13.4 Manufacture of Methanol from Synthesis Gas

13.4.1 Introduction

• Synthesis gas is H₂+CO
• When synthesis gas is subjected to high pressure and moderate temperature conditions, it converts to methanol.
• Followed by this, the methanol is separated using a series of phase separators and distillation columns.
• The process technology is relatively simple.

13.4.2 Reactions

• • Desired: CO + 2H₂→ CH₃ OH

• Side reactions: CO + 3H₂→ CH₄ + H₂ O
                       2CO + 2H₂→ CH₄ + CO₂

All above reactions are exothermic

• • Undesired reaction: zCO + aH₂→ alchohols + hydrocarbons.

• Catalyst: Mixed catalyst made of oxides of Zn, Cr, Mn, Al.

13.4.3 Process Technology (Figure 13.1)

Figure 13.1 Flow sheet of manufacture of Methanol from Synthesis Gas

• • H₂ and CO adjusted to molar ratio of 2.25.

• The mixture is compressed to 200 – 350 atms.

• Recycle gas (Unreacted feed) is also mixed and sent to the compressor.

• Then eventually the mixture is fed to a reactor. Steam is circulated in the heating tubes to maintain a temperature of 300 – 375°C.

• After reaction, the exit gases are cooled.

• After cooling, phase separation is allowed. In this phase separation operation methanol and other high molecular weight compounds enter the liquid phase and unreacted feed is produced as the gas phase.

• The gas phase stream is purged to remove inert components and most of the gas stream is sent as a recycle to the reactor.

• The liquid stream is further depressurized to about 14 atms to enter a second phase separator that produces fuel gas as the gaseous product and the liquid stream bereft of the fuel gas components is rich of the methanol component.

• The liquid stream then enters a mixer fed with KMNO₄ so as to remove traces of impurities such as ketones, aldehydes etc.

• Eventually, the liquid stream enters a distillation column that separates dimethyl ether as a top product.

• The bottom product from the first distillation column enters a fractionator that produces methanol, other high molecular weight alcohols and water as three different products.