Instrumentation & Control In Distilation Coloumn

INTRODUCTION :    Distillation is the most common class of separation processes

 and one of the better understood unit operations. It is an energy-separating-agent

 equilibrium process that uses the difference in relative volatility, or differences in

 boiling points, of the components to be separated. It is the most widely used 

method of separation in the process industries. The distillation process will 

most often be the choice of separation unless the following conditions exist: 

• Thermal damage can occur to the product.

• A separation factor is too close to unity.

• Extreme conditions of temperature or pressure are needed.

 • Economic value of products is low relative to energy costs. Control involves

 the manipulation of the material and energy balances in the distillation equipment

 to affect product composition and purity. 

Difficulties arise because of the multitude of potential variable interactions and

 disturbances that can exist in single-column fractionators and in the process that

 the column is a part of. Even seemingly identical columns will exhibit great diversity 

of operation in the field. Therefore, this section will not attempt to provide 

control strategies that can be applied.

Distillation separates a mixture by taking advantage of the difference in the composition

of a liquid and that of the vapor formed from that liquid. In the processing industries,

 distillation is widely used to isolate and purify volatile materials.

 Thus, good process control of the distillation process is vital to maximize 

the production of satisfactory purity end products. 

Although engineers often speak of controlling a distillation tower, 

many of the instruments actually are used to control the 

auxiliary equipment associated with the tower.

DISTILLATION EQUIPMENT 

There are some basic variations to the distillation process. One such basic 

difference is between continuous and batch distillation.

 The main difference between these processes is that in continuous distillation

 the feed concentration is relatively constant, while in batch distillation it is rich in light 

components at the beginning and lean in light components at the end.

 While batch distillation is also described in this section, the emphasis is on

 the continuous processes. Another basic difference is in the way the condenser 

heat is handled. The more common approach is to reject that heat into the cooling water

 and thereby waste it. This necessitates the use of “pay heat” at the reboiler, which 

usually is a large part of the total operating cost of the column. An alternate approach, 

also discussed in this section, is “vapor recompression” in which the heat taken out by 

the condenser is reused at the reboiler after a heat pump (compressor) elevates its temperature. 

While vapor recompression controls are also discussed in this section, the emphasis is on the 

traditional air- or water-cooled condenser designs.

The Column  The primary piece of distillation equipment is the main tower. 

Other terms for this piece of equipment are column and fractionator, and all three terms are 

used interchangeably. The tower, column, or fractionator has two purposes: 

First, it separates a feed into a vapor portion that ascends the column and a liquid portion 

that descends; second, it achieves intimate

In continuous distillation, the feed is introduced continuously into the side of the distillation column.

 If the feed is all liquid, the temperature at which it first starts to boil is called the bubble point. 

If the feed is all vapor, the temperature at which it first starts to condense is called the dew point. 

The feed entering the column is normally operated in a temperature range that is intermediate

 to the two extremes of dew point and bubble point. However, some optimization strategies may call for

designs where the feed is either superheated or subcooled. For effective separation of the feed,

 it is important that both vapor and liquid phases exist throughout the column. 

The separation of phases is accomplished by differences in vapor pressure,

 with the lighter vapor rising to the top of the column and the heavier liquid flowing to the bottom.

 The portion of the column above the feed is called the rectifying section and below the feed is called the stripping section

Batch Distillation

During the distillation process, the initial charge in the vessel continually depletes while building up

 the overhead product in the distillate receiver. Batch distillations are more common in smaller,

 multiproduct plants where the various products can only be manufactured at different times

, and where a number of different mixtures may be handled in the same equipment. is the 

basic equation that describes this operation: 

W = Wi − Dt 

 where W = amount remaining in the bottoms Wi = the initial charge D = distillate rate t = time period of operation

 The basic objective of the control system of this type of separation is to keep the composition of the distillate constant.

 Other goals include keeping the distillate flow constant or maximizing the total distillate production.

 The main goal of a batch distillation is to produce a product of specified composition at minimum cost. 

This often means that operating time must be reduced to some minimum while product purity or

 recovery is maintained within acceptable limits. If product removal is too fast, separation 

and the quantity of the product are reduced. Conversely, if the product is withdrawn to maintain separation,

 its withdrawal rate is reduced and operating time is increased. 

However, the set point to a composition controller can be programmed so that the average composition 

of the product will still be within specifications while withdrawal rate is maximized.

References 

1.       Lockett, M. J., Distillation Tray Fundamentals, Cambridge, MA: Cambridge Press, 1986

2.         Strigle, R. J., Jr., Random Packings and Packed Towers, Design, and Applications, Houston, TX: Gulf Publishing Company, 1987. 

3. Distillation: Basic Controls H. L. HOFFMAN, D. E. LUPFER (1970) L. A. KANE (1985) B. A. JENSEN (1995) B. A. JENSEN, B. G. LIPTÁK (2005)