Study Of The Anti-corrosion Performance Of Thiophosphate In Distillate

Nowadays, the acid number and the sulfur content of the crude oil and distillates increase with the heavy and inferior crude oil refined all around the world. So the naphthenic acidic corrosion(NAC) and the sulfide corrosion(SC) during the oil refining process can be the main reason to explain the corrosion of the facilities in refinery. However, currently the study on the high-temperature corrosion of the refinery equipment mainly focuses on NAC, and the study of SC and the corrosion resulted from NAC and SC can usually be ignored.The high-temperature corrosion inhibitors used in refinery could availably inhibit the corrosion by sulfur-containing compounds and/or naphthenic acids, among which phosphorus-based inhibitors such as thiophosphates are effective. Thiophosphates was synthesized with octanol and P2S5 and the influence of reaction conditions on synthesis of the thiophosphates was researched. The results showed that the percentages of dioctyl thiophosphate and octyl thiophosphate were 91% and 4% when the molar ration of octanol to P2S5, reaction temperature and reaction time were 4:1, 130℃ and 2.5h, respectively. The structure of the thiophosphates was also characterized by FT-IR.White oil added naphthenic acid or sulfides were used as the corrosive medium and the static weight loss method was used to investigate the corrosion of A3 steel in the white oils. The results showed that the corrosive rate increased with the total acid number(TAN) of white oil added naphthenic acid. Among the sulfides, mercaptans and alkyl sulfides both exhibit corrosion activity at high temperature, while thiophenes were non-reactive sulfides. The corrosive rates of A3 steels increased with the increase of sulfur content of white oils added mercaptan/alkyl sulfide/ dibenzothiophene, and the rate reached to maximum at 1.5% of sulfur content. The corrosion of A3 steel in white oil added both naphthenic acid and sulfides was researched, and the results showed that the corrosion caused by naphthenic acid could be inhibited by sulfur-containing compounds in higher content in white oil, which indicated that the corrosion in refineries processing the crude oils with higher content of sulfur-containing compounds could be decreased.The anti-corrosion performance of the thiophosphates product was investigated in white oils with different TAN, and the results showed that the thiophosphates presented excellent corrosion inhibition ability, and the corrosion rate could be reduced to 0.01 mm·a-1 when the concentration of thiophosphate in white oil is 400μg·g-1 and the TAN of white oil was 10 mg KOH·g-1. The surface morphology of the A3 steel was researched by SEM, and the results showed that a protective memrance was formed in the surface of A3 steel by thiophosphate at higher temperature, which inhibited the corrosion of naphthenic acid. The elemental compositions of the A3 steel surface were researched by EDAX and X-ray fluorescence spectrometry technique, and results showed that the contents of elemental oxygen and phosphorus were increased apparently after corrosive experiments with thiophosphate as corrosive inhibitor, which indicated that thiophosphate reacted with the steel or chemical adsorption of thiophosphate took placed in the steel surface.The anti-corrosion performances of the thiophosphates were investigated in three white oils with higher TAN/hihger sulfur content, higher TAN/lower sulfur content and lower TAN/higher sulfur content, and the corrosive rates of A3 steel in the three white oils were close to zero at 380-500μg·g-1of the thiophosphate concentration, which indicated that the thiophosphate presents excellent anti-corrosion performances.Triethylamine, triethanolamine, n-octadecylamine and tetraethylenepentamine were combined with the thiophosphate, and the anti-corrosion performances of the composites were also researched.The results showed that the composites of the thiophosphate with n-octadecylamine or tetraethylenepentamine presented obviously synergistic effect in white oil with 10 mg KOH·g-1 acid number. Moreover, the combination of thiophosphate with amines could inhibit the corrosion of thiophosphate itself to the metal vessels because of the strong acid sulfhydryl group(-SH) in the thiophosphate molecules.