HYDROGEN SEPARATION FROM PURGE GAS PERMEATION OPTIMAL DESIGN AND OPERATING CONDITIONS

Abstract

Gas permeation is increasingly being used and now it has become an important unit operation. Gas permeators are smaller in size and require small floor area. It has no moving parts, therefore it is leak proof and pollution free having low maintenance and operating cost. For the same duty it is having an advantage over adsorption, absorption and cryogenics process because of its low investment, low energy consumption and ease of operation. In ammonia plant, the recycled gas is purged out to keep the percentage of inerts constant. The purge gas contains about sixty percent hydrogen and rest are nitrogen, argon and methane. For the separation of hydrogen, gas permeators are increasingly being used economically in developed countries, whereas in our country cryogenics and pressure swing adsorption processes are common. However, there is scarcity of reported data on purge gas permeators, therefore, it was considered appropriate to study the performance of purge gas permeators for recovery of hydrogen. The hydrogen recovered from the purge gas can be recycled to the converter for ammonia synthesis and thus a lot of precious raw material, natural gas or naphtha, can be saved. The hollow fibre purge gas permeators considered for this study consist of a single stage, two stage or three stage system with asymmetric membrane. The feed outside mode, that is, feed flow on the shell side was considered because of its superiority to feed inside mode. Counter-current flow pattern was preferred over co-current because it was found to be better. Further, the advantage of recycling was considered in order to enhance recovery/permeate composition. (i) The work can be categorized into following parts : 1) A rigorous mathematical model for the single-stage hollow-fibre multicomponent gas mixture permeator may include a set of coupled nonlinear partial differential equations, by making an unsteady state mass, energy and momentum balance in three dimensional space along with the governing rate and equilibrium relations. However, in order to have the model relations in manageable form several simplifying assumptions were made. Based on this approach, for multicomponent gas permeation, plug flow model was reported in literature. In this study two mathematical models were developed for multicomponent gas permeation assuming that axial diffusion as well as deformation of fiber is taking place. Then a comparative study of plug flow model, deformation model and diffusion-deformation model was done in order to establish the significance of incorporating axial-diffusion and fibre deformation in the mathematical model. The model equations were manipulated to simplify computations. Analysis of the information flow structure of the model relations was done. It showed that assuming stagecut leads to noniterative computations. Here the stagecut is defined as a ratio of the permeate flow rate to the feed flow rate. Based on the solution algorithm an efficient computer program was developed to make a comparative study of the three models. (ii) The study showed that all the three models were predicting nearly the same performances, hence, the simpler plug flow model is good enough and reasonably accurate for investigation of purge gas permeation. 2) The single stage permeator was considered as a base case for comparison with other stage arrangement in order to establish optimal configuration. The effect of pressure ratio, stagecut/membrane area and the recycle of a fraction of the reject to the feed on recovery and permeate composition was studied. For the case of recycle the computation procedure required use of an efficient convergence technique. Analysis of the information flow structure suggested that the component flow rates that were input to the permeator may be taken as cut streams for faster convergence. Surprisingly, this technique resulted in a close match within three to five iterations. Based on the solution algorithm an efficient computer program was developed and used to investigate the performance of the single stage permeator with recycle. For this study the range of variables studied for both the stages were : stagecut 0.1 to 0.6, increment of 0.1; and pressure ratio 0.2 to 0.4, increment of 0.1. This required computations for a total of about 5000 data to obtain the converged results. The study showed that choice of membrane area is strongly dependent on pressure ratio and increase beyond a certain value is ineffective in improving (iii) the permeator performance. It was also observed that the recovery and permeate composition decreases, in general, with increase in pressure ratio and fraction of the reject recycled. 3) A two stage permeator was considered in which the reject of the first stage was the feed for the second stage and the permeate of the second stage was recycled and added to the feed of the first stage. Further, a three stage permeator was considered in which the reject of the first stage was input to the second stage and the reject of the second stage was input to third stage. The permeate of the third stage was recycled and mixed with the input to the second stage and the permeate of the second stage was recycled and mixed with the fresh feed. The performance of the two stage and three stage permeator with recycle was studied for separation of hydrogen from purge gas using hollow fiber membrane. The effect of pressure ratio, stagecut and membrane area on recovery and permeate composition was investigated. Based on the solution algorithm the computer program for single stage was modified and developed for two stage and three stage permeators in order to investigate the performance. For this study the range of variables studied for all the three stages were : stagecut 0.1 to 0.6, increment of 0.1: and pressure ratio 0.2 to 0.4, increment of 0.1. This required computations for a total of about 7500 data to obtain the converged results. (iv) •* The study showed that the permeate composition and recovery decreases with increase in pressure ratio for both tow stage and three stage permeator. It was also observed that the optimal number of stages in a multistage permeator are two, requiring optimal distribution of membrane area within the stages. For purge gas system, the total membrane area for a single fiber was found to be 3 cm2. Out of which first stage should have 1.25 cm2 and second stage should have 1.75 cm2. The related design and feed conditions were : feed pressure of 2500 kPa, feed temperature of 298 K, feed flow rate of 4.415*10"5 mol/s, number of fibers equal to ten, outer diameter and inner diameter of 3.0* 10-4 m and 1.5 * 104 m. respectively. (