Antifoulant for acrylic acid purification

The invention comprises a method of dispersing foulants, residues, gums, precipitates, polymeric tars and other highly oxidized carbonaceous tars which can be formed in the process of manufacture and recovery of acrylic acid. The method comprises adding to the liquid or gaseous phases passing through, or stored in acrylic acid process equipment, an effective antifouling amount of a dispersant which is stable in the environment within an acrylic acid process and is compatible with the equipment used for the manufacture of acrylic acid.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method of dispersing foulants which are 
generated in an acrylic acid production unit. 
2. Description of the Prior Art 
Foulant formed in acrylic acid processes typically comprises poly(acrylic 
acid) and acrylic acid dimer, trimer, tetramer, etc. (poly(ester)). 
Poly(acrylic acid) is typically formed by the free-radical polymerization 
of acrylic acid. The dimer and the polyester are typically formed by the 
addition of acrylic acid across the carbon-carbon double bond. Fouling due 
to poly(acrylic acid) and addition products, if not cracked to the acrylic 
acid starting material, could be treated with a dispersant. 
The use of dodecylbenzene sulfonic acid-containing products as dispersants 
in a vinyl acetate monomer production process is disclosed in U.S. Pat. 
No. 4,902,824, the disclosure of which is incorporated herein by 
reference. The '824 patent claims the use of benzene sulfonic acids in 
solvents as antifoulants for the vinyl acetate manufacturing process. 
Similar fouling problems occur in acrylic acid production processes. 
Although many hydrocarbon production processes are superficially similar, 
it is often the case that compositions which are effective for a given 
application in one process are totally unsuitable in another. As 
exemplified in the instant application, the fact that benzene sulfonic 
acids effectively disperse foulant in the vinyl acetate manufacturing 
process, is not dispositive of whether benzene sulfonic acids would also 
disperse foulant in acrylic acid manufacturing processes. The nature of 
the monomers and of the fouling species in these areas are very different 
as described below. 
The foulant in a vinyl acetate unit consists of poly(vinyl acetate), vinyl 
acetate oligomers, polymeric aldehydes, and oxidation products. Poly(vinyl 
acetate) and the vinyl acetate oligomers are formed by the free radical 
polymerization of vinyl acetate. Vinyl acetate dimers are formed by the 
addition of acetic acid across the carbon-carbon double bond of vinyl 
acetate. Aldol condensation of the aldehydes species result in oligomers 
and polymers. Other polymers related to the ethylene oxidation in the 
reactor section can also contribute to fouling. 
The foulant in the acrylic acid process consists of poly(acrylic acid), 
acrylic acid dimer, and higher molecular weight addition products. 
Poly(acrylic acid) is formed by the radical polymerization of acrylic 
acid. The dimer of acrylic acid and the poly(ester), which are trimers, 
tetramers, pentamers, etc., are formed by the addition of acrylic acid 
across the carbon-carbon double bond. 
The foulant species for each process are different. Poly(vinyl acetate) is 
soluble in its monomer, vinyl acetate. Poly(acrylic acid) is not soluble 
in its monomer, acrylic acid. The following tables, taken from the Polymer 
Handbook, list solvents and nonsolvents for poly(vinyl acetate), and 
poly(acrylic acid). Poly(vinyl acetate) is soluble in common organic 
solvents. Poly(acrylic acid) is soluble in polar solvents like water and 
DMF. The solvents for poly(vinyl acetate) have dipole moments in the range 
of 0-2 D. The solvents for poly(acrylic acid) have dipole moments in the 
range of 1.5-3.9 D. 
______________________________________ 
Solvents For Polymers of Vinyl Acetate and Acrylic Acid 
poly(vinyl acetate) solvents 
poly(acrylic acid) solvents 
______________________________________ 
2,4-dimethyl-3-pentanol 
alcohols 
allyl alcohol dilute alkali 
benzene dioxane/water 80:20 (isotactic) 
acetone DMF 
acetic acid formamide 
benzyl alcohol 1-methyl-2-pyridone 
carbon tetrachloride/ethanol 
chlorobenzene 
chloroform 
dichloroethylene/ethanol 20:80. 
Dioxane 
ethanol/water 
glycol ether esters 
glycol ethers 
lower aliphatic esters 
methanol 
tetrahydrofurfuryl alcohol 
THF 
toluene 
______________________________________ 
Polymer Handbook, 3rd ed., Branrup, J. and Immergut, E. H., eds., Wiley 
Interscience, New York, 1989. 
______________________________________ 
Nonsolvents For Polymers of Vinyl Acetate and Acrylic Acid 
poly(vinyl acetate) nonsolvents 
poly(acrylic acid) nonsolvents 
______________________________________ 
saturated hydrocarbons 
hydrocarbons 
mesitylene esters 
carbon tetrachloride 
ketones 
ethanol (anhydrous) 
dioxane at higher temperatures 
ethylene glycol 
cyclohexanol 
diethyl ether 
higher esters C &gt; 5 
carbon disulfide 
water 
dilute acids 
dilute alkalies 
______________________________________ 
Polymer Handbook, 3rd ed., Branrup, J. and Immergut, E. H., eds., Wiley 
Interscience, New York, 1989. 
Acrylic acid and poly(acrylic acid) are acidic species and exhibit the 
properties of an acidic species. They are polar, water soluble, and react 
with bases to form salts. Vinyl acetate is not an acidic species and does 
not exhibit these properties. 
In summary, the foulant species in the vinyl acetate and acrylic acid 
processes exhibit vastly different physical characteristics. These 
differences include physical properties, solubilities, and reactivities. 
The interaction of dispersants with foulant material is dependent on 
properties like polarity (which correlates with dipole moment) and 
solubility. 
Thus, the fact that dodecylbenzene sulfonic acid effectively disperses 
poly(vinyl acetate), a material that is soluble in vinyl acetate and 
common organic solvents, such as toluene, does not predict that it will 
effectively disperse poly(acrylic acid), a more polar material that is not 
soluble in its monomer, and is not soluble in any common organic solvent. 
It would therefore be an advance in the art if one could simply add an 
effective amount of a dispersant to acrylic acid process equipment, so as 
to minimize or eliminate the possibility of foulant deposition which may 
inhibit the ability to achieve efficient use of the acrylic acid process 
equipment. 
SUMMARY OF THE INVENTION 
The invention comprises a method of dispersing foulants, residues, gums, 
precipitates, polymeric tars and other highly oxidized carbonaceous tars 
which can be formed in the process of manufacture and recovery of acrylic 
acid. 
The method comprises adding to the liquid or gaseous phases passing 
through, or stored in acrylic acid process equipment, an effective 
antifouling amount of a dispersant which is stable in the environment 
within an acrylic acid process and is compatible with the equipment used 
for the manufacture of acrylic acid. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The dispersant which has been found that meets all of these requirements, 
i.e., a dispersant that effectively disperses foulant caused by polymeric 
tars, residues, highly oxidized carbonaceous tars and debris, and similar 
foulants as described above, while being compatible with the environment 
and chemicals used to manufacture acrylic acid, without causing 
difficulties in the manufacture of such monomer, are primarily those 
dispersants which are sulfonic acids. These sulfonic acids are exemplified 
by such materials as dodecylbenzene sulfonic acid, dioctyl sulfosuccinic 
acid, methane sulfonic acid and the like. These sulfonic acids may be used 
as is, or may be formulated in a compatible solvent, and may include, 
optionally, other dispersants, other surfactants, antifoaming agents, 
corrosion inhibitors, and similar ingredients. 
The antifoulant formulation preferably used is one that contains 
dodecylbenzene sulfonic acid, and/or its salts, admixed and/or dissolved 
in an organic polar solvent, such a butyl cellosolve, an alkyl capped 
diether material available in commerce. 
These antifoulants can contain from about 20 to about 100 weight percent 
sulfonic acid, preferably dodecylbenzene sulfonic acid, and/or its salts, 
admixed and/or dissolved in an organic polar solvent, such a butyl 
cellosolve, an alkyl capped diether material available in commerce. 
These antifoulants can contain from about 20 to about 100 weight percent 
sulfonic acid, preferably dodecylbenzene sulfonic acid, and from about 0 
to about 80 weight percent polar solvent, preferably butyl cellosolve. 
In addition, the dodecylbenzene sulfonic acid may be present as its salts, 
particularly its quaternary ammonium or amine salts by neutralizing the 
sulfonic acid with various bases or with various amines, including 
polyamines and the like. 
In addition to the dodecylbenzene sulfonic acid, other hydrocarbonaceous 
sulfonic acids may be used in the invention. These sulfonic acids may be 
alkyl or aryl sulfonic acids which may include, but are not limited to, 
such organic sulfonic acids as toluene sulfonic acid, methane sulfonic 
acid, dodecyl sulfosuccinic anhydride, dodecyl sulfosuccinic acid, and 
dioctyl sulfosuccinate. Representative of these sulfonic acids are those 
having the structure: 
EQU R(SO.sub.3).sub.n M 
wherein R is a hydrocarbonaceous group chosen from linear or branched alkyl 
groups, aromatic, cyclic, alkaryl, aralkyl, or alkenyl groups, and 
mixtures thereof; M is hydrogen, alkali metal, alkaline earth metal, 
ammonium cation, alkylamine cation, quaternary amine cation, and the like, 
or mixtures thereof; and n ranges from about 1 to about 6, preferably 
between about 1-4, and most preferably 1-2. 
Also included in such effective sulfonic acids are structures which include 
alkyl aromatic sulfonic acids or alkyl naphthenic sulfonic acids. 
The organic polar solvents to be used are solvents such as butyl cellosolve 
or any of the ethylene oxide based cellosolve capped ether solvents, and 
may also include such organic polar solvents as the diethyl ether of 
tetraethylene glycol, polyethylene and polypropylene oxide alkyl ethers, 
and generally may also include ketonic solvents such as acetone, or ester 
solvents such as ethyl acetate, or ether solvents such as diethyl ether or 
butyl cellosolve. In addition, other polar solvents that also function 
include certain organic acids such as acetic acid, or such other polar 
solvents as diacetone alcohol, linear alkyl and branched alkyl alcohols 
such as propanol, isopropanol, t-butanol, and the like. Admixtures of 
these polar solvents may also be used. 
The alkyl sulfonic acids described above are preferred when used in process 
streams at concentrations ranging from about 10 ppm to about 20,000 ppm, 
based on the weight ratios of the additive formulation to the bottom 
stream to which the formula is added. However, hydrocarbonaceous sulfonic 
acids, or their formulations can function as antifoulants at treatment 
concentrations ranging from about 10-20,000 ppm, preferably between about 
100-10,000 ppm and most preferably, between about 1000-7500 ppm treatment 
acid based on the process stream being treated. 
Also, as can be seen, although the alkyl sulfonic acids can be used as 
amine salts, the activities of some amines, such as the heavy amine 
condensate salts are not as good as the activities of the free acids. 
Therefore, it is most preferred to use the sulfonic acids of the instant 
invention as the free acids. 
______________________________________ 
Dispersant Descriptions 
Dispersant Description 
______________________________________ 
A poly(acrylate) 
B poly(acrylate) 
C phenolic polymer containing sulfonate 
groups 
D arylsulfonic acid/formaldehyde copolymer 
E a-olefin/ma1eic anhydride copolymer 
F aryl sulfonic acid 
G a-olefin/maleic anhydride copolymer 
H oxyalkylate 
______________________________________

The following examples are presented to describe preferred embodiments and 
utilities of the invention and are not meant to limit the invention unless 
otherwise stated in the claims appended hereto. 
EXAMPLE 1 
Eight dispersants were evaluated for compatibility in the process stream. 
Descriptions of these dispersants are given in the table. The aqueous 
dispersants, Dispersant A, Dispersant B, Dispersant C, and Dispersant D, 
precipitated when added to the process stream. Dispersant E also 
precipitated when added to this process stream. Dispersant F, Dispersant 
G, and Dispersant H were soluble in the stream and were used for further 
experiments. 
EXAMPLE 2 
The three soluble dispersants, Dispersant F, Dispersant G, and Dispersant 
H, were evaluated to determine their effectiveness at dispersing 
poly(acrylic acid), obtained from Aldrich Chemical Company, in the process 
stream. The polymer had a molecular weight of 2000. A slurry of 
poly(acrylic acid) in the process stream was prepared. This slurry (1 mL) 
was added to tubes containing 1000 ppm of the dispersants dissolved in the 
process stream. Poly(acrylic acid) settled to the bottom of the untreated 
tube. The poly(acrylic acid) dissolved in all the tubes containing the 
dispersants. An additional 0.1 g of solid poly(acrylic acid) was added to 
each tube. The polymer settled to the bottom of the untreated tube. The 
polymer began to dissolve in the tube containing Dispersant F. The polymer 
also began to dissolve in the tubes containing Dispersant G and Dispersant 
H, but not as quickly as that in the tube containing Dispersant F. 
In order to differentiate among the dispersants, a higher concentration of 
dispersant was used. Using a higher concentration magnified the 
effectiveness of each dispersant. Dispersant F, Dispersant G, and 
Dispersant H (0.5 mL) were dissolved in 8 mL of the process stream. The 
solution of Dispersant G in the process stream was turbid, indicating an 
insoluble component at this concentration. Poly(acrylic acid) with a 
molecular weight of 2000, obtained from the Aldrich Chemical Company, (0.5 
g) was added to each tube. All tubes were shaken. The tube containing 
Dispersant F had significantly less polymer settle to the bottom of the 
tube. Dispersant F appeared to increase the solubility of the poly(acrylic 
acid) in the process stream. There was no noticeable difference in the 
samples containing Dispersant G, Dispersant H, and no dispersant. 
EXAMPLE 3 
The dispersants were also tested using the foulant from an acrylic acid 
unit. The foulant was suspended in the process stream. This suspension (1 
mL) was added to tubes containing dispersants in the process stream. 
Dispersant concentration was 1000 ppm. All foulant in tubes containing the 
dispersants settled at a slower rate than the foulant in the untreated 
tube. Some foulant remained suspended in all tubes, including the 
untreated tube, after 20 minutes. The experiment was repeated using 10,000 
ppm dispersant with the same results. 
The experiment was repeated using a five per cent dispersant. Initially, 
all of the tubes looked the same. After 12 hours, the tube containing 
Dispersant F was darker than at time zero. The undispersed poly(acrylic 
acid) foulant had a swollen appearance. No significant changes occurred in 
the tubes containing Dispersant G and Dispersant H. Dispersant F appeared 
to increase the solubility of the foulant in the process stream. 
Changes may be made in the composition, operation and arrangement of the 
method of the present invention described herein without departing from 
the concept and scope of the invention as defined in the following claims: