Abstract
Among the adsorption-based separation processes for gaseous mixtures, those exploiting pressure variations, so-called Pressure Swing Adsorption (PSA) processes, are the most popular. In this work, we focus on the specific PSA configuration known as Dual Reflux-Pressure Swing Adsorption (DR-PSA) given its ability to achieve sharp separations. In the case of binary mixtures, an analytical approach based on Equilibrium Theory has been proposed to identify the operating conditions for complete separation under the assumption of linear isotherms. This same approach is not available when the separation is not complete. Accordingly, in this work we study the features of non-complete separations by solving numerically a general DR-PSA model with parameter values suitable to approach equilibrium conditions (no mass transport resistances, no axial mixing, isothermal conditions and no pressure drop), thus reproducing the analytical solution when complete separations are examined. Even for non-complete separations, triangularly shaped regions at constant purity can be identified on a plane whose axes correspond to suitable design parameters. Moreover, we found a general indication on how to select the lateral feed injection position to limit the loss in product purities when complete separation is not established, whatever is the composition of the feeding mixture. Finally, a sensitivity analysis with respect to pressure ratio, light reflux ratio and heavy product flowrate is proposed in order to assess how to recover product purities according to the specific degrees of freedom of a DR-PSA apparatus.
Highlights
In recent years, economical and energy-efficient separation processes are more and more welcome since they represent a large part of the costs in chemical, petrochemical and related industry (Sholl and Lively 2016)
Even if all the results reported in this work have been obtained through a model implementing linear adsorption isotherms and using parameter values suitable to fulfil most of the assumptions behind the analytical solution based on Equilibrium Theory, some interesting general conclusions can be offered:
The loose of purity outside the Triangular Operating Zone (TOZ) is much sharper on the left of the optimal feed position than on the right; this means that for a practical process design, the feed position located on the right of the optimal value predicted by the Equilibrium Theory makes the process more robust;
Summary
PU Purge step qi Amount of i on the solid adsorbent, mol kg q∗i rAiummoucnotnodfitiioonns,thmkegolsolid adsorbent in equilib-. T Temperature, K t Time,s t0 Starting time of a step, s u Superficial velocity, u volumeatrriecaflowrate , m s uOUT Superficial velocity in Z = L during a given step yi,feed Gas molar fraction of i in the lateral feed injection flow y Average gas molar fraction of i in i,H. RT t0,FE +tFE t0,FE uOUT FE PH RT dt of dt i in yi,OUTFE Gas molar fraction of i in Z = L during the FE step yi Gas molar fraction of i zfeed Feed injection position, dimensionless zfeed = Zfeed∕zrif z Axial coordinate, dimensionless z = Z∕zrif
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