PROCESS FOR PREPARING 1,3-BUTADIENE FROM N-BUTENES BY OXIDATIVE DEHYDROGENATION

Abstract

A process for preparing butadiene from n-butenes, comprising the steps of: A) providing an input gas stream comprising n-butenes; B) feeding the input gas stream comprising n-butenes and a gas containing at least oxygen into at least one oxidative dehydrogenation zone and oxidatively dehydrogenating n-butenes to butadiene, giving a product gas stream; Ca) cooling the product gas stream by contacting with a circulating cooling medium in at least one cooling zone; Cb) compressing the cooled product gas stream in at least one compression stage, giving at least one aqueous condensate stream c1 and one gas stream c2; D) removing uncondensable and low-boiling gas constituents comprising oxygen and low-boiling hydrocarbons as gas stream d2 from the gas stream c2 by absorbing the C.sub.4 hydrocarbons in an absorbent, giving an absorbent stream laden with C.sub.4 hydrocarbons and the gas stream d2, and then desorbing the C.sub.4 hydrocarbons from the laden absorbent stream, giving a C.sub.4 product gas stream d1; E) separating the C.sub.4 product stream d1 by extractive distillation; F) distilling the stream e1 into a stream f1 consisting essentially of the selective solvent and a stream f2 comprising butadiene; G) removing a portion of the aqueous phase of the cooling medium which circulates in step Ca) as aqueous purge stream g; H) distillatively separating the aqueous purge stream g into a fraction h1 and a fraction h2 depleted of organic constituents.

Claims

1.-21. (canceled)

22. A process for preparing butadiene from n-butenes, comprising the steps of: A) providing an input gas stream comprising n-butenes; B) feeding the input gas stream and a gas that includes oxygen into at least one oxidative dehydrogenation zone to oxidatively dehydrogenate the n-butenes to butadiene, and provided a product gas stream comprising the butadiene, unconverted n-butenes, water vapor, oxygen, low-boiling hydrocarbons and high-boiling secondary components, with or without carbon oxides and with or without inert gases; Ca) cooling the product gas stream by contact with a circulating cooling medium in at least one cooling zone, the cooling medium being at least partly recycled and having an aqueous phase and an organic phase that includes an organic solvent, wherein the solvent used in step Ca) is selected from the group consisting of toluene, o-, m- and p-xylene, mesitylene, mono-, di- and triethylbenzene, mono-, di- and triisopropylbenzene and mixtures thereof; Cb) compressing the cooled product gas stream, which is optionally depleted of high-boiling secondary components in at least one compression stage to provide at least one aqueous condensate stream c1 and one gas stream c2 comprising butadiene, n-butenes, water vapor, oxygen and low-boiling hydrocarbons, with or without carbon oxides and with or without inert gases; D) removing uncondensable and low-boiling gas constituents comprising oxygen and low-boiling hydrocarbons, with or without carbon oxides and with or without inert gases, as gas stream d2 from the gas stream c2 by absorbing the C.sub.4 hydrocarbons comprising butadiene and n-butenes in an absorbent to provide an absorbent stream laden with C.sub.4 hydrocarbons and the gas stream d2, and then desorbing the C.sub.4 hydrocarbons from the laden absorbent stream to provide a C.sub.4 product gas stream d1; E) separating the C.sub.4 product stream d1 by extractive distillation with a butadiene-selective solvent into a stream e1 that includes butadiene and the selective solvent, and a stream e2 comprising n-butenes; F) distilling the stream e1 to provide a stream f1 consisting essentially of the selective solvent and a stream f2 that includes butadiene; G) removing a portion of the aqueous phase of the cooling medium which circulates in step Ca) as aqueous purge stream g; H) separating by distillation the aqueous purge stream g into a fraction h1 enriched in organic constituents and a fraction h2 depleted of organic constituents; and provide I) at least one fraction i1 as product of value from the stream h1.

23. The process according to claim 22, where at least 90% by weight of the organic constituents present in the aqueous purge stream g are removed by distillation from the purge stream.

24. The process according to claim 22, further comprising J) separating at least one fraction j1 from the stream h1.

25. The process according to claim 22, wherein, in a further distillation step I), maleic acid and/or maleic anhydride as product of value is/are obtained as fraction i1 from the fraction h1 enriched in organic constituents.

26. The process according to claim 24, wherein the fraction j1 is sent to an incineration and the heat which arises in the incineration is utilized for operation of a distillation column or of an evaporator in which the aqueous purge stream g is distilled.

27. The process according to claim 26, wherein the portion of the aqueous phase of the cooling medium which is removed in step G) is such that the heat which arises in the incineration of fraction j1 is sufficient to operate the distillation column or the evaporator.

28. The process according to claim 27, wherein the portion of the aqueous phase of the cooling medium which is removed in step G) is 0.5% to 100% of the mass flow of the butenes supplied to the oxidative dehydrogenation zone.

29. The process according to claim 22, wherein the cooling medium is fed into the cooling zones through one or more nozzles.

30. The process according to claim 29, wherein a flow is generated in the nozzle(s), in which the Reynolds number Re of the two phases of the cooling medium is at least 100.

31. The process according to claim 22, wherein the volume-specific power input into the cooling medium is at least 10.sup.3 W/m.sup.3.

32. The process according to claim 22, wherein the coefficient of variation for each component of the cooling medium on entry into the cooling zones is less than 1.

33. The process according to claim 22, wherein stage Cb) comprises at least one compression stage Cba) and at least one cooling stage Cbb).

34. The process according to claim 33, wherein the coolant in the cooling stage Cbb) comprises the same organic solvent which is used in stage Ca) as the organic phase of the cooling medium.

35. The process according to claim 22, wherein stage Cb) comprises a plurality of compression stages Cba1) to Chan) and cooling stages Cbb1) to Cbbn).

36. The process according to claim 22, wherein step D) comprises steps Da) to Dc): Da) absorbing the C.sub.4 hydrocarbons comprising butadiene and n-butenes in a high-boiling absorbent, giving an absorbent stream laden with C.sub.4 hydrocarbons and the gas stream d2, Db) removing oxygen from the absorbent stream laden with C.sub.4 hydrocarbons from step Da) by stripping with an uncondensable gas stream, and Dc) desorbing the C.sub.4 hydrocarbons from the laden absorbent stream, giving a C.sub.4 product gas stream d1 comprising less than 100 ppm of oxygen.

37. The process according to claim 36, wherein the high-boiling absorbent used in step Da) is an aromatic hydrocarbon solvent.

Description

EXAMPLES

[0197] Dehydrogenation Experiments

[0198] Dehydrogenation experiments were conducted in a Miniplant reactor. The Miniplant reactor was a salt bath reactor having a length of 500 cm and an internal diameter of 29.7 mm, and a thermowell having an external diameter of 6 mm. On a catalyst support rested a 10 cm-long downstream bed consisting of 60 g of steatite rings of geometry 7 mm×7 mm×4 mm (external diameter×length×internal diameter). This was followed by 2705 g of an undiluted eggshell catalyst (active composition content 19.6% by weight; bed height 384 cm, bed volume in the reactor 2552 ml) in the form of hollow cylinders of dimensions 7 mm×3 mm×4 mm (external diameter×length×internal diameter). The catalyst bed was adjoined by an 85 cm-long upstream bed consisting of 494 g of steatite rings of geometry 7 mm×7 mm×4 mm (external diameter×length×internal diameter).

[0199] The temperature of the reaction tube was controlled over its entire length with a salt bath which flowed around it. The reaction gas mixture used was a mixture comprising a total of 8% by volume of 1-, cis-2- and trans-2-butenes, small amounts of i-butene, 2% by volume of butanes (n- and isobutane), 12% by volume of oxygen, 5% by volume of water and remainder of nitrogen. The space velocity through the reaction tube was 5500 l (STP)/h of total gas. The temperature of the salt after the startup, during stable operation, was 374° C. The conversion of butenes was 85%, and the selectivity for butadiene was likewise about 85%. Secondary components detected comprise acetic acid, methacrolein, methyl vinyl ketone, methyl ethyl ketone, crotonaldehyde, acrylic acid, propionic acid, methacrylic acid, vinylcyclohexane, maleic anhydride, ethylbenzene, styrene, furanone, benzaldehyde, benzoic acid, phthalic anhydride, fluorenone, anthraquinone, formaldehyde, carbon monoxide and carbon dioxide.

[0200] The ODH offgas was cooled to 200° C. by means of a heat exchanger. It can be run directly to a flare or introduced from above into the top of a quench column. The quench column is 920 mm in length and has an internal diameter of 56 mm. After 300 mm and after 610 mm, Venturi nozzles having a central hole diameter of 8 mm are installed. The coolant is supplied from above to the top of the quench through two full-cone nozzles on either side of the gas inlet tube (smallest free cross section 1.15 mm). The gas/coolant mixture passes through the quench column via the two Venturi nozzles and is collected at the bottom of the quench column, and two quench circulation streams are conveyed with the aid of two pumps through a heat exchanger back to the top of the column. The pressure is measured on the pressure side of the pumps. A portion of the laden coolant can be withdrawn from the circulation stream. In addition, coolant (water and/or mesitylene) can be fed to the circulation stream.

[0201] The product gas is sent to further workup.

Example 1

[0202] 1111 g/h of deionized water and 2584 g/h of mesitylene (1,3,5-trimethylbenzene) are supplied to the circulation stream of coolant around the quench column. The two circulation flow rates around the quench column are 60 l/h. The coolant level in the column bottom is kept constant by discharging coolant from the circulation system.

[0203] When the reactor has attained a stable state, the reactor offgas is diverted from the flare into the quench column. After a few hours, the coolant just upstream of one cone nozzle has a temperature of 75° C. and just upstream of the second cone nozzle has a temperature of 76° C., and in the column bottom has a temperature of 72° C. The temperature in the gas space above the liquid coolant is measured at 74° C. The pressure in the coolant circuit is 1.5 bar gauge. The content of water vapor and mesitylene in the gas which enters and leaves the quench column is measured by online GC. 709 g/h of laden coolant are discharged as purge stream. The total inventory of coolant in the quench column is 3000 g. The phase ratio in the quench circuit and in the purge stream, expressed as mass of water to mass of mesitylene, is 0.43. The reactor and quench ran for more than 2400 hours with a virtually constant coolant circulation flow rate, without any significant changes in the pressures upstream of the cone nozzles.

[0204] After operation for 10 days, a sample was taken from the coolant circuit. The aqueous fraction of the coolant was removed. At a sample pH of 2.6, the total organic carbon content was 4.4% by weight. The water content, determined via Karl Fischer titration, was 90.3% by weight. By means of capillary electrophoresis, 0.58% by weight of phthalic acid, 6.4% by weight of maleic acid, 0.18% by weight of benzoic acid, 0.43% by weight of acrylic acid and 0.44% by weight of acetic acid were found.

[0205] The calorific value of the sample was about 1050 kJ/kg.

Example 2

[0206] 1112 g/h of deionized water and 2173 g/h of mesitylene (1,3,5-trimethylbenzene) are fed to the circulation stream of coolant around the quench column (see table). The circulation flow rate around the quench column in each case is 60 l/h through 2 full-cone nozzles. The coolant level in the bottom is kept constant by discharging coolant from the circuit.

[0207] When the reactor has reached a steady state, the reactor offgas is diverted from the flare into the quench column. After a few hours, the coolant just upstream of the cone nozzle has reached a temperature of 75° C., and a temperature in the bottom of 73° C. The temperature of the gas space above the coolant level is measured at 75° C. The pressure in the coolant circuit is 1.5 bar gauge. The content of water vapor and mesitylene in the product gas which leaves the quench column is measured by online GC. The phase ratio in the quench circuit, expressed as the mass of water relative to the mass of mesitylene, is 0.53. The reactor and quench ran for more than 150 hours with a virtually constant coolant circulation flow rate, without any significant changes in the pressure.

Example 3

[0208] 1314 g/h of deionized water and 2083 g/h of mesitylene (1,3,5-trimethylbenzene) are fed to the circulation stream of coolant around the quench column (see table). The circulation flow rate around the quench column in each case 60 l/h through 2 full-cone nozzles. The coolant level in the bottom is kept constant by discharging coolant from the circuit.

[0209] When the reactor has reached a steady state, the reactor offgas is diverted from the flare into the quench column. After a few hours, the coolant just upstream of the cone nozzle has reached a temperature of 75° C., and a temperature in the bottom of 73° C. The temperature of the gas space above the coolant level is measured at 75° C. The pressure in the coolant circuit is 1.5 bar gauge. The content of water vapor and mesitylene in the product gas which leaves the quench column is measured by online GC. The phase ratio in the quench circuit, expressed as the mass of water relative to the mass of mesitylene, is 0.99. The reactor and quench ran for more than 150 hours with a virtually constant coolant circulation flow rate, without any significant changes in the pressure.

Example 4

[0210] 1111 g/h of deionized water and 1730 g/h of mesitylene (1,3,5-trimethylbenzene) are fed to the circulation stream of coolant around the quench column (see table). The circulation flow rate around the quench column in each case is 60 l/h through 2 full-cone nozzles. The coolant level in the bottom is kept constant by discharging coolant from the circuit.

[0211] When the reactor has reached a steady state, the reactor offgas is diverted from the flare into the quench column. After a few hours, the coolant just upstream of the cone nozzle has reached a temperature of 75° C., and a temperature in the bottom of 73° C. The temperature of the gas space above the coolant level is measured at 75° C. The pressure in the coolant circuit is 1.5 bar gauge. The content of water vapor and mesitylene in the product gas which leaves the quench column is measured by online GC. The phase ratio in the quench circuit, expressed as the mass of water relative to the mass of mesitylene, is 0.84. The reactor and quench ran for more than 330 hours with a virtually constant coolant circulation flow rate, without any significant changes in the pressure.

[0212] After 1 day of operation, a sample was taken from the coolant circuit. The aqueous fraction of the coolant was removed. At a pH of the sample of 2.6, the total content of organic carbon was 3.0% by weight. By means of capillary electrophoresis, 0.40% by weight of phthalic acid, 3.7% by weight of maleic acid, 0.06% by weight of benzoic acid, 0.34% by weight of acrylic acid and 0.51% by weight of acetic acid were found.

Example 5

[0213] 1465 g/h of deionized water and 1730 g/h of mesitylene (1,3,5-trimethylbenzene) are fed to the circulation stream of coolant around the quench column (see table). The circulation flow rate around the quench column in each case 60 l/h through 2 full-cone nozzles. The coolant level in the bottom is kept constant by discharging coolant from the circuit.

[0214] When the reactor has reached a steady state, the reactor offgas is diverted from the flare into the quench column. After a few hours, the coolant just upstream of the cone nozzle has reached a temperature of 75° C., and a temperature in the bottom of 73° C. The temperature of the gas space above the coolant level is measured at 75° C. The pressure in the coolant circuit is 1.5 bar gauge. The content of water vapor and mesitylene in the product gas which leaves the quench column is measured by online GC. The phase ratio in the quench circuit, expressed as the mass of water relative to the mass of mesitylene, is 0.99. The reactor and quench ran for more than 330 hours with a virtually constant coolant circulation flow rate, without any significant changes in the pressure.

[0215] After 14 days of operation, a sample was taken from the coolant circuit. The aqueous fraction of the coolant was removed. At a pH of the sample of 2.9, the total content of organic carbon was 1.7% by weight. By means of capillary electrophoresis, 0.17% by weight of phthalic acid, 1.7% by weight of maleic acid, 0.05% by weight of benzoic acid, 0.31% by weight of acrylic acid and 0.34% by weight of acetic acid were found.

TABLE-US-00001 Example 1 Example 2 Example 3 Example 4 Example 5 Feed of water vapor from 613 587 629 593 585 reactor [g/h] Feed of water to the 1111 1112 1314 1111 1465 circulation stream [g/h] Removal of water from -214 -302 -525 -490 -916 circulation stream [g/h] Feed of of mesitylene to the 1911 2173 2083 1730 1730 circulation stream [g/h] Removal of mesitylene -495 -574 -530 -583 -586 from circulation stream [g/h] Phase ratio in the 0.43 0.53 0.99 0.84 1.56 circulation stream [g.sub.water/g.sub.mesitylene]