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Distillation Control

  • Component segregation is achieved  by control of heat input 
  • Stream splitting is achieved by control of product flow
  • Column pressure is dictated by many factors as, condensing media temperature, heat sensitivity of the material, heating media temperature, relative volatility and so on.
  • Column pressure too high – feed system may be unable to input feed to the column. Allowable design pressure may exceed.
  • Column pressure too low – Product may not flow from system
  • Too high bottom temperature – Product degradation. 
  • Rapid Variation in Flow or pressure -Control of  u/s equipment becomes impossible.


Feedback control of pf outlet temperature

Distillation Control

Flow rate of HTM is adjusted as PF outlet temperature is perturbed.
Valve may be shown in either supply or return line.


Distillation Control 2

Feed - forward control of PF outlet temperature
PF outlet setpoint T2 and perturbations of PF input flow and 
Temperature are fed to monitor which adjusts flow rate of HTM to 
Maintain constant PF outlet temperature T2

To get constant top vapor product composition, condenser outlet temperature needs to be controlled

Control Philosophy

Temperature control

Measure of composition.

Temperature is controlled either at top or bottom depending on which product specification is important. 

For high purity operation temp. will be controlled at intermediate pt.

The pt. Where dT/dC is max. is the best place to control temp. 

Column top temp 

Controlled by manipulating reflux.

For partial condenser it is typical to control condenser outlet temperature instead of column top temp

Distillation Control 3

Condenser on temperature control of PF condensate
However, throttling of HTM flow could make it too hot at the outlet condenser

Distillation Control 4

Condenser on pressure control of HTM flow
However, throttling of HTM flow makes it too hot at the outlet

Distillation Control 5


Flow rate of condensate is controlled by PF vapour pressure
If column pressure rises, condensate flow rate increases

Distillation Control 6

Condenser with vapor bypass to reflux drum

Condenser and drum partially filled with sub-cooled condensate

When pressure falls, vapor flows directly to drum and heats liquid

Resulting increase in vapor pressure forces some of the liquid back into the Condenser so that the rate of condensation decreases and consequently the pressure  is restored to the preset value

With sufficient sub-cooling a difference of 10-15 ft. in levels of drum and condenser is sufficient for good control by this method

Distillation Control 7

Air flow rate is controllable with adjustable louvers, variable pitch fan Or variable-speed motors

Last two methods achieve some  power saving compared to louver design Multi speed motors used for change between day and night and between Winter and summer Automatic switching based on air temperature


Feed Temperature : This is critical as this decides the vapor & liquid flow in feed zone. Often amount of heat available in the bottom is close to the optimum feed preheat. 

Distillation Control  8


Flow of steam controlled off the PF outlet temperature, and condensate is  removed with  steam trap or under liquid level control.

Distillation Control  9

Temperature control on condensate removal has the effect of varying amount of flooding of heat-transfer surface and hence, the rate of condensation, because the flow of condensate through the valve is relatively slow. 

Distillation Control  11


Level in condensate pot can be  varied to change effective area of heat transfer 
Results in smoother operation
Flashing of condensate across level control valve avoided
Higher heat transfer area require     


Steam pressure responds quickly to upsets in steam supply conditions
More sluggish PF temperature at outlet is used to adjust pressure to maintain proper heat transfer


Distillation Control  13

FIG. -Feed preheating

Details : Feed flow control cascaded with level  in feed sure drum 
Temperature of feed to column controlled by manipulating steam flow to  heat exchanger      
Features : Feed temperature desired - 100°c
Condensate pot used to control level in feed-steam heater
Application : Light reformate  feed to DIH in FCCU (0.62% BZ in  gasoline project)


Column Bottom Temperature
Reboiler outlet temp. indicator is used for controlling. 
For smooth control cascade arrangement with FRC on heating media flow. The set point of which is manipulated by TRC at the reboiler outlet. 

Distillation Control  14

Vaporizer with HTM , flow rate controlled by temperature of  PF vapor HTM may be liquid or vapor.

Distillation Control  15


A constant rate of heat input is achieved by flow control of  HTM or steam
HTM may be either liquid or vapor. 

Distillation Control  16

Flow control on HTM supply responds  rapidly to changes in  heat supply System
More sluggish TC on PF vapour resets FC if needed to maintain PF vapour temperature

Distillation Control  17

Details : 1. Steam on flow control  2. Column bottom on level control
Features : Compared to kettle-type reboiler overall heat transfer coefficient is higher
Surface fouling is low
Shorter residence time prevents overheating of thermally sensitive materials
Positioning of thermo syphon reboiler relative to the column bottom at proper  elevation is critical
Application: N-hexane  Recovery column

Distillation Control  18


Cascade Control on Reboiler Steam Flow Rate
Details : Temp. control at a particular tray achieved by a temp. controller which resets flow controller on steam (HTM) line
Column bottom level controlled by adjusting bottoms flow rate
Features : Flow measurement of steam may not be accurate
  Flow element requires frequent calibration


Distillation Control  19
DETAIL

Column bottom temperature controlled by temperature control on HTM
Level controlled by level controller on bottoms (product) line
Liquid inlet to reboiler is on flow control  minimum flow bypass required but not shown

FEATURES

High rate of heat transfer and short residence time which are desirable for thermally sensitive materials, are achieved 
Better control of flow to reboiler

APPLICATION

  n-Hexane recovery column

Distillation Control  20



Details 

Temperature control at column bottom achieved with help of a control  valve on hot-oil supply to reboiler. 
Column level control achieved by adjusting bottoms flow rate.

Features 

Differential pressure controller between supply and return of hot oil  avoids build-up of excessive pressure if its flow to the reboiler is throttled  substantially.

DPC control system ensures constant flow in hot oil circuit.

up to 74

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