Flooded reboiler

Flooded condensers and flooded reboilers are sometimes used on distillation columns. In the sketch below, a hquid level is held in the condenser, covering some of the tubes. Thus a variable amount of heat transfer area is available to condense the vapor. Column pressure can be controlled by changing the distillate (or reflux) drawoff rate. 

Changes in steam flow are achieved by increasing or decreasing the area used for condensing steam in the reboiler. This variable-area flooded reboiler is used in some processes because it permits the use of lower-pressure steam. However, as you will show in your calculations (I hope), the dynamic performance of this configuration is distinctly poorer than direct manipulation of steam flow. 

EXAMPLE 9.1. Consider the process with a series cascade control system sketched in Fig. 9.le. A typical example is a secondary loop in which the flow rate of condensate from a flooded reboiler is the manipulated variable M, the secondary variable is the flow rate of steam to the reboiler, and the primary variable is the temperature in a distillation column. We assume that the secondary controller Gci and the primary controller Gci are both proportional only. 

A special version of a flooded reboiler designed for low-boiling materials (requiring low-temperature steam) is shown in Figure 4.4. Steam to the reboiler is throttled in a conventional fashion—usually flow controlled or flow-ratio controlled—but there is neither a trap nor a condensate pot. Instead condensate is removed through a loop seal whose top is vented to atmosphere. The height... 

Use of lowest pressure steam available. In many plants excess low-pressure steam is availhigh-pressure steam. Where reboiler AT might be too small if the steam were throttled, c ie may use a partially flooded reboiler (see Chapters 4 and 15) and throttle ccmdensate. Since low-pressure steam is seldom available at constant pressure or steam quality, pressure and temperature compensation of flow measurements is highly desirable if steam is throtded instead of condensate. 

Preliminaiy signal flow diagram for flooded reboiler... 

Design experience with flooded reboilers is limited but indicates that typical time constants are of the order of 2—5 minutes. Simulation studies show that substantial improvement in response speed may be achieved by lead-lag compensations with transfer functions such as ... 

In Chapter 4, Section 2, we discussed a variation in flooded reboiler design for low-boiling materials. As shown by Figure 4.4, we throttle the steam instead of the condensate. The static force—balance relationship is given by equation (4.1) ... 

COLUMN-BASE LEVEL CONTROL VIA CONDENSATE THROTTLING FROM A FLOODED REBOILER (CASCADE LEVEL—FLOW CONTROL)... 

As a final example, let us look at the control of column-base levd by throttling condensate flow fi om the reboiler. We will ignore possible column inverse response but will take swell into account. Column acoustic impedance, seen fi om the base, will be assumed to be resistive only. The basic, open-loop signal flow diagram for the flooded reboiler is given in Figure 15.13. We need in addition the following relationships ... 

For a small number of flooded reboilers examined to date, the k i term is essentially constant from low to high heat loads that is, the relationship between heat load and tube vapor volume is a straight line. The compression effect due to process-side liquid-elevation changes has not been checked but is beheved to be small it is nc ected in this analysis. 

Preliminary signal flow diagram for column base level control via condensate throttling from a flooded reboiler... 

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