methanol process - kongrespolskachemia.pl

methanol processm3000tm - m7000tm - m2000ctm

optimized processesfor any plant size and feedstock

methanol process m3000tm - m7000tm - m2000ctmcasale

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M3000™ and M7000™ are Casale's processes for natural gas-based methanol plants.M3000™ is used for plants having a capacity ranging from 1,000 to 3,000 MTD, and is optimized in order to reduce the investment cost and the energy consumption.M7000™ is used for plants having a capacity ranging from 3,000 to 7,000 MTD, and is optimized to reach very large capacities in the most efficient possible way.M2000C™ is Casale's process for coal-based methanol plants with capacity ranging from 1,000 to 2,500 MTD. The main difference between the three processes is the way the synthesis gas is generated.

key featUres of casale methanol processes

• Best design for any feedstock• Real single-line very large plants (more than 10’000 MTD with a single reactor)• Low steam to carbon ratio • Low energy consumption, less than 7.0 Gcal/MT for M3000TM and less than 6.7 Gkal/MT for M7000TM

• Optimized plant cost

synthesis gas generation

The capacity limit on a single steam reforming furnace is about 3,000 t/d of methanol. Duplicating reforming furnaces is an expensive option, particularly as they would still be producing a non-ideal synthesis gas.

Casale has therefore adopted a combination of primary and autothermal reforming, which optimizes the combination of energy consumption, stoichiometric number and plant cost.

A further feature, which reduces the duty on both the primary reformer and the ATR, is a pre-reformer, through which all of the natural gas feed is routed after desulfurization. The pre-reformed feed is then split into two fractions; one is sent to the primary reformer and then on to the ATR, the other directly to the ATR.

This process can produce synthesis gas that is correctly balanced for methanol synthesis, but a hydrogen recovery unit (HRU) is still provided, allowing the size of the primary reformer to be reduced further. Thus a single reforming furnace of manageable proportions can serve a 7,000-t/d plant.

Clearly, because the ATR is only handling part of the reforming duty, its oxygen requirement is smaller than that of a purely ATR installation, so the ASU is proportionately smaller, too.

m7000tm process oUtline

AIR oxygen

fuel

CH4

STeAM RefoRMeR

ATR

ASu

HeAT ReCoVeRy & CoolIng

CondenSATIon &

SepARATIon

HRu

SyngAS CoMpReSSoR

IMCConVeRTeR

3 ColuMnS dISTIllATIon

MeTHAnol

STeAM

o2

ngpRe

RefoRMeRpRIMARy

RefoRMeRATR

SynTeSISgAS

Main steps of M7000TM process

Combined reforming syngas generation

methanol process


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distillation sectionTo reach AA grade methanol, the final purification is performed by a two-step distillation: the topping column separates the lighter compoun-ds, while the refining section provides the final separation of water and higher ends from the pure product.

The refining section comprises two separate columns, one under pressure, the other atmospheric, which is the arrangement that consumes least distillation energy.

puRge gAS

H2

M.u.gAS

MpSTeAM

CIRCulAToR

SepARAToRMeoH

CondenSeR

CASAleIMC

ConVeRTeR

HydRogenReCoVeRy

unIT

gAS-gASexCHAngeR

SyngASCoMpReSSoR

methanol synthesis loopThe Casale synthesis loop is very simple. It features a single synthesis converter, a gas-gas exchanger to preheatthe converter feed and cool the converter effluent, a methanol condenser, a liquid methanol product separator,a purge gas HRU, a syngas compressor and a circulating compressor. The syngas and circulating compressor can be supplied as separate machines, or as different barrels on the same shaft, depending on the plant size and specific client requirements. The heart of the synthesis loop is the Casale IMC (Isothermal Methanol Converter), in which the heat of reaction is removed by means of a set of heat exchanging plates embedded in the catalyst, either preheating the incoming gas or boiler feed water or raising steam. The internal cooling system of the IMC converter is more efficient and more maintenance-friendly than in other designs such as, for instance, the old tube-cooled converters, even in very large units.Because of the high per-pass conversion rates attained in the Casale IMC synthesis converter and synthesis loop, the rate of gas recycle is proportionately lower, enabling equipment sizes in the loop to be reduced.

m3000tm process oUtlinesynthesis gas generation

In this case the generation of synthesis gas is obtained with a steam reformer only, thanks to the lower production rate requi-red. As in the M7000™ process, the desulfurized natural gas, mixed with steam, is first preheated in a coil in the convection section of the primary reformer and then passed through a pre-reformer. The pre-reformed gas is further pre-heated in the convection section before entering the reformer tubes. To minimize the plant’s overall gas consumption the steam-to-carbon ratio is set at 2.5-3.0.

The hot gas leaving the reformer is then routed to a train of exchangers to recover its heat value, while the process condensate is separated and sent to a steam stripper.

Synthesis gas generated in the M3000™ process has a large H2 excess with respect to methanol synthesis. If CO2 is availa-ble, then the synthesis gas composition can be adjusted as required.

Thanks to the optimized reforming conditions, the use of a pre-reformer to lower the amount of steam in the process gas, and the highly efficient IMC converter in the synthesis loop, a smaller primary reformer can be used than in rival processes. It can be built as a single unit for plant capacities of about 3,000 t/d.

Main steps of M3000™ process

fuel

CH4

STeAM RefoRMeR

ATRHeAT

ReCoVeRy & CoolIng

CondenSATIon &

SepARATIon

SyngAS CoMpReSSoR

IMCConVeRTeR


MeTHAnol

RAw MeoH

off gAS

STeAM boIleR

RefIned MeHo

ToppIng SeCTIon

RefInIng

SeCTIon

lp ColuMnHp ColuMn

boTToM wATeR

fuel oIl

pRoCeSS gAS

pRoCeSS gAS


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distillation section

The methanol distillation section is similar to that of the M7000™. The three-column arrangement is recommended when energy cost is high whilst, for smaller plants or low energy cost locations, a two-column distillation can be proposed.

raw methanol synthesisThe methanol synthesis is fundamentally similar to that of the M7000™ process, except there is no need for an HRU since the gas is already rich in hydrogen.

The methanol synthesis loop is designed once again around the Casale IMC plate-cooled synthesis converter. In spite of the excess of hydrogen in the synthesis gas, with the Casale IMC technology it is never necessary to have more than one converter vessel, which keeps the synthesis loop arrangement simple with the minimum number of pieces of equipment.

Because of the high conversion per pass in the IMC, it is possible to operate at a relatively low loop pressure (80 bar), reducing the duty on the synthesis gas compressor.

Another outstanding advantage of the IMC is the very low pressure drop (less than 1 bar), which is attributable to the axial-radial flow configuration of the catalyst bed. With the minimal number of pieces of equipment in the synthesis loop, this makes the overall loop pressure drop very low, so the size, cost and power consumption of the synthesis gas compressor are minimized.

Finally, all the reaction heat is recovered in the IMC converter as steam at the correct pressure for direct addition to the natural gas feed.

synthesis gas generation

The process for methanol synthesis from coal gasification is quite different from the natural gas route, as shown in the block flow diagram below.

In coal-based methanol plants the gasifier is third-party technology. Casale is involved as process licensor in the sections highlighted in red: CO shift, synthesis loop and distillation. Casale also supplies proprietary equipment for the sour-feed CO shift converters and the IMC methanol synthesis section.

m2000ctm process oUtline

synthesis gas work-UpThe raw syngas generated in a coal gasification based plant contains much carbon monoxide, a portion of which has to be converted to hydrogen to yield a more balanced gas for methanol synthesis.

One or two shift converters are usually installed for this purpose, downstream of the coal gasifier. These are designed to operate on the raw synthesis gas before it has been subjected to any treatment to remove its sulfur content and surplus carbon dioxide. In compensation for the excess of carbon and the high CO content, synthesis gas from coal gasification has a very low concentration of inerts.

In addition to these, the make-up gas typically contains trace elements, such as sulfur and arsenic. These compounds are normally washed away in the gas treatment upstream the synthesis loop (Rectisol), but there may be problems due to upset, maloperation or underperformance of some treatment units that may leave undesirable levels of impurities in the gas.

As these components, especially sulfur, poison methanol synthesis catalyst irreversibly, Casale provides a guard reactor, contain-ing a hydrolysis catalyst and a zinc oxide bed, either at the suction or at the discharge of the synthesis gas compressor (before the circulator). It can work in a temperature range from ambient up to 100°C.

raw methanol synthesisThe synthesis loop for the M2000C™ plant is fundamentally the same as for a natural gas-based plant, but with additional pieces of equipment and special features of process design such as the guard bed on the make-up gas line, as described above.

Secondly, the gas from coal gasification has a higher reactivity than synthesis gas from natural gas reforming, as the table clearly shows. This is something of a mixed blessing.

The fact that the make-up gas normally has a low inert content and is rich in carbon makes it possible to achieve high production rates with low recycle ratios and low catalyst volumes, provided that the converter design is appropriate. The catch is that, as the gas is very reactive, it can easily cause overheating of the catalyst and hot spots in the converter. That is obviously much less likely to happen with Casale’s IMC than with a reactor with adiabatic beds.

On account of the low recycle ratio, the loop equipment items are also small in relation to the methanol output, enabling very large capacities in a single train, much larger than ever achieved so far in any plant of any type, but only if the converter is of the right design.

distillation sectionThe methanol distillation section is similar to that of M3000™ or M7000™, and the selection of two or three columns again depends on the cost of energy.

syngas generation process - steam reforming combined reforming coal gasification

Plant feedstock - NG NG COAL

Specific production MTD/m3cat ~ 15 ~ 20 ~ 30

Converter outlet pressure bar g 80-100 80-100 70-90

CH3OH at converter outlet % mol 4 - 8 8 - 12 12 - 17

MUG H2/COx mol/mol ~ 3.3 ~ 2.4 ~ 2

Circulation/MUG mol/mol 4.5-7.5 3.0-4.0 2.5-3.5o2

CoAl gASIfIeR

ASu

Co-SHIfT

Co2 & H2S

AgR Synloop


MeTHAnol


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distinctiVe featUresof m3000tm and m7000tm

Casale axial radial catalyst bed

atr - aUtothermal reformer technology Casale has its own design of ATR, successfully in operation in several plants. The combustion chamber design of the Casale ATR unit is conceptually very simple. Oxygen is introduced axially at the top of the cylindrical combustion chamber. The process gas is introduced from one side at the top of the cylindrical chamber, before the burner tip. The high velocity of the oxygen increases mixing between the two and reduces the flame length because the combustion reaction takes place instantaneously after mixing. The short flame does not impinge on the catalyst surface, which would destroy its top layer, impairing performance and raising pressure drop.

The fluid-dynamic field inside the combustion chamber is de-signed to protect the refractory lining from the high-temperature core of the flame, preventing hot spots on the lining surface. The burner is provided with a water-cooling system.

This design, which is in service in methanol and ammonia plants, has the following features:

• high reliability and long durability, with no deterioration after several years of operation;• high conversion to synthesis gas, thanks to uniformity of temperature and composition;• longer lasting performance of the catalyst, thanks to the short flame length avoiding impingement on catalyst surface and consequent

damage;• total absence of soot formation, as evidenced by analysis and inspections carried out on ATR catalyst and downstream

equipment;• wide flexibility: it has been successfully operated at temperature conditions, composition and flow rates far from design.

pre-reforming reactor technologyCasale's pre-reforming reactor utilizes Casale’s extremely well-proven axial-radial flow catalyst bed technology.

Casale axial-radial technology has been extensively proved in commercial ammonia and methanol plants, having been used in more than 500 catalyst beds.

In an axial-radial catalyst bed, most of the gas passes through the catalyst bed in the radial direction, resulting in very low pressure drop. The balance passes down through a top layer of catalyst in an axial direction, thus eliminating the need for a top cover on the catalyst bed.

This layout overcomes the main drawbacks of axial-flow designs, namely the limitation in the maximum capacity that can be reached in a single converter on account of high pressure drop across the bed. It also addresses the main disadvantage of a urely radial-flow design, which is that the bed has to be overfilled with catalyst into ‘dead’ space at the top of the bed to ensure no short-circuiting over the top of the catalyst can take place as the catalyst mass settles in use.

The essential advantages of the axial-radial concept are:

• low pressure drop, stable with time and not constrained by the catalyst bed;• full utilization of installed catalyst volume;• possibility to use small-sized catalyst, more active and more resistant to poisons;• lower operating temperature of the vessel wall when the reaction is exothermic (e.g. shift, methanol and ammonia synthesis);• possibility to design slim vessels, with important capital cost savings, especially when high-grade construction materials are required,

and no size limitations.

Casale’s axial-radial design ensures a perfect gas distribution on the reacting side, controlled by perforated walls (the inlet and outlet collec-tors). Since the gas distribution is controlled by the walls and not by the catalyst bed, it is not affected by uneven loading, poor distribution or deterioration of the catalyst or by catalyst aging. At the same time, the gas pressure drop is low and stable throughout the life of the catalyst.

Another advantage that is specific to pre-reformers is that the small-sized catalyst has higher sulfur-resistance and higher activity: as a consequence, the catalyst volume is reduced for the same life, with consequent cost savings.

By converting the higher hydrocarbons contained in the natural gas to methane, the pre-reformer protects the plant against coke formation in the primary reformer feed preheat coil and catalyst tube inlet. That increases the plant reliability. It also allows the steam / carbon ratio to be reduced, which not only lowers energy consumption but also increases the attainable pre-heat temperature in the pre-reformed reformer feed, so reducing the radiant duty and cost of the primary reformer. Additionally, some steam reforming of the feed occurs in the pre-reformer, and that further reduces the load on the primary reformer.

imc - isothermal methanol conVerter synthesis technologyThe IMC is a quasi-isothermal converter in which the heat transfer surfaces are plates and the catalyst is loaded outside the cooling pla-tes. The Casale IMC design was first applied to methanol synthesis in 2002, and has achieved important successes in the last ten years.

The following are its main characteristics:• Perfect control of the temperature profile in the catalyst mass, ena-

bling operation of the converter according to the highest reaction rate temperature profile.

• The catalyst bed is continuous so it can easily be loaded from the top and unloaded from the bottom through drop-out pipes

• The converter can be designed to incorporate axial-radial flow ca-talyst bed configuration.

Thanks to the above features, it is possible to reach the maximum ef-ficiency thus minimizing the size and number of the loop equip-ment items and pipes, and optimizing the energy consumption and size of the front end.

There is no tube sheet, therefore there is no constraint on the conver-ter size, and the construction is light, consisting of a normal pressure vessel containing the catalyst bed and the plates.

The plates are fabricated by an automatic production process com-prising welding with a computer-controlled laser. With only minimal manual input, this results in consistently high quality.

The advantage of the IMC design is that much higher produc-tion rates are possible in a single converter, catalyst handling is easier, and because of the close temperature control in the catalyst mass, the operating life of the catalyst charge is prolonged.

For large methanol plants, the most suitable IMC converter configu-ration is the steam-raising axial-radial converter, as shown below in cutaway view. The radially-aligned heat exchanger plates are immer-sed in the catalyst bed and the gas flows inwards between them, imparting essentially all of its reaction heat to raise 25-35-bar steam. Since there is only a single pass, the pressure drop is very low, at about 1 bar. A pump circulates the water/steam mixture through the cooling plates.

The catalyst temperature is controlled by changing the saturated ste-am pressure according to the process requirements – for example, to compensate for declining catalyst activity.

Since the converter diameter is only marginally influenced by the cir-culation (for a given capacity and make-up condition), the loop recycle ratio can be adjusted to compensate for the declining activity of the catalyst between start-of-run (SOR) and end-of-run (EOR) and main-tain the desired rate of production. In an axial converter, on the other hand, the recycle rate is constrained by the vessel size, and produc-tion may suffer at EOR.

The IMC is also the converter of choice for the M3000™ process ei-ther in axial or axial radial configuration. Possibilities include using the IMC to raise steam or as a boiler feed water preheater, or as a conver-ter feed gas heater, or with other heat transfer fluids. A combination of different fluids is also possible.

In the axial gas-cooled IMC the plates are disposed radially, but the reacting gas flows downwards through the catalyst. In the example shown in the diagram, the cooling fluid is the fresh synthesis gas.


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axial-radial soUr co shift conVerterAs previously mentioned, in a coal-based methanol plant one or more CO shift stages are needed to adjust the balance of hydrogen and carbon oxides in the synthesis gas. The design of the CO shift section and converters has an important impact on the efficiency and reliability of a methanol plant.

Especially for large plants, low pressure drop in the shift section over the entire life of the catalyst is essential to minimize the power consumption of the downstream synthesis gas compressor. This is especially important when there are multiple shift con-verters arranged in series. Also, the shift converter design must be mechanically robust and able to withstand the challenging reaction environment. Once again, the axial-radial concept provides all the answers.

In Casale’s axial-radial sour CO-shift converter concept, the distribution of the gas over a wide area by the perforated walls ensures low pressure drop, which increases little as the catalyst ages or deteriorates and is also unaffected by caking of the top

layer of the catalyst resulting from carry-over of water droplets, which is not an uncommon occurrence. So, by delivering a higher suction pressure to the compressor throughout the life of the catalyst, the axial-radial sour shift technology achieves a higher production rate.

In conventional shift converters the pressure drop can build up with time to such an extent that the catalyst has to be changed before it has reached the end of its active life. Since the pressu-re drop in the axial-radial sour converter is virtually unaffected by the catalyst, premature change of the catalyst batch is avoided. Even in the case of the first sour shift converter, which is the one operating in the most severe conditions, the catalyst life can be four years or more.

On account of the low pressure drop, the Casale axial-radial sour shift converter can be designed with a much slimmer pressure vessel than is needed in an axial-flow design and, consequently, a thinner pressure vessel wall. That results in a significant capital cost saving, especially for the first shift converter.

That is not all. Owing to the high operating temperature, an axial sour shift reactor must be designed either with a stainless steel cartridge, and an annulus for flushing the pressure vessel, or with a refractory lined vessel. In the Casale axial-radial con-verter, on the other hand, the inward gas flow keeps hot gas at the center of the converter, away from the vessel wall, so that only the outlet nozzle is exposed to high temperatures, so the Casale sour shift converter is much easier, cheaper and robust in construction.

Casale axial-radial IMCPrinciple of Casale axial-radial IMC converter

Casale IMC cooling platesimc – ideal for coal-based plantsIn coal-based methanol plants, the amount of carbon mono-xide and carbon dioxide present in the synthesis gas is very significant. Normally the concentration of these gases is about 15 vol-% CO and 5 vol-% CO2 at the reactor inlet. At such concentrations the synthesis reaction is quite intense and an efficient heat sink is needed to control the temperature, not only to obtain the best equilibrium conversion but also to prevent damage to the catalyst.

Casale’s steam-raising IMC design is exactly what is required in these circumstances. The steam pressure of 25-30 bar is selected because its corresponding temperature level matches the operating temperature in the catalyst bed well, and the steam produced can be usefully utilized in the plant.

Steam production in coal-based methanol synthesis loops with the IMC usually ranges from 1.1 to 1.3 t/t CH

3OH, depending on the temperature of the boiler feed water.

distinctiVe featUresof m2000ctm

The Casale technologies applied in new coal-based methanol plants are the sour shift converter and, once again, the IMC synthesis technology.

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