Investigation of the effects of various parameters on mixing time in pilot stirred tank equipped single and dual rushton impeller

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1 Indan Journal of Chemcal Technology Vol. 4, May 07, pp Investgaton of the effects of varous parameters on mxng tme n plot strred tank equpped sngle and dual rushton mpeller Fruz Fakher*,, Jafarsadegh Moghaddas, Sajjad Feredon 3 & Hossen Attar 3 Member of Young Researchers and Elte Club, Islamc Azad Unversty, Scence and Research Branch, Tehran, Iran Transport Phenomena Research Center, Chemcal Engneerng Faculty, Sahand Unversty of Technology, P.O. Box 5335/996, Tabrz, Iran 3 Islamc Azad Unversty, Pharmaceutcal Scences Branch, Tehran, Iran. E-mal: eng.fakher@gmal.com Receved 5 October 05; accepted 5 March 06 The optmum postons for the addtve njecton and probe locaton n a dual-rushton strred tank have been nvestgated usng Response Surface Methodology. Therefore, by keepng these two parameters (probe locaton and addtve njecton pont) fxed durng consequent experments, the sngle Rushton mpeller has been compared wth a dual Rushton mpeller strred tank. The results have shown that the mxng tme for sngle mpeller systems, s lower than that for double mpeller systems, so to obtan a lower mxng tme n mult mpeller systems, t s necessary to obtan the relaton between the heght of the lqud and the number of mpellers. In the next part, the obtaned mxng tme s compared aganst smulaton results and publshed data based on expermental works for a Rushton mpeller n dfferent confguratons of a vessel. Also n ths work the effects of superfcal gas velocty and mpeller rotatonal speed for both systems have been nvestgated. Keywords: Strred tank, Mxng tme, RSM, Rushton turbne, Aeraton The mxng or agtaton of lquds n strred tanks are wdely used n the process ndustres and one of the most mportant unt n the many ndustres such as chemcal, food, ol and petrochemcal processng for mxng sngle or multphase fluds. Now a days, ths type of system s very sutable for the sold-lqud-gas dsperson n the lqud, wth or wthout chemcal reacton,. In many stuatons, mxng tme s an mportant factor to optmze mxng tank desgn wth sngle and multple mpellers 3. In fact, the product qualty and producton costs are affected by optmzaton of strred tank desgn. Mxng tme can be used as a comparatve factor of the mxers effcency, wth the same power nput,4. The deal geometry of the helcal mxng systems (ptch sze rato, clearance-wall, blade wdth, number of blades) has been nvestgated by Delaplace et al. 5 and they showed that the global mxng characterstcs of a helcal rbbon mxer are ndependent of Re for a lamnar flow. The studes on mxng tme have been carred out usng acd/base/ndcator reacton, temperature varaton, planar laser-nduced fluorescence (PLIF) technque and conductvty technque. The results obtaned for mxng tme by researchers s dfferent because the mxng tme s dependent on the type of mxng process, the geometrcs of the system, the defnton and method of measurement of the system 5. A comparson of the two types of methods has prevously been reported by researchers. Chomcharn 6 has compared between physcal (conductvty probe technque) and colormetrc methods for the unbaffled system and have ndcated that the colormetrc method produces results that were n agreement wth the conductvty method only when two probes are used. Paglant and Pntus 7 showed that mxng tme measured n chemcally reactng system were, on average, % shorter than the values measured usng the conductvty technque. In order to mxng tmes comparson the measurng method must be the same n all experments. Gas-lqud mxng tme measurements n several technques are dfferent, ths dfference may be due to the geometrcs of the mpeller, the probe locaton, the tracer njecton pont and the process condton and measurement technques. Thus, there are few studes for mxng tme under aerated condtons. Otomo et al. 8 measured the unaerated and aerated mxng tme by mprovement on a conductvty method. They found that the nfluence of gas flow rate on the mxng tme was not sgnfcant. Pandt and Josh 9 nvestgated mxng tmes under aerated condtons usng ph and conductvty probe. They showed that the mxng tmes ncreased wth an ncrease n the gas flow rate, but decreased under floodng rates and, among the data obtaned by both technques, there was no sgnfcant dfference. The parameters affectng the mxng tme n a fxed geometrcal condton n present study are: the

2 NOTES 345 mpellers rotatonal speed N, superfcal gas velocty v s, pont of bulk njecton tracer and conductvty probe locaton. For each experment, a pulse of electrolyte s added to the lqud bulk and the mxng tme requred to acheve 95% of the fnal concentraton of the tracer s measured by conductometer (t 95 ) 0. In ths work, two types of strred tank systems for determnng the mxng tme were used, ) the frst sets of experments were carred out n the strred tank equpped wth a sngle Rushton turbne and ) the second sets of experments were carred out n a Double-Rushton strred tank. The mxng tme obtaned from these two mxng systems has also been compared wth the prevous works. The objectve of ths work s the optmzaton of the probe locaton and the njecton pont n order to expermentally compare the lqud and gas-lqud mxng tme of these two types of systems as well as wth prevous works, and fnally, to obtan the relatonshp between the heghts of the lquds n these two systems of strred tank for measurng low mxng tmes. The optmzaton of these parameters requres many tests, whch s tme consumng and exorbtant n cost. In order to overcome ths problem, the optmzaton of these parameters has been carred out usng multvarate statstc technques. Now a days, expermental desgn methods, such as the factoral desgn, response surface methodology (RSM) and Taguch methods are wdely used for the optmzaton of response surface,. In ths work, response surface methodology (RSM) s used to fnd a reasonable approxmaton for the functonal relatonshp between a response varable and ndependent varables. Theory Response Surface Methodology For the varables measurable, the response surface can be expressed as follows: Y = f (x, x,..., x n ) () The goal s to optmze the response varable, Y (mxng tme) and x, the ndependent varables of called factors. The desgn conssts of factoral desgn ponts wth k runs, k axal or star ponts, and n replcatons at the center of the desgn,3. The correlaton of the ndependent varables and the response were estmated by a second-order polynomal Eq. and s expressed as 4 : k βx + k k Yˆ = β + β x x + β x + e = 0 j j = = j = = β 0 + x β + x βx + e () j where Yˆ s the mxng tme, β 0 s the value of ftted response at the center pont of the desgn, e the resdual (error) term, β, β and β j are the lnear, quadratc and nteracton terms respectvely, and x the dmensonless coded varables. Codfcaton of the levels of the varable Accordng to the Equaton below, t can be used to transform a real value (X ) nto a dmensonless coded value (x ) 5 : X X 0 x = =,,3,,k (3) X where x s the dmensonless coded value; X s the real value; X 0 s the real value at the center pont and X s the step change. Accordng to the Eq., the quadratc model ncludng lnear, squared and nteracton terms. The mportance of terms n the model by analyss of varance (ANOVA) are characterzed 3. Expermental Secton Instrumentaton and equpment The expermental setup s shown schematcally n Fg.. All experments were carred out n a transparent vessel wth a flat bottom, havng an nner dameter, T, of 0.3 m. The cylndrcal tank body was made of glass and the workng heghts lqud were 0.54 m (frst tank) and 0.3 m (second tank). The strred tank was ftted wth four wall mounted baffles havng a wdth of /0th and thckness /00th that of the tank dameter (fully baffled condton). In the frst system, two sx bladed Rushton turbne mpellers and n the second system, a sx-bladed Rushton turbne wth a dameter of D = T/3 were mounted on the shaft. The mpeller wdth, L, and the mpeller blade heght, W, were equal to D/4 and D/5. The off bottom clearance s C = 0.55T, and the upper mpeller s placed C = 0.7T above the lower one. An aeraton gas was ntroduced 5 cm under the bottom mpeller through a perforated plate dstrbutor havng a 7.5 cm dameter and 3 orfces. The strred tank s flled wth tap water as the man contnuous phase flud, whose surface dstances from the upper mpeller was C = 0.55T. A pump drves the k

3 346 INDIAN J. CHEM. TECHNOL., MAY 07 turbnes and the strrng speed s measured usng a calbrated dgtal osclloscope. A tachometer was used to measure the mpeller speed. A pulse of 50 ml of NaCl soluton (0 g/l) per 3 second has been njected at axal dstances n the tank wall and fve ports were used for tracer detecton. The poston of the probe and the tracer njecton pont were stuated n across to each other n the state of an axally parallel. The mxng tme was estmated for each of the probes as the tme requred to attan 95% of the fnal concentraton wthn ±5% of the average concentraton. Conductvty probe technque In exactly the same expermental condtons, conductometer probe response at dfferent measurement locatons was normalzed to the same overall concentraton change. The varance can be expressed as follows: 6 5 σ = ( C ( t) ) (4) 5 = where s the number of probe, for one probe σ = ( C ( t) ) and: C( t) C0 C( t) C ( t) = = (5) C( ) C C( ) 0 Fg. Schematc of the expermental set wth detals of the tanks. Table Expermental range and levels of the ndependent varable Impellers Superfcal Probe Tracer njecton rotatonal gas velocty locaton pont speed (rpm) (mm/s) Real Coded Real Coded Real Coded Real value value value value value value value X x X x X 3 x 3 X 4 x 4 Coded value where C (t) s the value of the normalzed response at tme t. Accordng to the Eq. 4 the obtaned mxng tme by usng varance gves overall mxng tme 6-8. The mxng tme s reached when the fnal concentraton fluctuaton crosses the lne Log (σ ) = Expermental desgn MINITAB software (a package from Mntab Inc.) statstcal package used n ths paper for calculaton and analyss of the second order polynomal coeffcents. The levels chosen for the ndependent varables, mpellers rotatonal speed (x ), superfcal gas velocty (x ), probe locaton (x 3 ) and tracer njecton pont (x 4 ) were shown n Table. Central composte rotatable desgn (CCRD) of the experments was used, where the values of

4 NOTES 347 ndependent varables were coded as the varables, x, n the range of + and - levels. Ths desgn was composed of a 4 factoral desgn (runs 6), 8 star ponts (runs 7 4) and 6 replcates (runs 5 30), thus 30 experments were needed n total. The mean value of the response (mxng tme) obtaned n 30 experments are summarzed n Table. The expermental data were analyzed usng central composte rotatable desgn to predct the mxng tme (see Table ). In order to estmate the curvature and expermental error of the model, the star ponts and the replcates were added to expermental desgn 9. Accordng to Eq. 3, the relatons between the coded and the central value were as gven below: x = ( X 400) 00, x = ( X.35) 0. 48, x = X 3) & x = X 3) (6) 3 ( 3 Results and Dscusson 4 ( 4 Estmated regresson coeffcent for response and analyss of varance (ANOVA) The coeffcents of the polynomal model (see Eq. ), T- values, P-values, and the standard error (SE) of the coeffcent were shown n Table 3, where the SE of the coeffcent s a measure of the varaton n estmatng the coeffcent. The rato of the coeffcent to the standard error can be characterzed by T-value. The mportance of the coeffcents of the polynomal model was determned by T-values and P-values, where the larger the magntude of T-value and the smaller the P-value ndcate the consderable mportance of the correspondng coeffcent 0. The results of the quadratc and lnearty model n the form of ANOVA showed a small probablty value (P < 0.05), ndcatng that the ndvdual terms n the model have a sgnfcant effect. The model equaton for the mxng tme obtaned after performng 30 experments wth a 96% confdence s as follows: Y ˆ = x x +.775x x +.565x +.x 3 4 (7) The predcted values and expermental results obtaned by the model equaton (see Eq. 7) are gven n Table 3. Optmzaton of model The second-order models nclude the crtcal ponts such as maxmum, mnmum, saddle, and rdge. The Table Parameter levels of CCRD (coded value) and the results expermental and predcted values for mxng tme. Run Block x x x 3 x 4 Mxng tme (s) order Expermental Predcted Table 3 Results of the regresson analyss of the CCRD Term Coef SE Coef T-value P-value Constant x x x x x *x x *x x 3 *x x 4 *x x *x x *x x *x x *x x *x x 3 *x statonary pont can be found by solvng matrx algebra. The ftted second-order model n matrx form s as follows : Y ˆ ˆ + = β + xb Bx (8) The dervatve of Yˆ wth respect to the elements of the vector x s

5 348 INDIAN J. CHEM. TECHNOL., MAY 07 Y ˆ = b + Bx = 0 x (9) Therefore, the soluton to the statonary pont s: x S = B b (0) ˆ β ˆ β where b = and M ˆ β q ˆ β ˆ ˆ, β, K, βq ˆ β, ˆ, ˆ β Kβ q B = M M ˆ sym., β qq As a result, the optmum response value can be calculated as: Yˆ = ˆ β + 0 xsb () S Optmum response values from the model are shown n Table 4. Accordng to the expermental desgn for the tracer njecton ponts and also the probe locaton, t s obvous that the optmum ponts are located n zone 3 as llustrated n Table 4. These zones are stuated n the opposte drecton to each other. Ths s due to the probe locaton and the tracer njecton pont, both of whch are located n the mddle of two mpellers, where the lqud velocty n ths regon s qute hgh, and the tracer can be quckly dspersed to the other regon. So by keepng these two parameters constant durng the next experments, the mpeller rotatonal speed and rate of aeraton for both confguratons was nvestgated. Comparson of mxng tmes The reasons for the dfferent measurement mxng tmes are due to dfferences n operatng condton, geometres of the mxng system, measurement technques and expermental procedures such as locaton of the njecton pont, probe locaton, vessel sze, mpeller clearance, etc. Paglant and Pntus 7 compared mxng tme usng two methods (conductvty technque and decolorzaton) and showed that the predcted mxng tme can be dfferent between these two technques. Bujalsk et al. successfully smulated the effect of the radal dstance of the tracer addton pont at lqud surface on mxng tme usng CFD wth a sldng mesh approach, and showed that the njecton pont sgnfcantly nfluenced mxng tme. Table 4 Response optmzaton of the nonlnear polynomal model Varables Optmum (Coded value) value(coded) Optmum (Real value) value(uncoded) Probe locaton Tracer njecton pont 0 3 Table 5 Comparson of the mxng tme obtaned from the expermental and smulaton values N.t 95% % Devaton Present study (sngle Rushton mpeller) 4 S.L.Yeoh et al. (CFD) H. Hartmann et al. 4 (CFD) Wang Zheng et al. 5 (CFD) 66.5 _ Sano and Usu 6 47 _ 0.64 Shue and Wong Fasano and Penney M. Lunden et al Magell et al. 3 nvestgated that the mxng tme s affected n number of dfferent geometrcal confguratons and szes of the strred vessels, mpeller types. Also, effect of probe locaton 4, locaton of njecton pont n varous dameters of the mpeller 4 and probe locaton and njecton pont at dfferent mpeller speeds 5 on the expermentally measured mxng tme has been nvestgated. In the present study the obtaned mxng tme was compared aganst smulaton results and publshed data based on expermental works for a Rushton mpeller n dfferent confguratons of a vessel (see Table 5). The best agreement (0.64%) was found wth the result n lterature 6. The maxmum devatons are 40%, 36.84% and 36.8% from the measured values of Shue and Wong 7, Fasano and Penney 8 and Zheng et al. 5 respectvely Effect of mpeller rotatonal speed Fgure shows that for both systems, mxng tme decreases because the turbulent ntensty and the lqud crculaton speed were ncreased wth an ncrease n mpeller speed. Zhang et al. 9 have made smlar observatons for the sngle mpeller systems. At hgh mpeller rotatonal speed, the varaton of mxng tme s slght. The expermental sngle phase mxng tmes obtaned for sngle and dual Rushton turbne strred tank were compared wth values from Lu 30 n Fg.. The measured mxng tmes for a sngle Rushton mpeller n Fg. showed lttle devaton wth values from Lu 30, whch s due to the hgh bottom clearance (C=T/) n ths system.

6 NOTES 349 In the turbulent regon (Re>0000) the dmensonless mxng tme, N.t m wll reman constant and the dmensonless mxng tme s ndependent of Reynolds number n turbulent flow whle other desgn and operaton factors are held fxed. Accordng to the expermental results, the dmensonless mxng tmes for sngle and dual Rushton mpeller strred tanks are 4 and 4.5 respectvely, and are approxmately equal. Lu 30 obtaned the followng correlaton by correlatng the mxng tme wth n b, B/T, Q g and N, for gas-lqud system equpped wth a sngle Rushton turbne mpeller 30. t B T g m = 55.7 nb ( ) ( ) (3) 3 A comparson between the predcted values from Eq. 3 and the expermental values of the gas-lqud mxng tme s shown n Fg. 3. It can be seen that the expermental results for Re>50000 are also n good agreement wth the correlaton data. As shown n Fgs. and 3, the Fg. Effect of mpeller rotatonal speed on mxng tme under non-aerated systems N Q D expermental results for sngle Rushton mpeller agree well wth dual Rushton mpeller strred tank. Effect of aeraton In gas-lqud strred tank besdes the axal turbulent lqud exchange and crculaton flow, nduced flow durng aeraton s found as a consequence of the densty dfference. Accordng to Eq. 4, ths flow s dependent on the gas flow rate and pumpng capacty, whch s dentfed wth gas nduced at axal flow rate 3,3. Q (4) Q = g IF K Pg 3 ND P U In gas-lqud agtated vessel, the mxng tme was affected by gas flow rate. In the prevous lterature revews, there are many contradctons to the effect of the gas flow rate on the mxng tme. An ncrease n gas flow rate can ncrease, decrease, or have no effect on the mxng tme 9. Zhang et al. 9 studed the mxng behavour n a strred tank wth a sngle Rushton turbne mpeller and found that the mxng tme ncreased when the gas flow rate ncreased at constant mpeller speed. Shewale and Pandt 33 also nvestgated the effect of gas flow rate on mxng tme; they found the mxng tme decreased when the gas flow rate ncreased. So that, the dfferent effect of aeraton on the mxng tme was found. The effects of the superfcal gas velocty on mxng tme nvestgated n ths work are shown n Fgs. 4a and 4b. The created turbulency n two phase mxng system depends on both of the mpeller speed and aeraton. Fg. 3 Effect of mpellers rotatonal speed on mxng tme under aerated systems (vs=3.4 mm/s). Fg. 4a Effect of superfcal gas velocty on mxng tme for dual mpeller system.

7 350 INDIAN J. CHEM. TECHNOL., MAY 07 Fg. 4b Effect of superfcal gas velocty on mxng tme for sngle mpeller system. Further more, for expressng the effect of gas phase on the mxng tme, t s essental to nvestgate the nteracton between gas flow rate and mpeller speed n dfferent flow regmes (floodng and loadng). From Fg. 4, t s obvous that ncreasng the superfcal gas velocty n the low rotatonal speed of mpellers ncreases pneumatcal agtaton power, consequently the mxng tme decreased. Agan by ncreasng the superfcal gas velocty (v s ) from 0 to.3 mm/s at 300 and 400 the rotatonal speed of the mpeller mxng tmes for both systems rse, whle for the rates between mm/s there s a downward trend. When N 600 rpm the value of the mxng tme leveled off wth an ncrease n the superfcal gas velocty. Further, the value of the superfcal gas velocty dd not affect the value of the mxng tme, snce the nfluence of the mpeller rotaton at hgh speed s hgher than the superfcal gas velocty. In hgher than 600 rpm, decreasng the mechancal agtated power, ncreases the mxng tme; but ncreasng the pneumatcal agtaton power decrease the mxng tme n ths state. Therefore, the mxng tme doesn t change sgnfcantly due to smultaneous nteracton of these effects. Also, at hgh superfcal gas veloctes the changes n mxng tme n all rotatonal speeds wll reman approxmately constant. Ths may be because the mpeller pumped a consderable amount of flud n the radal drecton. So, t can be sad the mpeller s flooded. As shown n Fg. 4a, the mxng tme of the gas lqud phase n dual Rushton mpeller tank was hgher than that of the sngle phase. Ths s because the mpeller pumpng capactes n gas-lqud mxng system are less than the sngle phase. By ncreasng the superfcal gas velocty from mm/s, the mxng tme ncreases because of the Fg. 5 Comparson between two mxng systems. drop n mechancal power due to reduced pumpng capacty. In fact, wthn ths range of aeraton rate, the nduced flow cannot remedy ths power loss. When v s.3 mm/s, the pneumatcal agtaton power domnate and mxng tme decreases. Fgure 5 shows the general comparson between two mxng systems. It s completely obvous that mxng tmes are approxmately equal for both systems. Consequently, wth havng mxng tmes constant n the same geometry for both systems, an optmzed lqud level for dual Rushton mpeller strred tank has been acheved n our study. To have mxng tmes fxed after addng a top mpeller to a sngle Rushton mpeller strred tank, the lqud level nto the second system should be adjusted.8 tmes ts correspondng value for the frst system. When the lqud level of the second system s not equal wth ths sze, the mxng tme and unformty mght be affected. Therefore, the correlaton of lqud level n mult-mpeller agtated tanks, wth n and T took the followng form: H D =.8( n ) n T (5) where n and H D are the number of mpellers and lqud level n dual mpeller agtated tanks respectvely. Concluson In ths paper, by Response Surface Methodology the optmum poston for njecton tracer and probe locaton n a dual-rushton strred tank has been nvestgated. Accordng to the expermental desgn for tracer njecton ponts and also probe locaton, t can

8 NOTES 35 be assumed that the optmum ponts for both parameters are opposed at the entrance of the tank (pont 3). Effects of superfcal gas velocty and mpeller rotatonal speed for both systems are nvestgated. Mxng tme decreases wth an ncrease n mpeller speed. When N 700rpm, the value of mxng tme leveled off wth the ncreasng mpeller rotatonal speed. It can be seen from the results that by ncreasng the superfcal gas velocty, the mxng tme at the low rotatonal speed (00 rpm) decreases; however, the reducton of mxng tme at the hgher rotatonal speed would be from.3 mm/s of superfcal gas velocty, whle for less than the mentoned superfcal gas velocty the reverse effect s seen. When N 600 rpm, the value of the mxng tme leveled off wth the ncreasng superfcal gas velocty, and the value of the superfcal gas velocty dd not affect the value of the mxng tme. Accordng to the expermental results, the dmensonless mxng tmes for sngle and dual Rushton mpeller strred tanks are 4 and 4.5 respectvely. Consequently, mxng tmes are equal for both systems. An effectve crculaton loop and optmum mxng tme can be acheved wth an optmzed lqud level n a dual Rushton mpeller tank that equals H D =.8T. Nomenclature N mpeller rotatonal speed [rpm] v s superfcal gas velocty [mm/s] Y response varable [-] x coded varable level [-] T tank dameter [m] D mpeller dameter [m] H D heght of the lqud n the mult-mpellers [m] tank C mpeller spacng [m] C off bottom clearance of the lower turbne [m] C(t) tracer concentraton at tme t [Kg/m 3 ] σ dmensonless varance [-] t m mxng tme [s] n b baffle number [-] B baffle wdth [m] P g power consumpton wth aeraton [W] P U power consumpton wthout aeraton [W] n number of mpeller [-] L mpeller blade wdth [m] W mpeller blade heght [m] References You S T, Abdul Raman A A, Raja Ehsan Shah R S S & Mohamad Nor M I, Rev Chem Eng, 30 (3) (04) 33. Zadghaffar R, Moghaddas J S & Revstedt J, Indan J Chem Eng, 5 (008) 3. 3 Bronarz Press L, Wozwodzk S & Ochowak M, Proceedngs of European Congress of Chemcal Engneerng (Copenhagen), Houcne I, Plasar E & Davd R, Chem Eng Technol, 3 (000) Manna L, Chem Eng J, 67(3) (997) Chomcharn N, Bologcal Pharmaceutcal Engneerng, M S Thess, Otto H York Department of Chemcal, Paglant A & Pntus S, Exp Fluds, 3 (00) Otomo N, Bujalsk W & Nenow A W, Process Inst Chem Eng Res Event, (995) Pandt A B & Josh J B, Chem Eng Sc, 38 (983) Ocheng A, Onyango M S, Chem Eng Process, 47 (008) 853. Bradley N, The response surface methodology, PhD Thess, Indana Unversty South Bend, 007. Das S R, Dhupal D & Kumar A, IJAE, 3 (03) Kwak J S, Int J Mach Tools Mfr, 45 (005) Aslan N, Fuel, 86 (007) Tanyldz M S, Özer D & Elbol M, Process Bochem, 40 (005) 9. 6 Kresta S M & Mao D, Chem Eng Sc, 6 (006) Paul E L, Atemo Obeng V A & Kresta S M, Handbook of Industral Mxng: Scence and Practce (Wley-IEEE, I Chem E, Rugby), Ruszkowsk S, Proceedngs of 8th European Mxng Conference (Cambrdge, UK), Açkaln K, Karaca F & Bolat, E, Fuel Process Technol, 87 (005) 7. 0 Thakkar A & Saraf M, Nat Prod Boprospect, 4(6) (04) 34. Yeoh S L, Papadaks G & Yannesks M, Chem Eng Sc, 60 (005) 93. Bujalsk J M, Jaworsk Z, Bujalsk W & Nenow A W, Chem Eng Res Des, 80 (00) Magell F, Montante G, Pnell D & Paglant A, Chem Eng Sc, 0 (03) 7. 4 Hartmann H, Derksen J J & Van den Akker H E A, A I Ch E J, 5 (006) Zheng W, Za-sha M A O & Xan-qan S, Chn J Process Eng, 6 (006) Sano Y & Usu H, J Chem Eng Jpn, 8 (985) Shue S J & Wong C W, Can J Chem Eng, 6 (984) Fasano J B & Penney W R, Chem Eng Prog, 87 (99) Zhang Q, Yong Y, Mao ZS, Yang C & Zhao C, Chem Eng Sc, 64 (009) Lu W M, Wu H Z & Ju M Y, Chem Eng Sc, 5 (997) Vasconcelos J M T, Alves S S, Nenow A W & Bujalsk W, Can J Chem Eng, 76 (009) Vrabel P, Van Der Lans R, Cu Y Q, Luyben K & Ch A M, Chem Eng Res Des, 77 (999) Shewale S D & Pandt A B, Chem Eng Sc, 6 (006) Lunden M, Stenberg O & Andersson B, Chem Eng Commun, 39 (995) 5.