Timothy Christopher Golden

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.

Novel Adsorption Distillation Hybrid Scheme for Propane/Propylene Separation

Abstract A novel adsorption–distillation hybrid scheme is proposed for propane/propylene separation. The suggested scheme has potential for saving up to ∼50% energy and ∼15–30% in capital costs as compared with current technology. The key concept of the proposed scheme is to separate olefins from alkanes by adsorption and then separate individual olefins and alkanes by simple distillation, thereby eliminating energy intensive and expensive olefin-alkane distillation. A conceptual flow schematic for the proposed hybrid scheme and potential savings are outlined.

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.

Activated carbon adsorbent for PSA driers

Abstract A Type I water adsorption isotherm by the Brunauer classification with a moderate strength of water adsorption is the preferred adsorption characteristic for applications in pressure swing adsorption (PSA) driers. Zeolitic adsorbents provide the Type I isotherm shape, but they hold water too tightly. Aluminas exhibit the Type IV isotherm shapes and they do not provide good water adsorption capacities. It is possible to oxidize the surface of a normally hydrophobic activated carbon in order to produce a hydrophilic carbon with the desired Type I water adsorption characteristics for PSA drier applications. Nitric acid can be used as the oxidizing agent. Fuming nitric acid treatment can cause physical disintegration of the carbon. A moderate concentration of nitric acid prevents this problem, but the rate of oxidation is slow. Addition of oxidation catalysts like copper (II) acetate during the oxidation reaction hastens the reaction rate and a variety of water isotherm shapes can be produced by controlling the reaction time, temperature, catalyst type, and concentration. The effects of these variables on the shape of the water adsorption isotherm of the treated carbon are studied and a procedure is described for production of a preferred Type I water isotherm for PSA drier applications.

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.

Carbon-based oxygen selective desiccants for use in nitrogen PSA

Abstract Techniques for the production of composite oxygen selective adsorbents are disclosed. These adsorbents are comprised of a carbon molecular sieve (CMS) which is kinetically selective for the adsorption of oxygen over nitrogen and an agent for the sorption of water such as LiCl or SiO2. The adsorption properties of the composite adsorbents and results obtained from pressure swing adsorption (PSA) process testing are presented. The composite adsorbents improve the nitrogen PSA process performance (recovery and productivity) over the use of conventional desiccants which do not exhibit oxygen selectivity. Using a standard nitrogen PSA process cycle, replacement of conventional inorganic desiccants like alumina with the current CMS-based desiccants improved air recovery 2 to 4 percentage points and increased nitrogen productivity 15 to 20% at 70°F and a nitrogen purity of 99.5%.

Synthetic heterogeneity in X zeolite for gas adsorption

Abstract Energetic heterogeneity for gas adsorption was synthetically created by physically admixing NaX and BaX crystals and by binary ion exchange of X zeolite framework with Na + and Ba 2+ ions. Isotherms for adsorption of pure N 2 and CO at two temperatures were measured on the single and mixed ion crystals in order to characterize their degree of heterogeneity. It was found that both NaX and BaX crystals acted like homogeneous substrates for adsorption of N 2 . The adsorption of CO on the NaX crystals also exhibited homogeneous behavior, but its adsorption of BaX was significantly heterogeneous. The adsorption of both gases on the simulated heterogeneous adsorbents could be described by simple weighted additivity of the adsorption contributions of each site, indicating that there was insignificant influence between the adsorption force fields of Na + and Ba 2+ ions even when they were in close proximity to each other within the unit cell of X zeolite framework. It was shown that a substrate can be energetically heterogeneous for a gas even when the isosteric heat of adsorption does not vary with surface coverage, which is the common evidence of the existence of adsorbent heterogeneity. The existence of heterogeneity can also be masked when the adsorbate is weakly adsorbed. Consequently, blind curve fitting of adsorption isotherms with a homogeneous adsorption model can be very misleading, especially when extrapolating isotherm data and predicting multicomponent adsorption equilibrium.