Shivaji Sircar

ACTIVATED CARBON FOR GAS SEPARATION AND STORAGE

Abstract Activated carbons offer a large spectrum of pore structures and surface chemistry for adsorption of gases, which are being used to design practical pressure swing and thermal swing adsorption processes for separation and purification of gas mixtures. The activated carbons are often preferred over the zeolitic adsorbents in a gas separation process because of their relatively moderate strengths of adsorption for gases, which facilitate the desorption process. Three commercial applications of activated carbons, (a) trace impurity removal from a contaminated gas, (b) production of hydrogen from a steam-methane reformer off gas, and (c) production of nitrogen from air, are reviewed. Four novel applications of activated carbons for gas separation and purification are also described. They include, (a) separation of hydrogen-hydrocarbon mixtures by selective surface flow of larger hydrocarbon molecules through a nanoporous carbon membrane produced by carbonization of a polymer matrix, (b) gas drying by pressure swing adsorption using a water selective microporous carbon adsorbent produced by surface oxidation of a hydrophobic carbon, (c) removal by selective adsorption and in-situ oxidation of trace volatile organic compounds from air by using a carbon adsorbent-catalyst composite, and (d) storage of compressed natural gas on high surface area carbons.

Purification of Hydrogen by Pressure Swing Adsorption

Pressure swing adsorption (PSA) processes are used for the production of high purity hydrogen from steam methane reforming off-gas (SMROG) and refinery off-gases (ROG). A variety of commercial PSA processes for the production of H2 with or without a by-product (CO2 from SMROG), as well as PSA processes for direct production of ammonia synthesis gas (from SMROG), are reviewed. The equilibrium ad(de)sorption characteristics of the components of SMROG and ROG feed gas on an activated carbon, a zeolite, and a silica gel are reported, and the criteria for adsorbent selection in these PSA processes are discussed. Recent ideas to increase the H2 recovery from these PSA processes by integrating them with selective surface flow membranes or other PSA units are reviewed.

SEPARATION OF MULTICOMPONENT GAS MIXTURES

Rectification is the main separation technology used for purifying the products in the chemical and petroleum industry. Development of distillation equipments and processes is important because of their vast energy needs and their incidence. Recently, systematic synthesis of separation sequences is developed significantly. This paper presents a case study of the separation of a multicomponent cracked gas. A separation system created by heuristic rules and an “optimal” structure characterized by minimal costs were compared. Rules that can be generalized based on the actual example of the separation sequence synthesis are emphasized. The separation structure developed by applying these generalized rules unambiguously depend on the thermodynamics properties of the multicomponent mixture and the specification of the products.

Hydrogen production by hybrid SMR–PSA–SSF membrane system

Abstract Pressure swing adsorption (PSA) processes are commonly used to produce pure hydrogen from the steam–methane reformer (SMR) off-gas. The typical hydrogen recoveries for PSA processes producing 99.999+% hydrogen are in the range 70–85%. The nanoporous selective surface flow (SSF) carbon membrane can be used to extract hydrogen from the low pressure waste gases of the PSA processes and the enriched hydrogen stream can be recycled as feed gas to the PSA process after recompression. The net result of this integration between the PSA process and the SSF membrane is increased hydrogen recovery from the SMR off-gas. The separation performance of the SSF membrane in producing a hydrogen-enriched gas from the PSA waste gas was experimentally evaluated and two different schemes to integrate the membrane with a specific PSA process for hydrogen purification were studied. The performance of the PSA process was simulated using a software package called SIMPAC. It is demonstrated that the integrated process can increase the net hydrogen recovery to 84–85% from a hydrogen recovery value of 77–78% by the base PSA process.

Separation of Methane and Carbon Dioxide Gas Mixtures by Pressure Swing Adsorption

Abstract Two pressure swing adsorption processes for separation of methane and carbon dioxide gas mixtures are described. One process simultaneously produces a high purity CH4 and a high purity CO2 product with high recoveries of both components from the feed gas. The other process only produces a high purity CH4 product with high recovery. Test data for these processes are reported and their relative advantages are discussed.

Excess properties and column dynamics of multicomponent gas adsorption

The Gibbsian surface excesses are the true experimental variables in measuring the extent of adsorption from gases. The surface excess variables can be conveniently used to describe the dynamic adsorption of gases in adsorbent columns without defining adsorbed phase or the adsorbent structure. The resulting mass and enthalpy balance equations are as practical to solve as those conventionally written by using the actual amounts adsorbed as the variables to define adsorption.

Nanoporous carbon membrane for gas separation

Abstract A novel method for production of nanoporous carbon membranes by carbonization of a polymer latex is described. The estimated pore size of the membrane is between 5.0 and 5.5 A (diameter). The membrane can separate H2 from mixtures with CO2, CH4, C2H6 and C3 H8 by selective adsorption and surface diffusion of the larger components. A moderate to high selectivity of the adsorbing components can be achieved through the membrane while maintaining fairly high permeabilities for these components even at a moderate feed-gas pressure. The membrane can be used to enrich H2 from a stream containing these components.

Heat of Adsorption

Separation of gas mixtures by pressure swing and thermal swing adsorption processes is an established unit operation in the chemical industry. Mathematical simulations of these processes require precise knowledge of multicomponent gas adsorption equilibria, kinetics, and heats for the system of interest over all conditions of pressure, temperature, gas composition and adsorbate loading encountered by the adsorber during the separation process. Unfortunately, the published data on heats of adsorption are often not adequate. Limited heat data are generally available for pure gas adsorption, heat data for binary gas mixtures are rare, and heat data for mixtures containing three or more components are nonexistent.

Selective Surface Flow Membrane for Gas Separation

The selective surface flow membrane is a nanoporous carbon membrane which separates gas mixtures by a selective adsorption–surface diffusion–desorption mechanism. It selectively permeates the larger and the more polar components of a feed gas mixture. The separation characteristics of several different gas mixtures by the membrane are described. The membrane has been field-tested at a refinery site for separation of hydrocarbon–hydrogen mixtures.

Simultaneous Production of Hydrogen and Carbon Dioxide from Steam Reformer Off-Gas by Pressure Swing Adsorption

Abstract Pressure swing adsorption (PSA) processes are used for the production of ultrapure hydrogen from a crude hydrogen stream containing H2O, CO2, CO, CH4, and N2 impurities which is produced by steam reformation of natural gas or naphtha. Two commercial PSA processes designed for this purpose are reviewed and a new commercial PSA process which simultaneously produces ultrapure hydrogen and high purity carbon dioxide products from the crude hydrogen with high recoveries of both components is described. Performance data for the new process are reported.