Reflux rate increasing

Increasing the tower-top reflux rate increases the rate (in ft3/s) of vapor flow through the trays, because of the combined, additive affect of factors 1 and 2. 

Refluxing improves the separation that is achieved in most distillation columns. Any reflux rate increase, however, requires an increase in the rate of vapor production at the bottom of the column and hence an increase in energy consumption. 

For total-reflux distillations carried out in packed columns, regions of loading and flooding are identified by their effects on mass-transfer efficiency, as shown in Fig. 14-47. Gas and liquid rate increase... 

Increase in the use of main column overhead reflux rate instead of top pumparound to control the top temperature... 

Variable reflux, where the reflux rate is varied throughout the distillation to produce a fixed overhead composition. The reflux ratio will need to be progressively increased as the fraction of the more volatile component in the base of the still decreases. 

Stadler and Kappe [52] re-examined this synthesis using a Milestone ETHOS 1600 series MW reactor [23] with on-line temperature and pressure control in order to investigate the existence of a nonthermal MW effect. They found, however, that when the reactions were performed under the action of MW heating using a reflux condenser no significant rate increase occurred over conventional reflux at the same temperature (80 °C). They confirmed that there was an increase in rate and yield when the reaction was performed in an open beaker, but this could readily be explained by evaporation of the solvent, causing an increase in temperature and concentration solution. It is to be noted that the same explanation applies for the appar-... 

As mentioned earlier, a number of reactions initially observed to show MW rate enhancements compared with conventionally heated reactions at the same temperature, have since, with more careful comparison, been shown to occur at the same rate under the two heating modes. Other reactions, such as Knoevenagel reactions in ethanol solution (vide supra, Schs. 15 and 16), have been shown to have modest rate enhancements, occurring typically 2 or 3 times faster under MW heating than under conventional heating at the same temperature. These rate increases are not surprising considering that solvents superheat by 10 °C or more on MW reflux, particularly as the reaction mixtures were not stirred. 

Calculation of the pressure drop and flooding rate is particularly important for vacuum columns, in which the pressure may increase severalfold from the top to the bottom of the column. When a heat-sensitive liquid is distilled, the maximum temperature, and hence the pressure, at the bottom of the column is limited and hence the vapour rate must not exceed a certain value. In a vacuum column, the throughput is very low because of the high specific volume of the vapour, and the liquid reflux rate is generally so low that the liquid flow has little effect on the pressure drop. The pressure drop can be calculated by applying equation 4.15 over a differential height and integrating. Thus ... 

Further increases in the reflux rate, then act to increase, rather than decrease, the butane content of the overhead propane product... 

Figure 1.10 illustrates this point, from plant test data obtained in a Texas refinery. Point A is called the incipient flood point, that point in the towers operation at which either an increase or a decrease in the reflux rate results in a loss of separation efficiency. You might call this the optimum reflux rate, that would be an alternate description of the incipient flood point, neglecting the energy cost of the reboiler steam. 

K = 0.18 to 0.25 tray operation close to its best efficiency point K = 0.35 to 0.40 tray suffering from entrainment—increase in reflux rate, noticeably reduces tray efficiency K = >0.5 tray is in fully developed flood—opening a vent on the overhead vapor line will blow out liquid, with the vapor K = 0.10 to 0.12 tray deck is suffering from low tray efficiency, due to tray deck leaking... 

Note that the relative volatility has increased by about 20 percent at the lower temperature and pressure. This increase in relative volatility allows one to make a better split at a given reflux rate, or to make the same split at a lower reflux rate. We can quantify this last statement as follows ... 

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. 

The statement that the mass, or weight flow of vapor through the trays, increases as the refluxed 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 ... 

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, then will always push the tray closer to, or even beyond, the point of incipient flood. 

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 ... 

Consider the following. When the operator raises the top reflux rate, what happens to the weight flow of vapor going to the top tray Recalling that the external heat input to this tower is constant, does the pounds per hour of vapor flowing to the top tray increase, remain the same, or decrease The correct answer is increase. But why ... 

But what is vaporizing The reflux, of course. The sensible-heat content of the vapors, which is reduced when the reflux rate is increased, is converted to latent heat, as the vapors partially vaporize the incremental reflux flow. 

As the reflux rate is raised, the weight flow of vapor through the top tray, and to a lesser extent through all the trays below (except for the bottom tray), increases. This increase in the weight flow of vapor occurs even though the external heat input to the preflash tower is constant. The weight flow of vapor to the bottom tray is presumed to be solely a function of the pounds of vapor in the feed. 

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-... 

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. 

Now, let s increase the reflux rate. Certainly, the result will 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. 

---