Additive for inhibiting acid corrosion and method of using the new additive

Abstract

The present invention relates to the field of corrosion inhibition in hydrocarbon fluid processing units. The present invention comprises a new additive for inhibiting acid corrosion comprising polymeric thiophosphate ester, which is obtained by reaction of a polymer compound having mono, di or poly hydroxyl group, preferably polymer compound which is hydroxyl-termination, more preferably said polymer compound comprising hydroxyl-terminated polyisobutylene or polybutene and phosphorous pentasulphide. Said polymeric thiophosphate ester is further reacted with any oxide selected from group consisting of ethylene oxide, butylenes oxide or propylene oxide or such other oxide, preferably ethylene oxide, capably forming ethylene oxide derivative of thiophosphate ester. The invention is useful effecting acid corrosion inhibition on the metal surfaces of a distillation unit, distillation column, trays, packing and pump around piping.

Claims

1. An additive for inhibiting high temperature naphthenic acid corrosion comprising polymeric thiophosphate ester, which is reaction product of reaction of hydroxyl terminated polyisobutylene or polybutene succinate ester with phosphorous pentasulphide.

2. An additive for inhibiting high temperature naphthenic acid corrosion comprising an ethylene oxide derivative of polymeric thiophosphate ester formed by reacting polymeric thiophosphate ester with ethylene oxide, wherein said polymeric thiophosphate ester is a reaction product of reaction of hydroxyl terminated polyisobutylene or polybutene succinate ester with phosphorous pentasulphide.

3. The additive, as claimed in claim 1, wherein said hydroxyl terminated polyisobutylene or polybutene succinate ester has from 40 to 2000 carbon atoms.

4. The additive, as claimed in claim 1, wherein said hydroxyl terminated polyisobutylene or polybutene succinate ester has molecular weight varying from 500 to 10000 dalton.

5. The additive, as claimed in claim 1, wherein mole ratio of said phosphorous pentasulphide to said polymer compound which is hydroxyl-terminated is 0.01 to 4 moles to 1 mole respectively.

6. The additive, as claimed in claim 1, wherein said polyisobutylene is normal or high reactive.

7. The additive, as claimed in claim 1, wherein the effective dosage of said additive comprises the effective dosage varying from 1 ppm to 2000 ppm.

8. The additive, as claimed in claim 1, wherein said hydroxyl terminated polyisobutylene or polybutene succinate ester has molecular weight varying from 800 to 1600 dalton.

9. The additive, as claimed in claim 1, wherein said hydroxyl terminated polyisobutylene or polybutene succinate ester has molecular weight varying from 950 to 1300 dalton.

10. The additive, as claimed in claim 1, wherein the effective dosage of said additive comprises the effective dosage varying from 2 ppm to 200 ppm.

11. An additive for inhibiting high temperature naphthenic acid corrosion comprising an oxide derivative of polymeric thiophosphate ester formed by reacting polymeric thiophosphate ester with an oxide selected from the group consisting of butylene oxide and propylene oxide, wherein said polymeric thiophosphate ester is a reaction product of reaction of hydroxyl terminated polyisobutylene or polybutene succinate ester with phosphorous pentasulphide.

12. A method of making an additive for inhibiting high temperature naphthenic acid corrosion, wherein said additive comprises polymeric hydroxyl terminated polyisobutylene thiophosphate ester, and is prepared by a process comprising the steps of: (a) reacting high reactive polyisobutylene with maleic anhydride to form polyisobutylene succinic anhydride; (b) reacting said polyisobutylene succinic anhydride of step (a) with a compound selected from glycols or polyols or polymeric alcohols to form hydroxyl-terminated polyisobutenyl succinate ester; (c) reacting resultant reaction compound of step (b) with phosphorous pentasulphide to form thiophosphate ester of polyisobutylene succinate ester, which is high temperature naphthenic acid corrosion inhibiting additive.

13. A method of making an additive for inhibiting high temperature naphthenic acid corrosion, wherein the additive comprises polymeric ethylene oxide treated derivative of polyisobutylene thiophosphate ester, and is produced by a process comprising the steps of: (a) reacting high reactive polyisobutylene with maleic anhydride to form polyisobutylene succinic anhydride; (b) reacting said polyisobutylene succinic anhydride of step (a) with a compound selected from glycols or polyols or polymeric alcohols to form hydroxyl-terminated polyisobutenyl succinate ester; (c) reacting resultant reaction compound of step (b) with phosphorous pentasulphide to form thiophosphate ester of polyisobutylene succinate ester; (d) reacting resultant reaction compound of step (c) with ethylene oxide to form ethylene oxide treated derivative of polyisobutylene thiophosphate ester, which is high temperature naphthenic acid corrosion inhibiting additive.

14. The method as claimed in claim 12, wherein said polyisobutylene succinic anhydride of step (a) is reacted with a compound selected from the group consisting of propylene glycol, butane diol, butylene glycol, butene diol, glycerin, trimethylol propane, polyethylene glycol, polypropylene glycol and polytetramethylene glycol.

15. The method as claimed in claim 12, wherein said polyisobutylene succinic anhydride of step (a) is reacted with ethylene glycol.

16. The method as claimed in claim 12, wherein said resultant reaction compound of step (c) is reacted with an oxide selected from the group consisting of butylene oxide and propylene oxide to form oxide derivative of said polymeric thiophosphate ester.

17. The method as claimed in claim 13, wherein said polyisobutylene succinic anhydride of step (a) is reacted with a compound selected from the group consisting of propylene glycol, butane diol, butylene glycol, butene diol, glycerin, trimethylol propane, polyethylene glycol, polypropylene glycol and polytetramethylene glycol.

18. The method as claimed in claim 13, wherein said polyisobutylene succinic anhydride of step (a) is reacted with ethylene glycol.

19. The method as claimed in claim 13, wherein said resultant reaction compound of step (c) is reacted with an oxide selected from the group consisting of butylene oxide and propylene oxide to form oxide derivative of said polymeric thiophosphate ester.

20. A method of using an additive for inhibiting high temperature naphthenic acid corrosion as claimed in claim 1, comprising the steps of: a. heating a hydrocarbon containing naphthenic acid to vaporize a portion of said hydrocarbon; b. allowing the hydrocarbon vapors to rise in a distillation column; c. condensing a portion of said hydrocarbon vapors passing through the distillation column to produce a distillate; d. adding to the distillate from 1 to 2000 ppm of the additive to form a mixture; e. allowing the mixture of step d to contact metal surfaces of said distillation column to form protective film on said surface whereby such surfaces are inhibited against corrosion.

21. A method of using an additive for inhibiting high temperature naphthenic acid corrosion as claimed in claim 2, comprising the steps of: a. heating a hydrocarbon containing naphthenic acid to vaporize a portion of said hydrocarbon; b. allowing the hydrocarbon vapors to rise in a distillation column; c. condensing a portion of said hydrocarbon vapors passing through the distillation column to produce a distillate; d. adding to the distillate from 1 to 2000 ppm of the ethylene oxide derivative of said polymeric thiophosphate ester to form a mixture; e. allowing the mixture of step d to contact metal surfaces of said distillation column to form protective film on said surface whereby such surfaces are inhibited against corrosion.

22. A method of using an additive for inhibiting high temperature naphthenic acid corrosion as claimed in claim 11, comprising the steps of: a. heating a hydrocarbon containing naphthenic acid to vaporize a portion of said hydrocarbon; b. allowing the hydrocarbon vapors to rise in a distillation column; c. condensing a portion of said hydrocarbon vapors passing through the distillation column to produce a distillate; d. adding to the distillate from 1 to 2000 ppm of the oxide derivative of said polymeric thiophosphate ester to form a mixture; e. allowing the mixture of step d to contact metal surfaces of said distillation column to form protective film on said surface hereby such surfaces are inhibited against corrosion.

Description

DESCRIPTION OF THE INVENTION

(1) The present invention uses the following reacted compound to be used as corrosion inhibitor for inhibiting high temperature naphthenic acid corrosion. This reacted compound working as effective corrosion inhibitor is obtained by reaction of a polymer compound having mono, di or poly hydroxyl group, preferably hydroxy-terminated polymer compound, more preferably hydroxyl-terminated polyisobutylene (PIB) compound or polybutene with phosphorous pentasulphide, resulting into formation of thiophosphate ester, which is polyisobutylene thiophosphate ester when polyisobutylene is used as a polymer.

(2) The effect of corrosion inhibition is also achieved by a compound obtained by further reacting polyisobutylene thiophosphate ester with any oxide selected from group consisting of ethylene oxide, butylene oxide or propylene oxide, preferably capably forming ethylene oxide derivative of polymeric thiophosphate ester.

(3) Conventional PIBs and so-called high-reactivity PIBs (see for example patent EP-B-0565285) are suitable for use in this invention. High reactivity in this context is defined as a PIB wherein at least 50%, preferably 70% or more, of the terminal olefinic double bonds are of the vinylidene type, for example the GLISSOPAL compounds available from BASF.

(4) In one aspect, the polymer used for preparing hydroxy-terminated polymer has between 40 and 2000 carbon atoms.

(5) In another aspect the abovementioned polymer has molecular weight of from 500 to 10000 dalton, preferably from 800 to 1600 dalton and more preferably from 950 to 1300 dalton.

(6) The mole ratio of P.sub.2S.sub.5 to hydroxyl-terminated polymer is preferably 0.01 to 4 mole of P.sub.2S.sub.5 to 1 mole of hydroxyl-terminated polymer.

(7) The mole ratio of P.sub.2S.sub.5 to PIB hydroxyl-terminated ester is preferably 0.01 to 4 mole of P.sub.2S.sub.5 to 1 mole of hydroxyl-terminated PIB ester. The PIB can be normal or highly reactive.

(8) It has been surprisingly discovered by the inventor of the present invention, that a polymer based thiophosphate ester, having low phosphorus content, low acidity and high thermal stability, and non-fouling nature gives very effective control of napthenic acid corrosion.

(9) The novel additive of the present invention is made in four basic steps. 1. High reactive PIB (Polyisobutylene) is reacted with Maleic Anhydride to make Polyisobutylene succinic anhydride (PIBSA) 2. The resultant reaction-compound of step No. 1 is further reacted with ethylene glycol to give a polymer having hydroxyl end groups which is hydroxyl-terminated polyisobutenyl succinate ester. Depending on the mole ratio of PIBSA and ethylene glycol, Mono ester or diesters are formed which leads to the formation of mono hyrdoxy or di hydroxy terminated polymer, respectively. Both these compound are found to be useful in this invention.

(10) Other glycols or polyols or polymeric alcohols can also be used in place of ethylene glycol. The examples of such useable compounds are propylene glycol, butane diol, butylenes glycol, butene diol, glycerine, trimethylol propane, triethylene glycol, pentaerythritol, polyethylene glycol, polypropylene glycol or any other hydroxyl terminated compounds. (This is one of the many ways of obtaining the hydroxyl-terminated polymer) 3. The resultant reaction-compound of step no. 2 is then reacted with phosphorus pentasulfide. The reaction can be carried out by using various mole ratios of hydroxyl-terminated polymer, for example, of PIB-ester of step 2 above with phosphorus pentasulfide. The resultant reaction compound obtained after completing step no. 3 is Thiophosphate ester of polyisobutenyl succinate ester. (The resulting reaction compound is effective in the present invention in inhibition of napthenic acid corrosion). 4. The resultant reaction-compound, obtained after completing step-3 is further reacted with oxides like ethylene oxide. The other common oxides like butylene oxide or propylene oxide also can be used in place of ethylene oxide. The resultant reaction compound obtained after completion of step-4 is ethylene oxide treated derivative of polyisobutylene thiophophate ester. This resulting reaction compound of step 4 is also effective in the present invention in inhibition of naphthenic acid corrosion.

(11) It should be noted that the above mentioned steps can be understood better by referring to the corresponding examples 1, 2, 4, and 5.

(12) The above mentioned steps describe only one illustrative example of the method of preparing invention compound. The hydroxyl-terminated polymer described in these steps can also be obtained by other appropriate methods.

(13) The present invention is directed to a method for inhibiting corrosion on the metal surfaces of the processing units which process hydrocarbons such as crude oil and its fractions containing naphthenic acid. The invention is explained in details in its simplest form wherein the following method steps are carried out, when it is used to process crude oil in process units such as distillation unit. Similar steps can be used in different processing units such as, pumparound piping, heat exchangers and such other processing units.

(14) These method steps are explained below: a) heating the hydrocarbon containing naphthenic acid to vaporize a portion of the hydrocarbon: b) allowing the hydrocarbon vapors to rise in a distillation column; c) condensing a portion of the hydrocarbon vapours passing through the distillation column to produce a distillate; d) adding to the distillate, from 1 to 2000 ppm, preferably from 2 to 200 ppm, of polymeric Thiophosphate ester or its oxide-treated derivatives or combination thereof, which is the required additive of present invention; e) allowing the distillate containing compound of step (d) to contact substantially the entire metal surfaces of the distillation unit to form protective film on such surface, whereby such surface is inhibited against corrosion.

(15) It is advantageous to treat distillation column, trays, pumparound piping and related equipment to prevent naphthenic acid corrosion, when condensed vapours from distilled hydrocarbon fluids contact metallic equipment at temperatures greater than 200 C., and preferably 400 C. The additive is generally added to the condensed distillate and the condensed distillate is allowed to contact the metallic surfaces of the distillation column, packing, trays, pump around piping and related equipment as the condensed distillate passes down the column and into the distillation vessel. The distillate may also be collected as product. The corrosion inhibitors of the instant invention remain in the resultant collected product.

(16) In commercial practice, the additives of this invention may be added to a distillate return to control corrosion in a draw tray and in the column packing while a second injection may be added to a spray oil return immediately below the draw trays to protect the tower packing and trays below the distillate draw tray. It is not so critical where the additive of the invention is added as long as it is added to distillate that is later returned to the distillation vessel, or which contact the metal interior surfaces of the distillation column, trays, pump around piping and related equipments.

(17) The method of using the additive compound of the present invention for achieving inhibition of high temperature naphthenic acid corrosion is explained below with the help of examples and tables.

(18) Thus it is seen that the additive compound of present invention used for corrosion-inhibition has the following important distinguishing features, as compared to the prior art. 1) The inventor of the present invention, after extensive experimentation, has surprisingly found that the additive compound used by the inventor, is the POLYMERIC ADDITIVE, which is highly effective in high temperature corrosion inhibition, as shown by the experimental results given in Tables 1 to 7. The prior-art does not teach or suggest use of, a polymeric thiophosphate ester or oxide-treated derivative thereof, additive in naphthenic acid corrosion inhibition or sulphur corrosion inhibition or any corrosion inhibition, in general. 2) Another distinguishing feature of the additive compound of present invention is that it has more thermal stability as compared to the additive compounds taught by the prior-art, due to the polymeric nature of the additive compound of present invention. Due to its high thermal stability the additive compound of present invention is very effective in high temperature naphthenic corrosion inhibition or high temperature sulphur corrosion inhibition. 3) Yet another distinguishing feature of the additive compound of present invention is that, it has very low acidity as compared to the additive compounds of prior art, for example, the phosphate esters of prior art has very high acidity. The phosphate esters of prior art are known to have a tendency to decompose, even at lower temperatures, to form phosphoric acids, which travel further along the hydrocarbon stream and react with metal surfaces of equipments such as packing of distillation column, to form solid iron phosphate or iron sulphide. These solids plug the holes of equipments and thereby lead to fouling of distillation column. The additive compound of the present invention does not have this deficiency. 4) Further distinguishing feature of the present invention is effective inhibition by the invention additive with even low phosphorus content.

Example 1

Synthesis of Polyisobutenyl Succinate Ester (PIB Ester-Hydroxyl Terminated Polymer Compound)

Step 1: Polyisobutenyl succinic anhydride

(19) TABLE-US-00001 Details of compound % wt 1 HRPIB (OLOA 16500) 89.48 2 Maleic anhydride 10.52 Total size 100.00

(20) Procedure 1. HRPIB (High Reactive Polyisobutylene) was charged into a clean and dry, four necked flask, equipped with nitrogen inlet, stirrer and thermometer. 2. Temperature was raised to 125 C. 3. N.sub.2 gas bubbling was started and continued for 10 minutes. 4. Rate of N.sub.2 gas bubbling was reduced and, sample for moisture content was taken. 5. Maleic anhydride was added to the flask. 6. After addition of maleic anhydride, temperature was raised to 170 C. and maintained for 2 hours with nitrogen bubbling. 7. After completion of maintaining of step 6 period, temperature was further raised to 205 C. and, heated at such a rate that it should reach205 C. from 170 C. in 3 hours (5 C./25 min). 8. The reaction mixture was then maintained for 6 hours at 205 C. 9. After end of 6 hours (at 205 C.) the reaction mixture was cooled to 170 C. 10. Vacuum was slowly applied and then temperature was raised to 205 C. 11. At 205 C. vacuum was continued (below 10 mm Hg). After 2 hours sample 1 was taken for estimating acid value and free maleic acid and after 3 hours sample 2 was taken for acid value and free maleic aid.

(21) The acid value of the product was between desired range of 70 to 120 mg KOH/g

Step II: PIB Ester

(22) TABLE-US-00002 Details of compound % wt Remarks 1 Reaction product of step 1 79.899 Sample diluted on Toluene to 85% strength 2 Mono ethylene glycol 20.101 Total size 100.00

(23) Procedure 1. Resultant product obtained at the end of step 1 was diluted in toluene to 85% strength and mono ethylene glycol were charged into a clean and dry four necked flask equipped with nitrogen inlet, stirrer and thermometer. 2. Temperature was raised to 190 C. (Toluene and water were removed to reach the temperature) with nitrogen gas bubbling. 3. Reaction was maintained at 190 C. till the required acid value was obtained.

(24) (The desired acid value should be preferably less than 5 mg KOH/g)

Example 2

Synthesis of Polymeric Thiophosphate Ester (Invention-Compound) Obtained by Reaction of Compound of Step II of Example 1 (with Various Mole Ratios) with Phosphorous Pentasulphide (with Various Phosphorous Contents)

(25) General Procedure for Making Polymeric Thiosulphate Ester 1. PIB ester was charged into a clean and dry four necked flask equipped with nitrogen inlet, stirrer and thermometer and, temperature was raised to 90 C. with nitrogen gas bubbling 2. Phosphorus pentasulfide was added at 90 C. slowly in one lot 3. After addition of phosphorus pentasulfide temperature was raised to 120 C. 4. Reaction mixture was maintained for 1 hour at 120 C. 5. After 1 hour at 120 C., temperature was slowly raised to 140 C. and maintained for 1 hour. Then it was cooled to 90 C. 6. Acid value of the as sample was measured as (45.61 mgKOH/g) 7. The reaction mixture was diluted with 1:1 Toluene 8. Temperature was raised to reflux point, nitrogen gas bubbling was started and continued for 6 hours. 9. The reaction mixture was cooled and filtered through hyflow at 60 C. 10. The reaction mixture was diluted to 50% by weight in solvent.

(26) (2-A) Reaction of PIB Ester with Phosphorus Pentasulfide (Phosphorous Content in the Final 100% Active Product P3.156%)

(27) TABLE-US-00003 Details of compound % wt Remarks 1 PIB Ester obtained after 88.701 EXAMPLE 1 STEP II completion of step II of Example 1 2 phosphorus pentasulfide 11.299 Total weight 100.00

(28) (2-B) (Phosphorous Content in the Final 100% Active Product P4.47%)

(29) TABLE-US-00004 Details of compound % wt Remarks 1 PIB Ester 83.981 EXAMPLE 1 STEP II 2 phosphorus pentasulfide 16.019 Total weight 100.00

(30) Acid value was between 64 and 73 mgKOH/g (Typically acid value ranges from 40 to 190 mg/g KOH)

(31) (2-C) (Phosphorous Content in the Final 100% Active Product P7.715)

(32) TABLE-US-00005 Details of compound % wt Remarks 1 PIB Ester 72.374 EXAMPLE 1 STEP 2 2 phosphorus pentasulfide 27.626 Total weight 100.00

(33) Acid value was 109.65 mgKOH/g (Typically acid value ranges from 90 to 190 mg KOH/g)

Example 3

High Temperature Naphthenic Acid Corrosion Test

(34) In this example, various amounts of a 50% formulation of the composition prepared in accordance, with Examples 1 to 3, were tested for corrosion inhibition efficiency on carbon steel coupons in hot neutral oil containing naphthenic acid. A weight loss coupon, immersion test was used to evaluate the invention compound for its effectiveness in inhibition of naphthenic acid corrosion at 290 C. temperature. Different dosage such as 300, 400 and 600 ppm of invention compound were used, as 50% active solution.

(35) A static test on carbon steel coupon was conducted without using any additive. This test provided a blank test reading.

(36) The reaction apparatus consisted of a one-liter four necked round bottom flask equipped with water condenser, N.sub.2 purger tube, thermometer pocket with thermometer and stirrer rod. 600 g (about 750 ml) paraffin hydrocarbon oil (D-130-fraction of higher than 290 C.) was taken in the flask. N.sub.2 gas purging was started with flow rate of 100 cc/minute and the temperature was raised to 100 C., which was maintained for 30 minutes.

(37) An additive compound of (2-A) in example 2 was added to the reaction mixture. The reaction mixture was stirred for 15 minutes at 100 C. temperature. After removing the stirrer, the temperature of the reaction mixture was raised to 290 C. A pre-weighed weight-loss carbon steel coupon CS 1010 with dimensions 76 mm . . . times 13 mm . . . times 1.6 mm was immersed. After maintaining this condition for 1 hour to 1.5 hours, 31 g of naphthenic acid (commercial grade with acid value of 230 mgKOH/g was added to the reaction mixture. A sample of one g weight of reaction mixture was collected for determination of acid value, which was found to be approximately 11.7 mgKOH/g. This condition was maintained for four hours. After this procedure, the metal coupon was removed, excess oil was rinsed away, the excess corrosion product was removed from the metal surface. Then the metal coupon was weighed and the corrosion rate was calculated in mils per year. Similar method of testing was used for each of the additive compounds of (2-B) and (2-C) of example 2, prior-art-additive of example 4 and ethylene-oxide-treated additives of (2-B) and (2-C) of example 2. The test results are presented in Tables 1 to 5-A. Similar studies were conducted for ethylene-oxide-treated additives of example 2, in which the passivation time was 4 hours and the duration of the test was 24 hours. The test results are shown in the Table 5-B.

(38) Calculation of Corrosion Inhibition Efficiency

(39) The method used in calculating Corrosion Inhibition Efficiency is given below. In this calculation, corrosion inhibition efficiency provided by additive compound is calculated by comparing weight loss due to additive with weight loss of blank coupon (without any additive).

(40) The corrosion rate in MPY (mils per year) is calculated by the formula,

(41) $MPY=\frac{534\mathrm{Weight}\mathrm{loss}\mathrm{in}\mathrm{mg}}{\left(\mathrm{Density}\mathrm{in}\mathrm{gm}\text{/}\mathrm{cc}\right)\left(\mathrm{Area}\mathrm{in}{\mathrm{in}}^2\right)\left(\mathrm{Time}\mathrm{of}\mathrm{test}\mathrm{in}\mathrm{hours}\right)}$$\mathrm{Corrosion}\mathrm{Inhibition}\mathrm{Efficiency}=\frac{\begin{array}{c}\left(\mathrm{Weight}\mathrm{loss}\mathrm{for}\mathrm{blank}\mathrm{without}\mathrm{additive}\right)-\\ \left(\mathrm{weight}\mathrm{loss}\mathrm{with}\mathrm{additive}\right)\end{array}}{\left(\mathrm{weight}\mathrm{loss}\mathrm{for}\mathrm{blank}\mathrm{without}\mathrm{additive}\right)}100$

(42) The calculated magnitudes are entered in the Tables in appropriate columns.

(43) The results of the experiments are presented in Tables 1, 2 and 3.

(44) TABLE-US-00006 TABLE 1 Phosphorous Content P = 3.145% (Duration of test 4 hours) Dosage Effective Weight Corrosion Corrosion Expt. Com- in Dosage Loss in Rate Inhibition No. pound Ppm in ppm mg MPY Efficiency 1 Blank 89 445 2 Resultant 600 300 1.8 mg 9 97.97 product of 2-A of example 2

(45) TABLE-US-00007 TABLE 2 Phosphorous Content P = 4.47% (Duration of test 4 hours) Dosage Effective Weight Corrosion Corrosion Expt. Com- in Dosage Loss in Rate Inhibition No. pound Ppm in ppm mg MPY Efficiency 1 Blank 89 445 3 Resultant 300 150 24.7 123 72.4 product of 2-B of example 2

(46) The experiments were conducted with different contents of phosphorous in the final 100% active product as per Example 2 with the results being presented in Table 1 to 3. It is seen that with phosphorous content of 3.145% the corrosion inhibition efficiency was 97.97% for effective dosage of inhibitor compound as 300 ppm. When the phosphorous content was increased to 7.75% and effective dosages were reduced to 200 ppm and 150 ppm, the corrosion inhibition efficiency was 99.6% and 95.84% respectively.

(47) TABLE-US-00008 TABLE 3 Phosphorous content P = 7.75% (Duration of test 4 hours) Dosage Effective Weight Corrosion Corrosion Expt. Com- in Dosage Loss in Rate Inhibition No. pound Ppm in ppm mg MPY Efficiency 1 Blank 89 445 4 Resultant 400 200 0.4 2 99.6 product of 2-C of example 2 5 Resultant 300 150 3.7 18.5 95.84 product of 2-C of example 2

(48) The Effect of Invention Compound (Polymeric Thiophosphate Ester Non Ethylene Oxide Treated) on the Naphthenic Acid Corrosion Inhibition. 4 Hours Test Duration

(49) TABLE-US-00009 Experi- Effective Total phosphorus Corrosion inhibi- ment no Compound dosage in ppm content in ppm tion efficiency in % 2 Resultant product of 2-A 300 3.145 3.00 = 9.435 97.97 of example 2 Phosphrous content 3.145% (invention compound) 5 Resultant product of 2-C 150 7.75 1.50 = 11.625 95.84 of example 2 Phosphrous content 7.75% (invention compound) 8 Resultant product of 150 9.75 1.5 = 14.625 89.88 example 4 Phosphorous content 9.75% (prior art)

(50) The results of use of effective dosages of additives from Table 1, Table 3, and Table 4 are compared in a tabular form given above. It is clearly seen that, in comparison with the prior art compound, with the same effective dosage of 150 ppm, the invention compound (example 2, experiment 5 in the above table, polymeric thiophosphate ester non ethylene oxide treated) provides higher corrosion inhibition efficiency of 95.84% with lower total phosphorous content of 11.625 ppm as compared to the efficiency of 89.88% with higher total phosphorous content of 14.625 ppm for prior art compound (octyl thiophosphate esterNon polymeric additive, experiment no 8 in the above table).

(51) By doubling the effective dosage of the above invention compound (example 2 experiment no 2 in the above tablepolymeric thiophosphate ester) to 300 ppm it is observed that still higher corrosion inhibition efficiency of 97.97% is obtained with much lower total phosphorous content of 9.435 ppm.

(52) It is well known to the person skilled in the art that use of higher phosphorous content compounds as corrosion inhibitors has been claimed to affect the function of various catalyst used to treat crude oil such as fixed bed hydrotreaters and hydrocracking units. These higher phosphorous compounds also act as poison for the catalyst. Another disadvantage of the non polymeric additive is that they tend to break down at higher temperature conditions giving out volatile products which tend to contaminate the other hydrocarbon streams.

(53) The above discussion clearly shows the advantage of use of invention compound over prior art compound for naphthenic acid corrosion inhibition.

Example 4

Synthesis of Octyl Thiophosphate Ester (Non-Polymeric Thiophosphate Ester as Anticorrosion Compound of Prior Art (U.S. Pat. No. 5,552,085)

(54) The clean four-necked-flask was equipped with stirrer, nitrogen gas inlet and condenser. N-noctanol weighing 400 g was charged in the flask. Phosphorous pentasulphide weighing 187 g, was then added to the flask in installments. The temperature of the flask was then increased to 110 C. The H.sub.2S gas was seen to be evolved after addition of P.sub.2S.sub.5. After one hour, the reaction mixture in the flask was heated to 140 C. and the flask was maintained at that temperature for one hour. The sample was cooled and filtered through 5 micron filter. The sample was heated to 90 C. The nitrogen gas was purged for 5 hours. The resulting sample, that is compound B2 was analyzed for its acid value, which was found to be between 110 to 130 mg/KOH. This compound was tested for its naphthenic acid corrosion efficiency. The corrosion inhibition efficiency is calculated as per method given in Example 3 and results of experiments are presented in table 4.

(55) TABLE-US-00010 TABLE 4 Octyl thiophosphate ester Non - polymeric thiophosphate ester as anticorrosion compound of prior art. Phosphrous content P = 9.75% (Duration of test 4 hours) Effec- Experi- Dosage tive Weight Corrosion Corrosion ment Com- in dosage loss in Rate inhibition No. pound ppm in ppm mg MPY Efficiency 1 Blank 89 445 6 Example 4 90 45 45 225 49.43 7 Example 4 180 90 22 110 75.28 8 Example 4 300 150 9 45 89.88

Example 5

Synthesis of Ethylene Oxide Derivatives

(56) The ethylene oxide derivatives of polymeric thiophosphate ester of polyisobutylene succinate ester were prepared as using below described procedure:

(57) Procedure

(58) The additive compound, which is the resultant product of 2-C of example 2, was transferred to the autoclave and ethylene oxide is added at 60 C. to 70 C., till the pressure in the autoclave remained constant. The reaction mixture was maintained at that temperature for 2 hours. The reaction mixture was cooled and the autoclave was flushed with nitrogen. The resultant additive, that is, ethylene oxide treated thiophosphate ester of polyisobutylene succinate ester, was used as additive for napthenic acid corrosion inhibition. The similar synthesis was carried out by using resultant product of 2-B of example 2. The weight percentages for 2-B, 2-C, and ethylene oxide are given below.

Example (5-A)

Ethylene Oxide Derivative of (2-C) of Example 2

(59) TABLE-US-00011 Details of compound % wt 1 Resultant Product of 2-C 44.1 2 Ethylene oxide 15.1 3 Aromatic Solvent 40.8

Example (5-B)

Ethylene Oxide Derivative of (2-B) of Example 2

(60) TABLE-US-00012 Details of compound % wt 1 Resultant Product of 2-B 45.4 2 Ethylene oxide 15.4 3 Aromatic Solvent 39.2

(61) It was noted that the acid value of resultant product 2-C used in the above mentioned synthesis process was 87.2 mg KOH/gm, whereas the acid values of ethylene oxide reacted product was 16 mg/gKOH. Similarly, the acid value of resultant product 2-B used in the above mentioned synthesis process was 56.8 mg KOH/gm, whereas the acid value of corresponding ethylene oxide reacted product was 3.98 mg KOH/g. Both these synthesis examples point to the desirable low-acid-values of the final products after synthesis is completed.

(62) The corrosion-inhibition-tests for these synthesized additive products were conducted as per procedure given in Example 3 (4 hours and 24 hours test duration) and test results are presented in Table 5-A and Table 5-B, respectively.

(63) TABLE-US-00013 TABLE 5-A Corrosion inhibition studies (static) for 4 hrs test duration. Experi- Active Mg loss MPY % efficiency ment No Details of compound Dosage ppm after test after test after test 1 Blank 89 445 a Invention compound 150 2.1 10.5 97.60 as per example 5-A b Invention compound 90 17 85 80.89 as per example 5-A c Invention compound 120 14.9 72.5 90.44 as per example 5-B Note: It can be seen from the results presented in Table 5-A, that the ethylene oxide derivative of the polymeric thiophosphate ester is also very effective in acid corrosion inhibition, as compared to results of Table 4 for prior-art-compound.

(64) TABLE-US-00014 TABLE 5-B Corrosion inhibition studies (static) for 24 hrs test duration. Experi- Active Mg loss MPY % efficiency ment No Details of compound Dosage ppm after test after test after test 9 Blank 313 261 10 Prior-art-additive 300 88.5 73.8 71.7 (as per example-4) 11 Invention compound 450 65 54.2 79.2 (as per example 2-B) 12 Invention compound 300 130 108.5 58.5 (as per example 2-C) 13 Invention compound 300 135 112.6 60.4 (as per example 2-B) 14 Invention compound 300 11 9.2 96.5 (as per example 5-A) 15 Invention compound 300 22.4 18.7 92.8 (as per example 5-B)

(65) Comparison of Effects of Invention CompoundPolymeric Thiophosphate Ester (with and without Ethylene Oxide Treatment) on Naphthenic Acid Corrosion Inhibition24 Hours Test Duration

(66) TABLE-US-00015 Expt. Effective Total phosphorous Corrosion No Compound dosage in ppm content in ppm Inhibition in % 10 Prior-art-additive (as 300 9.75 3 = 29.25 71.7 per example 4) (9.75) 11 Invention compound (as 450 4.47 4.5 = 20.115 79.2 per example 2-B) (4.47) 12 Invention compound (as 300 7.715 3 = 23.145 58.5 per example 2-C) (7.715) 13 Invention compound (as 300 4.47 3 = 13.41 60.4 per example 2-B) (4.47) 14 Invention compound (as 300 5.49 3 = 16.47 96.5 per example 5-A) (5.49) 15 Invention compound (as 300 3.15 3 = 9.45 92.8 per example 5-B) (3.15) Note: Invention compound is polymeric thiophosphate ester prepared by following steps given in example 2 and example 5. The values in the bracket indicates the phosphrous content of the inventive compound in percentage.

(67) The results of the use of effective dosages of example 5 are compared above in a tabular form with specific references to the total phosphorous content and efficiency of the corrosion inhibition.

(68) Comparing the results of experiment numbers 10, 12 and 14, of Table 5-B the surprising favorable technical effect of the ethylene oxide derivative of polymeric thiophosphate ester is clearly seen from much higher efficiency of 96.5% and much lower phosphorous content of 16.47 ppm (after ethylene oxide treatment) as compared to the efficiency of 58.5% and phosphorus content of 23.145 ppm (before ethylene oxide treatment) and efficiency of 71.7% and phosphorous content 29.25 ppm of prior art compound.

(69) Similarly comparing results of experiment 10, 13, and 15, of Table 5-B the surprising favorable technical effect of ethylene oxide treatment of polymeric thiophosphate ester is clearly seen with much higher efficiency of 92.8% and much lower phosphorous content of 9.45 ppm (after ethylene oxide treatment) as compared to the efficiency of 60.4% and phosphorous content 13.14 ppm (before ethylene oxide treatment) efficiency of 71.7% and phosphorous content 29.25 ppm of prior art compound.

(70) The person skilled in the art should be aware of the surprising favorable technical effect mentioned above.

(71) It is well known to the person skilled in the art that use of higher phosphorous content compounds as corrosion inhibitors has been claimed to affect the function of various catalyst used to treat crude oil such as fixed bed hydrotreaters and hydrocracking units. These higher phosphorous compounds also act as poison for the catalyst. Another disadvantage of the non polymeric additive is that they tend to break down at higher temperature conditions.

(72) The above discussion clearly shows the advantage of use of invention compound over prior art compound for naphthenic acid corrosion inhibition.

Example 6

High Temperature Naphthenic Acid Corrosion Inhibition (Dynamic Test)

(73) The dynamic testing was carried out by using rotating means provided in the temperature-controlled autoclave and was carried out by using passivated steel coupons. A dynamic test on steel coupon was conducted without using any additive. This test provided a blank test reading. The passivation procedure is explained below:

(74) 400 g of paraffin hydrocarbon oil (D-130) was taken in a autoclave. A pre-weighed weight-loss coupon CS 1010 with dimensions 76 mm . . . times 13 mm . . . times 1.6 mm was fixed to the stirrer of the autoclave. This was then immersed in the oil. N2 gas was purged. While carrying out passivation of steel coupon in separate dynamic tests, each of the invention compounds of examples 2-B, 5-A and prior-art-additive of example 4 is added separately, in each separate test, to the reaction mixture (and each final dynamic test carried out separately). The reaction mixture was stirred for 15 minutes at 100 C. temperature. Then autoclave blanketing with 1 kg/cm.sup.2 by nitrogen was carried out. The temperature of the reaction mixture was raised to. After maintaining this condition for 4 hours, the autoclave was cooled and the coupons were removed and rinsed to remove the oil and then dried. This formed the pre-passivated coupon. The dried coupon was then fixed to the stirrer again.

(75) The oil used for the passivation was removed and 400 g fresh oil containing 6.2 g of commercial napthenic acid (TAN VALUE 230 mgKOH/g) was added to the autoclave. The resultant TAN of the system was 3.5 mgKOH/g. The temperature of the autoclave was then raised to 315 C. and maintained at this temperature for 24 hrs. Example 1 to 3, were tested dynamically for corrosion inhibition efficiency on steel coupons in a hot oil containing naphthenic acid.

(76) The following test equipment and materials were used in the Dynamic Corrosion Test: 1. Temperature controlled autoclave 2. Preweighed weight-loss carbon steel coupons CS 1010 with dimensions 76 mm . . . times 13 mm . . . times 1.6 mm. 3. Means to rotate the coupon, to provide a peripheral velocity in excess of 3 m/second.

(77) After the test, the coupons were removed, excess oil was rinsed away, excess corrosion product was removed from the surface of coupons. The coupons were then weighed and the corrosion rate was calculated as mils/year. The results of this dynamic test are presented in Table 6.

(78) TABLE-US-00016 TABLE 6 High Temperature Naphthenic Acid Corrosion inhibition (Dynamic Test). Experi- Active Mg loss MPY % efficiency ment No. Details of compound Dosage ppm after test after test after test 16 Blank 61.2 51.1 17 Prior art additive 500 2.9 2.42 95.26 as per example 4 17-A Prior art additive 250 15.1 12.6 75.3 as per example 4 17-B Invention compound 250 0.45 0.38 99.25 as per example 5-A 18 Invention compound 500 0.85 0.71 98.6 as per example 2-B

Example 7

Thermal Analysis

(79) The thermal analysis test of the invention compounds and the prior art compound were carried out in the Mettler Toledo Thermo Gravimetric Analyzer. A known weight of the sample was heated in the analyzer from 35 C. to 600 C. at a rate of 10 C./minute under nitrogen atmosphere. The temperature at which 50% loss in weight of sample occurs is taken as the representative of thermal stability. The weight of the residue obtained at 600 C., and the temperature at 50% weight loss are presented in Table 7. The weight of the residue is indicative of the tendency of the additive, to deposit at high temperature zones of equipments like furnaces, which may cause fouling of the equipment in due course.

(80) TABLE-US-00017 TABLE 7 Thermal Analysis data Experi- Temperature Residue at ment No Details of compound at 50% loss 600 deg C. 19 Invention compound 393 21.2975 as per example 2-C 20 Invention compound 386 12.9567 as per example 2-B 21 Invention compound 395 12.8771 as per example 5-A 22 Invention compound 391 6.8389 as per example 5-B 23 Prior-art-additive 220 23.5795 as per example 4

(81) Discussion about Thermal Stability

(82) It can be seen from the above table that the invention compounds (experiment No 19 to experiment No 22) the temperature of 50% weight loss varies from 386 C. to 395 C. The invention compounds in the above table include Non EO treated and the EO treated derivative. These values are much higher when compared with the prior additive which has a value of only 220 C. These clearly indicates the higher thermal stability of the invention compounds when compared with the prior art compound. It is known to the person skilled in the art that it is desirable to have additives with higher thermal stability since these will not decompose to volatile products leading to fouling and contamination of other streams. The other advantage of thermally stable compound is they retain their corrosion inhibition efficiency at higher temperatures.

(83) It is also seen from the above table that it is advantageous to treat the invention compound further with ethylene oxide. EO treatment reduces phosphorous content and also the residue at 600 C. It is seen from the above table that the invention compounds leave much lower residues at 600 C. The residue obtained for the invention compounds (experiment 20 to 22 in the above table) is much lower than the prior additive which is 23.5% (experiment no 23 in the above table). The above data clearly indicates that the invention compounds will have least deposition tendency in the areas of furnace.

(84) It is apparent from the foregoing discussion that the present invention comprises the following items: 1. An additive for inhibiting acid corrosion comprising polymeric thiophosphate ester, which is obtained by reaction of a polymer compound having mono, di or poly hydroxyl group, preferably polymer compound which is hydroxyl-terminated, more preferably said polymer compound comprising hydroxyl terminated polyisobutylene or polybutene, with phosphorous pentasulphide. 2. An additive, as described in item 1, wherein said polymeric thiophosphate ester is further reacted with any one oxide selected from group consisting of ethylene oxide, butylene oxide or propylene oxide or such other oxide, preferably ethylene oxide, capably forming ethylene oxide derivative of said polymeric thiophosphate ester. 3. An additive, as described in items 1 and 2, wherein said polymer compound has from 40 to 2000 carbon atoms. 4. An additive, as described in items 1 and 2, wherein said polymer compound has molecular weight of from 500 to 10000 dalton, preferably from 800 to 1600 dalton and more preferably from 950 to 1300 dalton. 5. An additive, as described in items 1 and 2, wherein mole ratio of said phosphorous pentasulphide to said polymer compound which is hydroxyl-terminated is preferably 0.01 to 4 moles to 1 mole respectively. 6. An additive, as described in items 1, wherein said polyisobutylene is normal or high reactive. 7. An additive, as described in items 1 and 2, wherein the effective dosage of said additive is from 1 ppm to 2000 ppm, preferably from 2 ppm to 200 ppm. 8. A method of making a new additive for inhibiting acid corrosion, said additive comprising polymeric polyisobutylene thiophosphate ester, comprising the steps of: (a) reacting high reactive polyisobutylene with maleic anhydride, capably forming polyisobutylene succinic anhydride. (b) reacting said polyisobutylene succinic anhydride of step (a) with glycols or polyols or polymeric alcohols, preferably propylene glycol, butane diol, butylene glycol, butene diol, glycerin, trimethyl propane, polyethylene glycol, polypropylene glycol, polytetramethylene glycol, more preferably ethylene glycol, capably forming hydroxyl-terminated polyisobutenyl succinate ester; (c) reacting resultant reaction compound of step (b) with phosphorous pentasulphide, with various mole ratios of said hydroxyl-terminated polyisobutenyl succinic ester and phosphorous pentasulphide, capably forming thiophosphate ester of polyisobutylene succinate ester, which is acid corrosion inhibiting additive; (d) reacting optionally resultant reaction compound of step (c) with any one oxide selected from group consisting of ethylene oxide, butylene oxide or propylene oxide preferably with ethylene oxide, capably producing ethylene oxide treated derivative of polyisobutylene thiophosphate ester, which is acid corrosion inhibiting additive. 9. A method of using a new additive for inhibiting acid corrosion, comprising the step of: a. heating the hydrocarbon containing naphthenic acid to vaporize a portion of said hydrocarbon; b. allowing the hydrocarbon vapors to rise in a distillation column; c. condensing a portion of said hydrocarbon vapors passing through the distillation column to produce a distillate d. adding to the distillate from 1 to 2000 ppm, preferably 2 to 200 ppm, of polyisobutylene thiophosphate ester or ethylene oxide treated compound thereof; e. allowing the resultant mixture of step d to contact substantially the entire metal surfaces of said distillation column capably forming protective film on said surface whereby such surfaces are inhibited against corrosion.

(85) Although the invention has been described with reference to certain preferred embodiments, the invention is not meant to be limited to those preferred embodiments. Alterations to the preferred embodiments described are possible without departing from the spirit of the invention. However, the process and composition described above are intended to be illustrative only, and the novel characteristics of the invention may be incorporated in other forms without departing from the scope of the invention.