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BITS Pilani Pilani Campus
Lecture 19 Chemical Process Technology (CHE F419)_Dr Smita R
Examples of Homogenous catalysis: Acetic acid production
Direct liquid phase oxidation of acetaldehyde was
once preferred route to acetic acid coz of low cost of these hydrocarbons
Reaction mechanism is based on radicals
It follows the typical initiation, propagation & termination steps
But D/A is that; around 50% of feed goes in production of by products (formic acid, aldehydes, higher acids)
Preferred route for acetic acid .
Carbonylation of methanol which dates back to 1925
But the commercial plant came in 1963 when a new cobalt/iodide catalyst system was developed (BASF technology)
In 1968; Monsanto Chemical Co. introduced a new carbonylation process using a novel highly selective catalysts (rhodium iodide) which was commercialized 2 yrs later
Preferred route for acetic acid .
Preferred route for acetic acid .
Key to success is very active catalyst system which
allows operation at much less severe conditions than BASF process
Important part of catalyst system is iodide which is supplied as hydrogen iodide which is very corrosive
Methanol equilibrium conversion is almost complete at the reaction conditions employed. Indeed, at the temperatures employed the conversion is even nearly complete at atmospheric pressure.
What is the reason????
But rxn is carried out at elevated pressure,
however, in order to keep the reaction mixture in the liquid phase and
To generate and maintain the catalyst in its active form; the catalyst complex is not stable at low carbon monoxide pressure
Preferred route for acetic acid .
Important part of catalyst system is iodide which is
supplied as hydrogen iodide which is very corrosive
Rxn is
Formation of methyl iodide is start of catalytic cycle
kJ/mol -53.1H OHICHHIOHCH 233
Preferred route for acetic acid .
Formation of methyl iodide is start of catalytic cycle
Addition of methyl iodide to rhodium complex has been found to be rate determining step;
In fact rxn rate is independent of two reactants methanol & CO
Rate eqn is:
]IRh][CH[r 3COOHCH3 k
Preferred route for acetic acid .
Consequence of zero order dependence of rxn
mixture on concentrations is that at any conversion level, production rate is same
Hence, high conversions can be obtained even in _____________
So a question can be..
Recall design eqn of CSTR & show that only in the
case of 0th order reactions, complete conversion can be attained in a reactor of finite volume.
Preferred route for acetic acid .
Also methanol is diluted with water to suppress
formation of methyl acetate & dimethyl ether
Addition of water increases methanol conversion to acetic acid but leads to water gas shift rxn as well (CO2 gets formed)
Flow sheet for Monsanto process
Monsanto process
CO & methanol are fed to CSTR containing the catalyst
Reaction takes place in liquid phase under relatively mild conditions
Liquid reactor effluent is de pressurized in a flash vessel which results in gas & liquid phase
Liquid phase is send to light ends column
Monsanto process
Overhead is combined with gases from flash vessel & scrubbed with methanol to recover methyl iodide
Off gases from scrubber are flared
Bottom stream of light ends column (containing catalyst complex, water & AA) is recycled to reactor
Wet AA is taken as a side stream from light ends column & fed to drying column
Monsanto process
Dry AA is removed as bottom pdt
Overheads containing a mixture of AA (35% concentration) & water are recycled to reactor
Fixed amt of water & AA is continuously circulating in plant
Dry AA is fed to products column from which heavy byproducts are removed as bottoms
Monsanto process
Overheads from product column is sent to finishing column from which ultra pure AA is obtained as a side stream
Both overhead & bottoms from this column are recycled to process
Key points: Monsanto process
Imp requirement is small amt of water
Requires large no. of trays
Catalyst complex must be recovered with an extreme efficiency & returned to reactor as rhodium is harmful metal & is quite expensive
Nearly complete recovery of rhodium is effected in light ends column
Key points: Monsanto process
Volatile & toxic methyl iodide has to be retained in system
This is achieved by methanol scrubber
Overhead gases from all the columns are send to this scrubber
Homogenous catalysis
In homogeneous catalysis, soluble catalysts are applied
Homogenous catalysis
S No. Homogeneous catalyst Heterogeneous catalyst
1. Soluble catalysts are applied Solid catalyst are used
2. Reaction mixture contains catalysts complex in solution: meaning that all metal is exposed to reaction mixture
Metal is applied on a carrier material as a porous metal sponge type material & only surface atoms are active
3. In terms of activity per metal center, homogeneous catalyst are more active
Less active
4. One poison molecule only deactivates one metal complex
Poison molecule can block a pore containing many active sites
5. Capable of being more selective; as there is only one type of active site
Less selective as contain many different types of active sites
6. As carried out in liquid phase, temp. control is relatively easy
Temp. control is not that easy
Homogenous catalysis
If so many advantages, then why homogeneous catalysis is not used in every reactions?
High specificity is not always called for
For example: in many refinery processes; especially; complex feeds & products do not justify use of highly selective catalysts
Homogenous catalysis: Certain disadvantages
Separation of catalysts & products is difficult
Feasible for only low M Wt products
Use of solvents add an additional separation step
Complex are not often resistant to high temp.
Often precious metals are used & hence recovery is needed which may not be the case always
Homogenous catalysis
Hence developments needs the working together of chemists & chemical engineers
Hence organometallic chemistry deals with development of catalysts
Mild reaction conditions are advantage for homogeneous reactions