chemical engineering research and design 8 6 ( 2 0 0 8 ) 977–988

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Chemical Engineering Research and Design

journa l homepage: www.e lsev ier .com/ locate /cherd

Evaluation of control configurationsfor a depropaniser

I. Chawla, G.P. Rangaiah ∗

Department of Chemical and Biomolecular Engineering, National University of Singapore,Engineering Drive 4, Singapore 117576, Singapore

a b s t r a c t

Distillation columns are highly coupled and non-linear, and have major impact on the utilities consumption and

product quality. By proper design of their controls, energy consumption, product variability, operator intervention

and equipment downtime can be reduced. Thus, selection of proper controls for distillation columns is both chal-

lenging and critical. Although extensive literature have been published on various aspects of distillation control, viz.,

level controller tuning, ratioing manipulated variables and turndown operation, there is no comprehensive study on

control evaluation considering all aspects and rigorous simulation. In particular, turndown operation has received

little attention in control research. This work deals with the composition control of distillation columns considering

depropaniser as an example and using rigorous simulation. Effect of level controller tuning, ratioing the manipulated

variable and turndown operation on the performance of several control structures to reject step disturbances in feed

flow rate and composition, and sinusoidal disturbance in feed composition, is studied. Both single- and dual-ended

composition control of the depropaniser are considered. Results of this study show the need and importance of a

comprehensive and rigorous analysis including column operation far away from the design conditions, for optimal

design of column control.

© 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Depropaniser; Dynamic simulation; Control structure; Hysys; Composition control

1

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. Introduction

istillation continues to be the popular separation in chemi-al and related process industries. Distillation columns exhibitighly non-linear dynamics, and have major impact on util-

ties consumption and product quality. By proper design ofheir controls, energy consumption, product variability, oper-tor intervention and equipment downtime can be reduced.hus, selection of proper controls for distillation columns

s both challenging and critical. Column control based onngineering judgment alone may not give optimal perfor-ance.The composition control for distillation columns can

e broadly divided into single- and dual-ended controls.ingle-ended control where either overhead or bottom prod-ct composition is controlled, is widely used for industrial

olumns due to its simplicity, good disturbance rejectionnd minimum coupling. If the control structure is selected

∗ Corresponding author. Fax: +65 6779 1936.E-mail address: chegpr@nus.edu.sg (G.P. Rangaiah).Received 27 August 2007; Accepted 21 March 2008

263-8762/$ – see front matter © 2008 The Institution of Chemical Engioi:10.1016/j.cherd.2008.03.022

and tuned adequately, dual-ended control (where both over-head and bottom product compositions are controlled) hasadvantage over single-ended control in terms of reducedproduct variability and hence reduced energy consumptionbut at the cost of increased complexity, investment andcoupling. The distillation column experiences extensive cou-pling between overhead and bottom product compositionsas both the manipulated variables affect both the con-trolled variables. Hence, controller tuning should considerthis as well as the expected disturbances, namely, varia-tions in feed flow rate, feed composition, utility conditions,product purity specifications and environmental changes.The most severe disturbances like failure of power, cool-ing water, steam, instrument air, pumps, control valve andoperator errors are taken care by the column safety sys-

There are many books and vast literature on distillationdesign and control; only the main books and relevant papers

neers. Published by Elsevier B.V. All rights reserved.

and

The composition profiles in Fig. 1 indicate that trays around32 should be avoided for temperature control as they show

978 chemical engineering research

are briefly reviewed here. Shinskey’s (1984) book is excel-lent in giving insight into the distillation control behavior.Deshpande (1985) systematically takes the reader throughdistillation concepts, steady-state design and various con-trol strategies. Kister (1990) presented operational aspects ofcolumns and practical recommendations for troubleshootingthem. Luyben (1990) described the modeling and simulation ofa wide range of process systems and advanced control systemsincluding distillation columns. Ludwig (1997) presented designmethods for process design of many unit operations includingdistillation. Skogestad (1997) described various control config-urations for columns based on closed loop disturbance gain.Riggs (1998) gave a comprehensive description of various col-umn controls based on relative volatility and generalized thecontrol performance for each category. Alsop and Ferrer (2004,2006) validated the rigorous Hysys model with site data foran industrial propylene/propane column. The recent book byLuyben (2006) is an excellent resource for steady-state design,dynamic simulation and control of columns using Aspen Plus.

It is observed that there is little attention in the literatureconcerning the performance of column composition controlsat turndown operation (i.e., at lower throughputs). A distilla-tion column rarely operates at its design capacity/conditions;market demand and operational constraints necessitate itsoperation away from the original design conditions. Further,column throughput may vary significantly due to the effect ofrecycles in a process plant. So, turndown operation can occurin industrial columns frequently making controller redesignfor it impractical. Also, there is very limited research compar-ing the column configurations based on with and without flowratioing of the manipulated variables to feed flow rate. Buckleyet al. (1985) described the ratioing approach as ‘feed-forwardapproach’ and utilized it for composition control. Riggs (1998)suggested flow ratioing for all configurations. However, mea-suring feed flow is not always practicable especially if the feedis multi-phase fluid or if flashing of the liquid feed across themeasuring device can affect the flow measurement.

The literature discussing tuning of level controllers incolumns and their effect on composition control perfor-mance is generally not integrated with other aspects ofcontrol (e.g., turndown operation, ratioing with feed flow,etc.). Buckley et al. (1985) described that, for level control viareflux flow manipulation, it is necessary to sacrifice prod-uct flow smoothening in the interest of good compositioncontrol using tight level tuning. Lundstrom and Skogestad(1995) and Skogestad (1997) noted that composition controlperformance is independent of tuning of level loops for (L,V) configuration while, for other configurations, the levelloops can be tuned to minimize interaction between over-head and bottoms composition. Duvall (1999) and Hurowitzet al. (2003) tuned level controller for critically dampedresponse to keep level and composition control indepen-dent of each other, for studying several control structuresfor a depropaniser. Huang and Riggs (2002) tuned level con-trollers for slow response to avoid oscillations in the columnand amplify disturbances. In this paper, the effect of levelcontroller tuning, ratioing with feed flow and turndownoperation on the performance of several control configura-tions for a depropaniser is studied. Rigorous simulation ofthe depropaniser, several common disturbances and bothsingle- and dual-ended composition control are employed.

The results of this comprehensive study are presented anddiscussed in order to facilitate optimal design of column con-trols.

design 8 6 ( 2 0 0 8 ) 977–988

2. Depropaniser design and compositioncontrol

A depropaniser column design similar to that in Duvall (1999)and rigorous process simulator: Hysys 3.2 from Aspen Tech,have been used in this study. Feed to this column is satu-rated liquid containing ethane, propane, i-butane, n-butane,n-pentane and n-hexane. Propane (light key, LK) in the bottomproduct and i-butane (heavy key, HK) in the overhead prod-uct are both specified to be 0.5 mol%. Short-cut distillation inHysys assuming (external) reflux ratio of 1.2 times the min-imum reflux ratio and overall stage efficiency of 0.69, gives50 real trays, which matches with the design used by Duvall(Riggs, 1998). Further, it suggests tray 25 (from bottom) for feedentry. However, it is better to select the feed tray to minimizeboil-up or reflux ratio, which would minimize reboiler andcondenser duties via rigorous simulation (Lek et al., 2004). Thisstrategy gives tray 28 (from bottom) as the optimum feed tray.The liquid hold-up in both reboiler and condenser are basedon 5 min residence time of the related product stream. Sievetrays with maximum 85% flooding are considered. The col-umn turndown operation is assumed to be at 60% of designcapacity.

Temperature control is cheaper, reliable, faster and farmore popular instead of controlling product compositionsdirectly (Kister, 1990). This is due to significant sampling timeof composition analyzers, which adversely affects the compo-sition control of a column. The main issues with temperaturecontrol instead of composition control are sensitivity andcorrelation of temperature with composition. The composi-tion profile for the optimized column design described above(Fig. 1) along with Fig. 2 indicates that temperature is sensi-tive to composition of key components (propane and i-butane)between trays 10 and 47. For other trays, temperature is moresensitive to non-key components.

For the best location of temperature control, ±1% changein D/F (with reboiler duty kept constant) has been used forsensitivity studies, as suggested by Kister (1990). The resultsare shown in Fig. 2. The profiles in Figs. 1 and 2 are similar tothose provided by Kister (1990) for a depropaniser. Based onthese results, tray 16 is selected for the bottom compositioncontrol as it shows large temperature variation per unit com-position change. This agrees with Hori and Skogestad (2007)who suggested avoiding temperature control location close tobottom and just above feed for heavy non-key components.

Fig. 1 – Liquid composition vs. tray number counted fromthe column bottom.

chemical engineering research and de

Fig. 2 – Column temperature profile for base case and ±1%c

rffl3cid

lbt

l

Pau(t

hange in D/F.

etrograde distillation (Kister, 1990). Hence, tray 40 is selectedor the overhead composition control; this gives some marginor feed composition changes affecting the retrograde distil-ation region. Duvall (1999) used temperatures at tray 14 and4 for the bottom and overhead composition control, which isonsistent with Fig. 2; however, Fig. 1 gives some more insightnto the composition behavior thus affecting the selection asescribed above.

Duvall (1999) and Hurowitz et al. (2003) employed the fol-owing equation for estimating mole fraction of propane in theottom product and of i-butane in the overhead product, fromemperature of the respective, selected tray:

n(x) = A + B

T(1)

arameters A and B have been initialized from steady-statenalysis, by using few sets of data for x vs. T. During the col-

mn control, A is kept unchanged while B is adjusted using Eq.

1) based on the latest composition measurement. Set point forhe respective temperature control is then updated using the

Table 1 – Possible pairings of controlled and manipulated varia

Manipulated variables—configuration(note 1)

Overhead composition (note 2)

L, V LL, B LD, V DD, B DL/D, V L/DL/D, B L/DL, V/B LD, V/B DL/D, V/B L/DL/F, V/F L/FL/F, B/F L/FD/F, V/F D/FD/F, B/F D/FL/D, V/F L/DL/D, B/F L/DL/F, V/B L/FD/F, V/B D/F

Note 1: For dual-ended configuration (A, B), both overhead and bottoms coulated variables, respectively. For single-ended configuration, the configurwhile the other end is not. This is later referred to as A- or B-configurationA as the manipulated variable or bottoms composition is controlled withcontrolled but it is maintained by providing the set point (via Eq. (1)) to the

sign 8 6 ( 2 0 0 8 ) 977–988 979

latest B in Eq. (1). Sampling time for composition measure-ment is considered as 5 min.

3. Control configurations and tuning

Lundstrom and Skogestad (1995) observed that a distillationcolumn with one feed and two products can be viewed as a5 × 5 dynamic system with 5 manipulated variables (inputs)and 5 controlled variables (outputs). The manipulated vari-ables are reflux flow (L), reboiler duty (QR), condenser duty (QC),distillate flow (D) and bottoms flow (B), whereas the controlledvariables are distillate composition (xD), bottoms composi-tion (xB), condenser pressure (PD), condenser holdup (MD) orlevel, and reboiler holdup (MB) or base level. For a columnon pressure control (say, using condenser duty), this can bereduced to a 4 × 4 system, with 4! (=24) possible ways of pairingthe manipulated and controlled variables (Deshpande, 1985).However, many of these schemes can be discarded based onundesirable factors like control of reboiler level by L or D, con-trol of condenser level by QR or B, etc. Finally, one is left withthe first four schemes in Table 1. Additional schemes havebeen added in this table based on ratioing the inputs withrespect to F, D or B.

Depropaniser with (L/D, V/B) configuration built in Hysys isshown in Fig. 3. For single-ended control, one of the manip-ulated variables for composition control will be free and isnot adjusted. The ratios (L/D and V/B) are computed using thebuilt-in spreadsheet in Hysys. The sinusoidal disturbance infeed composition is added using both transfer functions andspreadsheet.

The level controllers are designed using two approaches—tight level control with proportional–integral (PI) controllerand sluggish level control with proportional-only (P) con-troller. The intention is to find the best approach for various

control configurations. Tight tuning can lower the liquidholdup requirements in reboiler and reflux drum, while slug-gish level tuning has the benefit of smoothening the product

bles

Controlled variables

Bottoms composition (note 2) Condenser level Base level

V D BB D VV L BB L VV L + D BB L + D VV/B L V + BV/B L V + BV/B D BV/F D BB/F D VV/F L BB/F L VV/F L + D BB/F L + D VV/B D V + BV/B L V + B

mpositions are automatically controlled with A and B as the manip-ation (A, B) indicates that the composition of one end is controlled,, which respectively means overhead composition is controlled with

B as the manipulated variable. Note 2: Composition is not directlytemperature control loop.

980 chemical engineering research and design 8 6 ( 2 0 0 8 ) 977–988

/D a

Fig. 3 – Depropaniser with L/D, V/B configuration in Hysys; Lspreadsheet (not shown in the figure).

flows. Auto-tune variation (ATV) technique available in Hysysis used for tight tuning of a PI controller. This would aimto maintain the reboiler and condenser levels at 50%. Forsluggish level tuning with P controller, the proportional gainis selected to maintain 50% level for design flow and 40%level for turndown operation. The level can rise above 50%for higher than design flow. The objective is to maintainlevel variation within 40–60% in order to keep sufficient mar-gin from level alarm levels which are usually set at 20 and80%.

The open-loop responses for a step change in L and V (withlevel loops closed and composition loops open) are analysed.As expected, they show first order plus time delay dynamics(Shinskey, 2002). The corresponding time constants are listedin Table 2. Duvall et al. (2000) obtained a time constant ofapproximately 150 min for the same design used in the presentstudy. Skogestad and Morari (1988) obtained dominant timeconstant of 194 min for a column with 40 trays and 1% impu-rity at both ends. Hence, the time constants calculated arereasonable for a depropaniser. Further, results in Table 2 indi-cate significant interaction between the overhead and bottom

composition loops; this implies that, for single-ended control,the uncontrolled end will have significant deviation from thedesired composition.

Table 2 – Time constants for composition response to a step ch

Step change in L

Design flow (min) Turndow

Overhead composition 160 300Bottoms composition 260 420

nd V/B ratios are calculated using and the built-in

For temperature (composition) control loops, propor-tional–integral (PI) controllers are tuned based on set-pointchanges, which provide a good compromise between perfor-mance and robustness (Riggs, 1998). Huang and Riggs (2002)utilized the ATV method along with Tyreus-Luyben settingsto arrive at initial PI parameters; then, the parameters werefine-tuned using a detuning factor which results in minimumIAE (integral of absolute error) for several set-point changes(±25% in product purity). In this study, a similar approach isfollowed, and the optimum detuning factor is calculated usinga macro-written in Visual Basic, as an interface with Hysys.PI parameters and detuning factors are given in Appendix(Chawla, 2007). For most of the configurations, the optimumdetuning factor is less than 1 when the loops interact, whichimplies more aggressive (than ATV) control action due tohigher gain and lower integral time. However, for some con-figurations, the optimum detuning factors are greater than1; thus, these configurations require less aggressive controlaction.

4. Single-ended composition control

Single-ended composition control is widely used for distilla-tion columns in industry as it is easier to implement, tune,

ange in L and V

Step change in V

n (min) Design flow (min) Turndown (min)

240 380200 310

nd de

c1dHs(ffis

4

Brlc

chemical engineering research a

ontrol and maintain than dual-composition control (Riggs,998). Feed flow and feed composition are the most commonisturbances a column can be subjected to for a long time.ence, these are used for evaluating each configuration of

ingle-ended composition control. The performance resultsIAE for step disturbances and maximum amplitude ratio, ARor the sinusoidal disturbance) of various single-ended con-gurations for overhead and bottoms composition control arehown in Table 3.

.1. Base case

ase case refers to operating the column at design flow

ate, with no ratioing of manipulated variable(s) and tightevel tuning. It is evident from Table 3 that, for overheadomposition control in the base case, configuration D is

Table 3 – Performance of various single-ended configurations fodisturbance, and Max AR for sinusoidal disturbance)

Manipulated variable Step disturbance infeed composition

S

Overhead Bottoms Overhea

L 0.61 12.8 0.18L/F 0.10 13.2 0.18L-TD 0.21 13.0 0.04L-SL 0.65 11.0 0.15L-SL TD 0.24 13.2 0.09L-SL LFT 0.62 10.4 0.18L-SL HFT 0.57 16.0 0.16

D 0.12 13.4 0.20D/F 0.10 13.5 0.33D-TD 0.08 12.8 0.07D-SL 0.10 7.4 0.24D-SL TD 0.13 13.0 0.04D-LFT 0.11 1.4 0.26D-HFT 0.08 15.9 0.27

L/D 0.07 13.4 0.26L/D-TD 0.08 14.9 0.08L/D-SL 0.07 16.7 0.17L/D-SL TD 3.0 16.3 0.07L/D-SL LFT 0.08 10.7 0.18L/D-SL HFT 0.08 15.9 0.28

V 50.1 1.4 84.3V/F 43.6 0.3 92.0V-TD 47.3 2.2 61.8V-SL 43.5 0.14 92.3V-SL TD 42.9 0.13 78.8V-SL LFT 46.7 0.21 115.1V-SL HFT 34.4 0.14 72.5

B 50.0 14.9 84.4B/F 45.1 19.1 90.2B-TD 40.3 0.658 85.1B-SL 35.0 0.236 90.7B-SL TD 43.3 0.142 79.9B-SL LFT 55.6 0.658 94.0B-SL HFT 36.7 0.246 68.9

V/B 47.1 0.302 84.6V/B-TD 43.2 0.210 79.2V/B-SL 51.9 0.333 71.7V/B-SL TD 41.3 0.190 83.5V/B-LFT 51.2 0.414 106.8V/B-HFT 35.3 0.249 72.1

Note: The notation used to define the configuration is: no suffix (e.g. L), bafeed tray is two trays below the optimum feed tray; and HFT, feed tray is tw

sign 8 6 ( 2 0 0 8 ) 977–988 981

the best and is marginally better than L/D. This findingis in agreement with the results of Skogestad (1997). Forbottoms composition control, B configuration is inferior, par-ticularly for step disturbances in feed composition, comparedto the other two configurations. For B-control, reboiler levelwill be controlled using V, which requires vaporizing theexcess level by supplying more energy and hence introducesadditional lag leading to poor control. Note that the flowthrough boil-up loop is driven by thermosiphon effect and itis not possible to add any restriction to directly control theboil-up rate. Compared to B-control, V configuration is sig-nificantly less sensitive to feed disturbances, and it can befurther improved by using V/B configuration. In fact, B con-

figuration is unstable and so not recommended for control.Temperature controller outputs (not presented for brevity)show that small changes in B result in large variation in

r overhead and bottoms composition control (IAE for step

tep disturbance infeed flow

Sinusoidal disturbancein feed composition

d Bottoms Overhead Bottoms

98.5 0.160 0.04684.9 0.030 0.04074.1 0.041 0.02889.1 0.130 0.03673.9 0.046 0.02898.5 0.122 0.03791.7 0.122 0.042

97.2 0.027 0.02685.1 0.025 0.04070.3 0.135 0.02891.8 0.130 0.03070.2 0.140 0.02875.8 0.026 0.03091.8 0.023 0.042

97.2 0.030 0.02583.3 0.030 0.02289.7 0.020 0.03574.6 0.018 0.02875.4 0.023 0.02898.5 0.032 0.040

0.16 0.200 0.0190.16 0.200 0.0252.1 0.250 0.0450.05 0.200 0.0150.03 0.150 0.0180.06 0.200 0.0150.06 0.200 0.013

1.5 0.300 0.0653.7 0.400 0.1400.54 0.160 0.0250.14 0.210 0.0260.05 0.142 0.0080.29 0.250 0.0450.24 0.200 0.024

0.14 0.200 0.0120.09 0.150 0.0080.12 0.210 0.0080.07 0.150 0.0060.15 0.210 0.0120.14 0.190 0.009

se case; TD, for turndown flow; SL, with sluggish level tuning; LFT,o trays above the optimum feed tray.

982 chemical engineering research and

Fig. 4 – Effect of configuration and frequency on amplituderatio for sinusoidal disturbance in feed composition forsingle-ended control.

composition, while large changes in V result in small com-position variation; V/B-control shows the most stable controlexcept for feed flow disturbance. Fig. 4 shows the typi-cal variation of AR (of amplitude of key component in thecontrolled product to the amplitude of key component inthe feed) for sinusoidal feed composition disturbance forsingle-ended control; many of the configurations are insen-sitive to this sinusoidal disturbance for frequencies outside0.02–0.3 rad/min. These results are similar to those of Duvall(1999).

4.2. Effect of level controller tuning

The base case model is modified to study the effect of slug-gish level (SL) tuning on the composition control performance.Table 3 shows that the performance of D configuration for over-head composition control to reject sinusoidal disturbances isdeteriorated by SL tuning while the performance of L and L/Dconfigurations is nearly independent of level tuning. For stepdisturbances, L configuration is unaffected by level tuning, L/Dconfiguration is slightly improved, while D configuration isslightly deteriorated. For D configuration, the overhead com-position is controlled by the distillate rate while the refluxdrum level is controlled by the reflux rate. For good com-position control, the reflux control should be quick. For SLtuning, the response of reflux rate is slow, thus affectingthe composition control. This shows that SL tuning is notalways the best choice, and is in agreement with Shinskey(1984).

For bottoms composition control, the performance of V/B-control is unaffected by level tuning. However, V and Bconfigurations show marked improvement in performance byusing SL control. Shinskey (1984) described that base levelcontrolled by bottoms product flow is stable and responsiveexcept for kettle reboilers or if the bottom flow is extremelysmall. Kettle reboiler is used in this study, and V-controlis better than B-control when SL tuning is used (Table 3),which is in agreement with Shinskey (1984). It can be con-cluded that for single-ended composition control with SLtuning, configuration (D, V) (which means D configuration for

overhead composition or V configuration for bottoms compo-sition) performs better than (L, B), with (L/D, V) being the bestchoice.

design 8 6 ( 2 0 0 8 ) 977–988

4.3. Effect of ratioing with feed flow

The base case model is updated to study the effect of ratioingthe manipulated variables with feed flow on the performanceof all control configurations for the depropaniser. Skogestad(1997) and Riggs (1998) described that ratioing the manipulatedvariable with feed flow provides self-regulation with respect tofeed flow and is equivalent to feed forward control, and henceit is always recommended. For overhead composition control,the response of L configuration to feed composition distur-bance improves substantially by using flow ratioing, whileresponse to feed flow disturbance is nearly unchanged. Theperformance of D-control for feed flow disturbance is slightlydeteriorated by flow ratioing. It can be concluded that L/F isleast sensitive to feed disturbances. This conclusion matcheswith the results of Skogestad (1997), Riggs (1998) and Duvall(1999). Note that L/F control performance is comparable to thatof D and L/D control.

For bottoms composition control, flow ratioing V givessignificant improvement in IAE for step disturbance in feedcomposition but similar IAE for the other two disturbances.Both B and B/F configurations are very sensitive to all dis-turbances. Overall, V/B performs the best. As noted earlier,Skogestad (1997) and Riggs (1998) recommended using ratioingthe manipulated variable with feed flow. Contrary to this, per-formance of D and B configurations for feed flow disturbanceis deteriorated by flow ratioing (i.e., D/F and B/F configura-tions in Table 3). This can be explained by the large effect thatmass balance has on the product composition than the refluxrate, as illustrated by Skogestad and Morari (1988). Hence,any changes in D or B based on changes in feed flow affectsthe product composition much before the feed disturbancereaches the product, and this causes bigger fluctuations inproduct composition.

4.4. Turndown operation

Both the base case (with tight level tuning) and the SL tuningmodels are modified to study the performance of the config-urations during the turndown operation (TD, at 60% of designflow). For this operation, set points for the tray temperatureshave been adjusted to meet the product specifications. Table 2shows that, during turndown, the dynamics could be affected;time constants have increased substantially due to increasedresidence time. Control during TD is studied with both tightand sluggish level tuning; for this, controllers tuned for designflow were used without re-tuning.

From Table 3, it can be seen that the performance of alloverhead composition control configurations for step distur-bances are nearly unaffected or improved at TD. For sinusoidaldisturbance in feed composition, L-control is improved at TD,while D-control is adversely affected. L/D control has inherentfeed flow compensation due to ratioing the variables; thus,it shows no significant change in performance for TD andany of the three disturbances. Results in Table 3 show thatthe change in performance at TD compared to design flow isnearly independent of level tuning. For bottoms compositioncontrol, performance of B-control is improved significantly(Figs. 5 and 6), while V-SL configuration is unaffected at TD.B-SL control improves, while V-control deteriorates at turn-down. Similar to L/D control, V/B control has inherent feed flow

compensation due to ratioing the variables; thus, it shows nosignificant change in performance for TD and any of the threedisturbances studied.

chemical engineering research and design 8 6 ( 2 0 0 8 ) 977–988 983

Fig. 5 – Comparison of closed loop response between basecase and turndown for step disturbance in feedc

4

Scptfmtfauc

5

Thpt

Fig. 6 – Comparison of closed loop response between basecase and turndown for step disturbance in feed flow.

omposition.

.5. Feed tray location

ince feed tray may not be optimal in some industrialolumns, the effect of changing the feed tray location on theerformance of control configurations is studied for the bet-er configuration. Simulation models are generated for lowereed tray (LFT, with feed tray located two trays below the opti-

al feed tray used for the above simulations) and higher feedray (HFT, with feed tray located two trays above the optimaleed tray used for the above simulations). Moving the feed trayffects the steady state and dynamic performance of the col-mn; however, there is no significant effect for single-endedomposition control.

. Dual-ended composition control

he major disadvantage with single-ended control is the

igher energy cost as the uncontrolled end may over-urify the product. Dual-ended control is designed to controlhe composition at both ends of the column. Similar

to single-ended control, feed flow and feed compositiondisturbances are used for evaluating dual-ended control con-figurations for the depropaniser. The performance results(IAE for step disturbances and max AR for sinusoidal dis-turbance) of various dual ended configurations are shown inTable 4.

5.1. Base case

Base case refers to operating the column at design flowrate,with no ratioing, and tight level tuning. It is evident fromTable 4 that configurations (D, V), (L, V/B) and (D, V/B) arestable for all disturbances, with (D, V) control being the bestperformer. The configurations (L, B), (L, V), (L/D, B), and (L/D,V/B) are not stable for at least one of the disturbances; hence,these are not recommended for the depropaniser studied.Configurations (L, V) and (L, B) are highly sensitive to distur-bances. Where L is used for overhead composition control,the reflux drum level is controlled by distillate rate. The tighttuning of level control adversely affects the capability of L to

control the composition. Where B is used for bottom compo-sition control, the reboiler level will be controlled using V,

984 chemical engineering research and design 8 6 ( 2 0 0 8 ) 977–988

Table 4 – Performance of various dual ended configurations (IAE for step disturbance, Max AR for sinusoidal disturbance)

Configuration Step disturbance in feedcomposition

Step disturbance in feedflow

Sinusoidal disturbancein feed composition

Overhead Bottoms Overhead Bottoms Overhead Bottoms

L, V 8.4 6.8 7.9 6.1 0.480 0.150L/F, V/F 0.38 0.43 0.13 0.06 0.040 0.012L, V-TD 0.37 0.27 0.06 0.04 0.037 0.015L/F, V/F-TD 0.41 0.40 0.16 0.09 0.041 0.011L, V-SL 0.51 0.52 0.16 0.08 0.036 0.019L, V-SL TD 2.0 2.2 0.05 0.03 0.100 0.052L, V-SL, LFT 0.60 0.56 0.19 0.11 0.037 0.023L, V-SL, HFT 0.30 0.24 0.19 0.09 0.035 0.015

D, V 0.54 0.46 0.50 0.20 0.050 0.023D/F, V/F 0.39 0.54 0.18 0.13 0.045 0.021D, V-TD 9.8 4.3 0.12 0.08 0.120 0.037D, V-SL 3.9 0.32 8.15 0.23 0.360 0.013D, V-SL TD 13.1 45.9 10.2 37.2 1.050 0.640D, V-TS 0.49 0.49 0.50 0.21 0.047 0.023D, V-TS LFT 0.58 0.59 0.21 0.11 0.060 0.025D, V-TS HFT 0.77 1.80 0.44 0.19 0.050 0.018

L, B 22.8 17.6 18.5 12.4 0.800 0.210L/F, B/F 18.8 17.7 6.2 4.9 0.450 0.162L, B-TD 15.8 14.3 20.3 17.2 0.360 0.122L, B-SL 19.9 20.4 16.9 15.2 0.560 0.210L, B-SL TD 6.9 8.6 10.0 10.9 0.250 0.120L, B-SL LFT 15.0 16.0 14.3 12.8 0.480 0.180L, B-SL HFT 22.6 21.9 16.9 14.8 0.700 0.230

L/D, V/B 1.2 0.91 0.81 0.28 0.080 0.015L/D, V/B-TD 1.1 0.62 0.36 0.19 0.041 0.012L/D, V/B-SL 0.25 0.57 0.11 0.10 0.030 0.024L/D, V/B-SL TD 0.22 0.27 0.16 0.10 0.026 0.013L/D, V/B-SL LFT 0.40 0.73 0.14 0.32 0.036 0.025L/D, V/B-SL HFT 0.12 0.46 0.09 0.13 0.024 0.019

L/D, V 1.4 0.49 0.41 0.30 0.100 0.013L/D, V/F 1.2 0.40 0.27 0.39 0.091 0.014L/D, V-TD Unstable Unstable 0.21 0.05 0.087 0.055L/D, V-SL 1.2 0.25 0.54 0.11 0.085 0.012L/D, V-SL TD Unstable Unstable 0.26 0.18 0.090 0.120L/D, V-SL LFT 0.49 0.41 0.30 0.17 0.060 0.015L/D, V-SL HFT 1.5 0.21 0.58 0.10 0.100 0.010

L/D, B 0.97 2.46 0.42 1.6 0.036 0.055L/D, B/F 1.24 3.10 0.27 0.65 0.034 0.062L/D, B-TD Unstable Unstable Unstable Unstable Unstable UnstableL/D, B-SL 1.2 9.5 0.70 14.1 0.036 0.130L/D, B-SL TD 3.62 6.7 0.93 3.2 0.062 0.055L/D, B-LFT 0.89 3.0 0.93 2.5 0.040 0.091L/D, B-SL HFT 3.0 11.3 1.1 8.7 0.140 0.240

L, V/B 0.69 0.54 0.24 0.18 0.100 0.025L/F, V/B 0.60 0.76 0.24 0.25 0.050 0.023L, V/B-TD 1.09 0.84 0.12 0.08 0.030 0.012L, V/B-SL 0.57 0.40 0.27 0.11 0.032 0.012L, V/B-SL TD 0.34 0.38 0.11 0.12 0.031 0.010L, V/B-LFT 0.71 0.58 0.49 0.20 0.055 0.013L, V/B-HFT 0.41 0.32 0.24 0.18 0.082 0.021

D, V/B 0.75 1.0 0.73 1.2 0.052 0.026D/F, V/B 0.39 2.0 0.22 1.4 0.037 0.033D, V/B-TD 0.63 2.8 0.73 1.2 1.400 0.620D, V/B-SL 1.5 1.0 2.25 0.73 0.040 0.020D, V/B-SL TD 0.33 1.0 0.34 0.41 0.085 0.038D, V/B-LFT 1.7 3.3 1.35 7.1 0.117 0.140D, V/B-HFT 0.26 0.65 0.31 0.37 0.055 0.027

TS: tight level tuning for overhead loop and sluggish level tuning for bottom loop. See footnote of Table 3 for other notation used.

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waLVtt((itat

F

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hich requires vaporizing the excess level, and hence cre-tes additional lag in control. It is interesting to note that-control for overhead composition is a good option when/B-control is used for bottoms composition. This can be due

o reduced disturbance propagation from the bottom sec-ion to top of the column. Temperature controller outputsChawla, 2007) show similar pattern to composition output.L, V) control shows large and nearly sustained fluctuationsn controller outputs owing to tight tuning of level con-

rollers. The changes required in B to maintain the set pointre small, which makes it a very sensitive variable for con-rol.

ig. 7 – Comparison of closed loop response between base config

sign 8 6 ( 2 0 0 8 ) 977–988 985

5.2. Effect of level controller tuning

The base case model is modified to study the effect of SL tun-ing on the composition control performance. Results in Table 4indicate that the performance of (L, V) and (L/D, V/B) is signif-icantly improved by SL tuning compared to tight level tuning.Some configurations like (D, V), (L/D, B) and (D, V/B) show dete-rioration with SL tuning. Configurations (L, B), (L/D, V), and (L,V/B) are unaffected by level tuning. With SL tuning, config-

urations (L, V), (L, V/B), and (L/D, V/B) are stable and betterconfigurations. The improvement in the performance of (L,V) is mainly due to overhead composition control. The reflux

uration and turndown for step disturbance in feed flow.

and

tion on each configuration is studied. From these results, it canbe concluded that the L/D (with sluggish tuning for reflux drum

Table A.1 – Controller parameters for single-endedcomposition control

Configuration Detuning factor Tuning parameters

Outer loop Inner loop

Kc Ti Kc Ti

L 0.4 1.6 262L/F 0.2 3.6 120 0.3 0.5L-SL 0.4 1.5 309L-SL LFT 0.4 1.5 309L-SL HFT 0.4 1.5 309D 0.2 18.4 97D/F 0.2 11.7 95 0.3 0.5D-SL 0.2 12.8 145D-TS LFT 0.2 20.3 87D-TS HFT 0.2 17.6 90L/D 0.2 22.5 107 0.2 1.1L/D-SL 0.2 24.8 101 0.1 0.8L/D-SL LFT 0.2 24.8 101L/D-SL HFT 0.2 21.5 103V 0.2 142.0 20V/F 1.3 2.9 264 1.9 7V-SL 0.2 134.0 23V-SL LFT 0.2 134.0 23V-SL HFT 0.2 134.0 23B 0.8 0.8 658B/F 1.3 0.4 1084 0.2 0.8B-SL 0.3 2.1 305B-SL LFT 0.4 1.6 406B-SL HFT 0.3 2.1 305V/B 0.4 5.2 172 0.7 11V/B-SL 0.5 4.6 199 0.7 12V/B-LFT 0.4 5.2 172V/B-HFT 0.4 5.2 172

Note 1: Units for Kc is %/% and Ti is s. Note 2: Final tuningKc = ATV tuning Kc/detuning factor; final tuning Ti = ATV tuningTi × detuning factor. Note 3: For turndown studies, controller-tuningparameters are kept at those values found good for design flow.Note 4: Ratioing schemes are implemented using cascade controlas shown in Fig. 3, with temperature control as the outer loop and

986 chemical engineering research

drum level is not held tight, thus minimizing the interac-tion between composition and level loops. The improvementin (L/D, V/B) is also for similar reason. For single-ended con-trol, L configuration is nearly independent of level tuning;however (L, V) configuration for dual-ended control contra-dicts this as there is large improvement by using SL tuningcompared to tight level tuning. This can be explained bythe increased interaction among the loops when all outputvariables are tightly controlled. The deterioration in (D, V) isdue to sluggish control of overhead composition. When D-control is used for overhead composition, the reflux drumlevel sets the reflux rate which directly affects the composi-tion. For good composition control, the reflux control shouldbe quick. For SL tuning, the response of reflux rate is slow, thusaffecting the composition control. When B-control is used forbottom composition, the control performance is not improvedby SL tuning. As explained earlier, for this configuration, thereboiler level will be controlled using V, which requires vapor-izing the excess level and hence creates additional lag incontrol.

5.3. Effect of ratioing with feed flow

Skogestad (1997) described that ratioing the manipulated vari-able with feed flow provides self-regulation with respect tofeed flow and is equivalent to feed forward control. Similarly,he noted that L/D and V/B configurations have self-regulationwith respect to feed flow. The base case model is modifiedto study the effect of ratioing the manipulated variables withfeed flow on the performance of various control configura-tions through rigorous simulation. The performance of (L,V) configuration is substantially improved by flow ratioing;however, flow ratioing has little effect on the performance ofall other configurations tested for the depropaniser (Table 4).Overall, with flow ratioing (L/F, V/F), (D/F, V/F), and (L/F, V/B)configurations are stable and good. (L/F, V/F)-control has theadvantage of good energy and material balance control, whichmakes it attractive over other configurations. It is interest-ing to note that (L/F, V/B) is also attractive as the materialbalance is set through L/F, while V/B minimizes the propa-gation of disturbances from the bottom section to the columntop. As observed earlier, configurations with B-control for bot-tom composition are very sensitive to disturbances and theycannot be improved by flow ratioing (Table 4).

5.4. Turndown operation

The base case model is modified to study the performanceof the dual-ended control configurations during the TD (i.e.,at 60% of design flow). For this, steady-state tray tempera-tures have been adjusted to meet the product specifications,which are 56.45 ◦C for overhead (base case: 55.94 ◦C) and 95.7 ◦Cfor bottoms (base case: 96.53 ◦C). Recall that column dynam-ics are affected during the TD (Table 2)—time constants haveincreased substantially. Control during TD is studied with bothtight and sluggish level tuning, and with the controllers tunedearlier for design flow. It can be observed from Table 4 that(L/F, V/F), (L/D, V/B-SL) and (L, V/B-SL) configurations are nearlyunaffected by the significant change in the feed flow rate dueto their inherent feed flow compensation by ratioing the vari-

ables; they are also the stable and better configurations. Fig. 7shows comparison of closed loop response between base con-figuration and turndown for step disturbance in feed flow.

design 8 6 ( 2 0 0 8 ) 977–988

5.5. Feed tray location

The base case model is updated to study the effect of changingthe feed tray location on the performance of control configu-rations for the same disturbances (Table 4). The configurations(D, V-TS), (L, B-SL), (L/D, V-SL), and (L/D, B-SL) show better per-formance for lower feed tray, while configurations (L, V-SL),(L/D, V/B-SL), (L, V/B), and (D, V/B) show better performancewith higher feed tray. Hence, it is recommended to providemore options for alternate feed tray location in design andthen select the best-feed tray based on field trials as simula-tion cannot completely replicate the plant operation.

6. Summary

Many configurations for single- and dual-ended composi-tion control of a depropaniser are evaluated for the commonfeed flow rate and composition disturbances, using a pro-cess simulator, namely, Hysys. Effect of level tuning, ratioingmanipulated variable to feed flow rate and turndown opera-

ratio control as the inner loop. The temperature controller gives aset point to the inner loop, which manipulates one stream flow rateto maintain the desired ratio.

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(cSa((

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evel) and V/B (with either tight or sluggish tuning for reboilerevel) are the best configurations for single-ended composi-ion control. For dual-ended composition control (L/F, V/F), (L,/B-SL) and (L/D, V/B-SL) are the best configurations consid-ring the range of operation. The additional findings of thistudy for the depropraniser control are: (a) turndown opera-ion adversely affects the performance of many dual-endedontrol configurations and (b) right level tuning is critical forood composition control.

For the depropaniser, Duvall (1999) observed that (L, V/B),L/D, V), and (L/D, V/B) configurations with flow ratioing andritically damped tuning for level control performed best.kogestad (1997) concluded that (L/D, V/B) is a good over-ll choice for all modes of operation. He also observed thatL/D, V) configuration behaves somewhere between (L, V) andL/D, V/B). The present results are generally in agreement

with these. However, two observations do not agree withthe earlier studies: dual-ended (L, V) and single-ended V-configurations show marked improvement in performance by

Table A.2 – Controller parameters for dual-ended composition c

Configuration De-tuning factor Overhead loo

Outer loop

Kc Ti K

L, V 0.3 2.1 196.2L/F, V/F 0.7 1.0 418.6 0.2L, V-SL 0.2 3.0 154.6L, V-SL, LFT 0.2 3.2 152.4L, V-SL, HFT 0.2 2.8 151.2D, V 0.5 7.4 243.5D/F, V/F 0.4 3.0 202.4 0.4D, V-SL 0.9 2.9 652.5D, V-TS 0.5 7.8 230.0D, V-TS LFT 0.5 8.1 218.0D, V-TS HFT 0.5 7.0 225.5L, B 1.5 0.5 450.0L/F, B/F 1.4 0.5 837.2 0.2L, B-SL 1.3 0.7 564.2L, B-SL LFT 1.3 0.8 564.2L, B-SL HFT 1.3 0.7 564.2L/D, V/B 0.4 4.9 207.2 0.1L/D, V/B-SL 0.4 7.5 90.8 0.0L/D, V/B-SL-LFT 0.4 8.0 91.2 0.0L/D, V/B-SL-HFT 0.4 7.5 90.8 0.0L/D, V 1 4.5 535.0 0.1L/D, V/F 1 5.0 498.0 0.1L/D, V-SL 1 5.0 504.0 0.1L/D, V-SL LFT 0.8 6.3 403.2 0.1L/D, V-SL HFT 1 4.3 516.0 0.1L/D, B 0.7 11.1 243.6 0.1L/D, B/F 0.8 9.3 285.6 0.1L/D, B-SL 0.7 9.4 277.9 0.1L/D, B-LFT 1 7.8 348.0L/D, B-SL HFT 1.4 4.7 555.8 0.1L, V/B 0.3 2.1 198.0L/F, V/B 0.8 0.9 480.0 0.2L, V/B-SL 0.7 1.6 179.2L, V/B-LFT 0.4 1.6 264.0L, V/B-HFT 0.3 2.1 198.0D, V/B 0.6 6.6 279.0D/F, V/B 0.4 3.0 202.4 0.4D, V/B-SL 0.4 6.4 292.4D, V/B-LFT 0.6 6.6 279.0D, V/B-HFT 0.5 7.9 232.5

Note: See the footnotes in Table A.1.

sign 8 6 ( 2 0 0 8 ) 977–988 987

using SL tuning, and flow ratioing does not always improvethe performance of control loops. The first result differsfrom the observation of Lundstrom and Skogestad (1995)and Skogestad (1997) that composition control performanceis independent of level loops for (L, V) configuration. Thesecond observation does not support the suggestion of flowratioing for all configurations by Riggs (1998). In general,the best configuration is likely to depend on the columndesign, expected disturbances and the operating envelope.Hence, rigorous dynamic simulation to study these aspectsand then selection of the best configuration are recom-mended.

Appendix A. Controller parameters for single-

and dual-ended composition control

See Appendix Tables A.1 and A.2.

ontrol

p Bottom loop

Inner loop Outer loop Inner loop

c Ti Kc Ti Kc Ti

93.0 28.464 0.479 5.3 142.0 1.93 7.18

134.0 23.0146.5 21.8134.0 23.0

28.6 70.564 0.565 5.0 72.4 1.93 7.18

22.4 122.429.4 76.532.4 72.044.8 59.5

0.4 1233.064 0.479 0.4 1167.6 0.186 0.775

0.5 1319.50.5 1279.20.5 1319.5

0.923 1.7 208.4 0.64 19.06 0.781 5.6 54.4 0.83 7.147 1.06 5.8 62.0 0.83 7.146 0.781 5.6 54.4 0.83 7.1466 1.06 27.1 99.07 1.05 5.3 128.4 1.85 7.067 1.05 29.2 102.07 1.05 36.5 81.69 1.19 29.2 102.066 1.06 0.9 634.27 1.05 0.7 700.8 0.186 0.77566 1.06 0.9 718.9

0.6 906.066 1.06 0.5 1437.8

6.9 129.0 0.683 11.464 0.479 11.9 150.4 0.658 13.7

3.3 277.95.3 172.07.0 129.0

13.1 110.4 0.683 11.464 0.565 17.0 79.6 0.658 13.7

11.7 90.0 0.719 11.613.1 110.415.7 92.0

and

Skogestad, S., 1997, Dynamics and control of distillationcolumns—a tutorial introduction. Trans IChemE Part A,

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