Procedure for exergetic analysis of processes for obtaining ultrapure products by low-temperature fractionation

Abstract

In manufacturing processes, the term "ultrapure products" (UPP) is applied to substances in which the total content of impurities is no more than a few parts per million [1]. Even with such low impurity contents, the behavior of the mixture can be described by the laws of thermodynamics, since the minimum concentrations at which these laws are applicable are on the order of 10 -ll ppm [2, 3]. High purities can be achieved by various methods, of which the most important (from the standpoint of scale of production) are fractional distiUation, adsorption, and catalytic purification.We will examine certain questions related to UPP production in air separation units (ASUs), i.e., production of these ultrapure materials by low-temperature fractionation.In many cases, UPPs are obtained in ASUs in specially designed sections connected in series ( Fig. 1) with the main air separation section (ASS), from which process streams are taken off with concentrations corresponding to the accepted standards. Ultrapure oxygen, argon, and nitrogen are currently obtained in this sort of a processing scheme [4, 5]. In obtaining a UPP from a stream of a mixture G m in which the mole fraction of the major component (O) is approximately ~m ~ = 0.990--0.999, impurity components must be removed; these are classed as high-boiling impurities (HI) and low-boiling impurities (LI). In the actual section for obtaining UPPs, the purification process takes place in a number of series-connocted independent separation units, the number of which will depend on the set of impurities to be removed [4]. In order to segregate from the total expenditure of power N E the fraction of expenditure for obtaining the UPP (Nuo P) in the combined separation of air, and also in order to determine the poss~le means of improvement of the UPP production section, exergetic analysis of the process is required.The basic principles of the exergetic method are well known, so that it can be used effectively in analyzing many specific energy-technology processes [6]. At the same time, analysis of UPP production processes on the basis of the exergetic method has not yet been worked out adequately. The problems that have arisen in determining Nup P have been examined in part only in [7, 8].In order to determine the paths to take in improving any manufacturing process, it is necessary to determine not only the efficiency of the process as a whole, but also the components d i of the losses of exergy d~: = Ed i. Let us dwell in detail on the calculation of d~ of a separation unit in application to processes of obtaining a UPP by low-temperature fractionation.In the general case,where df = qc(e)[7"e c(e) --"~e c(e)] are the losses (specific) due to adiabatic performance of the fractionation process and the consequent nonequilibration of streams on the tray; qc(e) is the specific heat flux supplied to the condenser (or evaporator) of the column (determined from a calculation of the fractionation process); T e c(e) = (Tc(e) --Tam)/Tc(e) and ~e c(e) = (T --Tam)IT are exergetic temper...