Vapor flow reflux effects

In the column model discussed in Chapters 3 through 13, the internal liquid and vapor flows were considered only from the standpoint of their effect on the thermodynamic performance of the column. The performance parameters that were investigated included quantities such as compositions, temperatures, pressures, enthalpies, and A -values. The column was assumed capable of physically handling any liquid or vapor flow rate, regardless of hydraulic effects or pressure drops. The only flow limitations that were taken into account involved minimum reflux ratio and conditions under which the liquid or vapor dried up, or approached zero flow in certain parts of the column. 

For given feed and product streams, only one of heat effects, or q, is independent and subject to choice by the designer or operator. In designing a column, q is usually chosen to correspond to the desired reflux ratio and moles of overhead vapor. Then q can be calculated from Eq. (18.44). However, in operating a column, q is often varied to change the vapor flow rate and reflux ratio, and changes in q then follow. 

In a sidestream column, changing the flow rates of liquid streams above or at a product withdrawal location affects the composition of that product and all other products below it. Changing the flow rate of the sidestream has little effect on the products above it. Therefore, sidestream composition can be controlled by vapor boilup, reflux flow rate, liquid split, or sidestream flow rate. As discussed later, we want to control a composition near the top of the prefractionator, and the logical manipulated variable to achieve this control is the liquid split. If reflux is used for distillate control, we are left with using either sidestream flow rate or vapor boilup for the control of the sidestream composition. 

However, the customer needs for component B change this picture completely. Component B is a reactant fed to multiple batch reactors that operate virtually independently of each other. It is also required to be very high purity. This means that the overhead vapor flow from C2 changes instantaneously at the will of the batch reactors since no storage of B can exist. The key process variables with the most significant effect on product quality are then identified to be the reflux to feed ratio in C2, the control temperature in C2 (for composition control of S), and the control temperature in Cl (for composition control of A). The control strategy must then be designed to keep these process variables on-aim and also to satisfy the on-demand requirement for production rate. 

The simple distillation curve is the temperature as a function of the per cent distilled in a simple or Rayleigh type of distillation. This type of distillation is approximated by the laboratory A.S.T.M. distillation which is widely used to characterize petroleum fractions. The A.S.T.M. procedure gives some reflux and rectification, and the results are not exactly equal to the simple batch distillation, although the difference is not large. The temperature normally measured is the condensation temperature of the vapor flowing from the still to the condenser. Curve A of Fig. 11-1 is typical for the simple distillation of a complex mixture. The temperature at any point is the averaged result of a large number of components and includes all the effects of nonideality in the solutions. Thus in most cases it is impossible to relate such a curve to the volatility of the individual components involved. As a result, such simple distillation curves are not of much direct value for the solution of rectification problems. 

With reflux on flow control, variations in vapor rate and reflux rate will both affect the flow of distillate. Earlier it was demonstrated that the distillate to feed ratio had the greatest effect on product quality. Therefore, it is imperative that distillate flow be controlled directly, instead of being subject to variations in vapor and reflux rates. If the flow of vapor leaving the top of the tower is the same as that generated... 

Scheme (2) is very effective in the absenee of noneondensable gases. Figure 11.10 shows how the heat transfer area ean be ehanged by flooding the condenser tubes with liquid. The reflux flow essentially sets the separation factor for the tower. If vapor flow into the condenser exceeds liquid flow out, condensate will rise to eover more heat transfer surface. This will cause a pressure rise, whieh in turn will reduce the heat input through the pressure controller. Beeause of the rapid response of vapor flow to heat input, this is a fairly fast loop. 

If the same disturbance happens while using the energy balance control scheme, the top quality controller detects a decrease in the distillate impurity and lowers the reflux flow. At the same time, the bottom quality controller increases the vapor flow, since the bottom impurity increases. The effect of a decrease in reflux and at the same time an increase in vapor flow, leads to a fast increase of the distillate flow. The result is that the new desired outpoint is reached fast, at the same time the changes in both quahties are more acceptable. 

Increasing the tower-top reflux rate increases the rate (in t /s) of vapor flow through the trays, because of the combined, additive effect of factors 1 and 2. 

In this chapter we will derive the dynamic relationships between internal reflux and both vapor rate and external reflux (or feed) as function of tray and column design. The basic tray hydraulic equations are based on the treatment by Van Winkle. First we discuss what happens on an individual tray, and then derive an approximate model for a combination of trays. Vapor flow will be assumed to occur without lags, and heat-storage effects will be assumed to be negligible. A discussion of reboiler dynamics will be deferred to Chapter 15. 

The design of distillation columns is commonly phrased in terms of vapor and liquid flows. For example, we will speak of vapor flow of 600 mol/sec of 99.7% propylene coming out of the top of a propylene propane distillation tower. We will consider the effect of changing the product purity by changing the reflux ratio, and hence the ratio of Hquid to vapor flows in the column. We will evaluate the effect of partially vaporizing some of our feed, and hence reducing the liquid flow in the lower, stripping section of our column. 

A flow diagram of an anhydrous phenol solvent extraction plant is shown in Figure 8. Raw distillate is passed through a tower in which it absorbs phenol from the recovery system vapor. The oil is then passed to the treating tower, generally a few sections above the bottom. Anhydrous phenol is introduced at the top of this tower. Phenolic water condensate from the solvent recovery system (about 9.5% phenol) is introduced at the bottom of the tower to effect reflux. A temperature gradient of 10° to 75° F. may be... 

When we increase the reflux rate, the tower-top temperature drops— let s say from 300 to 240°F. Actually, the temperature of the vapor leaving all the trays in the tower will decrease. The effect is bigger on the top tray, and gradually gets smaller, as the extra reflux flows down the tower. If the top-tray temperature has dropped by 60°F, then the vapor temperature leaving tray 9 might drop by only 5°F. Let s assume that the extra reflux causes the temperature of the vapor from tray 4 to decrease by 40°F. We can say that the sensible-heat content of the vapor has decreased. Sensible heat is a measure of the heat content of a vapor, due to its temperature. If the specific heat of the vapor is 0.5 Btu/[(lb)(°F)], then the decrease in the sensible-heat content of the vapor, when it cools by 40°F, is 20 Btu/lb. 

In an operating column, the effective reflux ratio will be increased by vapor condensed within the column due to heat leakage through the walls. With a well-lagged column, the heat loss will be small, and no allowance is normally made for this increased flow in design calculations. If a column is poorly insulated, changes in the internal reflux due to sudden changes in the external conditions, such as a sudden rainstorm, can have a noticeable effect on the column operation and control. 

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