Treatments to reduce aldol condensation and subsequent polymerization in monoethanolamine scrubbers

The present invention provides a large number of inhibiting agents that are useful to reduce aldol condensation in monoethanolamine scrubbers. Such agents include hydrazine, hydroperoxides, hydrogen peroxide, and dialkylketone oximes. Preferred inhibiting agents are hydrazines.

FIELD OF THE INVENTION 
The present invention relates to the use of inhibiting agents to reduce 
fouling caused by aldol condensation in monoethanolamine (MEA) scrubbers. 
Such agents include hydrazines, hydroperoxides, hydrogen peroxide, and 
dialkylketone oximes. Preferred inhibiting agents are hydrazines. 
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 carbonyls." 
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 
monoethanolamine scrubbers would be highly desirable. 
SUMMARY OF THE INVENTION 
The present invention provides a large number of inhibiting agents that are 
useful to reduce aldol condensation in monoethanolamine scrubbers. 
Suitable agents include hydrazine, hydroperoxides, hydrogen peroxide, and 
dialkylketone oximes. Preferred inhibiting agents are hydrazines.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is directed to reactions that cause fouling in 
monoethanolamine 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. 
Suitable inhibiting agents for MEA scrubbers include hydrazines, 
hydroperoxides, hydrogen peroxide, and dialkylketone oximes. Preferred 
inhibiting agents are hydrazines. 
Substantially any hydrazine containing an --NH.sub.2 group should function 
in the present invention. Suitable hydrazines generally have the following 
structure: 
EQU R.sub.2 N--NH.sub.2 
wherein R is independently selected from a hydrogen, a straight, branched, 
or cyclic alkyl group having between about 1-8 carbon atoms, an aryl 
group, an alkaryl group, or an aralkyl group. Hydrazine is commercially 
available from Elf Atochem North America, Inc., 2000 Market Street, 
Philadelphia, Pa. 
Preferred hydroperoxides are commercially available hydroperoxides, such as 
tertiary-butyl and cumene hydroperoxide, which may be obtained from Elf 
Atochem North America, Inc. 
A preferred form of hydrogen peroxide is a 30% solution of hydrogen 
peroxide, which may be obtained commercially from EM Science, a division 
of EM Industries, Inc., 5 Skyline Drive, Hawthorne, N.Y., and Elf Atochem 
North America, Inc. 
Suitable dialkyl ketone oximes are those in which the alkyl groups are 
selected from the group consisting of straight, branched, and cyclic alkyl 
groups having between about 1-8 carbon atoms. A preferred dialkylketone 
oxime is 2-butanone oxime, which may be obtained commercially from 
AlliedSignal, Inc., Morristown, N.J. 
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 
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 acid 
gas scrubber at ambient temperatures. Therefore, the hydrocarbon stream 
need not be heated. In the following examples, the samples are heated to 
between about 50.degree.-55.degree. C. (122.degree.-131.degree. F.) to 
hasten the polymerization reaction for facile candidate evaluation. In 
actual use, heating is neither necessary nor advisable. 
The invention will be more readily understood with reference to the 
following examples. 
EXAMPLE 1 
A number of screening tests were performed to ascertain agents that would 
reduce aldol condensation and/or oligomerization in MEA scrubbers. To 
perform the screening, 2 oz. bottles were dosed with 1.5 g or 1.5 ml of 
candidate--an amount in excess of 1.1 mmoles of candidate for every 1.0 
mmole of vinyl acetate. As seen below, the dosage of vinyl acetate that 
was added to each bottle is 600 .mu.l (6.51 mmoles). 
After dosing with the candidate, 25 ml of 5% MEA in NaCl saturated water 
(to render the aldol condensation product less soluble) was added to each 
bottle, and the bottles were shaken 100 times. Subsequently, 600 .mu.l of 
vinyl acetate (560 mg, or 6.51 mmoles) was added to each bottle. The 
bottles were shaken 50 times, and placed in an oven at 
50.degree.-55.degree. C. (122.degree.-131.degree. F.) and observed after 
24 hours for color and precipitate. Candidates that exhibited no color, or 
only slight color, and no precipitation were selected for further study. 
Approximately 26 candidates were selected for further testing. 
EXAMPLE 2 
The procedures of Example 1 were followed to test 26 candidates except that 
an exact mole ratio of candidate:vinyl acetate of 1.1 was used in each 
case, except for candidates 1 and 10. The amount of candidate added to 
each bottle is given in Table I: 
TABLE I 
__________________________________________________________________________ 
AMT. OF 
CANDI- 
DATE 
ADDED MMOLES OF 
SAMPLE 
CANDIDATE SOURCE 
MW (mg) CANDI-DATE 
__________________________________________________________________________ 
Blank -- -- -- -- -- 
1 70% tertiary- 
Arco 90 1,610 12.5 
butyl hydro- 
Chemical 
peroxide Co. 
2 H.sub.2 N--NH.sub.2 
Aldrich 
32 229 7.16 
Chemical 
Co. 
3 Benzyl Synthe- 
202 
1,450 7.16 
isothi- sized by 
uronium treating 
chloride benzyl 
chloride 
with 
thiourea 
in 
ethanol 
4* HO--CH.sub.2 CH.sub.2 --SH 
Aldrich 
78.1 
559 7.16 
Chemical 
Co. 
5* Acetalde- Allied 
59.1 
423 7.16 
hyde Signal 
Oxime 
6 Aniline Aldrich 
93.1 
666 7.16 
Chemical 
Co. 
7 Benzylamine 
Aldrich 
107 
766 7.16 
Chemical 
Co. 
8 Benzoic Aldrich 
136 
973 7.16 
hydrazide Chemical 
Co. 
9 2-butanone 
Allied- 
87 623 7.16 
oxime Signal 
10 Benzaldehyde 
Aldrich 
121 
697 5.76 
oxime Chemical 
Co. 
11 Dimethyl Aldrich 
144 
1,030 7.16 
maleate Chemical 
Co. 
12 Maleic Baker 98.1 
702 7.16 
Anhydride Perfor- 
mance 
Chemi- 
cals, 
Inc. 
13 Aceto- Aldrich 
120 
859 7.16 
phenone Chemical 
Co. 
14 Benzo- Aldrich 
182 
1,300 7.16 
phenone Chemical 
Co. 
15 Thiophenol 
Aldrich 
110 
788 7.16 
Chemical 
Co. 
16 Diethyl Aldrich 
160 
1,150 7.16 
malonate Chemical 
Co. 
17 Triphenyl- 
Aldrich 
262 
1,880 7.16 
phosphine Chemical 
Co. 
18 Triphenyl Aztec 310 
2,220 7.16 
phosphite Catalyst 
Co. 
19 Trimethyl Aldrich 
124 
888 7.16 
phosphite Chemical 
Co. 
20 Benzaldehyde 
Aldrich 
106 
759 7.16 
Chemical 
Co. 
21 2 Benz- -- 212/ 
1520/ 7.16 
aldehyde/1 58 415 (reaction 
acetone product) 
22 30% H.sub.2 O.sub.2 
EM 34 811 7.16 
Science 
23 25% Na.sub.2 CS.sub.3 
Baker 154 
4,410 7.16 
Perfor- 
mance 
Chemi- 
cals, 
Inc. 
__________________________________________________________________________ 
*Candidates 4 and 5 inadvertently may have been doubledosed with vinyl 
acetate. 
The bottles were observed for color and precipitation after 24 hours. The 
results are shown in Table II: 
TABLE II 
______________________________________ 
CANDI- 
DATE OBSERVATION AT 24 HOURS 
______________________________________ 
1 Dark red solution, red deposit on top 
2 Colorless, no haze, no precipitate 
3 Light yellow solution, no red precipitate; but 
insoluble candidate on bottom 
4 Yellow solution, insoluble candidate on bottom 
5 Dark red solution, very hazy, possible 
precipitate 
6 Red solution, unreacted candidate on top 
7 Red solution, unreacted candidate on top 
8 Yellow solution, voluminous amount of white, air- 
filled precipitate on top 
9 Light red solution, no haze or precipitate 
10 Light red solution, insoluble candidate on top 
11 Dark red solution, very hazy; lots of red 
precipitate 
12 Dark Red solution, very hazy; lots of red 
precipitate 
13 Red solution, small amount of insoluble candidate 
on bottom 
14 Red solution; no haze or precipitate, but 
substantial unreacted candidate on top 
15 Yellow solution, no haze or precipitate, but a 
lot of insoluble candidate on top 
16 Red solution, hazy but no observable precipitate 
17 Red solution, ho haze or precipitate, but 
substantial undissolved candidate 
18 Light yellow solution, no haze or precipitate, 
but substantial unreacted candidate 
19 Dark red solution, haze, but no obvious 
precipitate 
20 Red solution, no haze, no precipitate, but 
substantial sticky black precipitate on top 
21 Red solution, hazy, a lot of sticky black 
precipitate on top 
22 Dark red solution, no haze, no precipitate 
23 Light red solution, no haze; flaky, black 
precipitate on bottom 
______________________________________ 
Based on the foregoing, candidate 2 (hydrazine) is a preferred inhibiting 
agent for MEA scrubbers because the solution is colorless and no haze or 
precipitate is present. Candidates 1, 9, and 22--tertiary-butyl 
hydroperoxide, 2-butanone oxime, and hydrogen peroxide, 
respectively--appear to be suitable agents for inhibiting aldol 
condensation in MEA scrubbers. 
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.