Hydrogen can be produced from methanol by combining catalytic steam reforming with pressure swing adsorption (PSA). During the reforming stage, a mixture of methanol and water reacts over a catalyst at elevated temperature, generating a hydrogen-rich reformate. The gas is then cooled and separated before entering the PSA unit, where impurities are removed to obtain high-purity hydrogen. The resulting product can be used in applications such as hydrogen refuelling stations.

1. Principles of Methanol Steam Reforming

Methanol and water are first blended in a predetermined ratio. The mixture is then vaporised, superheated and introduced into a catalyst bed under carefully controlled conditions of temperature, pressure and flow. Within the reactor, methanol decomposition and the water-gas shift reaction proceed concurrently, producing a reformate composed mainly of hydrogen and carbon dioxide.

The conversion of methanol into hydrogen is a heterogeneous catalytic process involving several components and interconnected reactions. The principal reactions are as follows:

Methanol decomposition:

CH₃OH ⇌ CO + 2H₂  ΔH° = +90.7 kJ/mol

Water-gas shift reaction:

CO + H₂O ⇌ CO₂ + H₂  ΔH° = −41.2 kJ/mol

Overall methanol steam reforming reaction:

CH₃OH + H₂O ⇌ CO₂ + 3H₂  ΔH° = +49.5 kJ/mol

Following heat recovery, cooling and gas–liquid separation, the reformate typically contains about 74.5% hydrogen and 24.5% carbon dioxide. The concentration of carbon monoxide is generally maintained at or below 1.0%.

Under the specified operating conditions, more than 98% of the methanol is normally converted in a single pass. Any unreacted methanol, together with the recovered demineralised water, is returned to the feed system and reused. The hydrogen-rich reformate is subsequently directed to the PSA unit, where it is purified to the required product specification.

2. Principles of Pressure Swing Adsorption

Pressure swing adsorption is a well-established technology for purifying hydrogen and separating industrial gas mixtures. It removes carbon dioxide, carbon monoxide, nitrogen, methane and other contaminants by taking advantage of the different adsorption behaviours of the individual gas components.

Separation is carried out in fixed-bed vessels filled with one or more adsorbent materials. When the pressurised reformate enters an adsorption vessel, components with a stronger affinity for the adsorbent are captured within the bed. Hydrogen is adsorbed only weakly and therefore passes through the vessel, leaving as the purified product gas.

As the bed approaches its adsorption capacity, the vessel is taken offline and depressurised. Lowering the pressure releases the retained impurities and regenerates the adsorbent for the next cycle. Several vessels operate in parallel and move through adsorption, depressurisation, regeneration and repressurisation in a coordinated sequence. This arrangement allows hydrogen production to continue without interruption.

The achievable hydrogen purity depends on the system design, feed-gas composition, operating conditions and final product requirements. In practice, PSA units can produce hydrogen with purities ranging from 99% to 99.9999%.

3. Process Description

3.1 Methanol Conversion

Fresh methanol supplied from the methanol buffer tank is blended with the methanol and water recovered in the recycle buffer tank. The combined feed is prepared at a methanol-to-water mass ratio of 1:1. A metering pump then raises the feed pressure and delivers the mixture to the feed–reformate heat exchanger.

Inside the heat exchanger, the incoming feed recovers thermal energy from the hot reformate leaving the reactor. This heat-recovery step raises the feed temperature to approximately 160°C. The preheated mixture then passes into an evaporator heated by thermal oil, followed by a superheater that brings the vapour to the required reaction temperature.

At approximately 230°C, the superheated feed enters the reformer. Methanol steam reforming and the water-gas shift reaction take place simultaneously in the catalyst bed, converting the feed into hydrogen-rich reformate gas.

On leaving the reactor, the hot reformate returns to the feed–reformate heat exchanger. Its temperature falls to approximately 140°C as part of its heat is transferred to the incoming methanol–water mixture. The reformate then enters a condenser and is cooled further by circulating cooling water.

Once its temperature has fallen below 40°C, the reformate flows into a gas–liquid separator. The separated gas is routed to the PSA purification section. Condensed water and the small quantity of unreacted methanol are collected and returned to the recycle buffer tank for reuse.

3.2 PSA Hydrogen Purification

Reformate from the methanol hydrogen production section enters the bottom of each adsorption vessel. As it travels upward through the adsorbent bed, carbon dioxide, carbon monoxide, methane, residual methanol and other impurities are selectively retained. Hydrogen passes through the bed with comparatively little adsorption and is withdrawn from the top of the vessel as the product gas.

When an adsorption bed nears saturation, it is removed from the adsorption stage and depressurised. The reduction in pressure causes the retained impurities to desorb, restoring the bed’s adsorption capacity and preparing it for another cycle.

The system uses a 10-3-3 PSA configuration. It consists of ten adsorption vessels, with three vessels performing adsorption at any given time. The other vessels move through a coordinated series of pressure equalisation, co-current depressurisation, counter-current depressurisation, purging, repressurisation and standby operations.

Each complete cycle includes adsorption, three successive pressure-equalisation depressurisation steps, co-current depressurisation, counter-current depressurisation, purging, three pressure-equalisation repressurisation steps, standby and final repressurisation.

Continuous rotation of the ten vessels through these stages allows the PSA unit to receive reformate and deliver purified hydrogen without interrupting production.