Salil U. Rege

A novel FTIR method for studying mixed gas adsorption at low concentrations: H2O and CO2 on NaX zeolite and γ-alumina

Abstract The modeling and measurement of mixed gas adsorption at low concentrations (ppm level) is an important part of the design of an adsorptive purification process, but has received little attention so far. This work introduces a new technique for the measurement of the equilibrium adsorbed amounts of a multicomponent mixture of gases/vapors on an adsorbent pellet based on Fourier-transform infrared spectroscopy (FTIR). The technique consists of first calibrating the infrared absorption peak areas with known adsorbed amounts of the different components in the adsorbate mixture using single gases/vapors. By measuring the IR peak areas of the adsorbates on the sorbent in contact with the gas mixture, the actual amounts of the sorbate can be determined. The method is fairly simple, quick, and accurate even at low concentrations provided the adsorbates exhibit strong distinct peaks and the sorbent is at least partially infrared-transparent. The utility of this method is demonstrated by measuring the binary gas adsorption of CO 2 and H 2 O-vapor at low concentrations on two adsorbents: 13X (NaX) zeolite molecular sieve, and γ -Al 2 O 3 . In the low CO 2 concentration range, the amount of CO 2 adsorbed seemed to be significantly enhanced in the presence of H 2 O in trace amounts. The adsorption data was fit to the Doong–Yang potential theory model, and the ideal (and real) adsorbed solution theory (IAS) of Myers and Prausnitz, with the Dubinin–Astakhov equation as the basis.

Sorbents for air prepurification in air separation

Air fed to air separation units such as cryogenic distillation columns needs to be prepurified; that is, the concentration levels of air-borne impurities such as water vapor, CO2, and light hydrocarbons need to be brought down below the tolerable limits. This process is commonly carried out by using adsorptive methods such as pressure swing adsorption (PSA) or temperature swing adsorption (TSA). This work deals with the study of adsorption characteristics of two conventional microporous adsorbents, namely 13X zeolite molecular sieves, and activated γ-Al2O3, and three non-conventional adsorbents, namely a natural zeolite (clinoptilolite), and its K+- and Ca2+-ion exchanged forms. A noteworthy feature of this work is the measurement of adsorption isotherms at very low partial pressures of the adsorbate gas (to a few ppm). The relative merits of these adsorbents for the removal of trace amounts of water vapor, CO2, and hydrocarbons such as CH4, C2H4, and C2H6 are discussed. The isotherm data for 13X zeolite and γ-Al2O3 has been fit to the Langmuir–Freundlich, Toth, and Dubinin–Astakhov (DA, or potential theory) isotherm models. It has been found that the potential theory model is the most suitable one for description of low pressure or concentration data. The origin of the better fit by potential theory is that its corresponding energy distribution function follows a quasi-Gaussian distribution with a broadening at high adsorption energies, and the high-energy sites are important for adsorption at low pressures or concentrations. Finally, the possibility of using H2O adsorption isotherm to evaluate the pore size distribution by the Horvath–Kawazoe approach is discussed.

Air-prepurification by pressure swing adsorption using single/layered beds

Air fed to air separation units such as cryogenic distillation columns needs to be prepurified; that is, the concentration levels of air-borne impurities such as water vapor, CO2, and light hydrocarbons need to be brought down below the tolerable limits which are in the ppm or sub-ppm levels. This process is commonly carried out by using adsorptive methods such as pressure swing adsorption (PSA) or temperature swing adsorption (TSA). In this work, the adsorption characteristics of a natural zeolite (chabazite) for the trace removal of water vapor, CO2, and a hydrocarbon (CH4) were studied and compared with those of two conventional microporous adsorbents, namely 13X (NaX) zeolite molecular sieves, and activated alumina (γ-Al2O3). The low pressure isotherm data was fit to the Dubinin–Astakhov (DA, or potential theory) isotherm model and was extended to multicomponent mixtures using the maximum available micropore volume model of Doong and Yang (Ind. Eng. Chem. Res. 27(4) (1988) 630). An equilibrium based non-isothermal model was used to simulate the performance of the above sorbents for a typical four-step Skarstrom-type PSA cycle for the simultaneous removal of H2O and CO2 from feed N2. Two types of bed configurations were considered: single beds containing only one type of sorbent, and layered beds with contiguous layers of two different sorbents (alumina/13X zeolite). The relative amounts of sorbents required for the layered bed were optimized for particular bed operating conditions.

Molecular sieve sorbents for kinetic separation of propane/propylene

Abstract The separation of propane–propylene mixtures is one of great industrial importance. It is also one of the most difficult and costly to achieve. In this work, the feasibility of using sorbents based on kinetic (due to differences in diffusion rates) or steric (due to size exclusion) effect for gas-phase separation of propane–propylene was determined. Equilibrium isotherms and uptake curves were measured on NaA (4A), NaLiA, and AlPO4-14 zeolites. All of the sorbents were capable of selectively adsorbing propylene over propane. PSA simulations were used to compare these sorbents against π-complexation sorbents such as AgNO3/SiO2. The best sorbents were AlPO4-14 and AgNO3/SiO2. In both cases, over 99% propylene product purities could be obtained at reasonably high recoveries and throughputs.

Propane/propylene separation by pressure swing adsorption: sorbent comparison and multiplicity of cyclic steady states

Abstract The separation of propane/propylene mixtures is an important yet difficult separation. Two types of adsorbents, namely a π -complexation sorbent (AgNO 3 /SiO 2 ), and a steric (based on size exclusion) sorbent (AlPO 4 -14), were previously proposed to be highly effective materials for C 3 H 6 /C 3 H 8 separation using a vacuum pressure swing adsorption (PSA) cycle. In this work, high pressure C 3 H 6 and C 3 H 8 adsorption isotherms have been measured for the AgNO 3 /SiO 2 . The performance of the AgNO 3 /SiO 2 sorbent has been compared with that of AlPO 4 -14 by simulating a four-step PSA cycle with two different grades of feed. The adsorption step was carried out at 7 atm while desorption was conducted at either 1 or 0.2 atm . It was found that although both sorbents provided more than 99% product purity at reasonably high recovery, the performance of AgNO 3 /SiO 2 was better than the AlPO 4 -14 sorbent. The advantage of performing adsorption at a superatmospheric pressure was that the olefin product could be obtained at a higher pressure than that possible with a vacuum-swing cycle. For C 3 H 6 /C 3 H 8 separation using AlPO 4 -14 sorbent, multiplicity of cyclic steady states was observed under certain operating conditions. One unstable and two stable steady states were observed within these regions for identical cycle conditions depending upon the initial temperature or sorbate concentration in the PSA bed at startup. The exact initial bed temperature and concentration at which there was a switch in the stable steady states was identified.

Kinetic Separation of Oxygen and Argon Using Molecular Sieve Carbon

A pressure-swing adsorption (PSA) simulation study was performed for the separation of a mixture of 95% O2 and 5% Ar using a molecular sieve carbon (MSC) as the adsorbent. Two PSA cycles have been outlined to maximize the recovery of either argon or oxygen as a high purity product. The effect of cycle parameters such as cocurrent depressurization pressure, purge/feed ratio, pressure ratio and adsorption pressure on the separation of O2/Ar has been studied. It was found that it is feasible to obtain an argon product of purity in excess of 80% with reasonably high recovery using one of the cycles. The other cycle is capable of producing high purity oxygen (>99%) at high recovery (>50%) with reasonably high product throughputs. The PSA process can be conducted at room temperature and hence has an advantage over conventional processes like cryogenic distillation and cryogenic adsorption.

A SIMPLE PARAMETER FOR SELECTING AN ADSORBENT FOR GAS SEPARATION BY PRESSURE SWING ADSORPTION

A simple parameter is proposed for comparing the performance of two or more adsorbents for a particular binary gas separation by pressure swing adsorption. The parameter is most suitable for separations based on differences in equilibrium adsorption capacity rather than on differences in adsorption kinetics. The two main components of the parameter are the ratio of the delta loadings (differences in adsorbed amounts at high and low pressures) of the 2 gases (Δq 1/Δq 2) and the equilibrium selectivity (α1,2) of the sorbent for the strongly adsorbed species. A dimensionless sorbent selection parameter (S) is defined. For a given separation, the sorbent that yields the highest S value is the best sorbent. In the cases of sorbate-sorbent systems showing Langmuir-type isotherms, This parameter is simple to calculate and is sensitive to subtle differences in adsorption isotherms. The effectiveness of the proposed parameter was demonstrated through two examples of air separation using molecular-sieve zeolites.