CPT Lecture19 Monsanto Process

Lecture – 19 19 Chemical Process Technology (CHE F419)_Dr Smita R BITS Pilani Pilani Campus

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 mechanism is  Reaction mechanism

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 …. •

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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 ….


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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, •

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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


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Important part of catalyst system is iodide which is supplied as hydrogen iodide which is very corrosive Rxn is CH3OH  HI  CH3I  H 2O

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H 

-53.1 kJ/mol

Formation of methyl iodide is start of catalytic cycle


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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: r CH COOH  k [Rh][CH3I] 3


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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.


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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… •

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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


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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


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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


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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

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Requires large no. of trays

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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 •

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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

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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


If so many advantages, then why homogeneous catalysis is not used in every reactions? –

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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

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Feasible for only low M Wt products

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Use of solvents add an additional separation step

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Complex are not often resistant to high temp.

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Often precious metals are used & hence recovery is needed which may not be the case always…


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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

CPT Lecture19 Monsanto Process

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