Produzione del metanolo
P.J.A. Tijm et al. / Applied Catalysis A: General 221 (2001) 275–282 277
Both of the above reactions are exothermic and result However, over the time, catalysts may be poisoned
in a reduction in volume. The conversion reaction is, by sulfur, chlorine compounds, metal carbonyls or
therefore, favored by low temperatures and high pres- other compounds. More commonly, however, they
sures. Today’s synthesis processes take place at low are deactivated by thermal sintering (copper site clus-
pressures, some even close to the pressures at which tering) or carbon deposition. Converter designs take
the steam reforming production of synthesis gas op- this into account, being based on estimated catalyst
erates. These processes use far less energy than high activity. After an initial period of operation of cata-
pressure ones as the synthesis gas compression is a lyst, the converter should be able operate at its name-
costly operation. The reactions are promoted by the plate capacity even at these so-called “end-of-run”
use of catalysts, particularly the shift reaction, which conditions.
allows coal based synthesis gas to be used effectively. Developments to improve methanol synthesis are
Although the equilibrium conditions favor low tem- therefore composed of elements of catalyst system im-
peratures, methanol converters must be operated at provements and reactor improvements. In the follow-
temperatures in the range 200–300 C to ensure the ing sections, both elements will be addressed.
catalysts are active and to use the heat of reaction
effectively. As the synthesis reactions are strongly 2. Current catalysts [1,2]
exothermic, heat removal is an important part of the
As the conversion favors high pressures, low pres- In the last decade, Synetix has developed catalyst
sure processes tend to result in only a low fraction 51-7. Sintering is limited by the presence of MgO.
of the synthesis gas being converted in each pass Synetix claims that the catalyst has a 30% higher cop-
(typically some 10%). Therefore, the processes use a per surface area than other catalysts. Its research has
recycle loop to achieve adequate yields, with a purge also indicated that using the Synetix 51-7 and 51-3
gas to remove impurities that would otherwise build catalysts prevents a high proportion of CO in the syn-
up over time. The amount of purge depends on the thesis gas having a permanent effect on the catalyst
stoichiometric ratio of the reactants in the synthesis performance. Research findings have suggested that
gas. For example, when the gas is too rich in carbon carbon dioxide-rich conditions may cause irreversible
oxides, it may be necessary to remove the excess damage to other catalysts. In 1997, Synetix acquired
through absorption or adsorption in the form of CO the marketing and manufacturing right of BASF’s S
2 3-86 methanol catalyst.
If the gas is too rich in hydrogen, rejection via water Süd Chemie (in the US, Union Catalyst Inc.) recent
is required. In other schemes, though, CO injection is
2 catalyst development has taken a different course. It
contemplated. The role of CO in the reaction mech-
2 has stated that the damaging effect high CO levels
anism has been and still is a subject of discussion 2
appear to have on methanol catalysts is, in fact, a re-
between many scientists. Its contribution in reaction sult of the water formed by the synthesis reaction. The
models is certainly not well reflected. company has also argued that the increasing diversity
The current catalysts used in low-pressure methanol of converter types, feed-stock and operating condi-
synthesis are composed of copper oxide and zinc ox- tions found in methanol production has created the
ide on a carrier of aluminum oxide. The ratios of the need for tailor-made catalysts. With this in mind Süd
components vary from one manufacturer to another. Chemie has developed two new methanol catalysts to
As a rule, the proportion of CuO ranges between compliment its traditional C79-4 GL catalyst, for use
40 and 80%, that of ZnO between 10 and 30% and with different types of methanol plants. The two new
Al O from 5 to 10%. Additives such as MgO may
2 3 catalysts have a higher tolerance of carbon oxides
also be present. Such catalysts are manufactured by (CO and CO ) and are designed to show an optimum
Synetix (formally ICI Katalco), Süd Chemie (in the 2
balance between activity, selectivity and lifetimes, un-
US sold by United Catalyst Inc.), Haldor Topsoe, and der a range of different industrial conditions, as plant
Mitsubishi Gas Chemical. sizes increase. C79-4 GL shows the best selectivity
A good catalyst should remain active for several for isothermal reactors using synthesis gas obtained
(up to 4) years, so as to sustain high plant output.
278 P.J.A. Tijm et al. / Applied Catalysis A: General 221 (2001) 275–282
from partial oxidation of oil fractions or coal. The new 3.1. Converter developments
catalysts, C79-5 GL and C79-6 GL, have a differ- The most important section of the methanol synthe-
ent matrix structure, providing a more stable copper sis process is the reactor or converter. As the methanol
crystalline distribution. C79-5 GL has a long lifetime, synthesis reaction is exothermic, the primary task of
claimed to be up to 4 years, and is particularly suited all the reactors is to control the reaction temperature.
for operation in isothermal and adiabatic converters The reactor technologies that have been used exten-
that use synthesis gas obtained by steam reforming. sively in commercial settings fall into two categories.
C79-6 GL is designed specifically for use of synthesis
gas containing high levels of olefins (e.g. acetylene 3.1.1. Gas phase technologies
off-gases). The different options and technology developments
Haldor Topsoe produces a multi-purpose cata- are discussed as follows:
lyst, MK-101, that has gained industrial experience •
with most types of converters using synthesis gases Multiple catalyst bed reactors
obtained from variety different feed-stock and re- This reactor option controls the reaction temper-
forming technologies. Topsoe reports that the catalyst ature by separating the catalyst mass into several
has recently been used in an ammonia–methanol sections with cooling devices placed between the
co-production unit. sections. Bed sizes are generally designed to allow
the methanol reaction to reach equilibrium. The
cooling devices can work either by direct heat ex-
3. New catalyst developments change or by the injection of cool synthesis gas,
to limit the adiabatic temperature rise of the very
Most recently, Mukerjee, Sassinopoulos and exothermic reaction in each section.
Caradonna, announced their finding of a class of Single bed converters
bi-nuclear non-heme iron catalysts, like, e.g. iodosyl- In single bed designs, heat is continuously re-
benzene as a catalyst to convert methane directly into moved from the reactor by transfer to a heat re-
oxygenated hydrocarbons. However, it seems to be an moving medium. The reactor is run effectively as
impractical oxidative catalyst for any product volume a heat exchanger.
of importance. The key to their invention would be to The following paragraphs look at recent two phase
work in H O and with O .
2 2 (gas–solids) reactor or the so called “gas phase” reac-
In Japan, Maruyama of the Project Center for CO
2 tor technologies offered by a range of manufacturers.
Fixation and Utilization of the Research Institute of They are followed by new, innovative three phase
Innovative Technology for the Earth, reported the de- (gas–solids–liquid) or “liquid phase” technologies.
velopment of a Cu/ZnO type catalyst for methanol These liquid phase technologies are contributing to
synthesis with CO and hydrogen. A pellet type cat-
2 cost reduction in the methanol industry through the
alyst was reported to be under test-run operation in a simplicity of their converter design. Their potential
thirty-one litre reactor. may well be a driving force behind the methanol in-
Methanol conversion from carbon monoxide and dustry in the new millennium. In parallel, one also
hydrogen of almost 50% per pass, compared to about finds a similar trend in the Fischer–Tropsch develop-
10% for the conventional method, has been claimed ments where a shift from the fixed bed reactor to the
by researchers at the Central Research Institute of the liquid phase reactor has taken place.
Electric Power Institute (Tokyo).The improvement has
been made by means of a new catalyst, composed 184.108.40.206. Multiple bed converters.
of alkali metal alkoxide, chromium and copper ox-
◦ 220.127.116.11.1. Haldor Topsoe collect, mix, distribute
ides. The synthesis is done at 200–300 C and less converter. Haldor Topsoe’s CMD designs are aimed
than 50 bar. The catalyst is made by grinding the mix- at revamping conventional quench converters. They
ture in a ball mill to create an amorphous structure employ catalyst beds separated by support beams.
and very fine particles, which allow for the higher Gas leaving an upstream catalyst bed is collected and
yield. P.J.A. Tijm et al. / Applied Catalysis A: General 221 (2001) 275–282 279
are embedded in the catalyst bed. It resembles a lique-
mixed with the quench gas. This mixed gas stream fied natural gas heat exchangers with catalyst around
is then evenly spread over the downstream catalyst the tubes. This arrangement allows for some 50% more
bed. This has the effect of increasing the conversion catalyst loading per unit of reactor volume. The tubes
per pass rate of the synthesis reaction and allows the are wound in a multi-layer arrangement over spacers
reaction temperature to be lowered, extending the running the length of the unit. Boiling water circulates
catalysts life. Haldor Topsoe now has installed CMD through the tubes by natural draft to reach an integral
revamps in seven quench converters. steam drum at the top of the reactor. The heat transfer
18.104.22.168.2. Kellogg, Brown and Root’s (now Halli- on the catalyst side for a Linde isothermal reactor is
burton) adiabatic reactors in series. Kellogg, Brown significantly higher than designs with the catalyst in-
and Root offers technologies featuring more than one side the tubes. This results in a much smaller cooling
adiabatic, fixed-bed reactor in series. Each catalyst area being required, saving on material costs. Linde
layer is accommodated in a separate reactor vessel claims that it is possible that their reactor can be man-
with intercoolers located between each of the reactors. ufactured at such a scale as to produce a single-train
All of the make-up gas can be fed directly into the first capacity of 4000 t per day.
reactor. This increases the kinetic driving force for the
reaction and, as a result, the catalyst volume is signif- 22.214.171.124.2. Lurgi combination converter system.
icantly less than would be required by a quench-type The Lurgi (now metal gesellschaft) methanol reac-
reactor with the same output. The reactors themselves tor is a tube-based converter that contains its cata-
have a spherical geometry that allows the thickness lysts in fixed tubes. The reaction control temperature
of the pressure shell to be reduced, giving savings in is controlled by steam pressure control. The reac-
materials and structural costs. The distribution design tor is able to achieve high yields and low recycle
is simple and offers benefit in constructibility as it ratios.
lacks complicated internals for heat transfer or flow For high methanol capacities, Lurgi has devel-
distribution. (Multi-vessel adiabatic rector systems for oped a two-stage converter system that uses two
methanol production are also offered by Haldor Top- Lurgi methanol reactors in combination (the Lurgi
soe and Krupp Uhde.) combi-forming approach). The first converter can op-
erate at higher space velocities and temperatures than
126.96.36.199.3. Toyo Engineering Corporation’s a single-stage converter as it needs to achieve only
reactor. The reactor from Toyo partial conversion of synthesis gas to methanol. This
Engineering Corporation (TEC) is a multi-stage radial enables the converter to be smaller and the high tem-
flow reactor with intermediate cooling using bayonet peratures allow high-pressure steam to be produced,
boiler tubes. The indirect cooling system allows the saving energy costs. The exit gas containing methanol
temperature to be kept very close to the path of the is directed to a second reaction stage that operates at
maximum reaction rate curve, achieving maximum a lower reaction rate.
conversion per pass and reducing the catalyst require- The reaction temperature is reduced across the
ment by 30% compared to quench converters of the whole of the catalyst bed to maintain the equilibrium
same size. The design was first tested on driving force for the reaction. The remaining reaction
a large scale in 1993 and TEC is currently construct- heat is used to heat feed gas for the first converter. For
ing advanced versions of the reactor at a plant capacities up to 3000 t per day, the two stages
methanol plant in China to convert acetylene off gas can be accommodated in a single vessel. Larger ca-
into methanol. The company believes that the reactor pacities are better accommodated by two separate
can be developed to achieve single-train capacities of converters.
5000 t per day. 188.8.131.52.3. Mitsubishi Gas Chemical/Mitsubishi
184.108.40.206. Single bed reactors. Heavy Industry superconverter. The superconverter
has been jointly developed by Mitsubishi Gas Chem-
220.127.116.11.1. Linde isothermal reactor. The Linde ical (MGC) and Mitsubishi Heavy Industry (MHI). It
isothermal reactor is unique, as helically-coiled tubes
+1 anno fa
I contenuti di questa pagina costituiscono rielaborazioni personali del Publisher Atreyu di informazioni apprese con la frequenza delle lezioni di Chimica industriale II e studio autonomo di eventuali libri di riferimento in preparazione dell'esame finale o della tesi. Non devono intendersi come materiale ufficiale dell'università Parma - Unipr o del prof Moggi Pietro.
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