U.S. patent application number 11/547920 was filed with the patent office on 2007-07-19 for use of organic polysulfides against corrosion by acid crudes.
Invention is credited to Francis Humblot.
Application Number | 20070163922 11/547920 |
Document ID | / |
Family ID | 34944754 |
Filed Date | 2007-07-19 |
United States Patent
Application |
20070163922 |
Kind Code |
A1 |
Humblot; Francis |
July 19, 2007 |
Use of organic polysulfides against corrosion by acid crudes
Abstract
Method of combating the corrosion caused by naphthenic acids to
the metal walls of a refining unit, comprising the use of a
polysulphide having an alkyl radical containing between 2 and 5
carbon atoms.
Inventors: |
Humblot; Francis;
(Lanneplaa, FR) |
Correspondence
Address: |
ARKEMA INC.;PATENT DEPARTMENT - 26TH FLOOR
2000 MARKET STREET
PHILADELPHIA
PA
19103-3222
US
|
Family ID: |
34944754 |
Appl. No.: |
11/547920 |
Filed: |
April 8, 2005 |
PCT Filed: |
April 8, 2005 |
PCT NO: |
PCT/FR05/00861 |
371 Date: |
October 6, 2006 |
Current U.S.
Class: |
208/255 |
Current CPC
Class: |
C10G 75/02 20130101 |
Class at
Publication: |
208/255 |
International
Class: |
C10G 45/00 20060101
C10G045/00; C10G 17/00 20060101 C10G017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 13, 2004 |
FR |
0403838 |
Claims
1. Method of combating the corrosion caused by naphthenic acids to
the metal walls of a refining unit, characterized in that it
comprises the addition to the hydrocarbon stream for processing by
the unit of an effective amount of one or more hydrocarbon
compounds of formula ##STR2## in which n is an integer between 2
and 15 and the symbols R.sup.1 and R.sup.2, which are identical or
different, are selected from a linear or branched alkyl radical
containing between 2 and 5 carbon atoms, or a cycloalkyl radical
containing between 3 and 5 carbon atoms, it being possible for
these radicals to contain, optionally, one or more heteroatoms such
as oxygen or sulphur.
2. Method according to claim 1, characterized in that a compound of
formula (I) is used in which R.sup.1 and R.sup.2 are linear or
branched alkyl radicals and n is between 2 and 6.
3. Method according to claim 1, characterized in that a compound of
formula (I) is used in which the radicals R.sup.1 and R.sup.2 are
identical.
4. Method according to claim 1, characterized in that said one or
more hydrocarbon compounds comprises a mixture of
poly(di-tert-butyl sulphide)s in which the average value of n is
between 2 and 6.
5. Method according to claim 1, characterized in that the amount of
compound(s) of formula (I) corresponds to a concentration,
expressed by equivalent weight of sulphur relative to the weight of
the hydrocarbon stream, of between 1 and 5000 ppm.
6. Method according to claim 1, characterized in that the stream of
hydrocarbons to be processed has a TAN of more than 0.2.
7. Method according to claim 1, characterized in that it is carried
out at a temperature of between 200 and 450.degree. C.
8. Method according to claim 1, characterized in that the
hydrocarbon stream to be processed is selected from petroleum
crude, the residue from atmospheric distillation, the gas-oil cuts
obtained from atmospheric distillations, the gas-oil cuts obtained
from vacuum distillations, the vacuum residue obtained from vacuum
distillation or the distillate obtained from vacuum
distillation.
9. Method according to claim 1 characterized in that the amount of
compound(s) of formula (I) corresponds to a concentration,
expressed by equivalent weight of sulphur relative to the weight of
the hydrocarbon stream of between 5 and 500 ppm.
10. Method according to claim 1, characterized in that the stream
of hydrocarbons to be processed has a TAN of more than 1.
11. Method according to claim 1, characterized in that it is
carried out at a temperature of between 250 and 350.degree. C.
Description
[0001] The present invention pertains to the field of the treatment
of acidic crude petroleums in refineries. It relates more
especially to a method of combating corrosion in refining units
which process acidic crudes, comprising the use of specific
polysulphide compounds.
[0002] Petroleum refineries may be confronted with a serious
corrosion problem when they are required to process certain crudes
known as acidic crudes. These acidic crudes consist essentially of
naphthenic acids, which are the origin of this corrosion
phenomenon, which is a very particular phenomenon since it takes
place in a liquid medium which is a non-conductor of electrical
current. These naphthenic acids correspond to saturated cyclic
hydrocarbons which carry one or more carboxylic groups. The acidity
of a petroleum crude is described by a measurement standardized in
accordance with ASTM standard D 664-01. It is expressed in mg of
potassium hydroxide required to neutralize 1 g of petroleum and is
referred to as TAN (total acid number). It is known in this
technical field that a crude petroleum having a TAN of more than
0.2 is qualified as acidic, and may lead to damage within the units
of a refinery.
[0003] This corrosion reaction depends heavily on local conditions
such as, for example, the temperature and the metallic nature of
the walls in the unit concerned, the space velocity of the
hydrocarbon, and the presence of a gas/liquid interface.
Accordingly, even after major studies on the topic, refiners
encounter great difficulty in predicting the extent of the
corrosion reactions and their location.
[0004] One of the industrial solutions to this corrosion problem
involves using apparatus made of stainless steels--that is, alloys
of iron with, in particular, chromium and molybdenum. However, this
solution remains little used, owing to the high capital investment
cost. That choice, moreover, must preferably be considered during
the design of the refinery, since stainless steels have mechanical
properties inferior to those of the carbon steels normally used,
and require an appropriate infrastructure.
[0005] The existence of these technical difficulties in processing
acidic crudes therefore means that, in general, these crudes are
sold to refiners at a price level lower than that of the standard
crudes.
[0006] Another solution to the problem of processing an acidic
crude petroleum, which is used in practice by refiners, involves
diluting it with another, non-acidic petroleum crude, so as to give
a low average acidity, lower for example than the 0.2 TAN
threshold. In this case the concentration of naphthenic acid
becomes low enough to give rise to acceptable corrosion rates. This
solution remains limited in scope, however. The reason for this is
that certain acidic crudes have TANs of more than 2, which curtails
their use to not more than 10% of the total volume of crudes
entering the refinery. Moreover, certain blends of crudes sometimes
lead to the converse of the desired effect, even after dilution, in
other words to an acceleration of the corrosion reactions by
naphthenic acids.
[0007] An alternative approach for combating this corrosion problem
is to introduce, into the acidic crude petroleum to be processed,
chemical additives which inhibit or prevent the attack of the metal
walls of the unit in question. This route is often very economic in
comparison to that indicated above, involving the use of special
steels or alloys.
[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 petroleum, for the purpose
of reducing the corrosion by naphthenic acids. This solution,
however, is not applicable in the refinery, since the hydrogen
sulphide, which is gaseous at ambient temperature, is highly toxic,
thereby making the consequences of any leak extremely serious, and
limiting its use. Moreover, at even higher temperature, the
hydrogen sulphide itself becomes highly corrosive and, in other
parts of the refinery, will lead to aggravation of the generalized
corrosion.
[0009] U.S. Pat. No. 5,182,013 describes the use, for solving this
corrosion problem, of other sulphur compounds, namely polysulphides
having alkyl radicals containing from 6 to 30 carbon atoms.
[0010] EP Patent 742277 describes the inhibitory activity of a
combination of a trialkyl phosphate and an organic polysulphide.
U.S. Pat. No. 5,552,085 recommends the use of thiophosphorus
compounds such as organic thiophosphates or thiophosphites. AU
Patent 693975 discloses as inhibitor a mixture of trialkyl
phosphate and phosphoric esters of sulphurized phenol neutralized
with lime.
[0011] However, the handling of organophosphorus compounds is very
delicate, owing to their high toxicity. In addition, they are
poisons for the hydrotreating catalysts which are installed to
purify the hydrocarbon cuts obtained from atmospheric and vacuum
distillations. For these two reasons at least, their use in the
field of refining is undesirable.
[0012] Surprisingly it has been found that the use of a specific
class of organic polysulphides, namely poly-alkyl sulphides in
which the number of carbons in each alkyl radical is between 2 and
5, allows the corrosion caused by naphthenic acids to be inhibited
more effectively than using the organic polysulphides known to
date, and without the need to introduce phosphorus inhibitors as
well.
[0013] The invention accordingly provides a method of combating the
corrosion caused by naphthenic acids to the metal walls of a
refining unit, characterized in that it comprises the addition to
the hydrocarbon stream for processing by the unit of an effective
amount of one or more hydrocarbon compounds of formula ##STR1## in
which [0014] n is an integer between 2 and 15 and [0015] the
symbols R.sup.1 and R.sup.2, which are identical or different, each
represent a linear or branched alkyl radical containing between 2
and 5 carbon atoms, it being possible for these radicals to
contain, optionally, one or more heteroatoms such as oxygen or
sulphur; or [0016] R.sup.1 and R.sup.2, which are identical or
different, each represent a cycloalkyl radical containing between 3
and 5 carbon atoms, it being possible for these radicals to
contain, optionally, one or more heteroatoms such as oxygen or
sulphur.
[0017] The polysulphides of formula (I) are prepared according to
processes which are known per se, such as those described in
patents U.S. Pat. Nos. 2,708,199, 3,022,351 and 3,038,013. Some of
them are commercial products.
[0018] Preferably R.sup.1 and R.sup.2 are linear or branched alkyl
radicals and n is between 2 and 6.
[0019] According to another preferred version the radicals R.sup.1
and R.sup.2 are identical, owing to the improved stability of the
corresponding compound of formula (I).
[0020] According to a version which is even more preferred,
poly(di-tert-butyl sulphide)s are used as a mixture of compounds of
formula (I). These products, industrial in origin, are obtained for
example from the reaction of sulphur with tert-butyl mercaptan. The
reaction conditions allow industrial products to be prepared that
are composed of a mixture of polysulphides with a number of sulphur
atoms varying between 3 and 10, with a number-average value of
between 2 and 6.
[0021] The amount of compound(s) of formula (I) to be added to the
hydrocarbon stream for processing by the refining unit corresponds
generally to a concentration, expressed by equivalent weight of
sulphur of the said compound relative to the weight of the
hydrocarbon stream, of between 1 and 5000 ppm, preferably between 5
and 500 ppm. While remaining within this concentration range, it
will be possible to set a high content at the start-up of the
method according to the invention, then to reduce this content
subsequently to a maintenance level.
[0022] The method according to the invention makes it possible
advantageously to process hydrocarbon streams, and more
particularly crude petroleums, whose TAN is greater than 0.2 and
preferably greater than 1.
[0023] The temperature at which the method is employed corresponds
to that at which the corrosion reactions by naphthenic acids take
place, and is generally between 200 and 450.degree. C. and more
particularly between 250 and 350.degree. C.
[0024] The addition of the compound of formula (I) to the
hydrocarbon stream may be carried out in close proximity to where
the corrosion reaction occurs or else, at a lower temperature,
upstream of the process of the said unit. This addition may be
carried out by any means known to the skilled person which ensures
control of the injection rate and effective dispersion of the
additive in the hydrocarbon: for example, by means of a nozzle or
of a mixer.
[0025] The metal walls of the refining unit in which the corrosion
can be prevented by the method according to the invention are any
walls liable to come into contact with the stream of acidic
hydrocarbon to be processed. The walls involved may therefore
equally be the inner walls proper of units such as the atmospheric
and vacuum distillation towers, or the surface of internal elements
thereof, such as their plates or packings, or else peripheral
elements thereof, such as their offtake and entry lines, pumps,
preheating ovens or heat exchangers, in so far as these elements
are taken to a local temperature of between 200 and 450.degree.
C.
[0026] Non-limiting examples of hydrocarbon streams to be processed
in accordance with the method according to the invention include
the petroleum crude, the residue from atmospheric distillation, the
gas-oil cuts obtained from atmospheric and vacuum distillations,
and the vacuum residue and distillate obtained from vacuum
distillation.
[0027] The examples which follow are given purely to illustrate the
invention and should not be interpreted as limiting its scope.
[0028] In these examples a corrosion test is implemented whose
conditions are given below.
DESCRIPTION OF THE CORROSION TEST
[0029] This test employs an iron powder, which simulates 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 reactants are as follows: [0030] white
mineral oil having a density of 0.838 [0031] powder of spherical
iron particles having a size of -40+70 mesh (i.e. from
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 components are introduced into a 150 ml glass
reactor equipped with a dropping funnel and a water condenser and
fitted with a stirring system and a temperature-measurement system:
[0034] 70 ml (or 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 at a temperature of
250.degree. C. for 2 hours under an atmosphere of dry nitrogen, in
order to avoid oxidation reactions.
[0039] At the end of the test the concentration of iron dissolved
in the medium is determined by a conventional method employing
mineralization of a sample, the taking-up of the residue in
acidified water, and an assay using an electron torch.
[0040] This concentration of dissolved iron (expressed in ppm) is
directly proportional to the corrosion rate of the iron powder that
is generated by the mixture of naphthenic acids present in the
mineral oil.
EXAMPLE 1
Reference Test in the Absence of Inhibitor
[0041] The above test is employed without any compound of formula
(I) being added, with 2 repetitions.
[0042] The results are indicated in Table I below. TABLE-US-00001
TABLE I Iron concentration (ppm) Test 1 180 Test 2 227 Average
203.5
EXAMPLE 2
Tests in the Presence of Polyalkyl Sulphides
[0043] Example 1 is repeated with the addition of different types
of polyalkyl sulphides in mineral oil during the charging of the
reactor. The amount of these derivatives added is calculated so as
to give a concentration of 500 ppm, expressed in equivalent weight
of sulphur, in the mineral oil present in the reactor.
[0044] The results collated in Table II below are obtained.
[0045] Likewise indicated in this table is the degree of inhibition
of the corrosion brought about by the naphthenic acid mixture. This
degree is expressed in % and is defined by the following 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 equal to 203.5 ppm in accordance with
Example 1. TABLE-US-00002 TABLE II Iron Degree of Compound of
Commercial concentration inhibition formula (I) name* (ppm) (%)
Di-tert-butyl TPS 44 4 98% trisulphide Di-tert-butyl TPS 54 7 97%
tetrasulphide *supplier: ARKEMA
* * * * *