Engineering Optimization

43Case Study 8: Horizontal Tank Vapor–Liquid Separator

43.1 Description and Analysis

The objective is to design a horizontal cylindrical tank to disengage liquid and vapor (see Figure 43.1). Two‐phase flow enters the top of the tank on one side, the liquid drops out to the lower portion of the tank, and the vapor to the upper portion. They exit at the other end of the tank, separately. Liquid at the bottom. Vapor at the top. As the vapor flows from entrance to exit, mist droplets gravity‐fall from the vapor to the liquid. The higher the vertical dimension of the head space, or the higher the vapor flow rate, the longer the tank must be for droplets to have time to fall out of the vapor. This places a constraint relating tank length to diameter. There are two additional constraints: the vapor flow rate must be small enough so that it does not make waves on the liquid surface and re‐entrain droplets. And the liquid holdup must provide a reservoir to provide continuity of downstream flow rate in spite of inflow pulses. The process tank designer can choose tank aspect ratio (l/d) and liquid level in the tank (f, as a fraction of diameter), and then lower‐cost tank diameter will be a consequence of compliance to the limiting constraint.

Figure 43.1 Illustration of a vapor–liquid separator, a horizontal right circular tank.

The objective is to minimize the capital cost of the tank: ...