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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

process.

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-

2

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 3.1.1.1. Multiple bed converters.

of alkali metal alkoxide, chromium and copper ox-

◦ 3.1.1.1.1. 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

3.1.1.1.2. 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- 3.1.1.2.2. 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

3.1.1.1.3. Toyo Engineering Corporation’s a single-stage converter as it needs to achieve only

MRF-Z® MRF-Z®

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

MRF-Z®

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

MRF-Z®

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. 3.1.1.2.3. Mitsubishi Gas Chemical/Mitsubishi

3.1.1.2. Single bed reactors. Heavy Industry superconverter. The superconverter

has been jointly developed by Mitsubishi Gas Chem-

3.1.1.2.1. Linde isothermal reactor. The Linde ical (MGC) and Mitsubishi Heavy Industry (MHI). It

isothermal reactor is unique, as helically-coiled tubes


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DETTAGLI
Corso di laurea: Corso di laurea in chimica industriale e tecnologie del packaging
SSD:
Università: Parma - Unipr
A.A.: 2007-2008

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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