Page 233 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

232 Chapter 8 Interphase mass transfer

phenomenon, and it involves the transfer of specific component(s) from the gas/liquid phase to the
surface of the solid adsorbent, where it adheres due to preferential affinity. Leaching is the preferential
dissolution of one or more components from a solid mixture by contact with a liquid solvent. The
appropriately chosen solvent dissolves specific components so that the desired solute can be removed.
Creation of the phases: In case of distillation, the contacting vapor and liquid phases are (normally)
derived from the same source, that is, the process feed by addition of heat. The other mass transfer
processes use a stream different from the feed to create the additional phase. The additional phase is
created by the introduction of a liquid stream (absorbent/solvent) in case of absorption, leaching, and
extraction; an external vapor stream that is referred to as the inert stream in case of stripping and a solid
adsorbent contacting the fluid (liquid/gas) in case of adsorption. Thus, for separation of a binary
mixture, the minimum number of components is two for distillation and three for the other operations
(see Table 8.1), while the number of contacting phases involved is two for all the cases.
Recovery and recycle: Design of systems for processes using an external stream to create the
second phase (as outlined above) will usually include an additional facility for recovery/regen-
eration of the said stream for reuse/recycling. This is done to improve the economy of the process.
Thus, while the distillation system may be complete in itself, the other processes in order to be
economic will have absorbent/solvent recovery section or adsorbent regeneration section, as the
case may be.

8.2 Processes and equipment
Distillation separates the feed to streams that are relatively pure. This gives the distillation process an
edge over the other processes, where the substance to be separated is collected in a dilute form in the
auxiliary phase(s) and may require a costly recovery step. Distillation requires energy to be supplied in
the form of heat that can subsequently be easily recovered/removed from the streams.
Distillation, absorption, stripping, and extraction, normally use the contacting columns. The two
phases are brought into intimate contact either in a batch or a continuous flow device where the two
phases exhibit cross (extraction) or countercurrent flow (absorption, stripping). Several stages of
interphase mass transfer are present in the same physical column with counterflowing phases. Inter-
phase mass transfer occurs because the contacting phases are not in thermodynamic equilibrium. The
column may be fitted with internals (packings and trays) to promote contacting and also to improve
separation of phases (chimney trays and demister pads) for the streams leaving the equipment.
The contacting equipment can also be broadly classified based on the phase dispersed. The gas
phase is dispersed as bubbles in bubble columns (sparged vessels), mechanically agitated vessels
and tray towers. Liquid phase is dispersed as thin films in wetted wall columns, packed bed, and as
droplets in spray towers and venturi scrubbers. Tray and packed towers are the most common in-
dustrial equipment for gas-liquid and liquid-liquid contacting. They are used for both continuous
andbatch processes. Stirredtanks followedbysettler vessels are used for extraction and leaching
and also for solid adsorbents being contacted with the liquid phase. The phase that has the higher
mass transfer resistance is chosen as the dispersed phase. This is done to reduce the diffusion length
in the controlling phase and also to increase its interfacial area. Bubble columns are, therefore,
used when the resistance is gas-phase controlled and spray towers for larger resistance in the liquid
phase.
Typical features of the different mass transfer processes are shown in Table 8.1. It may, however, be
kept in mind that the entries in the table are only for general information and deviations/variations are
possible for specific applications.