Integration of Design & Control

CHEN 4470 – Process Design Practice

Dr. Mario Richard EdenDepartment of Chemical Engineering

Auburn University

Lecture No. 16 – Integration of Design and Control II

March 7, 2013

Contains Material Developed by Dr. Daniel R. Lewin, Technion, Israel

Plantwide Control Design

Luyben et al. (1999) suggest a method for the conceptual design of plant-wide control systems, which consists of the following steps: Step 1: Establish the control objectives. Step 2: Determine the control degrees of

freedom. Simply stated – the number of control valves – with additions if necessary.

Step 3: Establish the energy management system. Regulation of exothermic or endothermic reactors, and placement of controllers to attenuate temperature disturbances.

Step 4: Set the production rate. Step 5: Control the product quality and handle

safety, environmental, and operational constraints.

Plantwide Control Design

Step 6: Fix a flow rate in every recycle loop and control vapor and liquid inventories (vessel pressures and levels).

Step 7: Check component balances. Establish control to prevent the accumulation of individual chemical species in the process.

Step 8: Control the individual process units. Use remaining DOFs to improve local control, but only after resolving more important plant-wide issues.

Step 9: Optimize economics and improve dynamic controllability. Add nice-to-have options with any remaining DOFs.

Example 2: Acyclic Process

Maintain a constant production rate Achieve constant composition in the liquid effluent from

flash drum Keep the conversion of the plant at its highest permissible

value.

Steps 1 & 2: Establish the control objectives and DOFs:

Select V-7 for On-demand product flow

Select V-1 for fixed feed

Example 2: Acyclic Process

Need to control reactor temperature: Use V-2 Need to control reactor feed temperature: Use V-3

Step 3: Establish energy management system:

Example 2: Acyclic Process

For on-demand product: Use V-7

Step 4: Set the production rate:

Example 2: Acyclic Process

To regulate V-100 pressure: Use V-5 To regulate V-100 temperature: Use V-6

Step 5: Control product quality, and meet safety, environmental, and operational constraints:

Example 2: Acyclic Process

Need to control vapor inventory in V-100: Use V-5 (already installed)

Need to control liquid inventory in V-100: Use V-4 Need to control liquid inventory in R-100: Use V-1

Step 6: Fix recycle flow rates and vapor and liquid inventories :

Example 2: Acyclic Process

Install composition controller, cascaded with TC of reactor

Step 7: Check component balances

Step 8: Control the individual process units

Step 9: Optimization

N/A: Neither A or B can build up

N/A: All control valves in use

Example 2: Acyclic Process

The liquid levels in R-100 and V-100 are now controlled in the direction of the process flow, where before they were controlled in the reverse direction.

Differences: Only step 6 is different

Select V-1 for fixed feed

Example 2: Acyclic Process

Example 3: Cyclic Process

This control structure for fixed feed has an inherent problem.

Can you see what it is?

Example 3: Cyclic Process

F0

B

D

B

F0 + B

0

0

0

Combined molar f eed to the CSTR:

Molar material balance around the fl ash vessel:

Overall molar material balance:

F B

F B D B

F D

Example 3: Cyclic Process

AA A 0 A

11 Rtotal total

R

dnkx c x F B kx c V

V dtR Ttotalc V n

Molar balance on CSTR:

A 0 A A 0 A1 1 R Ttotalx F B kx c V x F B kx n

A 0 0

A

1

Tx F kn F

Bx

Rearranging:

20

0

T

FB

kn F

Substitute:

Balance on A for perfect separation:

Example 3: Cyclic Process

16.750

450150

208125

100100

4575

BF0

e.g., suppose knT = 200:2

0

0

T

FB

kn F

A more general result uses the dimensionless, Damköhler number: Da = knT/F0

giving:

0

1F

BDa

“Snowball” effect for Da 1

“Snowball” effect

Example 3: Cyclic Process

Maintain the production rate at a specified level Keep the conversion of the plant at its highest permissible

value.

Steps 1 & 2: Establish the control objectives and DOFs:

Example 3: Cyclic Process

Need to control reactor temperature: Use V-2

Step 3: Establish energy management system:

Example 3: Cyclic Process

For on-demand product: Use V-1

Step 4: Set the production rate:

Example 3: Cyclic Process

To regulate V-100 pressure: Use V-4 To regulate V-100 temperature: Use V-5

Step 5: Control product quality, and meet safety, environmental, and operational constraints:

Example 3: Cyclic Process

Need to control recycle flow rate: Use V-6 Need to control vapor inventory in V-100: Use V-4 (already

installed) Need to control liquid inventory in V-100: Use V-3 Need to control liquid inventory in R-100: Cascade to FC on

V-1

Step 6: Fix recycle flow rates and vapor and liquid inventories :

Example 3: Cyclic Process

Install composition controller, cascaded with TC of reactor

Step 7, 8 and 9: Improvements

Summary

Part I: Previous Lecture

Provided motivation for handling flowsheet controllability and resiliency as an integral part of the design process

Outlined qualitative approach for unit by unit control structure selection

Part II – This Lecture

Outlined a qualitative approach for plantwide control structure selection

• Next Lecture – March 19– Equipment sizing and pinch analysis

• Q&A Session with Consultant – March 21

– Bob Kline will participate via videoconference– Questions can be sent to Bob and/or me ahead

of time

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