Process for preparing acrylic acid purified by crystallization from hydroxypropionic acid and apparatus therefore

1. A process for the preparation of acrylic acid wherein the acrylic acid is made from a fluid F 1 containing a hydroxyproprionic acid wherein said fluid F 1 is obtained by a process comprising the process steps of i) preparation of hydroxypropionic acid from carbohydrates to give an aqueous phase containing hydroxypropionic acid and solids including microorganisms, and ii) separating off of the solids from said aqueous phase, wherein the process comprises the process steps:

(a1) provision of said fluid F 1 containing a hydroxypropionic acid;

wherein said fluid F 1 is aqueous, and has a composition comprising

(C1-1) from about 5 to about 20 wt. % of hydroxypropionic acid, salts thereof, or mixtures thereof,

(C1-2) from about 0.1 to about 5 wt. % of inorganic salts,

(C1-3) from about 0.1 to about 30 wt. % of organic compounds which differ from hydroxypropionic acid,

(C1-4) from 0 to about 50 wt. % of solids, and

(C1-5) from about 20 to about 90 wt. % of water,

wherein the sum of components (C1-1) to (C1-5) is 100 wt. %;

(a2) dehydration of said hydroxypropionic acid to give a fluid F 2 containing acrylic acid wherein the dehydration comprises a reactive distillation and the dehydration is carried out in a CO 2 atmosphere, or an inorganic acid, or both; and

(a3) purification of said fluid F 2 containing acrylic acid by crystallization to give a purified acrylic acid phase wherein said purified acrylic acid phase comprises acrylic acid having a purity of at least 70 wt. %.

2. The process according to claim 1 , wherein said dehydration is carried out in at least two reactors, comprising the following process steps:

i) heating of said fluid F 1 in at least one first reactor in the presence of a homogeneous or heterogeneous catalyst to give a first aqueous fluid containing acrylic acid under a pressure Π 1 ;

ii) introduction of said first aqueous fluid into a further reactor; and

iii) heating of said first aqueous fluid introduced into said further reactor in the presence of a catalyst under a pressure Π 2 to give said fluid F 2 ;

wherein said pressure Π 2 and said pressure Π 1 are not equal.

3. The process according to claim 2 , wherein said pressure Π 2 is lower than said pressure Π 1 .

4. The process according to claim 1 , wherein said crystallization is selected from the group consisting of a suspension crystallization or a layer crystallization.

5. The process according to claim 1 , further comprising cooling said fluid F 1 to a maximum to the temperature of the triple eutectic point of a composition of acrylic acid, water and hydroxypropionic acid.

6. The process according to claim 1 , wherein process step (a3) comprises obtaining a first crystal phase;

wherein said first crystal phase comprises:

i) at least about 5 wt. % of acrylic acid,

ii) at least about 40 wt. % of water, and

iii) at most about 10 wt. % of hydroxypropionic acid.

7. The process according to claim 1 , wherein the purification is carried out by an at least two-stage crystallization.

8. The process according to claim 1 , wherein process step (a3) comprises the following process steps:

(a3-1) crystallization of said fluid F 2 in a first crystallization stage to give a crystal phase K 1 and a mother liquor M 1 , wherein said crystal phase K 1 comprises:

i) from about 5 to about 60 wt. % of acrylic acid,

ii) from 39.9 to about 95 wt. % of water, and

iii) from about 0.1 to about 10 wt. % of by-products which differ from water and acrylic acid,

and wherein the sum of the amounts by weight of acrylic acid, water and by-products is 100 wt. %,

(a3-2) separating off of said crystal phase K 1 from said mother liquor M 1 ,

(a3-3) melting of said crystal phase K 1 from said first crystallization stage,

(a3-4) renewed crystallization of said melted crystal phase K 1 in a second crystallization stage to give a crystal phase K 2 and a mother liquor M 2 , wherein said crystal phase K 2 comprises:

i) from about 8 to about 35 wt. % of acrylic acid,

ii) from about 60 to about 90 wt. % of water, and

iii) from about 2 to about 5 wt. % of by-products which differ from water and acrylic acid,

wherein the sum of the amounts by weight of acrylic acid, water and by-products is 100 wt. %,

(a3-5) separating off of said crystal phase K 2 from said mother liquor M 2 ,

(a3-6) crystallization of said mother liquor M 2 in a third crystallization stage to give a crystal phase K 3 and a mother liquor M 3 , wherein said crystal phase K 3 comprises:

i) from about 25 to about 55 wt. % of acrylic acid,

ii) from 44.5 to about 70 wt. % of water, and

iii) from about 0.5 to about 5 wt. % of by-products which differ from water and acrylic acid,

wherein the sum of the amounts by weight of acrylic acid, water and by-products is 100 wt. %, and

(a3-7) separating off of said crystal phase K 3 from said mother liquor M 3 , a purified phase containing acrylic acid crystals being obtained.

9. The process according to claim 8 , wherein the mother liquor obtained in process step (a3-2) is fed back into step (a1).

10. The process according to claim 1 , wherein said purified acrylic acid phase has a purity of at least about 40% with respect to the acrylic acid.

11. The process according to claim 1 , wherein said hydroxypropionic acid in said fluid F 1 or the acrylic acid in said fluid F 2 is converted into its protonated form by the addition of acids before carrying out at least one of process steps (a2) or (a3).

12. The process according to claim 1 , wherein said hydroxypropionic acid is 2-hydroxypropionic acid.

13. The process according to claim 1 , wherein said hydroxypropionic acid is 3-hydroxypropionic acid.