Purity Predictive Model-based Control of Oxygen Vacuum Swing Adsorption Process

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1 8th eiterranean Conference on Control & Automation Congre Palace Hotel, arrakech, orocco June 3-5, 00 Purity Preictive oel-bae Control of Oxygen Vacuum Swing Aorption Proce J. acron, O. Roy, J. Pierquin, P. Rouchon Abtract The paper eal with both ynamic moeling an control of the oxygen vacuum wing aorption (VSA) proce. Thi proce i well known for it batch-like operation an inherent non-linearitie. The main objective are to etablih a robut ynamic moel an then to emontrate the practical application of a purity preictive moel-bae control to thi inutrial ytem. Experimental reult point out the efficiency of the propoe trategy. V I. INTRODUCTION SA i a metho for the prouction of relatively lowpurity oxygen (~ 90%) which i now a mature an wiely ue technology in many chemical engineering procee uch a gla, iron an aluminum melting [],[]. However, unlike other unit operation, VSA procee remain ifficult to unertan an, couple by the lack of theoretical evelopment in batch proce control, are till ifficult to control. Furthermore, the focu on previou reearch effort in VSA procee ha primarily been the etermination of cyclic teay-tate conition an ha often neglecte ynamic apect. Some attempt of ynamic control o exit but remaining purely theoretical an/or with limite practical application (uch a PID ecentralize control on pilot plant) [3], [4], [5]. The firt part of thi tuy eal with the ynamic moeling of an inutrial VSA unit. A implifie ynamic moel i evelope an then valiate on a plant elivering oxygen for a gla maker. It i bae on a reuce-orer repreentation of VSA phyical phenomenon an iffer from many previou tuie which were bae on empirical moeling technique or comprehenive aorption imulator limite to ingle-be configuration. The repone (oxygen purity, preure, an flow) of an inutrial VSA unit to perturbation in prouct elivery an weather conition are valiate in comparion with moel output. From thi ynamic moel, a preictive control of the oxygen purity variable (arguably the variable of mot importance on thi batch ytem) i eigne. The anucript receive January 3, 00. J. acron i with the Air Liquie Reearch Center, Le Loge en Joa, France (phone: ; jonathan.macron@liquie.com). O. Roy i the Air Liquie Stanar Plant Engineering, Vitry, France ( olivier.roy@liquie.com). J. Pierquin wa with the Air Liquie Reearch Center. He i now with Avenci, olheim, France ( joeph.pierquin@avenci.com). P. Rouchon i with the Ecole Nationale Supérieure e ine e Pari, Centre Automatique et Sytème, Pari, France ( pierre.rouchon@enmp.fr). performance of thi control for loa rejection, et point change an weather conition variation i evaluate. At lat, the practical implementation of the avance proce control i icue. II.. DYNAIC ODELING OF VSA PROCESS A. VSA ubytem & cycle ecription An inutrial VSA unit (epicte in Fig. ) i uually equippe with: Zone : two aorber upplie by an blower an connecte to a vacuum pump. Two layer of aorbing component are ue in each aorber: alumina, to catch humiity of tream, an zeolithe, to aorb nitrogen. Zone : a buffer able to tore a relatively large volume of oxygen (in comparion with aorber) an then to limit preure variation. Zone 3: a compreor to control preure an/or flow rate elivere to the final cutomer through a recycling valve. Zone 4: a cryogenic veel fille with Liqui Oxygen (LO), ue when cutomer eman excee VSA own capability. Fig.. Subytem of a VSA inutrial unit An operation cycle conit of 5 main tep (for type- Air Liquie VSA Unit). Both aorber are run in a phae-hifte manner in orer to obtain a quai-continuou prouction (Fig. ). Typical preure profile in both aorber (Zone Aorber & ) an buffer (Zone Buffer) are given in Table. A etaile ecription of the variou tep within the cycle, tep time, flow irection an control valve for a typical VSA can be foun in [] /0/$ IEEE 745

2 Fig.. Preure cycle in aorber & buffer Phae TABLE I 5-STEPS 0 VSA CYCLE B. VSA Purity: Average Dynamic oel The moel preente i thi ection i mainly bae on the application of ma balance equation to the ifferent ubytem (zone) an on the relate tranit time. The control trategy i preente in ection 3: it aim at controlling the purity (or 3 ) at the outlet of the VSA by ajuting the flow rate ref elivere to the cutomer. Therefore, for zone (aorber), ba balance equation i given by: ( N ) = v v () Where N i the total number of mole in zone which i equivalent to conier an average moel over an entire cycle. By coniering zone perfectly homogeneou, equation () become: N = ( ) v ( v ) () Which can be implifie a v << : N = ( v ) (3) A lat implification, bae on the following relation, give: = (4) Sequence A Air amittance & preurization of aorber B O prouction by aorber C O prouction by aorber (while purging aorber to regenerate it aorber) D Depreurization of aorber E Purging of aorber Where i the o-calle tranit time in aorber equal to: N = (5) A the control objective i to control the purity elivere to the cutomer it houl be note that the time contant relate to i: N = = = 95 r (6) Where r i the ratio between the oxygen in the (~0%) an the oxygen elivere to the cutomer (~90%). It houl be note that epen on (which can be coniere a contant at 475 Nm 3 /h) an which correpon to cutomer flow rate in teay tate ( = = 3 = 4 = 950 Nm 3 /h). By uppoing that the buffer compoition in zone i homogeneou, a imilar behavior can be ecribe by the following equation: ( N ) = (7) From which can be euce: = (9) Where i the tranit time in the buffer given by: N = = 95 (0) Zone 3 (compreor & re-circulating valve) oe not affect the prouct purity & flow. Then: = 3 =, = 3 = () The only ynamic phenomenon in thi zone i inuce by the flow control loop which can be coniere a a firt orer ytem, with a contant time (equal to 40 ): = ref () By combining an linearizing equation (4), (9) an (), a thir orer linear moel F() i euce to link ref an : = F( ) (3) C. VSA Purity: Average Dynamic oel The moel ecribe above ha been compare to ata from on an inutrial unit elivering oxygen for a gla maker in the Pari area. The variation of purity of the oxygen elivere to the cutomer have been oberve in repone to eman (flow) tep ( ref - ee Fig. 3). It i oberve that the oxygen purity ha an invere repone a ecribe in equation (4). The global experimental repone time i approximately 500, while the ominant contant time ( ) of the moel previouly efine wa cloer to

3 III. PREDICTIVE PURITY CONTROL A. Economical Interet of Oxygen Purity control Preictive Functional Control (PFC), belonging to the family of preictive control technique, ha been emontrate a a powerful algorithm for controlling proce plant. It i here implemente to control the VSA Oxygen purity which i arguably the variable of mot importance on the plant, epecially from the perpective of a cutomer an conequently eman tight control tolerance (typically ± 0.5% of et point). Fig. 3. VSA Oxygen Purity () repone to flow tep ( ref). Only variation are repreente. Nominal 950 Nm 3 /h Nominal 90% Thu, the oxygen purity exhibite a meaurable elaye reaction to the change (ea time ~ 00) an a ignificantly lower return to cyclic teay tate. It houl be recalle that the oxygen prouct flow into a large prouct tank an that the oxygen purity reporte here (an of interet for the cutomer) wa meaure owntream of the buffer. The oxygen repone wa therefore trongly influence by the mixing pattern in the prouct tank. If perfect mixing occurre, the repone time woul be longer an clearly of a ifferent type (exponential ecay to the teaytate value). It i therefore uggete that the primary reaon for the time elay in oxygen repone i the exitence of a compoition graient in the buffer. Equation (3) i then moifie to take thi elay into account: T = F( ) e (4) oel given by equation (3) i then valiate on experimental ata a hown in Fig. 4. From a proucer tanpoint, obtaining the require purity ha a trong economical interet. Inee, failure to achieve prouct purity require by the cutomer can inuce: - Prouct ga venting when purity target i not reache (VSA hutown). In that cae, the cutomer i upplie by the liqui oxygen backup. - Overconumption of LO in cae of purity below it contractual value. It ha been een that flow an purity have invere repone: any exce of cutomer eman ramatically reuce the purity provie by the VSA unit which ha to be compenate by LO injection. The mot common iturbance in oxygen purity control i change in ambient temperature, which are ue to both iurnal an eaonal fluctuation. Thi type of iturbance affect both the inlet tream temperature an the amount of heat lot or gaine by the aorbent be an can alter the aorptive capacity of the zeolite ieve. It i through thi mechanim that the performance of the proce can vary. B. Oxygen Preictive Control A ingle loop PFC operate on the following principle [6], [7]: internal moel, reference trajectory, auto-compenation, an calculation of the manipulate variable. The PFC implemente i bae on an internal moel uner a cacae form by oberving the fact that any m-th orer ytem can be ecompoe into a et of firt orer block [7]. Then, the average ynamic purity moel i repreente in Fig 5. ref K + e T + + r u y y y y elay Fig. 5. Internal moel in a cacae form Fig. 4. Experimental valiation of VSA oxygen purity moel 747

4 By uing the claical input/output formalim (u/y), the icrete time formulation of the moel zero-orer hol equivalent i given by [7]: G K ( + β z i i ( z) = Π i= αi z ) (5) With repect to the intant k, the moel output can be preicte a: The robutne of preictive control ha been emontrate on a two year teting perio. The main proce iturbance are both outie preure an temperature. From thi tanpoint, the efficiency of the control i confirme: epite the variation of temperature at the inlet of the ytem (correponing to the ay & night cycle), the purity remain table (Fig. 8) - range within a 0. % tolerance while the flow i maximize to optimize the unit prouctivity (Fig. 7). y k) = α y ( k ) + K y ( k) + K β y ( k ) (6) ( which can be ecompoe into free an force repone by calculating y (k) an y (k) in the ame way [7]. The reference trajectory ue here i bae on a econ-orer repone (Butterworth type) to enure a mooth flow rate variation. In PFC, the eire repone i normally pecifie a [7]: OLRT R r = (7) CLRT which efine the ratio of the Open Loop Repone Time (OLRT, the time to 90% of the final value) to the Cloe Loop Repone Time (CLRT). A ratio of 3 i choen for the VSA proce - which i typical of low procee [7]. Fig. 7. Day & Night cycle: flow optimization The way the elay i taken into account an compenate a well a further etail on the formulation of the control law in the cae of cacae firt orer block can be foun in [7]. C. Experimental Reult The tracking performance of oxygen control loop i given in Fig. 6. It how a fat repone time a expecte an a goo compenation of the elay. 9 0 Purity (%) 9,6 ref 9, 90,8 90, , Time (min) Fig. 6. Purity preictive control: tracking performance Fig. 8. Day & Night cycle: Purity control at ± 0.% To confirm the robutne of the control to a fater iturbance, a variation of the inlet temperature ha been applie by quickly increaing the temperature on the heat exchanger (Fig. 9). For thi wort cae cenario (which coul correpon to uen change of atmopheric conition - uch a a torm for intance), purity remain contant a well (Fig. 0). 748

5 IV. CONCLUSION The paper eal with both ynamic moeling an preictive control of the oxygen vacuum wing aorption. The performance of thi control for both tracking an iturbance rejection (mainly inuce by the variation of weather conition) i emontrate on an inutrial plant over a ignificant perio an le to both prouctivity increae & eae of operation. To our knowlege, the reult preente in thi paper are the firt one in the literature obtaine on an inutrial unit. Fig. 9. Inlet Heat Exchanger Temperature REFERENCES [] D.. Ruthven, S. Farooq, K.S. Knaebel, Preure Swing Aorption VCH Publiher, Weinheim, New York, Cambrige, 994. [] R.T. Yang, Ga Separation by Aorption Procee, Butterworth, Boton, 987. [3] Beh,C.C.K., P.A.Webley, Control of the Oxygen Vacuum Swing Aorption Proce. Part Single Loop Control, Inutrial an Engineering Chemitry Reearch, 004. [4] A.I. Shirley, A. I. LaCava, PSA Sytem with prouct turnown an purity control, US Patent n , The BOC Group. [5] Beh, C.C.K.; Wilon, S.; Webley, P.A.; He, J. The Control of the Vacuum Swing Aorption Proce for Air Separation, Proceeing of the Secon Pacific Bain Conference on Aorption Science an Technology, 000, D. Duong (e), Worl Scientific, Singapore, 663. [6] Richalet J, Pratique e la commane preictive, Hermè, Pari, 993. [7] ohame Tarek Khair; John V. Ringwoo, Extenion of firt orer preictive functional controller to hanle higher orer internal moel, Int. J. Appl. ath. Comput. Sci., 008, Vol. 8, No., Fig. 0. Oxygen purity uring heat exchanger temperature iturbance At lat, the PFC ha alo le to ignificant reuction of both energy an liqui oxygen (LO) conumption. LO Saving have been obtaine by maximizing the flow rate (Table ), a no manual intervention to ajut purity i neee anymore on both VSA. Thi manual ajutment ue to be conervative (i.e leaing to a higher purity than requete by the cutomer) to avoi any lo of purity which coul have prouce a plant hutown. TABLE LO SAVINGS ON VSA UNITS Unit LO Saving % LO Saving Nm 3 VSA 3,6% VSA 0,8% Total 39,%

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