1. San Francisco Refinery, Rodeo
DISTILLATION CONTROL
Dr. Prakash Karpe
Control & Elec. Eng. Supt.
ConocoPhillips
San Francisco Refinery, Rodeo

2. Distillation Column Control
Control Objectives Qc V L D
Rectification
Stages F
R = L/D
Stripping
Stages QH B
Two Control objectives
Inventory control
Composition control

3. Degrees of Freedom Analysis
From control perspective, degrees of freedom of a process is defined as number of variables that can or must be controlled.
Helps to avoid over- or under-control of processes.
Degrees of freedom (to control) = No. of rationally placed control valves
A control valve represents a manipulated variable (MV)

4. Degrees of Freedom Analysis Flash Vessel (Separator)
F,T,P,xi
Disturbances B

5. Inventory Control
For steady state operation of a process, all inventories must be controlled
Vapor inventories are maintained by pressure control
Liquid inventories are maintained by level control

6. Degrees of Freedom Analysis Flash Vessel (Separator)PC F
F,T,P,xi
Disturbances LC B

7. Degrees of Freedom Analysis Typical Distillation Column
Inventory Control PC V LD L D TD F TB QH LB LC B
Degrees of Freedom = 3

8. Liquid Inventory Control Level Control
Reflux drum level control
LD - L or LD – D?
Richardson’s rule:
Use the largest stream to control level.
Guidelines:
L/D < = 1 : Use LD – D pairing
L/D > = 5 : Use LD – L pairing
For 1 < L/D < 5, use scheme proposed by Rysjkamp
(L+D) – D and L/D – L pairings

9. Two Common Level Control Schemes
Level control dilemma
Tight flow control?
Oscillating level
Tight level control?
Oscillating product flow
Averaging or nonlinear level control
Tight level control

10. Common Level Control Schemes
Averaging (nonlinear) level control
Used when product is a feed to a downstream process
Examples
Train of lightends columns
Reflux drum level control
Tight level control
Used when product goes to tankage or a surge drum or process requires low hold up
Use P-only controller with KC = 4
Reboiler level control
FCC Main Frac and Vacuum column bottoms (coking concern)
Dirty wash oil draw level control
Control hydrostatic P in the draw line

11. Vapor Inventory Control Common Pressure Control Schemes Partial Condensers
Off gas rate > 0
Common Problems
If off gas is routed to a compressor, reflux drum P is controlled leading to tower P swings.

12. Common Pressure Control Schemes Partial Condensers
Off gas rate > 0 or = 0
Common Problems
If off gas is routed to a compressor, reflux drum P is controlled leading to tower P swings.
Inert gas, typically noncondesables, can cause downstream process problems

13. Common Pressure Control Schemes Total Condensers Flooded Condenser
Off gas rate = 0
Common Problems
If P equalizing line is not used, P in the reflux drum swings.
If condensed liquid is introduced into the drum from top w/o dip leg, vapor in the drum can collapse.

14. Common Pressure Control Schemes Total Condensers Hot Vapor Bypass
Off gas rate = 0
Common Problems
Bypass line inadequately sized
If drum top surface is not insulated, P can swing with ambient changes. The effect is less pronounced for high P columns.

15. Degrees of Freedom Analysis Typical Distillation Column
Composition Control PC V LD L D TD F TB QH LB LC B
Degrees of Freedom = 3

16. Composition Control Problem
Number of MV’s = 3
Reflux flow: L
Distillate flow: D
Reboiler heat: QH
Reflux ratio
Product/ feed ratio
Steam/ feed ratio
Need three controlled variables (CV’s)
Possible CV’s
Reflux drum level: LD
Distillate composition: xD
Appropriate temperature in rectification section (TD)
Bottoms composition: xB
Appropriate temperature in stripping section (TB)
Control problem
How do we pair CV’s and MV’s?

17. Composition Control
Fundamental manipulated variables
Feed split or cutpoint variable
Fraction of the feed that is taken overhead of out of the bottom
Increasing distillate flow will increase bottom purity and decrease distillate purity, etc.
Fractionation variable
Energy that is put into the column to achieve separation
Increasing the reflux ratio or the reboiler duty will increase both distillate and bottoms purity
Feed split has more pronounced impact on product purity than fractionation variable (exception low purity, < 90%, products)
It is almost impossible to control any composition in the column if the feed split is fixed.

18. Manipulation of Fundamental Variables for Composition Control
Fractionation Variables
L/D
QH/ F (steam to feed ratio)
L/F
High purity columns or dual product purity columns
DeC3’s, DeC4’s, DIB’s, etc.
Feed Split Variables
D or B flow (direct control scheme)
FCC Main Fracs, Crude and Vacuum column side cuts
L or QH (indirect control scheme)
Level adjusts the product flow indirectly

19. Controlled Variables for Composition Control
Stage temperature (Inferential control)
Useless for aij < 1.2
Online analyzer
High economic gains
aij < 1.2
Temperature control – Special cases
Difficult separations ( 1.2 < aij < 1.5)
Flat temperature profiles
Use differential temperatures ( DT = Tm – Tk) between stages for control
Example – HVGO quality control
Extremely easy separations (high aij)
Nonlinear in nature
Steep temperature profile
Use temperature profile control
Tavg = (Tk + Tm)/ 2 , etc.

20. Composition Control Temperature Sensor Location
Locate TI on the stage whose temperature shows maximum sensitivity to one of the available MV’s
From simulation calculate (dTi / dD)L,B, (dTi / dL)D,B , (dTi / dB)L,D and
(dTi / dQ)L,D where Ti is the temperature of stage i. Locate TI at the stage where (dTi / dD)L,B , etc., is maximum.
For calculating the derivatives, vary B, D, L and Q in the column specs only by small amount, e.g., by +0.5% and -0.5%. Calculate average derivative.
Scale each variable by dividing it by its span in order to calculate the derivatives. The derivative will be a dimensionless number.
Use high precision numbers

21. Optimum Temperature Sensor Location
Most common Mistake! TC

22. Optimum TI Location for Columns with Side Draws
Locate the TI in the vapor space one – two stages below the product draw for product EP control
This temperature (P-compensated) correlates well with the product EP
Example
Atmos column diesel 95% pt control

23. TI Location for Side DrawTC

24. Special Cases Draw Tray Control
Total Draw Tray
Control tray level by product draw
Control pumpback on flow control
Control p/a on flow control p/a duty as CV
In fuel vacuum columns maximize duty LC FC LT

25. Special Cases Draw Tray Control
Partial Draw Tray
Level on the tray is fixed by the outlet weir height. There is no level control FC FC LT FC

26. Special Cases Stripping Steam Flow
Bottom stripping steam
Maximize to 8 – 12 lb stm per bbl of product
Fixed flow control
Side stripping steam
Minimize to meat front end spec
Use steam/ product ratio control

27. Distillation Control Case Study: Deisobutanizer Control
Joyce Kaumeyer
Sr. Consulting Engineer
Prakash Karpe
Control & Elec. Eng. Supt.
ConocoPhillips
San Francisco Refinery, Rodeo

28. Deisobutanizer

29. Tower Pressure Control Tower Temperature Control Composition Control
Tower Operation
Tower Pressure Control
By Overhead Product Rate
Tower Temperature Control
Tray 45 By Condensate Level (Steam)
Composition Control
Operator Adjusts Reflux Rate Based on Lab / On-line Analyzer
Tower Feed from Various Upstream Units
Large Rate Swings

30. Deisobutanizer Control Objectives
Control IC4 Product, IC4 Concentration
Reduce Variability & Control Closer to Specification
Improve Tower Pressure Control
Reflux / Product Rate = 5 / 1
Change Existing Temperature / Composition Control
Reduce NC4 Product, IC4 Concentration

31. Deisobutanizer Modified Controls

32. IC4 Product On-line Analyzer Vs. Delta Temperature Correlation

33. IC4 Product IC4 / Delta Temperature Correlation
Process Dynamics
Deadtime: 19 minutes
Lagtime: minutes

34. Modified Tower Operation
Tower Pressure Control
By Reflux Rate
Tower Heat Input Control
By Condensate Level (Steam)
Composition Control
Operator Adjusts TDIC Setpoint Based on Lab / On-line Analyzer
Tower Feed from Various Upstream Units
Large Rate Swings

35. Tower Pressure Control Before and After

36. IC4 Product %IC4

37. NC4 Product %IC4

38. Future ARC -OR- DMC Add AIC Cascaded to TDIC
Requires improved analyzer performance
Add Heat Input Feed-Forward to AIC
-OR-
DMC
Hold for DCS platform conversion to Refinery Standard