Distillation ratios

In any plant distilling sea water directly to obtain a demineralized water, the ratios of pounds of water distilled per pound of product, and pounds of total material distilled per pound of product, are identical and are equal to unity. Therefore, if a competing process has a distillation ratio greater than unity, it is clearly uneconomical when compared to direct distillation. Table V shows either a three-stage or four-stage plant to fall in this category the ion exchange process, as visualized, involves more distillation than does direct distillation. 

A spinning band distillation setup is employed (800 Mirco Still, B/R Instrument Corporation, 3000 rpm, theorical plates 23-26). The mixture was heated under reflux for 3 hr to reach equilibrium before distillation (reflux/distillation ratio 5-10/1). 

The reflux ratio, R, for a column is the ratio of the liquid we reflux back to the column at the top stage relative to the distillate product flow i.e., R = L/D. where L is the liquid flow in the top section of our column. Suppose we operate our column with a fixed solvent-to-distillate ratio i.e., / , = S/D = constant. If R, and D are fixed, the location of the A point is fixed because... 

The control engineer has specified three control loops that he believes to be independent. One is to control ffie reflux/distillate ratio, the second to control the distillate/feed ratio, and the third to maintain top tray temperature. Comment on this proposed control scheme. 

Due to the above limitations, an energy balance control scheme such as scheme 16.7 is not recommended, but some situations exist where this scheme can offer better product composition control than many other alternatives. Superfractionators with a reflux to distillate ratio of 10 to 1 or more are one example. Here, distillate flow may be too small to satisfactorily control either accumulator level or column temperature. The author has experienced a satisfactory operation of scheme 16.7 in a propylene-propane splitter, with intermittent operator intervention to ac ust the material balance. The cycles in reflux and reboil (see above) could be tolerated, as the column was not operating close to its limits. In this column, scheme 16.7 gave tighter composition control than scheme 16.4d. 

Column acted like a strq>per, reflux-to-distillate ratio was 0.43. When reflux flow was on accumulator level control, a small change in heat input led to large changes in reflux flow. Reflux flow at times fell below the minimum required for tray wetting. 

Scheme 2. This scheme indirectly adjusts the material balance through the two level control loops. This arrangement has the advantage of reducing the ratio of effective dead time to total lag time within the composition loop. It has the disadvantage of allowing greater interaction between the material and energy balances because internal reflux is not held constant. This scheme should be considered when the reflux is smaller than other flows in the column and when the reflux to distillate ratio is 0.8 or less. It should also be considered for applications where reducing the ratio of effective dead time to total lag time in the composition loop is a significant and necessary consideration.

The highest energy consumption is at total reflux, that is, when all of the boilup is returned as reflux, and there is no feed to the column and no distillate or bottoms. A computer simulation for this separation at total reflux required a minimum number of 27.8 theoretical stages. This was accomplished by setting the reflux/distillate ratio to 10 million in a computer simulation. This is the shortest column that can achieve the desired separation. 

The reflux scheme can be difficult to start up because initially there may not be enough light components accumulated in the reflux drum, and the column temperature may be too hot so, the controller may want more reflux flow than is available. This can pump the reflux drum level down until the distillate flow stops and then proceed to pump the reflux drum empty. However, this situation can be handled with computer control by using a low-level constraint control that will constrain the reflux flow rate to maintain a low-level constraint setpoint until the column temperature is low enough, so the temperature controller calls for less reflux. This reflux scheme is recommended when the reflux/distillate ratio is less than... 

Because of the lambda effect, this system is operated at a reflux to distillate ratio of only 0.20 to 0.25. Although a slightly greater number of theoretical stages may be required at such a low reflux ratio, the HETP value realized will be lower [13]. With such a low reflux ratio, the liquid irrigation rate at the top of the column may be in the order of only 20 gal/ft h. A special design of liquid distributor is required to produce... 

Since reflux drum holdups are usually small compared with base holdups, a buffer tank in the top product line (not in the reflux line) is highly recommended. Top product composition may be controlled by trimming the steam/distillate ratio bottom composition may be controlled by trimming the bottom-prod-uct/distillate ratio. 

As shown in Figure 6.5, this scheme requires reflux/distillate and bottom-product/distillate ratio controls. A buffer tank in the top-product line is recommended. 

As indicated earlier, reflux drum level control via boilup is difficult unless a lot of holdup is available. Overhead composition may be controlled by trimming the reflux/distillate ratio base composition may be controlled by trimming the bottom-product/distillate ratio. Note that if either distillate or bottom product (or side product) is a demand flow, either reflux drum or base level control must manipulate feed rate. 

The method recommended here is an analytical solution for the reflux-to-distillate ratio which has been found to check very closely the values obtained from other more sophisticated calculation procedures. It has been of particular value in setting up and analyzing distillation problems on the computer. It involves a three-step procedure correlating key component vapor-liquid equilibrium at the feed point and the purity of the reflux liquid. 

---