Lecture 9 +10 - Distillation Control

Question 1

Process Control Objectives - Distillation

Answer 1

• Maintain energy balance (column pressure)
• Maintain mass balance (reflux drum and column levels)
• Maintain product composistions within spec
• Maximise recovery of more valuable product
• Minimise disturbances to downstream units
• Optimise product yield vs enegy consumption

Question 2

Basic Control Problems

Answer 2

• Inherently non-linear process
• Long deadtimes and complex dynamics
• Multiple disturbances
• Expensive Instrumentation

Question 3

Maintaining Energy Balance

Answer 3

Pressure is a good indication of energy balance
- pressure will rise if energy in > energy out
- quick response
Need to contril pressure anyway
Realistically, only condenser duty can be manipulated
- multiple schemes involve the manipulation of:
vapour condensation, rate at which vapour leaves column, vapour generation

Question 4

Manipulation of Vapour Condensation

Answer 4

• used standalone when there is normally no vapour product
  Manipulation of:
• coolant rate
• coolant temp
• condenser efficiency
• vapour by pass

Question 5

Manipulation of Coolant Rate

Answer 5

Manipulation of cooling water
- minimum flow often restricted by need to avoid high temps or due to issues with fouling
Manipulation of air flow
- fan speed, fan pitch, number of fans - can lead to mechanical problems

Question 6

Manipulatioon of Coolant Temp

Answer 6

each colum will need an additional pump - therefore expensive

Question 7

Flooded Condenser (Type of Condenser)

Answer 7

A flooded condenser is used in distillation columns where a portion of the condensing surface is covered by liquid
- This design results in less effective sub-cooling of refluc
- The pressure controller adjusts the liquid level in the condenser which changes the effective surface area for condensation
- The liquid level affects the heat transfer rate - uncovered area has a high heat transfer, covered area has a low heat transfer

Question 8

Manipulation of Vapour Rate

Answer 8

Must have significant flow of vapour
Fast Response
- Direct manipulation of a valve in the vapour line
- Manipulation of overhead gas compressor
- Manipulation of spillback around ejector on vacuum column - operated on manual valve with valve shut to minimise presure and improve product yield

Question 9

Manipulation of Vapour Generation

Answer 9

Can be used
- at minimum condenser duty
- at maximum condenser duty
- when condenser duty cannot be manipulated
May have no other choice
- Slower response to disturbances
- Poor composition control strategy

Question 10

Combination Methods for Distillation Control

Answer 10

Applicable when
- limited condenser capacity
- intermittent vapour
- compressor speed is minited
- upset conditions

Question 11

Maintaining Material Balance

Answer 11

Feed = distillate + bottoms
– Need to manipulate at least one
- changing levels of reflux drum or column base is the first indication of imbalance

Question 12

Cut Definition

Answer 12

The cut is the proportion of feed that is drawn off as distillate, it is determined by the mass balance and the light key balance of the components in the distillation column
- Only adjusted if the feeds light key composition changes or in response to change in feed or product comp

Question 13

Separation Definition

Answer 13

The separation refers to the purity of the components achieved in the distillate and bottoms products - the degree to which the different components are isolated from each other

Question 14

When is material balance control favoured

Answer 14

• Large reflux ratio (>5)
• When keeping cut constant is important

Question 15

When is energy balance control favoured

Answer 15

• Small reflux ratio
• Situations where distillate flow is required to change
• Large reflux drum

Question 16

What does Separation depend on

Answer 16

• Column design
• Relative volatility of components
• Operating conditions
  – Column design will limit the maximum achievable separation

Question 17

Composition Control

Answer 17

Aims to maintain product compositions within a limit - employs various strategies including the use of tray temperature, pressure compensation, inferential properties, and addressing interactions between control loops

Question 18

On- Stream Analysers

Answer 18

• Not always practical to directly measure product compositions
• On stream analysers are: expensive to install, expensive to maintain, slow, can be unreliable
  – may not be sufficient economic justification

Question 19

Tray Temperature Control

Answer 19

Provide some level control in the event of analyser failure
- Liquid on trays is at its bubble point, composition dependent
- Temperature gives an approximate indication of composition
- Temperature control approximately fixes cut

Question 20

Potential Issues with Tray Temperature Control

Answer 20

Issues if liquid on tray is not at its bubble point
- sub cooled reflux
- super heated vapour from reboiler
- liquid feed not at its bubble point
- vapour feed not at dew point
- change in feed comp
Issues if there is a non linear relationship between composition and temp or if temperature is insensitive to comp changes

Question 21

Pressure Compensation

Answer 21

A technique used to account for the impact of pressure changes on the relationship between tray temperature and composition
- Acts to adjust the temp to provide a pressure compensated temperature to maintain more accurate composition control

Question 22

Optimisation

Answer 22

Focus on maximizing the profitability - if one product is more valuable, may be economically beneficial to use more energy to recover a greater amount of the valuable product.
This incurs energy costs, and the relationship between additional yield and additional energy is non-linear
The aim is to identify an “optimum” point where the gain in valuable product no longer justifies the increased cost of the additional energy input.