Fundamentals of Distillation Column Control

Transcription

1 Fundamentals of Distillation Column Control by Terry Tolliver Standards Certification Education & Training Publishing Conferences & Exhibits ISA Automation Week 2011

2 PRESENTER Terry Tolliver Terry is a retired Senior Fellow from Solutia/Monsanto and presently provides contract engineering at ConocoPhillips Wood River refinery. He has 40+ years of experience in process control, simulation, operations, troubleshooting and optimization. Terry graduated from the Missouri University of Science and Technology with BS, MS & PhD ChE and was recently inducted into their Academy of Chemical Engineers in He was inducted into the CONTROL Automation Hall of Fame in He has received several ISA awards including the Distinguished Society Service Award in 1997, the E. G. Baily Award in 1993 and the Excellence in Documentation award in Terry has been an adjunct professor for Washington University from , a Lehigh University Biannual Lecturer from , an ISA Fellow in 1990, an AIChE Fellow in 2000 and a PE since

3 DISTILLATION CONTROL TOPICS Levels of control Classification and pairing of variables Control objectives and constraints Dynamic responses Material and energy balances Separation Pressure control Material balance control Temperature control References 3

4 DISTILLATION CONTROL BASIC Inventory Control Composition Control SUBOPTIMIZING Feedforward Control Two Point Composition Control OPTIMIZING Floating Pressure Maximum Profit 4

5 CLASSIFICATION and PAIRING of VARIABLES CONTROLLED VARIABLES distillate composition bottom composition accumulator level sump level column pressure Y X La Ls P P La Y Ls X 5

6 CLASSIFICATION and PAIRING of VARIABLES MANIPULATED VARIABLES distillate flow bottom flow reflux flow reboiler duty condenser duty D B R Qr Qc R Qc D Qr B 6

7 CLASSIFICATION and PAIRING of VARIABLES DISTURBANCE VARIABLES feed flow feed composition feed temperature reboiler heat supply condenser cooling supply and weather F Z q S W F Z q W S 7

8 CLASSIFICATION and PAIRING of VARIABLES CONTROLLED MANIPULATED DISTURBANCE VARIABLES VARIABLES VARIABLES distillate composition distillate flow feed flow bottom composition bottom flow feed composition accumulator level reflux flow feed temperature sump level reboiler duty reboiler heat supply column pressure condenser duty condenser cooling supply and weather 8

9 CONTROL OBJECTIVES Total Operating Cost $/F Cost of Energy Cost of Product Lost RECOVERY 9

10 COLUMN CONSTRAINTS Condenser Flood Reboiler Boilup rate Weep Vessel Limit Column Pressure 10

11 COLUMN INTERNALS TRAYS SIEVE TRAYS VALVE TRAYS BUBBLE CAP TRAYS DUAL FLOW TRAYS PACKING RANDOM PACKING STRUCTURED PACKING LIQUID & VAPOR DISTRIBUTORS 11

12 COLUMN DYNAMICS RELATIVE RESPONSE TIMES VAPOR - FAST LIQUID - MEDIUM COMPOSITION - SLOW Response time 12

13 MATERIAL & ENERGY BALANCES MATERIAL BALANCE F = D + B zf = yd + xb D/F = (z-x)/(y-x) ENERGY BALANCE hf + Qreb = hd + hb + Qcond 13

14 SEPARATION TOTAL REFLUX α n = [y(1-x)]/[x(1-y)] NORMAL OPERATION S = f (α, N, N F, V/F, z, E) 14

15 MATERIAL BALANCE and SEPARATION EFFECTS 1.0 Increasing S y Light Key Mole Fraction X Z Increasing S 0 0 D/F

16 MATERIAL BALANCE CONTROL LC TC LC 16

17 SEPARATION CONTROL LC TC NOT RECOMMENDED LC 17

18 PRESSURE CONTROL VENT PI LC 18

19 CONDENSER DUTY Tp Qcond = UA(Tp-Tc) COOLANT Tc VENT CONDENSATE 19

20 BLOCK & BLEED PRESSURE CONTROL INERTS BLEED VALVE VACUUM SOURCE BLOCK VALVE PC LC 20

21 MATERIAL BALANCE CONTROL - TYPE 1 LC TC LC 21

22 MATERIAL BALANCE CONTROL - TYPE 2 LC TC LC 22

23 MATERIAL BALANCE CONTROL - TYPE 3 LC TC LC 23

24 MATERIAL BALANCE CONTROL - TYPE 4 LC TC response LC time 24

25 PROCESS SIMULATION STEADY STATE PARAMETRIC CASES CONTROL STAGE LOCATION SENSITIVITY ANALYSIS DISTURBANCE ANALYSIS DYNAMIC STUDY CONTROL RESPONSE STARTUP AND SHUTDOWN 25

26 CONTROL STAGE LOCATION Benzene-Toluene Column Temperature Profiles Stage Number %D/F 4-1%D/F Temperature, DegC 26

27 TEMPERATURE SENSITIVITY DISTILLATE PRODUCT COMPOSITION, wt% BOTTOM PRODUCT CONTROL STAGE TEMPERATURE, degf 27

28 DISTURBANCE ANALYSIS Benzene-Toluene Column Load Effects Tray Number Delta Temperature, DegC 28

29 REFERENCES 29

30 REFERENCES 30