A METHOD OF PREVENTING POLYMER TAR BUILD-UP IN ACH PRODUCTION OF MMA AND MAA

Abstract

A method of preventing polymer tar build-up in ACH production of MAA and/or MMA is described. The method is characterised in that one or more surfactants are contacted with the hydrolysis and optional esterification stage reaction medium, the said surfactants are selected from: a) C.sub.10 to C.sub.30 alcohol ethoxylates with an average of 5 to 100 ethylene oxide units per molecule; b) alkyl, hydrogen, O[CH.sub.2CH.sub.2O].sub.xH and/or O[CH.sub.2CH.sub.2CH.sub.2O].sub.x H N-substituted alkylene di- or triamines with an average of 1 to 30 total ethylene oxide and propylene oxide repeating units per molecule and wherein x and x are from 1 to 30; and c) C.sub.10 to C.sub.30 alcohol ethoxylate, propoxylates with an average of 5 to 100 total propylene oxide and ethylene oxide units per molecule, which units may be in a random, block or alternating sequence or may be a combination thereof. The method is particularly useful for preventing build-up of oligomer and polymer tar-like deposits in reaction vessels, process equipment, pipework or other parts of the acetone cyanohydrin MMA and MAA production process.

Claims

1. A method of preventing polymer tar build-up in ACH production of methyl methacrylate (MMA) and/or methacrylic acid (MAA) in which one or more surfactants are contacted with a hydrolysis and optional esterification stage reaction medium, the one or more surfactants being selected from: a) C.sub.10 to C.sub.30 alcohol ethoxylates with an average of 5 to 100 ethylene oxide units per molecule; b) alkyl, hydrogen, O[CH.sub.2CH.sub.2O].sub.xH and/or O[CH.sub.2CH.sub.2CH.sub.2O].sub.xH N-substituted alkylene di- or triamines with an average of 1 to 30 total ethylene oxide and propylene oxide repeating units per molecule and wherein x and x are from 1 to 30; and c) C.sub.10 to C.sub.30 alcohol ethoxylate, propoxylates with an average of 5 to 100 total propylene oxide and ethylene oxide units per molecule, which units are in a random, block or alternating sequence or a combination thereof.

2. The method according to claim 1 wherein the surfactants are defined as: a) C.sub.10 to C.sub.30 alcohol ethoxylates with an average of 5 to 100 ethylene oxide units per molecule represented as formula I
RO[CH.sub.2CH.sub.2O].sub.nHI wherein R is a C.sub.10 to C.sub.30 linear or branched alkyl group, and n is on average 5-100, b) alkyl, hydrogen, O[CH.sub.2CH.sub.2O].sub.XH and/or O[CH.sub.2CH.sub.2CH.sub.2O].sub.XH N-substituted alkylene di- or triamines represented by formula II ##STR00004## wherein ##STR00005## wherein R is a C.sub.2H.sub.4 (ethylene) or C.sub.3H.sub.6 (propylene) group, each R is alkyl, hydrogen, O[CH.sub.2CH.sub.2O].sub.vH or O[CH.sub.2CH.sub.2CH.sub.2O].sub.vH, wherein at least one R group, but not more than two R groups, is alkyl, preferably branched or linear C.sub.10 to C.sub.30 alkyl, and wherein at least one R group is O[CH.sub.2CH.sub.2O].sub.vH or O[CH.sub.2CH.sub.2CH.sub.2O].sub.vH, and wherein the total average number of [CH.sub.2CH.sub.2O] and [CH.sub.2CH.sub.2CH.sub.2O] repeating units per molecule of alcohol is from 1 to 20, and wherein v and v are from 1 to 20 with the proviso that the total average number of such units does not exceed 20; and/or c) C.sub.10 to C.sub.30 alcohol ethoxylate, propoxylate is represented by general formula III
R-O[CH.sub.2CH.sub.2O].sub.n[CH.sub.2CH.sub.2CH.sub.2O].sub.mHIII where R is a linear or branched C.sub.10 to C.sub.30 alkyl group and n+m is on average 5-100 per molecule, wherein the CH.sub.2CH.sub.2O and CH.sub.2CH.sub.2CH.sub.2O units are in a random, block or alternating sequence or a combination thereof.

3. The method according to claim 1, wherein the substituted alkylene di- or triamine b) is of formula IIa ##STR00006## wherein R is derived from tallow fatty acid such as tallow alkyl and x+y+z is on average 5-15 per molecule.

4. The method according to claim 1, wherein the surfactant c) has an average of 20 to 50 total ethylene and propylene oxide units per alcohol and the alcohol group is a C.sub.13 to C.sub.15 alcohol represented as C.sub.13 to C.sub.15 alkoxy group.

5. The method according to claim 1, wherein in surfactant c), 1-99% of the units are propylene oxide units.

6. The method according to claim 2, wherein in surfactant c), n:m is between 1:4 to 1:19.

7. A method of producing methyl methacrylate (MMA) or methacrylic acid (MAA) comprising the steps of: converting acetone cyanohydrin to methacrylamide (MAM) using concentrated sulphuric acid and thermal treatment; hydrolysing the MAM to MAA; and optionally, esterifying MAA to MMA using methanol, wherein one or more surfactants are contacted with a hydrolysis and optional esterification stage reaction medium, the one or more surfactants being selected from: a) C.sub.10 to C.sub.30 alcohol ethoxylates with an average of 5 to 100 ethylene oxide units per molecule; b) alkyl, hydrogen, O[CH.sub.2CH.sub.2O].sub.XH and/or O[CH.sub.2CH.sub.2CH.sub.2O].sub.XH N-substituted alkylene di- or triamines with an average of 1 to 30 total ethylene oxide and propylene oxide repeating units per molecule and wherein x and x are from 1 to 30; and c) C.sub.10 to C.sub.30 alcohol ethoxylate, propoxylates with an average of 5 to 100 total propylene oxide and ethylene oxide units per molecule, which units are in a random, block or alternating sequence or are a combination thereof.

8. The method according to claim 7, wherein the hydrolysis or esterification stage reaction medium comprises a concentrated sulphuric acid solution containing MAM, streams of fresh water, optionally methanol and/or optionally recycle streams such as waste water or streams from refining stages of the process.

9. The method according to claim 7, wherein the one or more surfactants are added to the process in liquid form.

10. The method of claim 7, wherein addition of the one or more surfactant within a hydrolysis or esterification medium vessel is achieved by separate addition to the reaction medium, optionally, at the same time as one or more of the other process streams or a via a mixer placed in one of the other incoming process streams such as an in-line static mixer.

11. The method of according to claim 10, wherein a suitable other incoming stream is the fresh water and/or methanol stream or a recycle stream from the refining stage.

12. The method according to claim 2, wherein the substituted alkylene di- or triamine b) is of formula IIa ##STR00007## wherein R is derived from tallow fatty acid such as tallow alkyl and x+y+z is on average 5-15 per molecule.

13. The method according to claim 2, wherein the surfactant c) has an average of 20 to 50 total ethylene and propylene oxide units per alcohol and the alcohol group is a C.sub.13 to C.sub.15 alcohol represented as C.sub.13 to C.sub.15 alkoxy group.

14. The method according to claim 2, wherein in surfactant c), 1-99% of the units are propylene oxide units.

15. The method according to claim 4, wherein in surfactant c), 1-99% of the units are propylene oxide units.

16. The method according to claim 13, wherein in surfactant c), n:m is between 1:4 to 1:19.

17. The method according to claim 14, wherein in surfactant c), n:m is between 1:4 to 1:19.

18. The method according to claim 9, wherein the liquid is either a substantially pure liquid at the temperature of addition or in the form of a solution.

Description

EXAMPLES

[0050] In the case of the present invention, candidate surfactant types were screened using a series of lab scale tests. Descriptions of the tests used, and the type of results gained were as follows:

Example 1: Miscibility with the Liquid Medium and Interaction with Polymer Tar

[0051] A straightforward visual observation test was used, in which a 100 ml glass beaker containing spent acid at 120 C. was placed upon the stage of a low powered microscope, and observations made when a 5% solution of polymer tar in concentrated acid was added first to spent acid as a control experiment, and then to spent acid containing a small amount of surfactant. In the control experiment, and those experiments where the candidate materials were clearly immiscible with the spent acid medium, or showed no positive interaction with the polymer tar, the tar would quickly agglomerate forming large particles, or stick to the side of the glass beaker. In those experiments where the candidate surfactants showed a positive interaction with the polymer tar, the tar would remain well dispersed in the spent acid in the form of small droplets, and would not stick to the walls of the glass beaker. A very large number of materials were subjected to this test. Some illustrative examples are shown in the table 1, below, in which it can be seen that only three candidate materials were found which were miscible with the hot BPA liquid medium, and also showed a sufficiently positive interaction with polymer tar to be included in subsequent tests.

TABLE-US-00001 TABLE 1 Initial Screening of Candidate Materials Miscibility with Interaction with BPA Medium Polymer Tar Candidate Material (Yes or No) (Yes or No) Alkyl substituted Y N polyethylenediamines Tallow substituted Y N ethylene diamine Coconut oil N N Palm Kernel oil N N Nonylphenol ethoxylate, Y N average (av) 25 ethylene oxide (EO) units per alcohol Mixed alkylbenzenes, N N b.p. >150 C., (an industrial solvent) Alkyl-ether-carboxylic N N acid C7 alkyl substituted Y N phenylenediamine polyethylene Y N oxide/polypropylene block copolymer Alkylphosphate ester N N Alkylethoxylatephosphate N N esters Branched C13-15 alkyl Y Y ethoxylate (av 7 EO units per alcohol) N - ethoxylated, N - Y Y tallow substituted propylene diamine (av 10 EO units per molecule Branched C13-15 alkyl Y Y ethoxylate/propoxylate (av 20 to 30 PO/EO units per alcohol, of which 90% are PO)

Example 2: Foaming of Spent Acid

[0052] A purpose designed foam measurement device was used, which comprised a graduated glass tube with a sintered glass sparging device mounted within it such that the glass tube could be filled with liquid, and compressed air sparged in below the surface of the liquid. In the control case of spent acid with a few drops of polymer tar added, the flow of sparging air was adjusted until it caused a fixed height of foam to form above the liquid in the tube. To achieve the desired assessment, one drop of a candidate surfactant was then added to a foaming control experiment, and once the foam height had changed the new foam height was noted. Candidates which increased the height of the foam in the system were rejected. Results for the three successful candidate materials from Test 1 are shown in Table 2, below, in which it can be seen that all three candidates actually behaved as antifoam agents in the hot spent acid system.

TABLE-US-00002 TABLE 2 Results of Foaming Experiments Increase or Decrease of foam height Candidate Material on addition to spent acid medium Branched C13-15 alkyl ethoxylate Decrease (av 7 EO units per mol) N - ethoxylated, N - tallow Decrease substituted propylene diamine (av 10 EO units per molecule) Branched C13-15 alkyl Decrease ethoxylate/propoxylate (av 20 to 30 PO/EO units per alcohol, of which 90% are PO)

Example 3: Corrosion

[0053] Coupons of the materials of construction of the vessels and equipment used in the hydrolysis and esterification stages of the process were suspended in stirred spent acid, with added polymer tar, at process temperature, at lab scale in glass equipment. To achieve the desired assessment of the surfactants, the candidates were each added to spent acid in separate experiments at a level of 1% w/w. After 1 week of stirring at process temperature the coupons were removed, rinsed and examined microscopically for any signs of corrosion damage. The results are shown in Table 3, below, in which it can be seen that none of the candidate materials caused an observable effect on corrosion of the coupons.

TABLE-US-00003 TABLE 3 Corrosion Experiments Observations upon microscopic examination of coupons after exposure Candidate Material to spent acid Control experiment, no Slight very slight surface roughening surfactant present present, some black staining visible Branched C13 alkyl ethoxylate As control (av 7 EO units per alcohol) N - ethoxylated, N - tallow As control substituted propylene diamine (av 10 EO units per molecule) Branched C13 alkyl As control ethoxylate/propoxylate (av 20 to 30 PO/EO units per alcohol, of which 90% PO)

Example 4: Decomposition

[0054] The composition of the process liquid in the hydrolysis and esterification stages of the acetone cyanohydrin route process to MAA and MMA are understood to be highly acidic and corrosive. As such it is important that any materials that will come into contact with process liquid are stable against rapid, acid promoted chemical decomposition. To test the candidate surfactant materials, the beaker test described in Test 1 was repeated, but the duration of the test was extended. Any candidate showing only a temporary interaction with the polymer tar was rejected, as this was taken to be a sign that the material had decomposed in the hot, corrosive medium. The results of this testing on the three remaining candidates showed that all three had retained their effectiveness at dispersing the polymer tar after 30 minutes in the hot, spent acid medium.

Example 5: Product Quality

[0055] MAA and MMA are both traded as essentially pure products, with purity specifications of greater than 99.9%. It is therefore important that trace impurities are kept to a very low level in the pure commercial products. It follows that the presence of any new close boiling impurities, which may have arisen as a result of impurities in a new additive, or from the decomposition of a new additive that was being used in the process, would be highly undesirable. For this reason when a new additive is proposed for use in the acetone cyanohydrin route to MAA or MMA process, it is important that a test can be carried out that clearly shows that no new trace impurities are found in the product that can be ascribed to the use of the new additive. With this requirement in mind candidate surfactant materials were subjected to model esterification reactions, at laboratory scale. The candidate surfactants were each added to separate portions of a sample from the exit of the thermal converter stage of the amide stage of the process. Water and methanol were then added, and the mix was taken to esterification reaction temperature. Crude MMA was then distilled off from the mix, and this was analysed by GC and GC coupled with mass spectroscopy to look for the presence of any new trace impurities. The results are shown in Table 4, in which it can be seen that both candidates contained no trace materials.

TABLE-US-00004 TABLE 4 Product Quality Experiments Trace Impurities Found By GC - Mass Candidate Material Spectroscopic Analysis Control experiment, no Trace Impurities typical of ACH route surfactant present process, e.g. methanol, acetone, dimethyl ether, methacrylonitrile, methyl propionate, ethyl methacrylate, methylhydroxyisobuyrate N - ethoxylated, N - tallow As control substituted propylene diamine (av 10 EO units per molecule) Branched C13 alkyl As control ethoxylate/propoxylate (av 20 to 30 PO/EO units per alcohol, of which 90% PO)

Example 6: Minimum Effective Level

[0056] Two of the candidate surfactant materials were subjected to a further test to determine the lowest concentration that could be used while still observing a discernible effect on the polymer tar in the esterification stage. Further model esterification reactions were carried out by the lab scale batch esterification method outlined in the section above, except that iml of a solution containing 5% of polymer tar in concentrated sulphuric acid solvent had been added as a means of exaggerating the effect of the surfactant and making it easier to see. In the acetone cyanohydrin process for the continuous manufacture of MAA or MMA it is typical to express the concentrations or levels of other raw materials or additives to the process as a fraction of the feed-rate of the main raw material, the Acetone Cyanohydrin. In the present case the concentration of surfactants in the process was expressed in terms of parts per million ppm based on the ACH feed-rate. A series of batch esterification reactions was carried out at lab scale where the concentration of the surfactant in the first reaction was set at 5000 ppm. Subsequent reactions were done at gradually reducing levels of surfactant until the level at which there was no longer a discernible effect of polymer tar dispersal had been identified. The level was then increased to 2 this value, and second, confirmatory experiments carried out.

TABLE-US-00005 TABLE 5 Minimum Effective Level Minimum Effective Level Candidate Material (ppm, based on ACH) N - ethoxylated, N - tallow 250 substituted propylene diamine (av 10 EO units per molecule) Branched C13 alkyl 750 ethoxylate/propoxylate (av 20 to 30 PO/EO units per alcohol, of which 90% PO)

Example 7: Extension of Time Between Stoppages Caused by Polymer Tar Blockage at Production Scale

[0057] The candidate surfactant with the lowest minimum effective level was tested at production scale, by carrying out a trial with continuous addition for the complete period between stoppages for clean down. The trial was carried out on a continuous production plant, which was designed to operate at ACH feed-rates of up to 13 te/hr.

[0058] Those skilled in the art will recognise that on such plants there are many factors which can affect the rate at which polymer tar is produced, and also the number of blockages which are caused by accumulation of polymer tar. Factors that affect the number of stoppages caused by polymer tar include:

[0059] Average Plant Rate: The lower the average plant rate is, the longer the residence time in the amide stage of the process vessels becomes, and the more tar is produced

[0060] Number of Plant Hold Periods: Stopping and holding up of process material causes more tar generation because of the effect this has on extending the normal residence times of the material in the vessels, and also allows opportunities for accumulation and blockage due to disengagement of the tar, which is less dense and tends to float on the spent acid in the esterifiers.

[0061] Quality of the ACH raw material: Poor quality ACH has been shown to give rise to a greater level of generation of polymer tar

[0062] Levels and types of polymerisation inhibitors: This is particularly important in the amide stage of the process, where the majority of the components of polymer tar are formed

[0063] Levels of solvent-like components remaining in the spent acid at the end of the stripping stage: It is broadly recognised that the levels and types of solvent-like components in the spent acid, such as Methanol, Acetone, Methacrylic acid, Methylmethacrylate and Hydroxyisobutyric acid, have an effect on the nature of the polymer tar that is found in the esterification vessels. Higher levels of these components lead to polymer tar which is less viscous and sticky, with a lower tendency to accumulate and cause blockages.

[0064] The continuous production trial of the preferred candidate surfactant material was designed to take into account two periods of operation at similar production rates, with no surfactant addition. These periods were considered control periods for comparison purposes.

[0065] The period between stoppages for clean down, and the mass of polymer tar removed at the shut-down were used as indicators of the performance of the surfactant. The results of the trial are shown in Tables 6 and 7 below, in which it can be seen that a significant lengthening of the period between enforced stoppages, and a reduction in the mass of polymer tar removed are both evident, despite those factors which are known to cause accumulation of polymer tar being worse in the trial period compared with the two control periods.

TABLE-US-00006 TABLE 6 Conditions for Continuous Production Trial Control Period Control Period Surfactant Condition 1 2 Trial Period Average 10.0 9.1 8.7 Production rate (ACH feed-rate, te/hr) Number and Average of 2 As control As control duration of per week, short period 1 period 1 Stoppages duration associated with plant trips ACH Quality Average 0.8% As for control As for control (represented by acetone period 1, ACH period 1, ACH the major impurity from same from same % w/w acetone) stock used stock used Process 300 ppm As for control As for control Inhibition, amide Phenothiazine, period 1 ACH period 1 ACH stage (Type and dissolved in ACH from same from same level, expressed as prior to feed into stock used stock used ppm of ACH amide stage feed-rate) Levels of Average 0.45% As for control As for control solvent-like MAA in spent period 1 period 1 components in acid exit spent acid esterification (represented by % MAA)

TABLE-US-00007 TABLE 7 Results from Continuous Production Trial Control Period Control Period Surfactant Success Criteria 1 2 Trial Period Period 30 20 38 between enforced stoppages for clean- out of polymer tar (days) Mass of 10 12 6 polymer tar removed during clean down after period of operation (te)