Page 236 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

8.2 Processes and equipment 235

Most equipment used for these continuous processes involve multistage contacting.
There are two different design approaches for mass transfer equipment.
Equilibrium Stage approach: The number of equilibrium contacting stages (N Ideal ) required for
achieving the desired separation is estimated. Additional stages are provided to account for
nonequilibrium effects by considering a stage efficiency parameter h such that the actual number of
stages, N actual ¼ N Ideal /h. This ensures that the equipment is designed to achieve the desired perfor-
mance. The detail of the procedure and a discussion on stage efficiency is provided in Chapter 11 on
Distillation. This approach is adequate for binary mixtures and also for nearly ideal multi-component
mixtures.
Rate-based approach: The contacting efficiency (h) of a stage depends on the physical and
transport properties of the phases and the species getting transferred. It also depends on the hy-
drodynamic conditions (turbulence and mixing) resulting from the contactor geometry and flow
conditions (velocity, etc.) influencing the mass transfer coefficient (k) in the equipment. This
variation in k is taken care of in the rate-based approach. Unlike the equilibrium stage approach, it
considers nonequilibrium stages based on the physical details of the contacting stages (type, di-
mensions. etc.) and determines the number of actual stages (number of trays or packing height)
required for the desired separation. Accordingly, the mass and energy balances around each
equilibrium stage are replaced by separate balance equations for each phase around a stage.
Although the same equilibrium and enthalpy relations are used, phase equilibrium is considered to
exist only at the vapor-liquid interface on trays/packing and the enthalpies, concentrations, etc., are
evaluated at the conditions of phases exiting. Entrainment, occlusion, chemical reaction(s), etc.,
can also be added to the model. Rate-based approach is superior to the equilibrium-based models
particularly for nonideal systems and multicomponent mixtures. This is implemented in the
RATEFRAC module of the process simulator ASPEN and also in others like ChemSep Release 3.1
þ
and CHEMCAD.
The Design Problem: The common form in which a problem is posed states the requirement for
designing a separation system for an available feed stream. The feed stream composition and quantity
(in case of batch processing) or flow rate (in case of a continuous system) are known. Separation
performance target for the design is specified in terms of parameters, such as
(i) purity limits in terms of concentration of specific component(s) in the separated streams
(ii) recovery % of the desired component(s) from feed
(iii) limit of specific property values of the separated streams
The client often imposes additional constraints that may include but are not limited to:

(i) using a specific process or equipment (contactor) type
(ii) using specific solvent/inert/adsorbent
(iii) limitations on availability of hot and cold utilities
(iv) maximum limit of solvent/inert/adsorbent and/or utilities per unit of feed processed
(v) specified range of feed throughputeturndown ratio limit
(vi) maximum limit on investment required to build and operate the facility