Hierarchized Distributed Control System for a Gas Processing Unit

Transcription

1 Journal of Electrcal Engneerng, Electroncs, Control and Computer Scence JEEECCS, Volume 2, Issue 6, pages 25-35, 2016 Herarchzed Dstrbuted Control System for a Gas Processng Unt Maran POPESCU Automatc Control, Computers and Electroncs Department Petroleum-Gas Unversty of Ploest Ploest, Romana mpopescu@upg-ploest.ro Abstract The paper presents a study regardng the complex problems assocated wth the herarchcal and dstrbuted control of a gas processng unt. Ths unt conssts of three separaton columns, and for ths tran of dstllaton columns a herarchzed dstrbuted control system wth three levels s proposed. Frst herarchcal level s dedcated to conventonal control, second level s assocated wth advanced control and thrd level s the optmal control level. The levels of the herarchcal control systems for each column are characterzed and the proposed solutons are studed through smulaton. The evoluton of the dstllaton processes and the targets that must be acheved n ther operaton, as well as the development of the automaton equpment, mposed the herarchcal and dstrbuted structures n the control of these processes. In ths paper, the obect of the nvestgatons regardng ts control s a gas processng unt whch takes part from a catalytc crackng plant, and represents practcally a tran of (three) dstllaton columns, presented n Fg. 1. Keywords-herarchcal system; dstllaton column; feedforward control; decouplng; nternal model control; optmal control I. INTRODUCTION The process of dstllaton s probably the most used operaton of mxtures separaton nto pure components or components mxtures. In chemcal and petrochemcal ndustry approxmately 95% of the separaton processes are performed usng dstllaton, the unts n whch the dstllaton takes place consumng nearly 3% of the total energy consumpton n the world [1, 2]. The economc obectve of dstllaton s to obtan more valuable products relatve to the mxture beng processed. ecause the products values depend drectly on ther qualty, the assurance of the qualty specfcatons s of great mportance n dstllaton columns operaton. Also, the operaton methods must provde certan producton and proft [3]. The dstllaton process complexty, gven by ts dmenson and the multple obectves that must be acheved, nvolves a hgh effort n fndng the proper control solutons. The mportant problems of dstllaton control consst n: the control loops nteractons; the process nonlneartes; the tme varaton of process parameters; the slow dynamcs of the process; the large load changes whch may occur. The solutons to these problems could nclude feedback control, feedforward control, multvarable control or optmal control strateges [4]. The complexty of the control problem can be addressed through a herarchcal approach by: specfyng the control obectves wth dfferent tme scales, modellng the process from abstract to detaled, selectng the controlled and manpulated varables, enablng the confguraton of control structures [5]. Fgure 1. Smplfed structure of the gas processng unt. As t can be seen n Fg. 1, the depropanzer column separates the C 3 -C 4 fracton. The top product consstng manly n propane and propylene s dred and then sent to the propane-propylene separaton column (PPSC). The bottom product of the depropanzer represents the feed for the butanebutylene separaton column (SC). The propanepropylene separaton column separates the C 3 fracton, wth propylene at the top and propane at the bottom. The butane-butylene separaton column separates the C 4 fracton, the top product contans manly sobutane and sobutylene, and the bottom product s composed of nbutane, cs- and trans-butylene. The paper s structured n fve sectons brefly descrbed n the followng. Secton 2 presents the

2 26 Maran POPESCU proposed herarchcal system for the nvestgated gas processng unt. In secton 3 s descrbed the control system for the C3-C4 separaton column, the man contrbuton beng the proposal of an obectve functon at the optmal control level whch calculates the optmum values of the set-ponts for the flowrate and bolup flowrate control systems thus ensurng the best separaton. Secton 4 emphaszes the contrbutons regardng the herarchcal control system wth three levels for the propylene-propane separaton column, namely, the mplementaton of a feedforward control system at level 2, and the proposal of an obectve functon at level 3 whch ensure the recovery of the most valuable component wth low energy effort. In secton 5 s presented the herarchcal control system for the butylene-butane separaton column. The man contrbutons regardng ths system refer to the proposal of a nonlnear decoupler whch calculates automatcally ts parameters and type, the use of nternal model controllers for the two concentratons at the second herarchcal level and the proposal of an obectve functon at the thrd herarchcal level. II. PROPOSED HIERARCHICAL SYSTEM The general structure of the proposed herarchcal system for the gas processng unt conssts of three herarchcal levels, as t can be seen n Fg. 2. Fgure 2. The proposed herarchcal system: CCU central control unt; LCU local control unts; CCS conventonal control systems; cs control sgnals; v nformaton varables. Frst herarchcal level s assocated wth conventonal control, wth each CCS contanng the controllers assocated wth basc control for each dstllaton column, whch refers to the control of the followng parameters: column top pressure, levels from the reflux drum and column bottom, reflux flowrate and bottom product flowrate. The second herarchcal level s dedcated to advanced control (feedforward, IMC etc.), the control functons from ths level beng fulflled by the local control unts (LCU). The thrd herarchcal level s assocated wth optmal control, and refers to the fulfllment of techncal and economc obectves, ths level s functons beng mplemented n the central control unt (CCU). The structure from Fg. 2 s partally dstrbuted, the process beng consdered consstng of subprocesses (the three dstllaton columns), ths partton beng gven by both geographcally, but mostly functonal crtera. Ths control system s herarchzed on vertcal and dstrbuted on horzontal wthn each level. In ths paper, the herarchcal levels were studed through smulaton n SIMULINK. For each column, the frst herarchcal level s mplemented usng a nonlnear model [6, 7, 8] based on materal balance (total and component) (relatons (1-2)), equlbrum equatons (relaton (3)), hydraulc delays assocated to the vapor and lqud transport phenomena etc. dm dt d dt L ( M 1 FL x, 1 FV x ) FL V 1 y L x, FL, 1 V V L 1 1 FV y, FV L x V y, y x 1 ( 1) x ) /( where: M lqud holdup of tray; FL, FV external lqud and vapor flowrates of tray; L, V lqud and vapor flowrates leavng tray; x,fl concentraton of component n external lqud feed of tray; y,fv concentraton of component n external vapor feed of tray; x concentraton of component n lqud phase on tray; y concentraton of component n vapor phase on tray; - relatve volatlty.

3 Herarchzed Dstrbuted Control System for a Gas Processng Unt 27 The used model s for a bnary dstllaton column, so the two multcomponent columns (depropanzer and butane-butylene separaton column) were treated as pseudo-bnary ones, the PPSC beng mplctly bnary. The dynamc smulatons of the three columns was performed usng a smulator developed n SIMULINK around the nonlnear model prevously descrbed. The smulator assocated to the columns has the structure from Fg. 3. F obcl1 pr ( L, V ) ( pr D MM nc4 x D _ nc4 ' C3 x ( pr ' D _ C3 soc 4 ) MM x pr _ soc 4 pr C3 steam x D _ C3 r V ) Fgure 3. Structure of the column smulator. It can be observed that the structure of the smulator s based on closng the control loops assocated to the levels n reflux drum and column bottom. Gven that the studed columns control structures are LV and L, the flowrates for level control are dstllate D and bottom flowrate or bolup flowrate V, respectvely. Thus, the process model nputs are: the flowrates used as control agents (reflux flowrate L and V or ), flowrates used for levels control (D and or V), the two dsturbances (feed flowrate F and feed concentraton x F ), fracton of lqud n feed (q F ). The outputs of the model are concentratons x D and x, and the lqud holdups for reflux drum and column bottom (M RD and M ). In the followng sectons the herarchcal systems for each column are descrbed. III. THE HIERARCHICAL CONTROL SYSTEM FOR THE C 3 -C 4 SEPARATION COLUMN The control system assocated wth the C 3 -C 4 mxture separaton column s presented n Fg. 4 and conssts of the conventonal automaton level (LV control structure [9]) and the optmal control level. The LV control structure means that L (reflux flowrate) and V (bolup flowrate) are the control agents for ths column used for control of the two concentratons: x D the concentraton of the lght component (propane-propylene mxture) n the top product, and x - the concentraton of the lght component n the bottom product. The optmal controller mplements an obectve functon presented n (1) [10]. Fgure 4. Control system for C 3 -C 4 separaton column. The notatons used n (1) sgnfy: pr ' - prce of the propylene from the top of C 3 CL2 column [le/t]; pr - prce of the propane from the bottom of C 3 CL2 column [le/t]; pr soc 4 - prce of the sobutane-sobutylene mxture from the top of CL3 column [le/t]; pr nc 4 - prce of the nbutane-(cs+trans)-butylene mxture from the bottom of CL3 column [le/t]; pr - steam prce [le/t]; st x - percentage representaton of propylene ' D _ C 3 concentraton n the lght product from the top of CL1; x - percentage representaton of propane D _ C3 concentraton n the lght product from the top of CL1; x _ soc - percentage representaton of sobutanesobutylene concentraton n the heavy product 4 separated at the bottom of CL1; x - percentage representaton of nbutane- _ nc4 (cs+trans)-butylene concentraton n the heavy product separated at the bottom of CL1; MM D - molar mass of propylene-propane mxture [kg/kmole]; MM - molar mass of C 4 fracton [kg/kmole]; D - dstllate flowrate for CL1, D = V L [kmole/mn]; - bottom product flowrate for CL1, = F D = F + L V [kmole/mn]; r - rato between the vaporzaton latent heat of the mxture from CL1 bottom and condensaton latent heat of the steam [kg/kmole].

4 28 Maran POPESCU The obectve functon desgnates a (theoretcal) proft obtaned from sellng of the products from the CL2 and CL3 columns, from whch s deducted the costs assocated wth utltes (steam, n ths case) from CL1 column. The restrctons refer to the varables L and V. In order to calculate the obectve functon, t was developed a MATLA scrpt whch contans: a process model; the values for products and steam prces, F, MM D, MM and r; the relatons for x ', D _ C 3 x D _ C 3, x _ soc 4 and x _ nc 4 ; the expresson of the obectve functon from (1). The optmzaton problem s multdmensonal wth constrants and the solvng starts from an ntal estmaton of the soluton. In Fg. 5 s shown a 3D representaton of the obectve functon on the doman mposed by restrctons. In Fg. 6 t s presented the smulaton scheme of the herarchcal control system assocated to the C 3 -C 4 separaton column. The solver uses Runge-Kutta algorthm wth varable ntegraton step, and the nterval of ntegraton s [0 300] mn. Fg. 7 and 8 present the tme evolutons of concentratons x D and x to modfcatons of set-ponts L and V from current values kmole/mn and kmole/mn to optmal values calculated by the optmal controller, 13.2 kmole/mn and 17.5 kmole/mn respectvely. Fgure 7. x D evoluton to L and V changes. Fgure 5. Representaton of the obectve functon. The result of the obectve functon maxmzaton s the par of control sgnals [L opt V opt ] = [ ] [kmole/mn] whch ensure a separaton wth x D = mole fr. and x = mole fr., the proft beng of tens of thousands of le/h. The herarchcal control system presented n Fg. 4 was smulated n SIMULINK. The outputs of the optmal controller (L opt and V opt ) represent (practcally) the set-ponts for the reflux flowrate and bolup flowrate control systems from the frst level. Fgure 6. Smulaton scheme for C 3 -C 4 separaton column herarcahcal system. Fgure 8. x evoluton to L and V changes. The responses from Fg. 7-8 demonstrate that the applcaton of the optmal set-ponts for L and V leads to an ncrease of x D concentraton and a decrease of x concentraton thus ensurng an mprovement n separaton, whch means that the top product of the column contans very lttle of the butylene-butane mxture and the bottom product contans very lttle of the propylene-propane mxture. IV. HIERARCHICAL CONTROL OF THE PROPANE- PROPYLENE SEPARATION COLUMN The control system assocated wth the propanepropylene separaton column s presented n Fg. 9 and conssts of the conventonal automaton level (L control structure [9]), the advanced control level (ACS2) and the optmal control level.

5 Herarchzed Dstrbuted Control System for a Gas Processng Unt 29 The L control structure means that L and are the control agents for ths column, the top concentraton beng controlled usng the reflux flowrate (L) and for the bottom concentraton control s used the bottom product flowrate (). The dsturbances assocated wth ths column are feed flowrate (F) and feed concentraton (x F ). Fgure 11. Smulaton scheme of frst two herarchcal levels for PPSC. Fgure 9. Herarchcal control system for CL2 column. The second and thrd herarchcal levels are characterzed n the followng. The feedforward model (FUG) mplemented at the advanced control level s a steady-state one, and usng frst order dfferental equatons a dynamc component was added to the model. The feed flowrate (F) presents more mportant and frequent varatons than the feed concentraton x F. Takng nto account ths aspect resulted from analyss of data from a real column, the followng results refer to the system behavor only to F changes. In Fg are presented the evolutons of the two concentratons to modfcatons of F, n two cases: wthout and wth compensaton. A. Advanced control level for PPSC From studes of the propane-propylene separaton column [11, 12] t was observed that the two dsturbances of the column (F and x F ) have an mportant effect on concentratons x D and x. Thus, at the second level of herarchcal control s mplemented a feedforward control system, the block dagram of the control system beng presented n Fg. 10. Fgure 12. x D evoluton to a 2.5% ncrease of F. Fgure 10. lock dagram of frst and second herarchcal levels. The control mathematcal model s based on Fenske-Underwood-Gllland (FUG) relatons [13, 14, 15, 16] whch take nto account total and component materal balance, mnmum number of theoretcal trays, mnmum reflux rato and relatve volatlty. At the second herarchcal level the set-ponts are the concentratons of the lght component n dstllate and n the bottom product (x D and x ), the dsturbances are F and x F, and the outputs are setponts (L and ) for the reflux flowrate and bottom product flowrate control systems from the frst herarchcal level. Frst two herarchcal levels were smulated n SIMULINK (Fg. 11). Fgure 13. x evoluton to a 2.5% ncrease of F.

6 30 Maran POPESCU Fgure 14. x D evoluton to a 5% decrease of F. The product r (F + L ) represents the steam flowrate, wth r the rato between the vaporzaton latent heat of the propane and condensaton latent heat of the steam [kg/kmole]. The optmum value of x s obtaned by mnmzng the obectve functon from (2), based on the values of dsturbances F and x F and set-pont x D, whch are the nputs of the optmal control level, as t can be seen n Fg. 9. The controller from ths level contans the obectve functon, a control mathematcal model of the process and an algorthm for determnng the optmum (e.g. golden secton algorthm). A representaton of the obectve functon s shown n Fg. 16. Fgure 15. x evoluton to a 5% decrease of F. Analyzng the responses from Fg t can be observed that the feedforward controller compensates wth success the effect of the dsturbance feed flowrate on the two concentratons, these results beng confrmed also by [11, 12]. Fgure 16. Obectve functon representaton. y mnmzng the obectve functon, the optmum value of the set-pont for the bottom product concentraton was obtaned, x _opt = mole fr., a value whch ensure the recovery of the most valuable component wth low energy effort.. Optmal control level for PPSC The two products separated at ths column have dfferent qualty specfcatons. As the top product specfcaton s fxed (rgd) wth x D = 0.92 mole fr., the qualty specfcaton of the bottom product s x [ ]. The determnaton of the optmum value of the bottom product specfcaton (x ) takes nto account the recovery of the most valuable product (propylene) wth as low as possble energy effort. Ths requrement can be mplemented wth the followng obectve functon F obcl2 ( x pr ) MM st p x r ( F L ) where: pr st s steam prce, [le/kg]; p- prce dfference between propylene and propane, [le/kg]; - bottom product flowrate, [kmole/mn]; MM - molar mass of propane, [kg/kmole]. Fgure 17. Smulaton scheme of the herarchcal control system wth 3 levels for PPSC. The system wth three herarchcal levels (the column wth conventonal control at level 1, the feedforward controller at level 2 and the optmal controller at level 3) was smulated n SIMULINK (Fg. 17), and the tme response of the concentraton x to the change of the set-pont s presented n Fg. 18.

7 Herarchzed Dstrbuted Control System for a Gas Processng Unt 31 The second and thrd herarchcal levels are characterzed n the followng. Fgure 18. x evoluton to the change of x. From Fg. 18 t can be observed that the concentraton x s brought to the set-pont mposed by the optmal controller. V. HIERARCHICAL CONTROL OF THE UTANE- UTYLENE SEPARATION COLUMN The herarchcal control system for the butanebutylene separaton column s presented n Fg. 19 and has three levels: the conventonal automaton level (L control structure [9]), the advanced control level (ACS3) and the optmal control level. As n the PPSC case, the L control structure means that L and are the control agents for the control of the two concentratons (x D concentraton of sobutene-sobutylene mxture n the top product, and x - concentraton of sobutene-sobutylene mxture n the bottom product). The dsturbances assocated wth ths column are feed flowrate (F) and feed concentraton (x F ). A. Advanced control level for SC Dstllaton columns are multvarable systems whch present crossed nteractons that cannot be neglected and can create control dffcultes. The L control structure assocated wth SC can be consdered a 2x2 system wth L and as nputs and x D and x as outputs. From studes on ths column t was observed that L nfluences not only the top concentraton but also the bottom concentraton, and has an effect on x D not only on x. Consequently, a decoupler must be used n order to dmnsh or elmnate these crossed nteractons. The method used n ths paper s general, the decoupler havng a standard structure whch can be mplemented n four versons, one of these versons beng presented n (3) [17]. K d12 1 Td12s 1 G D1 ( s) K d 21 1 Td 21s 1 The form from (3) s used when the drect channels are faster than the crossed channels. The other versons are smlar, wth forms dependng on the process dynamc characterstcs, and havng the followng propertes: the gan on the drect channels s 1 and at least two of the four channels are statc. The decoupler gans (K d ) are obtaned from the steady-state decouplng condton and the tme constants (T d ) are calculated, for each verson, as dfferences between the process tme constants on the correspondng channels. The frst step n the decoupler desgn was the determnaton of the process gans (K p ) and the transent tmes (T tr ) for step changes of the control agents L and. The process tme constants (T p ) are obtaned as T tr / 4 [17]. Fgure 19. Herarchcal control system for CL3 column. Fgure 20. K p varaton accordng to L varaton on L x channel.

8 32 Maran POPESCU Fgure 21. T p varaton accordng to varaton on x D channel. Fg present two examples of dependences of K p and T p on L and varatons. All dependences of K p and T p accordng to L and varatons were approxmated wth regresson functons wth dfferent degrees. The next step n the decoupler desgn was to develop a MATLA functon whch automatcally compute the values of K p and T p dependng on current values of the nputs, through nterpolaton (f the varatons of the nputs are n the already consdered domans) or usng the prevously determned regresson relatons (f the nputs varatons are outsde the consdered domans). Ths functon also calculates the decoupler parameters (K d and T d ) and type, dependng on the process channels dynamcs. Accordng to the obtaned decoupler type, there are mplemented the prevously mentoned four versons of the decoupler. Fg. 22 syntheszes the descrbed algorthm. Fgure 23. x evoluton to a 10% ncrease of frst nput. Fgure 24. x D evoluton to a 3% ncrease of second nput. Fg emphasze the benefcal effect of the decoupler on the crossed channels. The man advantage of the decouplng s the possblty to consder the concentraton multvarable control system as two ndependent monovarable control systems, as t can be seen n Fg. 25. Fgure 25. Concentraton control system. Fgure 22. Decoupler synthetc representaton. Fg present some results obtaned by smulatng the system composed of the decoupler and the SC, system whch has as nputs the control sgnals c 1 and c 2 and as outputs the two concentratons x D and x. The decoupler together wth the two concentraton controllers (AC xd and AC x ) are mplemented at the second level of the herarchcal system for SC presented n Fg. 19. AC xd and AC x are feedback nternal model controllers. For the model based standard control method used n ths study, the controller s desgned so that the control system response to step set-pont s the same as the process step response. An mportant property of ths method s that for unt step set-pont the control sgnal s also a step functon [17]. Fg. 26 shows the structure of the standard nternal model control system.

9 Herarchzed Dstrbuted Control System for a Gas Processng Unt 33 Fgure 26. The standard nternal model control system: y setpont; e error; c control sgnal; d dsturbance; y output; G C (s) nternal model controller transfer functon; K C controller gan; G M (s) model transfer functon; K M model gan; G P (s) process transfer functon. The standard value of the controller gan K C s 1 [17]. The model of a stable and overdamped process, wth gan K p, transent tme T tr for step nput, and dead-tme T m, can be lke the one from (4) [17]. G M K ( s) ( T M M e Tms s 1) 2 Most often, practcal applcatons use a model gan K M equal wth process gan K p, and the model tme constant T M as the sxth part of transent tme T tr [17]. ecause the presented method can be appled only to overdamped processes, and the response of the studed system on channel c 1 x D s underdamped wth overshoot (as presented n Fg. 27), n seres wth the process a flter was used Fgure 28. Smulaton scheme of the concentratons control systems. At ths pont, the control system for x D concentraton can be studed usng the standard nternal model control method. The tunng parameters for the controller of ths system were determned to obtan a response wthout overshoot and wth adequate transent tme and a control sgnal close to a step form. The best tunng parameters were K C1 = 1, K M1 = 0.4, T M1 = 30 mn. and T m1 = 10 mn., the system response for a step change from mole fr. to 0.97 mole fr. of the set-pont x D beng presented n Fg G f ( s) T s 1 f Fgure 29. x D evoluton to a step change of set-pont x D. The process response on channel c 2 x s proper for the use of standard nternal model control method. The best tunng parameters obtaned n ths case were: K C2 = 1, K M2 = 1.8, T M2 = 70 mn. and T m2 = 10 mn., the system response for a step change from mole fr. to 0.06 mole fr. of the set-pont x beng presented n Fg. 30. Fgure 27. x D evoluton to c 1 changes. The flter tme constant T f was determned so that the process response to be overdamped, and the best obtaned value was 40 mn. The concentratons control system were smulated n SIMULINK, the smulaton dagram beng presented n Fg. 28. The ntegraton nterval s [0 1000] mn, and the solver uses ode45 functon whch mplements Runge-Kutta algorthm for solvng systems of dfferental equatons. Fgure 30. x evoluton to a step change of set-pont x.

10 34 Maran POPESCU Fg present two examples of the concentratons tme evolutons to changes of the two dsturbances (F and x F ). and nbutane (cs+trans)-butylene mxture, [le/kg]; - bottom product flowrate, [kmole/mn]; MM - molar mass of nbutane (cs+trans)-butylene mxture, [kg/kmole]. The obectve functon consders the optmum recovery of the sobutane sobutylene mxture wth as low as possble energy effort. The controller from ths herarchcal level contans a control mathematcal model of the process, the obectve functon, and an algorthm for determnng the optmum. Fg. 33 shows a representaton of the obectve functon. Fgure 31. x D evoluton to a 3% decrease of x F. Fgure 33. Obectve functon representaton. The optmzaton problem solvng led to the optmum value of x, namely x _opt = mole fr., whch s sent as set-pont of the control system for the x concentraton from the second herarchcal level, as t can be seen from the smulaton scheme n Fg. 34. Fgure 32. x evoluton to a 3% decrease of F. From Fg t can be observed that the two controllers work properly so that the errors produced by the two dsturbances are elmnated. Takng nto account the results presented n ths secton t can be stated that the standard nternal model control method s smple and robust, these results beng confrmed also by [18, 19, 20].. Optmal control level for SC The control system from ths level s smlar wth the one used for the PPSC. For ths column, the specfcaton for the overhead product s fxed, x D = 0.96 mole fr., and refers to the concentraton of the sobutene-sobutylene mxture n dstllate. The qualty specfcaton for the bottom product s assocated to the concentraton of the same mxture n the bottom product, and s flexble, x [ ] mole fr. In ths case, the obectve functon s Fgure 34. Smulaton scheme of the herarchcal control system wth 3 levels for SC. F ( obcl3 x ) MM pr st p x r ( F L ) wth: pr st prce of steam, [le/kg]; p - prce dfference between sobutane sobutylene mxture Fgure 35. x evoluton to the change of set-pont x.

11 Herarchzed Dstrbuted Control System for a Gas Processng Unt 35 From Fg. 35 t can be observed that the x control system brngs the concentraton to the optmum setpont value receved from the thrd herarchcal level. VI. CONCLUSIONS The paper presents a herarchzed dstrbuted control system of a gas processng unt consstng of three dstllaton columns. Ths system s herarchzed on vertcal and dstrbuted on horzontal wthn each level. The herarchcal system for C 3 -C 4 separaton column has two levels: conventonal control level (represented by LV control structure) and optmal control level. For ths column s proposed an obectve functon whch desgnates a (theoretcal) proft obtaned from the sellng of the separated products n PPSC and SC, from whch s deducted the costs assocated wth utltes (steam, n ths case) from the CL1 column. The maxmzaton of the obectve functon led to a good separaton wth x D = mole fr. and x = mole fr., the proft beng of tens of thousands of le/h. For the propane-propylene separaton column the proposed herarchcal system has three levels: conventonal control level (L control structure), advanced control level and optmal control level. ecause the dsturbances assocated wth ths column have an mportant nfluence on the concentratons, at the second herarchcal level s proposed a feedforward control system. The performed smulatons n ths case demonstrated that the dsturbances effects on concentratons are compensated wth success. At the optmal control level s mplemented an obectve functon whose mnmzaton led to x _opt = mole fr., whch ensure the recovery of the most valuable product wth as low as possble energy effort. The output of ths level s set as set-pont for the control system from level 2. The herarchcal system for the butane-butylene separaton column presents at the frst level the conventonal control for ths column (L control structure), the second level consst n an advanced control system and the thrd level s dedcated to optmal control. ecause the column presents crossed nteractons between control loops, frst a nonlnear decoupler s proposed to elmnate the effects of the mentoned nteractons. The smulaton results demonstrated the vablty of ths soluton. Consequently, for the concentratons control can be used monovarable controllers for each concentraton. The proposed controllers use standard nternal model algorthms. The IMC controllers proved ther robustness also to dsturbances changes, the errors beng completely elmnated. The decoupler together wth the IMC controllers are at the advanced control level. The obectve functon from the optmal control level s smlar to the one from the propane-propylene separaton column. 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