Untitled record

Method for purifying a dissolved acrylic acid ore

Full description

Background of the invention

The present invention relates to a process for purifying a melt of raw acrylic acid which contains, depending on its weight, not including the water present in the melt of raw acrylic acid,

≥ 80% by weight acrylic acid and, as various impurities from acrylic acid, at least

≥ 100 ppm by weight acetic acid

≥ 10 ppm by weight propionic acid, in which the dissolved material from the raw acrylic acid is converted, under the influence of low temperatures, into a suspension of raw acrylic acid consisting of acrylic acid crystals and the remaining dissolved material, the amount by weight of impurities different from the acrylic acid in the crystals Acrylic acid is less than, and the amount by weight of impurities different from acrylic acid in the remaining dissolved material is greater than, the amount by weight of impurities different from acrylic in the dissolved material is the raw acrylic acid. If necessary, part of the dissolved material separates. The remaining crude acrylic acid suspension is kinetically evacuated. The acrylic crystals in the remaining crude acrylic acid suspension are evacuated from the remaining dissolved material in a wash column.

Acrylic acid, as is or in the form of salts or components thereof, is particularly useful for preparing polymers for a very wide range of applications (e.g., adhesives, superabsorbents, binders).

Acrylic acid is obtained, among others, by catalytic gas-phase oxidation of propane, propene and/or acrolein. These starting gases, diluted primarily with carrier gases, such as nitrogen, carbon dioxide (CO2) and/or steam, are passed in mixture form with oxygen at high temperatures and, on demand, ultra-high atmospheric pressure, over transition metal mixed oxide catalysts and are transformed. By oxidation to a product mixture containing acrylic acid. By condensing the product mixture or by placing it in a suitable absorbent material (such as water or a mixture of 70 to 75% by weight diphenyl ether and 25 to

30% by weight biphenyl), a basic separation of acrylic acid from the product gas stream can be done (see, e.g. 445 297 EP-A and German patent DE-B-2136396).

By removing the sorbent (and, if required, previously removing impurities with minimal solubility in the sorbent, by removing, e.g., air) by extraction and/or distillation separations (e.g., removing water from the sorbent by distillation, at a constant boiling temperature Or separation by extraction of the acid from the aqueous solution and subsequent removal of the extraction medium by distillation) and/or after using other separation methods, we always obtain acrylic acid, which is referred to here as acrylic acid.

raw acrylic.

This raw acrylic acid is not a pure product. Also, it contains a range of different impurities typical of a gas-phase catalytic oxidation preparation method. In particular, these impurities are low-molecular-weight acrylic and propionic aldehydes (typically, the total content of low-molecular-weight aldehydes in the crude acrylic acid to be processed according to the invention is ≥ 100 ppm by weight, based on the weight of the crude acrylic acid calculated as Anhydrous; as a rule, the above mentioned aldehyde content is ≤ 1% by weight), such as propionaldehyde, methacrolein, acrolein, n-butyraldehyde, benzaldehyde, crotonaldehyde and furfurals. Depending on how the raw acrylic acid is prepared, it may also contain water as an additional impurity. Another typical component of raw acrylic acids are polymerization inhibitors. These are added during the separation processes used to prepare crude acrylic acid, where they are intended to inhibit potential free radical polymerization of the alpha-,beta-mono-ethylene-unsaturated acrylic acid, and are therefore also referred to as process stabilizers. PTZ operates (dibenzo-l,4-thiazine), 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl

(MEHQ) and p-methoxyphenol (4-OH-TEMPO) feature prominently among acrylic acid process stabilizers and may form a component of raw acrylic acid either individually, in pairs, or as a mixture of three substances. Typically, the total amount of polymerization inhibitors contained in raw acrylic acid is 0.1 to 2% by weight, depending on the weight of the raw acrylic acid (water in the raw acrylic acid is not included).

Additional undesirable impurities of acrylic acid present in the condensed phase are acrylic oligomers (Michael aggregates) formed by Michael adding acrylic acid to itself and to the acrylic acid dimers formed therein. Although these compounds are usually not physically present in freshly prepared crude acrylic acid (usually, their amount by weight is <1.01% by weight, by weight of raw acrylic calculated as anhydrous), they are formed in it when the acrylic acid leaves The aforementioned ore for several long. For statistical reasons, the formation of acrylic acid dimers is of particular interest, while larger acrylic acid oligomers (trimers' tetramers, etc.) are formed only in trace amounts.

The total amount of other byproducts likely to be contained in raw acrylic acid is as a rule no more than 10% by weight, based on the weight of raw acrylic acid calculated as no

water.

In this application, crude acrylic acid is therefore understood to mean specifically that crude acrylic acid which contains, based on its calculated weight as anhydrous, ≥ 80% by weight acrylic acid,

From ≥ 100 ppm by weight to ≤ 15% by weight acetic acid,

From ≥01 ppm by weight to ≤5% by weight propionic acid,

Up to 5% by weight low molecular weight aldehydes,

Up to 3% by weight polymerization inhibitors

Zero to 5% by weight acrylic acid oligomers (Michael aggregates). Therefore, the term raw acrylic acid used herein also includes specifically that raw acrylic acid which contains, based on its weight calculated as anhydrous, ≥ 90% by weight acrylic acid,

From ≥100 ppm by weight to ≤5% by weight acetic acid,

From ≥ 10 ppm 0N by weight to ≤2% by weight propionic acid,

Up to 2% by weight low molecular weight aldehydes,

Up to 2X the weight of polymerization inhibitors

Zero to 3% by weight acrylic acid oligomers (Michael aggregates).

Also, the term raw acrylic acid used herein therefore includes that raw acrylic acid that contains, by weight calculated as anhydrous, ≥ 95% by weight acrylic acid,

From ≥ 100 ppm by weight to ≤3% by weight - acetic,

From ≥10 ppm by weight to ≤2% by weight propionic acid,

Up to 2% by weight low molecular weight aldehydes,

Up to 2% by weight polymerization inhibitors

Zero to 2% by weight acrylic acid oligomers (Michael aggregates). Depending on the amount of acrylic acid present in the raw acrylic acid, raw acrylic acids do not always contain water, but in some cases they may also contain up to 5% by weight, up to 4% by weight, or up to 3%. By weight water.

Apart from acrylic, the ingredients present in raw acrylic acids mentioned above, most of them are proven to be unhelpful while using raw acrylic acid.

When crude acrylic acid is used, for example, to prepare esters of C1-C8 acrylic alkanols, the corresponding propionic esters and acetic acid esters will form

Also in secondary reactions, which reduces the yield of acrylic esters required, depending on the amount of alkanol used. When acrylic esters formed in the presence of low-molecular-weight aldehydes are used for free-radical polymerizations, their low-molecular-weight aldehydes content generally has an unhelpful effect, for example, because they affect, for example, the induction time of the polymerization reactions, i.e., the time between... Reaching the polymerization temperature and actually starting the polymerization. Furthermore it,

They generally affect the degree of polymerization and may also cause damage to the colors of polymers.

The above disadvantages can usually also apply when raw acrylic acid is used directly as an acrylic acid source in polymerizers.

Acrylic acid sources used to produce superabsorbents (=water absorbing materials based on polyacrylic acid and its salts) are specifically subject to the requirement that they may contain a very small amount of diacrylic acid and a very small amount of dibenzo-l,4-thiazine, because these two components are not Both are desirable in the production of superabsorbents (specifically dibenzo-l,4-thiazine

It causes problems due to its strong inhibitory effect on free radical polymerization in the production of superabsorbents) or in the use of superabsorbents (superabsorbents are specifically used in the health sector (e.g., infant diapers); the final stage of producing a superabsorbent material consists of cross-linking as a base High-temperature surfactant; at the cross-linking temperatures used, the copolymerized diacrylic acid will at least partially dissociate (reverse Michael addition) with the formation of monomeric acrylic acid; Not likely, however, in this range of use.)

In practice, separation processes involving refining are used in particular for further purification of crude acrylic acid (see, for example, 926 722 EPA).

The disadvantage of these separation processes is that they require a large amount of energy, specifically to separate components that have a boiling point similar to acrylic acid (such as propionic acid), because the procedure has to be performed at high boiling ratios and/or with refining columns with a large number of theoretical plates. Therefore attempts have also been made, for example, to synthesize acrylic acid free from propionic acid and/or acrylic acid with a suitable modification of the gas phase oxidation process (see, e.g., JPA 35519 11 and EPA 253409). Furthermore, thermal stress generally causes free radical polymerization that is not required for acrylic acid in the case of pretreatment processes involving refining.

Alternatively, the procedure of crystallizing a melt has become increasingly used in the very recent past to prepare pure acrylic acid (see, for example, EPA 616 998). Very generally, the contaminated raw acrylic acid (dissolved substance) partially hardens by post. Depending on the phase equilibrium, the corresponding acrylic crystals have a lower impurity content than the remaining satellite solute. The separation effect described above and clearly limited by thermodynamic factors is weakened by the inclusion of a liquid during the crystallization process and by the residual solute that remains in contact with the solid following the solid/liquid separation. In order to achieve greater purities, the use of a combination of successive crystallization stages is often necessary, even in the case of easily soluble systems, i.e., the crystals produced in the first crystallization stage are melted again and subjected to an additional crystallization step, etc. The disadvantage of this successive procedure is that, at each stage the crystallization temperature must be removed upon freezing and supplied again upon melting.

next. This has a negative impact on the cost effectiveness of separations involving crystallization. In order to make economic use of the melt crystallization process, it is therefore very important to achieve very high purity of crystals separated by very few crystallization stages.

For the purification of raw acrylic acid soluble materials by crystallization, the prior field has overwhelmingly favored the use of layer crystallization processes (see, for example, pending German patent application 2606364, EP-A 648 520, EP-A 616 998 and EP-A 776875 ).

In layer crystallization processes, the crystals freeze in the form of coalesced, thermally bonded layers. Solid/liquid separation is accomplished simply by allowing the remaining dissolved matter to flow away. The pure crystals then melt. Essentially, a distinction is made between static and dynamic layer crystallization processes.

In static processes, the raw acrylic acid melt to be purified is introduced, for example, into a tube-pack or modified plate-type temperature exchange device and then partially hardens by slowly lowering the temperature on the secondary side. Following solidification, the remaining solute is discharged and then, with a slowly increasing temperature, first the more and then the less contaminated sections of the crystal bed are melted until finally the highly pure product is melted. This process is referred to in the literature as secretion. It is true that static crystallization processes achieve remarkable purity in a single crystallization stage in the case of crude acrylic acids. However, the typically low time-space throughput in static crystallization processes is not advantageous because, in static crystallization of a melt, heat transfer and mass transfer to the deposition surfaces occur only by free convection.

Typically in the dynamic layer crystallization of the acrylic acid melt material the raw material is convection driven by the acrylic acid melt material. This can be done by circulating the raw acrylic acid solute by pumping it through tubing with a complete cross-sectional flow (e.g. pending German patent application 6364 0 26), adding the raw acrylic acid solute as a lean membrane distillation (e.g. 998 616 EP-A) or by passing An inert gas in a tube filled with a dissolved substance or by mixing.

The disadvantage of purifying raw acrylic acid soluble materials by dynamic layer crystallization is that purification in one crystallization stage with large impurity contents of raw acrylic acid soluble materials is not sufficient, for this reason, 998 616 EP-A recommends using the combination of static and dynamic layer crystallization to purify acrylic acid soluble materials. raw. However, the drawback of this procedure is that it requires a photo

Necessary Collection of Crystallization Stages A certain improvement can be achieved by using washing, recommended in 709 708 3 DE-A, of the deposited crystal layers having purer sections of the solute. Due to the small specific surface area of ​​the deposited layers however, washing is not completely sufficient.

998 616 EP-A guarantees the possibility of using suspension crystallization to purify raw acrylic solutes by crystallization, but no consideration has been given to washing the separated suspension crystals to remove the remaining attached solutes. Instead, a combination with static crystallization stages has been recommended, which is not sufficient given the deterministic multistage nature of the procedure.

In the process of crystallizing a suspension, as a rule, a crystallized suspension is produced consisting of crystals with a lower content of impurities and a residual solute with a greater content of impurities produced by cooling the starting melt containing the impurities. Solid crystals may form while directly in suspension or may be deposited as a layer on a cold wall, from which they are then scraped off and resuspended in the remaining dissolved material, i.e., solid formation may be carried out in cold stirred boilers, in surface scraped heat exchangers or In tablet crystallization devices, according to the description, for example, in:

Chem.-Ing.Techn. 57 (1985) No. 2, 91-102.

The subsequent required separation of the dissolved material remaining from the crystallizer can be carried out initially in a purely kinetic manner by pressure separation, filtration and/or centrifugation, see, for example:

Chem.-Ing.Techn. 57 (1985) No. 2, 91-102.

The disadvantage of this procedure with purely kinetic separation of crystals and residual solutes is that, due to the residual solutes remaining attached to the crystals, the resulting purification in a single separation step is not sufficient in the case of crude acrylic acid solutes.

Therefore, previous order 19926082 DE-A recommends that the kinetically removed acrylic acid suspension crystals be further washed with a washing liquid containing acrylic acid, preferably the washing liquid used being a dissolved substance of acrylic acid in which the quantity by weight of the impurities different from the acrylic acid is less of impurity content corresponding to the kinematically separated suspension crystals to be washed.

The disadvantage of the washing method used in 19926082 DE-A is that its purification is not completely sufficient. It is possible that this is precisely due to the fact that the contact between the crystals to be washed and the washing liquid is not completely sufficient.

It is now generally known that, in the case of a slurry of suspension crystals, separation of the suspension crystals and the residual solute may also occur either comprehensively, or after aggressive kinetic separation (particularly before the use of a kinetic wash column) of the residual solute, with a suitable washing liquid in the column A wash in which the washing liquid is passed in the form of a counterweight to the suspension crystals.

Basically, the types of wash column (see Figures 1 to 4) are divided into those with forced transport to the bottom of the suspension crystal and those with gravitational transport of the suspension crystals (a detailed description of the different types of wash column, among others, is given in: 50 57 (1985). No. 2, 91-102, in Chemical Engineering Science Chem.-Ing.-Techn. 1995, No. 17, 2712 to 2729, Elsevier Science Ltd., in Applied Thermal Engineering,.17, (1997) No. 8. 10, 879-888, Published by Elsevier Science Ltd

And the notes mentioned in the references mentioned above). In wash columns with a forced suspension crystal bed transition, at least one force other than gravity is used in the direction of migration to move the suspension crystal bed.

Inside the washing column, the suspension crystals move either from top to bottom or from bottom to top. The washing liquid is passed in the form of a counter-current to the suspension crystals in the washing cycles. In publications

Previous 19626839 DE-A, 19832962 DE-A 19829477, DE-A. 19740252 DE-A, 19833049 A and 19838845 DE-A among others. Water or aqueous acrylic acid are recommended as washing liquid for use in raw acrylic suspensions. However, the disadvantage of these washing liquids is that, on the one hand, the protection is not completely sufficient and, on the other hand, it leads to a loss of acrylic acid to a reasonable extent.

As an alternative to the above procedure, it is also possible to dissolve the crystals of the suspension arriving in the wash column in pure form at the end of their travel distance (the original liquid is removed as a base in the opposite part of the wash column), removing only a portion of the resulting pure solute and recirculating the remaining solute The pure water is transported as a dissolved detergent to the washing column, and this is done in the form of a counter-current

For suspension crystals to be fed to the wash column (in this publication, wash columns operated in this manner are narrowly referred to as wash-dissolve wash columns). Depending on the physical characteristics of the crystal suspension to be processed in the washing column, purification can be achieved either entirely or solely on some of the different mechanisms listed below:

Replacing the remaining dissolved substance (the original liquid) with the dissolved washing substance,

Removal by washing of the remaining dissolved material layer adhering to the suspension crystals using the dissolved washing material.

Diffusion washing of a few areas/non-washing of areas with permeate flow between suspension crystals (eg those in contact over large areas) with the dissolved washing agent,

crystallization of dissolved washing agent; Recirculated in the washing column, onto the suspension crystals fed in the form of a counter-current,

Secretion of suspension crystals in contact with the dissolved washing agent.

Adiabatic recrystallization of suspension crystals in contact with dissolved washing material.

The last three of the purification mechanisms mentioned above are referred to here as additional purification mechanisms.

according to:

Chemical Engineering Science 50, (1995) No. 17, 2717-2729, Elsevier Science Ltd.,

The contribution of individual protective mechanisms depends, among other things, on the contact time between the suspension crystals and the dissolved washing agent and on the shape and composition of the suspension crystals. Basically, none of the above-mentioned sterilization mechanisms can be excluded because, due to the smaller melting point drop due to impurities, the melting point of the pure crystals is greater than the temperature of the still unwashed crystals, which essentially matches the equilibrium temperature of the crude acrylic acid suspension. .

In the case of gravity wash-dissolve wash columns, the suspension crystals move through the gravity wash column in the form of a counter-current to the dissolved wash, which has a lower density and therefore a lower special ground profile (and therefore rises in the wash column). A slowly rotating stirrer column (usually <0.035 revolutions per second) always extends over the entire gravity wash-dissolution column and serves to prevent caking and/or channel formation in the descending crystal bed. Duration of residence of suspension crystals in a wash column.

Gravity dissolution is ≥ 1 hour (the difference between the density of the liquid phase and that of the solid phase is ≤ 15%). Furthermore, the minimum porosity within the crystal bed in a gravity wash-dissolution column is typically <0.45, often <0.65. The original liquid leaves the wash column Gravity-dissolved, usually by overflow. The advantage of a gravity wash-dissolution column is the longer residence times of the crystals, which allows the use of additional strengthening mechanisms to a certain extent. according to:

Applied Thermal Engineering 17, No. 8-10, (1997), 879-888, Elsevier Science Ltd.,

The weak point of the gravity wash-dissolution column is the need for large crystals

relatively.

In the case of wash-dissolve columns that have a forced transfer to the bottom of the suspension crystal, a distinction must be made, for example, between pressure columns (also referred to as hydraulic columns), in which the crystals and the dissolved wash are transferred; For example; to the outside by pumps and/or a hydrostatic level. The original liquid is generally forced out of the wash column by a filter (on the other side of the filters, there may be atmospheric pressure, low pressure or ultra-atmospheric pressure), and motor columns with means of transferring the kinetic force to the bottom of the crystal, e.g. Special presses, stirring tools, screws, Spirals or spirals Washing columns Kinetic washing-dissolution is particularly suitable for purifying crystallized suspensions containing a small amount of residual dissolved material. The original slope is removed in kinetic wash-dissolve wash columns, as a rule likewise by means of filters which are located either behind or in the kinetic driven conveyance.

Wash-dissolve wash columns with forced crystal bottom migration have much shorter residence times for crystals in the wash column compared to a gravity wash-dissolve wash column. The stated residence time is ≤ 30 minutes and is usually 10 to 15 minutes, always 2 to 8 minutes. Bonus

Therefore, the minimum porosity (= pore size/total volume) inside the crystal bottom of a driven-transfer wash-dissolve wash column is usually ≤0.45 according to:

Chemical Engineering Science 50, No. 17, (1995) 2717-2729, Elsevier Science Ltd.,

The residence times of crystals in wash-dissolve wash columns with driven migration are too short to allow for the possibility of additional purification mechanisms.

82210-7 JP-A discloses a process for purifying raw acrylic by crystallization, in which a suspension of raw acrylic acid is first produced from the raw acrylic acid dissolved in the presence of water under the influence of low temperatures, the purpose of the presence of water being to produce the low temperatures required by evaporative mail. 82210-7 JP-A Remember that in passing only the presence of water affects the formation of acrylic acid crystals so that particularly large crystals are formed.

In 82210-7 JP-A, the resulting crude acrylic acid suspension is finally subjected to purification treatment by gravity wash-dissolve cycles. Although the purification that occurs in 7-82210 JP-A with a single purification stage is sufficient, the space-time throughput is not sufficient. 82210-7 JP-A also states that the presence of water during the production of a crude acrylic acid suspension has a beneficial effect on the purity of acrylic crystals washed in a gravity wash-dissolve column. Use of a gravity wash-dissolve wash column is also recommended in 730893 EP-A.

In order to adequately purify crude acrylic acid by crystallization in a single stage purification (specifically to sufficiently remove propionic acid and/or acetic acid impurities) WO 99/06348 recommends first adding a polar substance to the crude acrylic acid, then producing an acrylic acid suspension by low-temperature exposure and washing the suspension. Mentioned in Column Washing - Kinetic Dissolution.

A disadvantage of this procedure is that it requires the addition of a polar organic solvent to the raw acrylic acid.

From a study from:

M. Nienoord, GJ Arkenbout and D. Verdoes on Experiences with the TNO-Hydraulic Wash Column& at the 4th BIWIC 94/Bremen International Workshop for Industrial Crystallization, Bremen, Sep. 8th-9th, 1994, at the University of Bremen, Ed.: J. Ulrich,

It is known that hydraulic wash-dissolve columns are mainly suitable for the purification of acrylic acid suspensions. However, the above-mentioned notice does not contain information about the composition of acrylic acid suspension or its preparation.

In light of the prior art mentioned above, it is for the purpose of the present invention to provide an improved process for purifying raw acrylic acid dissolved materials, in one aspect, capable of providing high-purity acrylic acids with a large time-space yield in only one purification stage and, in the other aspect, not need

Pre-addition of a polar organic solvent to the crude acrylic acid, specifically in order to sufficiently remove propionic and/or acetic acid.

General description of the invention

We have discovered that this purpose is achieved by a process for purifying a raw acrylic acid melt which, depending on its weight, does not include the water present in the raw acrylic acid melt,

≥80% by weight acrylic acid and, as different impurities from acrylic acid, at least ≥ 100 ppm by weight acrylic acid and ≥ 10 ppm by weight propionic acid, in which the dissolved material cools the raw acrylic acid, under the influence of low temperatures , for a crude acrylic acid suspension consisting of acrylic acid crystals and a residual solute, the amount by weight of impurities different from acrylic acid in the acrylic acid crystals is less than, and the amount by weight of impurities different from acrylic acid in the remaining solute is greater than, the amount by weight As for various impurities from acrylic acid in the raw acrylic acid dissolved material, when necessary a portion of the remaining dissolved material is separated kinetically from the raw acrylic acid suspension and the acrylic acid crystals in the remaining raw acrylic acid suspension are evacuated from the remaining dissolved material in a wash column, where:

(a) Acrylic acid crystals are produced from a suspension of raw acrylic acid in the presence of 0.20 to 10% by weight, depending on the weight of acrylic present in the dissolved material, raw acrylic acid, water,

(b) The wash column is a wash column with driven migration of acrylic acid crystals and (c) the wash liquid used is the dissolved material of acrylic acid crystals purified in the wash column.

Brief explanation of the drawings

Figure 1 shows the wash bottom column with gravity.

Figure 2 shows a washing column with a hydraulic crystal bottom transmission.

Figures 3 and 4 show a washing column with a mechanical bottom transition.

Figure 5 shows a process program for the gas-phase oxidation of propane to acrylic acid and the processing of the resulting crude acrylic acid crystals.

Figure 6 shows a hydraulic washing column for processing acrylic acid crystals.

Figure 7 shows a device for regulating the filtration front of a hydraulic washing column.

Figure 8 shows a device for regulating the specified amount of washing media for the hydraulic washing column using optical reflection sensors.

Figure 9 shows the regulation of the specified amount of wash media for a hydraulic wash column using an optical extinction sensor.

Figures 10 and 11 show a design for the application of a drain pipe.

Figure 21 shows the design of the replacement tool in the device from Figure 10.

Detailed description of the invention

Basically, the new process is suitable for all of the raw acrylic acids mentioned in this publication.

In all cases, the production of acrylic acid crystals from a suspension of crude acrylic acid can be accomplished in the presence of 0.20 to 10, 0.40 to 8, 0.60 to 5, 0.60 to 3, 0.60 to 2, or 0.75 to 1.5% of Weight, based on the weight of acrylic acid contained in raw acrylic acid, water.

The preparation of raw acrylic acid can be slowed down so that it contains this amount of water as a result of the preparation. Usually, however, the preparation produces crude acrylic acid that is essentially or completely free of water. In these cases or in cases where the amount of water contained in the raw acrylic acid as a result of the preparation is not sufficient, it is of course possible, according to the invention, to bring the water content to the desired level by adding water before preparing the raw acrylic acid suspensions.

In preparing the crude acrylic acid suspension required according to the invention, the solid crystals can be produced in such a way that they are present directly in the suspension. However, it is of course also possible for it to be deposited as a layer on a cold wall, which is then scraped off and resuspended in the remaining dissolved material.

This means that, according to the invention, the solid can be formed in cold stirred kettles, in surface-scraped heat exchangers or in tablet crystallization devices, as described, for example, in:

Chem.-Ing.-Techn. 57 (1985) No. 2, 91-102. In very general terms, all suspension crystallization devices mentioned in the publications mentioned in the previous section in this publication are suitable for the new process of preparing crude acrylic acid suspensions. Specifically, suspension crystallizers from the companies below can be used as suspension crystallizers:

<img file="SA1862B1_D0001.tif" />

Suspension crystallizers can be operated with a refrigerant and feed the raw acrylic acid simultaneously and in a counter-current manner. The latter is the rule.

As a rule, the acrylic acid crystals from a suspension of raw acrylic acid to be produced according to the invention have a cube-like geometry. The aspect ratios of the above geometric objects are always as follows: Length (L): Width (W): Height (H) = 1 to 1:1:5. The length L is always from 10 to 100 µm, in many cases from 100 to 800 µm, or up to

400 micrometer.

On the way from the crystallizer to the washing column, it is usually convenient to homogenize the crystallization suspension (eg by stirring and/or by suitable pumps).

Suitable wash-dissolve wash columns according to the invention that may be used are both hydraulic wash columns, for example those of:

TNO Institute in Alepdoom, The Netherlands (cf. Applied Thermal Engineering 17, No. 8-10, (1997), 879-888, or Chemical Engineering Science 50, No. 17, (1995) 2717-2729, Elsevier Science Ltd. , or 4th BIWIC 94/Bremen International Workshop for Industrial Crystallization, Bremen, Sep 8th-9th, 1994 at the University of Bremen, Ed.: J. Ulrich, or Trans.;I Chem E, 72, Part A. September 1994, pages 695 to 702, and Applied Thermal Engineering 17, Nos. 8-10, (1997), 879-888, Elsevier Science Ltd.)

and kinetic washing columns, for example those from:

Niro, Process Technology BV, Hertogenbsch, The Netherlands.

According to the invention, all tanks of wash-dissolve wash columns having driven transfer of acrylic acid crystals described in publications noted as the prior art may be used very generally in this publication. For example, publications can be mentioned:

Chem.-Ing.-Techn. 57 (1985) No. 2, 91-102, and Chem.-Ing.-Techn. 63 (1991), No.

99/6348 9, 881-891, and Wo Wash-dissolve wash columns that have a driven transition described in the patents of TNO, Niro, or other companies are also particularly useful according to the invention (see, e.g., EPA 97405, U.S. Pat. EP-A, 193226, EP-A, 98/27240, EPA 920894, EPA 398437, 373720

316 And US Patent No. 4,787,985). It is not important according to the invention that TNO and Niro systems are always designed primarily for removing water from liquid feeds or for extraction processes.

The raw acrylic suspension to be produced according to the invention can be produced with an acrylic acid crystal content from 10 to 80, always from 20 to 60, and in many cases from 30 to 50./' by weight, depending on the total weight of the raw acrylic acid suspension.

Crude acrylic acid suspensions thus obtained, either as is or only following kinetic removal of a portion of the residual solute contained therein, may be subjected to a new wash column process. Suitable means for kinetic separation of the crystalline state are presses, sieves, centrifuges and filters. For example, belt filters, drum filters, Seiner screws and curved sieves can be used. Of course, decanting and sedimentation techniques are also suitable. Invariably, the kinetic separation of the crystal phase from a suspension of crude acrylic acid, according to the invention, is performed in such a way that the crystal phase remains scented wet with the residual solute. The acrylic acid crystal phase separated from the crude acrylic acid suspension still contains 5 to 30, or up to 10% by weight, based on the total weight of the acrylic acid crystal phase and the residual solute, of the residual solute. For additional new purification of crystal phases

For those wetted acrylic acids, kinetic wash-dissolve wash columns are suitable

private.

According to the invention, it is important that, in order to achieve adequate purification, it is not necessary to subject the kinetically separated crystalline phase herein above to resuspension before further purifying it in a wash-dissolve wash column with a driven transition, as recommended, for example in WO 98/25889. Of course, the aforementioned resuspension can in any case be done before using a new washing column purification step.

Always, the new purification step with a wash-dissolve wash column is done in such a way that the difference between the temperature of the raw acrylic acid suspension fed to the wash column and that of the dissolved wash returned to the wash column (i.e., the temperature difference) is from 2 to 15 C, always 2 To 10 Celsius or from 2 to 4 Celsius.

According to the invention, it is surprising that, despite the very much shorter residence time of acrylic acid crystals which is always from 5 to 25, often from 10 to 15 and in many cases from 2 to 1 to 4, or from 2 to 8 minutes, in columns The wash-dissolve wash used according to the invention has a driven transfer, purification equal to that of a gravity wash-dissolve column is achieved. This is due to the special crystalline characteristics of acrylic crystals, which have these characteristics due to their production in the presence of water. Furthermore, pure acrylic acid appears to be extracted from these crystals, in a particularly impressive manner, identifying the acrylic and/or propionic acid contained therein, which is probably the reason, for the excellent success of the new procedure. The new process is also amazing in that it is able to separate low molecular weight aldehydes, such as liquid aldehydes, from crude acrylic acid in an excellent manner.

According to the invention; It is important that the new process does not require the addition of a polar organic substance to the raw acrylic acid before preparing the raw acrylic acid, as considered necessary in WO 99/06348.

However, the new procedure can of course also be used in conjunction with the 99/6348 WO procedure. In this case, the preparation of the crude acrylic acid suspension is done in the presence of water and after adding an organic liquid to the crude acrylic acid to be purified.

The new process can of course also be performed in such a way that the crude acrylic acid suspension to be washed according to the invention is the result of a fractionation, for example of the crystallization of a fractional suspension. However, it is important according to the invention that such fragmentation is not necessary for successful purification.

Of course, the crystallization required according to the invention can be accomplished by indirect cooling, e.g. shell cooling, and/or direct cooling (e.g., use of a refrigerant, such as CO2 and/or

propane, and/or water evaporation).

It is particularly useful to apply the new process to crude acrylic acid produced by the procedure described in DEA 19909923.

Of course, the new procedure can also be used many times in a row. It is also possible to recycle the waste solute (original liquid) removed from the wash column in the new process back to the process for, for example, the preparation of crude acrylic acid to a column for fractional condensation of a gas phase oxidation reaction gas mixture, as recommended in various places in the literature. .

For example, the new procedure could be integrated as follows in processes for preparing acrylic acid published in DE-A, 19909923, DE-A 19924532, 19924533, DE-A 19833049, 19740253 DE-A, 19.740252 and especially DE-19627847. A (The schematic integration is shown in Figure 5 attached to this publication, to which we relate the following headings).

Next, propene or propane in an oxidation zone undergoes catalytic gas phase oxidation in anisotropic manner with molecular oxygen at elevated temperatures to give acrylic acid (e.g., propene (8) and atmospheric oxygen (9,1) are fed into the recirculated reaction gas (11) It has a small content of acrylic acid, and propene is oxidized in the gas phase at high temperature by catalyzing two successive oxidation stages (1, 2) over polymetal oxide materials containing Mo, Bi, Fe (first stage), Mo, and V. (Second stage).

The reaction gas (2 1) arriving from the oxidation zone and rich in acrylic acid (e.g. a multistage condensation column (3)) undergoes fractional condensation (the column always operates at a constant temperature), in which an acrylic acid-rich liquid fraction is formed (typically containing 0.2 to 0.1% by weight, depending on acrylic acid present, water). The water content can be fixed in

Raw acrylic acid, for example, by recirculating some of the dilute acid solution generated at the top of the sintering column, inside and/or outside the sintering column, to the raw acrylic acid removal tray.

This crude acrylic acid (15) is removed (eg by side removal from the condensation column) and fed for further purification (preferably without prior addition of foreign materials) to the suspension crystallization purification stage. The unused gas stream (13), which, in addition to air components, contains substantially water and other low-boiling substances, is separated from the gas stream with a low acrylic acid content that exits at the top of the suffocating column. At the bottom of the stagnation column, a liquid fraction (4 1) rich in a high-boiling substance is removed.

The unused gas stream and the high-boiling fraction can be further treated as described in the above publications. In the crystallizer (4), acrylic acid crystals are precipitated from the preferred pre-precipitated crude acrylic acid (16) by heat withdrawal, thus producing a suspension containing, for example, 20 to 40% by weight of acrylic acid crystals in the remaining original liquid. .

This suspension (7 1) is fed in unchanged form, preferably as is, to its wash-dissolve wash column, in which the acrylic acid crystals are substantially removed from their original liquid by filtration and washing in a countercurrent.

The washed acrylic acid crystals are dissolved. In a dissolution cycle (9 1). A portion (in the case of a hydraulic wash column, typically 20% to 30% by weight) of this material dissolved in the wash column is used as wash medium for countercurrent washing and, in unspecified cases, the wash column is left with the original liquid (8 1). The other part of the dissolved habit is removed as comparably pure acrylic acid.

Pre-quenching of raw acrylic acid (5 1) is conveniently done indirectly in heat exchangers (6) and (7). The removed heat is used advantageously to dissolve acrylic acid crystals in the dissolution cycle (19, 6) and conveniently to preheat the original liquid (18) recirculated to the retardant column (3).

The following compositions are typical for inlet and outlet streams as described: Raw acrylic acid (5 6/1 1): 97.2% by weight acrylic acid

4000 ppmw acetic acid 619 ppmw propionic acid

5000 ppm by weight liquid aldehyde 703 ppm by weight benzalehyde 1500 ppm by weight maleic anhydride

0 0 2 parts per million by weight phenothiazine 1.5% by weight water

Original liquid (18): 96.4% by weight acrylic acid

0 0 6 ppmw by weight acetic acid 744 ppmw by weight propionic acid 7000 ppmw by weight aldehyde liquid 925 ppmw by weight benzalehyde 2000 ppmw by weight maleic anhydride 263 ppmw by weight phenothiazine

1.9% by weight is water

Pure acrylic acid: 99.7% by weight acrylic acid

1030 ppm by weight acetic acid 225 ppm by weight propionic acid 7 ppm by weight aldehyde liquid 1 ppm by weight benzalehyde 2 ppm by weight maleic anhydride &lt;1 ppm phenothiazine

0.1% by weight is water

Stated otherwise, the process comprises heterogeneous catalytic gas-phase oxidation of propene or propane to a gas mixture containing acrylic acid, fractional condensation of this product gas mixture with the removal of a crude acrylic acid containing from 0.2 to 10% by weight water ( Based on the acrylic acid present), crystallize a single-stage suspension of raw acrylic acid to give a crystallized suspension containing from 2 to 40% by weight of acrylic acid crystals and then wash to give a suspension

Crystallization according to the invention in a wash column having a driven crystal bed transition typically allows for minimal device costs, - with acrylic acid containing ≥99.5% by weight acrylic acid.

Suitable cooling media for all the indirect cooling described in this publication are mixtures of ethylene glycol and water or methanol and water.

The washing columns suitable according to the invention for incorporation herein are, as mentioned above, of specifically two types:

(a) Washing columns with hydraulic transmission of crystal beds.

(b) Wash columns with kinetic transport of crystal beds.

The crystal volume fraction at the bottom of the crystal generally reaches &gt;0.6 in both types of washing times. As a rule, values ​​from 0.7 to 0.75 can be reached. Some of these types of washing columns are presented below.

(a) Suitable washing columns with hydraulic transmission to the bottom of the crystal:

Figure 6 attached to this publication schematically shows the design of a hydraulic wash column suitable for the described integration. The suspension (1) of acrylic crystals is fed into the original liquid, separated from the suspension crystallizer, by a pump (8) and/or through a hydrostatic level under ultra-atmospheric pressure in the wash column (7). A dual-function fluid register is placed at the top of the washing column. The suspension is distributed across the cross-section of the washing column through passages (4 2) from the top to the bottom of the column. The adhesive inner part of the fluid register (23) serves as a collection device for the removed liquids (the original liquid and the washing liquid (2)). Drain tubes (4 1) are placed down into the fluid recorder (the tubes have a fixed cross section inside the concentration area; viewed in the direction of the suspension feed, this is the area that reaches the first filter) and are connected to the inner part (23). The discharge piping shall be provided, at a specified height, with at least one conventional filter (5 1) through which the original liquid (4) is removed from the wash column (the original liquid may be at atmospheric, ultra-atmospheric or low pressure). It forms the bottom of a fused crystal (5). The bottom of the crystal is displaced by the force resulting from the loss of hydraulic flow pressure of the original fluid past the filters into the washing area below the filters. Recirculating part of the original liquid in the column by the control pump (3 1) allows this transition force to be regulated. Variations in crystal content in the feed suspension or changes in crystal size distribution, which substantially affect the flow pressure loss, can thus be compensated for.

These differences are evident from a change in the position of the filter front (7 1), which can be identified by position detection tools (18).

At the lower end of the washing column the crystals are removed from the crystal bed by a rotating blade (61) and resuspended in the pure dissolved product, which can be excessively inhibited by p-methoxyphenol (MEHQ). This suspension is transported in a melt circulator (2 1) through a heat exchange device (9), by which the heat required to dissolve the crystals is indirectly introduced and removed from about 70 to 80% by weight, in advantageous cases (e.g. with Apparent recrystallization) also from &gt;80 to 100% by weight of the dissolved crystals as a pure product (3) of the solute cycle. The amount of pure product removed is controlled by a product control valve (10).

By flowing the remaining part of the dissolved product material as a washing medium (6), in a direction opposite to the direction of the crystal bed migration, to the filters (5 1), as a result of which washing with an anti-crystalline current occurs in the washing area. Crystal purification depends essentially on the replacement and dilution of the original liquid in the crystal bed voids by the washing liquid. Dilution here depends on mixing in the spaces between the crystals, through which flow occurs, and diffusion at contact points where there is no permeate flow, or at the peripheral layer of flow near the surface of the crystals.

In steady-state operation, the washer front end (19) is anchored at a specified height in the wash area. A concentration shift between the original liquid concentration (above the wash front) and the concentration of a pure solute (below the wash front) occurs when the wash front rises. The washer front end (19) must be placed above the rotating blade (16) in order to achieve adequate filtration at a lower height. The location (91) is established as a dynamic equilibrium of a traveling crystal mass tunnel (5) and a countercurrent wash medium flow (6). The amount of wash medium results from the amount of pure product removed.

With good purity compared to raw acrylic acid, the crystallization temperature in the suspension crystallizer is only 3 to 4 K below the melting point of the pure product. In the wash front area, recrystallization of the wash liquid therefore occurs only to a small degree when the temperature of the cold crystals is in equilibrium with the wash liquid. This limits the recovery of dissolved wash material by recrystallization as well as reducing the porosity of the crystal bed below the wash front by recrystallization. That

The lower porosity of the crystal bed will reduce the amount of washing medium required as well as recrystallization extraction.

With the good purity of the crude acrylic acid, it is convenient to feed the stabilized methoxyphenol (MEHQ) for storage into the solute cycle (12) of the wash column. For this purpose, MEHQ, dissolved in a pure product, is added by means of a metering pump (22) to the relatively warm solute cycle for stabilization. Pass the MEHQ, along with the removed mixture of the original liquid and the dissolved washing agent (2), into the column used for fractional condensation (Figure 5, No. 3) and install the aforementioned column.

In order to ensure stable operation of the hydraulic washing column in the context of specific space-time productivity and consistently good purification, it is appropriate to compensate for external disturbance variables, such as: variations in the amount of suspension,

Variation of crystal content in the suspension, variation of crystal size distribution and

Concentration variations in the feed material and/or in the original liquid by regulation:

(a) Front end of filtration (Figure 6, No. 17),

(b) The specific amount of washing medium (Figure 6, No. 6) and (c) The temperature of combination (Figure 6, No. 12).

(a) Front-end layout of the filter (headings used relate to Figure 7 attached to this publication)

Fixed position of the front end of the filtration. It confirms that the balance of an external mass of the washing column remains maintained at all times. It is preferable to determine its location using four visual reflection sensors (18) placed at specific heights in the wall of the column. Determination of the location by a line camera through a corresponding window in the wall of the column or radiometric reflection methods are suitable as additional possible methods of identification. Reflection methods are based on the fact that the intensity of reflected radiation depends on the position of the edge of the bottom. The line camera shows the full focus and wash area in a vertical line. The front end of the filtering is evident from a change in intensity in the line signal. The amount of original fluid recycled is regulated (0 3) and can be varied by the control pump (3 1), for example by a change in speed. When the crystal strength increases, the control bar increases (as a result the pressure drop increases); When the bottom drops, it...

It decreases. It is preferable to change the magnitude of the control current not suddenly but steadily, for example linearly as a function of time.

(b) Regulating the special amount of washing medium for the front of the washing)

The specific amount of wash media is the amount of wash media that depends on the stream of pure product effectively leaving the wash column and which must be used to achieve a specific separation effect. We relate to the following headings Figures 7 through 9 included with this publication.

Adjusting the front washer (19) below the filters (15)

The front end of the washing machine (9 1) is adjusted by regulation (No. 29 in Figure 7 or No. 31 in Figure 8)

Amount of washing medium by the product valve (10) to a specific position between the filter (5 1) and the rotating blade. In this procedure, the separation task is performed with minimal use of washing medium.

The front end of the wash may be specified, for example, with four or more optical reflection sensers (Figure 7, No. 20) or preferably with four or more temperature sensers arranged at the bottom of the crystal (Figure 8, No. 52).

Control concept 2:

External balance for the specific amount of washing medium

The specific amount of washing aid is set as a specific ratio to the amount of crystals fed, and this ratio must be estimated randomly. The ratio must be chosen to be large enough to ensure that the front end of the wash is firmly established. This is achieved by adjusting an excess of washing medium (based on Control Concept 1). The front end of the washer (9 1) is attached to the filter (in the area of ​​\u200b\u200bthe lower edge of the filter to the middle of the filter) (see, Fig. 9, which is attached to it. Also the following headings). Ratio regulation (34) depends on the flow measurement of the suspension (33), the crystal content of the suspension (32) and the amount of crude acrylic acid removed. The amount and speed of washing are controlled indirectly by the amount of raw acrylic acid product removed (33) = the amount of crystals - the amount of wash medium. To retain and control purification, for example the quality (27) of the raw acrylic acid product can be monitored. For this purpose, the measurement can be made with an optical polarization dark sensor in the 450 nm spectral range directly in

Product line or in the side link (presumably it detects discoloration due to phenothiazine which does not

still exists). The quality is measured in the solute cycle line and can also be used to start the washing column. Control Concept 2 is simpler to achieve but requires more washing media compared to Control Concept 1.

(c) Organization of the union part (titles relate to Figures 7 to 9)

Ensure that the correct amount of heat is introduced into the melt cycle to dissolve the crystals by regulating the temperature of the raw acrylic acid product (28) after a heat exchange device (9). The temperature in the melt cycle may be about 1 to 5 K above the melting point of the raw acrylic acid product.

The rotary blade (26) is operated conveniently at a constant speed (0 2 to 60 rpm).

Additional information on the organization of the front apron for filtering with a line camera and on the external balance of the special amount of wash medium in a hydraulic wash column is provided in 880 036 10 DE-A and 881 036 10 DE-A.

Other front end and union temperature regulations that must be taken care of during the operation of a hydraulic washer shaft according to the definition are described in detail, for example in:

Trans I Chem E, Vol. 72, Part A, September 1994

And in articles:

Hydraulic Wash Columns, Solid-liquid Seperation in Melt Cystallization by Lianne Von oord-knol, Technical University of Delft, Jun. 13, 2000 (ISBN 90-805709-1-5) or Fractional Suspension Crystallization of Organic Compounds by Pieter Johannes Jansens, Technical University of Delft Apr. 5, 1994 (ISBN 90-370-0097-5).

Procedure for starting the operation of the hydraulic washing column (titles relate to Figure 6) The suspension (1) arriving from the crystallizer is fed into the washing column (7) under high atmospheric pressure by means of a pump (8) or by hydrostatic pressure. In the washing column, one or more drain pipes (4 1) are arranged, each of which is provided at a specified height with at least one (5 1) filter, through which the original liquid (4) is removed from the washing column. When the washing column starts to operate, it is removed first

Only the original liquid and the remaining crystals in the washing column form a stable bottom with a porosity of about 30 to 40% by volume. The pores between the crystals are completely filled with the original liquid. Once a specific bed height (17) is reached, the rotating blade (16) located at the lower end of the washing column is rotated, and its task is to remove the crystal bed homogeneously at its lower end. The separated crystals pass by scraping under the rotating blade in the solute cycler (2 1) which has a circulation pump (1 1) and a free replacement device (9). The heat required to melt the crystals is provided by a heat exchange device (9). In order to ensure the conductive temperature gradient required for the melting process, the product temperature at the outlet of the heat exchanger (9) is set at about 1 to 5 K above the melting point of the pure product. The dissolved substance is primarily a mixture of dissolved crystals and an original liquid, with a mixing ratio that matches the porosity of the bottom of the crystal. The product control valve (01) remains closed initially and; Because the suspension is fed continuously, the solute (6) has a lower density and inevitably flows in a direction opposite the bottom of the crystal moving downward, toward the filter. As a result, crystal leaching occurs.

The original impure liquid transported in the pores at the bottom of the crystal is removed by washing in the form of a countercurrent by the purer dissolved substance, and the washing liquid (6) escapes through the filter (5 1). As a result, the purity of the bottom of the blouse in the washing zone, the area between the rotating blade (6 1) and the filter (5 1), now increases dramatically. Once the purity of the product (for measurement see Figure 7, No. 27) has reached its highest level, the product control valve opens to the point at which about 70 to 80% of the weight is left, and in preferable cases &gt;80 to 100% of the weight is left. ; of the dissolved material is washed off the column as a pure product and only the remaining portion (always about 20 to 30% by weight) is used as the wash medium.

(6) in the described manner and flows toward the filter. The washing column is now in steady state operation.

Design of drainage pipes (see attached figures 10 and 11)

Drain tubes serve to remove the original liquid and wash liquid from the bottom of the crystal. All drain pipes in hydraulic wash columns usually have the same design. It is conveniently composed of a combination of components that perform the following functions: conveying the filtration fluid

Heat insulation

Heat conduction replacement

In a simple model (Figure 01), the drain tube consists of a vacuum tube (36) which removes the original liquid and wash liquid removed from the filter (37) from the bottom of the crystal to the fluid register (see No. 23 in Figure 6) and serves as a replacement device (38 ) in maintaining the cross-sectional structure of the crystal bottom after the filter (37). In steady-state operation, the washer front is located at a height below the filter (37) in the replacer area (38). In order to inhibit cooling of the replacement device (38) below the wash front, by conducting heat from the warm area below the wash front to the cold area above the wash front, it is preferable to make said replacement device of a heat-insulating material such as Teflon. This prevents the replacer (38) from cooling to a temperature below the melting point of the wash medium (=pure product) and the wash medium from crystallizing below the wash front on the replacer (38).

Basically, drain pipe filter elements can be designed as shown in Figures 10 and 11 (37, 39, 41), as small-hole sieve filters or perforated sieve filters (see Figure 1, 1,

No. 43).

When the crystals at the bottom of the crystal form a wedge-shaped structure that the washing liquid can reach very poorly, it may be useful to design the replacement device, as shown in Figure 12, No. 44, in such a way that it expands down into a conical shape. As a result of this change in the cross section along the path of the crystal bottom, the latter becomes accustomed to an increasing shear stress, which leads to a relative movement of the crystals and thus the wedge edge of the crystal is exposed to the washing liquid. In other words, drain pipes generally have a constant cross section.

The length of the filter is usually 1 to 3 times the diameter of the drain pipe. The length of the drainage pipe concentration area is always from 2 to 50 cm. As a rule, the length of the washing area is 3 to 10 times as long as the distance from the drain pipe to the washing column housing.

In other words, the titles in Figures 5 to 9 accompanying this publication have the following meanings: Figure 5:

0 5 = oxidation stage 1

1 5 - Oxidation stage 2 52 = Condensation column 53. Suspension crystallization device 4 5 = Hydraulic washing column 55 = Heat exchange device for heat of combination 56 = Precooler for raw acrylic acid or preheater for the recirculated original liquid 57 = Propane feed

58=Air supply 1 59=Air supply 2

0 6 = recirculation gas with low acrylic acid content 1 6 0 recirculation gas rich in acrylic acid 62 = unused gas 63 = large boiler outlet 64 = side removal site for raw acrylic acid 65 = raw acrylic acid cooled in preparation for suspension crystallization 66 = Crystal suspension for hydraulic washing column 67 = Original liquid outlet

68 = Dissolution cycle of raw acrylic acid product from a hydraulic wash column 69 = Removal of raw acrylic acid product 70 = Original recirculated pre-stored liquid Figure 6:

1 = Crystal suspension feed 2 = Original liquid removal 3 = Pure acrylic acid product 4 = Internal original liquid stream 5 = Moving crystal bed 6 = Dissolved washing material

7 - Washing column 8 = suspension pump

9 = Heat exchange device for melting crystals

10=Control valve to adjust the proportion of dissolved detergent/remove product - pure acrylic 11=Circulation pump for dissolved substance cycle 12=Dissolved substance cycle 13=Control pump

14 = Drain tube for original liquid and washer liquid 15 = Filter

16 = Rotary blade for resuspension of washed crystals 17 = Filtration front (upper limit of the bottom of the crystal) 8 1 = Determination of the filtration front (4 optical reflection sensors) 19 = Washing front (liquid phase pure-impure concentration conversion) 20 = Determine the front of the wash (4 optical reflection sensors)

1 2 = inhibitor solution (MEHQ in pure acrylic acid product) 22 = metering pump for inhibitor solution

23 = Fluid register: Collection tray for original liquid and washing liquid 24 = Fluid register: Dispenser tray for crystal suspension 25 = Locating the front end of the wash (4 temperature sensers) Figures 7 and 8:

Such as Figure 6, and additionally:

26 = Rotary blade speed (10 to 60 rpm) 27 = Control of the quality of pure acrylic acid (measurement of optical extinction at 450 nm) 28 = Temperature regulation in the solute cycle (4 1 to 25 C) 29 = Location regulation 4- Points for the front end of the washing (visual locating) 30= Positioning 4- Points for the front end of the filtering (visual locating) 31= Positioning 4- Points for the front end of the washing (thermal locating)

Figure 9:

Such as Figures 7 and 9, and additionally:

32 0 Measurement of the density of the crystal suspension (crystalline portion of the suspension) 33= Measurement of the mass flow of the suspension

34 = Ratio regulation (mass of washing medium = factor x mass of crystals removed) 35 = Regulation of mass flow of washing medium; Setpoint value 34 Figures 10 to 12:

36 = discharge tube

37 = Filter for original liquid and wash media; Designed here as a small orifice filter 38 = Replacement device made of heat-insulating material (such as Teflon); It has a cylindrical design here 43 = filter design with hole geometry 44 = replacement tool with a conical design

The hydraulic wash column is preferably driven using pulsating fluid streams. These are flowing streams whose flow rate, but not direction, varies periodically as a function of time. It can be observed in a simple way, for example by periodically varying, as a function of time, either the flow rate of the dissolved detergent stream fed into the wash column or the amount...the number of bar flows of the original liquid removed from the wash column. Moreover, the flow rate of the suspension stream fed to the washing column can

It varies periodically as a function of time.

(b) Suitable washing columns with kinetic transport to the bottom of the crystal

Wash columns suitable for the described integration and having kinetic crystal bed transfer differ substantially from hydraulic wash columns in that the crystal bed transfer is accomplished by a kinetic device (eg a screw-blade rotor or oscillating piston), and so they do not have drain pipes. A conventional filter that prefers the original fluid is usually located either in the transmission (eg in the case of an oscillating piston) or behind the transmission (eg in the case of a screw-blade rotation). Schematic drawings of kinetic washing columns suitable for the described integration are shown in Figures 3 and 4 attached as an addendum to this patent.

The preferred material for the washing columns used according to the invention is stainless steel, specifically grade 1.4571 stainless steel. This1 also applies to filters.

The polymerization of the pure solute removed from the washing column is inhibited by a self-identified method by adding polymerization inhibitors. In the case of particularly high purity of acrylic acid crystals present in the crude acrylic acid suspension, the polymerization inhibitor is added by a self-identified method as early as possible when it becomes dissolved. This may be done, for example, by a procedure in which the monoethyl ether of hydroquinone is dissolved in the pure soluble product. (acrylic acid) (e.g. in an amount of 1000 millionths of a weight or more, depending on the solution) is added to this solution during dissolution (see for example 776875 EP-A).

The lowest porosity at the bottom of the crystal in the wash-dissolve column used according to the invention is usually ≤0.45, always from 0.15 to 0.35.

When using, according to the invention, a hydraulic wash-dissolve column (e.g. a column conforming to EP-A, 97405, 437 398 or U.S. Pat. 4735781

As mentioned earlier, the hydraulic suction as a rule is from 0.1 to 10, often from 1 to 3 bar. According to the invention, pulsed wash columns may also be used or the wash column may be operated with currents flowing in the form of pulses, as described, for example, in 97405 EP-A. As previously mentioned, Figures 2 through 4 illustrate the basis for some suitable wash-dissolve wash columns according to the invention (Figure 2 = hydraulic bed transfer, Figure 3 and Figure 4 = kinetic bed transfer and Figure 1 = gravity bed transfer).

The numbers 1 in these figures have the following meanings:

1: Pending

2: Residual dissolved material (original liquid) 3: Product (dissolved pure crystals) 4: Impure residual dissolved material 5: Moving crystal bed 6: Washing liquid (dissolved) 7: Washing column.

8: Suspension pump

9: A heat exchange device to dissolve crystals 10: A control valve to adjust the ratio of washing liquid (dissolved) to the product 11: A circulation pump for the dissolving substance cycle 2 1: A dissolving substance cycle 13: A stirrer 14: A filter tube 5 1: A filter

6 1: Rotary knife for resuspension of washed crystals 7 1: Oscillating press with end filtration surface and discharge of residual dissolved material 18: Rotary tool with threaded blade for transferring the crystal bed 19: Cylindrical replacement tool

Examples

By analogy to Example 1 in 19909923 DE-A, 150 kg/h of acrylic acid yields a raw material with the following composition:

Acrylic acid 98.5% by weight

Acetic acid 0.8% by weight

Propionic acid 500 ppm by weight

Liquid aldehyde 4800 ppm by weight

Maleic anhydride 40 ppm by weight

Benzaldehyde 680 ppm by weight

Water 1.5% by weight

Phenothiazine 0 0 2 parts per million by weight

The raw acrylic acid is fed into a suspension crystallizer. The suspension crystallization device is a cooling disc crystallization device (internal volume 100 L). The heat of crystallization is removed through the cooling surfaces of the container. The equilibrium temperature of the residual solute is 9.5°C. In independent experiments, the crude acrylic acid suspension obtained in crystallization (solids content approximately 20% by weight) is:

(a) Washed in a gravity wash-dissolution column;

(b) Washed in a hydraulic wash-dissolve column;

(c) Separate in batches into crystals of the original liquid in a centrifuge (0.80 specific gravity).

At a centrifugation time of 30 seconds. The crystals are then washed with dissolved (pre-washed) crystallizer (at a mass ratio of 1 part wash medium to 5 parts crystallizer) for 0.3 seconds at 80 specific gravity.

The vapor reflux ratio (the ratio of the amount of pure dissolved material removed per unit time to the amount of dissolved laundry material recirculated per unit time) and the difference in the temperature of the washing column are chosen - they are almost the same - the analysis of the washed crystals is as follows for cases (a) to (c). ):

<img file="SA1862B1_D0002.tif" />

We obtain better purity with methods (a) and (b) than with (c); The results of methods (a) and (b) are essentially indistinguishable. In particular, the crystalline acrylic acid obtained from (a) and (b) contains the least acetic acid.