Process for preparing acrylic acid from formaldehyde and acetic acid

Abstract

The invention relates to a process for preparing acrylic acid from formaldehyde and acetic acid, comprising (i) providing a gaseous stream S1 comprising formaldehyde, acetic acid and acrylic acid, where the molar ratio of acrylic acid to the sum total of formaldehyde and acetic acid in stream S1 is in the range from 0.005:1 to 0.3:1; (ii) contacting stream S1 with an aldol condensation catalyst in a reaction zone to obtain a gaseous stream S2 comprising acrylic acid.

Claims

1. A process for preparing acrylic acid from formaldehyde and acetic acid, comprising (i) providing a gaseous stream S1 comprising formaldehyde, acetic acid and acrylic acid, where a molar ratio of acrylic acid to a sum total of formaldehyde and acetic acid in stream S1 is in a range from 0.005:1 to 0.3:1; (ii) contacting stream S1 with an aldol condensation catalyst in a reaction zone to obtain a gaseous stream S2 comprising acrylic acid.

2. The process according to claim 1, wherein the molar ratio of acrylic acid to the sum total of formaldehyde and acetic acid in stream S1 in (i) is in the range from 0.02:1 to 0.1:1.

3. The process according to claim 1, wherein a molar ratio of acetic acid:formaldehyde in stream S1 in (i) is in a range from 0.25:1 to 4.4:1.

4. The process according to claim 1, wherein at least 65% by volume of stream S1 in (i) consists of formaldehyde, acetic acid, acrylic acid, water and inert gas.

5. The process according to claim 1, further comprising (iii) partly condensing stream S2 obtained in (ii) by cooling it down to a temperature in the range from 0 to 200 C., with separation of stream S2 into a condensed stream S2a and an uncondensed stream S2b, with optional intermediate storage of stream S2a in a buffer vessel.

6. The process according to claim 5, wherein stream S2b is at least partly recycled into the reaction zone in (ii).

7. The process according to claim 5, wherein the acrylic acid content of stream S2b is in a range from 0.01% to 0.5% by volume, based on a total volume of stream S2b.

8. The process according to claim 5, further comprising (iv) working up stream S2a to obtain a product stream SP comprising acrylic acid and a recycling stream SR comprising acrylic acid, where the recycling stream SR comprises not more than 10% of the acrylic acid present in stream S2.

9. The process according to claim 8, wherein at least a portion of the recycling stream SR is recycled into the reaction zone in (ii).

10. The process according to claim 1, wherein stream S1 comprises a stream comprising formaldehyde and acetic acid, of the recycling stream SR and optionally of stream S2b.

11. The process according to claim 8, wherein the workup in (iv) comprises (iv.1) removing a portion of the acrylic acid present in stream S2a from stream S2a to obtain a stream S3 depleted of acrylic acid relative to stream S2a, and a stream S4 enriched in acrylic acid relative to stream S2a, comprising acrylic acid and acetic acid; (iv.2) removing a portion of the acrylic acid present in stream S4 from stream S4 to obtain a stream S5 depleted of acrylic acid relative to stream S4, comprising acrylic acid and acetic acid, and a stream S6 enriched in acrylic acid relative to stream S4, comprising acrylic acid.

12. The process according to claim 11, wherein the acrylic acid content of stream S3 is in a range from 0.01% to 5% by weight, based on a total weight of stream S3.

13. The process according to claim 11, wherein the acrylic acid content of stream S5 is in a range from 0.1% to 30% by weight, based on a total weight of stream S5.

14. The process according to claim 11, wherein stream S5, at least in part, is at least part of the recycling stream SR which is recycled into the reaction zone in (ii).

15. The process according to claim 11, wherein stream S3, at least in part, is at least part of the recycling stream SR which is recycled into the reaction zone in (ii).

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows, in schematic form, a flow diagram of the process of the invention, i.e. including the experimental set up according to example 2, with a reaction unit comprising an aldol condensation catalyst and streams S1 to S8. As well as the recycling stream SR (not shown), the recycling stream S2b (S2b_rec) is preferably present. The recycling stream SR is preferably composed of stream S3, and any further streams S5 and/or S8. In the case of simultaneous use of S5 and S8, they can, as shown, be recycled via a common conduit; an alternative option is recycling via two separate conduits (not shown).

(2) FIG. 2 shows a plot of the acrylic acid yield based on the formaldehyde conversion in % (ordinate, from 0% to 35%) versus the molar ratio of acrylic acid to the sum total of formaldehyde and acetic acid (reactants) in stream S1 (abscissa, from 0 to 0.12 vol/vol) for experiment 1 with the results from tables 1-4.

(3) FIG. 3 shows a plot of the acrylic acid yield based on the acetic acid conversion in % (ordinate) versus the molar ratio of acrylic acid to the sum total of formaldehyde and acetic acid (reactants) in stream S1 (abscissa, from 0 to 0.12 vol/vol) for experiment 1 with the results from tables 1-4.

(4) FIG. 4 shows a plot of the relative preparation costs for acrylic acid (ordinate, from 100% to 110%) versus the molar ratio of acrylic acid to the sum total of formaldehyde and acetic acid in stream S1 (abscissa, 0 to 0.3 vol/vol).

(5) The present invention is illustrated in detail by the examples which follow.

EXAMPLES

(6) I. Analysis

(7) I.1 Gas Chromatography

(8) For gas chromatography, an instrument of the Agilent 7890 type with an FFAP column was used. The temperature program was as follows: hold at 40 C. for 10 min; heat to 90 C. at a heating rate of 2 K/min; heat to 200 C. at a heating rate of 6 K/min; heat to 250 C. at a heating rate of 25 K/min; hold at 250 C. for 10 min.
I.2 X-Ray Diffractometry (XRD)

(9) X-ray diffractograms (Cu K alpha radiation) were recorded on a D8 Advance series 2 diffractometer from Bruker AXS. The diffractometer was equipped with a divergence aperture opening of 0.1 and a Lynxeye detector. On the abscissa is plotted the angle (2 theta), and on the ordinate the signal intensity (Lin (counts)).

(10) I.3 BET Measurements

(11) The specific BET surface areas were determined by means of nitrogen adsorption at 77 K to DIN 66131.

(12) II. Preparation of the Catalysts

(13) II.1 Catalyst 1

(14) Oxidic catalyst comprising vanadium and phosphorus on silica support

(15) The catalyst was applied to a silica support by means of a two-stage incipient wetness impregnation. A vanadium oxalate solution was brought to a volume of 900 mL by adding 0.9 M oxalic acid to 1.1 mol of solid V.sub.2O.sub.5. The suspension was stirred and heated to 80 C. Solid oxalic acid dihydrate was added stepwise to the suspension until the color changed from orange to green to deep blue. The resulting solution was diluted to a total volume of one liter with 0.9 M oxalic acid. The final solution was 2.2 M with respect to vanadium (V).

(16) 41.71 mL of this vanadium oxalate solution were diluted to a volume of 42 mL with deionized water, corresponding to 100% of the liquid absorption capacity of the support. 50 g of silica (Cariact Q20-C, 1-1.6 mm gap) were impregnated with the vanadium solution. The resulting solid material was dried in a drying oven at 80 C. overnight. In a second step, 21.02 g of 85% phosphoric acid were diluted to 42 mL with deionized water and impregnated onto the solid material. The resulting solid material was dried in a drying oven at 80 C. overnight. The resulting solid material was calcined in accordance with the following temperature profile: i) heating from room temperature to 260 C. at a rate of 1 C. per minute; ii) heating at 260 C. for 2 hours.
II.2 Catalyst 2
Oxidic Catalyst Comprising Phosphorus and Tin on Beta-Zeolite Support
II.2.1 Preparation of a Boron-Containing Zeolitic Material Having a BEA Base Skeleton Structure

(17) 209 kg of deionized water were provided in a vessel. While stirring at 120 rpm (revolutions per minute), 355 kg of tetraethylammonium hydroxide were added and the suspension was stirred at room temperature for 10 minutes. Subsequently, 61 kg of boric acid were suspended in this water and the suspension was stirred at room temperature for a further 30 minutes. Subsequently, 555 kg of Ludox AS-40 were added and the resulting mixture was stirred at 70 rpm at room temperature for a further hour. The liquid gel had a pH of 11.8, as measured with a pH electrode. The final mixture obtained was transferred into a crystallization vessel and heated to 160 C., at a pressure of 7.2 bar, while stirring (140 rpm) within 6 h. Subsequently, 61 kg of boric acid were suspended in water and the suspension was stirred at room temperature for a further 30 minutes. Subsequently, 61 kg of boric acid were suspended in water and the suspension was stirred at room temperature for a further 30 minutes. Then the mixture was cooled to room temperature. The mixture was heated again to 160 C. within 6 h and stirred at 140 rpm for a further 55 h. The mixture was cooled down to room temperature and then heated to a temperature of 160 C. while stirring at 140 rpm for a further 45 h. 7800 kg of deionized water were added to 38 kg of this suspension. The suspension was stirred at 70 rpm, and 100 kg of a 10% by weight aqueous HNO.sub.3 solution were added. The boron-containing zeolite material having a BEA skeleton structure was separated from this suspension by filtration. The filtercake was washed with deionized water at room temperature until the wash water had a conductivity of less than 150 microsiemens/cm. The filtercake thus obtained was dried in a nitrogen stream.

(18) The zeolitic material thus obtained was subjected to a spray drying operation in a spray tower with the following spray drying conditions: Drying gas, nozzle gas: technical grade nitrogen Drying gas temperature: spray tower temperature (inside): 235 C. spray tower temperature (outside): 140 C. Nozzle: Top component nozzle supplied by Gerig; size 0 Nozzle gas temperature: room temperature Nozzle gas pressure: 1 bar Mode of operation: nitrogen direct Apparatus used: spray tower with a nozzle Configuration: spray tower-filter-scrubber Gas flow rate: 1500 kg/h Filter material: Nomex needle-felt 20 m.sup.2 Metering via flexible peristaltic pump: SP VF 15 (supplier: Verder)

(19) The spray tower comprised a vertical cylinder having a length of 2650 mm and a diameter of 1200 mm, with conical narrowing of the cylinder at the base. The length of the cone was 600 mm. At the top of the cylinder were disposed the atomization devices (a two-phase nozzle). The spray-dried material was separated from the drying gas in a filter downstream of the spray tower, and the drying gas was conducted through a scrubber. The suspension was conducted through the inner orifice of the nozzle, and the nozzle gas was conducted through the annular slot that surrounded the orifice.

(20) The spray-dried material was then calcined at 500 C. for 5 h. The calcined material had a molar B.sub.2O.sub.3:SiO.sub.2 ratio of 0.045, a total organic carbon (TOC) content of 0.08% by weight, a crystallinity determined by XRD of 100%, and a specific BET surface area determined to DIN 66131 of 498 m.sup.2/g.

(21) II.2.2 DeboronationFormation of Vacant Tetrahedral Sites

(22) 840 kg of deionized water were provided in a vessel provided with a reflux condenser. While stirring at 40 rpm, 28 kg of the spray-dried and calcined zeolitic material were added as described above in II.2.1. Subsequently, the vessel was closed and the reflux condenser was put into operation. The stirring rate was increased to 70 rpm. While stirring at 70 rpm, the contents of the vessel were heated to 100 C. within one hour and kept at this temperature for 20 h. Then the contents of the vessel were cooled to a temperature below 50 C.

(23) The resulting deboronated zeolitic material having a BEA skeleton structure was separated from the suspension by filtration under a nitrogen pressure of 2.5 bar and washed with deionized water four times at room temperature. After filtration, the filtercake was dried in a nitrogen stream for 6 h.

(24) The resulting deboronated zeolitic material, after resuspension in deionized water, was spray-dried under the conditions mentioned above in II.2.1. The solid content of aqueous suspension was 15% by weight, based on the total weight of the suspension. The zeolitic material obtained had a molar B.sub.2O.sub.3:SiO.sub.2 ratio of less than 0.002, a crystallinity determined by XRD of 77%, and a specific BET surface area determined to DIN 66131 of 489 m.sup.2/g.

(25) II.2.3 Synthesis of an Sn Beta-Zeolite

(26) 200 g of the deboronated zeolitic material having a BEA skeleton structure according to II.2.2 were combined in a mill (mill type: Microton MB550) with 56.8 g of tin(II) acetate (Sn(OAc).sub.2 [CAS no.: 638-39-1]), and the mixture was ground at 14 000 rpm (revolutions per minute) for 15 minutes. After the grinding, the mixture was transferred to a porcelain basket and calcined under air at 500 C. at a heating rate of 2 K/min for 3 h.

(27) The powder material obtained had an Sn content of 14.4% by weight, a silicon (Si) content of 38% by weight and a TOC of less than 0.1% by weight.

(28) II.2.4 Production of a Tin-Containing Material Having BEA Skeleton Structure with Acid Treatment

(29) 200 g of zeolitic material obtained according to II.2.3 were provided in a round-bottom flask, and 6000 g of 30% by weight aqueous HNO.sub.3 solution having a pH in the range from 0 to 1 were added. The mixture was stirred at a temperature of 100 C. for a time span of 20 h (200 rpm). The suspension was filtered and the filtercake was then washed with deionized water at room temperature until the wash water had a pH of about 7.

(30) The zeolitic material obtained was dried at 120 C. for 10 h and calcined by means of heating to 550 C. (2 K/min) and then heating at 550 C. for 10 h. The dried and calcined zeolitic material had an Si content of 36% by weight and an Sn content of 14.0% by weight. In addition, the zeolitic material had a specific BET surface area determined to DIN 66131 of 402 m.sup.2/g.

(31) II.2.5 Preparation of a P-Treated Sn-Containing Material Having a BEA Skeleton Structure

(32) 191 g of the zeolitic material obtained according to II.2.4 were mixed with 23.88 g of ammonium dihydrogenphosphate (NH.sub.4H.sub.2PO.sub.4). 149.6 g of deionized water were added and mixed carefully. The suspension was dried in a vacuum oven at 110 C. for 12 h. The dried material was calcined in an oven heated to 500 C. with a temperature ramp of 2 K/min under air for 5 h. Subsequently, the dried and calcined material was cooled to room temperature. 214 g of Sn-containing material having a BEA skeleton structure were obtained.

(33) The Sn-containing zeolitic material having BEA skeleton structure had the following composition: 12.7% by weight of Sn, 32% by weight of Si, <0.1% by weight of C (TOC), 2.8% by weight of P. The BET surface area was determined to be 267 m.sup.2/g in accordance with DIN 66131.

(34) II.2.6 Forming of the P-Treated Sn-Containing Material with BEA Skeleton Structure

(35) A kneader was charged with 200 g of the zeolitic material obtained according to II.2.5 and mixed with an acidic solution prepared from 6 g of HNO.sub.3 (65% by weight) dissolved in 20 mL of distilled water. The suspension was mixed (kneaded) for 10 min. Added to the resulting mixture were 10 g of Walocel and 26.3 g of Ludox AS-40, and the mixture was mixed for a further 30 min. Finally, 120 mL of distilled water were added to the mixture and mixed for a further 20 min. The paste was then extruded in a Loomis extruder. Extrudates of 2.0 mm were obtained in a static oven and dried at 120 C. for 5 h, followed by calcination at 500 C. for 5 h under air at a heating rate of 2 K/min. The resulting extrudates were divided into a fraction of 1.0-1.6 mm.

(36) The calcined extrudates had a bulk density of 490 g/L with a mechanical strength of 3 N. The elemental composition was Sn 12.7% by weight, Si 34% by weight and TOC<0.1% by weight and P 2.8% by weight.

(37) II.3 Catalyst 3

(38) Oxidic Catalyst Comprising Vanadium, Tungsten, Phosphorus and Bismuth on Silica Support

(39) 67 g of bismuth acetate were added to an aqueous citric acid solution (100 g of acid in 1 liter of deionized water). The mixture was heated to 80 C. and stirred for 30 minutes. 117.5 g of phosphoric acid (85%), 116 g of a colloidal silica suspension (Ludox AS 40) and 100 g of ethylene glycol were added successively. The mixture was stirred at 80 C. for a further 30 minutes. 110 g of ammonium metavanadate and 169 g of ammonium metatungstate were added successively. 20 g of acetyl cellulose were slurried with deionized water and added to the mixture. The final mixture was stirred at 80 C. for three hours. The mixture was concentrated in a rotary evaporator at 60 C. and 45 mbar. The resulting solid material was dried further in a drying oven at 100 C. for 16 h.

(40) The resulting solid material was calcined in accordance with the following temperature profile: i) heating from room temperature to 160 C. at a rate of 10 C. per minute; ii) heating at 160 C. for 2 hours; iii) heating from 160 C. to 250 C. at a rate of 3 C. per minute; iv) heating at 250 C. for 2 hours; v) heating from 250 C. to 300 C. at a rate of 3 C. per minute; vi) heating at 300 C. for 6 hours; vii) heating from 300 C. to 450 C. at a rate of 3 C. per minute; viii) heating at 450 C. for 6 hours.
II.4 Catalyst 4
Oxidic Catalyst Comprising Vanadium, Tungsten, Phosphorus and Bismuth on Silica Support

(41) 167.5 g of ammonium metavanadate were added to 3 liters of a 20% by weight aqueous solution of citric acid. The mixture was heated to 50 C. and stirred until dissolution was complete. 116 g of a colloidal silica suspension (Ludox AS 40) were added, followed by 227.8 g of ethylene glycol. The mixture was heated to 80 C. and stirred for 30 minutes. 35.3 g of ammonium metatungstate were dissolved in 500 mL of deionized water and added dropwise to the mixture. The mixture was then stirred at 80 C. for 15 minutes. 347.2 g of bismuth nitrate hexahydrate were dissolved in 480 mL of a 10% nitric acid solution. The acidic bismuth solution was added dropwise to the previous mixture and stirred at 80 C. for 30 minutes, then cooled down to 30 C. while stirring constantly. 1232 mL of a 2% solution of methyl cellulose were added and then the mixture was stirred for a further 30 minutes. Finally, 303.7 g of an 85% phosphoric acid solution were added and the mixture was stirred for 30 minutes. The resulting mixture was dried at 80 C. in a drying oven for 48 h.

(42) For safety reasons, the resulting solid material was calcined in an atmosphere having 3% by volume of O.sub.2/97% by volume of N.sub.2 in accordance with the following temperature profile: i) heating from room temperature to 160 C. at a rate of 10 C. per minute; ii) heating at 160 C. for 2 hours; iii) heating from 160 C. to 250 C. at a rate of 3 C. per minute; iv) heating at 250 C. for 2 hours; v) heating from 250 C. to 300 C. at a rate of 3 C. per minute; vi) heating at 300 C. for 6 hours; vii) heating from 300 C. to 450 C. at a rate of 3 C. per minute; viii) heating at 450 C. for 6 hours.
III. Setup and Operation of the Pilot Plant
III.1 Example 1: Determination of the Maximum Amount of Acrylic Acid in Stream S1

(43) The apparatus consisted of a fixed bed reactor (bed length about 90 cm, diameter 16 mm, 1.4541 stainless steel) heated in four zones and having 3 sampling points for online GC measurements (inlet, middle, outlet) and two reactant metering zones. In order to charge the plant with formaldehyde and acetic acid, the reservoir vessel was initially charged with acetic acid or acetic acid solution and formaldehyde or formalin solution.

(44) Formalin (49% by weight of formaldehyde in water) was conveyed by means of a Fink HPLC pump and evaporated completely by means of a microevaporator (passage length 60 mm, passage width 0.2 mm, alloy 22, 2.4602) (wall temperature about 280 C.). In order to prevent paraformaldehyde from precipitating out in the cold conduit, the reservoir vessel and the distance up to the evaporator were heated to 60 C. By means of a three-way tap, it was possible to run formalin either in a circuit back into the vessel or else in the evaporator direction.

(45) A Fink HPLC pump was used to pump acetic acid into a helical tube evaporator (diameter 8 mm, length about 2 m, 1.4571 stainless steel), which completely evaporated therein (wall temperature about 200 C.) and mixed with a stream comprising nitrogen.

(46) The stream comprising the evaporated formalin and the stream comprising the evaporated acetic acid and nitrogen were combined and passed as stream S1 via a pipeline heated to 150-200 C. through a static mixer (diameter 10 mm, length 80 mm, 1.4541 stainless steel) containing wire mesh into the reactor heated to 320 C. (outer wall) (WHSV: 1.4 kg/kg/h). After passing through an unfilled region (length 2.8 cm), the gas stream arrived at a first steatite bed (mass 33 g, bed height 16 cm, 4-5 mm balls). The downstream catalyst bed was divided into two (mass of each 40 g, bed height 23 cm) and was interspersed with a second steatite bed (mass 42 g, bed height 20 cm, 4-5 mm balls). The overall bed rested on a catalyst support of about 3 cm in height, with a third steatite bed (mass 14 g, bed height 7 cm, 4-5 mm balls) concluding the reactor outlet. Within the reactor was a thermowell of thickness 3.17 mm, which was used to measure a temperature profile along the reactor. The reaction was conducted at a pressure of 1100 mbar (absolute).

(47) The reactor offgas was passed to a total combustion unit downstream of the reactor outlet. For protection against blockages by catalyst dusts, a filter station was installed downstream of the reactor outlet. In the total combustion unit, all components were incinerated with air metered in additionally (about 2000 L (STP)/h) and nitrogen which can be metered in additionally (about 1000 L (STP)/h) to give water and carbon dioxide. Constant pressure conditions in the reactor over different test runs were established by partly throttling the valves in the filter station. The total combustion unit air was heated to 300-400 C. by means of heating sleeves. The combustion temperature in the combustion catalyst bed varied with the organic carbon loading of the reactor offgases and was between 250 C. and 500 C. The offgas from the total combustion unit was passed through a separator (T=5-15 C.). The offgas that remains thereafter was passed into the offgas conduit.

(48) Acrylic acid (ACR) was added to the acetic acid-comprising stream in different contents (ACR content, ACR input). Various catalysts were used. The individual streams were analyzed by gas chromatography. The results, and details of ACR contents, are shown in tables 1 to 4 below, and presented in graph form in FIGS. 2 and 3. Since stream S1 was entirely gaseous, rather than the molar figures for the ratio of acrylic acid to the sum total of formaldehyde+acetic acid, the figures are given in % by volume.

(49) TABLE-US-00001 TABLE 1 Catalyst 1 Ratio Yield of Yield Selectivity Selectivity of acrylic acid acrylic of acrylic for acrylic for acrylic to sum total of acid based acid acid based acid formaldehyde + on formal- based on Formal- Acetic on formal- based on acetic acid at dehyde acetic acid dehyde acid dehyde acetic acid Exper- reactor inlet conversion conversion conversion conversion conversion conversion iment [vol/vol] Mean [%] Mean [%] Mean [%] Mean [%] Mean [%] Mean [%] Cat1-A 0.014 30.77 36.78 46.96 48.45 65.52 75.91 Cat1-B 0.036 30.37 33.19 46.62 45.03 65.14 73.71 Cat1-C 0.063 29.95 32.53 44.53 44.61 67.26 72.92 Cat1-D 0.090 23.95 26.53 44.12 41.24 54.28 64.33

(50) TABLE-US-00002 TABLE 2 Catalyst 2 Ratio Yield of Yield Selectivity Selectivity of acrylic acid acrylic of acrylic for acrylic for acrylic to sum total of acid based acid acid based acid formaldehyde + on formal- based on Formal- Acetic on formal- based on acetic acid at dehyde acetic acid dehyde acid dehyde acetic acid Exper- reactor inlet conversion conversion conversion conversion conversion conversion iment [vol/vol] Mean [%] Mean [%] Mean [%] Mean [%] Mean [%] Mean [%] Cat2-A 0.032 2.7 3.07 2.5 9.9 32.43 Cat2-B 0.072 1.98 1.83 6.14 10.38 36.30 18.00 Cat2-C 0.098 1.35 1.45 1.38 7.58 34.20 24.20 Cat2-D 0.018 3.628 3.158 8.38 11.08 40.26 32.15

(51) TABLE-US-00003 TABLE 3 Catalyst 3 Ratio Yield of Yield Selectivity Selectivity of acrylic acid acrylic of acrylic for acrylic for acrylic to sum total of acid based acid acid based acid formaldehyde + on formal- based on Formal- Acetic on formal- based on acetic acid at dehyde acetic acid dehyde acid dehyde acetic acid Exper- reactor inlet conversion conversion conversion conversion conversion conversion iment [vol/vol] Mean [%] Mean [%] Mean [%] Mean [%] Mean [%] Mean [%] Cat3-A 0.025 18.43 17.46 32.26 30.9 60.36 56.81 Cat3-B 0.048 14.6 20.51 21.76 29.33 77.61 75.20 Cat3-C 0.068 14.4 20.02 28.68 31.71 55.63 66.33 Cat3-D 0.090 12.32 17.83 38.98 31.13 37.04 61.43 Cat3-E 0.027 14.53 19.14 23.03 31.47 56.34 62.03

(52) TABLE-US-00004 TABLE 4 Catalyst 4 Ratio Yield of Yield Selectivity Selectivity of acrylic acid acrylic of acrylic for acrylic for acrylic to sum total of acid based acid acid based acid formaldehyde + on formal- based on Formal- Acetic on formal- based on acetic acid at dehyde acetic acid dehyde acid dehyde acetic acid Exper- reactor inlet conversion conversion conversion conversion conversion conversion iment [vol/vol] Mean [%] Mean [%] Mean [%] Mean [%] Mean [%] Mean [%] Cat4-A 0.030 27.83 27.21 42.63 40.28 65.34 67.55 Cat4-B 0.053 29.12 30.11 38.11 38.93 77.53 77.26 Cat4-C 0.076 26.80 32.73 32.59 37.31 82.55 82.34 Cat4-D 0.038 24.54 29.79 34.35 39.60 71.85 75.72 Cat4-E 0.102 12.91 14.93 45.56 51.47 30.21 32.42

(53) As can be seen from the above tables 1-4 and especially shown by FIGS. 2 and 3, the presence of acrylic acid in stream S1 was acceptable for the acrylic acid production in a molar ratio relative to the sum total of the reactants, formaldehyde and acetic acid, up to a value of 0.3:1. It was apparent that preferred molar ratios of acrylic acid to the sum total of the reactants, formaldehyde and acetic acid, in stream S1, were in the range of up to 0.1:1, more preferably of up to 0.09:1, further preferably of up to 0.08:1, further preferably of up to 0.07:1.

(54) III.2 Example 2: Determination of the Minimum Acrylic Acid Content in Stream S1

(55) The example which follows was run with the aid of the process simulation program CHEMASIM from BASF. The essential compositions and properties of the streams shown in FIG. 1 can be found in tables 5 and 6. Mass balances are completed by any offgas streams not mentioned/shown.

(56) The acetic acid and formalin solution reactants (49% by weight of formaldehyde, 49% by weight of water, 2% by weight of methanol) were subjected to total evaporation (i) in a suitable heat transferer, diluted with inert gas (nitrogen), and fed as stream S1, optionally after mixing with the recycled streams S2b_rec and/or S3 and/or S5 and/or S8, in gaseous form to the reaction zone (ii), charged with the aldol condensation reactor.

(57) In the reaction zone (ii), stream S1 was contacted at 370 C. and 1.1 bar absolute with a catalyst of the empirical formula VO(PO).sub.4 shaped into cylindrical extrudates having a cross-sectional area diameter of 3 mm and an average length of 20 mm. This was done using a shell and tube reactor, with the catalytically active fixed bed within the catalyst tubes, around which fluid heat carrier flowed.

(58) The gaseous reactor output S2 was cooled down to about 40 C. in a suitable heat transferer in (iii), and partly condensed at the same time. The uncondensed portion S2b which comprised predominantly low-boiling components and inert gases, after removing at least a portion of S2b, S2b_Purge, was recycled upstream of the reactor in (ii) as S2b_Rec.

(59) The condensed portion of S2, S2a, was guided into a distillation column in (iv.1). This column was designed as a tray column equipped with a number of crossflow trays equivalent to about 30 theoretical plates, and was operated in rectificative mode. The feed stream was at about the 10th theoretical plate. A return stream consisting of at least a portion of S3 (not shown in FIG. 1) was applied to the uppermost tray. The vapor from the evaporator (not shown in FIG. 1) which was executed as a shell and tube circulation evaporator and was operated with 4 bar steam as heat carrier was conducted into the column below the first tray. The column in (iv.1) was operated at a top pressure of 1.3 bar absolute; the bottom temperature was about 140 C., and the top temperature about 105 C. The vapors from the column were at least partly condensed in a shell and tube apparatus (not shown in FIG. 1), with conduction of the liquid component into a distillate collection vessel and division thereof into return stream and distillate draw stream S3 therein. At the bottom of the column in (iv.1), a liquid bottom stream S4 was withdrawn.

(60) Stream S4 was passed into a distillation column in (iv.2). This column was designed as a tray column equipped with a number of dual-flow trays equivalent to about 20 theoretical plates, and was operated in rectificative mode. The feed stream was at about the 8th theoretical plate. A return stream consisting of at least a portion of S5 (not shown in FIG. 1) was applied to the uppermost tray. The vapor from the evaporator (not shown in FIG. 1) which was executed as a shell and tube circulation evaporator and was operated with 4 bar steam as heat carrier was conducted into the column below the first tray. The column in (iv.2) was operated at a top pressure of 100 mbar absolute; the bottom temperature was about 105 C., and the top temperature about 40 C. The vapors from the column were at least partly condensed in a shell and tube apparatus (not shown in FIG. 1), with conduction of the liquid component into a distillate collection vessel and division thereof into return stream and distillate draw stream S5 therein. Stream S5 was recycled upstream of the reactor in (ii). Acrylic acid was drawn off in liquid form as S6 in the bottom of the column in (iv.2).

(61) Stream S3 was passed into a distillation column in (iv.3). This column was designed as a column with random packing, equipped with a random packing bed height equivalent to about 20 theoretical plates, and was operated in rectificative mode. The feed stream was at about the 5th theoretical plate. A return stream consisting of at least a portion of S7 (not shown in FIG. 1) was applied to the uppermost tray. The vapor from the evaporator (not shown in FIG. 1) which was executed as a shell and tube circulation evaporator and was operated with 4 bar steam as heat carrier was conducted into the column below the first tray. The column in (iv.3) was operated at a top pressure of 90 mbar absolute; the bottom temperature was about 60 C., and the top temperature about 40 C. The vapors from the column were at least partly condensed in a shell and tube apparatus (not shown in FIG. 1), with conduction of the liquid component into a distillate collection vessel and division thereof into return stream and distillate draw stream S7 therein. Stream S7 was disposed of as wastewater in need of treatment. At the bottom of the column in (iv.3), a liquid bottom stream S8 was withdrawn and recycled completely upstream of the reactor in (ii).

(62) With reference to the overall simulation of the process described in example 2, the influence of the amount of acrylic acid recycled on the economic viability of the process was illustrated. With the aid of the CHEMASIM process simulator and an in-house BASF SE tool for realistic assessment of capital and operating costs of chemical processes, the preparation costs for acrylic acid by the process described in example 2 were examined as a function of the amount of acrylic acid permitted in the recycle streams. The relative value estimated for the acrylic acid preparation costs (based on the costs at a molar ratio of acrylic acid to the sum total of formaldehyde and acetic acid of 0.3:1 in the reactor inlet) as a function of the molar ratio of acrylic acid to the sum total of formaldehyde and acetic acid in the reactor inlet (S1) is shown in FIG. 4. The rise in the preparation costs with a smaller permitted ratio of acrylic acid to the sum total of the reactants at the reactor inlet was attributable to a crucial degree to the rising energy costs which are caused by the higher distillative separation intensity and hence rising demand for steam and cooling water in the column (iv.2).

(63) It is apparent from the thermodynamic simulation that the lower limit in the molar ratio of acrylic acid to reactants (formaldehyde+acetic acid) in stream S1 was 0.005:1; the preferred lower limit was seen to be a molar ratio of acrylic acid to reactant in stream S1 of 0.02:1. Since stream S1 was entirely gaseous, rather than the molar figures for the ratio of acrylic acid to the sum total of formaldehyde+acetic acid, the figures were given in % by volume.

(64) TABLE-US-00005 TABLE 5 Stream bar (1/2) M Stream S1 Stream S2 Stream S2a Abbre- [kg/ [% by [% by [% by viation kmol] [kg/h] wt.] [kg/h] wt.] [kg/h] wt.] Formaldehyde FA 30.03 20769 8.73 8307.7 3.49 8033.4 9.35 Acetic acid ACE 60.05 41540 17.46 16616 6.98 15672 18.24 Acrylic acid ACR 72.07 5538.7 2.33 32680 13.73 31475 36.63 Water H2O 18.02 24332 10.23 33565 14.11 30023 34.94 Methanol MEOH 32.04 552.1 0.23 220.8 0.09 180.5 0.21 Formic acid FAC 46.03 564.0 0.24 564.0 0.24 531.9 0.62 Propionic acid PRA 74.08 10.9 0.00 10.9 0.00 10.7 0.01 Carbon dioxide CO2 44.01 22160 9.31 27685 11.64 Oxygen O2 32.00 6763.4 2.84 2580.6 1.08 Carbon monoxide CO 28.01 Hydrogen H2 2.02 Nitrogen N2 28.01 115703 48.63 115703 48.63 Sum total 237933 100.0 237933 100.0 85927 100.0 Volumetric flow rate V m.sup.3/h 293553 375736 82.64 Density kg/m.sup.3 0.811 0.633 1039.7 Viscosity eta mPa s 0.027 0.027 0.997 Specific heat c_p kJ/kg/K 1.387 1.398 2.901 Surface tension N/m 0.039 Mean molar mass M kg/kmol 31.0 30.8 32.4 Temperature T C. 370.0 370.0 40.0 Boiling pressure BP bar Pressure p bar 1.400 1.100 1.400 Stream Stream Stream S2b S2b_Purge S2b_Rec Stream S3 [% by [% by [% by [% by [kg/h] wt.] [kg/h] wt.] [kg/h] wt.] [kg/h] wt.] Formaldehyde 274.3 0.18 51.7 0.17 206.6 0.17 8030.6 19.33 Acetic acid 943.8 0.62 188.8 0.62 755.0 0.62 3287.0 7.91 Acrylic acid 1205.1 0.79 241.0 0.79 964.1 0.79 472.1 1.14 Water 3541.7 2.33 707.1 2.33 2828.5 2.33 29579 71.19 Methanol 40.4 0.03 6.8 0.02 27.2 0.02 179.9 0.43 Formic acid 32.2 0.02 6.4 0.02 25.7 0.02 0.1 0.00 Propionic acid 0.2 0.00 0.0 0.00 0.2 0.00 0.0 0.00 Carbon dioxide 27685 18.21 5537.0 18.21 22148 18.21 Oxygen 2580.6 1.70 516.1 1.70 2064.5 1.70 Carbon monoxide Hydrogen Nitrogen 115703 76.12 23141 76.12 92563 76.12 Sum total 152007 100.0 30401 100.0 121605 100.0 41549 100.0 Volumetric flow rate 94258 18852 75406 41.41 Density 1.613 1.613 1.613 1003.3 Viscosity eta 0.018 0.018 0.018 0.412 Specific heat 1.095 1.095 1.095 3.636 Surface tension 0.049 Mean molar mass 29.9 29.9 29.9 21.0 Temperature 40.0 40.0 40.0 105.7 Boiling pressure 1.300 Pressure 1.400 1.400 1.400 1.300

(65) TABLE-US-00006 TABLE 6 Stream bar (2/2) M Stream 54 Stream S5 Stream S6 Stream S7 Stream S8 Abbre- [kg/ [% by [% by [% by [% by [% by viation kmol] [kg/h] wt.] [kg/h] wt.] [kg/h] wt.] [kg/h] wt.] [kg/h] wt.] Formaldehyde FA 30.03 2.8 0.01 2.8 0.02 109.7 0.50 7920.9 40.40 Acetic acid ACE 60.05 12385 27.91 12331 70.82 54 0.20 0.0 0.00 3287.0 16.77 Acrylic acid ACR 72.07 31003 69.86 4102.4 23.56 26901 99.76 0.0 0.00 4721 2.41 Water H2O 18.02 443.8 1.00 443.8 2.55 0.0 0.00 21657 98.70 7920.9 40.40 Methanol MEOH 32.04 0.6 0.00 0.6 0.00 175.9 0.80 3.9 0.02 Formic acid FAC 46.03 531.7 1.20 531.7 3.05 0.0 0.00 0.1 0.00 Propionic acid PRA 74.08 10.7 0.02 0.5 0.00 10 0.01 0.0 0.00 Carbon dioxide CO2 44.01 Oxygen O2 32.00 Carbon monoxide CO 28.01 Hydrogen H2 2.02 Nitrogen N2 28.01 Sum total 44378 100.0 17413 100.0 26965 100.0 21943 100.0 19605 100.0 Volumetric flow rate V m.sup.3/h 48.80 16.92 28.40 22.23 17.81 Density kg/m.sup.3 909.3 1028.9 949.6 987.1 1101.0 Viscosity eta mPa s 0.293 0.880 0.377 0.665 1.284 Specific heat c_p kJ/kg/K 2.900 2.449 2.348 4.166 2.929 Surface tension N/m 0.016 0.027 0.019 0.069 0.044 Mean molar mass M kg/kmol 66.0 58.3 72.0 18.1 25.6 Temperature T C. 139.6 40.0 106.4 40.0 62.0 Boiling pressure BP bar 1.490 0.310 Pressure p bar 1.490 0.100 0.310 0.090 0.185