Instrumentation And Control Of Distillation Towers

INDUSTRIAL PROCESS CONTROL: CONTROL OF DISTILLATION TOWERS





Distillation separates mixture on the basis of a difference in composition between liquid and vapour formed from the liquid. In the process industry, distillation is widely used to isolate and purify volatile materials.

Distillation Equipment 

The tower or column has two purposes: It separates the feed into vapour portion which ascends the column, and into liquid portion which descends the column. Achieves intimate mixing between the two counter current flowing phases.

1)

Column

Feed Pump

Preheat er

2)



Purpose of mixing is to get an effective transfer of the more volatile components into the ascending vapour and a corresponding

Distillation Equipment 

Column

Feed Pump

Preheat er





If the feed is liquid, the temperature at which it first starts to boil is called the bubble point. If the feed is vapour, the temperature at which it first starts to condense is called the dew point. The column is operated in a temperature range that usually intermediate to the two extremes of dew point and bubble point. 

For effective separation of the feed,

Distillation Equipment 

Column

Feed Pump

Preheat er

This interaction Condens between the counter er flow of vapour and liquid phase gives the perfect separation of Accumulator volatile components from liquid to vapour. Reflux

Reflux Pump Distillate

Boil-up

The overhead vapour leaving the column is sent to a cooler, and collected as a liquid in receiver.  



The bottom liquid leaving the column is heated in a reboiler. 

Reboiler 

Residue

A part is withdrawn as product or distillate. Remaining part is returned to the column as reflux.

Part of the liquid is withdrawn as bottom product or residue. Remaining part is vaporized and injected into the

Variables to Control 

Independent variables are: Column pressure.  Feed flow rate.  Feed composition.  Feed temperature.  Reboiler (boil-up) control.  Reflux control. 



Other variables are: 

Level control. 

Depends on the performance of independent variable controllers.

Pressure Control Most distillation control systems are based upon maintaining the column pressure at some constant value. 



Variation of pressure will upset the control system by changing the equilibrium condition of the material.

The set-point of pressure is the compromise between two extremes. 



The pressure must be high enough to cause condensation of the overhead vapour by heat exchange with the cooling medium usually water. The pressure must be low enough to cause vaporization of the bottom liquid by heat exchange with the heating medium usually steam.

Column Pressure Control with Liquid Distillate and Inerts Vent

PT PIC

PR C

Column pressure can be controlled by manipulating the  Presence of large heat transfer area inside the percentage of inert gas condenser. If the pressure is less, the pressure If the rate complicates of condensate large, of by liquid in this control isvalve is level closed accumulatorcontrol increases, which is sensed by the level using PRC so that liquid level problem. transmitter and opens valve, so the that the inside the the pipecontrol line just above  Uncondensable inert must liquid level remains constant. CV increases, which will decrease be removed otherwise it the heat transfer area, so that will and As the pressure condensation passed a accumulate certain point (due rateoperating is decreased blanket off gas), thepressure condensing to the presence of large PIC opens (and the inert column is this control valve so that inert gas with very small portion of surface. increased). vapour is removed (because, the density of inert gas is LIC very small, and LT ascends the condenser tube, which is not condensed). This control valve is an ON-OFF control valve.

Column Pressure Control with Liquid Distillate and Negligible Inert  

Water

PT



Most common. Maintenance costs are low because the valve is on the water line. For properly designed condenser, PRC need only proportional controller, as narrow throttling range is sufficient.

Here the heat transfer area is constant. Column  As the residence time of the water increases, it will increase the time lag by of the system. pressure is regulated manipulating the rate of  Wide proportional band is not sufficient. cooling medium. 

PR C

 

Need automatic reset to compensate for the load changes.

Control the condensate which will maintain the water rate constant. Keep the condenser 10 to 15ft below accumulator.

LT

LIC

Column Pressure Control with Vapour Distillate and Inert 

Water

PT

PR C

 LIC

LT

System pressure will quickly respond to changes in flow of cooling medium. Condenser will condense only enough condensate to provide the column with reflux.  Condenser

require a short residence time.

Vacuum System 

For some liquid mixtures, the temperature required to vaporize the feed would need to be so high that decomposition would result.  To

avoid this, it is necessary to operate the pressure below atmosphere.

Column Pressure Control with Liquid Distillate and Inerts Stea m

PR C

PT

PIC

LT

Control air Control valve valve of of air stream by PT, streamis isopened closed by so pressure PT, thatso air that air becomes greater than pressure becomes the accumulator Measurement of accumulator zero. pressure. pressure (which is almost  Column vapour is  equal Air to stream is flowed column pressure) flowed through the through less the ejector. introduces time lag than ejector. direct measurement of  the Column pressure  increases Column pressure column pressure decreases. 

Air

LIC

Click here to know the effect if column pressure P1 is close to the too small (or more atmospheric pressure vacuum).

Feed Control 

Only six variables need to be fixed inorder to predetermine all conditions in a distillation column. 



The column pressure can be the first controlled variable.

If fixed feed conditions are assumed, three more independent variables are determined.   

Feed rate. Feed composition. Feed temperature.

Feed Flow Rate 

FR C

FT

By holding the flow rate constant, best means of operation stability is obtained.  Narrow

is used. Feed

proportional band

Feed Composition 



Feed composition have greater influence on operation. Feed composition is seldom subject to adjustment.  Changes

are made elsewhere.

Feed Temperature 

To Column TR C

For efficient separation, it usually is desirable to have the feed at its bubble point when it enters the column. 

TT

S FR P C

FT



Steam

Constant temperature feed does not necessarily mean constant feed quality.  

Fee d

Unless the feed comes directly from some preceding distillation step, an outside source of heat is required.

If feed composition varies, its bubble point also varies. Set the temperature control at a point which is equivalent to the bubble point of heaviest feed.

Reboiler (Boil-up) Control 

 FR C

FT

Steam LT

LIC

Boil-up rate is controlled by setting the flow of heat to the reboiler. The amount of lighter boiling material in the bottom product is determined by the setpoint. 

Setting more steam will driven large amount of lighter material as boil-up.

Reflux Control 

PR C PR C

PT

 PT

FR C

FT LT

Since distillation achieves separation of materials by counter currently contacting vapour and liquid, reflux furnishes the continuous supply of liquid to the column. Level control shown is not an independent variable because, it reflux the difference between what comes into accumulator and what goes as reflux. LIC

Variable Column Feed 

LT

LR C

 FT

FR C

Any inadvertent changes which occurs in the first column would be reflected in the quantity and composition of the feed to the second column. Liquid level controllers can have a wide proportional band so that changes in the reboiler / accumulator level can swing over a wide range without drastically upsetting the flow of products. 

Second column will receive varying flow rate, which must be avoided.

Thank You