Method for Minimizing Fouling of Heat Exchanges

This invention relates to antifoulants and to a method of inhibiting fouling in petroleum or petroleum derivative processing equipment by injecting an antifoulant composition into a feed stream of the material being processed.

Fouling of heat transfer surfaces of petroleum processing equipment occurs continuously during the period when petroleum or its derivatives are being processed in the equipment. The fouling is caused by the gradual buildup of a layer of high molecular weight polymeric material resulting from the thermal polymerization of unsaturated materials which are present in the petroleum. As time goes by, fouling continues with the attendant loss of heat transfer until finally the point is reached where it becomes necessary to take the equipment out of service for cleaning. Cleaning is expensive and time consuming, consequently methods of preventing fouling, or at least significantly reducing the rate of fouling, are constantly being sought.

The most economical method of reducing the fouling rate in process heat transfer equipment is to add chemicals which inhibit fouling, called "antifoulants", to the feed stream being processed. Among the more interesting classes of chemical compounds which exhibit antifoulant activity are the dialkylhydroxylamines. Their use to inhibit polymerization has been described in several patents.

U.S. Pat. No. 3,148,225, issued to Albert, employs dialkylhydroxylamines for inhibiting popcorn polymer formation in styrene-butadiene rubbers. The dialkylhydroxylamine compounds appear to react with and terminate free radicals which cause undesired formation of polymer. U.S. Pat. No. 2,965,685, issued to Campbell, discloses inhibiting polymerization by adding about 5 ppm to 5 percent dialkylyhydroxyamine to styrene monomer. Sato et al, in U.S. Pat. No. 4,849,498, teach the use of diethylhydroxylamine as a polymerization inhibitor for an alcoholic solution of unsaturated aldehydes. Mayer-Mader et al, U.S. Pat. No. 3,878,181, employ diethylhydroxylamine either alone or in combination with a water soluble amine such as triethanolamine to terminate the aqueous emulsion polymerization of chloroprene. Albert, U.S. Pat. No. 3,333,001 discusses use of 2-mercaptobenzothiazole as a stabilizer for aqueous solution of dialkylhydroxylamines. Gross U.S. Pat. No. 3,426,063 uses an arylhydroxylamine to inhibit popcorn polymer formation. Miller, U.S. Pat. No. 3,105,810 describes the use of dodecylbenzenesulfonic acid as a surfactant in antifoulant compositions. Gonzales, U.S. Pat. No. 3,271,295 descloses substituted succinicimides as surfactants in antifoulant compositions.

It has now been discovered that mixtures of N,N-dialkylhydroxylamines and organic surfactants provide outstanding antifoulant protection for petroleum and petroleum derivative processing equipment. Thus, because of the synergistic effect of these mixtures it is now possible to provide unexpectedly superior antifouling protection with the same total equivalent weight of N,N-dialkylhydroxylamines and surfactant mixtures than can be obtained by the use of members of either of these groups of compounds by themselves.

Accordingly, it is an object of the invention to present new petroleum processing equipment antifoulant compositions. It is another object of the invenion to present a method of enhancing the antifouling protection of petroleum processing equipment. These and other objects of the invention are set forth in the following description and examples of the invention.

The improved antifoulant compositions of the invention are comprised of mixtures of one or more dialkylhydroxylamines and one or more organic surfactants. Preferred dialkylhydroxylamines are those having 2 to 10 carbon atoms in each alkyl group. Preferred organic surfactants are the polymeric surfactants prepared from vinyl pyridine or vinyl pyrrolidone with ethylene or propylene or both ethylene and propylene and the alkyl benzene sulfonic acids or amine-neutralized sulfonates. In a preferred embodiment of the process of the invention the antifoulant composition is dissolved in an organic solvent and the resulting solution is continuously injected into a stream of petroleum at a point which is upstream from the equipment which is to be protected.

The N,N-dialkylhydroxylamine compounds used in the invention have the structural formula

wherein R and R' are the same or different straight or branched-chain alkyl groups having 2 to about 10, and preferably 2 to 6, carbons atoms. Although N,N-dialkylhydroxylamines having more than about 10 carbon atoms in each alkyl group may be useful in the invention it is preferred that compounds containing 10 or fewer carbon atoms in each alkyl group be used in the invention because the latter compounds are commercially available. Mixtures of two or more N,N-dialkylhydroxylamines can also be advantageously used in the compositions of the invention.

Suitable N,N-alkylhydroxylamines include N,N-diethydroxylamine, N,N-dibutylhydroxylamine, N,N-butylethylhydroxylamine, N,N-didecylhydroxylamine, N,N-2-ethylbutyloctylhydroxylamine, etc. Examples of preferred N,N-dialkylhydroxylamines include N,N-diethylhydroxylamine and N,N-dibutylhydroxylamine. As noted above, two or more of these compounds may be used in combination, if desired.

The organic surfactants can be any of the anionic, cationic and non-ionic compounds which exhibit surfactant activity.

Representative of the anionic surfactants useable in the invention are the organic sulfonates such as the alkyl benzene sulfonic acids and their amine-neutralized salts and the alkylated naphthalene sulfonic acids.

Alkyl benzene sulfonic acids particularly useful in the invention are those having the structural formula ##STR1## atoms in R may vary from 1 to 20 or more. The total number of carbon atoms in R may exceed 20 but no particular advantage is derived from the use of such high molecular weight compounds. The alkyl groups may be straight or branched-chain. Preferred alkyl benzene sulfonates are those in which the total number of carbon atoms in R in the above formula is 4 to 14. Mixtures of two or more alkyl benzene sulfonates may be used in the invention if desired.

Amine-neutralized alkyl benzene sulfonic acids may also be used in the invention. Suitable neutralizing amines are the alkyl and cycloalkyl amines having up to 20 carbon atoms. Preferred amines are those having 1 to 10 carbon atoms.

Suitable alkyl benzene sulfonic acids include p-t-butylbenzene sulfonic acid, p-t-amylbenzene sulfonic acid, octylbenzene sulfonic acid and dodecylbenzene sulfonic acid. Preferred alkylbenzene sulfonic acids include p-t-butylbenzene sulfonic acid, dodecylbenzene sulfonic acid. etc.

Alkylated naphthalene sulfonates useful in the invention are those having the structural formula ##STR2## wherein R" and R'" are the same or different substituents selected from H and alkyl groups and the total number of carbon atoms in each of R" and R'" may vary from 3 to 20 or more. The total number of carbon atoms in R" and R'" may exceed 20 but no particular advantage is derived from the use of such high molecular weight compounds. When R" and/or R'" are alkyl groups they may be straight or branched-chain. Preferred alkyl naphthalene sulfonates are those in which the total number of carbon atoms in each of R" and R'" in the above formula is 3 to 14. Mixtures of two or more alkyl naphthalene sulfonates may be used in the invention if desires.

Suitable alkylated naphthalene sulfonic acids include n-butyl naphthalene sulfonic acid, t-butyl naphthalene sulfonic acid, di-t-butyl naphthalene sulfonic acid, hexyl naphthalene sulfonic acid, 4,5-didodecyl naphthalene sulfonic acid, 4-decyl-5-octadecyl naphthalene sulfonic acid, etc.

Typical of the cationic surfactants useful in the invention are the oxidized and unoxidized graft copolymers of ethylene and vinyl pyridine, propylene and vinyl pyridine and ethylene, propylene and vinyl pyridine and polymethacrylates, such as polymethylmethacrylate. Mixtures of two or more cationic surfactants may be used in the invention, if desired. Preferred cationic copolymers are the copolymers of ethylene and/or propylene with vinyl pyridine having number average molecular weights of about 5,000 to 300,000.

Nonionic surfactants useful in the invention include the oxidized and non-oxidized graft copolymers of ethylene and vinyl pyrrolidone, propylene and vinyl pyrrolidone and ethylene, propylene and vinyl pyrrolidone; polyimides, such as polysuccinimide and polyamides, such as diethyleneaminediamide. Mixtures of two or more-ionic surfactants may be used in the invention, if desired. Preferred nonionic surfactants include the oxidized or non-oxidized copolymers of ethylene and/or propylene with vinyl pyrrolidone having number average molecular weights of about 5,000 to 300,000 and polysuccimides having number average molecular weights of about 5,000 to 300,000.

Some N,N-dialkylhydroxylamines, such as N,N-diethylhydroxylamine, and many surfactants, such as dodecylbenzene sulfonic acids, polysuccinimide and ethylenepropylene-vinyl pyrrolidone graft terpolymer are available commercially. Those N,N-dialkylhydroxylamines and surfactants which are not commercially available may be prepared by any of the well known techniques. The preparation of these compounds forms no part of the present invention.

The concentrations of N,N-dialkylhydroxylamine and surfactant in the compositions of the invention are generally in the range of about 10 to 90 weight percent N,N-dialkylhydroxylamine and 90 to 10 weight percent of surfactant, based on the total combined weight of these components. In preferred embodiments the concentrations generally fall in the range of about 25 to 75 weight percent N,N-dialkylhydroxylamine and 75-25% surfactant based on the total combined weight of these components.

The antifoulant compositions of the invention may include other additives, if desired. For example, other antifoulants may be used in combination with the above antifoulants of this invention, or corrosion inhibitors, etc. may be combined with the antifoulant compositions to improve the efficiency of these compositions or to provide additional protection to the process equipment.

The antifoulant compositions of the invention can be introduced into the equipment to be protected by any conventional method. It is generally introduced just upstream of the point of desired application by any suitable means, such as by use of proportionating pump. The antifoulant composition may be added as a concentrate but it is preferable to add it as a solution or a slurry in a liquid diluent or solvent which is compatible with the stream being treated. Suitable solvents include kerosene, petroleum distillate, naphtha, the lower alkanes such as hexane, aromatic solvents, such as toluene, etc. The concentration of antifoulant in the solvent is desirably in the range of about 1 to 30 weight percent and preferably about 5 to 20 weight percent based on the total weight of antifoulant and solvent.

The antifoulant is used at the concentration which is effective to provide the desired protection against fouling. It has been determined that amounts of antifoulant in the range of about 0.5 to 1000 ppm based on the weight of the petroleum or petroleum derivative stream being treated afford ample protection against fouling. For most applications the inhibitor is used in amounts in the range of about 1 to 100 ppm.

The following examples will serve to further illustrate the invention. Unless otherwise stated. parts and percentages are on a weight basis.

In the examples the thermal fouling determinations were made using a Thermal Fouling Tester marketed by Alcor, Inc. In general the apparatus consists of a reservoir to hold the hydrocarbon liquid being tested, an electrically heated tubular heater and a precision stainless steel filter. Tubular conduit connects the reservoir with the heater and the heater with the filter. Pressure gauges are provided for measuring the pressure drop across the filter. A thermocouple and a temperature controller are provided for precise control of the temperature of the liquid passing through the heater.

In operation, a hydrocarbon oil is pumped through the heater, which has adequate heat transfer surface to maintain the heater effluent at a predetermined temperature in the range of about 250.degree. to 900.degree. F. As the hydrocarbon passes through the heater a film of polymeric residue builds up on the inside of the heater. Particles of the residue slough off the surface of the heater tube and are caught in the filter. As the filter clogs up the pressure drop across the filter increases. The fouling rate in the heater is approximated by measuring the rate of pressure build-up across the filter or by measuring the temperature decrease due to fouling in the exchanger and calculating the change in heat transfer coefficient (U). U is determined by the following equation:

Where Q is the heat input to the oil, BTU/hr; U is the heat transfer coefficient, BTU/hr-ft.sup.2 ; A is the area of the heat transfer surface, ft.sup.2 ; and t is the difference between the oil temperature and the wall temperature. .degree.F. The equipment is dismantled and thoroughly cleaned after each test run.

A series of antifoulant effectiveness tests were conducted using crude oil as the hydrocarbon carrier liquid. During the tests the flow rate of the crude oil through the heated annular zone was maintained at 240 ml per hour, the heater inlet temperature was maintained at 149.degree. C. and the heat exchanger skin temperature was maintained at 399.degree. C. Run 1 was carried out using uninhibited crude oil. Run 2 was carried out using 25 ppm (based on the weight of neutralized acid) of a commercial dodecylbenzene sulfonic acid neutralized with a primary alkyl amine (sold by Rohm and Haas Company under the trademark Primene.RTM. 81R). Run 3 was carried out using 25 ppm (based on the total weight of neutralized acid) of a 1:1 equivalent weight ratio mixture of diethylhydroxylamine and a commercial dodecylbenzene sulfonic acid. The temperature and heat transfer data for these three experiments are tabulated in Table I as Runs 1, 2 and 3 respectively.

This example illustrates the benefits derived from using one of the antifoulant compositions of the invention in a crude oil stream. Run 1, in which no antifoulant was used showed a significant decrease in heat transfer efficiency after three hours of operation. The results obtained in Run 2 in which a benzene sulfonic acid neutralized by a primary amine was used, showed slightly less fouling (6.7%) after three hours than that which occurred in the Run 1 test. On the other hand, Run 3 in which one of the antifoulant compositions of the invention was used, showed 57% less fouling after three hours of operation, compared to the untreated crude oil run.

A sample of depropanized thermally cracked gas oil was evaluated in the Thermal Fouling Tester described above. Fouling was measured by periodically measuring the pressure drop across the filter. The test was conducted for 3 hours (180 minutes). Pressure drop readings were made every 30 minutes. During the test the heated annular zone was maintained at a temperature of 260.degree. C. The results are reported in Table II as Run 1.

The procedure of Example II was repeated except that the depropanized feedstock was modified by the addition of 50 ppm of a kerosene solution containing 20% weight percent of an ethylene-propylene-vinyl pyridine graft interpolymer (sold by Rohm and Haas Company under the trademark Acryloid.RTM. 1159) and 4 weight percent of a diethylene triamine polyamide having a molecular weight of <1000 (sold by ChemLink, Inc. under the trademark IPC2010.TM.). The fouling rate data are reported in Table II as Run 2.

The procedure of Example II was repeated except that the depropanized feedstock was modified by the addition of 50 ppm diethylhydroxylamine. The fouling data are reported in Table II as Run 3.

The procedure of Example II was repeated except that the depropanized feedstock was modified by the addition of 50 ppm of a kerosene solution containing 20 weight percent Acryloid.RTM. 1159 graft copolymer, 4 weight percent IPC 2010.TM. polyamide and 10 weight percent diethylhydroxylamine. Fouling data are reported in Table II as Run 4.

The results tabulated in Table II show the improvement obtained when one of the compositions of this invention was added to a depropanized thermally cracked gas oil feedstock which was subsequently subjected to prolonged exposure to elevated temperatures. The Run 1 test showed that a significant pressure resulted when an untreated sample of the depropanized feedstock was tested for 3 hours. Run 2, in which a commercial surfactant was added to the depropanized also showed significant pressure drop after three hours. Run 3 showed a considerable improvement using diethylhydroxylamine as the antifoulant. However, in the last half hour the pressure drop increased significantly in this test also. In the Run 4 test, which illustrates an embodiment of this invention, no measurable drop resulted after three hours.

The foregoing examples illustrate specific examples of the invention. Modifications of these examples is considered to be within the scope of the invention. For example, the antifoulant composition of the invention may be combined with other antifoulants. Furthermore, the additives can be added to other organic liquids which cause thermal fouling. The scope of the invention is limited only by the breadth of the appended claims.