It was as long ago as 1931 that thyssenkrupp Industrial Solutions first designed and constructed a methanol plant. This plant employed a high pressure methanol synthesis process, the syngas feed being generated from coal.
thyssenkrupp Industrial Solutions later constructed the first low-pressure (LP) methanol plant using the copper-based catalyst and providing coal as feedstock. The first modern methanol plant - using steam reforming and a low-pressure synthesis process (50 bar) - was designed and supervised by thyssenkrupp Industrial Solutions in Romania in 1972/1974. Even at that time, the plant had all the characteristic features of a modern methanol plant, i.e.:
- Steam reformer technology
- Waste heat recovery system generating 120-bar steam
- Low-pressure synthesis loop (50 bar)
- Efficient grade AA methanol distillation.
Further commercial-scale methanol plants were subsequently engineered for instance in Bahrain (1,250 mtpd) and Libya (2 x 1,250 mtpd).
Commercial experience
Methanol plant Schwarze Pumpe, Germany
The first methanol plant using steam reforming and a low-pressure synthesis process (50 bar) was designed and constructed by Uhde in Romania in the early seventies. Even at that time, this plant had all the characteristic features of a modern methanol plant.
Since then, eleven plants have been engineered and commissioned or revamped by thyssenkrupp Industrial Solutions world wide.
Process
The standard methanol plant concept consists of the following process steps:
feed purification, steam reforming, syngas compression, methanol synthesis and crude methanol distillation.
The feedstock (natural gas, for example) is desulphurised, mixed with steam and converted to synthesis gas in the reformer over nickel catalysts at 20 bar to 35 bar pressure and at temperatures of 800 °C to 950 °C. The Uhde steam reformer is a top-fired reformer with tubes made of centrifugally-cast high alloy steel and a proprietary "cold outlet manifold system" to enhance reliability.
The reformed gas at the reformer outlet is a mixture of hydrogen, carbon oxides and residual methane. It is cooled from approximately 880 °C to ambient temperature. Most of the heat from the synthesis gas is recovered by steam generation, BFW preheating, heating of the crude methanol distillation section and by demineralised water preheating.
Also, heat from the flue gas is recovered by feed/feed-steam preheating, steam generation and superheating as well as combustion air preheating. After final cooling, the synthesis gas is compressed to synthesis pressure, which ranges from 40-110 bar (depending on plant capacity) before entering the synthesis loop. The synthesis loop consists of a recycle compressor, feed/effluent exchanger, methanol reactor, final cooler and crude methanol separator.
Crude methanol, which is condensed downstream of the methanol reactor, is separated from unreacted gas in the separator and routed via an expansion drum to the crude methanol distillation. Water and minor quantities of by-products formed in the synthesis and contained in the crude methanol are removed by a distillation system.
Uhde offers isothermal and adiabatic reactors. The isothermal reactor is the most efficient system, as the heat of reaction is directly utilised at reaction temperature level to generate medium-pressure steam.
Uhde's isothermal reactor is a tubular reactor with a copper catalyst contained in vertical tubes and boiling water on the shell side. The methanol reaction heat is removed by partial evaporation of the boiler feed water, thus generating 1 metric ton of medium-pressure steam per 1.4 metric tons of methanol.
The advantages of this reactor type are: low by-product formation due to almost isothermal reaction conditions, high reaction heat recovery, and easy temperature control by regulating steam pressure.
To avoid the build-up of inert components in the loop, a purge is withdrawn from the recycle gas and used as fuel for the reformer.
The axial radial multi-bed adiabatic quench is a low cost reactor concept. It is normally used for plants which require no steam generation in synthesis units, due to the fact, that surplus steam is produced during syngas generation (for instance steam reforming).
Uhde has developed various concepts to match the energy requirements of the distillation section with energy available from the front end.
The conventional distillation unit consists of a topping and a refining section. The light ends present in the raw methanol are removed in the topping column. The stabilised raw methanol, consisting of methanol, water and minor amounts of higher alcohols, is fractionated in the refining section to produce grade AA methanol.
In this conventional two-column distillation unit, the heat requirement (i.e. the consumption of LP steam) is the highest for a given methanol yield. However, Uhde's multi-column design maximises the yield and minimises the consumption of LP steam.
The multi-column distillation design consists of three or four columns - one topping column and two refining columns, augmented by an additional recovery column in the case of the four-column design concept.
The appropriate design for the distillation section depends primarily on the plant capacity, the heat available in the process plant and the energy export requirements.
To meet the growing demand for methanol, future methanol plants will incorporate large capacities coupled with low production costs, high energy efficiency and the lowest possible environmental pollution. Autothermal reforming combined with energy-efficient synthesis and distillation processes could be the answer to these requirements.