An Algebraic Modeling of Dual Reflux PSA Process for High Enrichment and Recovery of Dilute Adsorbate

A dual reflux PSA process that has two refluxes and an intermediate feed inlet position is analyzed theoretically by means of a simple analytic model to investigate the effect of the operating variables such as the feed inlet position and the reflux ratios. The model is based on the short cycle time approximation and gives a simple semi-algebraic solution. The optimum feed inlet position is mathematically proved to be the point where the adsorbate concentration in the column is equal to that in the feed gas. In addition, this optimum condition is not affected by the operating parameters and a form of adsorption isotherm. The effect of the reflux ratio is analyzed keeping the feed inlet position optimum. This analysis can hardly be carried out in experimental studies because the concentration distribution in the column is changed by the reflux ratio. This result shows that the reflux ratio greatly affects on the process performance and has an optimum value. The effect of the form of adsorption isotherm is also examined. This result suggests that there is an optimum form of isotherm which is related to the operating parameters. Introduction Pressure Swing Adsorption (PSA) is a gas separation technology that has rapidly developed mainly for air drying, air separation and hydrogen purification in the last 30 years. Recently, the application of PSA processes to other kinds of gases, such as carbon dioxide or organic compounds has been attracting the attention along with the development of new adsorbents. Some of the reasons for the rapid development of PSA are that the mechanism of PSA is simple and the operation of PSA processes is so easy that unattended operation is possible. Although PSA is convenient as a gas separation technology like this, it has some disadvantages. One of them is that the enrichment of adsorbable components has a limit. The limit is determined by a pressure ratio between an adsorption step and a desorption step, which is known as a thermodynamic limitation. The enrichment ratio to the adsorbate concentration in feed gas cannot exceed the pressure ratio. Therefore, the enrichment is very difficult when the adsorbate concentration in feed gas is low. This is caused by the PSA cycle steps consisted of a high pressure feed step and a low pressure purge step. These steps have been applying to most PSA processes. This type of process is called stripping reflux PSA (SR-PSA) by analogy with distillation. Generally, there is no limit in the enrichment of non-adsorbable components in the SR-PSA cycle. Thus, PSA is mostly used for the purification of the non-adsorbable components. For these reasons, PSA is rarely used for enriching dilute adsorbate. To produce pure adsorbate gas from a dilute mixture, it is necessary to increase the number of adsorption columns and utilize more complicated cycle sequences and a very high pressure ratio. However, these modifications cause an increase in initial and operating costs. To solve this problem, an interesting PSA process cycle has been proposed. In this process, the feed gas is supplied to a column at low pressure and part of the product gas is supplied to another column at high pressure as purge gas. In other words, this process has the inverse configuration to the SR-PSA configuration in terms of the pressure. This process is called enriching reflux PSA (ER-PSA). In the ER-PSA process, the adsorbable component can be enriched up to very high concentration and the enrichment is not limited by the pressure ratio anymore. This fact has been demonstrated experimentally by some recent studies . Also, it has been analyzed theoretically by a simplified model. However, contrary to the SR-PSA, the enrichment of the non-adsorbable components is limited by the pressure ratio. Eventually, the recovery of the adsorbate in the feed gas does not become high. Each of the above two processes respectively has a limit caused by the pressure ratio. The SR-PSA process can not enrich the adsorbable component, whereas, the ER-PSA process can not obtain high recovery of the adsorbate. Therefore neither of the two processes can simultaneously achieve the high enrichment and the high recovery of dilute adsorbate. In order to overcome this limitation, an amazing PSA process called Dual Reflux PSA (DR-PSA) was proposed by Diagne and co-workers. This process has two refluxes at both ends of the column and the feed gas is supplied to an intermediate position of the column. In a word, this is a combination of the SR and ER-PSA processes. By reprocessing adsorbate-enriched gas leaving the SRPSA in ER-PSA and inert-enriched gas leaving the ER-PSA in SR-PSA, the limitation is removed. Hence, the enrichment and the recovery of the adsorbate are no longer limited by the pressure ratio and it is determined simply by mass balance. This fact has been demonstrated experimentally and theoretically by recent studies. Few published studies on the theoretical analysis of the DR-PSA have considered the effects of a finite mass transfer rate and a non-linear adsorption isotherm. Therefore, we analyze the DR-PSA theoretically by means of a method called the short cycle time approximation. The method is a highly simplified model, but it involves a finite mass transfer rate and a non-linear isotherm. In the previous experimental studies, the feed inlet position and the reflux ratio were reported to have a great impact on the performance of the DR-PSA. These parameters also reported to have optimum values. The principal objective of this paper is to investigate the effects of the feed inlet position and the reflux ratio. In addition, the effect of the form of adsorption isotherm is examined. Process configuration A schematic diagram of the DR-PSA is shown in figure 1 for the system under consideration. By analogy with distillation, each column is divided into two sections (rectifying or enriching section and stripping section) at the feed inlet position. For the sake of simplicity, the DR-PSA process can be considered as a four-column and two-step (adsorption and desorption step) process. In the first step, the high pressure feed gas is supplied to an intermediate position of the high pressure column, at which this gas is added to the gas stream leaving column 4. The mixed gas is drawn into column 1, and then inert-enriched gas is obtained at the bottom of column 1. Part of the inert-enriched gas is recycled to column 2 after depressurization as a stripping reflux. Then adsorbate-enriched gas is flowed out of the top of column 2 and the gas is enriched further up to very high concentration in column 3 over the limitation of the pressure ratio. Part of the adsorbate-enriched gas produced at the top of column 3 is recycled to column 4 after compression. Next, in the second step, the first step is repeated with Bed 1, 4 and Bed 2, 3 changing roles. Each of the steps is switched after specific cycle time. Since the feed inlet position can be varied at any position, it is not necessary that all columns (1-4) have the same size. A stripping reflux ratio RS is defined as the ratio between the flow rate of the recycled inert-enriched gas and that of the inert-enriched gas product. An enriching reflux ratio RE is defined as the ratio between the flow rate of the recycled adsorbate-enriched gas and that of the adsorbate-enriched gas product.