Model Predictive Control Approach to Improve the Control Performance in Pasteurized Milk Process

Transcription

1 Paper code: pc IChE International Conference Noveber, at Hatyai, Songkhla HAIAND odel Predictive Control Approach to Iprove the Control Perforance in Pasteurized ilk Process Sathit Niasuwan, Paisan Kittisupakorn *, Iqbal. ujtaba Departent of Cheical Engineering, Faculty of Engineering, Chulalongkorn niversity, Bangkok, 33, hailand School of Engineering, Design and echnology, niversity of Bradford, Bradford, BD7 DP, nited Kingdo *e-ail: Abstract In a pasteurized ilk process, the ilk ust be heated to above 7 C and subsequently cooled to a low teperature about C in order to ensure a food safety. According to the real pasteurized ilk plant, individually single variable control approach including anual control with butterfly valve and autoatic control by on-off controller have been carried out to control ilk level at storage s, hot water teperature at the boiler, and iced water teperature at the ripple plate, respectively. Although the on-off controllers adequately control the ilk teperature at holding tube and cooling section of plate pasteurizer () at process requireents, the dynaic profiles of ilk teperature fluctuate around desired value. his leads to insufficient energy consuption for pasteurized ilk production. o handle this, a ulti-variables control approach with odel predictive control () has been proposed owing to an interaction between anipulated variables. he control perforance is siulated based on AAB software. he siulation results show that the can control the ilk and water teperature at the plant s requireents with less drastic control action. his shows that the is applicable to control the ilk and water teperature. Keyword: odel predictive control, ulti-variables control, atheatical odel, Pasteurized ilk process, Energy cost saving

2 IChE International Conference Noveber, at Hatyai, Songkhla HAIAND. Introduction Pasteurized ilk is a dairy product that has a shelf life of 8 to days in an unopened package. In pasteurized ilk process, a priary concern is food safety involving with the ilk teperature at every stage in the process. In particular, at the heat treatent process, one of the food safety is guaranteed by the requireents of above 7 C heating teperature at the outlet of the holding tube and below C cooling teperature at the cooling stage of the. Consequently, the control syste has been designed to ensure the safety of pasteruized ilk. According to the real pasteurized ilk plant, individually single variable control approaches have been carried out to process equipent involved including utilities. At interediate and product storage s, ilk levels are anually control by butterfly valves. eanwhile, the traditional on-off controllers have been applied to control hot water teperature of boiler and iced water teperature of ripple plate, respectively. Even though the control perforance is satisfied by the conventional controllers, the teperature profiles of the water and the ilk still fluctuate around desired set points. his leads to insufficient energy consuption for pasteurized ilk production. oreover, the pasteurized ilk plant encounters the reduction of production capital owing to increasingly high enegy cost. hese becoe the focus of further iproveent in the control syste design. In literatures, autoatic control strategy focused on the control of pasteruized teperature. he historic controllers used the outlet teperature at the holding tube as easured variable to pneuatic controller which anipulated the stea valve to heat the hot water []. Fluctuation of the ilk teperature was caused by disturbances such as inlet ilk teperature, ilk flowrate, hot water teperature, hot water flowrate etc. o handle this, the single or ultivariable control with cascade standard PID (propotional-integral-derivative) controllers was proposed [], [3]. Both algoriths gave good disturbance rejection at the. Practically, not only the teperature control but also the level control was required for process equipents. A prograable logic controller (PC), prograed in ladder logic, was built up a total control to ake a whole diary plant ore productive []. On account of an interaction between anipulated variables, these algoriths gave the poor control perforance and continuously cycling action at final control eleents. odel predictive control () is a technique to provide the set of control actions based reference odel. he has been required to handle highly nonlinear and interacting cheical process [], [5], []. his work is aied at applying the controller design by the to iprove the control perforance for the pasteurized ilk production. Coparison ade with the conventional control syste is perforeed in this paper.. Process odel he atheatical odels of pasteurized ilk process have been studied here [7]. Fig. has been shown process diagra over the unit process involved including utilities: silo, five storage s,, holding tube, boiler, cooling tower, ripple plate, and three water s. he eaning of letters and sybols are given in noenclature. he physical properties and process data are given in able. Raw ilk Silo h F Storage h 3 F Holding tube F Storage ilk Hot water h3 Storage h Iced water ap water Water Water Water Boiler PG, f 9 5 Cooling tower Ripple plate Electrical current, I 3 F 3 evel sensor Pasteurized ilk eperature sensor anipulated value for final control eleent Fig.. Pasteurized ilk process diagra including the syste. pc-

3 IChE International Conference Noveber, at Hatyai, Songkhla HAIAND he ass and energy balance in the unit process involved including utilities can be written as follows. dh uf dt A and d dt C d d dt p a dh dt uf F A u F a d h Cpd (,) (3) () dj F j j A for j=,7 (5) dt V C V p dj F j j A for j=9,, () dt V C V p Fh u3fh 5 j dj HAH for j=7,8,9 dt 3V C V H h p, h H, d Ft C F DFcw dt V C V w p w w w C t p, t C tw p, tw C () EtwV ha a (3) C V d ufi u n d 3 f h A dt V C V s fg f R i p, i R A R R a C V i p, i R u F 5.5 (7) d 3 h A dt V hwcphv I h A u n d 38 fg f s f Ft dh3 u (8) F F3 dh u F F3 and t pt dt A dt A 3 ufc 3 d3 uf 3 3 a (9) c pc dt d h Cp, d P AP j a d u F 3 a () d A dt V C V d A dt V C V j j C p V P for j=3,5,8, u 5 5 f HV 5 () d5 F A dt V C V C V h B B a B hw p, h B h p, h B able. he physical properties and process data for the siulation study. ransfer area of at regenerative stage ( ).89 Overall heat transfer coefficient of dt d h 3 C d p, ransfer area of at heating stage ( ).89 - Regenerative stage (W/ K) 9 ransfer area of at pre-cooling stage ( ).89 - Heating stage (W/ K) 9 ransfer area of at cooling stage ( ) Pre-cooling stage (W/ K) 95 ransfer area of heating coil at water ( ).57 - Cooling stage (W/ K), Volue of boiler ( ). Overall heat transfer coefficient of heating coil at water Volue of water basin at ripple plate ( ).5 - ank (W/ K) Volue of water basin at cooling tower ( ).5 - ank (W/ K) 9 Holding tube diaeter () ank 3 (W/ K) 38 ength of holding tube () Overall heat transfer coefficient of device surface Rate of pasteurized ilk production (kg/day) x - Holding tube (W/ K) 95 ilk flowrate at raw ilk ( 3 /s). - - Silo (W/ K) ilk flowrate at ( 3 /s) - - Storage (W/ K) 3 Rate of filling process ( 3 /s) Water (W/ K) 5 Hot water flowrate at ( 3 /s) Boiler (W/ K) Water flowrate at ( 3 /s) Ripple plate (W/ K) Iced water flowrate at ( 3 /s).9-3 Heat transfer coefficient of convection between water and air (W/K) Hot water flowrate return to the boiler ( 3 /s) Specifiaction of copressor at the ripple plate Water ake up teperature ( C) 7 - Heat vaporization of refrigerant (kj/kg) 7, () (5) (,7) (8) (9) pc-3

4 IChE International Conference Noveber, at Hatyai, Songkhla HAIAND Abient air teperature ( C) 3 - Rotating speed of ipeller (rp) 5, Rate of circulation water at cooling tower ( 3 /s) Diaeter of ipeller (). Drift loss at cooling tower (%) - Periphereal flow area ( ). PG consuption rate at boiler (kg/s) Specific volue of refrigerant at the exit.5 3. odel Predictive Control he basis concept of the is that it calculates future controls based on current easureents via the solution of predictive cotrol strategy, but only the first eleent of controls is applied to the process [8]. herefore, the objective function of the predictive control strategy has been fored as follows: P in W Y p( k l ) Y sp( k )... u n( k ),...,u n( k ) n l... W u n Subject to process odels () to (9) () un,in u n( k l ) un,ax for l=,,..., () Y Y( k l ) Y for l=,,...,p () p,in p,ax Y( k p ) Y sp (3) he objective function of the (Eq. ) is to iniize the su of squares of the errors between the predicted outputs and the set point values and also the control oveents evaluated over the prediction horizon. Bounded controls and controlled variable constraints are represented by Eqs. and. oreover, Eq. 3 is included to ensure that the controlled variables are forced to desired set points at the terinal tie. he optiization proble is classified as ulti-variables constraint optiization and solved by the sequential quadratic prograing (SQP) in AAB software. Set point Y sp at tie=k Objective function Process odel Constraints tie=k u(), u(),, u() anipulated variable at tie=k+, u() Process Fig. illustrates the inforation flowchart of the algorith. A control trajectory u(k) referring to set point (k) for an entire horizon is coputed on-line based on current state. he initial value of controls is ipleented to the syste. his eans that the control action at tie (k+) is the control u() referring to set point () of future controls calculate at tie k. Soe feedback control is provided by easureents of the state at the next interval and repeating the calculation. Otherwise, easureents are copared to a set point or predicted value. he error between the easureents and set points can be utilized within the algorith. he produces the future controls, which iniizes this error.. Siulation Results and Discussion o achieve the ain goal, the tuning paraeters: control horizon (), prediction horizon (P), and weighting factors (W, W ) are given in able. able. uning paraeter of algorith = 3 P = W =, W = he ain purpose of this study is to iprove the control perforance copared with the conventional control syste. Here, the algorith has been studied to control the ilk level (h and h 3 ), the teperature of heat treatent process ( 8 and ), the hot water teperature ( 5 ), and the iced water teperature ( ) to desired set points by adjusting the ilk flowrate, hot water and iced water flowrate at the, liquefied petroleu gas (PG) feeding rate at the boiler, and electrical current at the ripple plate. he siulations are ade corresponding to ilk flowrates (F, and F,3 ), teperature disturbances ( 5 ), and shifted backward in tie by transporting delay as illustrated in Fig. 3. easured variable at tie=k Y=[h, h 3, 8,, 5, ] Fig.. Inforation flowchart of the algorith. pc-

5 elecritcal current (ap) teperature (C) teperature (C) teperature (C) PG feeding rate (kg/s) height () height () teperature (C) flow rate ( 3 /s) flow rate ( 3 /s) teperature (C) flow rate ( 3 /s) flow rate ( 3 /s) teperature (C) IChE International Conference Noveber, at Hatyai, Songkhla HAIAND x - F, x F, x process tie Conventional (sec.) syste x process tie Conventional (sec.) syste x process tie (sec.) x.5 Fig. 3. A step disturbance based on actually process operation. interediate storage product storage boiler x x x x process tie (sec.) x - x x process tie (sec.) x 8 x Fig.. he response of the and the conventional control syste. 8 7 x -3 x process tie (sec.) x ripple plate holding tube of cooling stage of process tie (sec.) x 8 process tie (sec.) x 8 x process tie (sec.) x Fig. shows the control perforances of the and the conventional control syste. As seen in dashed line, the is designed to bring the ilk level, the ilk teperature, the hot water teperature, and the iced water teperature to the desired set points fro initial values. he desired set points of the level at the interediate storage and the product storage are specified at.. and.9., respectively, and the desired set points of the teperature are also set at 75 C in the holding tube, 3 C in the cooling stage, 9 C in the boiler, and C in the ripple plate, respectively. Suitable control has been found to drive the process responses to the following set points without overshoot and oscillation and with less drastic control actions. his is because of the accurate representation of the process odels. Coparison is also ade with the conventional control syste. he level control at the storage s by the anual butterfly valve and the teperature control at the boiler and the ripple plate using the on-off controller show the poor perforances as displayed in solid line of Fig.. hey show oscillation of the controlled variables. able 3 suarizes the integral absolute error (IAE) values using the and the conventional control syste for the pasteurized ilk process. hey indicate that the gives soother and better control perforance than the conventional control syste with sall IAE values when disturbances are introduced into the syste. able 3. Perforance coparison between the and the conventional control syste. nit Conventional IAE values Interediate storage Product storage..9 pc-5

6 IChE International Conference Noveber, at Hatyai, Songkhla HAIAND Holding tube of Cooling stage of Boiler Ripple plate Conclusions he ulti-variable control approach with the has been studied here. In this work, the has been applied to control the teperature of the pasteurized ilk process. Siulation results have deonstrated that the can control the ilk level and teperature, the hot water teperature, and the iced water teperature at the desired set points without overshoot and oscillations and with less drastic control actions. his shows that ulti-variable control approach with the is applicable to control the level and the teperature of unit process involved including utilities for pasteurized ilk process.. Acknowledgeents he authors would like to thank hailand Research Fund (RF) under he Royal Golden Jubilee Ph.D. Progra (IG5K5) for financial support and Sarakhakaset Co., td. for inforation support during the research. A i 7. Noenclature transferred area of unit i A peripheral flow area of copressor f, p C px, heat capacity of fluid x d E ipeller diaeter of copressor evaporation rate in cooling tower F x, j voluetric flowrate of fluid x at strea j h A heat transfer coefficient at water surface h latent heat vaporization of refrigerant h i I fg ilk level at unit i electrical current D echanical drift loss in cooling tower HV low heating value of fuel cobustion f n ns P j t fueling rate at boiler control horizon saple the process outputs ipeller s rotation speed prediction horizon teperature of fluid at strea j tie t f terinal tie of horizon u anipulated variables i Y overall heat transfer coefficient at unit i easured variables Y sp desired set points W weighting factor Greek etter x V density of fluid x specific volue of refrigerant at the exit latent heat vaporization of water teperature difference between both sides References [] G. Bylund. Diary Processing Handbook (etra Pack Processing Syste, 995). [] A. Negiz. A. Cinar. J. E. Schlesser. P. Raanauskas. D. J. Arstrong. W. Stroup, Autoated control of teperature short tie pasteurization, Food Control (99) [3] K. R. orison, Steady-State Control of Plate Pasteurisers, Food Control (5) 3-3. [] P. Kittisupakorn, P. Ruksawid, Application of odel Predictive Control for a Batch Cheical Reactor, KK Eng. J. 5 (998) -. [5] P. Kittisupakorn,. A. Hussain, odel Predictive Control for the Reactant Concentration Control of a Reactor, Korean J. Che. Eng. 7 () [] P. Kittisupakorn, P. hitiyasook,. A. Hussain, W. Daosud, Neural Network Based odel Predictive Control for a Steel Pickling Process, J. Pro. Cont. 9 (9) [7] S. Niasuwan, P. Kittisupakorn, I.. ujtaba, Optiization Approach to iniize Energy Consuption in Pasteurized ilk Process, International Conference on Cheistry and Cheical Process. Bangkok, hailand, ay 8-9 (). [8] P. Kittisupakorn. odel Based Control for Batch Cheical Process. (Chula Press, 8). pc-