## Archive for the ‘Tower Pressure’ Category

## Effect of Feed Preheat

Up to this point, we have suggested that the weight flow of vapor up the tower is a function of the reboiler duty only. Certainly, this cannot be completely true. If we look at Fig. 6.2, it certainly seems that increasing the heat duty on the feed preheater will reduce the reboiler duty.

Let us assume that both the reflux rate and the overhead propane product rate are constant. This means that the total heat flow into the tower is constant. Or the sum of the reboiler duty plus the feed preheater duty is constant. If the steam flow to the feed preheater is increased, then it follows that the reboiler duty will fall. How does this increase in feed preheat affect the flow of vapor through the trays and the fractionation efficiency of the trays?

The bottom part of the tower in Fig. 6.2—that is, the portion below the feed inlet—is called the stripping section. The upper part of the tower—that is, the portion above the feed inlet—is called the absorption section.

Since both the reflux flow and the overhead product flow are constant in this problem, it follows that the weight flow of vapor leaving the top tray is also constant, regardless of the feed preheater duty. Actually, this statement is approximately true for all the trays in the top or absorption part of the tower. Another way of saying this is that the heat input to the tower above the feed tray is a constant.

But for the bottom stripping section trays, a reduction in reboiler duty will directly reduce the vapor flow from the reboiler to the bottom tray. This statement is approximately valid for all the trays in the stripping section of the tower.

As the flow of vapor through the absorption section trays is unaffected by feed preheat, the fractionation efficiency of the trays in the upper part of the tower will not change as feed preheat is increased. On the other hand, the reduced vapor flow through the stripping section may increase or decrease fractionation efficiency—but why?

## Heat-Balance Calculations

If you have read this far, and understood what you have read, you will readily understand the following calculation. It is simply a repetition, with numbers, of the discussion previously presented. However, you will require the following values to perform the calculations:

• Latent heat of condensation of alcohol vapors = 400 Btu/lb

• Latent heat of condensation of water vapors = 1000 Btu/lb

• Specific heat of alcohol (vapor or liquid) = 0.6 Btu/[(lb)(°F)]

• Specific heat of water = 1.0 Btu/[(lb)(°F)]

The term specific heat refers to the sensible-heat content of either vapor or liquid. The specific heat is the amount of heat needed to raise the temperature on one pound of the vapor or liquid by 1°F. The term latent heat refers to the heat of vaporization, or the heat of condensation, needed to vaporize or condense one pound of liquid or vapor at constant temperature. Note that the heat of condensation is equal to the heat of vaporization. Each is referred to as the latent heat. The sum of the sensible heat, plus the latent heat, is called the total heat content, or enthalpy.

Returning to our example in Fig. 6.1, we wish first to determine the reboiler duty. To do this, we have to supply three heat requirements:

A. Heat 9000 lb/h of water from the 100°F feed temperature to the tower-bottom temperature of 220°F.

B. Heat 1000 lb/h of alcohol from the 100°F feed temperature (where the alcohol is a liquid) to the tower overhead temperature of 160°F (where the alcohol is a vapor).

C. Vaporize 10,000 lb/h of reflux from the 150°F reflux drum temperature to the tower overhead temperature of 160°F.

Solution to step A:

9000 lb/h x 1.0 Btu/[(lb)(°F)] x (220°F – 100°F) = 1,080,000 Btu/h

Solution to step B:

1000 lb/h x 0.6 Btu/[(lb)(°F)] x (160°F – 100°F) + 1000 lb/h x 400 Btu/lb = 36,000 Btu/h + 400,000 Btu/h = 436,000 Btu/h

Solution to step C:

10,000 lb/h x 0.6 Btu/[(lb)(°F)] x (160°F – 150°F) + 10,000 lb/h x 400 Btu/lb = 60,000 Btu/h + 4,000,000 Btu/h = 4,060,000 Btu/h

The reboiler duty is then the sum of A + B + C = 5,576,000 Btu/h.

The next part of the problem is to determine the vapor flow to the bottom tray. If we assume that the vapor leaving the reboiler is essentially steam, then the latent heat of condensation of this vapor is 1000 Btu/lb. Hence the flow of vapor (all steam) to the bottom tray is

= 5,576,000 Btu/h / 1000 Btu/lb = 5576 lb/h

How about the condenser duty? That is calculated as follows:

11,000 lb/h x 0.6 Btu/[(lb)(°F)] x (160°F – 150°F) + 11,000 lb/h x 400 BTU/lb = 66,000 BTU/h + 4,400,000 BTU/h = 4,466,000 BTU/h

We can draw the following conclusions from this example:

• The condenser duty is usually a little smaller than the reboiler duty.

• Most of the reboiler heat duty usually goes into generating reflux.

• The flow of vapor up the tower is created by the reboiler.

For other applications, these statements may be less appropriate. This is especially so when the reflux rate is much smaller than the feed rate. But if you can grasp these calculations, then you can appreciate the concept of the reboiler acting as the engine to drive the distillation column.

## The Reboiler

All machines have drivers. A distillation column is also a machine, driven by a reboiler. It is the heat duty of the reboiler, supplemented by the heat content (enthalpy) of the feed, that provides the energy to make a split between light and heavy components. A useful example of the importance of the reboiler in distillation comes from the venerable use of sugar cane in my home state of Louisiana.

If the cut cane is left in the fields for a few months, its sugar content ferments to alcohol. Squeezing the cane then produces a rather lowproof alcoholic drink. Of course, one would naturally wish to concentrate the alcohol content by distillation, in the still shown in Fig. 6.1.

The alcohol is called the light component, because it boils at a lower temperature than water; the water is called the heavy component, because it boils at a higher temperature than alcohol. Raising the top reflux rate will lower the tower-top temperature and reduce the amount of the heavier component, water, in the overhead alcohol product. But what happens to the weight of vapor flowing up through the trays? Does the flow go up, go down, or remain the same?

There are two ways to answer this question. Let’s first look at the reboiler. As the tower-top temperature shown in Fig. 6.1 goes down, more of the lighter, lower-boiling-point alcohol is refluxed down the tower. The tower-bottom temperature begins to drop, and the steam flow to the reboiler is automatically increased by the action of the temperature recorder controller (TRC). As the steam flow to the reboiler increases, so does the reboiler duty (or energy injected into the tower in the form of heat). Almost all the reboiler heat or duty is converted to vaporization. We will prove this statement mathematically later in this chapter. The increased vapor leaving the reboiler then bubbles up through the trays, and hence the flow of vapor is seen to increase as the reflux rate is raised.

Now let’s look at the reflux drum. The incremental reflux flow comes from this drum. But the liquid in this drum comes from the condenser. The feed to the condenser is vapor from the top of the tower. Hence, as we increase the reflux flow, the vapor rate from the top of the tower must increase. One way of summarizing these results is to say that the reflux comes from the reboiler.

The statement that the mass, or weight flow of vapor through the trays, increases as the reflux rate is raised is based on the reboiler being on automatic temperature control. If the reboiler were on manual control, then the flow of steam and the reboiler heat duty would remain constant as the reflux rate was increased, and the weight flow of vapor up the tower would remain constant as the top reflux rate was increased. But the liquid level in the reflux drum would begin to drop. The reflux drum level recorder controller (LRC) would close off to catch to falling level, and the overhead product rate would drop in proportion to the increase in reflux rate. We can now draw some conclusions from the foregoing discussion:

• The flow of vapor leaving the top tray of the tower is equal to the flow of reflux, plus the flow of the alcohol overhead

product.

• The overhead condenser heat-removal duty is proportional to the reboiler heat duty.

• The weight flow of vapor in a tower is controlled by one factor and one factor only: heat.

An increase in reflux rate, assuming that the reboiler is on automatic temperature control, increases both the tray weir loading and the vapor velocity through the tray deck. This increases both the total tray pressure drop and the height of liquid in the tray’s downcomer. Increasing reflux rates, with the reboiler on automatic temperature control, will always push the tray closer to or even beyond the point of incipient flood.

## The Phase Rule in Distillation

This is perhaps an idea you remember from high school, but never quite understood. The phase rule corresponds to determining how many independent variables we can fix in a process before all the other variables become dependent variables. In a reflux drum, we can fix the temperature and composition of the liquid in the drum. The temperature and composition are called independent variables. The pressure in the drum could now be calculated from the chart in Fig. 5.4. The pressure is a dependent variable. The phase rule for the reflux drum system states that we can select any two variables arbitrarily (temperature, pressure, or composition), but then the remaining variable is fixed.

A simple distillation tower, like that shown in Fig. 5.2, also must obey its own phase rule. Here, because the distillation tower is a more complex system than the reflux drum, there are three independent variables that must be specified. The operator can choose from a large number of variables, but must select no more than three from the following list:

• Tower pressure

• Reflux rate, or reflux ratio

• Reboiler duty

• Tower-top temperature

• Tower-bottom temperature

• Overhead product rate

• Bottoms product rate

• Overhead product composition

• Bottoms product composition

## Incipient Flood Point

As an operator reduces the tower pressure, three effects occur simultaneously:

• Relative volatility increases.

• Tray deck leakage decreases.

• Entrainment, or spray height, increases.

The first two factors help make fractionation better, the last factor makes fractionation worse. How can an operator select the optimum tower pressure to maximize the benefits of enhanced relative volatility, and reduced tray deck dumping without unduly promoting jet flooding due to entrainment?

To answer this fundamental question, we should realize that reducing the tower pressure will also reduce both the tower-top temperature and the tower-bottom temperature. So the change in these temperatures, by themselves, is not particularly informative. But if we look at the difference between the bottom and top temperatures, this difference is an excellent indication of fractionation efficiency. The bigger this temperature difference, the better the split. For instance, if the tower-top and tower-bottom temperatures are the same for a 25-tray tower, what is the average tray efficiency? (Answer: 100 percent / 25 = 4 percent.)

Figure 5.5 illustrates this relationship. Point A is the incipient flood point. In this case, the incipient flood point is defined as the operating pressure that maximizes the temperature difference across the tower at a particular reflux rate. How, then, do we select the optimum tower pressure to obtain the best efficiency point for the trays? Answer: Look at the temperature profile across the column.

## Selecting an Optimum Tower Pressure

The process design engineer selects the tower design operating pressure as follows:

1. Determines the maximum cooling water or ambient air temperature that is typically expected on a hot summer day

in the locale where the plant is to be built.

2. Calculates the condenser outlet, or reflux drum temperature, that would result from the above water or air temperature.

3. Referring to Fig. 5.2, the designer calculates the pressure in the reflux drum, assuming that the condensed liquid is at its bubble point. Adding 5 or 10 psig to this pressure, for pressure loss in the overhead condenser and associated piping, the designer then determines the tower-top pressure.

Of course, the unit operator can physically deviate from this design pressure, but to what purpose?

## Optimizing Tower Operating Pressure

Why are distillation towers designed with controls that fix the tower pressure?

Naturally, we do not want to overpressure the tower and pop open the safety relief valve. Alternatively, if the tower pressure gets too low, we could not condense the reflux. Then the liquid level in the reflux drum would fall and the reflux pump would lose suction and cavitate. But assuming that we have plenty of condensing capacity and are operating well below the relief valve set pressure, why do we attempt to fix the tower pressure? Further, how do we know what pressure target to select?

I well remember one pentane-hexane splitter in Toronto. The tower simply could not make a decent split, regardless of the feed or reflux rate selected. The tower-top pressure was swinging between 12 and 20 psig. The flooded condenser pressure control valve, shown in Fig. 5.1, was operating between 5 and 15 percent open, and hence it was responding in a nonlinear fashion (most control valves work properly only at 20 to 75 percent open). The problem may be explained as follows.

The liquid on the tray deck was at its bubble, or boiling, point. A sudden decrease in the tower pressure caused the liquid to boil violently. The resulting surge in vapor flow promoted jet entrainment, or flooding.

Alternately, the vapor flowing between trays was at its dew point. A sudden increase in tower pressure caused a rapid condensation of this vapor and a loss in vapor velocity through the tray deck holes. The resulting loss in vapor flow caused the tray decks to dump.

Either way, erratic tower pressure results in alternating flooding and dumping, and therefore reduced tray efficiency. While gradual swings in pressure are quite acceptable, no tower can be expected to make a decent split with a rapidly fluctuating pressure.