DISTILLATION CONTROL
Dr. Prakash Karpe
Control & Elec. Eng. Supt.
ConocoPhillips
San Francisco Refinery, Rodeo
1
Distillation Column Control
Control Objectives
Qc
L
V
D
Rectification
Stages
F
R = L/D
Stripping
Stages
QH
B
• Two Control objectives
– Inventory control
– Composition control
2
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)
3
Degrees of Freedom Analysis
Flash Vessel (Separator)
V
F
F,T,P,xi
Disturbances
B
4
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
5
Degrees of Freedom Analysis
Flash Vessel (Separator)
Degrees of Freedom = 0
V
PC
F
F,T,P,xi
Disturbances
LC
B
6
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
7
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
8
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
9
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
• Examples
– 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
10
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.
11
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
12
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.
13
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.
14
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
15
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?
16
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.
17
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
18
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.
19
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
20
Optimum Temperature Sensor
Location
Most common Mistake!
TC
21
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
22
TI Location for Side Draw
L
TI
D
TC
F
23
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
FC
LC
LT
FC
24
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
25
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
26
Distillation Control
Case Study:
Deisobutanizer Control
Joyce Kaumeyer
Sr. Consulting Engineer
Prakash Karpe
Control & Elec. Eng. Supt.
ConocoPhillips
San Francisco Refinery, Rodeo
27
Deisobutanizer
PIC
A
PIC
B
SW
Fuel Gas
Partially
Flooded
Condenser
Flooded
Accumulator
LIC
IC4
TI
OVHD
Low Level
Override
FIC
RFLX
SS
Tray 1
FI
IC4
Tray 13
IC4
TI
13
Feed 1
AI
Tray 25
IC4
Feed 2
Tray 37
Tray 45
TIC
45
Tray 60
LIC
FIC
NC4
NC4
NC4
AI
PIC
STM
Steam
NC4
Partially
Flooded
Reboiler
LIC
COND
FI
STM
Condensate
28
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
29
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
30
Deisobutanizer
Modified Controls
PIC
A
PIC
B
SW
Fuel Gas
Partially
Flooded
Condenser
Flooded
Accumulator
TI
TDIC
OVHD
OVHD
LIC
IC4
Low Level
Override
FIC
RFLX
SS
Tray 1
FIC
IC4
Tray 13
IC4
TI
13
Feed 1
AI
Tray 25
IC4
Feed 2
Tray 37
Tray 45
TI
45
Tray 60
LIC
FIC
NC4
NC4
NC4
AI
UIC
PIC
STM
Steam
Partially
Flooded
Reboiler
NC4
BTU
LIC
COND
FI
STM
Condensate
31
IC4 Product
On-line Analyzer Vs. Delta Temperature Correlation
100.0
98.0
%
96.0
94.0
92.0
90.0
Analyzer IC4
DT Predicted IC4
32
IC4 Product
IC4 / Delta Temperature
Correlation
%IC4 = 100.3 – 1.4464 * (Delta T)
Process Dynamics
• Deadtime: 19 minutes
• Lagtime: 102 minutes
33
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
34
Tower Pressure Control
Before and After
Before
After
35
IC4 Product
%IC4
IC4 Product
100.0
98.0
96.0
94.0
92.0
90.0
High Pentanes
88.0
Steam Increase
Start New Control
IC4
36
NC4 Product
%IC4
Isobutane Giveaway in n-Butane
6.0
%
Start New Control
Initial Implementation phase
Operator Training
5.0
4.0
3.0
2.0
1.0
0.0
37
Future
• ARC
– Add AIC Cascaded to TDIC
• Requires improved analyzer
performance
– Add Heat Input Feed-Forward to AIC
-OR• DMC
– Requires improved analyzer
performance
– Hold for DCS platform conversion to
Refinery Standard
38