WO2014099236A1 - Method for reliable inhibitor delivery - Google Patents

Abstract

A process comprising feeding to a chemical process a solution of a vapor phase inhibitor in a solvent via an inhibitor feed system, wherein the inhibitor solution comprises at least one specified impurity, and wherein the impurity concentration is low and/or the inhibitor feed system is operated under conditions sufficient to maintain the specified impurity in solution.

Description

METHOD FOR RELIABLE INHIBITOR DELIVERY

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from provisional application serial number

61/738,678, filed December 18, 2012, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to an improved method for the formulation and delivery of a polymerization inhibitor to process equipment used for the manufacture of ethylenically unsaturated monomers.

In the processing and distillation of ethylenically unsaturated monomers such as acrylic acid, undesirable polymerization is reduced by the addition to the process liquid of compounds that inhibit the unwanted polymerization and fouling of the process equipment. For example, Shimizu, et.al. in US 4,021,310 describe the stabilization of acrylic acid in the presence of copper dibutyldithiocarbamate, hydroquinone and molecular oxygen. Other molecules known to those skilled in the art to be effective polymerization inhibitors include, but are not limited to, phenothiazine (PTZ), 4-hydroxyanisole (MeHQ), 1,4-benzoquinone (BQ), methylene blue, nitrosobenzenes (R-NO), diphenylamines and other such molecules. These are known as liquid phase polymerization inhibitors that help protect process equipment surfaces that are effectively wetted by the process liquid phase.

However, in the processing of ethylenically unsaturated monomers there are often hard to reach regions in distillation towers, e.g., under down-comers and underneath distillation tray sections, that are not effectively wetted by the process liquid. These areas, often referred to as vapor spaces, do not consistently get wetted and replenished by the process liquid. This frequently results in the formation, e.g., via condensation, of regions of uninhibited liquid monomer that can polymerize, and then serve to seed the tower or other process equipment with polymer. This condition results in an accelerated fouling rate and leads to costly cleanouts and lost productivity due to process equipment downtime.

This effect can be countered by the use of so-called vapor phase polymerization inhibitors, whose relative volatility is such that it enables them or their decomposition products to travel with the vapor traffic in a distillation tower to reach the areas where the process liquid does not adequately inhibit the process equipment. For example, Schumacher et. al. describe in US 4,113,574 the use of 2,4-pentadione as an inhibitor of relatively low vapor pressure that codistills in the distillation of acrylic acid and prevents the fouling of heat exchangers in the manufacturing process. N-nitrosophenylhydroxylamine (NPH) and its salts were described by Gros et. al. in US 3,426,063 as effective inhibitors of vapor space polymerization in the distillation of ethylenically substituted monomers. Likewise, Varwig et.al. describe in US 4,898,976 the application of the ethanolamine salt of NPH as an effective inhibitor of the vapor phase polymerization of acrylic acid and related esters. The constant and reliable application of vapor phase inhibitors to process equipment is critical in reducing fouling in many large scale manufacturing processes.

The ethanolamine salt of NPH is typically formulated as a concentrated aqueous solution with excess ethanolamine solvent to maintain a pH that promotes storage stability of the inhibitor. Concentration ranges of 30 to 60 wt active ingredient, 15 to 30 wt free ethanolamine and 30 to 40 wt water are typical. Application of this inhibitor involves dilution of this formulated raw material concentrate with water or another suitable solvent compatible with the process stream to which it will be added to achieve the correct active ingredient concentration that will result in effective polymer inhibition of the tower or process equipment vapor spaces.

A common problem in the industry with this inhibitor is the intermittent formation of solids that form in the narrow inhibitor lines, valves and related metering equipment that feed this inhibitor to the process equipment. For example, a loss of effectiveness characterized by product discoloration, precipitates and turbid mixtures was reported by Harris et. al. in US 6,018,078.

These solids are insidious in that they travel through filters and accumulate in protected equipment. This fouling can extend to the inhibitor feed system, resulting in intermittent interruptions of the inhibitor feed to the monomer processing and refining equipment, in addition to requiring frequent cleanouts of the inhibitor metering system. In the more severe cases, shutdown of the affected process equipment is required to prevent a larger fouling event of the monomer manufacturing process. This condition leads to prohibitively costly losses in production. It would be desirable to have a more reliable means of delivering this inhibitor and reducing the risk of a loss of inhibitor feed with the resulting increased risk of fouling the distillation towers and other process equipment used for monomer manufacture.

SUMMARY OF THE INVENTION

The process of the invention is a process comprising feeding to a chemical process a solution of a vapor phase inhibitor in a solvent via an inhibitor feed system, the inhibitor comprising at least one NPH compound from the group consisting of NPH and salts of the formula I:

I wherein the inhibitor solution comprises at least one of the impurities of formulas Π an

II III with the proviso that at least one of the following conditions is true: (1) the concentration of the impurity of formula II in the solution is less than 1,000 ppm and the concentration of the impurity of formula ΙΠ in the solution is less than 1,000 ppm, or (2) the inhibitor feed system is operated under conditions sufficient to maintain the impurities in solution.

Surprisingly, it was discovered that impurities in the inhibitor are the source of problematic emulsions that lead to plugging of inhibitor feed systems and subsequent manufacturing problems.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a plot of turbidity vs. impurity concentration. DETAILED DESCRIPTION OF THE INVENTION

As used herein, "a," "an," "the," "at least one," and "one or more" are used interchangeably. The terms "comprises," "includes," and variations thereof do not have a limiting meaning where these terms appear in the description and claims. Thus, for example, an aqueous composition that includes particles of "a" hydrophobic polymer can be interpreted to mean that the composition includes particles of "one or more" hydrophobic polymers.

Also herein, the recitations of numerical ranges by endpoints include all numbers subsumed in that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.). For the purposes of the invention, it is to be understood, consistent with what one of ordinary skill in the art would understand, that a numerical range is intended to include and support all possible subranges that are included in that range. For example, the range from 1 to 100 is intended to convey from 1.01 to 100, from 1 to 99.99, from 1.01 to 99.99, from 40 to 60, from 1 to 55, etc. Also herein, the recitations of numerical ranges and/or numerical values, including such recitations in the claims, can be read to include the term "about." In such instances the term "about" refers to numerical ranges and/or numerical values that are substantially the same as those recited herein.

As used herein, the use of the term "(meth)" followed by another term such as acrylate refers to both acrylates and methacrylates. For example, the term "(meth)acrylate" refers to either acrylate or methacrylate; the term "(meth)acrylic" refers to either acrylic or methacrylic; and the term "(meth)acrylic acid" refers to either acrylic acid or methacrylic acid.

Unless stated to the contrary, or implicit from the context, all parts and percentages are based on weight and all test methods are current as of the filing date of this application. For purposes of United States patent practice, the contents of any referenced patent, patent application or publication are incorporated by reference in their entirety (or its equivalent U.S. version is so incorporated by reference) especially with respect to the disclosure of definitions (to the extent not inconsistent with any definitions specifically provided in this disclosure) and general knowledge in the art. The process of the invention employs a solution comprising a solvent and a vapor phase polymerization inhibitor, wherein the polymerization inhibitor comprises an N- nitrosophenylhydroxylamine (NPH) compound.

The solvent functions to solubilize the NPH compound. Examples of suitable solvents include: N-methyl-2-pyrrolidone (NMP); dimethylformamide (DMF); dimethyl sulfoxide (DMSO); alkyl and higher order ethanolamines such as methyl-diethanolamine, diethylenetriamine, and other di and tri-ethanol amines; alcohols, such as methanol; diols such as ethylene glycol and propylene glycol; triols such as glycerol; cyclic and acyclic ethers and polyethers such as dioxane, diglyme, triglyme etc; and glycol ethers such as triethylene glycol, ethylene glycol monomethyl ether, and derivatives thereof. Mixtures of solvents can be employed. For example, water can be employed as a solvent, but in that case it is preferred to employ a water-miscible solvent as part of the solvent composition.

The NPH compound can be NPH itself or can be a salt of formula I:

I

The NPH compound acts as a vapor phase polymerization inhibitor that is used to prevent undesirable polymerization in the manufacture of acrylic acid and other monomers. NPH compounds are commercially available, typically in the form of a concentrated solution that is diluted prior to use. For example, one commercially available form of NPH compound is a solution in water and monoethanolamine, which is a water-miscible organic co-solvent.

The invention is based on the discovery that the cause of NPH inhibitor problems is that trace amounts of the condensation products azobenzene (AZB) (see Formula II) and azoxybenzene (AZXB) (see Formula ΠΙ)

N=

in the inhibitor give rise to the unwanted solids that foul the metering equipment and disable the inhibitor feed to the distillation and process equipment. These impurities can be characterized using modern spectroscopic methods including Fourier Transform Nuclear Magnetic Resonance Spectrometry (FT-NMR) and Fourier Transform Infrared Spectroscopy (FT-IR) as well as chromatographic methods including Ultra Performance Liquid

Chromatography (UPLC).

According to a process of the invention, at least one of the following conditions is true: (1) the concentration of the impurity of formula Π in the inhibitor solution is less than 1,000 ppm and the concentration of the impurity of formula ΠΙ in the inhibitor solution is less than 1,000 ppm, or (2) the inhibitor feed system is operated under conditions sufficient to maintain the impurities in solution Typical concentrations of these impurities are from 500 to 5,000 ppmw based on the weight of the inhibitor solution fed to the monomer manufacturing process. In one embodiment of the invention, the concentration of AZB is less than 500 ppm and the concentration of AZXB is less than 750 ppm. In one

embodiment of the invention, the concentration of the AZB and AZXB impurities is from 300 to 500 ppmw for AZB and 1,000 to 1,200 ppmw for AZXB. In one embodiment of the invention, the concentration of azobenzene in the inhibitor solution is from 150 to 1,000 ppm based the weight of the inhibitor solution. In one embodiment of the invention, the concentration of azoxybenzene in the inhibitor solution is from 250 to 1,000 ppm based on the weight of the inhibitor solution. Unlike the active ingredient NPH salt, the impurities AZB and AZXB are insoluble to sparingly soluble in water. Upon dilution with solvents such as de-ionized water, these impurities form a suspension that can traverse filter barriers and eventually allow precipitation of solids in delicate polymerization inhibitor metering equipment. Therefore, by controlling the properties and/or environment of an inhibitor solution so that impurities AZB and AZXB are reduced or made to remain in solution, the formation of their corresponding precipitates can be prevented, thereby resulting in a more consistent and reliable inhibitor feed system that reduces unwanted fouling and cleanout costs of acrylic acid distillation towers and other processing equipment. There are a variety of ways to accomplish the process of the invention. For example, maintaining or increasing the temperature of the inhibitor solution increases the solubility of the impurities and reduces their ability to form an undesirable emulsion. Advantageously, the temperature of the inhibitor feed solution can be above room temperature when the solution is fed to the monomer manufacturing process, and preferably the temperature is at least 55°C.

In addition, the properties of the inhibitor solution that we discovered can be controlled to prevent these impurities from forming unwanted solids include, for example: (1) reducing the overall concentration of these impurities in the inhibitor concentrate solution, (2) changing the degree of dilution in the inhibitor feed to the process, and (3) adding a co-solvent to the inhibitor solution.

Another way to reduce problems caused by the impurities is to avoid or reduce the use of water as a solvent. For example, a solvent other than water can be employed, with or without water as a cosolvent. The nonaqueous solvent advantageously is compatible with the monomer manufacturing process, and advantageously is miscible with water. In one embodiment of the invention, the solvent is essentially free of water.

The design and mechanical make up of the inhibitor feed system is not particularly critical. Inhibitor feed systems are well known to those skilled in the art. For example, an inhibitor feed system can comprise one or more mix tanks, one or more storage tanks, and associated piping, pumps, and control systems as needed.

SPECIFIC EMBODIMENTS OF THE INVENTION

The following examples are given to illustrate the invention and should not be construed as limiting its scope.

Turbidity is measured on a Hach AN2100 Turbidimeter and reported as

Nephelometric Turbidity Units (NTU). The turbidimeter is calibrated using a preprogrammed calibration method and 6 turbidity standards ranging from 0.1 to 7500 NTU. Sample measurements are performed by rigorously swirling each sample, then transferring 25mL of sample using an Eppendorf pipette to a 30mL turbidimeter sample cell. Turbidity measurements are collected at ambient temperature.

Experiments 1-7. Control experiments; turbidity measurements as a measure of precipitates.

Dilution ladder studies of a 45 wt ethanolamine salt solution of NPH with deionized (DI) water are conducted at ambient temperature and the resulting mixtures are analyzed using a turbidity meter to develop a quantitative measure of the extent of solids formation as a function of added DI water. Upon dilution with DI water, the appearance of a turbid mixture is noted. Upon standing for 24 to 48 hours at ambient temperature, the turbidity subsides and the appearance of red brown solids is noted. Hence, turbidity of the mixture is used as a convenient indicator of initial solids generation upon dilution with DI water.

Samples of 25 grams of a 45 wt ethanolamine salt solution of NPH are (1) diluted with increasingly larger amounts of DI water, (2) agitated manually, and (3) immediately sampled for turbidity measurements. Samples with no or little DI water added appear clear and free from any evidence of turbidity. As the amount of added DI water is increased, turbidity is observed to increase via visual inspection. Turbidity readings increased accordingly. The results are summarized in Table 1.

Table 1. Turbidity of NPH as a function of added water

Experiments 8-17. Variable azo(xy)benzene concentration experiments.

Solutions of 45 wt NPH containing variable amounts of azobenzene or azoxybenzene are prepared by spiking laboratory prepared low azo(xy)benzene NPH (ethanolamine salt) with known amounts of standard samples of azobenzene and azoxybenzene. The resulting concentrations of 45 wt NPH solution spiked with azobenzene are summarized in Table 2 (Experiments 8 to 12). The equivalent experiments for azoxybenzene are summarized in Table 3 (Experiments 13 to 17). Spiked NPH 45 wt samples typically require stirring overnight at ambient temperature to achieve homogeneity. Approximately 14 grams of azo(xy)benzene-spiked NPH 45 wt solutions are diluted with DI water to simulate dilution to an active ingredient NPH solution of 25 wt NPH. The NPH 25 wt solutions spiked with azobenzene become opaque upon visual examination at the 500 ppm azobenzene level. Those spiked with azoxybenzene likewise lose visual clarity at the 750 ppm level. Turbidity measurements are conducted on fresh samples (Tables 2 & 3) for the spiked NPH 45 wt% solutions for Experiments 8 to 17 upon dilution to 25% with de-ionized water. The resulting turbidities are summarized in Tables 2 & 3 and illustrated in Figure 1. Solids are formed after the samples are allowed to stand at ambient room temperature over 12 to 18 hours. The amount of solids formed upon standing is directly proportional to the spiked concentration of azo(xy)benzene.

Table 2. Azobenzene spiked 45% NPH dilutions.

Experiments 18 - 28. Co-solvent effect; Effect of ethanolamine concentration on solids formation upon dilution of NPH with de-ionized water.

Experiments are carried out to understand what concentrations of NPH, free ethanolamine and water could be achieved without resulting in turbid or hazy mixtures. The concentration of a stock NPH solution containing 45 wt% of NPH active ingredient, 19 wt% of free monoethanolamine (MEA) and 36 wt% water is adjusted by charging different amounts of MEA to the NPH stock material. The initial NPH 45 wt% solution is found to contain 416 ppm of azobenzene and 1065 ppm of azoxybenzene as determined by liquid chromatographic methods using standard samples of known composition for calibration purposes. The resulting solution concentrations are summarized in Table 4. A 45 wt% NPH solution diluted to an active ingredient concentration of 25 wt% exhibited a turbidity value of 1700 NTU as seen in Exp 18. This solution appeared hazy and cloudy upon visual inspection and deposited dark brown solids after standing at ambient temperature for approximately 18 hours. Experiments 19 to 21 show that high MEA concentrations afford clear solutions with no evidence of solids deposition after 18 hours at ambient temperature.

Table 4. Turbidity of I as a function of added monoethanolamine (MEA)

Experiment 29. Temperature effect.

An experiment is conducted in which several samples consisting of 50 mL of a 45 wt% stock solution of NPH inhibitor are mixed with 25 mL of deionized water to reach an inhibitor active ingredient (NPH) concentration of 25 wt%. As before, a cloudy, colloidal- type dispersion results from the water dilution in all cases. Four of these mixtures are prepared and immediately placed in three constant temperature baths at 35, 45 & 55°C respectively with an ambient temperature (~ 26°C) sample at the side for comparison purposes. The samples are allowed to equilibrate in the temperature baths and are subsequently removed and visually examined. The cloudiness and haziness of the mixtures is observed to gradually disappear as the mixture temperature increases, clearing up substantially at 55°C. Solids do not precipitate from the mixture kept at 55°C. The general observation made is that warmer solution temperatures appear to reduce the turbidity of the mixture and, by inference, the formation or stability of the colloidal dispersion formed by AZB and AZXB. By preventing or reversing the formation of the dispersion, the formation of undesired solids is avoided.

Claims

WHAT IS CLAIMED IS:

1. A process comprising feeding to a chemical process a solution of a vapor phase inhibitor in a solvent via an inhibitor feed system, the inhibitor comprising at least one NPH compound from the group consisting of NPH and salts of the formula I:

I wherein the inhibitor solution comprises at least one of the impurities of formulas Π an

II III with the proviso that at least one of the following conditions is true: (1) the concentration of the impurity of formula II in the solution is less than 1,000 ppmw and the concentration of the impurity of formula ΙΠ in the solution is less than 1,000 ppmw, or (2) the inhibitor feed system is operated under conditions sufficient to maintain the impurities in solution.

2. The process of any of the preceding claims wherein the total combined amount of impurities II and ΙΠ is from greater than 2,000 up to 5,000 ppmw based on the total weight of the inhibitor solution.

3. The process of any of the preceding claims wherein the inhibitor feed system is maintained at a temperature sufficient to keep the impurities in solution.

4. The process of any of the preceding claims wherein the inhibitor feed system is maintained such that the concentration of the impurities is sufficiently low to keep the impurities in solution.

5. The process of any of the preceding claims wherein the solvent comprises an inert, water-miscible organic solvent that is present in the inhibitor feed system in an amount sufficient to keep the impurities in solution.

6. The process of any of the preceding claims wherein the chemical process is a process for the preparation of an unsaturated monomer.

7. The process of any of the preceding claims where the temperature of the inhibitor solution is at least room temperature.

8. The process of any of the preceding claims where the temperature of the inhibitor solution is at least 55°C.

9. The process of claim 6 where the water-miscible organic co-solvent is monoethanolamine .

10. The process of any of the preceding claims where the monomer comprises styrene, vinyl acetate, a (meth)acrylic acid, or a (meth)acrylic ester.

11. The process of any of the preceding claims where the monomer comprises at least one of acrylic acid or methacrylic acid.