Madhukar Bhaskara Rao

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.

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.

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.

Multicomponent gas separation by selective surface flow (SSF) and poly-trimethylsilylpropyne (PTMSP) membranes

Abstract A selective surface flow (SSF) membrane consisting of a thin layer of a nanoporous carbon was produced in a tubular form using a macroporous alumina support. The membrane was tested for hydrogen enrichment applications. Simulated waste gases from a petrochemical refinery and a hydrogen pressure swing adsorption unit were used as the feed gas to the membrane. Very high rejections of C 1 C 3 hydrocarbons (saturated and unsaturated) and carbon dioxide over hydrogen were exhibited by the membrane at low feed gas pressures. The hydrogen enriched stream was produced at the feed gas pressure. The separation characteristics of a polymeric poly-trimethylsilylpropyne (PTMSP) membrane in a tubular form was also tested for the same applications using identical conditions of operation. This membrane also selectively rejected heavier components of the feed gas mixture over hydrogen and produced the hydrogen enriched stream at the feed gas pressure. The SSF membrane exhibited much higher hydrogen recovery and hydrocarbon rejections than the PTMSP membrane for these applications under identical conditions of operations using identical support materials.

Fractionation of air by zeolites

Abstract Fractionation of air by selective adsorption of N 2 on zeolites has become a common industrial practice. Many different zeolites and air separation processes have been developed for this purpose. Pure gas isotherms for adsorption of N 2 and O 2 on five commercial zeolites (NaX, 5A, Na- Mordenite, CaX and CaLSX) at two different temperatures are reported. The isotherms can be described by the Langmuir model in the range of the data. Mixed gas Langmuir model is used to evaluate the relative N 2 adsorption and desorption characteristics for these zeolites in connection with air separation application by the pressure swing adsorption (PSA) concepts. Nine different PSA processes for air separation using zeolites are reviewed and their process performances are compared. These processes can be designed to produce low (23–50 mole%) and medium purity (90–95 mole%) O 2 -enriched air and high purity (98+ mole%) N 2 -enriched air. Processes can be tailor made to match the adsorptive properties of the zeolite for a given separation need or vice versa.

Scale-Up of Selective Surface Flow Membrane for Gas Separation

Abstract The Selective Surface Flow (SSF) membrane, consisting of a nanoporous carbon layer supported on a macroporous alumina tube, can be used to enrich hydrogen from a feed gas containing hydrogen and hydrocarbon mixtures. The membrane produces a hydrogen-enriched product stream at feed gas pressure by selectively rejecting the hydrocarbons to the low pressure side of the membrane. Bench-scale testing of the membrane showed that very high rejections of C+ 2 hydrocarbons can be achieved from a feed gas containing low concentrations of hydrogen at moderate pressure. The membrane has been scaled-up in length and field-tested in modular form using a real refinery waste gas under actual operating conditions. It successfully tracked the performance of the bench-scale unit under field conditions. Both bench-scale and field-scale performance data are described. Six months of continuous operation in the field did not exhibit any degradation of membrane performance.

Diffusion through carbon micropores-4 years later

Abstract The activated diffusion through micropores is a subject of considerable importance in the practical application of carbon adsorbents, molecular sieves, and catalyst supports. A theoretical model for the interaction of complex nonspherical molecules with graphite was used to characterize the interaction of He, Ar, CO, CO2, O2, N2, and H2 with idealized carbon micropores. The calculations yielded values for the critical pore dimension below which the diffusion for these species becomes activated; these values agree well with relative diffusion rates observed experimentally. In addition, estimates of the relative ease of diffusion of these gases were made from the magnitude of the various diffusional energy barriers within the micropores.

Investigating the Compatibility of Ruthenium Liners with Copper Interconnects

Ruthenium is proposed as an alternative liner material in sub 30-nm line-width device technology. Ruthenium is a noble metal and does not oxidize readily. Hence, it is possible to directly plate copper on ruthenium. Because of very low solubility of copper in ruthenium, ruthenium also offers good barrier properties. However, the same noble electrochemical characteristics that prevent the oxidation of ruthenium and so useful for being able to directly plate copper provides challenges in terms of corrosion of copper lines in subsequent wet processing. Copper is electrochemically active to ruthenium. As a result of which, in a galvanic couple with ruthenium copper corrosion will be accelerated. In this paper we will examine the kinetics of galvanic corrosion using novel techniques of in-situ AFM analysis and demonstrate that through chemistry optimization, galvanic corrosion can be minimized.

Chapter 2.12 Drying of gases and liquids by activated alumina

Publisher Summary Removal of trace and bulk water from a fluid (gas or liquid) stream is a major unit operation in the chemical and petrochemical industries. The drying process is necessary to (1) prevent condensation and freeze-out of water in plant pipeline and equipment, (2) eliminate corrosion in process equipment, (3) protect against undesirable chemical reactions such as hydration and hydrolysis, (4) prevent catalyst poisoning, and (5) meet product fluid composition specification. Selective adsorption of water on a solid desiccant such as zeolites, silica gels, and activated aluminas is often used as the method of drying the fluid stream. Various forms of cyclic pressure swing adsorption (PSA) and thermal swing adsorption (TSA) concepts are generally used as the drying process. These processes utilize regenerative schemes consisting of adsorption and desorption steps so that the adsorbent can be repeatedly used for drying the fluid stream. The design and cost of operation of these processes demand certain properties for adsorption of water by the adsorbent that facilitate the adsorption and desorption steps. Activated aluminas often provide a large spectrum of desirable adsorptive properties for such drying applications. These properties include adsorption equilibria, adsorption kinetics, heats of adsorption, and adsorption and desorption column dynamics, which govern the performance of the drying process. This chapter briefly describes these properties for adsorption of water on various forms of alumina and illustrates several conventional drying processes using alumina.

Combined pattern collapse and LWR control at the 70 nm node through application of novel surface conditioner solutions

As pattern collapse and line width roughness (LWR) become critical lithography challenges, there is growing interest in applying surface conditioner solutions during the post-develop process to address BOTH these issues. In this paper, we patterned 90nm 1:1.2 lines/spaces (L/S) on 200mm wafers and 70nm dense lines on 300mm wafers to evaluate the combined performance of pattern collapse and LWR using newly formulated surface conditioners. The performance of each conditioner was compared to the standard formulation, which is capable of significant pattern collapse reduction, but affords no LWR improvement. These newly improved formulations enabled a ~20% LWR reduction for 90nm features and a ~10% LWR reduction for 70nm dense lines. In addition, the new formulations significantly enlarged the LWR and CD process windows for 70nm dense lines, as demonstrated by a 50% increase of maximum depth of focus (DOF) over the standard formulation.