New reactor can produce pure stream of hydrogen

Article by Amanda Doyle

A CHEMICAL reactor has been designed that can produce hydrogen as a pure product stream.

Most reactors are thermodynamically irreversible as the reactants become mixed with the products, making a separation step necessary. Thermodynamic losses occur, as energy will be needed to separate the reactants from the products. There is also an incomplete conversion of reactants due to the reaction being limited by equilibrium. This means that when the system reaches equilibrium, the concentrations no longer change, and the net reaction rate drops to zero.

A conventional water gas shift reactor converts water and carbon monoxide to hydrogen and carbon dioxide. This reaction is used on a large scale industrially, but it is limited by thermodynamics so that the hydrogen product is mixed with other products.

Now, a new reactor known as a Hydrogen Memory Reactor, which has been developed by a team led by Newcastle University, UK, is capable of approaching a thermodynamically-reversible operation. The team uses a solid-phase oxygen carrier material (OCM) which can transfer oxygen back and forth between the two streams, so that the reactor switches between the forward and reverse reactions in half cycles.

The H2O is passed over the OCM which accepts oxygen from the H2O, therefore producing H2. Pure hydrogen is produced as a product stream until the bed becomes fully oxidised, at which point unreacted water exits the bed. The CO is then passed over the OCM, where the oxygen is removed to produce CO2. The bed begins as fully oxidised from the previous half cycle, therefore only pure CO2 is produced. Eventually the bed becomes completely reduced, and unreacted CO exits. However, by shortening the duration of the half cycles so that the bed doesn’t become fully oxidised or reduced it is possible to only have the two pure product streams of H2 and CO2 produced from different ends of the reactor bed without any unreacted H2O or CO making its way through the bed.

The OCM essentially transfers chemical information between the two half cycles, retaining a “memory” of the oxygen chemical potential of the oxidising and reducing gases. Each half cycle must be short enough for the OCM to ‘remember’ the chemical potential associated with each feed stream. It then becomes a thermodynamically reversible operation between the two half cycles.

“Chemical changes are usually performed via mixed reactions whereby multiple reactants are mixed together and heated,” said Ian Metcalfe, lead author and Professor of Chemical Engineering at Newcastle. “But this leads to losses, incomplete conversion of reactants and a final mixture of products that need to be separated. With our Hydrogen Memory Reactor we can produce pure, separated products.  You could call it the perfect reactor.”

Conventional H2 production requires two reactors and a separation step, however the memory reactor accomplishes all steps in a single unit. It is also possible to apply the memory reactor to other processes.

A video describing the process can be found here.

Nature Chemistry  

Article by Amanda Doyle

Staff Reporter, The Chemical Engineer

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