Methanol Process Description

The Leading Concept Methanol process in use at the Coogee Methanol Plant has various advantages compared to the conventional methanol processes. Some of those advantages are that it is efficient and compact and substantially reduces waste through the internal recycling of process effluents.

Natural gas feedstock is delivered to the plant via a pipeline from the main Melbourne-to-Geelong trunkline carrying Bass Strait gas. The gas is first compressed and then purified by removing sulphur compounds. The purified natural gas is saturated with heated and recycled process waste water. The mixed natural gas and water vapour then goes to the gas heated reformer to be partially converted to synthesis gas, a mixture of carbon dioxide, carbon monoxide and hydrogen. This partially converted gas is then completely converted to synthesis gas by reaction with oxygen in the secondary reformer.

The synthesis gas is then converted to crude methanol in the catalytic synthesis converter. The crude methanol is purified to standard quality specifications by removing water and organic impurities through distillation. The water and organic impurities are recycled.

The Coogee Energy plant is designed to produce 164 tonnes per day of methanol from about 5 TJ/day of Bass Strait natural gas.

COMPONENT	Mol %
Methane	90.7
Ethane	5.9
Propane	0.6
Butane	0.1
Nitrogen	0.9
Carbon Dioxide	1.8

The plant consists of four main process steps : feed gas preparation, synthesis gas generation, methanol synthesis and distillation supported by utilities and offsite units. Click here to view a simple process flow diagram of the Coogee Methanol Plant.

Natural gas is compressed to about 45 bar and sulphur removed by hydrodesulphurisation in the purifier. The desulphurising gas is cooled and flows to the saturator where it contacts with hot water over a bed of packing. The saturated gas leaving the vessel contains about 92% of the steam required for reforming. Saturator make up is 90% process condensate and the balance refining column bottoms water. Prior to leaving the saturator the gas stream is contacted with recycled fusel oil where waste products from methanol synthesis are stripped off. A blowdown stream is required to control dissolved solids. Additional steam generated in the boiler is made up to the gas stream to achieve 3.0:1 steam to carbon ratio for reforming.

The total feed stream is then heated in the gas heated reformer pre heater. Both the pre heater and boiler are fired with a mixture of synthesis loop purge gas and natural gas.

Steam reforming - CH4 + H2O = CO + 3H2
Shift reaction - CO + H2O = CO2 + H2
Combustion - 2H2 + O2 = 2H2O

The net effect of these reactions is the production of a synthesis gas stream which is composed of carbon monoxide (CO), carbon dioxide (CO2) and hydrogen (H2).

Preheated gas flows from the pre heater to the tube side of the advanced gas heated reformer (AGHR). The feedstock is heated from the feed temperature of 425° C as it passed down through the catalyst and the reforming reactions start. The AGHR contains 19 reforming tubes which contain the reforming catalyst.

Hot reformed gas exits the bottom of the reforming tubes and flows to the tube side exit of the AGHR at about 700°C. The heat required for the endothermic reforming reaction is derived from cooling the secondary reformer effluent in the shell side of the AGHR. About one quarter of the methane is reformed in the AGHR.

The partly formed gas flows from the AGHR to the combustor/secondary reformer where the bulk of the reforming takes place. The heat required for the endothermic reforming in both the AGHR and secondary reformer is provided by partially burning the AGHR effluent with pure oxygen in the combustor located integrally at the top of the secondary reformer. Oxygen is injected into the gas via a specially designed gun. About 0.50 tonne of oxygen per tonne of methanol is required.

The oxygen is completely consumed and the resulting hot gas stream passes over the secondary reforming catalyst. Reforming reactions continue and the gas leaves the secondary reformer at up to 1000°C with less than 0.5% methane slip. The secondary effluent passes to the AGHR shell and thence through the heat recovery train to provide heat for the saturator circuit and distillation reboilers. The process condensate which condenses out of the reformed gas is recycled back to the saturator. After heat recovery the reformed gas is finally cooled and then compressed to about 70 barg in the synthesis gas compressor to be fed as synthesis gas to the synthesis loop.

Bass Strait natural gas contains about 91 mol% of methane, 6 mol% of ethane with the balance being predominantly propane, nitrogen and carbon dioxide. On an offshore facility with less sophisticated gas separation facilities there may be higher levels of higher hydrocarbons such as components but the oxygen consumption would increase.

The net effect of these reactions is the production of a crude methanol stream which is about 80% methanol and 20% water.

The synthesis gas joins the synthesis loop recycle gas from the circulator to pass through the loop interchanger and be fed to the methanol converter at about 130° C. The converter is a tubular cooled converter design where the gas is preheated to reaction temperatures inside the tubes as it flows up through the hot catalyst bed. This type of converter maximises catalyst efficiency as it enables a temperature profile to be maintained inside the converter that is close to the maximum reaction rate curve. The hot reacted gas leaves the converter and provides heat to the saturator water circuit and the loop interchanger before finally being cooled. Crude methanol is separated from the uncondensed gases in the loop catch pot and the gases recirculated back to the converter via the circulator.

Distillation
Crude methanol from the loop catch pot is filtered to remove traces of wax, let down in pressure and fed to the product purification section. This section consists of a topping column and a refining column. Unlike most methanol distillation columns these columns are packed with structured packing. Reboiler duty is provided by reformed gas. The product methanol specification is for a water content of less that 0.10 wt %. The water bottoms from the refining column has a specification of less than 100 ppm of methanol and is recycled back to the saturator. Other synthesis by products such as higher alcohols are collected as fusel oil and recycled back the saturator.

Utilities and Offsites
An air compressor, demineralisation, cooling water system, flare, firewater system , crude and refined methanol tanks provide the supporting utilities and offsite facilities. All rotating equipment is motor driven and the power demand of the plant is about 2.2 MW.