Process for the distillation of acrylic acid with split-dosing of polymerization inhibitor

Abstract

A process for the distillation and / or fractional condensation of a mixture comprising acrylic acid, water, and noncondensable gaseous components, comprises feeding the mixture into a column having a plurality of separatory trays; withdrawing acrylic acid from the column via a side draw; condensing water vapor generated in the column to obtain an aqueous condensate, and returning part of the condensate as reflux liquid into the column; and metering a polymerization inhibitor to the column at a height between the height of withdrawing the acrylic acid and the height of returning the aqueous condensate; wherein the polymerization inhibitor is simultaneously metered to at least two trays.

Description

[0001] Process for the Distillation of Acrylic Acid with Split-Dosing of Polymerization Inhibitor

[0002] The present invention relates to a process for the distillation of a mixture comprising acrylic acid, water, and noncondensable components, in particular of a product gas mixture obtained by gas-phase partial oxidation of a C3 precursor of acrylic acid with molecular oxygen under heterogeneous catalysis, and in particular to a method for effectively preventing generation of polymers, which often causes problems during distillation of liquids containing acrylic acid.

[0003] Acrylic acid is prepared, for example, by catalytically oxidizing propylene and / or acrolein (herein referred to as "C3 precursors”) with a molecular oxygen-containing gas in gas phase, collecting an aqueous acrylic acid solution by condensing the gaseous reaction products obtained, and then distilling the aqueous acrylic acid solution.

[0004] As liquid phase acrylic acid has a tendency to unwanted free-radical polymerization, especially under the action of thermal energy, it is required to suppress generation of polymers during distillation for stable operation thereof. In addition, it is needed to maintain the concentration of polymerization inhibitors added into the distillation column at a certain level for securing the potential of the polymerization inhibitors, and hence to add the polymerization inhibitors continuously into the distillation column, as the polymerization inhibitors are also removed continuously from the distillation column

[0005] US 6,646, 161 discloses a process for the fractional condensation of a gas mixture comprising acrylic acid or methacrylic acid, at least one further condensable component, and additionally a high proportion of one or more noncondensable components, the process comprising passing the gas mixture through a column having separatory internals, condensing the condensable components by cooling, and removing acrylic acid or methacrylic acid having a purity of more than 95% by weight from the column. A stabilizer, in particular phenothiazine, should be added to the column at one or more points which can be readily determined by a person skilled in the art, in order to avoid polymerization. The reference does not specifically identify the points to which the stabilizer should be added.

[0006] While injection of polymerization inhibitor to a single column tray would constitute the simplest way of polymerization inhibitor addition, practice shows that such single-point addition tends to cause uneven distribution of polymerization inhibitor with localized inhibitor overdosing and inhibitor deficits in other spots, hampering efficient purification of raw acrylic acid.

[0007] Therefore, there is a need for a polymerization inhibitor dosage regime that allows for even distribution of the inhibitor within the relevant regions of the distillation column and effectively prevents unwanted polymerization.

[0008] The invention relates to a process for the distillation of a mixture comprising acrylic acid, water, and noncondensable gaseous components, comprising feeding the mixture into a column having a plurality of separatory trays; withdrawing acrylic acid from the column via a side draw; condensing water vapor generated in the column to obtain an aqueous condensate, and returning part of the condensate as reflux liquid into the column; and metering a polymerization inhibitor to the column at a height between the height of withdrawing the acrylic acid and the height of returning the aqueous condensate; wherein the polymerization inhibitor is simultaneously metered to at least two trays.

[0009] Generally, during distillation the rising vapor and the liquid flowing down from the top of the column are brought into contact with each other with a low-boiling component being distilled from the top of the column and a high-boiling component being removed from the bottom of the column. Where a tray-type distillation column is used, the composition of the liquid present on a tray varies depending on the height of the tray, containing an increasingly higher proportion of the low-boiling component towards the top of the column. Accordingly, in the distillation of an aqueous acrylic acid mixture, the composition and water concentration of the liquid on the trays are not constant. Addition of a polymerization inhibitor, in particular an inhibitor which is sparingly soluble in water such as phenothiazine, may lead to the precipitation of the inhibitor, especially at the trays with relatively high ratio of water to acrylic acid. The lacking availability of inhibitor caused by the precipitation can lead to fouling due to polymerization.

[0010] The problem is solved by the process according to the invention. The process involves simultaneous addition of polymerization inhibitor to at least two trays. Since the total amount of polymerization inhibitor is divided into at least two trays, the local concentration of polymerization inhibitor injected at an individual tray is lower. Thus, the risk of precipitation is significantly reduced, leading to stable operation with reduced fouling.

[0011] According to the invention, the polymerization inhibitor is simultaneously metered to at least two trays, preferably to at least three trays, in particular three trays.

[0012] The trays to which polymerization inhibitor is metered may be in direct proximity or may be separated by other trays. In preferred embodiments, the trays to which polymerization inhibitor is metered are spaced apart by at least one tray, preferably by at least two trays.

[0013] Generally, the lowermost tray to which polymerization inhibitor is metered is spaced apart from the height of withdrawing the acrylic acid by at least 20 trays, preferably at least 30 trays, more preferably 30 to 40 trays. The uppermost tray to which polymerization inhibitor is metered is generally spaced apart from the height of returning the aqueous condensate by at least 10 trays, preferably 10 to 18 trays, more preferably 10 to 15 trays.

[0014] Conveniently, the polymerization inhibitor is metered as a solution in an appropriate solvent, such as acrylic acid, for example crude acrylic acid (containing 90% by weight or more of acrylic acid) or pure acrylic acid (containing 99% by weight or more of acrylic acid).

[0015] Metering preferably involves measuring the amount of polymerization inhibitor solution that is being metered with flowmeters. Suitably, the flowmeters are installed in the pipeline connecting a polymerization inhibitor solution storage tank and each tray of the purification column, to which polymerization inhibitor solution is added. Due to the measurement of the PTZ polymerization inhibitor added, the influence of the polymerization inhibitor solution addition on pressure in the column may be precisely monitored.

[0016] In a preferred embodiment, the process comprises sensing the temperature on a tray and adjusting, depending on the temperature of the tray, the amount of polymerization inhibitor to be metered to the tray.

[0017] The mixtures as used here as feed are regularly complex mixtures of numerous different individual components. Feed mass flow rate, feed composition, and feed temperature are subject to variation over time. Controlling the quality of the acrylic acid withdrawn in such a way that the content of foreign compounds therein is not too big, requires adjustments to control elements, especially valves, for the reflux rate, bottoms withdrawal rate and the like. A multitude of process parameters may be measured and entered into a computer, which uses these and predetermined target values to calculate signals that are transmitted to the control elements. There is a pronounced temperature sensitivity of the distillation column in the event of changes in the reflux rate, in particular in the section of the distillation column above the acrylic acid side draw.

[0018] In that connection, a lower temperature is indicative of a higher water content in the liquid on the tray. A higher water content, in turn, is associated with the risk of polymerization inhibitor precipitating as a solid, resulting in increasing pressure losses, and lack of polymerization inhibitor when the temperature rises again.

[0019] The process of the invention envisages to adapt the relative polymerization inhibitor dosing rate for each of the individual trays onto which polymerization inhibitor is metered. If the temperature on a tray rises, the amount of polymerization inhibitor to be metered to the tray is increased. If the temperature on a tray sinks, the amount of polymerization inhibitor to be metered to the tray is decreased.

[0020] In a preferred embodiment, there is provided a plurality of temperature sensing means at several trays, and a current temperature profile along at least a portion of the height of the column is created based on the temperature detected by each of the temperature sensing means. The current temperature profile can be compared to a reference temperature profile that is, e.g., stored in a computer database. If the current temperature profile is shifted downwards as compared to the reference temperature profile, more inhibitor is dosed to the lower trays and less to the higher trays, and vice versa.

[0021] In a still more preferred embodiment, the process comprises establishing a database having stored therein at least one reference temperature profile in the column and polymerization inhibitor solubilities as a function of temperature, and determining the amount of polymerization inhibitor to be metered to a tray, from data extracted from the database.

[0022] The polymerization inhibitor that is metered to the column at a height between the height of withdrawing the acrylic acid and the height of returning the aqueous condensate may be any of the polymerization inhibitors conventionally used for that purpose. The polymerization inhibitor may be a single polymerization inhibitor or a combination of two or more polymerization inhibitors. In a preferred embodiment, the polymerization inhibitor is phenothiazine. Other polymerization inhibitors that may be used are stable nitroxyl radicals. The stable nitroxyl radical may be selected from the group consisting of: di-tert-butylnitroxide; di-tert-amylnitroxide; 2,2,6,6-tetramethyl-piperidinyloxyl; 4-hydroxy-

[0023] 2.2.6.6-tetramethyl-piperidinyloxyl (HTEMPO); 4-oxo-2,2,6,6-tetramethyl-piperidinyloxyl; 4-dimethylamino-

[0024] 2.2.6.6-tetramethylpiperidinyloxyl; 4-amino-2,2,6,6-tetramethylpiperidinyloxyl; 4-ethanoyloxy-2, 2,6,6- tetramethylpiperidinyloxyl; 2,2,5,5-tetramethylpyrrolidinyloxyl; 3-amino-2,2,5,5-tetramethylpyrrolindinyloxyl; 2,2,5,5-tetramethyl-1-oxa-3-azacyclopentyl-3-oxyl; 2,2,5,5-tetramethyl-3-pyrrolinyl-1-oxy-3-carboxylic acid;

[0025] 2.2.3.3.5.5.6.6-octamethyl-1,4-diazacyclohexyl-1,4-dioxyl; and combinations thereof. A preferred stable nitroxyl radical is 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxyl (HTEMPO).

[0026] The column which can be used for the process is not subject to any particular restriction. In principle, all columns having separatory internals are suitable. The column has a plurality of separatory trays as separatory internals. Among the separatory trays, bubble trays, sieve trays, valve trays and / or dual -flow trays are preferred.

[0027] In the process of the invention, acrylic acid is recovered from a mixture comprising acrylic acid, water and noncondensable gaseous components by treatment in a column by a series of rectification and stripping stages in a column having separatory trays. The treatment may be a distillation and / or fractional condensation. Fractional condensation occurs if the mixture needs not be evaporated, but condensable components are condensed by cooling. Acrylic acid is suitably recovered via a side draw. The purity level of the withdrawn acrylic acid is generally 94 wt.-% or more, such as 95 wt.-% or more. These purity levels of acrylic acids are commonly referred to as crude acrylic acid or technical grade acrylic acid. Additional purification may be achieved via crystallization or further distillation to obtain glacial acrylic acid.

[0028] The noncondensable gaseous components are normally gaseous components that are not condensed during passage of the column. The noncondensable gaseous components may include one or more of nitrogen, carbon monoxide, carbon dioxide, oxygen, methane, propane and propylene.

[0029] The method of the invention involves condensing water vapor generated in the column to obtain an aqueous condensate, and returning part of the condensate as reflux liquid into the column. The aqueous condensate is also known as "acid water” due to the content of acrylic acid and acetic acid it usually contains.

[0030] Condensing the water vapor may comprise providing a quench section in the column, through which the noncondensable gaseous components and water vapor ascend. An aqueous condensate is collected at the lower end of the quench section, for example by use of a collecting tray. The aqueous condensate is partially circulated into the quench section through a circulation line to condense the water vapor by direct heat exchange. Another part of the aqueous condensate is returned to the column underneath the quench section as the reflux liquid. Generally, the aqueous condensate is cooled before being circulated into the quench section. Still another part of the aqueous condensate, which predominantly contains water, acetic acid and formaldehyde, is removed and discharged.

[0031] In order to avoid fouling in the condenser and the associated parts of the acid water recirculation line, it is preferred to meter additional polymerization inhibitor into the aqueous condensate. Suitably, a water-soluble polymerization inhibitor such as MEHQ, is metered into the aqueous condensate. Small amounts of other polymerization inhibitor such as phenothiazine me also be metered into the aqueous condensate in addition. Generally, the mixture which is passed into the column is a product gas mixture obtained by gas-phase partial oxidation of C3 precursors of acrylic acid with molecular oxygen under heterogeneous catalysis, which mixture may be partially condensed.

[0032] A particularly suitable hot gas mixture is the reaction gas mixture as formed in the catalytic gas-phase oxidation of propylene and / or acrolein by known processes. Generally in the preparation of acrylic acid, the C3 precursor and oxygen are diluted with inert gases, such as nitrogen, CO2, saturated Ci-Ce-hydrocarbons and / or steam, passed at elevated temperatures, usually from 200 to 450° C, and, if required, super- atmospheric pressure, over transition metal mixed oxide catalysts, and oxidized to acrylic acid. The mixed oxide catalysts contain, for example, Mo, V, W and / or Fe, see for example DE 44 05 059 A1, EP 0 253 409 A2, EP 0 092 097 A1 and DE 44 31 949 A1 . These reactions are carried out, for example, in one or more stages.

[0033] In addition to the desired acrylic acid, the resulting reaction gas mixture contains byproducts such as unconverted acrolein and / or propene, steam, carbon monoxide, carbon dioxide, nitrogen, oxygen, acetic acid, propionic acid, formaldehyde, further aldehydes.

[0034] Usually, the reaction gas mixture contains, based on the total reaction gas mixture:

[0035] 1 to 30% by weight of acrylic acid,

[0036] 0.05 to 1 % by weight of propene

[0037] 0.05 to 1 % by weight of acrolein,

[0038] 0.05 to 10% by weight of oxygen,

[0039] 0.05 to 2% by weight of acetic acid,

[0040] 0.01 to 2% by weight of propionic acid,

[0041] 0.05 to 1 % by weight of formaldehyde,

[0042] 0.05 to 2% by weight of aldehydes, and

[0043] 20 to 98% by weight, preferably 50 to 90% by weight, of noncondensable gases.

[0044] Generally, the reaction gas mixture is fed to the bottom or the lower region of the column.

[0045] In a preferred embodiment, the hot gas mixture is cooled directly or indirectly before feeding the mixture into a column. This can be effected by indirect cooling, for example by means of a gas cooler. The cooling can be carried out in a gas cooler, a quench or a flash vessel. During the cooling, the hot gas mixture is cooled to 50 to 300° C, in particular to 70 to 200° C. Alternatively, the hot gas mixture is integrated in the bottom of the column.

[0046] The invention is further illustrated by the appended drawing and the examples that follow.

[0047] Fig. 1 schematically shows a distillation apparatus useful for implementing the process according to the invention. According to Fig. 1 , the hot gas mixture is passed via line 101 into a quench cooler 102 and cooled by direct cooling with a high boiler fraction. The quenched mixture is fed via line 103 to the bottom region of the column 105. The condensed high boiler fraction for cooling the hot gas mixture is recycled to the quench via line 104.

[0048] Acrylic acid is withdrawn via side take-off 106.

[0049] The condensation of the water vapor from the gas stream fed in an upward direction is carried out by an external cooling circulation 107, 108, 109. Heat of condensation is removed externally by means of a heat exchanger 108 with, for example, water as coolant, by collecting and removing aqueous condensate via line 107, cooling the aqueous condensate and recycling a part of the cooled, aqueous condensate via line 109 to the column, while the other part is removed via line 111. Another part of the aqueous condensate is returned to the column as the reflux liquid via line 110.

[0050] The uncondensed gases are taken off at the top of the column via line 112.

[0051] A stabilizer, in particular phenothiazine or another stabilizer disclosed in EP-A-0 765 856, is added to the column at three trays, as indicated by 113.

[0052] Reference Example

[0053] The solubility of phenothiazine in acrylic acid with varying water contents was examined. The results are indicated in the table below:

[0054] Example 1

[0055] A distillation apparatus according to Fig. 1 is used for the recovery of acrylic acid from a product gas mixture obtained by a two-stage gas-phase partial oxidation of propylene with molecular oxygen under heterogeneous catalysis. The pre-quenched mixture is introduced into the bottom of a distillation column having 78 separatory trays.

[0056] Acrylic acid is collected on a chimney tray between trays 15 and 16 and withdrawn as a side-draw. Acid water is collected on a chimney tray disposed above tray 68 and partially returned to the column as the reflux liquid on tray 68. A solution of phenothiazine is added to the trays 47, 51, and 55.

[0057] The column is operated stably without phenothiazine precipitation and without fouling, even during periods of fluctuating working load and during start-up.

[0058] Comparative Example 2

[0059] In a set-up as described in Example 1, a solution of phenothiazine is added to either tray 47, or tray 51.

[0060] Phenothiazine precipitation and increased fouling is observed during periods of low working load (where reflux of acid water is increased) and during start-up.

Claims

Claims1 . A process for the distillation and / or fractional condensation of a mixture comprising acrylic acid, water, and noncondensable gaseous components, comprising feeding the mixture into a column having a plurality of separatory trays; withdrawing acrylic acid from the column via a side draw; condensing water vapor generated in the column to obtain an aqueous condensate, and returning part of the condensate as reflux liquid into the column; and metering a polymerization inhibitor to the column at a height between the height of withdrawing the acrylic acid and the height of returning the aqueous condensate; wherein the polymerization inhibitor is simultaneously metered to at least two trays.

2. The process according to claim 1, wherein the polymerization inhibitor is simultaneously metered to at least three trays, preferably to three trays.

3. The process according to claim 1 or 2, wherein the trays to which polymerization inhibitor is metered are spaced apart by at least one tray, preferably by at least two trays.

4. The process according to any one of the preceding claims, wherein the lowermost tray to which polymerization inhibitor is metered is spaced apart from the height of withdrawing the acrylic acid by at least 20 trays and the uppermost tray to which polymerization inhibitor is metered is spaced apart from the height of returning the aqueous condensate by at least 10 trays.

5. The process according to any one of the preceding claims, wherein the polymerization inhibitor is phenothiazine.

6. The process according to any one of the preceding claims, wherein additional polymerization inhibitor, preferably MEHQ, is metered in the aqueous condensate returned to the column as reflux liquid.

7. The process according to any one of the preceding claims, wherein condensing the water vapor comprises: providing a quench section in the column, collecting an aqueous condensate at the lower end of the quench section, partially circulating the aqueous condensate into the quench section through a circulation line, and partially returning the aqueous condensate to the column underneath the quench section as the reflux liquid.

8. The process according to claim 7, wherein the aqueous condensate is cooled before being circulated into the quench section.

9. The process according to any one of the preceding claims, wherein the mixture is a product gas mixture obtained by gas-phase partial oxidation of a C3 precursor of acrylic acid with molecular oxygen under heterogeneous catalysis, which mixture may be partially condensed.

10. The process according to any one of the preceding claims, comprising sensing the temperature on a tray and adjusting, depending on the temperature of the tray, the amount of polymerization inhibitor to be metered to the tray.

11. The process according to claim 10, comprising establishing a database having stored therein at least one reference temperature profile in the column and polymerization inhibitor solubilities as a function of temperature, and determining the amount of polymerization inhibitor to be metered to a tray, from data extracted from the database.

Images