Field of the Invention

The present invention relates to the use of agents to reduce fouling caused by aldol condensation in caustic acid gas scrubbers ("caustic scrubbers"). Background of the Invention

Refineries employ atmospheric and vacuum distillation towers to separate crude oil into narrower boiling fractions. These fractions then are converted into fuel products, such as motor gasoline, distillate fuels (diesel and heating oils), and bunker (residual) fuel oils. Some of the low boiling fractions from various units of the refinery are directed to petrochemical plants, where they are further processed into highly refined chemical feedstocks to be used as raw materials in the manufacture of other types of products, such as plastics and basic chemicals.

During the refining of petroleum, hydrocarbon streams typically are treated in an amine scrubber, such as a DEA or MEA scrubber, to remove acid gases, such as hydrogen sulfide and carbon dioxide. In a petrochemical plant, hydrocarbon streams typically are treated for the same purpose in a caustic scrubber. All of these scrubbers herein are referred to as

"acid gas" scrubbers. The hydrocarbon stream entering an acid gas scrubber may contain aldehydes and ketones, their precursors, such as vinyl acetate, or other impurities, that are hydrolyzed or otherwise converted to aldehydes and salts of organic acids in the highly alkaline environment of an acid gas scrubber. Such compounds will herein be referred to as "reactive compounds." These reactive compounds either (a) contain carbonyls, or (b) form carbonyls under highly alkaline conditions that are susceptible to classic aldol condensation reactions. Carbonyls that are susceptible to classic aldol condensation reactions hereinafter will be referred to as "reactive carbony1s."

Under highly alkaline conditions, lower molecular weight aldehydes, such as propionaldehyde (propanal) and especially acetaldehyde (ethanal), readily undergo base catalyzed aldol condensation at ambient temperatures. The result is the formation of oligomers and polymers which precipitate out of the scrubbing solution as viscous oils, polymeric gums, and solids. These precipitates can foul the processing equipment and result in the reduction of processing throughput and costly equipment maintenance or repair.

In the past, organic reducing agents or organic and inorganic oxidizing agents have been proposed to prevent such polymerization. These organic agents might successfully retard polymerization in acid gas scrubbers; however, the organic

agents also tend to undergo other reactions which can reduce their effectiveness as aldol condensation inhibitors.

Effective and economical methods for retarding aldol condensation in acid gas scrubbers would be highly desirable. Summary of the Invention

The present invention provides a number of agents that are useful to inhibit aldol condensation in caustic scrubbers. Preferred agents include, but are not necessarily limited to benzoic hydrazide, hydroperoxides, hydrogen peroxide, salts of hypochlorous acid, nitroalkanes, monoethanolamine, peroxyesters, and N,N-dialkylhydroxylamines. Detailed Description of the Invention

The present invention is directed to reactions that cause fouling in caustic scrubbers. Without limiting the present invention, it is believed that the red precipitate that forms in acid gas scrubbers is the result of several aldol condensation/ dehydration steps. As used herein, the term "aldol condensation" is intended to refer to the reactions that ultimately result in the formation of a precipitate in acid gas scrubbers. The inhibiting agents of the present invention are believed to inhibit fouling by inhibiting such aldol condensation.

Agents that inhibit aldol condensation in caustic scrubbers include benzoic hydrazide, hydroperoxides, hydrogen peroxide, salts of hypochlorous acid, such as NaOCl and Ca(0Cl) ₂ , nitroalkanes, monoethanolamine, peroxyesters, 3-

methoxypropylamine, and N,N-dialkylhydroxylamines. Preferred inhibiting agents are benzoic hydrazide, tertiary-butyl hydroperoxide, hydrogen peroxide, NaOCl (commonly available as a 3.62% solution), nitromethane, monoethanolamine, tertiary- butyl perbenzoate, and N,N-diethylhydroxylamine. Most preferred agents are benzoic hydrazide and t-butyl hydroperoxide.

Preferred hydroperoxides are tertiary-butyl or cumene hydroperoxide, which are commercially available from Elf Atochem North America, Inc., 2000 Market Street, Philadelphia, PA. A preferred form of hydrogen peroxide is a 30% solution of hydrogen peroxide, available from EM Science, a division of EM Industries, Inc., 5 Skyline Drive, Hawthorne, NY.

Preferred nitroalkanes have the structure R-N0 ₂ , wherein R is a straight, branched, or cyclic alkyl group having between about 1-8 carbon atoms. A preferred nitroalkane is nitromethane, which is commercially available from Angus

Chemical Co., 1500 East Lake Cook Road, Buffalo Grove, Illinois.

Preferred peroxyesters are tertiary-butyl perbenzoate, tertiary-butyl peroxyacetate, tertiary-butyl peroxypivalate, tertiary-butyl peroxy-2-ethylhexanoate, and tertiary-amyl peroxy-2-ethylhexanoate, which are commercially available from

Elf Atochem North America Inc.

Preferred N,N-dialkylhydroxylamines have the general formula R ₂ N-0H, wherein R is selected from the group consisting of straight, branched, and cyclic alkyl groups having between about 1-10 carbon atoms, aryl groups, alkaryl groups, and

aralkyl groups. A preferred N,N-dialkylhydroxylamine is N,N- diethylhydroxylamine, which is commercially available from Elf Atochem.

Certain agents inhibit aldol condensation in caustic scrubbers, but are not as soluble in the caustic scrubbing solution, or form a slight precipitate. These agents are less preferred, but still function in the present invention. These agents include isothiuronium salts, primary amines, acetophenone, dibutylamine, and iodine. A preferred isothiuronium salt is benzyl isothiuronium chloride, which may be synthesized as shown in Example 1. Persons of skill in the art can alter the procedures of Example 1 to synthesize other isothiuronium salts.

Preferred primary amines have the general structure R-NH ₂ , wherein R is selected from the group consisting of straight, branched, and cyclic alkyl groups, preferably having between about 1-20 carbon atoms, wherein one or more of said carbon atoms can be replaced by an oxygen atom to result in an "ethereal amine," aryl, alkaryl, and aralkyl groups, and heterocyclic alkyl groups containing oxygen or tertiary nitrogen as a ring constituent. Preferred primary amines include, but are not necessarily limited to arylamines, aralkylamines, ethereal amines, and cyclic alkyl amines.

A preferred arylamine is aniline; a preferred aralkylamine is benzylamine; a preferred ethereal amine is 3- methoxypropylamine; and, a preferred cyclic alkyl amine is

cyclohexylamine. Aniline is commercially available from Uniroyal Chemical co.. Inc., Specialty Chemicals, Middlebury, CT. Benzylamine, 3-methoxypropylamine, and cyclohexylamine are commercially available from BASF Corp., 3000 Continental Drive, North Mount Olive, NJ.

Acetophenone and iodine are commercially available from a number of sources. Dibutylamine is commercially available from BASF Corp.

Preferably, the inhibiting agent should be injected into the scrubber slowly, on an "as-needed" basis. Due to substantially continuous mode of operation of most scrubbers, it is believed that the agent should reach a steady state during processing. An equimolar ratio of inhibiting agent to active carbonyl containing compound should be sufficient to inhibit aldol condensation. Even less than a 1:1 ratio may be sufficient. An excess of the inhibiting agent also may be added, if desired.

The agents of the present invention will react with the reactive carbonyls, or a condensation product of two or more reactive carbonyls, in the caustic scrubber at ambient temperatures. Therefore, the hydrocarbon stream need not be heated, and heating is neither necessary nor advisable.

The invention will be more readily understood with reference to the following examples.

Example 1

Thiourea (7.60 g; 100 mmoles) was added to 150 ml of ethanol in a 250 ml Erlenmeyer flask. The mixture was heated to reflux to dissolve almost all of the thiourea. After cooling the contents to 40°C, benzyl chloride (12.6 g; 100 mmoles) was added dropwise over three minutes. The slightly hazy solution was refluxed overnight, causing it to become clear. In the morning, the ethanol was stripped off on the rotary evaporator leaving a good yield of a white solid. NMR analysis of the white solid showed it to be benzyl-isothiuronium chloride.

Example 2 A number of screening tests were performed to ascertain agents that would reduce aldol condensation and fouling in caustic scrubbers. To perform the screening, 2 oz. bottles were dosed with at least 3.24 mmoles of each candidate, followed by 25 ml of 10% NaOH. Each bottle was then shaken. Subsequently, 100 μl of vinyl acetate (4000 ppm, or 1.08 mmoles) was added to each bottle, and the appearance of each solution was noted at 4 hours, 26 hours, and 72 hours. The solutions were evaluated for color and precipitate. Candidates that exhibited no color, or only slight color, and no precipitation were selected for further study. Approximately 20 candidates were further tested in the following examples.

Example 3 In the following experiment, with the exception of candidate No. 1, 2 oz. bottles were dosed with 1.19 mmoles of candidate, as shown in Table I.

TABLE I

SAMPLE CANDIDATE SOURCE MW AMT. OF MMOLES OF

CANDIDATE CANDIDATE

ADDED ADDED

1 70% tertiary- Arco 90 268 μl 2.08 butyl hydro¬ Chemi¬ peroxide cal Co.

2 tertiary-butyl Aztec 194 164 μl 1.19 perbenzoate Cata¬ lyst Co.

3 Synthe¬

Benzyl- sized isothiuronium as in 202 240 mg 1.19 chloride Ex. 1

*In the tables. "Ph" stands for phenyl.

After dosing the bottles, 25 ml of 10% NaOH was added to each. Subsequently, 100 μl of vinyl acetate (1.08 mmoles) was added to each bottle, and the color of the solution and any precipitate was noted after 6.5 hours and again after 21.5 hours. The results are reflected in Table II.

TABLE II

SAMPLE 6.5 HOURS 21.5 HOURS

Blank Dark yellow; heavy haze Yellow solution; red flocculated precipitate

1 Light yellow; no haze or Light yellow; no haze or precipitate precipitate

SAMPLE 6.5 HOURS 21.5 HOURS

2 Bright yellow; no haze Dark yellow; no haze or or precipitate precipitate

3 Yellow and hazy Light yellow; insoluble candidate; no red precipitate

Based on the foregoing, candidates 1 (t-butyl hydroperoxide) and 2 (t-butyl perbenzoate) appear to be suitable agents for inhibiting aldol condensation in caustic scrubbers because, although the solution is colored, no haze or precipitate is formed. Candidate 3 (benzyl isothiuronium chloride) may be suitable for use in caustic scrubbers, although unreacted candidate remains insoluble in the scrubber substrate.

EXAMPLE 4

The procedures of Example 3 were followed to test a second batch of target candidates given in Table III:

TABLE III

SAMPLE CANDIDATE SOURCE MW AMT. OF MMOLES CANDI¬ OF DATE CANDI¬ ADDED DATE (mg)

4 Acetalde¬ Allied 59.1 70.3 1.19 hyde Signal (D=0.969) Oxime

5 methyl- Aldrich 87.1 104 1.19 ethyl- Chemi¬ ketoxime cal Co.

6 aceto- Aldrich 120 143 1.19 phenone Chemi¬ (D=1.03) cal Co.

7 CH ₃ -N0 ₂ Aldrich 61.0 73 1.19 Chemi¬ (D=1.13) cal Co.

8 (H ₂ N) ₂ C=S Mal- 76.1 91 1.19 linck- rodt

9 Bu ₂ NH BASF 129 154 1.19 Corp. (D=0.767)

The results are shown in Table IV:

TABLE IV

SAMPLE 3 HOURS 21.5 HOURS

5 Yellow and hazy Yellow and red flocculent precipitate

6 Light yellow and hazy Yellow; no red (could be due to precipitate; green haze insolubility of due to insoluble candidate) candidate

7 Almost colorless; no Yellow, no flocculent or haze or precipitate haze

8 Yellow and hazy Yellow with red flocculent precipitate

9 Light yellow and haze Yellow; no red (could be due to Bu ₂ NH) precipitate; red oil on top due to insoluble candidate

Based on the foregoing, candidate 3 (benzoic hydrazide) is a preferred agent for inhibiting aldol condensation in caustic scrubbers because the solution is colorless and no haze or precipitate is formed. Candidate 7 (nitromethane) appears to be a suitable agent for inhibiting aldol condensation in caustic scrubbers because, although the solution is colored, no haze or precipitate is formed. Candidates 1, 2, 6, and 9—aniline, benzylamine, acetophenone, and dibutylamine, respectively—may be suitable for use in caustic scrubbers, although unreacted candidate remains insoluble in the scrubber substrate.

EXAMPLE 5 The procedures of Example 3 were followed to test a third batch of target candidates given in Table V:

TABLE V

SAMPLE CANDIDATE SOURCE MW AMT. OF MMOLES

CANDIDATE OF

ADDED (mg) CANDI¬ DATE

Blank

1 (PhO) ₃ P Bright 310 369 1.19

& Wilson

2 (MeO) ₃ P Aldrich 124 148 1.19 Chemi¬ cal Co.

3 DEA Aldrich 105 125 1.19 Chemi¬ cal Co.

4 MEA Fisher 61.1 73 1.19 Scien¬ tific

5 Aldrich 106 126 1.19

Benzalde- Chemi¬ hyde cal Co.

6 2 moles 106/58 252/69 1.19 (of benzalde- reaction hyde/1 produc ) mole acetone

7 30% H ₂ 0 ₂ EM 34 135 1.19 Science

8 3.62% Commer¬ 74.4 2,440 1.19 NaOCl cial Bleach

9 ι ₂ Mal- 254 302 1.19 linck- rodt

The results are shown in Table VI:

TABLE VI

SAMPLE 1 HOUR 21.5 HOURS

Blank Bright yellow and slight Yellow and red haze flocculent precipitate

1 Bright yellow and slight Yellow and red haze flocculent precipitate

2 Light yellow and hazy (due Yellow and red to insolubility of flocculent precipitate candidate)

3 Yellow and slight haze Yellow and red flocculent precipitate

4 Colorless, no haze or Yellow and no haze or precipitate precipitate

5 Yellow and cloudy (could Yellow, red haze and be due to insolubility of some red precipitate candidate)

6 Bright yellow and cloudy Yellow, red haze and (could be due to insolu¬ some red precipitate bility of candidate)

7 Colorless; no haze or Yellow, but no haze or precipitate precipitate

8 Light yellow, no haze or Yellow, no haze or precipitate precipitate

9 Yellow, no haze or Dark yellow, plus precipitate slight haze

Based on the foregoing, candidate 4 (MEA), 7 (hydrogen peroxide), and 8 (NaOCl) appear to be suitable agents for inhibiting aldol condensation in caustic scrubbers because, although the solution is colored, no haze or precipitate is formed. Candidate 9 (iodine) may be suitable for use in caustic scrubbers in spite of the small amount of haze present.

EXAMPLE 6 The following experiment was designed to test the promising candidates from the final candidate selection tests using recycled caustic solution from an actual caustic scrubber. A one-half quart sample of a customer's "recycle sodium hydroxide solution" was yellow and contained particulates. The caustic solution was vacuum filtered to remove particulates. Except for candidate 4, 2 oz. bottles were dosed to achieve a final concentration of at least 1.1 moles of candidate per 1.0 moles of vinyl acetate. Approximately 1.18-1.19 mmoles of candidate were added to each bottle. 25 ml of the filtered caustic solution was added to each bottle and the contents were shaken 100 times. The bottles were dosed with 100 μl (1.08 mmoles) of vinyl acetate, and the bottles were shaken 50 times. The candidates and the amount of each candidate added is given in Table VII:

TABLE VII

SAMPLE CANDIDATE SOURCE MW AMOUNT MMOLES OF OF

CANDI¬ CANDI¬ DATE DATE ADDED

Blank

1 Benzoic Aldrich 136 162 mg 1.19 hydrazide Chemi¬ cal Co.

2 70% Arco 90 268 mg 2.08 tertiary- Chemi¬ butyl hydro¬ cal Co. peroxide

3 tertiary-butyl Aztec 194 231 mg 1.19 perbenzoate Cata¬ lyst Co.

4 CH ₃ N0 ₂ Aldrich 61.0 73 mg 1.20 Chemi¬ cal Co.

5 MEA Fisher 61.1 73 mg 1.19 Scien¬ tific

6 30% H ₂ 0 ₂ EM 34 135 mg 1.19 Science

7 3.62% NaOCl Commer¬ 74.4 2,440 1.19 cial mg Bleach

The bottles were observed for color and precipitation at 3.5 hours and again at 24 hours. The results are shown in Table VIII:

TABLE VIII

SAMPLE 3.5 HOURS 24 HOURS

Blank Yellow and very hazy Light red, hazy, and red flocculent precipitate.

1 Light yellow, no haze or Light yellow, no haze or precipitate precipitate

2 Light yellow, no haze or Light yellow, no haze or precipitate precipitate

3 Yellow, no haze or Yellow, no haze or precipitate (undissolved precipitate (undissolved candidate on top) candidate on top)

4 Light yellow, no haze or Yellow, no haze or precipitate precipitate

5 Light yellow, no haze or Yellow, no haze or precipitate precipitate

6 Light yellow, no haze or Yellow, no haze or precipitate precipitate

7 Yellow, no haze or Yellow, no haze or precipitate precipitate

Based on the foregoing candidates 1 (benzoic hydrazide), and 2 (t-butyl hydroperoxide) are preferred agents for inhibiting aldol condensation in caustic scrubbers because the solution has only very slight color, and no haze or precipitate is present. Candidates 4, 5, 6, and 7— nitromethane, MEA, hydrogen peroxide, and NaOCl, respectively- -appear to be suitable agents for inhibiting aldol condensation in caustic scrubbers because, although the solution is colored, no haze or precipitate is formed.

Candidate 3 (t-butyl perbenzoate) may be suitable for use in caustic scrubbers even though some precipitate was present.

Note that no precipitate was observed using t-butyl perbenzoate in Example 3.

EXAMPLE 7 The procedures of Example 3 were followed, except that a molar ratio of candidate of 1.5 was used instead of a molar ratio of 1.1, to test several additional candidates given in Table IX:

TABLE IX

SAMPLE CANDIDATE SOURCE MW AMOUNT OF MMOLES

CANDIDATE OF

(mg) CANDI¬ DATE

Blank — — — — —

1 3-methoxy- Union 89.1 144 1.62 propyl- Carbide amine

2 Cyclohexyl Aldrich 99.2 161 1.62 -amine

3 N,N- Aldrich 89.1 144 1.62 diethyl- hydroxyl- amine

The results are shown in Table X:

TABLE X

SAMPLE OBSERVATION AT 67 HOURS

Blank Red solution; heavy red haze.

1 Yellow solution; very slight haze.

2 Light yellow solution; slight haze; no precipitate; some insoluble candidate on top.

3 Dark yellow solution; no haze or precipitate.

Based on the foregoing, candidates 1 (3- methoxypropylamine), 2 (cyclohexylamine), and 3 (N,N- diethylhydroxylamine) all appear to be a suitable agents for inhibiting aldol condensation in caustic scrubbers. Candidate 1 (3-methoxypropylamine) is suitable because, even though a slight haze was produced, the haze was much less than that produced in the blank. Candidate 2 (cyclohexylamine) is suitable because no precipitate was formed and the slight haze appeared to be candidate. Candidate 3 (N,N- diethylhydroxylamine) is suitable because, although the solution is colored, no haze or precipitate is formed. Persons of skill in the art will appreciate that many modifications may be made to the embodiments described herein without departing from the spirit of the present invention. Accordingly, the embodiments described herein are illustrative only and are not intended to limit the scope of the present invention.