SELECTING A DISTILLATION COLUMN CONTROL STRATEGY (a basic guide). Dr. M.J. Willis Department of Chemical and Process Engineering, University of Newcastle e-mail:
[email protected] Wri Written:
December, 1999 - March, 2000
Aims and objectives The aim of these notes is to provide some basic ideas and rules that may be used to select a distillation control strategy. Separate notes will discuss more complex mathematical techniques that may also be used as part of a 'toolbox' of methods that have evolved as aids in distillation control strategy selection.
Introduction The effective operation of a binary distillation column is determined by the control of many variables. Generally, the variables in table 1 need to be controlled. cv
reason
composition of the distillate stream, xD composition of the bottoms stream, xB. liquid le level in in th the re reflux dr drum. liquid liquid level level at the base of the column. column. pressure in the column
product quality product quality maiintain in ma inventory (ensure ma material ba balance) maintai maintain n inve inventor ntory y (ensure (ensure materia materiall balance balance)) maintain inventory (ensure energy balance). Equilibrium relationship is affected by changes in pressure.
Table 1. Typical variables that have to be maintained in a distillation column.
The two main disturbances that affect a column are: •
feed flowrate, F
•
feed composition, zf
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So called 'manipulated variables' are adjusted to counter-act the effect of disturbances and ensure desired operation. But what are the manipulated variables ? Relationships between inputs (mv's and dv's) and outputs (cv's) are quantified by steady-state material and energy balances. To simplify preliminary discussions consider ‘perfect control’ of pressure (i.e. the energy balance equations are not considered).
Steady-state material balances around a distillation column The following figure is a material balance diagram for a typical distillation column:
Rectifying Section
Vn, yn
Condenser
Reflux Ln,xD
Accumulator
n
Feed F, z F Stripping Section
D, xD Distillate
Ln,xn Vn-1,yn-1 m Lm,xm Vm-1,ym-1
Boilup VB,yB Reboiler
Bottom Product
LB,xB
B, xB The column feed is F (kmol/min) and the concentration of the more volatile component in liquid is zf. The distillate flow is D (kmol /min) with overhead product concentration xD and the bottom product flowrate is B (kmol / min) of concentration xB. For a binary column, the two independent overall balances are: •
the total material balance:
F=D+B
(1)
2
•
the component balance:
Fzf = DxD + BxB
(2)
Eliminating either B or D from these equations gives the following: D F
=
z f
−
x D
−
xB
B x D and / or = xB F x D
−
z f
−
xB
(3)
The equations (3) define the “cut”, i.e what percentage of the total feedflow exits the column as distillate and bottoms product for specified inlet and outlet concentrations. From equations (3) it is apparent that distillate (D) and bottoms flow (B) are related to top and bottom product compositions (xD and xB) and are therefore potential manipulated variables. As expected, changes in F and zf will also affect xD and xB. Around the condenser and accumulator assuming a total condenser, the material balances are: • •
the material balance: the component balance:
Vn = D + Ln Vnyn = DxD + LnxD
(4) (5)
Lm = Vb + B Lmxm = Vbyb + BxB
(6) (7)
and around the reboiler, • •
the material balance: the component balance:
For a liquid feed Lm = Ln+ F Assuming that the molar flows of liquid and vapour are constant through the column (constant molal overflow) then, L = Ln = Ln+1……etc. V = Vn = Vn+1 = ….etc. Therefore:
D = V - L and B = F + L - V
(8)
Equations (3) demonstrate that D and B may be used to regulated x D and xB, based upon the relationships (equation 8) it is obvious that L and V will also affect the product compositions.
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Summary: the potential manipulated variables for product compositions are D, B, L and V.
Column control strategies (an introduction) A ‘bottom – up’ approach should be adopted whereby variables that are essential to operation are regulated before quality variables. In other words, pressure and then level must be adequately controlled before attention is focused on control of composition. Pressure control Required as a change in pressure will affect relative volatility (α), the temperature difference across the reboiler and condenser as well as process safety. A common pressure control loop is shown below: Pressure Control
PC
V flare
P L
Accumulator D, xD Distillate
Feed F, zF
Boilup V
Steam in
B, xB Condensate
Figure 1. A common pressure control loop (PC = pressure controller).
Here, pressure is regulated using the flowrate of coolant to the condenser. Increasing or decreasing the water flowrate will alter the temperature of the condensing liquid and hence the amount of vapour in the column. This, in turn, alters the pressure in the column. This will be a slow loop as the dynamics effects of the cooling can be slow in comparison to simply venting the system by e.g. opening a valve (the figure also shows this option, which may be required as a safety mechanism, in case a situation of excessive pressures arose).
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Note: There are numerous pressure control strategies that should be considered. Further information may be found in: Chin, T.G. 'Guide to distillation pressure control methods', Hydrocarbon processing, October 1979, pp145 - 153 . For the purposes of this lecture attention is restricted to the strategy detailed above. Level control There will be two level loops on a distillation column as: • •
the column base level must be maintained at an acceptable value. the reflux drum level must be maintained at an acceptable value.
The possible schemes that may be employed to do this are summarised below: Controlling level at column base with: F D L B V Controlling reflux drum level with:
F D L B V
NP1
x U U
x NP4 NP2 NP3
NP5 x NP3*
(1) (3) x NP3
(2) NP3* NP2 x
KEY: Flows: NP: U: x:
F = Feed; D = Distillate; L = Reflux; B = Bottoms; V = Vapour Not Practical Unusual to control level at column base by manipulating the feed the same mv cannot be used to control both levels.
Reason why strategy is not practical: 1
Feed Flow would not be used to control reflux drum level. Bottoms flow would not be used to control reflux drum level. 3 Vapour flow would not be used to control reflux drum level. 3* This scheme violates the mass balance relationships therefore cannot be used (the reason why will be explained later in the notes) . 4 Distillate flowrate would not be used to control level at the column base. 5 Reflux flow would not be used to control level at the column base. 2
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From the matrix of 25 possible alternatives, there are actually only 3 schemes that offer acceptable input-output combinations (from a practical viewpoint). Scheme (I ) •
control level in the column base via manipulation of the bottom product flowrate (by automatically adjusting value ).
•
control level in the reflux drum by manipulation of distillate flowrate. a flow controller has been placed on the reflux line (to ensure steady flow of reflux to the column).
•
Configuring a control strategy: scheme 1 (the energy balance control scheme)
V
SP
PC
flare
P LC
L
D, xD Distillate
Feed F, zF
SP
LC
SP
FC
SP
Boilup V
Steam in
B, xB Condensate
Figure 2. Scheme (I), inventory control (PC = pressure control, LC = level control, FC = flow control).
Scheme (II ) •
control level in the column base by manipulation of the vapour boil-up through the energy input to the reboiler (in practice this is achieved by automatically adjusting the pressure/ flow of the heating medium to the reboiler ).
•
control level in the reflux drum by manipulation of distillate flowrate. a flow controller has been placed on the reflux line (to ensure steady flow of reflux to the column).
•
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Configuring a control strategy: scheme II (a material balance control scheme)
V
SP
PC
flare
P SP
LC
L
D, xD Distillate
Feed F, zF
FC
SP
Boilup SP
LC
SP
FC
V
B, xB Condensate
Figure 3. Scheme (II), inventory control (PC = pressure control, LC = level control, FC = flow control).
Scheme (III ) • • •
control level in the column base by manipulation of bottoms flowrate. control level in the reflux drum by manipulation of reflux flowrate. a flow controller has been placed on the distillate line (to ensure steady flow of product). Configuring a control strategy: scheme III (a material balance control scheme)
V
SP
PC
flare
P FC
L
D, xD Distillate
Feed F, zF
SP
LC
SP
LC
SP
Boilup V
Steam in
B, xB Condensate
Figure 4. Scheme (III), inventory control (PC = pressure control, LC = level control, FC = flow control).
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Selecting an appropriate distillation column control strategy Basic ‘rules of thumb’ can be used to develop feasible strategies. The methodology (as well as some rules) is explained below:
Rule of thumb 1: ‘flow control the smallest product flow’ (as this will leave a large flow stream to manipulate level). Example 1: Suppose that there is a large bottoms flowrate (B) and a small distillate flowrate (D). Using this information the control strategy may be developed as follows: •
flow control the distillate flow (D)
•
ensure that the material balance is maintained around the reflux drum. Recall that V = D + L, for a constant V, if D changes then there must be an equal and opposite change in L or the level in the reflux drum will either drop or start to increase. To ensure that the level remains constant (and that an appropriate change is made to L) a level controller is required on the reflux drum the manipulated variable being L. ensure that the material balance is maintained around the column base. Recall that F = D + B so if, for a constant F, D changes then there must be an equal and opposite change in B or the level in the base of the column will either drop or start to increase. To ensure that the level remains constant (and that an appropriate change is made to B) a level controller is required with its manipulated variable being B.
•
This control scheme corresponds to scheme III and is one of the more popular control schemes. It is often referred to as a material balance control scheme . Example 2: Suppose that there is a small bottoms flowrate (B) and a large distillate flowrate (D). Using this information the strategy may be developed as follows: • •
flow control the bottoms flow (B). ensure that the material balance is maintained around the column, F = D + B. For constant F, if B changes there must be an equal and opposite change in D or liquid inventory will change (e.g. level may rise in the reflux
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•
drum, column base, or both). To maintain constant inventory, a level controller is used to make an appropriate change to D. ensure that the material balance is maintained around the column base. Recall that at the column base F+ L - B = V, for a constant F and L, if B changes then there must be an equal and opposite change in V or the level in the base of the column will either drop or increase. To ensure that the level remains constant (and that an appropriate change is made to V) a level controller is required (the mv being V).
This control scheme corresponds to scheme II and it should be noted that the control of level using V may have weird dynamic effects and therefore is not a favourite. Again, this control scheme is often referred to as a material balance control scheme .
Rule of thumb 2: ‘material balance control scheme (III) should be favoured if there is a large reflux ratio, i.e. (L/D) > 5’ (if L is large in comparison to D then relatively small changes in L will ensure good level control, i.e. the process gain is large). Rule of thumb 3: ‘control scheme (I), often referred to as the energy balance control scheme, should be favoured if there is a small reflux ratio, i.e. (L/D) < 1’ (if L is small in comparison to D then relatively small changes in D will ensure good level control, i.e. the process gain is large).
Composition control On-line analysers are rarely used as the installed cost will normally be in the range of £100 K per instrument. Therefore composition is often regulated indirectly using temperature (at constant pressure there is a direct relationship between temperature and composition for a binary mixture). Using a liquid temperature near the base of the column for bottom composition and a liquid temperature near the top of the column for top product composition, the remaining mv’s (i.e. those not used for the purposes of level and pressure control) may be used to regulate composition. This leads to the following schemes:
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Scheme (I ) •
•
top product composition (through a liquid temperature near the top of the column) is regulated by adjusting reflux flow, L. bottom product composition (through a liquid temperature near the bottom of the column) is regulated by adjusting vapour flow, V (indirectly via steam flow).
This gives rise to an alternative name for this control strategy: the LV configuration. Composition Control: scheme 1 (the energy balance control scheme)
V
SP
PC
flare
P LC
L
SP
D, xD Distillate
Feed F, zF
FC
SP
TC
SP
SP
TC
LC
SP
V
B, xB Condensate
Figure 5. Scheme (I), inventory & composition control (PC = pressure control, LC = level control, FC = Flow control and TC = temperature control). This scheme is also known as the LV configuration.
Scheme (II ) •
•
top product composition (through a liquid temperature near the top of the column) is regulated by adjusting reflux flow, L. bottom product composition (through a liquid temperature near the bottom of the column) is regulated by adjusting bottoms flow, B.
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This gives rise to an alternative name for this control strategy: the LB configuration. Composition Control: scheme II (a material balance control scheme)
V
SP
PC
flare
P LC
L
D, xD Distillate
Feed F, zF
SP
SP
LC
SP
FC
SP
TC
V
FC
B, xB Condensate
Figure 6. Scheme (II), inventory & composition control (PC = pressure control, LC = level control, FC = Flow control and TC = temperature control). This scheme is also known as the LB configuration.
Scheme (III ) •
•
top product composition (through a liquid temperature near the top of the column) is regulated by adjusting distillate flow, D. bottom product composition (through a liquid temperature near the bottom of the column) is regulated by adjusting vapour flow, V.
This gives rise to an alternative name for this control strategy: the DV configuration.
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Composition Control: scheme III (a material balance control scheme)
V
SP
PC
flare
P FC
L
SP D, xD
Feed
SP Distillate
F, zF
LC
SP
TC SP
SP
LC
TC V
B, xB Condensate
Figure 6. Scheme (III), inventory & composition control (PC = pressure control, LC = level control, FC = Flow control and TC = temperature control). This scheme is also known as the DV configuration.
Worked example: a methanol / water column. 50 / 50 wt % methanol / water mixture is to be separated in a 10 stage column. The feedrate is 65 kg/hr entering at stage 5. The objective is to separate the mixture into a top product of 95 wt% methanol and a bottom product of 5 wt %. The feed is liquid at its boiling point. The condenser is a total condenser. The reflux flow is 36 kg/hr. a) What are the material flows through this system (external liquid and internal liquid and vapour flows) ? b) Suggest a possible control strategy for this column.
Summary Rules of thumb, common sense and a basic knowledge of chemical engineering can generally be used to specify an appropriate manipulated variables and hence the control scheme of a distillation column. However, this basic knowledge should also be complemented by rigorous systems analysis. To do this it is necessary to consider distillation column modelling in greater detail.
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