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There is an economic incentive to substitute energy and capital-intensive conventional gas separation schemes based on cryogenic distillation. Absorption has potential advantages over low-temperature schemes as it does not rely on high refrigeration requirements to perform the separation. An optimisation-based synthesis framework has been developed that integrates distillation and absorption-desorption schemes. This methodology is able to quantitatively resolve the numerous tradeoffs between the various capital and operating factors and systematically suggest new design configurations. A multilevel modelling approach enables the accommodation of absorption-desorption separation options in the distillation orientated framework supported by COLOM® (©Centre for Process Integration, University of Manchester). Improved shortcut models for reboiled absorption and distillation columns have been proposed, which are suitable for exploitation in the developed synthesis framework. A new methodology for heat integration is proposed that achieves efficient heat recovery and proposes a configuration of the heat exchanger network. This methodology works in harmony with the optimisation framework. Simultaneous optimisation of the separation system, the heat exchanger network and the refrigeration system offers the opportunity of achieving a superior overall configuration. The structural and operating variables of the separation system are optimised by Simulated Annealing. As a stochastic optimisation method, SA can deal with the large scale of the problem and its discontinuous and non-linear nature imposed by the feasibility limits of the separations and the model equations. The optimal separation configurations are selected on the grounds of minimum capital and operating costs. An analysis of costing methods is provided which aims at rationalising the basis for cost estimation. The application of the developed synthesis methodology is illustrated by a number of examples of relevance to the natural gas processing and refinery gas processing. Results will emphasise the functionality of the methodology as a tool for quantitative evaluation of preliminary designs and realisation of highly integrated and efficient process concepts.

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Thesis (Ph. D.)--Lehigh University, 1999. / Includes vita. Includes bibliographical references (leaves 248-262).

Thesis (Ph. D.)--University of Texas at Austin, 2002. / Vita. Includes bibliographical references. Available also from UMI Company.

Gas absorption in hollow fibre contactors is being increasingly used due to their enormous surface area/volume ratio. The capability of the hollow fibre membrane modules for the removal of CO 2 and SO2 from a binary gas mixture has been investigated experimentally. Four different modules were used in this study. The membranes in modules one and two were made of microporous polypropylene. The third module was made of non-porous silicone rubber (polydimethylsiloxane) while the latter one was a polyvinylidenefluoride (PVDF) asymmetric hollow fibre membrane. The gas mixtures used in the experiments were composed of 9.5% CO2 and 1% SO2 in N 2 , which was introduced into the hollow fibre lumen, while the absorbent liquid was fed into the shell side of module. The absorbent liquids used were water, aqueous solutions of diethanolamine (DBA) and ammonia at different concentrations (5, 10 and 20 wt%). The effects of different operating conditions on the permeation process have been investigated for co-current and counter-current flow patterns. In addition, to improve the silicone rubber hollow fibre membrane performance, baffles were installed within the shell of the permeator to increase liquid fibre contact. The results obtained showed that the use of baffles within the shell of the permeator improved the separation performance of the non-porous membrane module without any substantial increase in the physical size of the contacting device. Studies also showed that improved performance was observed when the system was operated under a counter-current flow pattern. The results showed that the use of an absorbent liquid in the permeate side of the fibres increased the selectivity of the membranes used, and reduced the need to maintain a high pressure ratio across the membrane. A decrease in the feed gas flow rate or increase in liquid flow rate generally improved the removal of gases. The results showed that the use of aqueous reactive solutions as an absorbing medium in the permeate side of the hollow fibre permeator can significantly improve CO2 removal from the gas mixture. However, the main problem when using a microporous membrane coupled with aqueous solutions of diethanolamine as absorbent was wetting of the microporous membrane by amine solutions. For 862 separation, the highest removal was attained using the microporous membrane coupled with water as absorbent liquid. This demonstrates that a hollow fibre based device can be a very efficient gas liquid contactor. The separation process was simulated with a numerical model based on the effective permeabilities of gases and compared with the experimental results. The model simulations showed good agreement with the experimental observations.

Polybenzimidazoles, a class of aromatic heterocyclic polymers, are well known due to their remarkable thermal stability, mechanical properties and chemical resistance which are often required in extreme operation conditions. Because of these properties, polybenzimidazoles are excellent candidates in various application areas including proton exchange membrane fuel cells, gas separation membranes, reverse osmosis and nanofiltration, and high performance coatings. The following studies are focused on the synthesis, characterization and related properties of polybenzimidazoles and polybenzimidazole based materials. A novel sulfonyl-containing tetraamino-substituted monomer (3,3',4,4'-tetraaminodiphenylsulfone) was synthesized and polymerized with three different diacid monomers to make polybenzimidazoles. The new monomer synthesis route with reduced steps relative to the existing literature method increased the overall yield by a factor of three. The sulfonyl-containing polybenzimidazoles have enhanced solubilities in common organic solvents including dimthylsulfoxide, dimethylacetamide and N-methyl-2-pyrrolidone in comparison with the commercial polybenzimidazole, Celazole®, poly(2,2'-(m-phenylene)-5,5'-bibenzimidazole). The improvements in solubility are attributed to the introduction of polar sulfonyl linking moiety in the monomer. Remarkable thermal stabilities (high T_g, > 428 °C) were demonstrated through Dynamic Mechanical Analysis (DMA) and Thermogravimetric Analysis (TGA). A well designed film casting process was investigated and established. Polybenzimidazoles were fabricated into transparent thin films (20-30 μm thick) for gas transport measurements. These novel polybenzimidazole films exhibited extraordinary gas separation properties, especially for H₂/CO₂ separation. There is a trade-off relationship between gas permeability and selectivity through dense, non-porous polymer membranes that was discovered by Robeson in 1991. The ultimate goal for developing gas separation membranes is to improve both permeability and selectivity simultaneously. Gas permeability is related to the free volume between polymer chains. In order to improve gas permeability, we hypothesized a concept that increasing free volume could be achieved by thermally degrading sacrificial components and volatilizing their byproducts from a glassy matrix. Volatile components were introduced into the films to preoccupy the spaces between polymer chains. Once they were degraded and removed through the thermal treatment, it was hypothesized that the preoccupied spaces would remain empty due to the glassy nature of the matrix at the heat treatment temperature, thus resulting in more free volume. Two post- modification strategies including grafting and blending were utilized to incorporate the volatile components, poly(propylene oxide) and poly(ethylene oxide). Post-modified polybenzimidazole films impressively showed significant enhancements in both gas permeability and selectivity for H₂/CO₂ separation. The H₂ permeability of the post-modified TADPS-OBA polybenzimidazole increased from 3.1-6.2 Barrers to 5.2-7.5 Barrers (up to 66% increase). The selectivity for H₂/CO₂ increased from 7.5-10.5 to 10.1-13.0 (up to 33% increase). The study on the potential effects of water vapor on the separation performance of PBI membranes was discussed in the appendix. / Ph. D. / Polybenzimidazoles represent a class of polymeric high performance materials due to their remarkable thermal stability, mechanical properties and chemical resistance. They are competitive material candidates for applications involving extreme conditions including high pressure and high temperature. The following studies are focused on the synthesis, characterization and properties of polybenzimidazoles and polybenzimidazole based copolymers and blends. Of particular importance to this dissertation are the gas transport properties. The new materials are excellent candidates for making non-porous membranes that can separate very small molecules such as nitrogen, oxygen, carbon dioxide, and hydrogen. The non-porous membranes achieve separations of such small molecules by having the gases solubilize in the upstream side of a membrane, diffuse through it, then evaporate from the downstream side. This mechanism is known as the solution-diffusion mechanism. The monomer, 3,3’,4,4’-tetraaminodiphenylsulfone, was synthesized via our designed synthesis method that was simpler than previous methods described in the literature and with a 3 times higher yield. A series of polybenzimidazoles with systematically varied chemical structures were prepared and it was demonstrated that they all had enhanced solubilities in common organic solvents over the only known commercial polybenzimidazole, Celazole®. This is particularly important for membrane materials because they must be fabricated into thin films from solution. Remarkable thermal stabilities for polymeric materials with glass transition temperatures above 400 °C were found for these polybenzimidazoles. A well designed film casting process was investigated and established. Polybenzimidazoles were fabricated into transparent thin films (20-30 µm thick) and their gas transport properties were measured. These novel polybenzimidazole films exhibited extraordinary gas separation properties, especially for H₂/CO₂ separation. The gas transport properties involve two important parameters, permeability and selectivity. A trade-off relationship between the two parameters was discovered by Robeson in 1991. The ultimate goal for developing gas separation membranes is to improve permeability and selectivity at the same time. In order to improve gas permeability, we hypothesized a concept that increasing permeability could be achieved by creating more spaces between the polymer chains in non-porous films. Sacrificial components were introduced into the films, then thermally degraded and the byproducts were volatilized to remove them from the film. It was further hypothesized that conducting the heat treatment process at a temperature where the matrix polymer was in the glassy state would allow the matrix polymer to preserve the free volume introduced by the volatization. Two post-modification strategies including grafting and blending were utilized to incorporate the volatile components, poly(propylene oxide) and poly(ethylene oxide). Post-modified polybenzimidazole films impressively showed significant enhancements in both gas permeability and selectivity for H₂/CO₂ separation. This is an important separation that could economically be carried out at elevated temperatures (~250°C) if the polymer membrane would withstand such a temperature. It could be utilized to separate H₂ from CO₂ in pre-combustion syngas. This is the major method for H₂ production worldwide. The study on the potential effects of water vapor on the separation performance of PBI membranes was discussed in the appendix.