578
due to exothermicity of reaction, high investment, and operating costs due to multiple recycling. Also, these traditional reactors are not preferable for coal-gasified
synthesis gas as stoichiometric number S ((moles H 2 -moles CO 2 )/(moles
CO 2 + moles CO)) is found to be less than two indicating hydrogen deficiency
which promotes catalyst deactivation because of coking. To deal with these problems, three phase (gas-solid-liquid) slurry and trickle bed reactors stand out to be
one of the major solutions and represent future of the methanol industry [33].
3.2.1 Trickle Bed Reactors
Catalyst synthesis and loading in trickle bed reactors has been proposed in similar
fashion as fixed bed reactors. The feed gas and solvent/liquid enters into the reactor
in a co-current manner. Inert solvents with high heat capacity like mineral oil/
hydrocarbons are used for suspension of catalyst, and these solvents facilitate good
heat transfer that ensures homogeneity in temperature during reaction. Removal of
product during reaction reduces recycling costs and striking the thermodynamic
limitations [10].
3.3 Membrane Reactors
The thermodynamic limitations of methanol synthesis reaction reduce the overall
yield of conventional fixed bed reactor systems. The overall yield of methanol can
be increased either by recycling of the unreacted feed or by in situ removal of products. Membrane reactors can be used for selective adsorption of methanol or water
or both for in situ removal of products. The in situ separation of methanol is helpful
to achieve high purity of product, whereas in situ separation of water shifts the equilibrium towards methanol synthesis reaction (Le Chatelier’s principle). In addition,
water removal avoids secondary reaction like methanol reforming and catalyst
deactivation via sintering in Cu/ZnO catalyst.
Struis et al. [103] reported lithiated Nafion membrane (polymeric membrane) for
in situ methanol separation. The removal of methanol enhanced the performance of
membrane reactors over the conventional fixed bed reactor system. The same group
developed a mathematical model combining methanol synthesis and permeation.
The model predicted an improved single pass reactor yield by 40% at 200 °C,
GHSV = 5000 h
−1
, and ∆P = 39 bar with 10μm lithiated Nafion membrane [104].
However, low thermal stability of membrane (150 °C) limited the industrial application of membrane reactors. The industrial methanol synthesis is at around
240–300 °C; thus, the application of polymeric membrane is difficult; thus, the
research focus has been synthesis and application of inorganic membrane in the
context of methanol synthesis membrane reactors.
Zeolite can separate molecules based on their size because of their well- structured
narrow pore size distribution. In addition, thermal, mechanical, and chemical
K. Tripathi et al.
due to exothermicity of reaction, high investment, and operating costs due to multiple recycling. Also, these traditional reactors are not preferable for coal-gasified
synthesis gas as stoichiometric number S ((moles H 2 -moles CO 2 )/(moles
CO 2 + moles CO)) is found to be less than two indicating hydrogen deficiency
which promotes catalyst deactivation because of coking. To deal with these problems, three phase (gas-solid-liquid) slurry and trickle bed reactors stand out to be
one of the major solutions and represent future of the methanol industry [33].
3.2.1 Trickle Bed Reactors
Catalyst synthesis and loading in trickle bed reactors has been proposed in similar
fashion as fixed bed reactors. The feed gas and solvent/liquid enters into the reactor
in a co-current manner. Inert solvents with high heat capacity like mineral oil/
hydrocarbons are used for suspension of catalyst, and these solvents facilitate good
heat transfer that ensures homogeneity in temperature during reaction. Removal of
product during reaction reduces recycling costs and striking the thermodynamic
limitations [10].
3.3 Membrane Reactors
The thermodynamic limitations of methanol synthesis reaction reduce the overall
yield of conventional fixed bed reactor systems. The overall yield of methanol can
be increased either by recycling of the unreacted feed or by in situ removal of products. Membrane reactors can be used for selective adsorption of methanol or water
or both for in situ removal of products. The in situ separation of methanol is helpful
to achieve high purity of product, whereas in situ separation of water shifts the equilibrium towards methanol synthesis reaction (Le Chatelier’s principle). In addition,
water removal avoids secondary reaction like methanol reforming and catalyst
deactivation via sintering in Cu/ZnO catalyst.
Struis et al. [103] reported lithiated Nafion membrane (polymeric membrane) for
in situ methanol separation. The removal of methanol enhanced the performance of
membrane reactors over the conventional fixed bed reactor system. The same group
developed a mathematical model combining methanol synthesis and permeation.
The model predicted an improved single pass reactor yield by 40% at 200 °C,
GHSV = 5000 h
−1
, and ∆P = 39 bar with 10μm lithiated Nafion membrane [104].
However, low thermal stability of membrane (150 °C) limited the industrial application of membrane reactors. The industrial methanol synthesis is at around
240–300 °C; thus, the application of polymeric membrane is difficult; thus, the
research focus has been synthesis and application of inorganic membrane in the
context of methanol synthesis membrane reactors.
Zeolite can separate molecules based on their size because of their well- structured
narrow pore size distribution. In addition, thermal, mechanical, and chemical
K. Tripathi et al.