Methanol synthesis by Reactive
Distillation

Shashwata Ghosh^1,*, Srinivas Seethamraju¹

¹Department of Energy Science
and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076,
India.

^*Corresponding author: Shashwata Ghosh

1.      
Introduction

Methanol, an important intermediate in the chemical
industry is conventionally produced from syngas over solid Cu-ZnO-Al₂O₃
catalyst. Since methanol synthesis is exothermic, cooling arrangements are
necessary to remove the heat of reaction. Therefore, the conventional reactors
remove the reaction exotherm either by cold syngas shots in between catalyst
beds in the reactor (ICI process), or using a multi-tubular reactor cooled by
boiling water on the shell side (Lurgi process). Another process, referred to
as the liquid phase process

[1]

uses an inert solvent to remove the exothermic heat of reaction. In this
process, syngas is passed through a liquid phase reactor in which methanol synthesis
takes place on solid catalyst slurried in the solvent. Though not
commercialized, the liquid phase process gave higher per pass reactants’ conversions
than the two phase process in which conversion was limited by chemical
equilibrium. Reactive distillation (RD), an emerging process is used for
synthesis of methyl acetate, MTBE, etc. and can be applied to synthesis of
methanol by using an inert solvent, as in the liquid phase process. Nemphos et
al.

[2]

demonstrated RD for methanol synthesis using n-octane as an inert solvent. RD
for methanol synthesis is expected to offer advantages like utilization of the
reaction exotherm for separation, reduction of plant size due to combination of
reaction and distillation in single equipment, reduced deactivation of catalyst
by thermal sintering due to presence of the solvent which acts as the heat
sink, etc. In the present study, a flowsheet for the production of methanol by
reaction distillation was simulated to produce 489 tons/day of methanol. Aspen
Plus was chosen for simulation of the process due to its strong thermodynamic
database and robust numerical solvers.

2.      
Methodology

The schematic of the simulated process is given in

Figure
1

.
2290 kmol/h of syngas containing 22.97 mol% CO, 6.86 mol% CO₂, 67.46
mol% H₂ and 2.71 mol% CH₄ is fed to the process. The RD
column is operated at 50 bars – it has 45 stages (a non-reactive condenser and
44 reactive stages with equal catalyst loading). Squalane (C₃₀H₆₂)
is used as the inert solvent. The kinetics of methanol synthesis and water gas
shift (side reaction) were modeled using the power law formulations of van der
Laan et al.

[3]

and Kobl et al.

[4]

,
respectively. The column is modeled using the “RadFrac” module of the
simulator, which performs rigorous distillation calculations based on the
“Equilibrium stage model” by solving the MESH equations

[5]

.
The RD configuration used in this work is referred to as a “Refluxed Rectifier”
as it does not use a reboiler and the vapor load to the column is provided by
the syngas fed at the bottom of the column. The solvent is fed at the top of
the RR on stage 2. The temperature of both the syngas and solvent streams
entering the column is 200°C. The vapor stream leaving the column exchanges
heat with the recycle gas stream and is cooled in a condenser operating at
40°C. The vapor and liquid distillate streams from the condenser contain
unreacted gases and methanol-water mixture, respectively. The bottom product
stream from the RR is rich in the solvent and is recycled back to the column.
While the vapor distillate is recycled back to the column, the liquid
distillate is further flashed at a pressure of 6.8 bars to release the dissolved
gases (mainly CO₂, CH₄ and H₂) and is
fractionated into methanol and water in a distillation column operating at 1
bar. Additionally, cooling duties are also used on four of the reactive stages
– 10, 15, 20 and 25 to recover heat and increase methanol formation also by
removing the reaction exotherm.

Figure

1

.
Schematic of process for methanol synthesis by reactive distillation

3.      
Results and Discussion

The performance of the process is evaluated in terms
of per pass and overall conversions of the reactants, methanol production and
heat recovery. The per pass conversions of CO, CO₂ and H₂
are 41.67%, 7.76% and 15.45%, respectively. The corresponding overall
conversions are 97.26%, 77.28% and 90.11%, respectively with a methanol productivity
of 14.63 mol/kg_catalyst.h. The product (methanol) stream from the
methanol-water distillation column has a flow rate of 636 kmol/h (489 tons/day)
with 99.4% methanol purity. This production rate of methanol is comparable to
the production rate from a gas-solid fixed bed reactor simulation reported by
Luyben

[6]

.
The work reports methanol production of 3310.9 kmol/h of methanol production
(98.9% purity) from 11450 kmol/h of syngas, which corresponds to 662.2 kmol/h
of methanol from 2290 kmol/h of syngas. 10 MW of heat is removed from the RR by
means of cooling duties on the reactive stages and is utilized to produce steam
for power generation. The off-gas from the process is used in a boiler to provide
heat for heating feed streams of the process, supplying heat to the reboiler of
the methanol-water column and to produce steam for generating power. The total
amount of power generated in the process is 5.1 MW – this can be used to run
the syngas and recycle compressors, which together consume 3.5 MW of power as
well as meet other power requirements in the plant.

4.      
Conclusions and Future Work

This study provides an estimate of the methanol
production and reactant conversions that can be obtained by means of RD for
methanol synthesis. The production rate of methanol by RD is comparable to the
conventional process. It is also seen that heat removed from the RD column can
be used for raising steam to generate power for the process. However, an
economic analysis of the process needs to be performed
following energy analysis to evaluate the applicability of reactive
distillation for methanol synthesis – this work is in progress.

References

[1]      R. L. Espino, T.S. Pletzke, Methanol
Production, United States Patent 3,888,896, 1975.

[2]      S.P. Nemphos, W.A. Groten, J.R. Adams,
Process for production of methanol, United States Patent 5886055, 1999.

[3]      G.P. van der Laan, A.A.C.M. Beenackers, B.
Ding, J.C. Strikwerda, Liquid-phase methanol synthesis in apolar (squalane) and
polar (tetraethylene glycol dimethylether) solvents, Catal. Today. 48 (1999)
93–100.

[4]      K. Kobl, S. Thomas, Y. Zimmermann, K.
Parkhomenko, A.C. Roger, Power-law kinetics of methanol synthesis from carbon
dioxide and hydrogen on copper-zinc oxide catalysts with alumina or zirconia
supports, Catal. Today. 270 (2016) 31–42.

[5]      R. Taylor, R. Krishna, Modelling reactive
distillation, Chem. Eng. Sci. 55 (2000) 5183–5229.

[6]      W.L. Luyben, Design and Control of a
Methanol Reactor/Column Process, Ind. Eng. Chem. Res. 49 (2010) 6150–6163.