what is selectivity and conversion in catalyst bed reactor and can anybody explain me about LHSV in reactor?
Answers
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04/07/2009
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Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
Selectivity and conversion are very important terms in conversion processes. Multiple and series reactions take place in the reactor giving a slate of various products. Some product may be most desirable while others may be less desirable and some may be undesirable. So the selectivity may be defined as the conversion of the reactant to the desirable product divided by the overall conversion of the reactant. It may also be stated as the rate of conversion of the feed to the desired product divided by the overall conversion rate of the feed.
Conversion means disappearance of the reactants. For feed stocks where there are thousands of reactants some convenient property, like boiling cut points in case of VGO FCC and Hydrocracking in a refinery is chosen. You may refer to W L Nelsion book on Petroleum Refinery Engineering. If A is one of the reactant ,
% Coversion of A= {(A in feed - A in product) /A in feed} * 100
LHSV iw an indicator of space time. In common terms it is defined as LHSV= (volume of feed in cubic meters per hour at 15degree C) / volume of catalyst loaded in reactor in cubic meters
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24/06/2009
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Kavirayani R Murthy, K R Murthy Consultants, krmurthy_ipcl@yahoo.com
When a chemical reaction takes place, some portion of the reactants disappear and gets converted to various products. The disappearance of reactants at a set of operating conditions is the conversion. For example, when Naphtha is reformed to produce aromatics like benzene, toluene and xylenes, we pump a calculated quantity of naphtha feed of a specific boiling range in to the reactor under a set of operating conditions like temperature, pressure, WHSV (weight hourly space velocity) H2/HC mole ratio, by maintaining proper reformer environment consisting of sulphur content, chloride content and moisture content in a recycle gas.
In a reformer several reactions may take place and the chemical compounds in the feed disappears and gets converted to various other chemical compounds may not be for our required product and appear in the reformate. Simplistically, conversion is the disappearance of reactants during reaction.
When we talk about selectivity we concentrate on the formation of a specific compound (Selective compound) during the process. For example, in Reforming process, say for production of Benzene, several reactions as given below may produce benzene.
• Benzene present in feed will have a idle ride and reappear in the product
• Cyclohexanes in feed gets dehydrogenated to benzene
• N-hexane gets dehydrocyclized to cyclohexanes and then dehydrogenated to benzene
• Methyl cyclopentanes gets dehydroisomerized to cyclohexanes and then gets dehydrogenated to benzene
• Methyl cyclohexanes gets dealkylated to cyclohexanes then dehydrogenated to benzene.
Several other reactions may also takes place and contribute for the formation of Benzene.
Selectivity calculation thus shows how a selective product in this case benzene is formed during the reaction.
This is calculated by finding out the wt/wt % yield of the liquid (Reformate) obtained during the reforming process multiplied by the concentration of the selective component in (wt%) which we are looking for.
Liquid hourly space velocity (LHSV) will be expressed on volume basis, i.e. volume of liquid flowing per hour per unit volume of the catalyst. Mostly this is expressed on weight basis then it is called weight hourly space velocity (WHSV) that is calculated by weight of liquid flow per unit weight of the catalyst per hour.
Inverse of space velocity is the contact time for the liquid to be in contact with the catalyst.
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24/04/2008
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Kamaljit Medhi, Indian Oil Corporation Ltd, medhik@indianoil.in
Selectivity is the ability of a particular catalyst to favour the desirable reactions rather than the undesirable reactions. For example in the Catalytic Reformer Unit, the aim is to convert the Paraffins & Naphthenes into Iso-Paraffins and Aromatics. However, in the process, coke formation also takes place which is not desirable. So the catalyst which gives better C5+ Reformate yield and lower coke formation is having better selectivity.
Fractional Conversion is defined as say XA = (NA0 - NA) / NA0
where XA is the conversion rate of a reactant A, NA0 is the initial concentration of A & NA is the final concentration of A after the reaction takes place.
LHSV: I suppose you are talking of liquid hourly space velocity which can be calculated by dividing the hourly volume of feed at 15o C to a reactor by the volume of the catalyst in the reactor.
So inverse of LHSV is linked to the residence time. Lower LHSV means increase of the residence time and higher severity of the reactor. Similarly with increasing LHSV will reduce the residence time and hence lower severity
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