Regulatory control layer for CO2 capturing...

Plantwide Control Course

Regulatory control layer for CO2 Capturing process

Ref.: M. Panahi and S. Skogestad, Economically efficient operation of CO2 capturing process. Part II. Design of control layer, Chemical Engineering and Processing, 52, 112-124 (2012)

by: Mehdi Panahi

Department of Chemical Engineering Ferdowsi University of Mashhad

Plantwide Control Course, Ferdowsi University of Mashhad, M. Panahi

Plantwide control Hierarchy

Real time optimization (RTO)

Model Predictive control (MPC)

Primary controlled variables, economic variablescs=y1s

Secondary Controlled variables, stabilizing variablescs=y2s

Systematic plantwide control procedure of Skogestad

I Top Down • Step S1: Define operational objective (cost) and constraints• Step S2: Identify degrees of freedom and optimize operation

for disturbances• Step S3: Implementation of optimal operation

– What to control ? (primary CV’s) (self-optimizing control)• Step S4: Where set the production rate? (Inventory control)

II Bottom Up • Step S5: Regulatory control: What more to control (secondary

CV’s) ?• Step S6: Supervisory control• Step S7: Real-time optimization

Proposed control structure with given flue gas flowrate (region I)

Region II: in presence of large flowrates of flue gas (+30%)

Flowrateof flue gas

(kmol/hr)

Pumpsduty(kW)

Self-optimizing CVs in region I CoolerDuty(kW)

Reboilerduty(kW)

Objectivefunction

(USD/ton)CO2 recovery

%Temperature

oftray no. 16

°COptimal nominal

point219.3 3.85 95.26 106.9 321.90 1161 2.49

+5% feedrate 230.3 4.24 95.26 106.9 347.3 1222 2.49

+10% feedrate 241.2 4.22 95.26 106.9 371.0 1279 2.49

+15% feedrate 252.2 4.64 95.26 106.9 473.3 1339 2.49

+19.38% feedrate,reboiler duty

saturates

261.8 4.56(+18.44%)

95.26 106.9 419.4 (+30.29%)

1393(+20%)

2.50

+30% feedrate (reoptimized)

285.1 4.61 91.60 103.3 359.3 1393 2.65

Saturation of reboiler duty (new operations region, region II); one unconstrained degree of freedom left

Maximum gain rule for finding the best CV: 37 candidates

Temp. of tray no. 13 in the stripper: the largest scaled gain

RGA analysis for selection of pairings

2 2

dyn. -2s

2 2

6.85s+1.74 -0.76s 0.0382400s +107s+119.7s +11.4s+1G (s)=

(-9.51s-1.02)e 0.45s+0.0754218s +17.3s+1 205s +18.8s+1

+é ùê úê úê úê úë û

dyn.RGA (0)=0.77 0.230.23 0.77

é ùê úë û

10-3

10-2

10-1

100

101

102

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Frequency [rad/min]

||RG

A -

I||su

m

Diagonal pairing alt.1

Off-diagonal pairing alt.2Recycle amine Reboiler duty

CO2 recovery

Temp. no.16 in the stripper

2SS

0.5232 1.48G 10

8.47 5.17- -é ù

= ´ ê ú-ë û

SSRGA =0.27 1.271.27 0.27

é ùê úë û

- ++ -

1. Dynamic RGA

2. Steady-State RGA

”Break through” of CO2 at the top of the absorber (UniSim simulation)

Liquid mole fraction of CO2 in trays of the Absorber

0,015

0,02

0,025

0,03

0,035

0,04

0,045

0,05

0,055

0 50 100 150 200 250 300 350 400 450

Time (min)

mol

e fr

actio

n

tray 15tray 14tray 13tray 12tray 11tray 10tray 9tray 8tray 7tray 6tray 5tray 4tray 3tray 2tray 1

tray 1

tray 15

Proposed control structure with given flue gas flowrate, Alternative 1

Performance of the proposed control structure, Alternative 1

Proposed control structure with given flue gas flowrate,Alternative 2 (reverse pairing)

Performance of the proposed control structure, Alternative 2

Proposed control structure in region II, Alternative 3

Performance of the proposed control structure, Alternative 3

Modified Alternative 2 = Alternative 4

Performance of the proposed control structure, Alternative 4

Performance of the proposed control structure, MPC