EP1711584B1 - Method for corrosion control of refining units by acidic crudes - Google Patents

Abstract

Inhibition of corrosion, due to the presence of naphthenic acids, in crude petroleum oils in refinery plant is achieved by addition of alkyl mercaptan(s) in which the mercaptan function is adjacent to a tertiary carbon. Inhibition of corrosion, due to the presence of naphthenic acids, in crude petroleum oils in refinery plant is achieved by addition of 4-20C alkyl mercaptan(s) of formula (I) . R 1-R 9 (identical or different) : H or alkyl, aryl or alkylaryl (optionally containing one or more heteroatoms such as O or S). [Image].

Description

The present invention relates to the field of treating crude acid oils in refineries. More specifically, it relates to a corrosion control process of refining units that process acid crudes, including the implementation of specific sulfur compounds.

Oil refineries may face a serious problem of corrosion when they have to treat certain so-called acidic crudes. These acidic crudes contain essentially naphthenic acids which are at the origin of this very particular corrosion phenomenon, since it occurs in a non-conductive liquid medium of electric current. These naphthenic acids correspond to saturated cyclic hydrocarbons bearing one or more carboxylic groups. The acidity of a crude oil is described by a standard measurement according to ASTM D 664-01. It is expressed in mg of potash necessary to neutralize 1 g of oil and is called TAN (Total Acid Number). It is known in this technical field that a crude oil having a TAN greater than 0.2 is termed acid, and can lead to damage in the units of a refinery.

This corrosion reaction strongly depends on local conditions such as, for example, the temperature and the metallic nature of the wall in the unit concerned, the space velocity of the hydrocarbon, and the presence of a gas-liquid interface. Thus, even after extensive work on the subject, refiners have great difficulty in predicting the importance of corrosion reactions and their location.

One of the industrial solutions to this corrosion problem is to use stainless steel equipment, namely iron alloys including chromium and molybdenum. However, this solution remains little used because of the high investment cost. This choice, moreover, should preferably be considered during the design of the refinery because the stainless steels have lower mechanical properties than the carbon steels that are normally used and require a suitable infrastructure.

The existence of these technical difficulties to treat acid crudes has the consequence that these crudes are generally sold to refiners at a lower price level than standard crudes.

Another solution to the problem of treating an acid crude oil, used by the refiners in practice, is to dilute it with another non-acidic crude oil, so as to obtain a low average acidity, for example below the threshold of 0. , 2 of TAN. In this case, the naphthenic acid concentration becomes sufficiently low to generate acceptable corrosion rates. This solution, however, remains limited in scope. Indeed, some acidic crude has TAN greater than 2, which limits their use to at most 10% of the total volume of crudes entering the refinery. On the other hand, some mixtures of crudes sometimes lead to the desired reverse effect even after dilution, that is to say to an acceleration of corrosion reactions by naphthenic acids.

Another approach to combat this problem of corrosion is the introduction into the acidic crude oil to treat chemical additives inhibiting or preventing the attack of the metal wall of the unit concerned. This route is often very economical compared to that of using the steels or special alloys indicated above.

Laboratory work, such as that of Turnbull (Corrosion-November 1998 in Corrosion, Volume 54, No. 11, page 922) envisaged adding small amounts (of the order of 0.1%) of hydrogen sulphide in crude oil, to reduce corrosion by naphthenic acids. This solution is, however, not applicable in the refinery because hydrogen sulphide, gaseous at room temperature, is very toxic which makes the consequences of an extremely serious leakage and limits its use. In addition, at higher temperatures, the hydrogen sulphide itself becomes very corrosive and will lead, in other parts of the refinery, to a worsening of the generalized corrosion.

The patent US 5182013 described to solve the same problem of corrosion the use of other sulfur compounds, namely polysulfides of alkyl radicals of 6 to 30 carbon atoms.

More recently, the use of sulfur and phosphorus corrosion inhibitors has also been described.

Thus, the patent EP 742277 describes the inhibitory action of a combination of a trialkyl phosphate and an organic polysulfide. The patent US 5552085 recommends the use of thiophosphorus compounds such as organo thiophosphates or thiophosphites. The patent AU 693975 discloses as an inhibitor a mixture of trialkyl phosphate and phosphoric esters of sulfurized phenol lime neutralized.

However the organophosporés are of a very delicate handling, because of their high toxicity. In addition, they are poisons for hydrotreatment catalysts installed to purify hydrocarbon cuts from atmospheric and vacuum distillations. For these two reasons at least, their use in the field of refining is undesirable.

Petroleum crudes contain a wide variety of organosulfur compounds including alkyl mercaptans. Surprisingly, it has been found that a particular family of alkyl mercaptans, the compounds whose mercaptan function is carried by a tertiary carbon, make it possible to inhibit corrosion by naphthenic acids, more effectively than polysulfides. organic compounds, and without the need for further phosphorus inhibitors.

dans laquelle les symboles R₁, R₂, R₃, R₄, R₅, R₆, R₇, R₈ et R₉, identiques ou différents, représentent chacun un atome d'hydrogène ou un radical alkyle, linéaire ou ramifié, aryle ou alkylaryle, ces radicaux pouvant contenir éventuellement un ou plusieurs hétéroatomes tels que l'oxygène ou le soufre.The subject of the invention is therefore a method for combating corrosion by naphthenic acids of the metal walls of a refining unit in which a hydrocarbon stream is treated in the absence of oxygen, characterized in that comprises adding to said stream an effective amount of one or more hydrocarbon compound (s) comprising from 4 to 20 carbon atoms, of formula:

in which the symbols R ₁ , R ₂ , R ₃ , R ₄ , R ₅ , R ₆ , R ₇ , R ₈ and R ₉ , which are identical or different, each represent a hydrogen atom or a linear or branched alkyl radical; aryl or alkylaryl, these radicals possibly containing one or more heteroatoms such as oxygen or sulfur.

The mercaptans whose use is preferred according to the invention are tertiary mercaptans of formula C _n H _{2n + 1} -SH in which n is between 8 and 14.

The tertiododecylmercaptan is a compound of formula (I) more particularly preferred, taken alone or in the form of a complex mixture comprising tertiary mercaptans of 10 to 14 carbon atoms in which it is present at a content greater than 50% by weight. Such a mixture is generally prepared industrially by the addition of hydrogen sulfide on an olefinic cut such as tetrapropylene, and sold under the name of tertiododecylmercaptan.

The amount of compound (s) of formula (I) to be added to the hydrocarbon stream to be treated by the refining unit generally corresponds to a concentration, expressed as the equivalent weight of sulfur of said compound relative to the weight of the hydrocarbon stream. between 1 and 1000 ppm, preferably between 5 and 200 ppm. It will be possible while remaining in this concentration range, to set a high content at the start of the process according to the invention, and then to reduce this content to a maintenance dose.

The process according to the invention advantageously makes it possible to treat hydrocarbon streams, especially crude oils, whose TAN is greater than 0.2, and preferably greater than 2.

The operating temperature of the process corresponds to that at which corrosion reactions occur with naphthenic acids, and is generally between 200 and 450 ° C., and more particularly between 250 and 350 ° C.

The addition of the compound of formula (I) in the hydrocarbon stream can be carried out either at the inlet of the unit (simultaneously with the hydrocarbon stream to be treated), for an overall treatment of corrosion, or in the part of the unit where the corrosion reaction takes place for a spot treatment. This addition can be carried out by any means known to those skilled in the art, providing a control of the injection rate and a good dispersion of the additive in the hydrocarbon, for example by means of a nozzle or a mixer .

The term "metal walls" of the refining unit, the corrosion of which can be prevented by the process according to the invention, means all the walls that may be in contact with the acidic hydrocarbon stream to be treated. It can thus be as well of the internal wall proper of units such as the atmospheric and vacuum distillation towers, as the surface of the elements internal to them such as their trays or packings, or else peripheral elements to these, such as their withdrawal and inlet lines, pumps, preheating furnaces, or heat exchangers, provided that these elements are brought to a local temperature of between 200 and 450 ° C.

The metal used for the manufacture of the walls of the refining unit is generally a carbon steel, optionally comprising up to 10% by weight of chromium and / or molybdenum, preferably up to 5%.

As a nonlimiting example of the hydrocarbon stream to be treated according to the process according to the invention, there is the crude oil, the residue of atmospheric distillation, the gas oil cuts resulting from atmospheric and vacuum distillations, as well as the distillate and the residue under vacuum from vacuum distillation.

The following examples are given purely by way of illustration of the invention and can not be interpreted with a view to limiting its scope.

In these examples, a corrosion test is carried out, the conditions of which are given below.

Description of the corrosion test:

• huile minérale blanche ayant pour densité 0,838
• poudre de particules de fer sphériques, ayant une granulométrie de - 40+70 mesh (soit d'environ 212 à 425 µm)
• mélange d'acides naphthéniques ayant de 10 à 18 atomes de carbone, un point d'ébullition compris entre 270 et 324°C et une masse molaire moyenne de 244 g/mol.

This test uses an iron powder simulating a metal surface, and a mineral oil in which is dissolved a mixture of naphthenic acids, simulating an acidic crude stream. The characteristics of these reagents are as follows:

• white mineral oil having a density of 0.838
• powder of spherical iron particles having a particle size of -40 + 70 mesh (ie about 212 to 425 μm)
• mixture of naphthenic acids having 10 to 18 carbon atoms, a boiling point of 270 to 324 ° C and an average molar mass of 244 g / mol.

• 70 ml (soit 58,8 g) de l'huile minérale,
• 2 g de la poudre de fer,
• 2,8 g du mélange d'acide naphthénique.

It is introduced into a 150 ml glass reactor, equipped with a dropping funnel and a water cooler, and equipped with a stirring system and measuring the temperature:

• 70 ml (58.8 g) of the mineral oil,
• 2 g of the iron powder,
• 2.8 g of the naphthenic acid mixture.

The initial TAN of the reaction mixture is 10.

These reagents are kept in contact for 2 hours at a temperature of 250 ° C. under a dry nitrogen atmosphere in order to avoid oxidation reactions.

At the end of the test, the dissolved iron concentration in the medium is determined by a conventional method using a mineralization of a sample, a recovery of the residue in acidified water and the measurement by a plasma torch. .

This dissolved iron concentration (expressed in ppm) is directly proportional to the rate of corrosion of the iron powder generated by the mixture of naphthenic acids present in the mineral oil.

EXAMPLE 1 (Comparative): Reference test in the absence of inhibitor

The preceding test is carried out without addition of compound of formula (1), with 2 repetitions.

The results are shown in Table (I) below. <b><u> Table I </ u></b> Iron concentration (ppm) test 1 180 test 2 227 Average 203.5

EXAMPLE 2 Tests in the presence of tertiary alkyl mercaptans

Example 1 is repeated by adding to the mineral oil, during the charging of the reactor, tertiononylmercaptan or tertiododecylmercaptan. These products are mixtures of tertiary alkyl mercaptans centered respectively on the compounds containing 9 and 12 carbons. The content of these derivatives is calculated so as to obtain a corresponding concentration of 500 ppm by mass of sulfur in the mineral oil present in the reactor.

The results summarized in the following Table II are obtained.

dans laquelle [Fer] est la concentration en fer dissous mesurée avec ou sans inhibiteur, la concentration en fer sans inhibiteur étant égale à 203,5 ppm conformément à l'exemple 1. Tableau II Composé de formule (I) Concentration en fer (ppm) Taux d'inhibition tertiononylmercaptan 48 76 % tertiododécylmercaptan <0,2 > 99,9 % In this table was also indicated the rate of corrosion inhibition caused by the naphthenic acid mixture. This rate is expressed in% and is defined by the formula: $$\mathit{inhibition}\left(\%\right)=\left(1-\frac{\left[\mathit{Iron}\right]\mathit{with\; inhibitor}}{\left[\mathit{Iron}\right]\mathit{without\; inhibitor}}\right)\times 100$$

wherein [Iron] is the dissolved iron concentration measured with or without an inhibitor, the concentration of iron without inhibitor being equal to 203.5 ppm according to Example 1. <b><u> Table II </ u></b> Compound of formula (I) Iron concentration (ppm) Inhibition rate tertiononylmercaptan 48 76% tertiododecylmercaptan <0.2 > 99.9%

Claims (8)

1. Process for combating the corrosion by naphthenic acids of the metal walls of a refining plant in which a hydrocarbon stream is treated in the absence of oxygen, characterized in that it comprises the addition to the said stream of an effective amount of one or more hydrocarbon compound(s) comprising from 4 to 20 carbon atoms of formula:

   

   in which the symbols R₁, R₂, R₃, R₄, R₅, R₆, R₇, R₈ and R₉, which are identical or different, each represent a hydrogen atom or a linear or branched alkyl radical, an aryl radical or an alkylaryl radical, it being possible for these radicals optionally to comprise one or more heteroatoms, such as oxygen or sulphur.
2. Process according to Claim 1, characterized in that use is made, as compound of formula (I), of a tertiary mercaptan of empirical formula C_nH_2n+1-SH in which n is between 8 and 14.
3. Process according to either of Claims 1 and 2, characterized in that use is made, as compound of formula (I), of tert-dodecyl mercaptan.
4. Process according to one of Claims 1 to 3, characterized in that the amount of compound of formula (I) corresponds to a concentration, expressed as equivalent weight of sulphur with respect to the weight of the hydrocarbon stream, of between 1 and 1000 ppm, preferably between 5 and 200 ppm.
5. Process according to one of Claims 1 to 4, characterized in that the hydrocarbon stream to be treated has a TAN of greater than 0.2 and preferably of greater than 2.
6. Process according to one of Claims 1 to 5, characterized in that it is implemented at a temperature of between 200 and 450°C and more particularly between 250 and 350°C.
7. Process according to one of Claims 1 to 6, characterized in that the metal employed in the manufacture of the walls of the refining plant is a carbon steel optionally comprising up to 10% by weight of chromium and/or molybdenum, preferably up to 5%.
8. Process according to one of Claims 1 to 7, characterized in that the hydrocarbon stream to be treated is chosen from crude oil, atmospheric distillation residue, the gas oil fractions resulting from atmospheric and vacuum distillations, and the corresponding vacuum residue and distillate.