Tower-top temperature

To answer this fundamental question, we should realize that reducing the tower pressure will also reduce both the tower-top temperature... 

The alcohol is called the light component, because it boils at a lower temperature then water the water is called the heavy component, because it boils at a higher temperature then 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. 4.1 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. 

When we raise the top reflux rate to our preflash tower, the tower-top temperature goes down. This is a sign that we are washing out from the upflowing vapors, more of the heavier or higher-molecular-weight, components in the overhead product. Of course, that is why we raised the reflux rate. So the reduction in tower-top temperature is good. 

In this equation, if the weight of gas goes up by 10 percent, and the molecular weight of the gas goes down by 8 percent, then the volume of gas goes up by 18 percent. The reduction in the tower-top temperature of 20°F does shrink the gas by about 2 percent, as a result of the temperature reduction, so that the net effect of raising the reflux rate is to increase the gas volume through the top tray by 16 percent (i.e., 18 per-... 

It is normal to assume that the vapor leaving the top of a tower is at its dew point. That is, it is at equilibrium with the liquid on the top tray of the tower. Unfortunately, this assumption falls apart if the tower is flooding and liquid is being entrained overhead from the column, with the vapor. However, assuming a normal, nonflooded condition, we will guess that the tower-top temperature is 140°F. Using the vapor-pressure curves provided in Fig. 9.1, we would calculate as follows ... 

The tower-top pressure is still 190 psia. The tower-top temperature will have to be reduced. Let s guess that it will be reduced to 130°F ... 

Or PT = 196 psia. But this is the calculated tower pressure. The actual tower pressure is only 190 psia. Try to repeat this calculation to get the correct tower-top temperature (answer 128°F). 

The 200°F stripper tower-top temperature is the dew point of the vapors leaving the top tray. Most of these vapors are steam, and that is why the tower-top temperature is so high. The high steam content of the overhead vapors causes a water stripper to behave in a strange way When the top reflux rate is increased, the tower-top temperature goes up, not down. This odd behavior is easily understood if we note that there is no liquid product made from the reflux drum. Therefore, the only way to increase the reflux rate, without losing the level in the reflux drum, is to increase the steam rate to the bottom of the stripper. The extra stripping steam drives up the tower-top temperature. 

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. 

The chief operator also insisted that lowering the vacuum tower-top temperature too much would hurt the vacuum. But why There is no doubt that the colder the tower-top temperature, the less the heat-duty load for the precondenser to absorb. Hence, cooling the vacuum tower-top temperature should, and did, reduce the precondenser vapor outlet temperature. This should have reduced the vapor load to the downstream jet. But it didn t. Here is why ... 

Increasing the tower-top temperature would distill over more pounds of naphtha. 

Of course, if the vacuum tower-top temperature became too high, the increase in the precondenser vapor outlet temperature would increase the vapor pressure of water. This factor would then limit the minimum pressure in the precondenser. 

Figure 1.3 shows a single drum overhead system. Double drum systems are also used. The difference between the two systems is the reflux temperature at the top of the tower. In the single drum system, total liquid condensation occurs in the overhead condensers. The reflux will be cool and will keep the tower top cool. It is advisable to check the hydrochloric acid dew point vs partial pressure to determine the anticipated location of corrosion. For example, tower top temperatures above 250° F (120°C) can transfer corrosion to the cold reflux. Where dew point conditions exist in the tower, It may be desirable to add ammonia to the reflux to neutralize the acid. 

A double drum system operates with a high tower top temperature that is above the dew point of the water-hydrogen chloride solution. A heat exchange with crude or another stream condenses only hydrocarbon in the first drum. This hydrocarbon, usually called heavy naphtha, is a hot reflux that controls... 

F (10.5 psia) to the tower top temperature of 111°F (1.3 psia). To transfer this heat through the reboilers, steam at 233°F (22 psia) was required. Because this heat is needed only at relatively low temperatures, it is very inefficient to obtain it by burning fuel under boilers, without making use of the heat at higher temperatures first. Two possible ways of providing low-temperature heat more efficiently are these ... 

Overhead Temperature In hot weather, the tower overhead fin fan condenser could be limited and thus the tower top temperature can go up. As a result, valuable components could be vaporized into overhead vapor leading to yield loss. There are a number of ways to reduce the overhead temperature such as increasing cooling water rate, turning on spare overhead fan for air cooler, and... 

Reduce the tower top reflux flow maintain the tower top temperature. 

Low flash-zone temperature. Have the instrument mechanic check the furnace outlet thermocouple. The optimum tower top temperature for a vacuum tower equipped with a precondenser is usually not the minimum temjjerature. As the tower top temperature is raised, heavy naphtha boiling-range materials are flashed overhead into the precondenser. Acting as an absorption oil, they absorb a portion of the light hydrocarbons that would otherwise overload the jets. However, getting the vacuum tower top too hot can overload the precondensers. By field trials, find the tower top temperature (usually 230°F to 280°F), that minimizes flash-zone pressure. 

Now suppose the refinery crude unit that contributes feed to the butane splitter suddenly increases the propane content of its butane product. Assume this change raises the propane content in the splitter s feed by 20%. If the tower top temperature is maintained at 140°F, the isobutane product composition would be 13% propane, 66% isobutane, and 21% normal butane. 

The tower top temperature and gross volume of overhead vapors are not high enough to force the moisture in the feed to be evaporated overhead. The tower bottom temperature and boil-up rate are too high to permit all... 

Operators may attempt to maximize heavy naphtha production at the expense of light naphtha by increasing the fractionator top reflux rate, which drops the tower top temperature. The water vapor in the overhead hydrocarbon vapors begins condensing at its dew point. If the tower top temperature is too low, water will condense on the top trays. This water is corrosive and will eat holes in the tray decks. Over a period of months, the degree of separation between light and heavy naphtha will consequently deteriorate. 

A sudden rise in vacuum tower top temperature, which occurs simultaneously with the onset of cold weather, can be due to freeze-ups of the seal legs. Plugging with waxy deposits is also possible. 

Vapor pressure of water at the tower top temperature, psia (look this up in a steam table)... 

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.)... 

There are two ways to answer this question. Let s first look at the reboiler. As the tower-top temperature shown in Fig. 7.1 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... 

A reduction in tower-top temperature of 20°F would increase the weight flow of vapor by roughly 10 percent. But the composition of... 

Solving for P, we find the calculated tower-top pressure equals 167 psia. But the actual tower-top pressure is 190 psia. Evidently, we guessed too low a temperature. Try the calculation again yourself with a better guess for the tower-top temperature (answer 146°F). 

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