Design and Analysis of a Classical Controller to the Residual Oil in a Small Scale Semibatch Extractor

Transcription

1 Euopean Symposium on Compute Aded Aided Pocess Engineeing 15 L. Puigjane and A. Espuña (Editos) 25 Elsevie Science B.V. All ights eseved. Design and Analysis of a Classical Contolle to the Residual Oil in a Small Scale Semibatch Extacto Aline F. Custódio, Daiton F. Rezende, M. R. Wolf Maciel and Rubens Maciel Filho Chemical Engineeing School - Univesidade Estadual de Campinas: UNICAMP Cidade Univesitáia Zefeino Vaz CEP , CP. 666, Campinas, S.P., Basil. alinefc@feq.unicamp.b Abstact The main effects of the vaiables concened with the isothemal and isobaic supecitical extaction of gape oil ae detemined using a two levels factoial design. It is veified that the solvent velocity can be manipulated to contol the esidual oil content in the solids. Distubances in the mean diamete of the paticles can not be contolled duing the extaction pocess. Paamete sensitivity suppoted the design and the pefomance investigation of the classical contolle to the esidual oil in the solid mateial. The poposed pocedue was able to contol the extaction pocess unde individual and simultaneous step distubances in the initial content of oil in the solids and in the solvent inlet concentation. Keywods: Supecitical Extaction, Semibatch Extacto, Classical Contolle 1. Intoduction The solvent extaction pocess fo oilseeds has been consideed the mo st efficient way fo extacting vegetable oils fom its solid matixes. The success of this technology is due to its ability in educing to a minimum the esidual oil in the extacted solid mateial. The hexane has been the widest used solvent, but because of its toxicity and flammability thee exist incentive to eplace it. Supecitical cabon dioxide has been successfully expeienced fo extacting natual poducts, due to its atoxicity, chemical stability, availability and low costs. While too much attention has been diected to know the supecitical - CO 2 pefomance fo extacting a vaiety of oil-beaing mateials, less effot has been dedicated to the optimization and to the contol of this extactive pocess. Seveal appoaches have been poposed and analysed fo contol of semibatch pocesses (Gacia-Munoz et. al 24; Asteasuain et. al. 24; Sinivasan et. al., 23; Gau and Puigjane, 2; Floes-Ceillo and MacGego, 24) and can be identified the benefits of such appoaches to opeate the systems at highe pefomance when compaed to manual opeation. Howeve, the CO 2 semibatch extactos have peculia chaacteistics, specially in thei quick dynamic and opeating vaiable inteactions, so that the esults and conclusions of othe types of system may not be diectly used. It is impotant to conside that a good contol stategy may lead the pocess to be opeated at high pefomance, which has a diect impact on the pocess viability, specially fo supecitical extaction, that is an exp ensive pocess. The pesent wok intends to contibute to this unexploed field though paametic sensitivity analysis and

2 investigation of the pefomance of a classical contolle. The fist equisite to accomplish this analysis is the availability of a mathematical model and, so, extensive simulation can be caied out. 2. Supecitical Extaction Model A eview on the mechanisms of the solvent extaction pocess of oilseeds showed that thee ae at least fou steps that can potentially dictate the ate at witch the oil is emoved fom its solid matixes. They ae: 1-solvent penetation in the intapaticle poes; 2-dissolution of the oil adheed to the solid paticles by the solvent that fulfill the intapaticle poes; 3-diffusion of the dissolved oil though the solvent in the intapaticle poes until the paticle suface; 4-convection of the oil fom the suface of the solid paticles to the bulk phase of the solvent. A mathematical extaction model that includes all the steps mentioned above equies a consideable numbe of paametes. A common pocedue (Sovová,1994 and Sovová et al. 1994), hee also adopted, is to take in consideation only the two last steps. The following consideations, in addiction to the above mentioned, ae made: the extaction occus in a fixed bed of solid paticles; the solvent flows continuously though the bed of solids; the oil is teated as a single specie and no axial dispesion and isothemal and isobaic opeation ae consideed. The equations descibing the model ae composed by mateial balances of oil in the solid and in the fluid phases: x ρ S = (1) t Solid Phase: ( 1 ε) J( x, y) y y ρ f f = (2) t h Fluid Phase: ε + ρ U J( x, y) whee: t-time of extaction ; ρs -density of the solid paticles; ε -bed poosity ; x-oil mass faction in the solids in an oil fee basis ; y-oil mass faction of the solvent in an oil fee basis; J(x,y)-ate of extaction; ρf -solvent density ; U-solvent velocity ; h -axial distance in the bed. J(x,y) depends on a citical esidual oil concentation in the solid paticles, x k. This concentation makes the tansition of a peiod of extaction dictated by convection in the film suounding the solid paticles to a peiod dictated by diffusion though the solvent inside the paticles. The fast extaction ates, in the convection peiod, ae descibed by: J ( x, y) = k a ρ ( y y) (3) f x x ; f when k y oil solubility in solvent; k f a - mass tansfe coefficient based on the solvent phase. The slow extaction ates, in the diffusion peiod, ae descibed by: J x, y) = k a ρ x(1 y y ) (4) ( s s x x ; when k k s a : mass tansfe coefficient based on the solid phase. Equations (1), (2), (3) and (4) ae subjected to the following initial and bounday x h, t = x y h, t = y y h, t t lim = y conditions: ( = ), ( = ), ( )

3 whee: x -initial oil content in the solids; y -inlet solvent oil content; y -equilibium solubility of the oil in the solvent; t lim- minimum time equied to the no satuated solvent each a point h of the bed. Equations (1) to (4) wee solved numeically with the fouth ode Runge-Kutta method in espect to time ( Δt =. 1s) and finite diffeences wee used fo the spatial coodinate discetization ( Δ h =. 5 m). Values fo the mass tansfe paametes may be found in Sovová et al. (1994) fo the supecitical extaction of gape oil. The pessue is 28 bas and the tempeatue is 4 C. The laboatoy scale extacto is 5. cm of useful length. 3. Solvent and Solid Oil Contents at the Extacto Exit Impotant infomation fo the equipment design as well as fo the definition of the opeating stategy is that elated to the extaction time egading to the esidual oil contents, in both, solids and solvent. Figues 1 and 2 show, espectively, the esidual oil in the solids and in the solvent at the extacto exit as a function of the extaction time. The magnitude of the numeical eos in pesented in the ight y-axis of Figue 1 (dashed line). In tems of extaction, it can be seen that in the initial phase of the convection peiod, the extaction is vey slow, as consequence of the high solvent oil content. As soon as fesh solvent achieves the extacto exit, the extaction becomes faste. This zone, in the convection peiod, chaacteized by fast ates of extaction, is called extaction font egion. When the esidual oil citical concentation in the solids is attained, at the diffusion peiod of extaction, the extaction ate deceases, and the esidual oil content in the solids changes vey slowly. The design and pefomance evaluation of contol stategies equie the knowledge of the effects of the vaiables concened with the extaction pocess. To ensue that the obtained effects ae meaningful in a computational solution, it is impotant to know the magnitude of the numeical eos. As can be seen in Figue 1, the elative numeical eos in the computation of the esidual oil in the solids is smalle than about 2.5% E-3 RESIDUAL OIL IN THE SOLIDS (kg of oil pe kg of oil fee soli ds) NUMERICAL ERROR (%) SOLVENT OIL CONTENT (kg of oil pe kg of oil fee solvent) 6.E-3 4.E-3 2.E-3.E EXTRACTION TIM E(s) Figue 1.Solid esidual oil at the extacto exit EX TRA CTION TIME (s) Figue 2.Solvent oil content at the extacto exit as a function of extaction time.

4 4. Paamete Sensitivity Analysis The objective of this section is to undestand how the vaiables affect the esidual oil content in the solids (time dependent) at the extacto exit. The vaiables analyzed wee: mean solvent velocity (U); oil mass faction of the solid mateial (x ); oil mass faction of the solvent at the extacto entance (y ), and mean solid paticle diamete (dp). The values of the vaiables elated to the extaction pocess in a standad condition ae: U=5.E-4 m/s; x =.144; y =. and dp=2.7 E-4 m. Figue 3 shows the main effects as a function of the extaction time. It can be seen that, in the initial extaction time (limited by convection), only the effect of the initial oil content of the solid paticles is not null. In the extaction font egion, howeve, all the vaiables have significant effects. The effects of the solvent velocity and of the initial oil content in the solids have positive effects on the esidual oil, while the solvent velocity has a negative effect. The mean solid paticle diamete shows an invesion in the signal of the main effect, in the egion of the font of extaction. Fistly, the signal is negative, indicating that the esidual oil in the extacto exit is lowe fo geate paticles. Afte, the signal becomes stongly positive, signifying that as bigge the paticles, geate will be the esidual oil content in the solids. In the peiod of extaction ates limited by diffusion inside the solid paticles, the paticle mean diamete is the unique vaiable with impotant effect on the esidual oil in the solids. The analysis made above indicates that the solvent velocity may be manipulated to contol the esidual oil concentation at the extacto exit, afte distubances in the solid and inlet solvent initial oil contents. The analysis of the impact of changes in the paticle mean diamete is also useful, since although it may not be contolled duing the extaction, an appopiated teatment in the paticles befoe the extaction will be an effective pocedue fo high pefomance pocess opeation. Figue 3: Effects of vaiables on the esidual oil in the solids at the extacto exit.

5 5. Pefomance of a Classical Popotional Feedback Contolle As the semi-batch extaction pocess is intinsically tansient, it was necessay to define a efeence extaction cuve as a guide fo the contolle. This extaction cuve was adopted as the time dependent esidual oil at the extacto exit, fo the vaiable levels consideed in Figue 3. The effect of the sampling peiod on contolle's pefomance was obseved in Rezende (1998). The lage sampling peiod that still leads to a good pefomance of the contolle was 2 seconds. This was adopted in this wok. It was, also, noted that the addition of the integal and deivative actions could not impove the classical contolle pefomance. The contolle is investigated fo keeping a time dependent esidual oil concentation at the level dictated by the efeence extaction cuve, afte individual and simultaneous step distubances (5 and 15 %) in the initial solid oil content and in the inlet solvent oil concentation. The effect of the popotional contolle on a time dependent cuve fo the esidual oil concentation at the extacto exit, afte positive step distubances (5 and 15 %) in the initial solid oil content is such that, afte 14 seconds, the pofiles of the contolled extaction joins the efeence extaction one. Concened with the solvent velocity afte distubances in x, it can be seen that as soon as the peiod of slow ates of extaction is eached, the velocity backs to its standad value although the eo is essentially null, and this velocity is no moe an efficient vaiable to contol the extaction. The pefomance of the popotional contolle afte distubances of 5 and 15 % in the solvent concentation, at the extacto entance, is pesented in Figue 4a. Cuves epesenting the contolled extactions ae seen to follow the efeence extaction cuve. The behavio of the manipulated vaiable (the solvent velocity) can be obseved in Figue 4b. Again, when the final peiod of slow ates of extaction is eached, the standad value of the solvent velocity is ecoveed. The pefomance of the popotional contolle is shown in Figue 5a. In spite of distubances in these two vaiables, the contolle is still able to guide the extaction to the neighbohood of the efeence extaction cuve. The behavio of the solvent velocity is shown in Figue 5b. 7.E-4 7.5E-4 Distubance of 5. % in x Distubance of 15, % in x Dist. of 5. % in y 6.5E-4 7.E-4 Dist. of 15. % in y SOLVENTSUPERFICIAL VELOCITY (m/s) 6.E-4 5.5E-4 SOLVENT VELOCITY (m/ s) 6.5E-4 6.E-4 5.E-4 5.5E-4 4.5E-4 5.E EXTRACTION TIME (s) Figue 4a. Pefomance of the popotional contolle afte distubances of 5 and 15% in the entance solvent oil content EXTRACTION TIME (s) Figue 4b. Popotional contolle in the solvent velocity afte distubances in the inlet solvent concentation.

6 RESIDUAL OIL IN THE SO LIDS (kg of oil pe kg of oli fee sol ids) % in x and y Refeence Extaction Cuve Open Loop Closed Loop 15% in x and y SOLVENT VELOCITY (m/ s) 2.E-3 1.6E-3 1.2E-3 8.E-4 4.E-4 Dist. s of 5. % in x and y Dist. s of 15. % in x and y..e EXTRACTION TIME (s) EXTRACTION TIM E(s) Figue 5a. Pefomance of the popotional contolle afte simultaneous distubances of 5 and 15%. Figue 5b. Popotional contolle in the solvent velocity afte simultaneous distubances. 6. Conclusions The paamete sensitivity analysis equied to the design of contol systems showed impotant featues fo the peiod of extaction contolled by the oil convection at the vicinity of the solid paticles. The popotional contolle is able to dive the time dependent cuve fo the esidual oil in the solids towads a pedefined extaction patten, duing the peiod of extaction limited by the oil convection. This contol action is made manipulating the solvent velocity afte individual o simultaneous distubances in the initial solid oil content and in the solvent inlet concentation. Refeences Asteasuain M., Bandolin A., Samoia C., et al. Simultaneous Design and Contol of a Semibatch Styene Polymeization Reacto, Industial & Engineeing Chemisty Reseach, 24, vol 43, No. 17, Floes-Ceillo J., MacGego J.F. Contol of Batch Poduct Quality by Tajectoy Manipulation using Latent Vaiable Models, Jounal of Pocess Contol, 24, vol 14, No. 5, Gacia-Munoz S., Kouti T., MacGego JF. Model Pedictive Monitoing fo Batch Pocesses, Industial & Engineeing Chemisty Reseach, 24, vol 43, No. 18, Gau M.D., Puigjane L. Batch and Semibatch Reactos Modelling and Validation Based on Online ph Measuement, Chemical Engineeing Communications, 2, vol 178, Rezende, D.F. PhD Thesis (in potuguese), LOPCA/FEQ/UNICAMP, S.P. Basil, 174p., Sovová, H., Rate of Vegetable Oil Extaction with Supecitical CO2 - I. Modelling of Extaction Cuves, Chemical Engineeing Sciense, Feb. 1994, vol. 49, No. 3, Sovová, H., Kucea, J. & Jez, J. Rate of Vegetable Oil Extaction with Supecitical CO2 - II. Extaction of Gape Oil, Chemical Engineeing Sciense, Feb. 1994, vol. 49, No. 3, Sinivasan B., Bonvin D., Visse E., et al. Dynamic Optimization of Batch Pocess II. Role of Measuements in Handling Uncetainty, Computes & Chemical Engineeing, 23, vol 27, No. 1,