Influence Of Operating Conditions To The Effectiveness Of Extractive Distillation Columns

Transcription

1 Influence Of Operatng Condtons To The Effectveness Of Extractve Dstllaton Columns N.A. Vyazmna Moscov State Unversty Of Envrnmental Engneerng, Department Of Chemcal Engneerng Ul. Staraya Basmannaya 21/4, , Moscow, Russa; e-mal: The nfluence of operatng condtons to the effectveness of extractve dstllaton process was nvestgated. The experment took place on an ndustral vacuum plant of purfcaton ethanol. It s obtaned that the concentraton of dfferent mpurtes at the selecton zones depends on the volumetrc flow rate of extractve solvent. It gves an opportunty to affect of ethanol purfcaton from the varous volatlty mpurtes. Proposed method allows to choose the extractve dstllaton regme that guarantees the necessary qualty of ethanol. The values of operatng condtons of extractve dstllaton for the best total purfcaton from the attendant mcro mpurtes are estmated. 1. Introducton One method of producton of ethyl alcohol s based on the dstllaton of alcohol-water soluton obtaned n the fermentaton. In ths process, n addton to the ncrease of concentraton of alcohol, purfcaton from mpurtes occurs whch were formed durng fermentaton and whch are harmful (e.g., methyl, propyl, and other alcohols, ethers, acds, aldehydes, etc.). Accordng to the volatlty to ethanol, these mpurtes are subdvded nto volatle, low-volatle, and ntermedate ones. It s dffcult to separate the latter class of mpurtes because they form azeotropc solutons wth ethanol. The exstng methods of dstllaton column computaton enable to calculate the concentraton of ethyl alcohol along the column heght to a good approxmaton. As a rule, these methods assume that the mxture to be separated contans only ethyl alcohol and water whle the mpurtes (whch total content s less than 1%) do not affect the separaton (Tsgankov P.S. and Tsgankov S.P., 2002). In practce, the desred purfcaton s acheved by usng several dstllaton columns n seres. The ntensty of ethanol purfcaton from every mpurty must be dfferent. It s mportant to remove the mpurtes whch worsen of ethanol qualty very strong. To separate a volatle and a part of ntermedate mpurtes, a specal dstllaton column of prelmnary purfcaton s used (Tsgankov P.S. and Tsgankov S.P., 2002). In order to make t more effcent, one uses a prncple of extractve dstllaton (Anshten, 2003; Kogan, 1971) consstng n the extractve solvent (water) feed on one of the plates n the concentraton part of the column. Ths enables to lower the concentraton of ethanol and to shft the equlbrum for some mpurtes to the transton from lqud to the vapor

2 Fg. 1. The process flowsheet of extractve dstllaton for ethanol producton: 1 prelmnary purfcaton column, 2 concentraton column, 3 condensers, 4 bolers phase. Extractve dstllaton consderably enhances the capabltes for the creatng of zones of concentraton of mpurtes along the column and ther separaton from ethanol. The process flowsheet of ndustral unts for ethanol producton s shown n Fg. 1. Unrefned ethanol alcohol (C) contanng volatle (A) and ntermedate (B) mpurtes feed on the prelmnary column 1. As rule volatle mpurtes (A) are concentrated n top part of the column and separated from ts condenser 3. The water (extractve solvent D) feed on one of the plates n the concentraton part of the column. Impurtes (A) and (B) are concentrated near the plate n the enterng water. Selecton of lqud phase from ths zone gve possblty to decrease the concentratons of the mpurtes (B) n product (C) drected to concentraton column 2. Water from column 2 s returned to column 1. It s essental that many problems connected wth purfcaton of ethanol are common for chemcal engneerng. In ths case we deal wth the problem of usng the new technque for separaton of multcomponent and polyazeotropc mxtures. 2. Condtons Of The Experments The effect of the parameters on the purfcaton degree was studed on a commercal dstllaton column of the ethanol producton plant by measurng amounts of mpurtes n the products of dstllaton usng hghly effectve gas chromatograph. The dameter of the column s 1200 mm. It has 39 bubblecap plants. The column works n contnuous runnng and has the stablzng systems for pressure n barometrcal condenser and for pressure of secondary vapor. The reflux rato for concentraton part of the column s 50. Unrefned water-ethanol mxture feed on the plate number 23. An overhead dstllate contaned volatle mpurtes are separated from a condenser. The product are drected to a concentraton column for contnue of a rectfcaton. The process of extractve dstllaton conssts of feedng of the water to plate number 32 and n selecton the ethanol mxture together wth mpurtes from plate number 33 (sde cut dstllate). The unrefned water-ethanol mxture s characterzed by volumetrc flow rate Q f, ethanol concentraton n the feed x f, and concentratons of mpurtes α f. Here subscrpt ndcates on specfc mpurty. The water (extractve solvent) s characterzed by volumetrc flow rate Q h. The overhead and sde cut dstllates are characterzed by

3 volumetrc flow rates Q d and Q p, ethanol concentratons x d and x p, and mpurtes concentratons α d and α p. A qualtatve and quanttatve allowance of mpurtes n these flows depends on effectveness of the extractve dstllery process. Specfcty of the column operaton does not allow drect measure a volumetrc flow rate of the product produced on the column Q w. It may be calculate from equaton of materal balance of column Q w =Q f Q h -Q d -Q p. (1) The mpurtes concentratons n product α w can not be measured because the sensblty of the gas chromatographc method s nsuffcent. One can calculate ts values from the equatons of materal balance of column for every mpurty α α w ( α α ) ( α α ) x ( Q Q Q Q ) x Q x Q x Q f f d f d d p f p p = f w f h d p (2) Durng of experments the technologcal parameters of column are retaned as a constant. In ths tme output of rectfcaton plant s nvarable. The orgnal gas chromatographc method of determnaton of quanttatve mpurtes allowance n ethanol mxes was used (Vyazmna and Savchuk. 2002). Selecton and determnaton of the mpurtes concentratons was carred out by usng gas chromatograph wth the flame-onzaton detector and the capllary column 50m 0.32 mm havng motonless phase HP-FFAP. 3. Influence Of Volumetrc Flow Rate Of Extractve Solvent On The Column Effcency The results of expermental determnaton of the relatve concentratons of some mpurtes presented n unpurfed ethanol (n convertng on absolute alcohol) n the overhead and sde cut dstllates on the volumetrc flow rate of the extractve solvent are shown n Fg. 2. For subsequent calculatons, t s convenent to express the expermental data from Fg. 2 n the form of emprcal formulas. Ths may be performed by ther nterpolaton wth polynomals wthn the range under study. Because all dependences are monotonous, let us use the second-order polynomals for the nterpolaton Y =A X 2 B XC, (3) where Y are ether α d /α f or α p /α f, X s the value of Q h ; A, B, and C are emprc constants whch are defned both by the partcular mpurty n the overhead and sde cut dstllates. For the nterpolaton of expermental data by polynomals (3) t s necessary to obtan the values of emprcal constants. The technque of least squares was used for ths purpose. In each case, the problem of calculaton of the coeffcents reduces to the soluton of the set of ordnary lnear equatons

4 n n n n = j j j= 1 j= 1 j= 1 j= 1 A X B X C X Y X n n n n 3 2 j j j = j j j= 1 j= 1 j= 1 j= 1 (4) A X B X C X Y X n n n 2 = j j= 1 j= 1 j= 1 A X B X Cn Y where n s the number of ponts used for the nterpolaton. If the calculaton results have expermental ponts, devatng from nterpolaton date more than for ±5%, these ponts were dscarded, and the calculatons were repeated. The results of fttng the expermental data shown n Fg. 2 are resumed n Table. 1. By usng expresson (3) wth the coeffcents gven n Table 1 and relatonshp (2) one s able to calculate the correspondng dependence of the relatve concentraton of each mpurty n the product, α w /α f, on the volumetrc flow rate of the extractve solvent feed, Q h. The correspondng plots are shown n Fg. 3 for typcal for a real mode of operaton of the prelmnary purfcaton column parameters: Q f = m 3 /s, Q d = m 3 /s; Q p = m 3 /s; x f =x w =0,33; x d =0,95; x p =0,88. Fg. 2. Dependence of the relatve concentratons of mpurtes n the overhead dstllate α d /α f (left) and n sde cut dstllate α p /α f (rght) on the volumetrc flow rate of extractve solvent (water) used for the extractve dstllaton, Q h. Impurtes: 1 ethyl acetate, 2 - acetaldehyde, 3 methanol, 4 - -amyl acetate, 5 - ethyl octanoate. No ethyl octanoate was found n the overhead dstllate. Table 1. The values of emprc constants A, B, and C for varous mpurtes Impurty Overhead dstllate Sde cut dstllate A d B d C d A p B p C p 1. Ethyl acetate Acetaldehyde Methanol I-amyl acetate Ethyl octanoate

5 The values of process parameter Q h, for each mpurty should be chosen n such a way that ts α w /α f s mnmum value. However, n the case of multcomponent mxture the concentraton of -th mpurty s mnmum value for ts own Q h. As can be seen from Fg. 3, at ncreasng Q h, concentratons of some mpurtes decrease whereas they ncreases for other mpurtes. Thus, the choce of process parameter Q h s not smple. Let us consder a column as a sngle apparatus. As a parameter characterzng effcency of purfcaton of a partcular substance t s natural to consder the rato of total mpurty content n the product to the total mpurty content n the ntal mxture m m ( α α ) w f = 1 = 1, (5) I = k m k where m s the number of mpurtes, whch are removed n ths column and k = α α. In the example under study m = 5. The coeffcent k f f1 shows how much the amount of -th mpurty n the ntal mxture s hgher than the concentraton of one of the frst. Values of coeffcents k f used n the experments are gven n Table 2. Ths coeffcent s a normalzng factor whch shows the effect of purfcaton from -th mpurty on the effcency column. Fg. 3. Dependence of relatve concentratons of mpurtes n the product, α w /α f, on the volumetrc flow rate of extractve solvent, Q h. For notaton see Fg. 2. Table 2. The values of k and γ for varous mpurtes Impurty k γ 1. Ethyl acetate Acetaldehyde Methanol I-amyl acetate Ethyl octanoate

6 Fg. 4. Dependence of I on Q h. Curve 1: for a sngle column, 2: for the whole unt. The results of calculaton of the I dependence on Q h accordng to the data gven n Tables 1 and 2, are presented by curve 1 n Fg. 4. As follows, n a separately taken column a slght decrease of the total (wth respect to all mpurtes) degree of purfcaton s observed f the volumetrc flow rate of extractve solvent ncreases. The purfcaton of ethanol s performed n multcolumn dstllaton unts, when there s no need to reach ts maxmum purfcaton n the frst column. One should selectvely address the problem of purfcaton from each mpurty, by takng account ts effect on the qualtatve characterstcs of the fnshed product and the possblty to remove t n the next columns. In ths case, nstead k of one should use n (5) a coeffcent k% = γ k, where γ s the weghted factor requred for the -th mpurty, ths factor varyng from 0 to 1. The valuesγ, obtaned by expert evaluaton, are gven n Table 2. The results of calculaton of the dependence I on Q h wth the account taken on the correcton for the selectvty of purfcaton are shown by curve 2 n Fg. 4. It follows that by choosng the column operatng mode wth takng account the possblty of further purfcaton, one may acheve a sgnfcant enhancement of the total degree of extracton wth respect to all mpurtes by smply ncreasng the volumetrc flow rate of extractve solvent. The best result s obtaned when the volumetrc flow rate s about 10% from the volumetrc flow rate of aqueous-alcoholc mxture gven as column feed. The further ncrease only slghtly affects the effcency of column. References Anshten V.G., 2003, General Course on Processes and Apparatus of Chemcal Engneerng. Logos, Moscow (n Russan). Kogan V.V., 1971, Azeotropc and Extractve Dstllaton. Khmya, Lenngrad. Tsgankov P.S. and Tsgankov S.P., 2002, Ethanol Dstllaton Gude. Pschepromzdat, Moscow (n Russan). Vyazmna N.A. and Savchuk S.A., 2002, Applcaton of gas chromatography to dentfcaton of the orgnal of alcohols, J. Anal. Chem. 57,