Acetic acid can be produced industrially by a number of methods, some having discrepancies while others are modern methods. By methanol carbonylation acetic acid is made by two major methods .The purpose of this chapter is to show how the best among them t is selected. The methods or the technologies used to manufacture acetic acid are as follows.
- Monsanto Process.
- Cativa Process
One by one these methods are discussed below.
1. Monsanto process
The carbonylation of methanol produces acetic acid:
150 – 200 oC
30 – 60 atmosphere CH3OH + CO CH3COOH
This is the second largest industrial homogeneous carbonylation process with over 7 billion pounds of acetic acid produced each year using this technology. Prior to 1970, acetic acid was made using cobalt catalysts (BASF process) requiring rather severe conditions. In 1970 Monsanto commercialized a rhodium carbonyl iodide catalyst that is commonly called the Monsanto Acetic Acid Process (developed in the late 60’s by James Roth and his research team at the corporate research center in St. Louis). In 1986 Monsanto sold the acetic acid plant and technology to British Petroleum (BP), but it is still commonly referred to as the Monsanto Acetic Acid process.
- Introduced methanol carbonylation which resulted in huge reduction of waste.
- The whole process uses less energy as compared to non methanol carbonylation processes.
- Uses methanol, a cheaper feedstock.
- Although methanol is usually manufactured from synthesis gas, produced from oil, it can also be produced from biomass (wood), municipal wastes and sewage. This may eventually lead to the process being no longer dependent oil.
- Rhodium metal is very expensive more expensive than gold.
- Rhodium and iodide form insoluble salts like RhI3. So water level in the reactor vessel has to be kept high in order to prevent this.
- A final distillation step has to be added in order to remove water. Adding to the costs and energy demands.
- Rhodium is capable of catalyzing a lot of side reactions.
2. CATIVA PROCESS:
Since 1997, ethanoic acid is increasingly being produced by the BP Cativa process, also involving the carbonylation of methanol. The difference is that it uses an iridium metal iodide complex ion as a catalyst, with promoters.
CH3OH + CO CH3COOH
Iridium Iodide Complex & promoter
- The mechanism involving iridium is different to that of rhodium as catalyst, as iridium works best under different conditions. The Cativa process also uses ruthenium compounds as promoters in the reaction. These increase the reaction rate by three times, even though ruthenium on its own has negligible catalytic activity in this system.
- Iridium costs only about one fifth as much as rhodium.
- The process is faster and more effective, requiring less catalyst to be used.
- Iridium is even more selective for methanol, which increases the overall yield and reduces by- products, resulting in lower purification costs and reduced waste.
- Iridium complexes are more soluble in the reaction mixture than rhodium complexes. This means that the catalyst is not lost by precipitation and does not need replacing so frequently.
- The energy needed at the distillation and purifying stages.
- Existing plant can be modified to run the Cativa process at half the cost of building a new plant. This is referred to as retrofitting.
- Cativa plants have a higher throughput a single plant can produce up to 75% more ethanoic acid than was previously possible using the Monsanto process.