Process for preparing acrylic acid

Abstract

The present invention provides an apparatus comprising a feed arranged below the middle of the apparatus, a quenching agent inlet arranged above the feed, an outlet arranged below the feed and a draw arranged above the quenching agent inlet, the apparatus having a region provided with one or more packing elements, wherein at least one means which ensures the formation of a liquid layer having a height of at least 1 cm is present between the quenching agent inlet and a packing element present between the quenching agent inlet and feed. The present invention also provides a process for preparing acrylic acid, which includes a step of oxidizing acrolein, wherein the reaction mixture obtained in this oxidation is contacted with water as quenching agent in an inventive apparatus, and a composition obtainable as bottom product in the process according to the invention, comprising acrylic acid and water.

Claims

1. An apparatus consisting essentially of a feed containing a reaction mixture arranged below the middle of the apparatus, a quenching agent inlet arranged above the feed, an outlet arranged below the feed and a draw arranged above the quenching agent inlet, the apparatus having, between the quenching agent inlet and the feed containing the reaction mixture, a region provided with one or more packing elements, wherein 2 or 3 sieve trays or ripple trays which ensure the formation of a liquid layer having a height of at least 1 cm is present between the quenching agent inlet and a packing element present between the quenching agent inlet and feed containing the reaction mixture, the liquid layer being arranged in the apparatus such that gaseous substances which pass through the feed containing the reaction mixture into the apparatus have to pass through the liquid layer in order to get into the top of the apparatus.

2. The apparatus according to claim 1, wherein the liquid layer has a height of 2 to 10 cm.

3. The apparatus according to claim 1 wherein the packing element is a structured packing element or a fabric packing element.

4. The apparatus according to claim 1, wherein the packing element has a height of 50 to 85% of the internal height of the apparatus.

5. The apparatus according to claim 1, wherein a distributor present between a lowermost sieve trays or ripple trays and the uppermost packing element distributes the liquid phase flowing downward from the lowermost sieve trays or ripple trays over the packing element.

6. The apparatus according to claim 5, wherein a collector arranged between the lowermost sieve trays or ripple trays and distributor collects the liquid phase flowing downward from the lowermost sieve trays or ripple trays and feeds liquid phase to the distributor.

7. A process for preparing acrylic acid, including a step of oxidizing acrolein, wherein the reaction mixture obtained in this oxidation is contacted with water as quenching agent in an apparatus consisting essentially of a feed containing a reaction mixture arranged below the middle of the apparatus, a quenching agent inlet arranged above the feed, an outlet arranged below the feed and a draw arranged above the quenching agent inlet, the apparatus having, between the quenching agent inlet and the feed containing the reaction mixture, a region provided with one or more packing elements, wherein 2 or 3 sieve trays or ripple trays which ensure the formation of a liquid layer having a height of at least 1 cm is present between the quenching agent inlet and a packing element present between the quenching agent inlet and feed containing the reaction mixture, the liquid layer being arranged in the apparatus such that gaseous substances which pass through the feed containing the reaction mixture into the apparatus have to pass through the liquid layer in order to get into the top of the apparatus.

8. The process according to claim 7, wherein the ratio of the volume flow rates of the quenching agent which is fed to the apparatus and the reaction mixture which is fed to the apparatus is from 1:2 to 1:20.

9. The process according to claim 7, wherein the temperature of the liquid phase on the sieve trays or ripple trays is from 50 C. to 100 C.

10. The process according to claim 7, wherein the pressure differential between the pressure measured at the top of the apparatus and the pressure measured in the gas space of the liquid phase in the bottom of the apparatus is from 10 mbar to 60 mbar.

11. The process according to claim 7 wherein the apparatus is fed with 210 to 380 kg of reaction mixture per hour per m.sup.3 of apparatus volume.

12. The apparatus according to claim 2, wherein the liquid has a height of 3 to 8 cm.

13. The apparatus according to claim 3, wherein the packing element is a structured packing.

14. The process according to claim 8, wherein the ratio of the volume flow rates of the quenching agent which is fed to the apparatus and the reaction mixture which is fed to the apparatus is from 1:5 to 1:10.

15. The process according to claim 9, wherein the temperature of the liquid phase on the sieve trays or ripple trays is from 70 C. to 80 C.

16. The process according to claim 10, wherein the apparatus is operated in such a way that the pressure differential between the pressure measured at the top of the apparatus and the pressure measured in the gas space of the liquid phase in the bottom of the apparatus is from 30 mbar to 40 mbar.

Description

(1) The present invention is illustrated in detail by the figures, FIG. 1 and FIG. 2, without any intention that the invention be restricted to these embodiments.

(2) FIG. 1 shows, in schematic form (customary apparatuses, for example slide vanes, taps, pumps or heat exchangers are not shown for the sake of better clarity), a plant for preparation of acrylic acid from propene. Propene P, steam (WD) and air (L) are passed into the first oxidation reactor R1. The reaction product comprising acrolein is passed into a second oxidation reactor R2 in which the acrolein is oxidized to acrylic acid. The gaseous reaction product from reactor R2 is run into the side of the quench tower Q in which water is introduced from the top. The gaseous top product from the quench tower can be sent partly as offgas to a further workup or disposal operation, and partly returned to the reactor R1 as cycle gas. The bottom product of the quench tower SQ is transferred into the side of the distillation column A, in which an azeotroping agent/azeotropic distillation is conducted. The top product KA is condensed and transferred into a separator S in which the top product is separated into an aqueous phase and a phase including the azeotroping agent. The azeotroping agent is recycled into the distillation column A above the feed of the bottom product SQ. The aqueous phase W obtained in the separator S is recycled into the quench tower above the feed of the reaction product from the reactor R2. The bottom product SA of column A can be sent to a further processing operation D.

(3) FIG. 2 shows, in schematic form, one setup of an inventive apparatus (of a quench tower): The apparatus has been provided with a feed for the reaction mixture ZR, an inlet for the quenching agent ZQ, an outlet in the bottom SQ and a draw for the top product KQ. Provided therein is a packing element P, above which is arranged a means M which assures the formation of a liquid layer having a height of at least 1 cm.

(4) The examples adduced below illustrate the present invention by way of example, without any intention of restricting the invention, the scope of application of which is apparent from the entirety of the description and the claims, to the embodiments specified in the examples.

EXAMPLES

(5) Measurement Method:

(6) The proportion of phthalic acid, isophthalic acid and terephthalic acid in the bottom product of the apparatus was determined by means of GC-MS as dimethyl esters. For this purpose, the samples were first admixed with diazomethane for esterification. The samples thus prepared were sent to the GC-MS (described hereinafter for terephthalic acid, to be conducted in an equivalent manner for phthalic acid/isophthalic acid). The following instruments were used: liquid sampler, PAL injector 80; Agilent GC 7890 gas chromatograph and Agilent MSD 5975 mass spectrometer.

(7) The measurements were effected with the following parameters:

(8) Capillary column: 30 m DB-5; internal diameter 0.25 mm; film 0.25 m

(9) Carrier gas: helium

(10) Column flow rate: approx. 1 ml/min

(11) Injector: 300 C. (0.5 min splitless, then 50 ml/min)

(12) Oven temperature: 60 C.-8 C./min-250 C. (15 min)

(13) Injection volume: 1.0 l

(14) Detector (MSD): single ion monitoring (SIM)

(15) Group 1 (6 min to 12 min): (91.0; 100 msec), (119.0; 100 msec), (150; 100 msec)

(16) Group 2 (from 12 min): (163.0; 100 msec), (194.0; 100 msec)

(17) The reagents used were:

(18) diazomethane derivatizing agent

(19) methanol

(20) terephthalic acid dopant solution (concentration=1 mg/ml, dissolved in methanol)

(21) Sample Preparation

(22) Derivatization of the Sample

(23) 25 mg of sample were dissolved in 1 ml of methanol and derivatized with diazomethane. The reaction mixture is then concentrated down to 1.5 ml in a water bath. The overall solution is transferred into a 2 ml standard flask and made up to the mark with methanol.

(24) Derivatization of the terephthalic acid-doped Sample

(25) 25 mg of sample were dissolved in 1 ml of methanol, 10 l of the terephthalic acid dopant solution were added, and then derivatization was effected with diazomethane. The reaction mixture is then concentrated down to 1.5 ml in a water bath. The overall solution is transferred into a 2 ml standard flask and made up to the mark with methanol.

(26) Evaluation

(27) Terephthalic acid was detected in the GC-MS analysis as dimethyl terephthalate. The dimethyl terephthalate peak is identified on the basis of the run time and on the basis of the ratio of the two ions measured, m/z 163 and m/z 194 (two characteristic ions for dimethyl terephthalate). The mass content of terephthalic acid in the sample is calculated using the measured values from the measurement of the undoped sample and from the doped sample:

(28) $\mathrm{wT}=\frac{\left(m,\mathrm{Tdot}.\right)}{\left(\left(\frac{(A\left(T,\mathrm{dot}.\right)E}{E\left(\mathrm{dot}.\right)A\left(T\right)}\right)-1\right)E\left(\mathrm{dot}.\right)}$

(29) w(T): proportion by mass of terephthalic acid in the sample (in mg/g)

(30) m(T, dot.): mass of terephthalic acid dopant in the doped sample (in g)

(31) A(T, dot.): peak area of the dimethyl terephthalate in the doped sample

(32) E(dot.): starting sample weight in the doped measurement (in mg)

(33) A(T): peak area of dimethyl terephthalate in the undoped sample

(34) E: starting sample weight in the undoped measurement (in mg)

(35) Apparatus:

(36) The apparatus used was a quench tower having a capacity of 300 m.sup.3 and an internal height of 35.05 m. Installed in the apparatus were 19 packing elements of the Montz-Pak B1-125.60 type at an internal height of 3.96 to 8.102 m, 42 packing elements of the Montz-Pak B1-250 type at an internal height of 10.975 to 19.585 m, and 32 packing elements of the Montz-Pak B1-250 type at an internal height of 21.97 to 28.53 m. The inlet for the quenching agent was installed at an internal height of 32.598 m; the reaction mixture was fed in at an internal height of 5.048 m. In the comparative example, a quench tower of this kind which did not have any sieve trays was used. The inventive quench tower had, at an internal height of 28.3 m, a sieve tray with which a liquid layer of 3 cm was achievable. The size of the uppermost packing element was reduced, such that it extended from an internal height of 21.97 to 27.3 m. The quench tower additionally had liquid distributors, gas distributors, collectors and standard support grids for the packing elements. A top draw was present at the top of the quench tower, and a bottoms outlet in the bottom of the column.

Comparative Example 1: Acrylic Acid Preparation Without the Use of an Inventive Quench Tower

(37) In a plant as shown schematically in FIG. 1 for preparation of acrylic acid, a reaction mixture stream which included 6.24% by volume of acrylic acid and a content of the sum total of phthalic acid, isophthalic acid and terephthalic acid of 0.0144% by weight was introduced into the quench tower at 71 141 kg/h. Via the inlet for the quenching agent, 8200 kg/h of water were fed in. Via the bottoms outlet, a product stream of 18 252 kg/h was withdrawn. Via the top draw, an offgas stream of 61 089 kg/h was withdrawn. The bottom temperature was set to 79.1 C. and the top temperature to 71.6 C. After three hours of continuous operation, the samples were taken from the bottoms. The results of the analysis are reported in Table 1.

(38) The plant had to be shut down after 33 days of operation, since the pressure drop between the reaction mixture feed and top draw had risen by more than 50% compared to the value on startup of the plant.

Example 1: Preparation of Acrylic Acid Using an Inventive Quench Tower

(39) In a plant as described in the comparative example, instead of the quench tower described therein, an inventive quench tower which had a sieve tray was used. After three hours of continuous operation, the samples were taken from the bottoms. The results of the analysis are again reported in Table 1.

(40) In contrast to the comparative experiment, it was still possible to conduct the preparation of acrylic acid without any problem even after 2 months. In a standard inspection of the plant after 3 months, absolutely no deposits attributable to phthalic acid, terephthalic acid or isophthalic acid or conversion products thereof were observed in the top of the quench tower.

Comparative Example 2: Quench Tower Without Packing Elements and Without Sieve Trays

(41) Comparative example 1 was repeated, except using a quench tower which, rather than the B1-250 packing elements, had 25 Thormann trays and, below that, instead of the B1-125.60 packing elements, 7 segmented cascade trays.

(42) TABLE-US-00001 TABLE 1 Content of diacids in the bottom product based on the overall composition of the bottom product Sum total of Reaction phthalic acid and Isophthalic mixture stream terephthalic acid acid throughput Comparative 0.016% 0.026% 71 141 kg/h example 1 by weight by weight Example 1 0.030% 0.026% 71 141 kg/h by weight by weight Comparative 0.028% 0.026% 51 100 kg/h example 2 by weight by weight

(43) As can be inferred from Table 1, the bottom product which was obtained in an inventive apparatus (Example 1) has a proportion of phthalic acid and terephthalic acid well above the proportion which was determined for Comparative Example 1. Using a quench tower without packing elements (Comparative Example 2), it is possible to achieve similar proportions of phthalic acid and terephthalic acid to those reported for the inventive apparatus, but only with acceptance of a distinct decrease in throughput.