Advanced Process Modelling for separation systems

Pressure-Swing Adsorption (PSA): using modelling to optimise the cyclic steady state

Pressure-Swing Adsorption is a technology widely used in gas production. However it is a difficult process to model accurately; it is inherently dynamic, and the adsorption process, if properly modelled, involves complex diffusion relationships.

Process Systems Enterprise's gPROMS Advanced Process Modelling package provides powerful technology for modelling Cyclic Separation Processes of all kinds, including PSA systems and the closely-related Vacuum-Swing Adsorption (VSA).

Key applications of gPROMS to Cyclic Separation Process are:

Optimise the cyclic steady state
Minimise overdesign
Verify designs and establish viability
Accelerate implementation
Optimise safe and effective start-up procedures
Define operating envelopes
Perform control design
Maximise operational flexibility
Troubleshoot poor operation.

gPROMS modelling advantages

gPROMS's capabilities for modelling of PSA and VSA systems include:

A general-purpose modelling language within an equation-oriented framework that allows rate-based modelling of detailed pore diffusion, reverse-flow hydraulics and all the other key effects, within a flowsheet representation.
Dynamic modelling, including dynamic simulation, as a standard capability
A powerful task language that allows easy description of complex operating procedures in parameterised form, enabling subsequent optimisation.
Dynamic optimisation, to allow optimisation of both the equipment physical attributes and operating procedures required in order to reach the optimal cyclic steady state.
Rapid and robust solution techniques, essential in order to handle the complex mathematical solution.

Example 1: RPSA process using Zeolite adsorbent

Example 1 involves a simple simulation of the production of oxygen-enriched air using a zeolite 5A adsorbent in a rapid pressure-swing adsorption (RPSA) process. Key equations used in the model are:

The process was simulated to analyse the steady-state behaviour. Sample results are shown below.

Example 2: Modified Skarstrom cycle

Example 2 involves the simultaneous optimisation of the design and operation of a more complex 2-bed 6-step PSA for producing nitrogen from air using a molecular carbon sieve adsorbent.

Simulation of Skarstrom cycle

6-step cycle
The 6-step cycle involves:
Pressurisation	2s
Adsorption	60s
1st pressure equalisation	2s
Blowdown	2s
Desorption	60s
2nd pressure equalisation	2s
Total	128s

The sequence for the two beds is shown on the right.

Dynamic optimisation was used to determine:

Optimisation of Skarstrom cycle

Optimal values for equipment design parameters such as bed length
Optimal operating condition values such as feed pressure
Optimal times for each step of the operation.

The resulting design maximises the production of nitrogen at cyclic steady state.

The results of the optimisation are shown in the table above.