Chemical Engineering Plant Design

Lek WanthaLecture 07

Separation System

A Hierarchical Approach to Conceptual Process Design

1. Decide whether the process will be batch or continuous

2. Identify the input-output structure of the process

3. Identify and define the recycle structure of the process

4. Identify and design the general structure of the separation system

5. Identify and design the heat exchanger network or process energy recovery system

General Structure of the System

General Structure of the System

• Reactor Effluent is Fluid - reactor effluent is liquid - reactor effluent is a two-phase mixture - reactor effluent is all vapor• Reactor Effluent is Solid-Fluid

Reactor Effluent is Liquid

Reactor Effluent is Liquid-Vapor

Reactor Effluent is Vapor

• Keep in mind that major product should be in liquid phase before it is sent to the separation system. The conditions of liquid phase should be the same conditions as required for separation system.

Reactor Effluent is Solid-Fluid

• Adjust the conditions of reactor effluent stream so that it is ready for solid-fluid separation. After separation, solid part will be sent to purification system and fluid part will be sent to separation system for further separation (if necessary).

Approximate Flash Calculations

• See accompanied sheet

Vapor Recovery System

• What is the best location?• What type of vapor recovery system is cheapest?

Location of Vapor Recovery System

• The purge stream• The gas-recycle stream• The flash vapor stream• None

Location of Vapor Recovery System

Location of Vapor Recovery System

• Condensation• Absorption• Adsorption• Membrane separation process• Reaction systems

Location of Vapor Recovery System

• We design the vapor recovery system before we consider the liquid separation system because each of vapor recovery processes usually generates a liquid stream that must be further purified. For the case of a gas absorber, where we need to supply a solvent to absorber, we also introduce a new recycle loop between the separation systems.

Location of Vapor Recovery System

Liquid Separation System

• How should light ends be removed if they might contaminate the product?• What should be the destination of the light ends?• Do we recycle components that form azeotropes with the reactants, or do we split azeotropes?• What separations can be made by distillation?

Liquid Separation System

• What sequence of columns do we use?• How should we accomplish separations if distillation is not feasible?

Light Ends Removal

• Drop the pressure or increase the temperature of a stream, and remove the light ends in a phase splitter. • Use a partial condenser on the product column.• Use a pasteurization section on the product column.• Use a stabilizer column before the product column.

Light Ends Removal

Light Ends Destination

• vent them (through a flare system if it causes air pollution problem)• send the light ends to fuel• recycle the light ends to the vapor recovery system or flash drum

Azeotropes with Reactants

• recycle the azeotrope• splitting and just recycling the reactant

Liquid Separation System

• Distillation• Liquid-Liquid Extraction• Crystallization• Adsorption• Membrane Separation• etc.

Heuristics for Design Separation Process

• Remove dangerous and/or corrosive species first.• Do not use distillation when the relative volatility between the key components is less than 1.1.• Use extractive distillation only if the relative volatility between the key component is much better than for regular distillation – say 6 times better.

Heuristics for Design Separation Process

• Do the easy splits (i.e., those having the largest relative volatilities) first in the sequence.• Place the next split to lead to the removal of the major component.• Remove the most volatile component next (i.e., choose the direct sequence).

Heuristics for Design Separation Process

• The species leading to desired products should appear in a distillate product somewhere in the sequence if at all possible.• These heuristics are listed in order of important.

Heuristics for Distillation

• The relative volatility between the two selected key components for the separation in each column is greater than 1.1.• The reboiler duty is not excessive.• The tower pressure does not cause the mixture to approach its critical pressure.• The overhead vapor can be at least partially condensed at the column pressure to provide reflux without excessive refrigeration requirements.

Heuristics for Distillation

• The bottoms temperature for the tower pressure is not so high that chemical decomposition occurs.• Azeotropes do not prevent the desired separation.• Column pressure drop is tolerable (not excess 10 psi), particularly if operation is under vacuum.

General Heuristics for Column Sequencing

• Remove corrosive components as soon as possible.• Remove reactive components or monomers as soon as possible.• Remove product as distillate.• Remove recycle streams as distillates, particularly if they are recycled to a packed bed reactor.

Heuristics for Column Sequencing

• Most plentiful first.• Lightest first.• High-recovery separations last.• Difficult separations last.• Favor equimolar splits.• Next separation should be cheapest.

Heuristics for Column Sequencing

• Remove thermally unstable, corrosive, or chemically reactive components early in the sequence.• Remove final products one by one as distillates.• Sequence separation points to remove, early in the sequence, those components of greatest molar percentage in the feed.

Heuristics for Column Sequencing

• Sequence separation points in the order of decreasing relative volatility so that the most difficult splits are made in the absence of the other components.• Sequence separation points to leave last those separations that give the highest purity products.• Sequence separation points that favor near equimolar amounts of distillate and bottoms in each column.

Column Sequencing – Simple Columns

Column Sequencing – Simple Columns

Column Sequencing – Simple Columns

Select the sequence that minimizes the number of columns in a recycle loop.

Select the sequence that minimizes the number of columns in a recycle loop.

Complex columns

Complex columns

Extraction

Extractive Distillation

Azeotropic Distillation

Reactive Distillation

Crystallization

HAD Process

HDA Process