27
BITS Pilani Pilani Campus Lecture 19 Chemical Process Technology (CHE F419)_Dr Smita R

CPT Lecture19 Monsanto Process

Embed Size (px)

DESCRIPTION

Thermo

Citation preview

  • 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