Arun S. Moharir

Heuristic design of pressure swing adsorption: a preliminary study

Abstract Due to its complicated nature and multiple decision parameters including plant dimensionality and operation condition, the design of pressure swing adsorption (PSA) processes is not a trivial task. Most previous studies on PSA design have been made through rigorous modeling and experimental investigation for specific separation tasks. General heuristics for a preliminary design of PSA processes are necessary but not well investigated so far. In this paper, we attempt to develop easy-to-use rules for PSA process design, based on analysis of the inherent properties of adsorbate–adsorbent systems (i.e. equilibrium isotherm, adsorption kinetics, shape of breakthrough curves, etc.) and simulation results. These rules include the selection of adsorbent, particle size, bed size, bed configuration, purge volume, pressure equalization and vacuum swing adsorption. Results of two case studies are presented to verify the rules proposed in this preliminary study.

Generic Mathematical Model for PSA Process

Adsorptive separations have gained considerable importance in process industry. A generic simulator has been developed in this work for Pressure Swing Adsorption (PSA) processes. Several simple and complex, conventional and unconventional PSA cycles have been studied using this simulator. Distinction has been made between PSA processes where a raffinate stream richer in the weakly adsorbed component as compared to the feed is the desired product as against processes where extract stream richer in the strongly adsorbed component is the desired product. These are termed as raffinate PSA and extract PSA respectively. Extract PSA is an unconventional process variation. The studies include several simple and complex PSA cycles for raffinate and extract PSA of industrial importance. The studies are aided by a generic simulator for all PSA process variations developed for the purpose. The simulator is equally applicable to Vacuum Swing Adsorption (VSA), Pressure Vacuum Swing Adsorption (PVSA), rapid cycle PSA processes, etc.

Natural Gas Treatment Using Adsorptive Separation

Natural Gas (NG) is used both as a process gas as well as a fuel. The two uses make contrary demands on its composition. Natural Gas is a mixture of Methane(C1), Ethane (C2), Propane (C3), Butane isomers (C4) and Pentanes, to name only the more significant components. It can have Carbon-Dioxide, Hexanes etc. as other components. Higher hydrocarbons are there as the gas comes off the well, but are removed in the gas treatment plants. Similarly, water, if any, is removed. Winterization and dehydration are two major aspects of gas treatment before it is transported to the user through pipelines [1].

Sr2+ Exchanged Zeolite X as an Adsorbent Material for Chromatographic Separation of Argon-Oxygen Gaseous Mixture

The separation of argon and oxygen from their gaseous mixture is very difficult to accomplish by the adsorption process using zeolite adsorbents due to the closeness of their molecular properties and adsorption behavior in the zeolites. Strontium exchanged zeolite X (SrX) showed the adsorption selectivity for oxygen over argon at ambient temperature and demonstrated its potential as a column–packing material for the gas chromatographic analysis of argon-oxygen mixtures. The adsorption capacities and Henrys law constants for oxygen and argon increased in SrX compared to NaX at ambient temperature. Activation of the SrX adsorbent is shown to play a highly significant role in the separation of argon-oxygen mixture due to the low hydrothermal stability of SrX.

Optimal Non-isothermal Reactor Network for Van de Vusse Reaction

For complex reactions the optimal reactor networks can involve several reactors operating at various temperature profiles. The often-reported strategy of optimizing parameters of a heuristically predetermined reactor system (Super-structure Approach) falls short of obtaining true solution due to the presence of multiple local optima. Attainable set method gives Global optimum but requires study of each reaction scheme in depth. Here one such study using phase-plane analysis (instead of convexity based analysis) is reported for finding globally optimal non-isothermal reactor network for van de Vusse reaction (A -> B -> C, 2A -> D, objective is to maximize yield of B). Compared to two-reactor networks proposed earlier, it is found that up to 5 reactors (CSTR with/without bypass of feed, Isothermal PFR, Non-isothermal PFR, CSTR, Isothermal PFR) may be required to get the highest yield of the desired intermediate. The proposed method involves only elementary calculus. The detailed solution algorithm has been described using analogy with highways. Three cases with the values of reaction constants reported in the literature have been solved.

Generalized reactor model: An object oriented approach to reactor modeling

Publisher Summary Industrial reactors differ from each other in many aspects, such as geometry, hydrodynamics, and reaction kinetics. This chapter focuses on the development of a generalized modeling tool that can accommodate these variations and help in the analysis, design, and synthesis of more novel reactor configurations. An object-oriented framework has been used for the reactor modeling and analysis. In the proposed framework, the design was accomplished by deconstructing the reactor model into simpler objects, such as reactor geometry, reaction network, and reaction kinetics, and then defining the state and behavior of these objects as well as their interrelationships. The implementation of the generalized model has been done in a Microsoft Component Object Model (COM) framework that allows the model to be used by any application or programming language that can function as a COM client. Case studies, involving industrially significant applications, have demonstrated the potential of the framework to solve reactor design and synthesis problems effectively.

Multi-cell model for pressure swing adsorption process

Pressure Swing Adsorption process is a discrete–continuous system by nature and it is extremely time consuming to simulate steady state performance for a given set of design and operating parameters. A multitude of design variations is offered by the configuration of Pressure Swing Adsorption cycle in terms of choice, sequence, and durations of various possible component steps implemented on two or more adsorber beds. Often, simplifying assumptions are made to speed up each simulation. These assumptions erode the quality of match between reality and simulation and make the resultant design approximate. Use of assumptions like no adsorption/desorption during the pressurization and blowdown steps, constancy in volumetric flow during the adsorption and purge steps makes the model computationally lighter but raises questions on its predictive power. A new modeling approach, namely Multi-cell Model is presented in this work. It is shown to avoid the extensive time taken with equation-based simulations and to have better predictive power. The model is used to study a representative Pressure Swing Adsorption process for nitrogen enrichment from the air. Numerical convergence with respect to the spatial and temporal step sizes and mass balance closure is verified. The model is generic in nature and is valid for any multi-bed, multi-adsorbent, multi-component Pressure Swing Adsorption process executing any combination of component steps.

Isothermal PFR/PMR Networks

Combinations of Plug Flow Reactor (PFR) and continuous Perfectly Mixed Reactor (PMR, also known as Continuous Stirred Tank Reactor - CSTR) are widely known to give superior performance over a single reactor especially when multiple reactions take place in the reactor. The occurrence of a PMR in an optimal reactor network requires presence of inflection condition in the space of variables describing the reactor objective. The mathematical equation for inflection of multi-dimensional trajectories is derived and applied to five cases of well-known models of kinetic schemes. The previously known results are confirmed or improved upon by applying the technique. A general algorithm for PFR/PMR network synthesis for arbitrary kinetic models is presented.