Continuous cocurrent processes in the nonsteady state

Abstract

Although continuous heat- and mass-transfer processes are usually conducted in countercurrent flow, there are certain cases in which the cocurrent flow can be used to some advantages from an operation point of view. For example, in the packed adsorption column, flooding due to high vapor or gas flow rate can be eliminated if cocurrent flow is employed. Use of cocurrent flow can also reduce the pressure drop requirement in the packed column. Reiss (1967) discussed several instances in which cocurrent mode operation may be favored over countercurrent flow. In the case of heat transfer, cocurrent flow is also suitable for some special situations. For instance, if it is necessary to limit the maximum temperature of cooler fluid or if it is important to change the temperature of at least one fluid rapidly (McCabe et al., 1985). The transient behaviors of both cocurrent and countercurrent processes are important in the startup and control of mass- and heat-transfer operations. Previously, numerical solutions based on a method of characteristics was used to study the transients of countercurrent processes (Tan and Spinner, 1984). Owing to the split boundary conditions, analytic solutions are difficult to obtain even for the linear system. For cocurrent flow, compact analytic solutions in terms of well-known tabulated function can be derived. The purpose of this note is to present the nonsteadystate behavior of continuous cocurrent flow of two contacting phases. The transient responses to the inlet disturbances are derived based on a simple mathematical model. This simplified model is equally applicable to either mass- or heat-transfer processes. Laplace transformation method is used to obtain compact analytic solutions. These solutions can be easily evaluated with the aid of the known tabulated mathematical functions or charts. Both the derivation and the final form of solutions are easier to apply in comparison to the work of Li (1986).