Antifoulant for inorganic fouling

A salt of polyacrylic acid having a molecular weight of between 1,000 and 50,000, preferably 1,000 to 5,000, is used to inhibit inorganic fouling in crude oil.

BACKGROUND OF THE INVENTION 
This invention relates generally to the treatment of crude oil streams to 
prevent fouling by inorganic materials. In one aspect, the invention 
relates to the prevention or reduction of inorganic fouling in crude oil 
heat exchangers using the salts of polyacrylic acid and its derivatives. 
The fouling of refinery equipment by crude oil has long been recognized. 
Such fouling not only damages equipment and reduces the effectiveness of 
heat exchange equipment, but also adds expense to the operation in that 
frequent shutdown times may be required to remove the fouling deposits. 
The fouling by crude oil at the heat exchanger upstream of the refinery 
distillation unit is particularly critical since the heat exchange tubes 
are generally small and cannot tolerate the buildup of solids. 
Fouling may be due to a variety of causes, including fouling by 
asphaltenes, fouling by polymerization of olefins, and fouling by 
oxidation, and fouling by deposition of inorganic impurities in the crude. 
Antifoulants for crude oil are formulated to combat the type of fouling 
encountered. For example, a variety of polyamines are available for 
inhibiting asphaltene fouling (see U.S. Pat. No. 4,619,756, column 4). 
Nitroxide compounds and antipolymerization additives are available for 
treating monomers to prevent polymer fouling (see U.S. Pat. No. 
4,670,131). Antioxidants such as phenylene diamine and phenolic 
antioxidants are available for combating oxidation fouling. 
Very few antifoulants for use in crude oil streams have been developed 
specifically for combating fouling by inorganic materials. These materials 
are present as impurities in the water phase of crude oil and include 
salts, silica, alumina, and corrosion by-products,s etc. Upon passing 
through the heat exchangers of the refinery these materials deposit on the 
heat transfer surfaces, reducing the heat transfer efficiency and/or 
plugging the equipment. All crudes have some water present. Even after 
desalting, there may be a trace to 0.5 percent of water which contains the 
inorganic impurities. 
Fouling by the inorganic impurities in crude oil has long been recognized, 
as evidenced by U.S. Pat. Nos. 3,546,097 and 3,776,835. U.S. Pat. No. 
3,558,470 describes the problems of inorganic impurities as causing 
complexes with organic deposits and oxidize derivatives resulting in the 
deposition of these materials as oil insoluble polymers and complexes. 
This patent suggests the use of an ashless dispersant and a phosphite type 
of antioxidant-antipolymerant compound. U.S. Pat. No. 3,776,835 identifies 
inorganic salts as foulants in crude oil and suggests the use of certain 
amines and amides in a hydrogen environment. 
Both of these solutions appear to address the problem of asphaltene and/or 
unsaturated olefin fouling and oxidation fouling, with the role of 
inorganic materials as being part of the complexes, polymers, and 
oxidation products. 
SUMMARY OF THE INVENTION 
Surprisingly, it has been discovered that by treating crude oil to 
specifically inhibit fouling by inorganic impurities, the fouling of the 
crude oil heat exchanger is greatly reduced. The antifoulant used in 
accordance with the present invention is a salt of polyacrylic acid or its 
derivatives having the following formula: 
##STR1## 
where R is H or a C.sub.1 -C.sub.3 alkyl group; 
M is an alkaline earth metal, preferably Na and K, most preferably Na; 
n is an integer ranging from about 10 to 200, preferably 15 to 100, most 
preferably 20 to 40. 
The antifoulant is water soluble and is used at a treating rate of 5 to 
1000 ppm, preferably 10 to 500 ppm, most preferably 20 to 200 ppm. The 
antifoulant preferably is introduced into the crude stream immediately 
upstream of the preheater of the distillation unit. 
The inorganic antifoulant may be used alone or with other antifoulants 
designed to treat other forms of fouling. Such other antifoulants include 
amines such as polyisobutylene succinic anhydride-polyamine adduct 
(PIBSA-PAM), which is used to treat mainly organic fouling, such as that 
disclosed in U.S. Pat. No. 4,619,756. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The treatment of crude oil with salts of polyacrylic acid for the reduction 
of inorganic fouling in accordance with the present invention is 
preferably carried out with crude oils having a water content of less than 
5%, preferably 2% or less. Thus, the method of the invention can be used 
with desalted crudes, or low water content crudes. The method of the 
invention thus is directed mainly at protecting the preheater of the 
distillation units where the flow tubes are relatively small, compared to 
downstream facilities. Unlike many of the prior art treatments, the 
treatment in accordance with the present invention is directed solely at 
reducing fouling by inorganic impurities in the crude oil. Such impurities 
include sodium chloride, magnesium chloride, calcium sulfate salts, and 
silica alumina. 
The salts of the polyacrylic acids may have molecular weights ranging from 
1,000 to 50,000, but preferably are within the range of 1,000 to 5,000 and 
most preferably between 1,800 to 3,600. 
The salts of acrylic acid and its derivatives may have the following 
formula: 
##STR2## 
where R is H or a C.sub.1 -C.sub.3 alkyl group; 
M is an alkaline earth metal, preferably Na and K, most preferably Na; 
n is an integer ranging from about 10 to 200, preferably 15 to 100, most 
preferably 20 to 40. 
The preferred treating compound includes the following polyacrylic acid 
derivatives: the C.sub.1 -C.sub.3 alkyl substituted salts of polyacrylic 
acid such as salt of polymethacrylic acid (e.g. sodium methacrylate and 
potassium methacrylate polymers). The most preferred compound is sodium 
polyacrylate. 
Polyacrylic acid derivatives which can be used in the present invention may 
be prepared by methods well known in the art. See, for example, 
Encyclopedia of Polymer Science and Engineering, Volume 1, pages 169,211 
and 234, the disclosures of which are incorporated herein by reference. 
Alternatively, many of these compounds are commercially available. A 
polymeric salt that has performed quite well is a product marketed by ALCO 
Chemical Company under trade designation AR910. 
These polyacrylate salts are available in concentrations of 5 to 50 wt % in 
a suitable carrier such as water.

EXPERIMENTS 
The following experiments demonstrate the significant role of inorganic 
fouling in the fouling of refinery equipment, specifically the preheater, 
and the effectiveness of treatments using the antifoulant described 
herein. 
All experiments were carried out using a laboratory test apparatus known as 
the thermal fouling tester (TFT). The TFT is a modification of the Alcor 
Tester described in ASTM Vol. 50, D-3241. It is configured to allow 
measurement of the fluid temperature at the exit of the heat exchanger 
while the metal temperature of the heated tube is controlled. The test 
thus measures the change in temperature of a fluid which has been pumped 
across a heated metal surface. The outlet temperature is directly related 
to the heat transferred to the fluid. If fouling occurs, a deposit adheres 
to the heated metal surface and insulates a portion of the surface from 
the test fluid. The insulating deposit reduces the rate of heat transfer 
to the fluid and its temperature decreases. The rate of change in the 
fluid temperature is a measure of the rate of fouling. 
The time over which temperature measurements are recorded was set at 3 
hours. By doing this, the changes in temperatures of several fluids can be 
used as a measure of their relative fouling tendencies. 
Experiment I Series 
Three samples of a crude oil at various stages of dehydration were tested. 
The crude oil was highly aromatic which exhibited little or no asphaltene 
fouling. The crude oil samples were as follows: 
Sample 1--crude oil emulsion 
Sample 2--dewatered crude oil 
Sample 3--desalted crude oil 
The results are presented in Table I. 
TABLE I 
______________________________________ 
Crude Oil 
Dewatered Desalted 
Emulsion 
Crude Oil Crude Oil 
______________________________________ 
Water Present (wt %) 
5 1-2 0.1-0.2 
Antifoulant None None None 
Antifoulant (PPM) 
None None None 
TFT Heater Temp (.degree.F.) 
700 700 700 
Unit Pressure (psig) 
500 500 500 
Test Time (min.) 
180 180 180 
Oil Flow (cc/min.) 
3.0 3.0 3.0 
Thermal Fouling 
44 19 0.00 
(.DELTA.T, .degree.F.) 
______________________________________ 
The Table I data demonstrate that inorganic fouling is present and that the 
fouling appears to be due to the presence of water. 
Experiment II Series 
Additional TFT tests were run to determine the effect of the use of the 
polyacrylate antifoulant on the same crude used in Experiment I. The raw 
crude had 20-30 pounds of salt per 1000 barrels of crude. The antifoulant 
was ALCO's AR910 (sodium polyacrylate); MW of 2000-3300 and 28% active in 
water. 
Tests were run varying the amount of the antifoulant. The results of these 
tests are presented in Table II. 
TABLE II 
______________________________________ 
Antifoulant None Yes Yes 
Active Antifoulant (PPM) 
None 250 50 
Heater Metal Temp. (.degree.F.) 
450 450 450 
Liquid Temp .degree.F.) 
404 373 402 
Unit Pressure (psig) 
550 550 550 
Oil Flow (cc/min.) 
6 6 6 
Test Time (min.) 180 180 180 
Thermal Fouling (.DELTA.T, .degree.F.) 
30 8 12 
______________________________________ 
The above laboratory experiments clearly demonstrate the effectiveness of 
the use of the polyacrylate antifoulant in mitigating the effects of 
inorganic fouling. Additional experiments were carried out in a refinery 
which experienced severe fouling in the preheater of the distillation 
unit. The crude oil was raw dewatered crude oil and had a salt content of 
200 per thousand barrels and filterable solids of 300 to 500 PPM based on 
ASTM D 4807-88. Filterable solids is a measure of the presence of 
inorganic material. When filterable solids exceed 150 ppm, fouling becomes 
a serious problem. 
The antifoulant (AR 910) was injected upstream of the preheater at a rate 
of about 12.5 ppm. The treatment was continued for 60 days without 
plugging. This increased the run time of the preheater by 200%. This test 
demonstrated that the present invention can be used in crude oil that has 
not been desalted. In fact, the present invention may delay or avoid the 
need for desalting.