originate from four manufacturers: BASF AG, ICI-Katalco, Haldor Topsoe a/s, and United Catalysts Inc. The work is performed in a small pilot plant (1−2 kg of biomass fed/h) with three reactors in series: gasifier, guard bed of dolomite, and full flow catalytic bed. Samples of gas are taken before and after the catalytic bed at different times-on-stream. It is shown how the H2, CO, CO2, CH4 and steam contents in the flue gas change because of the catalytic bed approaching contents near to the ones corresponding to the equilibrium state. Variations in the heating value of the gas and gas yield as a result of the catalytic bed are also reported.

Efficient Recovery of Carbon Dioxide from Flue Gases of Coal-Fired Power Plants by Cyclic Fixed-Bed Operations over K2CO3-on-Carbon

Abstract: An efficient chemical absorption method capable of cyclic fixed-bed operations under moist conditions for the recovery of carbon dioxide from flue gases has been proposed employing K2CO3-on-carbon. Carbon dioxide was chemically absorbed by the reaction K2CO3 + CO2 + H2O ⇄ 2KHCO3 to form potassium hydrogencarbonate. Moisture, usually contained as high as 8−17% in flue gases, badly affects the capacity of conventional adsorbents such as zeolites, but the present technology has no concern with moisture; water is rather necessary in principle as shown in the equation above. Deliquescent potassium carbonate should be supported on an appropriate porous material to adapt for fixed-bed operations. After breakthrough of carbon dioxide, the entrapped carbon dioxide was released by the decomposition of hydrogencarbonate to shift the reaction in Eq.1 in reverse on flushing with steam, which could be condensed by cooling to afford carbon dioxide in high purity. Among various preparations of alkaline-earth carbonates (...

Synthesis and Permeation Properties of SAPO-34 Tubular Membranes

Abstract: A SAPO-34 membrane was prepared on an alumina tubular support. This membrane appears to exhibit molecular sieving properties with permeances that decrease as the kinetic diameter increases. The room-temperature permeances of H2 and n-C4H10 were 2.4 × 10-8 and 1.9 × 10-10 mol/(m2 s Pa), respectively, and the permeances were in the order H2 > CO2 > N2 > CH4 > n-C4H10. As the temperature increased, the single gas permeances of H2 and N2 exhibited minima, whereas the permeance of CO2 decreased and that of CH4 increased. As the pressure increased with a constant pressure drop across the membrane, the permeances of H2, CO2, N2, and CH4 decreased. The H2/CH4, CO2/CH4, H2/N2, and CO2/N2 ideal selectivities at 300 K and 270 kPa feed pressure with a 138 kPa pressure drop were 25, 19, 7.4, and 5.7, respectively, and these selectivities decreased with increasing temperature and increased with increasing pressure. The ideal selectivity of N2/CH4 was 3.4 at the same conditions and decreased with increasing temperature ...

Phase Equilibria for Systems Containing Hydrocarbons, Water, and Salt: An Extended Peng−Robinson Equation of State

Abstract: The empirical success of the Peng−Robinson equation of state (PREOS) is unmatched; there is no other simple equation of state of the van der Waals form that has shown such wide and reliable applicability to the calculation of vapor−liquid equilibria (VLE) for systems containing light hydrocarbons, permanent gases, carbon dioxide, and hydrogen sulfide. PREOS is one of the great success stories of applied chemical engineering thermodynamics. However, PREOS also has limitations; two of these are addressed here. Theoretically based extensions are presented, first, for including water and aqueous hydrocarbon mixtures and, second, for including salt in water and in aqueous hydrocarbon mixtures. Illustrative examples show that these extensions are successful for VLE (where L refers to a hydrocarbon liquid dilute in water) and for VLE (where L refers to saline water). Because the extension to include water, based on SAFT (statistical association fluid theory), cannot properly take into account the well-documented...

The Yielding of Waxy Crude Oils

Abstract: The yielding process of statically cooled waxy crude oil was examined in detail. Three direct measurementsa controlled stress test, a creep−recovery test, and an oscillatory testwere employed using a controlled stress rheometer to investigate the yielding of two distinctly different statically cooled waxy crude oils. The results showed that yielding of waxy crude oil occurs by an initial elastic response, followed by viscoelastic creep and a final fracture. A model with an elastic-limit yield stress, a static yield stress, and a dynamic yield stress was introduced to describe the yielding process. The three yield stresses were determined by means of different techniques. The effect of the time scale in different measurements on the three yield stresses was studied. It was shown from both the creep−recovery and oscillatory tests that the elastic-limit yield stress was independent of the time scale. The static yield stress was found to be dependent on the time scale in the three tests. The dynamic yield str...

Supercritical Water Oxidation of NH3 over a MnO2/CeO2 Catalyst

Abstract: Catalytic oxidation of ammonia in supercritical water (SCW) was studied using a continuous-flow, packed-bed reactor at temperatures ranging from 410 to 470 °C, a nominal pressure of 27.6 MPa, and reactor residence times of less than 1 s. The kinetics and catalyst performance of MnO2/CeO2 for oxidation of ammonia in SCW was evaluated. In this reaction environment, ammonia was predominantly converted into molecular nitrogen (N2), and the rate of ammonia conversion was enhanced by MnO2/CeO2. For example, 40% of the ammonia was converted when using the MnO2/CeO2 catalyst at a temperature of 450 °C and a reactor residence time of 0.8 s. It was reported that, without a catalyst, essentially no ammonia conversion was observed below 525 °C (Helling, R. K.; Tester, J. W. Environ. Sci. Technol. 1988, 22 (11), 1319) and 10% of the ammonia was converted at a temperature of 680 °C, a pressure of 24.6 MPa, and a reactor residence time of 10 s (Webley, P. A.; Tester, J. W.; Holgate, H. R. Ind. Eng. Chem. Res. 1991, 30 (...

Removal of arsenic(V) from aqueous solution using industrial solid waste : Adsorption rates and equilibrium studies

Abstract: Industrial solid wastes can be recycled as nonconventional adsorbents, if they are nontoxic, to reduce the cost of wastewater treatments. ``Waste`` Fe(III)/Cr(III) hydroxide, generated electrolytically in the treatment of Cr(VI) containing wastewaters in a fertilizer industry was used for the adsorption of arsenic(V) from aqueous solution. Parameters studied include arsenic-(V) concentration, agitation time, adsorbent dosage, adsorbent particle size, temperature, and pH. The adsorption capacity was evaluated by using both Langmuir and Freundlich isotherm models. The adsorption followed a first-order rate expression. Adsorption of As(V) was independent of the initial pH (3--10) of the aqueous solution. Temperature studies showed that the adsorption process was endothermic in nature. Desorption of As(V) from spent adsorbent was also investigated using NaOH solutions.

TEOM: A Unique Technique for Measuring Adsorption Properties. Light Alkanes in Silicalite-1

Abstract: A new and fast technique, the tapered element oscillating microbalance, TEOM, has been applied to accurately measure adsorption in microporous materials at conditions relevant for practical applications. With this technique, the experimental data are not influenced by factors such as buoyancy and flow patterns, which are encountered with conventional gravimetric methods. The equilibrium adsorption of light alkanes, methane, ethane, propane, n-butane, and i-butane in silicalite-1 has been investigated using the TEOM technique. Single-component adsorption isotherms are reported at temperatures in the range from 303 to 473 K and at pressures of up to 500 kPa. Either a conventional or a double Langmuir isotherm appropriately describes the equilibrium data. Thermodynamic properties calculated from the isotherms are in excellent agreement with literature data from other techniques. Isobaric and isothermal experiments demonstrate that i-butane exhibits a two-step adsorption behavior. Based on geometry and entrop...

Effect of operating conditions and membrane quality on the separation performance of composite silicalite-1 membranes

Abstract: The separation capacity of silicalite-1 membranes for various hydrocarbon mixtures is determined as a function of membrane quality, operating conditions, and orientation of the composite membrane with respect to the feed side. The quality of the membranes is judged on the basis of the n-butane/i-butane permselectivity. Membranes with a different n-butane/i-butane permselectivity showed an identical separation capacity for ethane/methane mixtures, but the quality difference was affecting separation of hydrogen from the butane isomers. The selectivity of the membrane is significantly affected by the operating conditions, such as mixture composition, temperature, and absolute pressure. These effects are shown for ethane/methane, propene/ethene, and n-butane/i-butane mixtures. The selectivity for ethane in ethane/methane mixtures, found when the zeolite layer is facing the feed side, is completely lost when the orientation of the composite membrane is reversed, due to concentration polarization. Depending on the membrane orientation, the major resistance of the composite is in the support layer or in the zeolite layer.