control loop configuration of interacting units

Control Loop Configuration of Interacting Units

Dr. S. JanaDepartment of Chemical Engineering

Banasthali University

What is an Interacting Unit?• Several units interact with each other through material or

energy flows.• How to determine the feasible loop configuration in interacting

units?Steps:Divide the process into separate blocks.Determine the degree of freedom and no of controlled and manipulated variables for each block.Determine the feasible loop configurations for each and every block.Recombine the blocks with their loop configurations.Eliminate the conflicts among the control system of the various blocks.

We will study …… …..

An exothermic liquid phase reaction conducted in a CSTR and separated/purified in a flash drum.

An exothermic gas phase reaction conducted in a tube and shell heat exchanger and separated/purified in a combined system of flash drum and distillation column.

The control configuration of:

Liquid Phase Reaction in CSTR

Flow diagram of the process

• Objectives/Operational Goals:


To keep the conversion in the reactor at its highest permissible limit.

To maintain a constant production rate.

To achieve constant composition in the liquid product of the flash drum.

• STEP – 1: Dividing the process into blocks.


Liquid Phase Reaction in CSTR• STEP – 2: Determine the degree of freedom and as well as the

number of manipulated and controlled variable.

Total number of variables 08 Pcf, Tcf, Fc1, Tc1, Fc2, Tc2, Fc, Tco.

Number of modeling equation 04

Heat balance on cooling branch Heat balance on heating branch Heat balance on the mixing junction of

the two branches Mass balance on mixing junction

Degree of freedom 04

Number of externally specified variables 02 Pcf, Tcf,

Number of controlled and manipulated variables 02

Controlled Variable: Fc, Tco

Manipulated Variable: Fc, Tco

COOLANT SYSTEM

DETERMINED BY QUALITATIVE ARGUMENTS



REACTOR SYSTEM

Total number of variables 09 V, Tr, cA, cAi, Ti, Fi, Fc, Tc, Tco.


Component A balance around the reactor Energy balance on reacting mixture Energy balance on the coolant in the

jacket

Degree of freedom 06Number of externally specified variables 04 cAi, Ti, Fi and (Fc, OR Tco)


Controlled variable: Tr and cA. Manipulated variable: Fi and (Fc, OR Tco)



FEED PREHEATING SYSTEM

Total number of variables 06 Ws, To, Tr, Ti, Tint, Fi.


Heat balance of steam heater Heat balance of feed effluent heat

exchanger

Degree of freedom 04Number of externally specified variables 03 To, Tr, Fi.


Controlled variable: Ti

Manipulated variable: Ws.



FLASH DRUM

Total number of variables 13 Tint, zA, Fi, FV, FL, pf, Tf, h, FW, xA, yA, xB, yB.


Mass balance equation Component balance of A Heat balance equation VLE relationship of A VLE relationship of B Consistency relationship of liquid phase Consistency relationship of gaseous phase

Degree of freedom 06Number of externally specified variables 02 Tint, zA.


Controlled variable: Fi, pf, Tf, h. Manipulated variable: Fi , FV, FL, FW.

Liquid Phase Reaction in CSTR• STEP – 3: Determine the feasible loop configuration of each

block.COOLANT SYSTEM (comparison of different loops)

SERIAL NOLOOP CONFIGURATIONS

Fc controlled by Tco controlled by

01 Fc Fc1 and Fc2

02 Fc1+ Fc2 Fc1/ Fc2

03 Fc1+ Fc2 Fc1

04 Fc1+ Fc2 Fc2

05 Fc2 Fc1

06 Fc1 Fc2


block.COOLANT SYSTEM (best configuration)


block.REACTOR SYSTEM (comparison of different loops)


cA controlled by Tr controlled by

01 Fi Fc or Tco

02 Fc or Tco Fi

03 FiFc with Tc (cascade

configuration)


block.REACTOR SYSTEM (best configuration)


block.FEED PREHEATING SYSTEM (comparison of different loops)

SERIAL NO LOOP CONFIGURATIONS (WS controlled by)

01 Ti

FEED PREHEATING SYSTEM (best configuration)


block.FLASH DRUM (comparison of different loops)


Fi controlled by pf controlled by Tf controlled by h controlled by01 Fi FV FL FW

02 FV Fi FL FW

03 Fi FV FW FL

04 Fi FL FW FV

05 FL Fi FV FW

06 Etc..


block.FLASH DRUM (best configuration)

Liquid Phase Reaction in CSTR• STEP – 4: Recombine the blocks with their loop configurations.

Liquid Phase Reaction in CSTR• STEP – 5: Eliminate the conflicts among the control system of

the various blocks.

FINAL CONFIGURATION

Gas Phase Reaction Conducted in a Tube and Shell Heat Exchanger

Flow diagram of the process


• STEP – 1: Dividing the process into blocks.


• STEP – 2 & STEP – 3:Due to the complicacy of the system, it is analyzed in an qualitative manner. In spite of going through the process used in the last example we will try to analyze the configurations according to the experience and understanding of the system.

Lets try to analyze each and every block separately for finding the best loop configuration for each of them.


• STEP – 2 & STEP – 3:Compressors for fresh feed gas A (C-1) and recycled gas A (C-2) from the flash drum:

Simple feedback pressure controller would be sufficient.The pressure measurement and manipulation have to be done at the outlet of the compressor.Recycle of the feed gas to the inlet line is necessary as the reactant cannot be wasted.


• STEP – 2 & STEP – 3:Mixing drum for the fresh feed B and the recycled stream from the bottom of the distillation column:

Inlet and outlet flow could be controlled by feed-forward controller because the mathematical model of the mixing drum is not much complicated.

The level of the drum is another important parameter to be controlled. It can be controlled by the flow rate of feed B (stream 2) or by the recycled stream from the distillation column (stream 3).

Controlling stream 2 is a better choice because change in the flow rate of stream 3 could affect the performance of the distillation column.


• STEP – 2 & STEP – 3:Feed vaporizing and preheating:

The feed flow rate of stream 8 or 9 and the temperature of stream 9 are the controlled output.

The feed flow rate could be controlled by a feed-forward controller.

Only one manipulated variable (steam flow rate) is available for controlling the temperature of stream 9.


• STEP – 2 & STEP – 3:Reactor system:

The inlet reactant flow rate and the temperature of the reactor are the controlled variable.

The flow rate could be controlled by the feed-forward controller.

The temperature of the reactor can only controlled by manipulating the flow rate of coolant only.


• STEP – 2 & STEP – 3:Flash drum system:

The inlet stream flow rate and temperature are the obvious controlled parameter.Another two important controlled parameters are the pressure and the level of the flash drum. The inlet flow rate (stream 11) can be controlled by a feed-forward controller. Temperature of the inlet stream is controlled by the coolant water by feedback controller. Using of a feed-forward controller could be difficult because the temperature of the cooling water is unknown.


• STEP – 2 & STEP – 3:Flash drum system:

The best way to control the pressure of the flash drum is manipulating the flow rate of top gaseous product.

The level of the flash drum is controlled by the outlet flow rate of the liquid (stream 15).


• STEP – 2 & STEP – 3:Distillation column:

Inlet feed flow rate, re-boiler duty, concentration of the overhead product and the level of the column are the controlled variables.Inlet feed flow rate could be easily controlled by a feed-forward controller.The best way to control the level of the column is manipulating the flow rate of the bottom product (contains mainly of B). The bottom product is recycled to the mixing drum.


• STEP – 2 & STEP – 3:Distillation column:

As the flow rate of bottom product is already controlled, the stream 17 will be an uncontrolled entity for obtaining the desired separation/concentration of the product.The flow rate of the overhead product is directly related with the level of the condenser. So, a feedback controller will be sufficient for the same.The concentration of the overhead product is controlled by manipulating the reflux (stream 18).


• STEP – 4: Recombine the blocks with their loop configurations


• STEP – 5: Eliminate the conflicts among the control system of the various blocks.

REFERENCE:

Chemical Process Control -- An Introduction to Theory and Practices By George Stephanopoulos, PHI Learning Pvt. Ltd.