METHODS FOR INHIBITING HIGH TEMPERATURE CORROSION FIELD OF THE INVENTION The present invention relates to methods for inhibiting corrosion of process equipment in high temperature crude oil processing. More particularly, the present invention relates to the periodic addition of a film forming phosphorous-containing compound to the crude oil, crude oil fractions and residua to provide for a tenacious durable film on the surfaces of the processing equipment without the need for continuous maintenance dosages.

BACKGROUND OF THE INVENTION Hydrocarbon and petroleum refining operations suffer corrosion problems due in part to naphthenic acid constituents and sulfur compounds in crude oils. This corrosion is particularly severe in atmospheric and vacuum distillation units operating at temperatures between about 4000 and 7900F. The amount of naphthenic acid constituents and sulfur compounds, the velocity and turbulence of the

crude oil flow, and the location of the unit (e.g., liquid/vapor interface) all contribute to the corrosivity of crudes, fractions and residua.

In the distillation refining of crude oils, the crude oil is passed successively through a furnace and one or more fractionators such as an atmospheric tower and a vacuum tower. In most operations, naphthenic acid corrosion is not a problem at operating temperatures below about 4000F. Traditional nitrogen-based filming corrosion inhibitors are not effective at temperatures above 4000F. Furthermore, other corrosion inhibiting approaches such as neutralization present operational problems or are simply not effective.

Other efforts to minimize or prevent the corrosion caused by naphthenic acids and sulfur compounds include the use of corrosion inhibitors; the blending of higher naphthenic acid content crudes with crudes having lower quantities of naphthenic acids; and the neutralization and removal of naphthenic acids from the crudes, fractions and residua.

Given that these approaches have not been entirely satisfactory, the accepted approach in the industry is to construct the distillation units or at least the portions exposed to naphthenic acid and sulfur compound corrosion, with corrosion resistant metals such as high quality stainless steel or alloys containing higher amounts of chromium and molybdenum.

However, in units not so constructed, there is a need to provide against the corrosion caused by naphthenic acids and sulfur compounds.

Additionally, there is a desire to reduce chemical costs in inhibiting this corrosion as well as downstream costs associated with the contamination of catalysts.

The present inventors have demonstrated through use of the present invention that inhibitor usage and cost are reduced by as much as 80% over continuous treatment addition.

DESCRIPTION OF THE RELATED ART U.S. Pat. No. 4,941,994 teaches methods for inhibiting metal corrosion in hot acidic liquid hydrocarbons comprising adding to the hydrocarbons a dialkyl and/or trialkyl phosphite compound and, optionally, a thiazoline compound. The '994 patent notes that a high initial dosage of inhibitor is preferred for a short time to build up a protective coating on the metal surfaces. Once the protective surface is established, the dosage rate may be lowered to maintain the protective surface.

U.S. Pat. No. 5,500,107 teaches methods for inhibiting the corrosion of the metal surfaces of equipment used in processing crude oil comprising adding to the crude oil of a phosphite compound containing at least one aryl group. This patent also states that it is preferred to add a high initial dosage rate and to maintain this level for a very short time.

This induces a build-up of a protective coating which, once established, can be maintained with a lower rate of addition.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph of corrosion rate versus time for 101 OCS (Carbon Steel) at 6000F in heavy vacuum gas oil (HVGO) without an inhibitor added.

Figure 2 is a graph of corrosion rate versus time for 101 OCS at 6000F in HVGO. Fresh HVGO was added after 24 hours to replace the original, uninhibited HVGO.

Figure 3 is a graph of corrosion rate versus time for 1010CS at 6000F in HVGO with 100 ppm active of inhibitor added.

Figure 4 is a graph of corrosion rate versus time for 101 OCS at 6000F in HVGO with 100 ppm active of inhibitor added after precorrosion of the probe.

Figure 5 is a graph of corrosion rate versus time for 101 OCS at 6000F in HVGO with 200 ppm active of inhibitor added after precorrosion of the probe.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides for an improved method for providing a durable corrosion inhibiting film on the surface of metallic components in a crude oil processing system comprising adding in a periodic manner to the crude oil an effective film forming amount of a film forming phosphorus-containing compound.

For purposes of the present invention, the term "film forming phosphorus-containing compound" can be defined as any phosphorous- containing compound that when added to crude oil will form a durable corrosion inhibiting film on metallic surfaces.

Representative phosphorus-containing compounds include but are not limited to trialkyl phosphates having an alkyl moiety of Ci to 012.

Preferred trialkyl phosphates are selected from the group consisting of trimethyl phosphate, triethyl phosphate, try propyl phosphate, tributyl phosphate (TBP) and tripentyl phosphate.

Other representative phosphate-containing compounds include phosphite compounds containing at least one aryl group represented by the formulas: wherein R1, R2 and R3 are Ce to C12 aryl or alkyl and at least one R group is aryl.

Exemplary aryl containing phosphites include triphenyl phosphite, diphenyl phosphite, diphenyl isodecyl phosphite, diphenyl isooctyl phosphite, and phenyl diisodecyl phosphite (PDDP). These compounds are commercially available from GE Specialty Chemicals Company.

Other phosphorus-containing compounds include phosphate ester compounds such as mono- and di-(2-ethylhexyl) phosphate esters and mixtures thereof which are available from Chemax, Inc. as Chemfac PA- 080.

Alkyl phosphonate phenate sulfides are also useful in the present invention. These compounds are synthesized beginning with the reaction of an alkyl phenol of the formula with sulfur monochloride or sulfur dichloride. This reaction is known and is reported in U.S. Pat No. 2,916,454, Bradley et al., the contents of which is wholly incorporated by reference herein. The preferred alkyl phosphonate phenate sulfide is a calcium overbased phosphonate phenate sulfide which is available from Ethyl Corp. as HITEC E686.

Preparation of these alkaline earth metal alkyl phosphonate phenate sulfide is described in U.S. Pat. No. 4,123,369, Miller et al., the contents of which are wholly incorporated by reference herein.

For purposes of the present invention, the term "an effective film forming amount" is defined as that amount of phosphorus-containing compound that when added to a crude oil will form a durable, corrosion inhibiting film on the metallic surfaces in contact with the crude oil.

Preferably, this amount will range from about 10 part per million to about 10,000 parts per million parts of crude with a range of about 10 parts to about 250 parts per million more preferred. For purposes of the present invention, phosphorus-containing compound can include one or more compounds or a mixture thereof.

This amount will vary with local operating conditions and the particular hydrocarbon being processed. Temperature and amounts of naphthenic acids and sulfur compounds will also affect the amount of phosphorus-containing compounds added. Typical processing temperatures range from about 3500 to 1 0000F with a range of 4000 to 7900F more preferred.

The long lasting and durable nature of the film formed will allow for the addition of the film forming phosphorous-containing compounds on a periodic basis. One measure of time between additions may be based on when the film formed begins to lose effectiveness. This interval will also be dictated by the above-varied operating conditions and by economy of usage.

For purposes of the present invention, durable may be defined as the length of time that is measured in terms of hours to days rather than in seconds to minutes.

For purposes of the present invention, crude oils comprise crude oil and its fractions or residua produced or left after normal refinery processing steps, such as desalting, distillation, cracking, coking, extraction, hydrogenation, isomerization, or alkylation.

The invention will now be further described with reference to the following examples which are intended for illustration purposes and should not be construed as limiting the invention.

EXAMPLES Tests were conducted in an autoclave outfitted with an electrical resistance (ER) probe. The ER probe has a thin element of 101 OCS (Carbon Steel). As the element corrodes, it becomes thinner which causes an increase in Ohmic resistance. A transmitter which contains a Wheatstone Bridge Circuit is used to measure the resistance, convert it into "probe units", and generate a 4-20 mA signal proportional to the resistance. The 4-20 mA signal is continuously measured using a data acquisition system (DAQ). Probe units, temperatures, and pressures are stored every 5 minutes by the DAQ. Since the probe units are proportional to the element thickness, the slope of the probe unit versus time curve is proportional to the corrosion rate which is expressed in mils per year (mpy).

Plots of the corrosion rate of 1010 CS at 600C F versus time were used to demonstrate the ability of the film to protect in the absence of treatment. The results of this testing are presented in Table I.

Table I Example Treatment Corrosion Rate Approximate No. (Pom) (mpv) Time (Hours) 1 0 9.3 120 2 0 8.4 Before Fluid Change 24 8.1 After Fluid Change 21 3 100 1.4 Before Fluid Change 25 1.2 After Fluid Change 43

Table I (Continued) Example Treatment Corrosion Rate Approximate No. (Ppm} (mpv) Time (Hours) 4 0 5.6 Precorrosion 23 100 1.4 Before Fluid Change 23 0 0.1 After Fluid Change Film Persistency Step 76 0 2.3 Film Failure Step 23 5 0 7.0 Precorrosion 23 200 1.0 Before Fluid Change 23 0 1.2 After Fluid Change Film 92 Persistency Step 0 7.4 Film Failure Step 20 As demonstrated by Figures 1 to 5 and the results of Table I, the use of a phosphorus-containing compound proved effective at providing a durable, long lasting film on the 1010 CS metal. Example 1 and Figure 1 show the corrosion behavior of uninhibited Heavy Vacuum Gas Oil from a southern refinery, which is fairly constant over a five-day period.

Example 2 and Figure 2 demonstrate the effects of fluid change after 24 hours where the uninhibited fluid was removed by N2 pressure.

Corrosion rates between the two were essentially the same.

In Example 3 and Figure 3, 100 ppm of Chemfac PA-080 was injected into the HVGO shortly after the test temperature was attained.

The inhibitor was allowed to film the probe for about 25 hours. At that time, the treated fluid was replaced with fresh, untreated fluid. As shown in Figure 3, excellent corrosion inhibition was observed both when the inhibitor was present in the fluid and after the fluid change when no inhibitor was added to the fluid.

Figures 4 and 5 (Examples 4 and 5) demonstrated that films can be formed over a one-day period in an acidic environment (TAN about 2 mg KOH/g, TAN is Total Acid Number by ASTM D-974) and then persist in that same environment for at least three days. The precorrosion of the coupons did not produce a protective film in the southern refinery HVGO.

However, the inhibitor still provided a significant persistency without maintenance. In fact, a reduction of chemical usage of as much as 80% was achieved relative to a continuous feed approach.

A two-step coupon test was also performed utilizing a gas oil from a southern refinery having a TAN of 5.6 mg KOH/g and a blend of gas oils from the same refinery having a TAN of about 0.06 mg KOH/g.

During the first step, referred to as the pretreatment step, coupons were treated with an inhibitor in a "low acid blend" of gas oils for 18 to 20 hours. During the second step, referred to as the maintenance step, the same coupons were exposed to a "high acid" gas oil with or without additional inhibitor addition for an additional 18 to 20 hours.

The two steps were carried out using the same set of coupons in a single autoclave by forcing fluids into or out of the autoclave under nitrogen pressure and at temperature. Thus, the total amount of corrosion occurring in both steps was measured and a pretreatment step was run to correct the two step corrosion for that which occurred during the pretreatment step. The corrosion measured for pretreatment step was prorated and subtracted from the total corrosion to determine the amount of corrosion which occurred during the maintenance step. The results of this testing are presented in Table II.

Table II Two Step Weight Loss Method Pretreatment Data - High Acid Gas Oil Dosage (ppm) Maintenance PretreatmenV Corrosion Inhibitor Maintenance Rate (mpv) Low Acid Gas Oil Blank -/0 5.9 High Acid Gas Oil Blank -/0 14.1 High Acid Gas Oil Control 0/0 5.4 PDDP 200/0 2.4 PDDP 400/0 1.4 PDDP 4001100 1.0 TBP/E-686 400/0 0.3 TBP/E-686 4001100 1.0 TBP 62/0 1.2 TMT-3H 10010 8.4 PDDP is phenyl diisodecyl phosphite.

TBP is tributyl phosphate.

E-686 is a calcium overbased phosphonate phenate sulfide, available from Ethyl Corp. as HITEC E686.

TMT-3H is 2, 4, 6-trimercapto- 1, 3, 5-triazine, available from Degussa.

As demonstrated in Table II, the untreated, low acid blend gave a corrosion rate of 5.9 mpy. When coupons exposed to the untreated, low acid blend were subsequently exposed to the high acid gas oil, the calculated high acid gas oil corrosion rate was 5.4 mpy. Comparing this value to the value of 14.1 mpy measured in the high acid gas oil using unexposed coupons indicates that the sulfide film formed during the low acid blend exposure provide significant protection against corrosion.

It is apparent from these data that all of the phosphorous compounds form films which provide protection without the need for any maintenance dosage. This is a surprising result as it is traditional practice with high temperature corrosion inhibitors to either add an inhibitor continuously at one dosage or to add a maintenance dosage to repair defects in the protective film established at an initially high dose.

The results of Table II are especially dramatic when compared to inhibition levels for the same inhibitors when added directly (one step method) to the high acid gas oil. These results are reported in Table Ill.

Table Ill One step weight loss method Corrosion Inhibitor Dosacie (Dpm) Rate (mPv) High acid Gas Oil Blank -/0 14.1 PDDP 100 29.6 PDDP 400 7.4 PDDP 1200 4.9 PDDP 2000 4.1 TBP/E-686 400 15.3 TBP/E-686 1200 8.0 PDDP is phenyl diisodecyl phosphite.

TBP is tributyl phosphate.

E-686 is a calcium overbased phosphonate phenate sulfide, available from Ethyl Corp. as HITEC E686.

As demonstrated in Table II, the effective dosage to reduce corrosion to acceptable levels can be lowered several-fold under low acid conditions. The total amount of chemical can be further reduced by as

much as 80% by eliminating the maintenance dosage. In Table II, the films formed were able to last for about 24 hours at a minimum without maintenance dosages.

While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of this invention will be obvious to those skilled in the art.

The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.