Part-stream distillation

Abstract

A continuous process for the preparation of propylene oxide, comprising (a) reacting propene, optionally admixed with propane, with hydrogen peroxide in a reaction apparatus in the presence of acetonitrile as solvent, obtaining a stream S0 containing propylene oxide, acetonitrile, water, at least one further component B, optionally propene and optionally propane, wherein the normal boiling point of the at least one component B is higher than the normal boiling point of acetonitrile and wherein the decadic logarithm of the octanol-water partition coefficient (log K ow ) of the at least one component B is greater than zero; (b) separating propylene oxide from S0, obtaining a stream S1 containing acetonitrile, water and the at least one further component B; (c) dividing S1 into two streams S2 and S3; (d) subjecting S3 to a vapor-liquid fractionation in a fractionation unit, obtaining a vapor fraction stream S4 being depleted of the at least one component B; (e) recycling at least a portion of S4, optionally after work-up, to (a).

1 . A continuous process for preparation of propylene oxide, the process comprising
 (a) reacting propene with hydrogen peroxide in a reaction apparatus in the presence of acetonitrile as solvent, obtaining a stream S0 leaving the reaction apparatus, S0 comprising propylene oxide, acetonitrile, water, at least one further component B, wherein a normal boiling point of the at least one component B is higher than a normal boiling point of acetonitrile and wherein a decadic logarithm of an octanol-water partition coefficient represented by log K ow  of the at least one component B is greater than zero;   (b) separating propylene oxide from S0, obtaining a stream S1 comprising acetonitrile, water and the at least one further component B;   (c) dividing S1 into two streams S2 and S3, wherein a total weight of S3 relative to a total weight of S1 is in a range of from 0.01 to 25%;   (d) subjecting S3 to a vapor-liquid fractionation in a fractionation unit, obtaining a vapor fraction stream S4 being depleted of the at least one component B, and obtaining a liquid bottoms stream S4b being depleted of acetonitrile; and   (e) recycling at least a portion of S4.   
     
     
         2 . The process of  claim 1 , wherein in (c), the total weight of S3 relative to the total weight of S1 is in a range of from 0.05 to 20%. 
     
     
         3 . The process of  claim 1 , wherein from 90 to 99.9 weight % of S1 consist of acetonitrile and water and wherein from 0.01 to 5 weight-% of S1 consist of the at least one component B. 
     
     
         4 . The process of  claim 1 , wherein
 in (d), vapor-liquid fractionation is carried out in the fractionation unit so that from 10 to 30 weight % of the liquid bottoms stream S4b consist of acetonitrile and from 0.1 to 10 weight % of the liquid bottoms stream S4b consist of the at least one further component B.   
     
     
         5 . The process of  claim 1 , wherein in (d), vapor-liquid fractionation is carried out in the fractionation unit at an absolute pressure in a range of from 0.1 to 10 bar. 
     
     
         6 . The process of  claim 1 , wherein in (d), a number of theoretical trays of the fractionation unit is in a range of from 1 to 100. 
     
     
         7 . The process of  claim 1 , wherein a fraction of S4 is used after condensation as reflux. 
     
     
         8 . The process of  claim 1 , wherein the fractionation unit is operated without reflux and S3 is fed to a top of the fractionation unit. 
     
     
         9 . The process of  claim 1 , wherein from 95 to 99.99 weight % of S4 consist of acetonitrile and water, and wherein from 0.0001 to 0.2 weight % of S4 consist of the at least one component B. 
     
     
         10 . The process of  claim 1 , wherein (e) comprises recycling at least a portion of S4 to (a), and recycling at least a portion of S2 to (a). 
     
     
         11 . The process of  claim 1 ,
 wherein (e) comprises working-up S4, said working-up comprising combining at least a portion of S4 with S2 obtaining a liquid stream, subjecting said liquid stream to acetonitrile-water separation obtaining a stream enriched in acetonitrile, and recycling said stream enriched in acetonitrile to (a).   
     
     
         12 . The process of  claim 11 , wherein (e) comprises
 (i) preparing a liquid stream S5 by adding a liquid stream P to S2, or to at least a portion of S4, or to the liquid stream obtained from combining S2 and at least the portion of S4,   wherein P comprises at least 95 weight-% of C3, based on a total weight of P,   wherein C3 is propene;   (ii) subjecting S5 to a temperature of 92° C. at most and a pressure of at least 10 bar, obtaining a first liquid phase L1 and a second liquid phase L2,   wherein at least 95 weight % of L1 consist of C3, acetonitrile, water and the at least one component B, the water content of L1 being less than 10 weight % based on a total weight of L1, and   wherein at least 95 weight % of L2 consist of C3, acetonitrile, water and the at least one component B, a C3 content of L2 being 5 weight-% at most, based on a total weight of L2, and an acetonitrile content of L2 being less than 45 weight-%, based on the total weight of L2;   (iii) separating L1 from L2; and   (iv) recycling L1 as the stream enriched in acetonitrile to (a).   
     
     
         13 . The process of  claim 12 , further comprising working up L1, said working-up comprising subjecting L1 to a distillation stage wherefrom a bottoms stream BL1 is obtained, wherein at least 95 weight % of BL1 consist of C3, acetonitrile, water and the at least one component B, wherein a C3 content of BL1 is in a range of from 7 to 18 weight-%, and recycling BL1 as the stream enriched in acetonitrile to (a). 
     
     
         14 . The process of  claim 13 , wherein from 0.01 to 5 weight % of BL1 consist of the at least one component B. 
     
     
         15 . The process of  claim 1 , wherein (b) comprises
 (I) separating propene from S0, obtaining a stream S01 enriched in propylene oxide, acetonitrile, water, and the at least one component B; and   (II) separating propylene oxide from S01, obtaining a stream S02 enriched in acetonitrile, water and the at least one component B and wherein a weight ratio of acetonitrile relative to water is greater than 1:1.   
     
     
         16 . The process of  claim 15 , wherein (b) further comprises
 (IIIa) subjecting S02 obtained from (II) to hydrogenation to obtain a hydrogenated stream; and/or   (IIIb) subjecting the stream obtained from (II) or (IIIa) to distillation to obtain a bottoms stream,   wherein the hydrogenated stream obtained from (IIIa) or the bottoms stream obtained from (IIIb) is subjected to (c) as S1.   
     
     
         17 . The process of  claim 1 , wherein in (a), propene is reacted with hydrogen peroxide in the presence of a heterogeneous catalyst. 
     
     
         18 . The process of  claim 1 , wherein from 90 to 97 weight % of S0 consist of acetonitrile, water, and propylene oxide, and wherein from 0.01 to 3 weight % of S0 consist of the at least one component B. 
     
     
         19 . The process of  claim 1 , wherein the at least one component B is propionitrile, 1-nitropropane, 2-nitropropane, 3-methylbutanenitrile, n-pentanenitrile, 1-pentanol, 2-pentanol, 2-butanone, 2-pentanone, 2-hexanone, 4-methyl-2-heptanone, 2,6-dimethyl-4-heptanol, 4,6-dimethyl-2-heptanol, 2,6-dimethyl-4-heptanone, 4,6-dimethyl-2-heptanone, 2,6-dimethyl-4,6-heptandiol, 2,4-dimethyloxazoline, 2,5-dimethyloxazoline, cis-2,4-dimethyl-1,3-dioxolane, trans-2,4-dimethyl-1,3-dioxolane, at least one impurity contained in the hydrogen peroxide employed in (a), or a combination of two or more of these compounds. 
     
     
         20 . The process of  claim 19 , wherein the at least one impurity contained in the hydrogen peroxide employed in (a) is an alkyl phosphate, a nonyl alcohol, an alkylcyclohexanol ester, an N,N-dialkyl carbonamide, an N-alkyl-N-aryl carbonamide, an N,N-dialkyl carbamate, a tetraalkyl urea, a cycloalkyl urea, a phenylalkyl urea, an N-alkyl-2-pyrrolidone, an N-alkyl caprolactam, or a combination of two or more of these compounds. 
     
     
         21 . A composition, comprising at least 99.999 weight-% of propylene oxide obtained by the process according to  claim 15 .