U.S. patent application number 10/588017 was filed with the patent office on 2007-05-17 for method for corrosion control of refining units by acidic crudes.
Invention is credited to Francis Humblot.
Application Number | 20070108099 10/588017 |
Document ID | / |
Family ID | 34778563 |
Filed Date | 2007-05-17 |
United States Patent
Application |
20070108099 |
Kind Code |
A1 |
Humblot; Francis |
May 17, 2007 |
Method for corrosion control of refining units by acidic crudes
Abstract
Process for combating the corrosion by naphthenic acids of the
metal walls of a refining plant, comprising the use of a tertiary
mercaptan of empirical formula C.sub.nH.sub.2n+1--SH in which n is
between 8 and 14.
Inventors: |
Humblot; Francis;
(Lanneplaa, FR) |
Correspondence
Address: |
ARKEMA INC.;PATENT DEPARTMENT - 26TH FLOOR
2000 MARKET STREET
PHILADELPHIA
PA
19103-3222
US
|
Family ID: |
34778563 |
Appl. No.: |
10/588017 |
Filed: |
January 28, 2005 |
PCT Filed: |
January 28, 2005 |
PCT NO: |
PCT/FR05/00190 |
371 Date: |
August 1, 2006 |
Current U.S.
Class: |
208/48AA |
Current CPC
Class: |
C10G 75/02 20130101 |
Class at
Publication: |
208/048.0AA |
International
Class: |
C10G 9/16 20060101
C10G009/16 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 6, 2004 |
FR |
0401156 |
Claims
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: ##STR2## in which the symbols R.sub.1, R.sub.2,
R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8 and R.sub.9,
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.sub.nH.sub.2n+1--SH in which n is between 8 and 14.
3. Process according to claim 1, characterized in that use is made,
as compound of formula (I), of tert-dodecyl mercaptan.
4. Process according to claim 1, 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.
5. Process according to claim 1, characterized in that the
hydrocarbon stream to be treated has a TAN of greater than 0.2.
6. Process according to claim 1, characterized in that it is
implemented at a temperature of between 200 and 450.degree. C.
7. Process according to claim 1, 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.
8. Process according to claim 1, characterized in that the
hydrocarbon stream to be treated is selected from crude oil,
atmospheric distillation residue, the gas oil fractions resulting
from atmospheric distillations, the gas oil fractions resulting
from vacuum distillations, vacuum residue or distillate.
9. Process according to claim 1, 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 5 and 200 ppm.
10. Process according to claim 1, characterized in that the
hydrocarbon stream to be treated has a TAN of greater than 2.
11. Process according to claim 1, characterized in that it is
implemented at a temperature of between 250 and 350.degree. C.
12. Process according to claim 1, characterized in that the metal
employed in the manufacture of the walls of the refining plant is a
carbon steel optionally comprising up to 5% by weight of chromium
and/or molybdenum.
Description
[0001] The present invention relates to the field of the treatment
of acid crude oils in refineries. A more particular subject-matter
of the invention is a process for combating the corrosion of
refining plants which treat acid crudes, comprising the use of
specific sulphur compounds.
[0002] Oil refineries may be confronted with a serious problem of
corrosion when they are forced to treat certain "acid" crudes.
These acid crudes essentially comprise naphthenic acids, which are
the source of this very specific corrosion phenomenon since it
occurs in a liquid medium which does not conduct electrical
current. These naphthenic acids correspond to saturated cyclic
hydrocarbons carrying one or more carboxyl groups. The acidity of a
crude oil is described by a standardized measurement according to
Standard ASTM D 664-01. It is expressed in mg of potassium
hydroxide necessary to neutralize 1 g of oil and is referred to as
TAN (Total Acid Number). It is known in this technical field that a
crude oil having a TAN of greater than 0.2 is described as acidic
and can result in damage in the plants of a refinery.
[0003] This corrosion reaction is highly dependent on the local
conditions, such as, for example, the temperature and the metallic
nature of the wall in the plant concerned, the space velocity of
the hydrocarbon and the presence of a gas-liquid interface. Thus,
even after many studies on the subject, refiners encounter great
difficulties in predicting the scale of the corrosion reactions and
their location.
[0004] One of the industrial solutions to this corrosion problem
consists in using installations made of stainless steels or alloys
of iron with in particular chromium and molybdenum. However, this
solution is not used to any great extent because of the high
capital cost. Furthermore, this choice preferably has to be
contemplated during the design of the refinery as stainless steels
exhibit inferior mechanical properties to those of the carbon
steels which are normally used and require an appropriate
infrastructure.
[0005] The consequence of the existence of these technical
difficulties in treating acid crudes is thus that these crudes are
generally sold to refiners at a lower price level than that of
standard crudes.
[0006] Another solution to the problem of the treatment of an acid
crude oil, used by refiners in practice, consists in diluting it
with another non-acid crude oil, so as to obtain a low mean
acidity, for example less than the threshold for TAN of 0.2. In
this case, the concentration of naphthenic acid becomes
sufficiently low to generate acceptable rates of corrosion.
However, this solution remains limited in scope. This is because
some acid crudes exhibit TAN values of greater than 2, which puts
an upper limit on their use of at most 10% of the total volume of
crudes entering the refinery. Moreover, some mixtures of crudes
sometimes result in the reverse effect desired, even after
dilution, that is to say in an acceleration in the corrosion
reactions by naphthenic acids.
[0007] Another approach in combating this corrosion problem is the
introduction into the acid crude oil to be treated of chemical
additives which inhibit or prevent attack on the metal wall of the
plant concerned. This route is often very economical in comparison
with that consisting in using the special alloys or steels
indicated above.
[0008] Laboratory studies, such as that of Turnbull
(Corrosion--November 1998 in Corrosion, Volume 54, No. 11, page
922), have envisaged the addition of small amounts (of the order of
0.1%) of hydrogen sulphide to the crude oil in order to reduce
corrosion by naphthenic acids. However, this solution is not
applicable in a refinery as hydrogen sulphide, which is a gas at
ambient temperature, is highly toxic, which renders the
consequences of an escape extremely serious and restricts the use
thereof. Furthermore, at a higher temperature, hydrogen sulphide
itself becomes highly corrosive and will result, in other parts of
the refinery, in a worsening of generalized corrosion.
[0009] Patent U.S. Pat. No. 5,182,013 discloses the use of other
sulphur compounds in solving this same corrosion problem, namely
polysulphides with alkyl radicals comprising 6 to 30 carbon
atoms.
[0010] More recently, the use of corrosion inhibitors based on
sulphur and on phosphorus has also been disclosed.
[0011] Thus, Patent EP 742 277 discloses the inhibiting action of a
combination of a trialkyl phosphate and of an organic polysulphide.
Patent U.S. Pat. No. 5,552,085 recommends the use of thiophosphorus
compounds, such as organothiophosphates or organothiophosphites.
Patent AU 693 975 discloses, as inhibitor, a mixture of trialkyl
phosphate and of phosphoric esters of sulphurized phenol
neutralized with lime.
[0012] However, organophosphorus compounds are very problematic to
handle due to their high toxicity. Furthermore, they poison the
hydrotreating catalysts installed to purify the hydrocarbon
fractions resulting from atmospheric and vacuum distillations. For
these two reasons, at least, their use in the field of refining is
not desirable.
[0013] Crude oils comprise a great variety of organosulphur
compounds, including alkyl mercaptans. Surprisingly, it has been
found that a specific family of alkyl mercaptans, the compounds in
which the mercaptan functional group is carried by a tertiary
carbon, make it possible to inhibit corrosion by naphthenic acids
more effectively than organic polysulphides and without it being
necessary in addition to introduce phosphorus-comprising
inhibitors.
[0014] A subject-matter of the invention is thus a 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: ##STR1##
[0015] in which the symbols R.sub.1, R.sub.2, R.sub.3, R.sub.4,
R.sub.5, R.sub.6, R.sub.7, R.sub.8 and R.sub.9, 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.
[0016] The mercaptans preferably used according to the invention
are tertiary mercaptans of empirical formula C.sub.nH.sub.2n+1--SH
in which n is between 8 and 14.
[0017] tert-Dodecyl mercaptan is a more particularly preferred
compound of formula (I), taken in isolation 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 of greater than 50% by
weight. Such a mixture is generally prepared industrially by
addition of hydrogen sulphide to an olefin fraction, such as
tetrapropylene, and is sold under the name tert-dodecyl
mercaptan.
[0018] The amount of compound(s) of formula (I) to be added to the
hydrocarbon stream to be treated by the refining plant generally
corresponds to a concentration, expressed as equivalent weight of
sulphur in the said compound with respect to the weight of the
hydrocarbon stream, of between 1 and 1000 ppm, preferably between 5
and 200 ppm. It will be possible, while remaining within this
concentration range, to set a high content at the start of the
process according to the invention and then subsequently to reduce
this content to a maintenance dose.
[0019] The process according to the invention advantageously makes
it possible to treat hydrocarbon streams, in particular crude oils,
having a TAN of greater than 0.2 and preferably of greater than
2.
[0020] The temperature for implementing the process corresponds to
that at which the corrosion reactions by naphthenic acids occur and
is generally between 200 and 450.degree. C. and more particularly
between 250 and 350.degree. C.
[0021] The addition of the compound of formula (I) to the
hydrocarbon stream can be carried out either at the actual inlet of
the plant (simultaneously with the hydrocarbon stream to be
treated), for an overall treatment of the corrosion, or in the part
of the plant where the corrosion reaction takes place, for
localized treatment. This addition can be carried out by any means
known to a person skilled in the art which provides control of the
injection flow rate and good dispersion of the additive in the
hydrocarbon, for example using a nozzle or a mixer.
[0022] The term "metal walls of the refining plant", the corrosion
of which can be prevented by the process according to the
invention, is understood to mean all the walls liable to be in
contact with the acid hydrocarbon stream to be treated. The wall
can thus be equally well the inner wall proper of plants, such as
atmospheric and vacuum distillation towers, and the surface of the
components internal to the latter, such as their plates or
packings, or else of the components peripheral to the latter, such
as their withdrawal and inlet lines or the pumps, preheat furnaces
or heat exchangers, provided that these components are brought to a
local temperature of between 200 and 450.degree. C.
[0023] The metal employed in the manufacture of the walls of the
refining plant is generally a carbon steel optionally comprising up
to 10% by weight of chromium and/or of molybdenum, preferably up to
5%.
[0024] Nonlimiting examples of the hydrocarbon stream to be treated
in accordance with the process according to the invention are crude
oil, atmospheric distillation residue, the gas oil fractions
resulting from atmospheric and vacuum distillations, and the vacuum
residue and distillate resulting from vacuum distillation.
[0025] The following examples are given purely by way of
illustration of the invention and should not be interpreted with
the aim of limiting its scope.
[0026] In these examples, use is made of a corrosion test, the
conditions of which are given below.
[0027] Description of the Corrosion Test:
[0028] This test employs an iron powder, which simulates a metal
surface, and a mineral oil in which a mixture of naphthenic acids
is dissolved, which simulates an acid crude stream. The
characteristics of these reactants are as follows:
[0029] white mineral oil having a density of 0.838,
[0030] powder formed of spherical iron particles having a particle
size of
[0031] -40+70 mesh (i.e. of approximately 212 to 425 .mu.m),
[0032] mixture of naphthenic acids having from 10 to 18 carbon
atoms, a boiling point of between 270 and 324.degree. C. and an
average molar mass of 244 g/mol.
[0033] The following are introduced into a 150 ml glass reactor
equipped with a dropping funnel and a water-cooled reflux condenser
and provided with a stirring system and a system for measuring the
temperature:
[0034] 70 ml (i.e. 58.8 g) of the mineral oil,
[0035] 2 g of the iron powder,
[0036] 2.8 g of the naphthenic acid mixture.
[0037] The initial TAN of the reaction mixture is 10.
[0038] These reactants are kept in contact for 2 hours at a
temperature of 250.degree. C. under a dry nitrogen atmosphere to
prevent oxidation reactions.
[0039] At the end of the test, the concentration of dissolved iron
in the medium is determined by a conventional method in which a
sample is digested, the residue is taken up in acidified water and
assaying is carried out using a plasma torch.
[0040] This concentration of dissolved iron (expressed in ppm) is
directly proportional to the rate of the 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
[0041] The preceding test is carried out without addition of
compound of formula (I), with 2 repetitions.
[0042] The results are shown in Table I below. TABLE-US-00001 TABLE
I Iron concentration (ppm) Test 1 180 Test 2 227 Mean 203.5
EXAMPLE 2
Tests in the Presence of Tertiary Alkyl Mercaptans
[0043] Example 1 is repeated while adding to the mineral oil,
during the charging of the reactor, tert-nonyl mercaptan or
tert-dodecyl mercaptan. These products are mixtures of tertiary
alkyl mercaptans centred respectively on the compounds comprising 9
and 12 carbons. The content of these derivatives is calculated so
as to obtain a corresponding concentration of 500 ppm by weight of
sulphur in the mineral oil present in the reactor.
[0044] The results collated in the following Table II are
obtained.
[0045] The degree of inhibition of the corrosion brought about by
the naphthenic acid mixture has also been shown in this table. This
degree is expressed in % and is defined by the formula: inhibition
.times. .times. ( % ) = ( 1 - [ Iron ] .times. .times. with .times.
.times. inhibitor [ Iron ] .times. .times. without .times. .times.
inhibitor ) .times. 100 ##EQU1##
[0046] in which [Iron] is the concentration of dissolved iron,
measured with or without inhibitor, the concentration of iron
without inhibitor being 203.5 ppm in accordance with Example 1.
TABLE-US-00002 TABLE II Iron Degree of concentration inhibition
Compound of formula (I) (ppm) (%) tert-Nonyl mercaptan 48 76%
tert-Dodecyl mercaptan <0.2 >99.9%
* * * * *