METHOD FOR PREPARING METHYL MERCAPTAN
[OOOI] The present invention relates to a process for preparing mercaptans, in particular
methyl mercaptan, from a hydrocarbon charge, hydrogen sulphide and optionally sulphur.
[0002] Mercaptans are of great industrial interest and are very widely used nowadays by
the chemical industries, particularly as precursors or starting materials for the synthesis of
more complex organic molecules. For example, methyl mercaptan (CH3SH) is used as
starting material in the synthesis of methionine, an essential amino acid which is used to a
high degree in animal feeding. Methyl mercaptan is also used for the synthesis of dialkyl
disulphides, and particularly for the synthesis of dimethyl disulphide (DMDS), an additive
for sulphurizing hydrotreating catalyst for petroleum fractions, among other applications.
[0003] Methyl mercaptan is nowadays commonly produced industrially on the tonne scale
from methanol (CH30H) and hydrogen sulphide (HzS) in accordance with the following
reaction (1):
CH30H + H2S 3 CH3SH + Hz0 (1)
[0004] This synthesis pathway, however, has a number of drawbacks, including that of
using methanol, which necessitates a supplementary step, since methanol is prepared from
hydrocarbon charges, and the drawback of leading to secondaly products, typified
especially by dimethyl ether (CHJOCH~), dimethyl sulphide (CHJSCH~), and cracking
products (such as, for example, carbon monoxide and carbon dioxide), and water, to state
only some of the drawbacks. Moreover, the presence of secondary products of these kinds
results in a large number of purification steps for the methyl mercaptan, to the detriment of
high productivity and high selectivity and therefore of an optimum yield.
[0005] This synthesis pathway, and also some improvements thereto, are described for
example in documents W02004/096760, W02006/015668, W020071028708,
W020081118925 and W02013/092129.
[0006] Other synthesis processes do away with the need to use methanol, and include
the preparation of methyl mercaptan from carbon monoxide (CO) in accordance with the
following reaction (2):
CO + 2H2 + HzS + CH3SH +Hz0 (2)
[0007] However, the use of carbon monoxide (CO) is not free of drawbacks, since CO
originates essentially from synthesis gas, which is a COIHz mixture, and which consequently
necessitates:
- a supplementary step of steam reforming of a hydrocarbon source for the purpose of
obtaining a synthesis gas,
- the availability of a synthesis gas having appropriate proportions of carbon monoxide (CO)
and hydrogen (HZ), without any need for adjustment to the COIH2 ratio by means of the
water-gas shift reaction, defined as follows in reaction (A):
CO + Hz0 3 C02 + Hz (A)
[0008] Moreover, the processes in accordance with reaction (2) above have the drawback
of giving rise to secondary products, such as carbon dioxide (COz), methane (CH.,), dirnethyl
sulphide (CH3SCH3) and water (H20). These processes are described for example in
documents US2007213564, US2008293974, US2010094059, and US2010286448.
[0009] Yet other processes are described in the literature, and combine different
reactions, such as:
-formation of CS2 and H2S from methane and sulphur, in accordance with reaction (3):
CH4 + 4 S + CS2 + 2 HzS (3)
- hydrogenation of CS2, using the hydrogen formed above, in accordance with reaction (4):
CS2 + 3 Hz 4 CHaSH + H2S (4)
[OOIO] These processes evidently combine the drawbacks described for reactions (1) and
(2) with the additional difficulty of necessitating a supplementary source of hydrogen in order
to perform reaction (4).
[ O O I I ] Yet another method is disclosed in document W02010/046607 and involves the
hydrogenation of sulphur compounds which carry C=S unsaturation, and more particularly
the hydrogenation of carbon disulphide (CS2), to methyl mercaptan (CH3SH) in accordance
with reaction (4) above.
[0012] However, the process performed in this document employs carbon disulphide
(CS2), which is a dangerous, toxic product which can be used industrially with the installation
of severe safety means; all enterprises and factories do not wish to or are unable to develop
a plant meeting the safety standards required for the holding of carbon disulphide.
[0013] International patent application W02001/96290 proposes a process for synthesis
of methyl mercaptan directly from methane (CH4) and H2S with co-production of hydrogen.
This direct reaction between methane and H2S is accomplished by means of a pulsed
plasma with corona discharge. This patent application does not describe any example of
synthesis, and it does not appear possible to envisage the industrial-scale implementation
of this process for synthesizing methyl mercaptan. Moreover, this process requires the
synthesis of H2S if the latter is unavailable.
[0014] Today, therefore, there is a need for a process for synthesis of methyl mercaptan
that does not'exhibit the drawbacks encountered in the known processes, thus being more
environmentally friendly and less eco-toxic, but also safer, while preserving the high yields
and selectivities, or even with yields and selectivities which are improved relative to those
of the known processes, while being a process which is operated as economically as
possible.
[0015] It has now been found that it is possible to remove the aforementioned drawbacks,
entirely or at least partly, by virtue of the process for preparing methyl mercaptan in
accordance with the present invention, which is detailed in the description hereinafter. The
process of the present invention makes it possible more particularly to resolve a large
number of the drawbacks in the processes based on reactions (1) and (2) as described
above.
[0016] Accordingly, in a first aspect, the present invention provides a process for
preparing methyl mercaptan, batchwise or continuously, preferably continuously, said
process comprising at least the following steps:
a) reaction of at least one hydrocarbon charge in the presence of hydrogen sulphide (H2S)
and optionally of sulphur (S) to form carbon disulphide (CS2) and hydrogen (HZ),
b) hydrogenation reaction of said carbon disulphide (CS2) in the presence of said
hydrogen (HZ), both obtained in step a), to form methyl mercaptan (CHaSH), hydrogen
sulphide (HzS) and optionally hydrogen (HZ),
c) optionally, but preferably, recycling said hydrogen sulphide (HzS) formed in step b) to
step a), and
d) recovery of the methyl mercaptan.
[0017] This process has the very great advantage of consuming the hydrogen sulphide
(H2S) which is produced during the reaction, and in some cases even doing so
stoichiometrically as indicated later on below, hence meaning that all the hydrogen sulphide
consumed in the process of the invention is produced by said process. Therefore, the
process of the invention avoids any addition, and even in some cases any removal, of
hydrogen sulphide (H2S) in excess, or else avoids the additional synthesis of hydrogen
sulphide (HzS), as is sometimes required with the known processes of the prior art.
[0018] Moreover, the process of the present invention is a process which is simple to
perform, is of low eco-toxicity and is economical. The process of the invention also makes
it possible to obtain a high yield and a high selectivity in terms of methyl mercaptan. In the
present description, unless othetwise stated, the percentages referred to are percentages
by weight.
[OOIQ] The process according to the invention is a process of two consecutive reaction
steps (steps a) and b) above), without any need for intermediate purification between the
two steps. Schematically, the first step of the process (step a) is a reaction, carried out
preferably at high temperature, between a hydrocarbon charge (illustrated here: methane)
and hydrogen sulphide (HzS) in accordance with reaction (5):
CH4 + 2 H2S 4 CS2 + 4 H2 (5)
[0020] In the second step (step b)), the carbon disulphide formed in step a) is subjected
to catalytic hydrogenation with the hydrogen likewise formed in step a), in accordance with
reaction (6):
CS2 + 4 Hz 4 CH3SH + H2S + H2 (6)
[0021] This succession of two reaction steps is notable in that the number of mols of
methyl mercaptan formed is identical to the number of mols of methane consumed, and that
step a) (reaction 5) requires twice as many mols of hydrogen sulphide as formed in step b)
(reaction 6).
100221 In one especially advantageous embodiment of the present invention, the
hydrogen sulphide formed in step b) is recycled into step a). In this embodiment, it is seen
that all of the hydrogen sulphide formed may therefore be re-used in step a), avoiding the
storage of said hydrogen sulphide formed.
[0023] In another embodiment, the sulphurized hydrogen (or hydrogen sulphide) formed
at the end of step b) may not be recycled into step a), and may be recovered for subsequent
use.
[0024] As indicated above, the amount of hydrogen sulphide produced in step b) (reaction
(6) above) is not sufficient in molar amount for the implementation of reaction (5) in step a),
and a further amount of hydrogen sulphide must be supplied in order for step a) to be
conducted.
[0025] In one particularly advantageous embodiment, consideration may be given to
synthesizing the missing amount of hydrogen sulphide, especially from the hydrogen
formed in step b) reacted with sulphur according to the process described for example in
document W020041022482, in accordance with the following reaction (B):
S + Hz 4 H2S (B)
[0026] As a variant, the hydrogen produced may be used to supply thermal energy by
combustion, and this thermal energy may advantageously be used for the requirements of
the process, especially in step a), which requires high temperatures for industrially
acceptable performance levels.
[0027] With this further reaction (B) above, for synthesis of hydrogen sulphide introduced
in step a), and taking account of the overall mass balance, the process according to the
invention has the very great advantage of producing one mole of methyl mercaptan per
mole of methane consumed, while consuming exactly the amount of hydrogen and of
hydrogen sulphide produced. This embodiment of the present invention may consequently
be schematized as is illustrated by reaction scheme (a) below:
--. CS, + 4 H,
H2S + Hz +
[0028] According to a variant of the process of the invention, the sulphur may be
introduced in the first step (step a)). The balanced reaction may then be written in
accordance with scheme (7):
CH4 + S + H2S + CSz + 3 H2 (7)
[0029] Step b) of the process may then be illustrated by the reaction scheme (4):
CS2 + 3 H2 + CH3SH + HzS (4)
[0030] In this case, the entirety of the hydrogen sulphide produced in step b) in
accordance with the reaction of scheme (4) may advantageously be recycled entirely
(stoichiometry respected) in step a) (in scheme (7)), this avoiding the supplementary
synthesis of hydrogen sulphide with supplementary equipment. This embodiment of the
present invention may therefore be schematized as is illustrated by reaction scheme (0)
below:
+ H,S + S - CS, + 3 H, L H*S + F I II (P)
[0031] All of the above reactions involve methane (CH4) as initial hydrocarbon charge, but
the process of the invention may be carried out in a similar manner from any type of
hydrocarbon charge. For a hydrocarbon charge of type CaH2b, the general equation
corresponding to the reaction of the first step (step a)) then becomes:
CaHzb + 2a H2S + a CS2 + (2a+b) H2
where a is an integer preferably between 1 and 30, end points included, more preferably
between 1 and 20, end points included, more preferably between 1 and 10, end points
included, and b represents an integer between 3/2 and 2(3 + I), end points included, with
the restriction that when a represents 1, then b represents 2.
[0032] Accordingly, and as an illustrative example, when the hydrocarbon charge is
propane (C~HB3, = 3 and b = 4), for example, the potential products obtained would be 3
CS2 and 10 H2.
[0033] Accordingly, the hydrocarbon charge reacted with hydrogen sulphide (H2S) in step
a) may be any type of charge known to the skilled person, and is generally a hydrocarbon
charge in gaseous, liquid or solid form, preferably in gaseous or liquid form, more preferably
in gaseous form, and comprises at least one hydrocarbon having a hydrocarbon chain in
saturated or unsaturated linear, branched or cyclic form.
[0034] More preferably, the hydrocarbon filler comprises at least one alkane, preferably
at least methane (CHd), ethane, propane or butane, and very preferably methane. With
further advantage the hydrocarbon charge is pure, meaning that it contains a single
compound, for example an alkane, and preferably methane (CHd), ethane, propane or
butane, and very preferably methane.
[0035] The above-defined hydrocarbon charge may come from numerous sources, all of
which are known to the skilled person, whether natural, artificial or synthetic, for example
from natural sources, but also by direct synthesis, by metathesis, etc. Examples of sources
of hydrocarbon charge which can be used in the process of the present invention include,
illustratively and not limitatively, biomass, petroleum, charcoal, coal, bituminous shales,
bituminous sands, and others.
[0036] According to one especially preferred aspect, the hydrocarbon charge employed
in step a) is selected from natural gas, shale gas and shale oil. The sources of hydrocarbon
charges are preferably selected from natural gas, shale gas and biogas.
[0037] Other examples of sources of hydrocarbon charges which may advantageously be
used in the context of the present invention include naphthas, crude petroleum distillation
products, petroleum fractions, preferably demetallized, deoxygenated andlor
denitrogenated, decomposition products, and more particularly products of the natural or
industrial methanization of biomass.
[0038] In the context of the present invention, preference is given to using methane as
initial hydrocarbon charge, primarily for economic reasons, and with the recent
developments in the exploitation of shale gas.
[0039] The methane used as initial hydrocarbon charge may be employed with one or
more other gases, different from the hydrocarbon charges as described above, although for
obvious reasons of subsequent purification, and ease of implementation of the process (risk
of accumulation with any recycling operations), preference will be given to using only
nlixtures of hydrocarbon charges or pure methane.
[0040] Where pure methane is used, but also when the initial hydrocarbon charge is other
than methane alone, there are no real constraints in terms of the molar H2S/CH4 ratio, or
HSlhydrocarbon charge ratio, that can be used in step a), since the excess of H2S is
advantageously recycled at the end of step h). If a hydrogen sulphide is used in a substoichiometric
amount, the effect will be seen in the conversion of the methane, or of the
hydrocarbon charge, respectively, and of the production of hydrogen.
[0041] It is also possible to consider a first step without H2S, and generating the required
H2S in situ by reacting the hydrocarbon charge with sulphur according to reaction (3) defined
above. The molar H2Slhydrocarbon charge ratio may therefore be 0 (if sulphur is present)
and may be up to about 100, the molar ratio preferably being between 0.5 and 10 and more
preferably between 1 and 3, these ranges of values being understood with end points
included. These values are particularly suitable when the initial hydrocarbon charge is
methane or comprises methane.
[0042] The hydrocarbon charge and the hydrogen sulphide are advantageously provided
continuously or discontinuously in the reactor or reactors in which the processlthe invention
is implemented, depending more particularly on whether the process is implemented
continuously or batchwise. The hydrocarbon charge and H2S are advantageously in liquid
or solid or gaseous form, preferably in gaseous form.
[0043] According to one embodiment, step a) is implemented in the absence of sulphur.
According to another embodiment, step a) is implemented in the presence of sulphur. In
this embodiment, the sulphur is in liquid, solid or gaseous form, preferably in liquid or
gaseous form.
[0044] According to one embodiment, step a) is carried out in the presence of a catalyst.
In this embodiment, said catalyst advantageously comprises a transition metal selected
from the elements of groups 6 to 11 of the Periodic Table of the elements (groups VIE, VIIB,
VIIIB), preferably from the elements of groups 6, 9 and 10, and more preferably the catalyst
comprises one or more transition metals selected from platinum, rhodium, chromium and
palladium. More preferably the catalyst comprises one or more transition metals selected
from platinum, rhodium, chromium or palladium, and very preferably the catalyst comprises
platinum.
[0045] Accordingly, the catalyst of step a) comprises a metal or metals, it being possible
for these metals to be in the form of a mixture, and it being possible for said metal (or metals)
to be in metallic form, but also in the form of oxide(s) or sulphide(s). When the catalyst is
present in the form of a metal oxide, a sulphurizing step may advantageously be carried out
according to the methods known to the skilled person.
[0046] The catalyst used in step a) is preferably a supported catalyst, the support being
selected preferably from alumina, silica, zeolites, activated carbons, titanium oxide,
zirconium oxides, clays, hydrotalcite, hydroxyapatite, magnesia, and others. The catalyst
may be favourably used in a fixed, fluid, circulating or ebullating bed. The catalyst is
preferably used in a fixed bed. According to another embodiment, step a) is carried out in
the absence of catalyst.
[0047] The reaction temperature in step a) is advantageously between 500°C and
1300"C, preferably between 700°C and 110O0C, more preferably between 800°C and
1000°C. For conversion reasons, for the lower limit, and for reasons of resistance of
materials, for the upper limit, preference is given to a temperature range between 700°C
and 1 10O0C, preferably between 800°C and 1000°C.
[0048] The reaction of step a) may be carried out alternatively at atmospheric pressure,
under superatmospheric pressure, or even under subatmospheric pressure; the skilled
person is aware of how to adapt the reaction pressure conditions according to the nature of
the reactants employed, the reaction temperatures selected, the rates of circulation of the
streams, and the intended degrees of conversion and intended yields.
[0049] Generally speaking, step a) may be carried out under a pressure of between
50 mbar and 100 bar (i.e. between 5x103 and lx107 Pa), more preferably between
atmospheric pressure and 50 bar (or 5x106 Pa), and advantageously between atmospheric
pressure and 15 bar (or 15x105 Pa).
[0050] The duration of the reaction in step a) may vary within wide proportions, depending
in particular on the nature and amount of each of the reactants, the nature and amount of
catalyst used, and the selected temperature and pressure. Generally speaking, the reaction
time in step a) may vary between several seconds to several minutes.
[0051] When sulphur is present for the implementation of the reaction in step a), the molar
sulphurlCH4 ratio is preferably between 0 and 4, end points excluded, or more generally the
molar sulphur/hydrocarbon charge ratio is preferably between 0 and (2a+b), end points
excluded, where 3 and I! are as defined above.
[0052] A molar sulphurICH4 ratio of 4 or more, in view of reaction (3), could bring about
the complete conversion of the methane to CSZ and Has, this being undesirable for step b)
of the process, which requires hydrogen. According to one preferred aspect of the present
invention, therefore, the sulphurICH4 ratio is between 0 and 4, end points excluded.
preferably between 0 and 2.5, end points excluded, and more preferably between 0 and
1.5, end points excluded.
[0053] As indicated above, the process according to the invention removes the need for
a purification step between steps a) and b). The reason for this is that during the
implementation of step b), the hydrogen (HZ) and the carbon disulphide (CS2) that are
obtained in step a) react together directly to form hydrogen sulphide (HzS) and methyl
mercaptan (CHzSH), and optionally hydrogen (HZ). Accordingly, the respective ratios of the
reactants employed in step b) are directly dependent on the ratios of the products obtained
at the end of step a).
[0054] The conduct of the reaction in step b) is known to the skilled person and is
described for example in international patent application W02010/046607. This reaction is
therefore known to lead to a conversion of CS2 of 100% for a selectivity in terms of methyl
niercaptan of loo%, if hydrogen is present at stoichiometry or in excess. The consequence
is that the methyl mercaptan produced in this step b) is very easy to separate from the
reaction mixture, since this mixture contains only methyl mercaptan, HzS, hydrogen, if it was
in excess, and optionally the hydrocarbon charge which may be in excess in step a), to
have a complete conversion of the sulphur. It should be noted that the hydrocarbon charge
in excess, after passing inertly into step b) and after separation of the methyl mercaptan
formed, may be recycled to step a) with the H2S.
[0055] According to one embodiment, step b) may be carried out in the presence of a
catalyst. In one preferred embodiment, a catalyst is used for the hydrogenation of the carbon
disulphide to methyl mercaptan. The catalyst which may be used may be of any type known
to the skilled person as a hydrogenation catalyst. Advantageously, the catalyst used for step
b) of the process according to the present invention may be selected from those described
in international patent application W020101046607, in which said hydrogenation catalyst
comprises at least one metal doped with at least one alkali metal or alkaline-earth metal
hydroxide or oxide.
[0056] The metal present in the catalyst of the invention may be any metal from group 6
andlor 8 of the Periodic Table of the classification of the Elements (IUPAC), and is
preferably selected from the group consisting of nickel (Ni), cobalt (Co), palladium (Pd),
rhodium (Rh), platinum (Pt), molybdenum (Mo), tungsten (W), chromium (Cr), iron (Fe) and
combinations of two or more of these, preferably combinations of two of these metals, and
more particularly CoIMo, NilMo, NiNV, WIMo, with very particular preference being given to
the combinations of nickel and molybdenum.
[0057] The metal or metals present in the catalyst of the invention may also be present
directly in the form of metal sulphides. These metal sulphides may also be obtained from
the corresponding oxides by any method known to the skilled person.
[0058] The catalyst of the invention is advantageously supported, conventionally, on any
type of support generally used within this field, and for example on a support selected from
alumina, silica, titanium dioxide (TiO4, zeolites, carbon, zirconium, magnesia (MgO), clays,
hydrotalcites and others, and also mixtures of two or more thereof.
[0059] As for the catalyst used in step a), the catalyst employed is favourably used in a
fixed, fluidized, circulating or ebullating bed. The catalyst is preferably in a fixed bed.
[0060] The amount of catalyst used in step a) and the amount of catalyst used in step b)
are dependent on the amount of methyl mercaptan it is desired to obtain. Accordingly, the
amounts of catalyst(s) employed in steps a) and b) are adjusted with the aim of obtaining a
methyl mercaptan productivity of from 0.1 kg.h-' to 20 kg.h-' per litre of catalyst. In this
configuration, the process according to the present invention has proved to be of particular
interest in terms of industrial and economic profitability. According to another embodiment,
step b) is carried out without catalyst.
[0061] The reaction temperature in step b) is generally lower than that used in step a),
and is commonly between 100°C and 400°C and preferably between 200°C and 300°C, a
temperature range within which the maximum selectivity in terms of methyl mercaptan is
observed, for an optimum conversion.
[0062] As for step a), step b) may be carried out under any pressure, preferably of
between 50 mbar and 100 bar (i.e. between 5x103 Pa and 1x107 Pa) more preferably of
between atmospheric pressure and 50 bar (or 5x106 Pa) and advantageously between
atmospheric pressure and 15 bar (or 15x105 Pa).
(00631 The hydrogenation time varies according to the nature and amount of each of the
reactants, and the nature and amount of catalyst used. For example, the reaction varies
between several seconds and several minutes.
[0064] Steps a) and b) are implemented in any type of reactor suitable for receiving hightemperature
reactions, as for example reactors made of alloy, of Hastelloy, lncoloy and
other types.
[0065] According to one preferred embodiment, steps a) and b) are each employed in a
separate reactor. According to another embodiment, step a) and step b) are carried out in
succession in the same reactor.
[0066] As indicated above, the process according to the invention optionally, but
preferably, comprises a step c) of recycling of the hydrogen sulphide formed at the end of
step b), which is re-introduced into the initial charge for realization of step a). This step c)
of recycling of the hydrogen sulphide formed has the advantage that in this way it is possible
to avoid the ex situ synthesis of hydrogen sulphide.
[0067] Hydrogen sulphide may therefore be recycled afler separation from the reaction
mixture from step b), according to any method known to the skilled person, and, for
example, by distillation, preferably under pressure, by freezing, by membrane separation,
etc.
[0068] According to another embodiment, methyl mercaptan is isolated from the reaction
mixture of step b) by any means known per se, and, for example, by degassing of the more
volatile compounds, such as hydrogen and hydrogen sulphide. Any unconverted
hydrocarbon charge, and also any unconverted carbon disulphide, are separated from the
methyl mercaptan by distillation.
[0069] The entirety of the remaining reaction mixture from step b) (from which the methyl
mercaptan has been removed) may advantageously be re-introduced recycled into step a)
of the process. This embodiment has the advantage of also recycling the initial hydrocarbon
charge, thereby allowing a substantial improvement in the methyl mercaptan production
yield in relation to the hydrocarbon charge introduced at the start. The process is optimized
in this way, since each carbon atom present in the initial hydrocarbon charge is converted
into one molecule of methyl mercaptan.
[0070] Accordingly, and in accordance with one variant, the process according to the
invention comprises not only the recycling of the hydrogen sulphide but also the recycling
of the residual compounds, in other unreacted compounds, these being carbon disulphide,
optionally hydrogen, optionally the hydrocarbon charge, optionally sulphur, and optionally
impurities. Generally speaking, the recycling is carried out according to techniques which
are well known to the skilled person.
I00711 It has been observed, moreover, that when coke is formed during the
implementation of the process according to the invention, it reacts with the hydrogen
sulphide (and optionally the sulphur present) to form hydrogen and carbon disulphide. The
process according to the invention therefore has the very great advantage of being operated
as a perfectly autonomous system, without emission of hydrogen sulphide, and without
harmful formation of coke in the reactor, in spite of the presence of hydrocarbon charge at
high temperature. Moreover, according to the process of the present invention, the
production yield of methyl mercaptan is thus 100% relative to the initial hydrocarbon charge.
100721 Step d) of the process of the invention corresponds to the recovery of the methyl
mercaptan formed, it being possible for this recovery to be carried out, as indicated earlier,
by any known method of the prior art - for example, by condensation, cooling under
pressure, as indicated above. Cooling under pressure, which is used advantageously,
results in the separation of the methyl mercaptan from the reaction mixture by the
liquefaction of the latter.
[0073] The present invention therefore offers an industrial process for preparation of
methyl mercaptan that is completely autonomous, has a high yield, and is more
environment-friendly and more economical than the methods known in the prior art.
[0074] In one variant of the process of the invention, when the by-products are not
recycled, or when only the hydrogen sulphide is recycled, it is possible to exploit said byproducts
- hydrogen sulphide, hydrogen and optionally carbon disulphide. One use of
particular interest for the hydrogen formed during the process of the invention is its use with
liquid sulphur to form hydrogen sulphide, which may therefore be used in the process of the
invention for preparing methyl mercaptan, as already indicated above.
[0075] By virtue of the aforementioned advantages of the process according to the
present invention, it is possible to achieve a high methyl mercaptan productivity, generally
of the order of 0.1 kg to 20 kg of methyl mercaptan per hour per litre of catalyst in step b).
[0076] The methyl mercaptan thus produced by the process of the present invention may
be used in all fields known to the skilled person, and, for example, as indicated earlier, as
starting material in the synthesis of methionine and of bio-methionine, as for example
described in W020131029690. The methyl mercaptan may also be used for the synthesis
of dialkyl disulphides, and particularly of dimethyl disulphide (DMDS), a disulphide which is
widely used nowadays as a sulphurizing additive for catalysts, especially catalysts for
hydrotreating petroleum fractions, or else as a soil fumigation agent in agriculture.
[0077] Methyl mercaptan, indeed, is readily convertible to dimethyl disulphide under the
action of sulphur, as for example described in EP0976726, and in accordance with the
reaction scheme (10) below:
2 CH3SH + S + CHsSSCH3 + HzS (10)
[0078] The reaction of methyl mercaptan with sulphur to form dimethyl disulphide is very
advantageous, not least economically, owing to the simultaneous formation of hydrogen
sulphide, which can quite advantageously be recycled into step a) of the process of the
invention for synthesis of methyl mercaptan.
[0079] Accordingly, and in accordance with a further aspect, the present invention relates
to the reaction of the methyl mercaptan prepared as described above with sulphur to form
dimethyl disulphide.
[0080] More specifically, and according to a further subject, the present invention relates
to the process for preparing dimethyl disulphide that comprises at least the following steps:
a) reaction of at least one hydrocarbon charge in the presence of hydrogen
sulphide (HzS) and optionally of sulphur (S) to form carbon disulphide (CS2) and hydrogen
(Hz),
b) hydrogenation reaction of said carbon disulphide (CSz) in the presence of
said hydrogen (HZ), both obtained in step a), to form methyl mercaptan (CH3SH), hydrogen
sulphide (H2S) and optionally hydrogen (HZ),
C) optionally, but preferably, recycling said hydrogen sulphide (H2S) formed
in step b) into step a),
' e) reaction of the methyl mercaptan formed in. step c) with sulphur to form
dimethyl disulphide and hydrogen sulphide,
9 optionally recycling into step a) of the hydrogen sulphide formed in step
e), and
9) recovery of the dimethyl disulphide.
[0081] According to one preferred embodiment, the hydrogen sulphide from step c) is
recycled into step a). According to another preferred embodiment, the hydrogen sulphide in
step f) is recycled into step a). According to yet another preferred embodiment, the hydrogen
sulphide from step c) and the hydrogen sulphide from step f) are recycled into step a).
[0082] Thus the process for synthesis of dimethyl disulphide according to the invention
offers the advantages listed above for the synthesis of methyl mercaptan, that is to say not
requiring the use of methanol, resulting in lower production costs.
[0083] Schematically, the process for synthesizing dimethyl disulphide according to the
present invention may be represented by the following reactions:
2 CH4 + S + 3 H2S + 2 CS2 + 7 H2 (8)
~ C S ~ + ~ H Z + ~ C H B H + ~ H Z S + H ~ ( ~ )
2 CHSH + S + CH3SSCH3 + HzS (10)
[0084] It should be noted that the hydrogen sulphide formed in each of reactions (9) and
(10) (2 moles and 1 mole respectively) corresponds exactly to the requirements of reaction
(8). Therefore, and advantageously, and as in the case of the process for preparing methyl
mercaptan as described above, the entirety of the hydrogen sulphide formed may be
recycled for use as a stariing material in step a) of the process for preparing dimethyl
disulphide.
[0085] Moreover, and again as in the case of the process for preparing methyl mercaptan
as described above, the reaction (9) co-produces hydrogen, which can be used for reaction
with sulphur in order to prepare hydrogen sulphide or else in order to provide thermal energy
by combustion, this thermal energy being able to be used advantageously for the
requirements of the process, particularly in reaction (8), which requires high temperatures
(between about 900°C and about 1100°C) for industrially acceptable performance levels.
[0086] The present invention is now illustrated using examples below, which do not have
any limitative character and which therefore cannot be understood as being able to restrict
the scope of the invention as it is claimed.
EXAMPLES
[0087] For each of the examples, the reaction products and the products which have not
reacted are vaporized and analysed by gas chromatography with a capillary column
equipped with a detector (microGC, screenlPPU column in series with a PoraPLOT column
from Agilent Technologies, pTCD detector).
[0088] In the examples below, the degrees of conversion and selectivity are determined
as follows:
Degree of molar conversion of CH4 (%CCH~):
%CCH=~ [ ( ~ o c-H nc~H 4 residual) 1 ~ O C*H 10~0]
where nacH4 is the initial number of moles of CH4 and nc~4re sid,e~i s the number of moles of
unreacted CH4.
Degree of molar conversion of CS2 (%C 4:
%CCS=~ [ (~OC-S nZ ~res~idua2l) I ~ O C* SI0~0]
where nocs2 is the initial number of moles of CS2 and ncs2 is the number of moles of
unreacted CS2.
0 Molar selectivity for CH3SH (%SCH3SH):
%SCHJS=H [ ~CHSIS( H~ O C- nS c~s2 residual)] * 100
where nc~~sist tth e number of.moles of CHJSH produced during the process according to
the invention.
. Molar selectivity for CS2:
%S C S=~ [ n cs2 I( ~ O C- nHc~~ 4res idue^)] * 100
where ncs2 is the number of moles of CS2 produced in the process of the invention
Example 1 :
[0089] An l n c o l o ~80 0 HT reactor containing 12 grams of catalyst containing 0.5% by
weight of platinum on alumina, sold by STREM is placed in an oven. The catalyst is
intercalated between two layers of carborundum.
[0030] The reactor is supplied with 20 NL.h-I (or 893 mmo1.h-I) of hydrogen sulphide (H2S)
and 10 NL.h-' (or 446 mmo1.h-I) of methane (CH3. These two gases are preheated
independently to 500°C before entering the reactor. The reactor is brought to a temperature
of 900°C by means of the oven, and the pressure at the outlet of the reactor is regulated at
3 bar absolute. The flow rate of the exiting gases, taken under the standard conditions of
temperature and pressure, in other words 0°C and 1 atmosphere (101325 Pa), is
37.5 NL.h-'.
[0091] Gas-chromatographic analysis of the exiting gases indicates the presence of four
gases: unconverted CH4 and HzS, and also CS2 and Hz, which have been produced with a
molar H21CS2 ratio of 4. Under these conditions, the molar conversion of CH4 is 32%, with
a selectivity for CS2 of 100%.
100921 These exiting gases, after cooling to a regulated temperature of 250°C, are
introduced into a second reactor, containing 50 inL of NiMoIalumina catalyst (HR448, sold
by Axens), doped with 11.6% of KzO (according to the "Cata 3" preparation described in
patent application W020101046607). The pressure is 3 bar (0.3 MPa) absolute in the oven,
at 250°C. Gas-chromatographic analysis of the exiting gases shows that the CS2 has been
completely converted (100%) with a selectivity of 100% for methyl mercaptan, in other
words each molecule of carbon disulphide has been converted to methyl mercaptan in
accordance with reaction (4). The reaction mixture also comprises hydrogen sulphide,
hydrogen, and the unreacted methane. The entirety of these compounds may be recycled
into step a).
Exam~le2:
[0093] Example 1 was repeated, this time adding 5.7 g.h-' of sulphur (or 178 mmo1.h-l) to
the 10 NL.h-I of methane (or 446 mmo1.h-') and with a reduction in the 20 NL.h-I of HzS to
10 NL.h-I (446 mmo1.h-'). The sulphur is introduced in liquid form at 130°C with the other
reactants, at the top of the reactor, the internal reactor temperature being maintained at
900°C and the internal pressure at 3 bar (3x105 Pa) absolute. The flow rate of the exiting
gases, taken under standard conditions of temperature and pressure, is 28 NL.h-I.
[0094] Gas-chromatographic analysis of the exiting gases indicates the following molar
composition: CH4: 21% (or 262 mmo1.h-I), HzS: 22% (or 275 mmo1.h-I), CSZ: 14% (or
175 mmo1.h-I) and HZ: 43% (or 537 mmo1.h-l).
[0095] The mass balance realized with these analyses indicates that sulphur has been
converted to loo%, that methane has been converted to 39% to carbon disulphide (CSZ),
and that CSZ and hydrogen (HZ) have been produced with a molar HzICSZ ratio of 3.07.
[0096] In the same way as in example 1, the exiting gases, afler cooling to a regulated
temperature of 250°C, are introduced into the second reactor, which contains 50 mL of
NiMoIalumina catalyst (HR448 from Axens) doped with 11.6% of KzO. The pressure is 3
bar absolute.
[0097] Gas-chromatographic analysis of the exiting gases indicates that CS2 has been
converted to 100% with a 100% selectivity for methyl mercaptan (or 175 mmo1.h-I).
Moreover, the amount of HzS recovered at the end of this second step corresponds, within
the margins of measurement error, to the amount required for the first step (or approximately
450 mmol.h-I). The process according to the invention is an autonomous system which
advantageously allows the recycling of the residual compounds to step I), for example H2S.
CS2 and hydrogen were unquantifiable.
[0098] This example shows that it is entirely possible to envisage a process for synthesis
of methyl mercaptan in vdhich the entirety of the HzS produced could be recycled, and there
would be no need for it to be synthesized for the purposes of said process for synthesis of
methyl mercaptan.
[0099] The examples below further illustrate the process of the present invention as
indicated in example 1 above, but with the first step reproduced with different catalysts.
Example 3:
[OOIOO] The catalyst of the first step from example 1 was replaced by 30 mL of a catalyst
containing 2% by weight of palladium on alumina (Engelhard). The reaction was
subsequently carried out at 700°C, 800°C and 900°C. The results are collated in Table 1.
Example 4:
[OOIOI] The catalyst from the first step in example 1 was replaced by 60 cm of platinum
wire with a diameter of 0.4 mm. The reaction was subsequently carried out at 900°C. The
results are collated in Table 1.
Example 5:
[00102] The catalyst from the first step in example 1 was replaced by 20 superposed sheets
(thickness of one sheet = 0.152 mm, volume of 20 sheets = 0.611 mL) made of platinum
and rhodium and sold by Umicore. The reaction was subsequently carried out at 90O0C,
1000°C and 1100°C. The results are collated in Table 1.
Example 6:
[00103] The catalyst in the first step in example 1 is replaced by 30 mL of catalyst
containing 19% by weight of chromium oxide (Cr203) on alumina (T2777, sold by Siid-
Chemie). The catalyst underwent a prior sulphurizing treatment with a stream of H2S
(20 NL.h-I) for four hours at 900°C, so as to convert the Cr203 into C~ZSaSnd to prevent the
formation of oxygenous products during the main reaction of the methane with the H2S.
These oxygenous products might interfere in the steps of subsequent recovely of the methyl
mercaptan. The reaction was subsequently carried out at 900°C. The results are collated in
Table 1 below:
-- Table 1 --
-18-
CLAIMS
1. Process for preparing methyl mercaptan, batchwise or continuously, preferably
continuously, said process comprising at least the following steps:
a) reaction of at least one hydrocarbon charge in the presence of hydrogen sulphide (HzS)
and optionally of sulphur (S) to form carbon disulphide (CS*) and hydrogen (HZ),
b) hydrogenation reaction of said carbon disulphide (CS2) in the presence of said hydrogen
(HZ), both obtained in step a), to form methyl mercaptan (CHsSH), hydrogen sulphide
(HzS) and optionally hydrogen (HZ),
c) optionally, recycling said hydrogen sulphide (HzS) formed in step b) to step a), and
d) recovery of the methyl mercaptan.
2. Process according to Claim I, wherein the hydrocarbon charge is a hydrocarbon
charge in gaseous, liquid or solid form, preferably in gaseous or liquid form, more preferably
in gaseous form, and comprises at least one hydrocarbon having a hydrocarbon chain in
saturated or unsaturated linear, branched or cyclic form.
3. Process according to Claim 1 or Claim 2, wherein the hydrocarbon charge
comprises at least one alkane, preferably at least methane (CH4), ethane, propane or
butane.
4. Process according to any one of the preceding claims, wherein the hydrocarbon
charge is methane.
5. Process according to any one of the preceding claims, wherein the hydrogen
sulphide formed in step b) is recycled into step a).
6. Process according to any one of the preceding claims, wherein the hydrogen
optionally formed in step b) may be reacted with sulphur to form hydrogen sulphide.
7. Process according to any one of the preceding claims, wherein the molar
HzS/hydrocarbon charge ratio is between 0 and about 100, preferably between 0.5 and 10
and more preferably between 1 and 3, endpoints included.
8. Process according to any one of the preceding claims, wherein the reaction
temperature in step a) is advantageously between 500°C and 1300°C, preferably between
700°C and 1 10O0C, more preferably between 800°C and 1000°C.
9. Process according to any one of the preceding claims, wherein the reaction
temperature in step b) is between 100°C and 400°C and preferably between 200°C and
300°C.
10. Process according to any 'one of the preceding claims, wherein the methyl
mercaptan is reacted with sulphur to form dimethyl disulphide.