fundamentals of distillation column control control basic – inventory control – composition...
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Fundamentals of Distillation Column Control
by Terry Tolliver
ISA Automation Week 2011
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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 2009. He was inducted into the CONTROL Automation Hall of Fame in 2002.
• 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 1987.
• Terry has been an adjunct professor for Washington University from 2004-2008, a Lehigh University Biannual Lecturer from 1974-1986, an ISA Fellow in 1990, an AIChE Fellow in 2000 and a PE since 1974.
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
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DISTILLATION CONTROL
BASIC –Inventory Control –Composition Control
OPTIMIZING –Floating Pressure –Maximum Profit
SUBOPTIMIZING –Feedforward Control –Two Point Composition Control
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Distillation control can be discussed on at least three levels: Basic, Suboptimizing, and Optimizing. Good Basic column control is most important because the more advanced schemes build upon it. Basic column control must provide stable operation while maintaining column vapor and liquid inventories and one composition, usually inferred at some location in the column. Feedforward control is a widely applicable Suboptimizing control technique which improves the performance of the Basic column control by reducing composition variability resulting from measured disturbances.
CLASSIFICATION and PAIRING of VARIABLES
CONTROLLED VARIABLES distillate composition bottom composition accumulator level sump level column pressure
Y Y
X
X
La
La
Ls
Ls
P
P
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Basic distillation control is primarily concerned with the proper pairing of controlled and manipulated variables in order to best reject the effect of disturbance variables. Feed flow is often controlled to a desired flow rate, and then its setpoint is manipulated to establish a production rate, which thus makes the feed flow a disturbance variable to the operation of the distillation column. In fact, feed flow changes are often the most abrupt and severe disturbances that must be handled. Fortunately, feed flow rate is usually a measured disturbance and thus suitable for Feedforward control to be applied.
MANIPULATED VARIABLES distillate flow bottom flow reflux flow reboiler duty condenser duty
D D B
B
R
R
Qr
Qr
Qc
Qc
CLASSIFICATION and PAIRING of VARIABLES
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Presentation Notes
Basic distillation control is primarily concerned with the proper pairing of controlled and manipulated variables in order to best reject the effect of disturbance variables. Feed flow is often controlled to a desired flow rate, and then its setpoint is manipulated to establish a production rate, which thus makes the feed flow a disturbance variable to the operation of the distillation column. In fact, feed flow changes are often the most abrupt and severe disturbances that must be handled. Fortunately, feed flow rate is usually a measured disturbance and thus suitable for Feedforward control to be applied.
CLASSIFICATION and PAIRING of VARIABLES
DISTURBANCE VARIABLES feed flow feed composition feed temperature reboiler heat supply condenser cooling supply and weather
F F Z Z q q S
S
W
W
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Presentation Notes
Basic distillation control is primarily concerned with the proper pairing of controlled and manipulated variables in order to best reject the effect of disturbance variables. Feed flow is often controlled to a desired flow rate, and then its setpoint is manipulated to establish a production rate, which thus makes the feed flow a disturbance variable to the operation of the distillation column. In fact, feed flow changes are often the most abrupt and severe disturbances that must be handled. Fortunately, feed flow rate is usually a measured disturbance and thus suitable for Feedforward control to be applied.
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
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Presentation Notes
Basic distillation control is primarily concerned with the proper pairing of controlled and manipulated variables in order to best reject the effect of disturbance variables. Feed flow is often controlled to a desired flow rate, and then its setpoint is manipulated to establish a production rate, which thus makes the feed flow a disturbance variable to the operation of the distillation column. In fact, feed flow changes are often the most abrupt and severe disturbances that must be handled. Fortunately, feed flow rate is usually a measured disturbance and thus suitable for Feedforward control to be applied.
CONTROL OBJECTIVES
$/F
RECOVERY
Cost of Product Lost
Cost of Energy
Total Operating Cost
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Primary and secondary objectives must be identified as one of the first steps toward selection of a basic control scheme. Typically, the primary objective is to produce one product within its quality specifications, while the secondary objective is to maximize profit by minimizing the total operating costs. When a process is running soldout, the secondary objective to maximize profit usually equates to maximizing throughput.
COLUMN CONSTRAINTS
Column Pressure
Boilu
p ra
te
Vessel Limit
Weep
Flood
Reboiler
Condenser
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Operating constraints must be considered. Proper design of the column should place the normal operating point in the center of the operating window. Flooding is indicated by high pressure drop across the column, rapid accumulator level increase, sump level decrease or an abnormal temperature profile. Weeping gradually decreases separation efficiency. Condenser duty is limited by the temperature difference between the coolant and process temperature. Reboiler duty is limited by the temperature difference between the heat supply and process temperature.
COLUMN INTERNALS
TRAYS – SIEVE TRAYS – VALVE TRAYS – BUBBLE CAP TRAYS – DUAL FLOW TRAYS
PACKING – RANDOM PACKING – STRUCTURED PACKING – LIQUID & VAPOR DISTRIBUTORS
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TRAYS Sieve trays are the most common tray with 70% turndown capability. Valve trays are proprietary and provide 50% turndown. Bubble cap trays have even greater turndown, although used widely in the past, they are now used almost exclusively for columns requiring reaction residence time. Dual flow trays have greater capacity, but they provide less efficiency and the least turndown capability. PACKING Random packing like pall rings or saddles may provide an HETP equivalent to normally spaced trays but with greater capacity. Structured packing provides better HETP, capacity and lower pressure drop also. Vapor and liquid distributors are required at every stream entry or withdrawal point and every 20-25 feet to redistribute the flows. Column turndown is usually limited by the liquid distributor design and levelness of installation. V-notch weir, square-notch weir and orifice hole designs provide increasing turndown capability .
COLUMN DYNAMICS
RELATIVE RESPONSE TIMES
time
VAPOR - FAST LIQUID - MEDIUM COMPOSITION - SLOW
Response
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There are three distinct dynamic responses in both trayed and packed distillation columns. The vapor response is relatively fast and occurs within several seconds from bottom to top. The liquid response is medium, occurring in a few minutes from top to bottom, with the response being predominately deadtime. The composition response is slow, measured in hours throughout the column, with the response being predominately a slow first order lag plus a short deadtime roughly equal to the corresponding liquid response. Packed columns have similar, but often slightly faster dynamics than equivalent trayed columns.
MATERIAL & ENERGY BALANCES
MATERIAL BALANCE F = D + B zF = yD + xB
ENERGY BALANCE hF + Qreb = hD + hB + Qcond
D/F = (z-x)/(y-x)
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A Material Balance equation can be developed by combining the overall material balance equation with any component material balance equation. The notation used above applies to the more volatile component in a binary system. The resulting D/F equation shows that for a given feed composition there is a relationship between the two product compositions and the material balance “split”. A second relationship is required to uniquely determine the two product compositions. The Energy Balance equation is usually dominated by the reboiler and condenser duties since the difference in sensible heat for the feed and product streams is often small compared to the latent heat of vaporization of the feed. The reboiler and condenser duties represent the creation and removal of vapor, which directly affects the column pressure. For this reason, column pressure control usually is paired to manipulate one of these duties.
TOTAL REFLUX αn = [y(1-x)]/[x(1-y)]
SEPARATION
NORMAL OPERATION
S = (α, N, NF, V/F, z, E) f
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Separation is the second relationship which is required to uniquely determine the two product compositions. Separation is defined by the “Fenske-Underwood” equation for a column operating at total reflux. In this relationship, the average relative volatility raised to the number of stages of separation is equated to the ratio of product purity to product impurity. Under normal operation this relationship is much more complex, but the concept of a Separation factor can still be defined as the ratio of product purity to product impurity which is primarily a function of V/F, vapor per unit feed. Thus, for given Separation (V/F) and Material Balance (D/F) relationships, the product compositions are uniquely defined.
MATERIAL BALANCE and SEPARATION EFFECTS
D/F
Light
Key
Mol
e Fr
actio
n
0 1.0 0
1.0
y
X
Z
Increasing S
Increasing S
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The effect of varying the Material Balance (D/F) and Separation (S) can be visualized for a given column feed composition. Changes in D/F have a much stronger effect on product compositions than changes in Separation, except perhaps at very high product purities.
MATERIAL BALANCE CONTROL
FC
FC
FC
FC
FC
LC
LC
TC
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Material Balance control schemes manipulate the D/F ratio either directly or indirectly. In this example the ratio is directly manipulated by the temperature controller adjusting the setpoint on the distillate flow controller. Separation is fixed by a manual setpoint on the steam flow controller at the reboiler.
SEPARATION CONTROL
FC
FC
FC
FC
FC
LC
LC
TC NOT RECOMMENDED
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Separation control is not recommended because it fixes the material balance and requires much larger adjustments to the V/F, energy per unit feed. As a result, its range of control is also severely limited. In this example, the separation is manipulated by the temperature controller adjusting the setpoint on the reboiler steam flow controller. The material balance is fixed by a manual setpoint on the distillate flow controller.
PRESSURE CONTROL
FC FC LC
VENT
PI
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Column pressure is a measure of the moles of material in the vapor phase. If the feed and product streams are liquid, then only the reboiler duty and the condenser duty significantly affect the amount of material in the vapor phase. Condenser duty is usually manipulated to control pressure, unless it represents the limiting capacity constraint or uses a more costly utility than the reboiler. The simplest form of pressure control is at atmospheric pressure to connect the downstream side of the condenser to a suitable vent. This vent may lead directly to the atmosphere or to an inert header. Sometimes an additional vent condenser with a refrigerated coolant is used to minimize product vapor leaving with the inerts.
CONDENSER DUTY
Qcond = UA(Tp-Tc)
VENT
CONDENSATE
COOLANT
Tc
Tp
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The vent allows inerts to enter or leave the condenser as required to change the condenser duty. The condenser duty must satisfy the energy balance in order to keep the pressure constant. Condenser duty can be determined as: Qcond = UA(Tp-Tc) where - Qcond is the condenser duty U is the heat transfer coefficient A is the area available for heat transfer Tp is the condensing temperature Tc is the coolant temperature The area available for condensing vapor can be manipulated by adding or removing inert gas from the tubes. The area of the tubes that is blanked off by the inert gas provides subcooling to the condensate. If more vapor is being created than is being removed, then the pressure will rise and push out inerts until increased area available for condensing restores the energy balance. Conversly, if the pressure is dropping, inerts will enter the condenser until decreased area available for condensing restores the energy balance.
“BLOCK & BLEED” PRESSURE CONTROL
FC FC LC
VACUUM SOURCE
PC
BLOCK VALVE
BLEE
D VA
LVE
INERTS
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For columns operating under vacuum, the vent is connected to some vacuum source such as an steam jet or vacuum pump. A BLOCK and BLEED split-range valve arrangement is often used for pressure control in this case. The BLOCK valve is on the line to the vacuum source and the BLEED valve is on the inert supply line. The split-range is set up such that from 0-50% IVP the BLEED valve closes, and from 50-100% IVP the BLOCK valve opens. Normally, the BLOCK valve will be throttling the flow of naturally occurring inerts to the vacuum source and the BLEED valve will be fully closed. When pressure rises, the BLOCK valve will open further to let more inerts out. When pressure drops, the BLOCK valve will close allowing inerts to accumulate. If the BLOCK valve becomes fully closed, then the BLEED valve begins to open supplementing inerts. If there are sufficient inerts present naturally, then the BLEED valve and inert supply may not be required. For a column operating at very low pressure (< 20 mmHG), the BLOCK valve may be eliminated in order to minimize the pressure drop in the line to the vacuum source. In this case, the BLEED valve continually adds inerts, and the small pressure drop limits the flow out.
MATERIAL BALANCE CONTROL - TYPE 1
FC
FC
FC
FC
FC
LC
LC
TC
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Scheme 1 directly adjusts the material balance by manipulation of the distillate flow. If the distillate flow is increased, then the reflux accumulator level controller decreases the reflux flow. As less liquid proceeds to flow down to the sump, the sump level controller decreases the bottoms flow a like amount. The separation is held constant by manually setting the reboiler steam flow to maintain a constant energy per unit feed. This scheme is recommended when the distillate flow is one of the smaller flows in the column, particularly when the reflux ratio is large (R/D > 3). Also, it is important that the reflux accumulator level control can be tightly tuned and that the liquid holdup is not too large (< 5 minutes). This scheme has the least interaction with the energy balance, as it provides a good range of control with only small changes in the distillate flow, and it also provides a form of automatic internal reflux control. If the reflux becomes more subcooled, initially additional vapors will be condensed inside the column. However, the overhead vapors will be reduced by exactly the same amount, and with tight level control, the reflux will then be reduced accordingly. Another advantage of this scheme is that it lends itself readily to the application of feedforward control in order to maintain the D/F ratio for measured changes in feed flow. This feedforward signal would be trimmed by the feedback signal from the temperature controller .
MATERIAL BALANCE CONTROL - TYPE 2
FC
FC
FC
FC
FC
LC
LC
TC
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Scheme 2 indirectly adjusts the material balance through the two level control loops. If the reflux flow is increased, then the reflux accumulator level controller decreases the distillate flow. As the additional liquid proceeds to flow down to the sump, the sump level controller increases the bottoms flow a like amount. The separation is held constant by manually setting the reboiler steam flow to maintain a constant energy per unit feed. This scheme is recommended for columns with a small reflux ratio (R/D < 1). This scheme also offers improved dynamics, which may be required, particularly if the column has a large horizontal reflux accumulator.
MATERIAL BALANCE CONTROL - TYPE 3
FC
FC
FC
FC
FC
LC
LC
TC
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Scheme 3 indirectly adjust the material balance through the two level loops. If the steam flow is increased, then the sump level controller decreases the bottom flow. As the additional vapors go overhead and condense, the reflux accumulator level control increases the distillate flow a like amount. The separation is held constant by manually setting the reflux flow to maintain a relatively constant energy per unit feed. This scheme is recommended for columns with a small energy per unit feed (V/F < 2). This scheme also offers the fastest dynamics.
MATERIAL BALANCE CONTROL - TYPE 4
FC
FC
FC
FC
FC
LC
LC
TC
time
response
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Scheme 4 directly adjust the material balance by manipulation of the bottom flow. If the bottom flow is decreased, then the sump level controller increases the reboiler steam flow. As the additional vapors go overhead and condense, the reflux accumulator level control increases the distillate flow a like amount. The separation is held constant by manually setting the reflux flow to maintain a relatively constant energy per unit feed. This scheme is recommended when the bottom flow is one of the smaller flows in the column, particularly when the bottom flow is less than 20% of the vapor boilup. This scheme has little interaction with the energy balance, as it provides a good range of control with only small changes in the bottom flow. However, the tuning of the sump level loop usually makes this scheme slower than the others. An inverse response is also possible with this sump level control loop. This type of response occurs when an increase in steam flow temporarily cause the sump level to increase before it begins to decrease. If this occurs, the level loop must be detuned even more. An advantage of this scheme is that it lends itself readily to the application of feedforward control in order to maintain the B/F ratio for measured changes in feed flow. This feedforward signal would be trimmed by the feedback signal from the temperature controller.
PROCESS SIMULATION
STEADY STATE • PARAMETRIC CASES • CONTROL STAGE LOCATION • SENSITIVITY ANALYSIS • DISTURBANCE ANALYSIS
DYNAMIC • STUDY CONTROL RESPONSE • STARTUP AND SHUTDOWN
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HYSYS is a rigorous material and energy balance program which can provide both steady state and dynamic simulations. Parametric steady state cases can provide valuable insight into the operating characteristics of a particular column. HYSYS allows alternative independent parameters and their values to be specified, which makes it easy to study the effect of holding some parameters constant while varying others. Holding product compositions constant while varying feed composition would simulate perfect two point composition control. These parametric cases would indicate how various parameters must change in order to keep the product compositions constant, which can further guide in the selection of the basic column control scheme. Additional parametric steady state cases can be used to locate the control stage, determine the temperature sensitivity and evaluate the effect of disturbances. The basic column control scheme may then be implemented as part of the dynamic model and response studies can further evaluate the effectiveness of the control. Startup and shutdown scenarios may also be studied.
CONTROL STAGE LOCATION
Benzene-Toluene Column Temperature Profiles
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
70.0 75.0 80.0 85.0 90.0 95.0 100.0 105.0 110.0 115.0 120.0
Temperature, DegC
Stag
e Nu
mbe
r
+1%D/F
-1%D/F
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Once a basic column control scheme is chosen, parametric steady state cases, which hold the separation variable constant and adjust the material balance variable, can be used to determine the best control stage location. Temperature profiles from these parametric cases should be plotted together as shown. The best control stage location is where the largest, most symmetrical temperature deviation from the base case occurs. It is recommended that alternative thermowells be installed one stage above and below this location because of uncertainty in tray efficiencies and VLE data.
TEMPERATURE SENSITIVITY
CONTROL STAGE TEMPERATURE, degF
98
98.2
98.4
98.6
98.8
99
99.2
99.4
99.6
99.8
100
85 90 95 100 105 110
CO
MPO
SITIO
N, w
t%
BOTTOM PRODUCT
DISTILLATE PRODUCT
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Product compositions from these same parametric cases can be plotted against the temperature occurring on the uncontrolled control stage as shown. The effect of temperature variations due to measurement errors can be determined in this manner.
DISTURBANCE ANALYSIS
Benzene-Toluene Column Load Effects
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
-5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0
Delta Temperature, DegC
Tray
Num
ber
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Additional parametric cases can evaluate the steady state effect of disturbances on the proposed basic control scheme and control stage location. Temperature deviation profiles as shown illustrate the steady state changes that occur throughout the column at these different disturbance conditions.
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REFERENCES
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REFERENCES