Distillation Control
Whitehouse Consulting
Redway House, East Lane, Merstone, Isle of Wight, PO30 3DJ, United Kingdom
tel:
+44 (0)1983 529931
fax:
+44 (0)1983 530651
e-mail:
[email protected]
web site: www.whitehouse-consulting.com
Contents
• Basic control strategies
– energy balance
– material balance
• Composition control
– cut and separation
– use of tray temperature
– pressure compensation
– inferential properties
– interactions
• Optimisation
2
1
Process
3
Process control objectives
• Maintain energy balance (column pressure)
• Maintain mass balance (reflux drum and column
levels)
• Maintain product compositions within specifications
• Maximise recovery of more valuable product
• Minimise disturbances to downstream unit
• Optimise product yields versus energy consumption
4
2
Instrumentation
5
Basic control problems
• Pairing PV’s with MV’s:
PV’s
pressure
reflux drum level
column bottoms level
top product composition
bottom product composition
MV’s
distillate flow
bottoms flow
reboiler duty
condenser duty
reflux flow
• Theoretically 120 (5!) possible combinations
6
3
Basic control problems
• Inherently non-linear process
• Long process deadtimes with complex dynamics
• Multiple disturbances
– feed rate
– feed enthalpy
– feed composition
– reboil duty
– condenser duty
– column pressure
– product composition targets
• Expensive instrumentation (e.g. on-stream analysers)
7
Maintaining the energy balance
• Pressure is a good indication of energy balance
– rises if energy in > energy out
– quick response
• Need to control pressure in any case since it affects
– bubble points
– tray loading
– relative volatility
– temperature difference across reboiler and
condenser
– process safety
8
4
Maintaining the energy balance
+
.
feed enthalpy
reboiler duty
= +
+
product enthalpy
condenser duty
losses
• Realistically only condenser duty (and sometimes
reboiler duty) can be manipulated
• Multiple schemes, involving manipulation of:
– vapour condensation
– rate at which vapour leaves the column
– vapour generation rate
9
Manipulation of vapour condensation
• Used standalone when there is normally no vapour
product
• Several methods involving manipulation of
– coolant rate
– coolant temperature
– condenser efficiency
– vapour bypass
10
5
Manipulation of coolant rate
• Manipulation of cooling
water
– minimum flow often
restricted by need to
avoid high temperatures
or because of fouling
problem
– response can be slow
and non-linear
11
Manipulation of coolant rate
• Manipulation of air flow (fan speed, fan pitch, number
of fans or louvre position)
• Mechanical problems (deadband, hysteresis,
reliability and linearity)
12
6
Manipulation of coolant temperature
• Costly – each column needs additional pump
13
Flooded condenser
• Liquid covers part of condensing surface so that only
much less effective sub-cooling of reflux takes place
• Pressure controller changes liquid level and therefore
effective surface area of condenser
14
7
Hot vapour bypass
• Condenser at ground
level gives maintenance
advantages
• Because bypass line is
smaller than main line to
condenser, tendency to
locate control valve in
bypass to reduce cost
• Can give inverse
response problem
15
Inverse response
70
process parameter (% of range)
60
50
40
BYPASS VALVE
30
PRESSURE
20
10
0
0
5
10
15
time (minutes)
20
25
16
8
Hot vapour bypass
• Minimum SP limit in dPC
to ensure sufficient to
overcome liquid head
• Consider logic to disable
PC if dPC switched to
manual
• Controllers will interact reduce by using PC SP
(rather than PV) in dPC
and tune dPC to be faster
than PC
17
Manipulation of vapour rate
• Must have significant flow of vapour
• Fast response
• Several approaches
– direct manipulation of valve in vapour line
– manipulation of overhead gas compressor
– manipulation of spillback around ejector on
vacuum columns
18
9
Manipulation of vapour rate
19
Overheads gas compressor
20
10
Vacuum column ejector
• Often operated on manual with valve shut to minimise
pressure and improve product yield
21
Manipulation of vapour generation
• May have no other choice
– at minimum condenser duty (e.g. all fans switched
off on in cold weather)
– at maximum condenser duty (e.g. all fans switched
on in hot weather)
– cannot manipulate condenser duty (e.g.
mechanical problems on air condenser)
• Slower response to disturbances
• May result in poor composition control strategy
22
11
Manipulation of vapour generation
23
Combination methods
• Can use “split range” techniques if manipulating only
one variable does not give sufficient range of control
• Examples
– limited condenser capacity
– intermittent vapour
– compressor speed limits
– upset conditions
24
12
Valve positioner calibration
120
VALVE POSITION (% open)
100
80
60
40
20
0
-20
0
10
20
30
40
50
60
70
80
90
CONTROLLER OUTPUT (% of range)
100
25
Split range valves
120
VALVE POSITION (% open)
100
80
60
40
20
0
-20
0
10
20
30
40
50
60
70
CONTROLLER OUTPUT (% of range)
80
90
100
26
13
Combination methods
• Split range on column
with limited condenser
capacity
• Pressure too high - first
open cooling water then
open off-gas
• Can have problems
• non-linearity
• deadband or overlap
27
Combination methods
• Better to use two
pressure controllers
• Slightly higher set-point
in off-gas controller
• Must use the same
measurement since any
discrepancy will cause
instability
28
14
Maintaining the material balance
feed = distillate + bottoms
• Need to manipulate at least one
• Changing level is first indication of imbalance
– reflux drum
– column base
• If we cannot manipulate column feed rate there
remain theoretically 12 possible schemes, but only
two or three are practical
29
Maintaining the material balance
reflux drum level
manipulated variable
column level
controller
manipulated
variable
reflux flow
distillate
flow
bottoms
flow
“material
balance”
scheme
“energy
balance”
scheme
reboiler
duty
cannot
work
“material
balance”
scheme
30
15
Exercise
Set up the basic controls for T2.
– from the overview, click on T2
– click on Controls, then Answers
– enter the password ‘QC2’ and click on Reveal, then
Copy and then Yes
– switch drum level controller (LC2) to manipulate
reflux (FC6)
– switch column level controller (LC3) to manipulate
reboiler steam (FC7)
– increase feed rate (FC9) to 105 m3/hr and click Start
– examine the product flows and explain what has
happened
31
Violating the material balance
32
16
Exercise
Set up one of the Material Balance strategies
– switch drum level controller (LC2) to manipulate
distillate (FC5)
– switch column level controller (LC3) to manipulate
reboiler steam (FC7)
– trend LC3PV and initialise
– decrease column reflux (FC6) by 10 m3/hr and click
Start
– explain the problem with LC3
– what happens if reflux is decreased by 12 m3/hr?
– under what circumstances would this scheme be
required?
33
“Material balance” (bottoms small)
34
17
tight level control not possible, potential plant trip if decrease in
reflux was larger so need to correct product composition slowly
35
Exercise
Set up the Energy Balance scheme
– switch drum level controller (LC2) to distillate
(FC5)
– switch column level controller (LC3) to manipulate
bottoms (FC8)
– click on Controls, then LC2 and select averaging
control
– trend LC2PV and FC5PV on a 4 hour trend
– reduce reflux (FC6) by 10 m3/hr and click Start
– what advantage do you see?
– why must tight level control be used with the
Material Balance strategy?
36
18
“Energy balance”
37
averaging LC uses available surge capacity and changes
downstream flow as slowly as possible
38
19
tight LC changes distillate flow immediately as reflux is changed
39
“Material balance” (distillate small)
40
20
averaging LC changes reflux flow slowly as
distillate is changed and would result in
poor product composition control
41
tight LC changes reflux flow immediately as
distillate is changed and causes fast change
in product composition
42
21
tight LC changes bottoms flow immediately as effect of reduced
reflux reaches column base
43
little benefit on this column from using averaging LC on column
base because surge capacity is small
44
22
Cut and separation
45
Cut and separation
• Mass balance:
F  DB
• Light key balance:
F .LK f  D.LK d  B.LK b
• Eliminate B to define distillate “cut”:
D LK f  LK b

F LK d  LK b
46
23
Cut and separation
• Fixing LKf , LKb and LKd fixes cut
• Example:
LKf = 45%
LKd > 95%
LKb < 10%
D 45  10

 0.41
F 95  10
• i.e. 41% of feed must be drawn as distillate to achieve
desired compositions exactly
47
Cut and separation
• Normally want to keep HKd and LKb constant
• Since LKd = 100 – HKd – LLKd this is equivalent to
keeping LKd constant (assuming small changes in
non-key component)
• Should therefore only change cut if LKf changes
• In other words, only change distillate flow if there are
changes in:
– feed rate
– feed composition
– product composition targets
– non-key component content
48
24
Maintaining the material balance
• Material balance scheme favoured when
– large reflux ratio (R/D>5)
• more scope for drum level controller to
manipulate reflux rather than distillate flow
– situations where keeping cut constant is important
• frequent disturbances to energy balance (feed
enthalpy, reboiler duty, condenser duty)
• tray temperature unsuitable as inferential
composition control (components with similar
bubble points)
• high product purity (small changes in cut give
large changes in composition)
49
Maintaining the material balance
• Energy balance favoured when
– small reflux ratio (R/D<1)
• more scope for drum level controller to
manipulate distillate rather than reflux flow
– situations where distillate flow is required to
change
• frequent disturbances to feed rate and/or
composition
– large reflux drum
• potential to use surge capacity to benefit
downstream unit
50
25
Maintaining the material balance
• For many columns there is not a clear choice between
energy and material balance strategies
• Advanced controls deal with the weaknesses of both
approaches and so make choice less important, for
example
– addition of distillate-to-feed ratio to material
balance strategy overcomes problem of cut
changing
– addition of feed composition feedforward to
material balance strategy changes cut as required
– additional of internal reflux and reboil duty control
schemes to energy balance strategy helps it deal
with energy balance disturbances
51
Feed rate feedforward (material balance)
52
26
Feed composition feedforward
(material balance)
D LK f  LK b

F LK d  LK b
53
Internal reflux and reboiler duty
(energy balance)
54
27
Cut and separation
• Fixing cut does not fix compositions - necessary but
not sufficient condition
• Rearranging:
D
LK f    LK d
F
LK b 
D
1  
F
D LK f  LK b

F LK d  LK b
• In our example:
LK b 
45  0.41 LK d
 76.5  0.697 LK d
1  0.41
55
Cut and separation
45
40
35
D/F = 0.30
LKbb (mol
(mol %)
%)
30
25
D/F = 0.41
example of reduced separation
20
15
10
D/F = 0.50
5
0
45
50
55
60
65
70
75
LK d (mol %)
80
85
90
95
100
56
28
Cut and separation
57
Cut and separation
• Fixing D/F at 41% permits multiple product
compositions
–
–
–
–
LKd = 95%
LKd = 75%
LKd = 65%
etc.
LKb = 10%
LKb = 24%
LKb = 31%
is only one solution
is another
is another
• Need also to fix “separation”
58
29
Cut and separation
• Fenske equation:
N min 
log(S )
log( )
 LK d 


HK
d

S
 LK b 


 HK b 
• Normally used to determine the minimum number of
theoretical trays (Nmin) at total reflux
59
Target separation
60
30
Reduced separation
61
Cut and separation
• Separation depends on:
– column design (number of trays, tray efficiency,
location of feed tray etc.)
– relative volatility of components
– operating conditions (feed composition, feed
enthalpy, reboiler duty etc.)
• Once the column is built the only parameter which
can usually be changed is the energy used
(“fractionation”) which is set by reflux or reboil
• Column will limit maximum achievable separation
62
31
Tray loading constraints
63
Composition control
• We have two remaining process variables
– HK in distillate
– LK in bottoms
• Material balance scheme
– remaining manipulated variables are reboil and
distillate rate (usually)
• Energy balance scheme
– remaining manipulated variables are reboil and
reflux
64
32
Material balance
65
Energy balance
66
33
On-stream analysers
• Not always practical to directly measure product
compositions
• Technology may not exist for property required
• On-stream analysers are usually:
– expensive to install
– expensive to maintain
– slow
– potentially unreliable
• May not be sufficient economic justification for either
or both analysers
67
Tray temperature control
• Even if analysers justified we are likely to need a
faster control strategy
• Will also provide some level of control in the event of
analyser failure
• Can often be provided by tray temperature control
– liquid on trays is at its bubble point, which is
composition dependent
– temperature gives an approximate indication of
composition
– temperature control approximately fixes cut
68
34
Top tray TC (material balance)
69
Bottom tray TC (material balance)
70
35
Bottom tray TC (energy balance)
71
Top tray TC (energy balance)
72
36
Potential tray TC problems
• Liquid on tray not at its bubble point
– sub-cooled reflux
– superheated vapour from reboiler
– liquid feed not at its bubble point
– vapour feed not at its dew point
– change in feed composition
• Need to allow a few trays for equilibrium to be
reached (i.e. avoid trays at top or bottom of column,
or close to feed tray)
73
Sensitive to feed composition
30
Temperature
controlled at
68.8oC to meet
5% target
C4 in distillate (mol %)
25
20
15
Change in feed
composition
causes C4 to
drop to 1%
10
LKf -5%
5
0
64
66
68
70
tray temperature (OC)
72
74
76
74
37
Potential tray TC problems
• Non-linear relationship between composition and
temperature (or between temperature and
manipulated flow)
• Temperature insensitive to composition changes if
components have similar boiling points
• Variation in content of off-key components will
change the relationship between key composition and
temperature
75
Non-linear relationship
30
C4 in distillate (mol %)
25
Large process gain
if composition
higher than target
20
15
Small process gain
if composition less
than target
10
5
0
64
66
68
70
72
74
76
O
tray temperature ( C)
76
38
Insensitive temperature (C3/C3= splitter)
1.4
C3 in distillate (mol %)
1.2
1.0
0.8
0.6
Doubling C3 content
increases tray
temperature by 0.4oC
0.4
0.2
0.0
59.0
59.2
59.4
59.6
59.8
60.0
60.2
60.4
60.6
tray temperature (OC)
77
Locating the temperature sensor
• Many problems can be avoided by correctly locating
the temperature sensor
• Requires simulation study
– plot column temperature profile for normal
operating conditions
– vary chosen manipulated variable by about 5%,
keeping the others constant and plot new profile
– repeat in opposite direction
78
39
Effect of reboil (energy balance - reflux fixed)
20
19
+0.5 t/hr steam
18
17
16
-0.5 t/hr steam
15
tray number
14
13
12
11
10
9
8
7
6
5
4
3
2
1
40
50
60
70
80
tray temperature (OC)
90
79
Locating the temperature sensor
• Selection criteria for likely control point(s):
– upper section of column for distillate composition
– lower section of column for bottoms composition
– avoid regions close to feed tray (affected by
changes in feed composition)
– avoid regions too close to top and bottom of
column (most affected by sub-cooled reflux,
superheated reboil and off-key components)
– where temperature varies most (for sensitivity)
– base case temperature is midway (for linearity)
• May need to compromise between criteria
80
40
Effect of reboil (energy balance - reflux fixed)
20
19
18
17
16
15
tray number
14
+0.5 t/hr steam
-0.5 t/hr steam
13
12
11
10
9
8
7
6
5
4
3
2
1
-8
-6
-4
-2
0
2
4
6
change in tray temperature (OC)
8
81
Linearity/sensitivity of lower tray
90
tray temperature (OC)
88
86
tray 3
84
82
tray 4
80
78
76
13.4
13.6
13.8
14.0
reboil steam (t/hr)
14.2
14.4
14.6
82
41
Bottoms composition vs. temperature
9
8
C3 in bottoms (mol %)
7
-5% LKf
-5% LKf
6
tray 3
5
tray 4
4
3
2
1
0
76
78
80
82
84
86
88
tray temperature (OC)
90
83
Locating the temperature sensor
• Check suitability of selected point(s):
– plot temperature against manipulated variable (e.g.
reboil) and check sensitivity and linearity
– plot product composition versus temperature and
check linearity
– plot again with a different feed composition and
check that correlation only changes little
• If problem encountered then consider relocating the
sensor
• If this does not resolve problem then an analyser may
be the only answer
84
42
Checking for linearity and sensitivity
QC
TC
LKb
FC
lower tray
LKf
lower tray
reboil
85
Effect of pressure on bottoms composition
10
9
compensating
change to tray
temperature to
maintain quality
C3 in bottoms (mol%)
8
7
6
5
+1 bar
4
disturbance to
quality if tray
temperature
kept constant
3
2
-1 bar
1
0
80
81
82
83
84
85
86
tray temperature (OC)
87
88
89
90
86
43
Pressure compensation
• Although the pressure controller should work well
there will be disturbances and we may wish to change
the pressure often
• If pressure changes, need to modify temperature (T)
PV or SP to give pressure compensated temperature
(PCT)
• Simplest form is linear correction:
PV  TPV  K ( PPV  Pref )  PCT
• K (dT/dP) can be determined from plant data or
theoretically
• Pressure measurement should strictly be at same
location as TI but not normally necessary
87
Exercise
•
•
•
•
•
Commission the Energy Balance scheme
Cascade TC2 to FC6
Initialise and note the readings of TC2 and QC2
Change the column pressure by 1 bar
Adjust the SP of TC2 until QC2 returns to its starting
value
• Calculate dT/dP
88
44
Exercise
Calculate dT/dP from the Antoine equation:
ln( P )  A 
B
T C
P = pressure (bara)
component
A

dT
B

dP P( A  ln( P)) 2
T = temperature (OC)
B
C
propane
(C3H8)
9.04654
1850.841
246.99
butane
(C4H10) 9.05800
2154.897
238.73
89
Inferential properties
• ‘Soft sensors’ or ‘virtual analysers’ predict product
composition from basic measurements
• Two approaches
– ‘first principles’ using chemical engineering theory
– regress previously collected process data
• Typical form of regressed inferential
LKb  a0  a1.tray temperature  a2 .pressure  a3
bottoms flow
reboil duty
• Give dynamic advantage
– on-stream analyser used to update a0
– or replace on-stream analyser altogether
90
45
Interactions
• So far our control design has only considered
controlling either distillate or bottoms composition not both
• Problem is that cut and separation affect both
compositions
• Correcting one composition will affect the other
• In many cases the interaction may be sufficient for
the two composition controllers to become unstable
• Usually need some decoupling technique
(‘multivariable control’) which compensates one
controller so that the action of the other has no effect
91
Effect of cut (fractionation fixed)
25
20
C3H8 in bottoms
target operation
15
mol %
C4H10 in distillate
10
5
0
30
32
34
36
38
40
42
44
46
48
50
3
distillate (m /hr)
92
46
Effect of fractionation (cut fixed)
18
16
14
target operation
mol %
12
10
C3H8 in bottoms
8
6
C4H10 in distillate
4
2
0
9
10
11
12
13
14
reboiler steam (t/hr)
15
93
Optimisation
• Common error is to try to produce both distillate and
bottoms exactly at specification
– minimum fractionation saves energy
• If one product more valuable than the other then it
may be better to over-fractionate to recover more
product
– additional energy cost for additional yield
– relationship very non-linear - point will be reached
where additional yield does not justify additional
energy
– need to identify optimum
94
47
Feasible operating region
75
70
LKb>5%
reflux (m3/hr)
65
60
55
HKd>5%
50
45
13.0
13.5
14.0
14.5
15.0
15.5
16.0
16.5
reboil steam (t/hr)
17.0
95
Energy/yield optimisation
400
revenue
300
profit
profitability
200
optimum:
steam = 15.7 t/hr
bottoms
offgrade
100
0
-100
cost
-200
10
12
14
16
reboil steam (t/hr)
18
20
22
96
48
Energy/yield optimisation
300
profitability
290
280
optimum:
C3 in bottoms = 1 mol %
270
profit
260
250
0
1
2
3
C3 in bottoms (mol %)
4
5
6
97
49