Page 232 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

CHAPTER

8
Interphase mass transfer

8.1 Introduction
Interphase mass transfer is the basic transport process involved in all separation processes based on
concentration difference. In practice mass transfer is seldom stand-alone and is accompanied by heat
and momentum transport and/or chemical reaction. For conventional separation processes, namely
distillation, absorption, stripping, adsorption, extraction, and leaching mass transfer governs the
overall rate and their design is based primarily on mass transfer and equilibrium considerations. On the
other hand, in processes like drying, humidification, dehumidification, evaporative water cooling (in
cooling towers), and membrane separation, the respective overall rates are not limited by mass transfer
and their design is dealt with other transport processes (commonly heat transfer) as the controlling
phenomena. Heterogeneous reaction systems which may involve mass transfer that sometimes maybe
the rate-limiting step. Design of such reaction systems uses a different approach based on apparent/
overall reaction kinetic rate that includes the effect of mass transfer.
In this section of the book, we focus only on the design of traditional mass transfer processes, that is,
absorption, stripping, distillation, adsorption and extraction. Three steps are always involved in these
processes - (i) creation of a two-phase system, (ii) mass transfer between phases and (iii) separation of
the phases. Each mass transfer system in its basic configuration is built around an arrangement for
contacting the phases and auxiliary systems for supply/removal of heat and handling the fluids.
Distillationexploitsthe difference involatilityofthe componentstobeseparated,while absorptionand
stripping are separation processes based on the difference in solubility of the gaseous constituent(s) in the
contacting liquid phase. Absorption refers to the transfer of one or more components from the gas to the
liquid phase in which it is soluble. Stripping is the reverse operation, where the component transfer is
from the liquid to the gas phase. In distillation, the vapor and the liquid phases of the same component(s)
are contacted, and separation occurs by the transfer of the more volatile component(s) from the liquid to
the vapor phase and the less volatile component(s) from the vapor to the liquid phase. Accordingly, all
the components are present in both vapor and liquid phase during distillation. Extraction involves mass
transfer between two immiscible liquid phases. An external stream, which is a liquid solvent, is added (to
the feed) to create the immiscible phases. It is often used when the breaking of an azeotrope is difficult,
or the volatilities of the components are too close, making the separation by distillation nearly impossible
or uneconomical. An example is the separation of aromatic and paraffinic hydrocarbons of nearly the
same molecular weight present in kerosene. Industrially, these aromatic compounds are separated by
extraction using liquid sulfur dioxide as a solvent. Both adsorption and leaching involve solid-fluid
(liquid/gas) mass transfer and are governed by fluid-solid equilibria. Adsorption is a surface
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