material balance for multi-unit operations

8/10/2019 Material Balance for Multi-Unit Operations

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Material Balance for

Multi-Unit Operations


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Process Flow sheet Flow Chart)

1. Process Information: key properties data, market conditions, techniinformation, etc., are collected.

2. Input /Output Diagram: All major inputs and outputs streams and thstoichiometric balances are illustrated, according to the potential chemipathways.

3. Functions Diagram: All the major functions of the process and materiflow to and from those functions are specified. The functions include reacti

boxes, separation boxes, finishing boxes, etc.

4. Operations Diagram: Detail technologies used such as types of reactoperating conditions, etc. are specified.

5. Process Flow sheet: Details for each function are described, ready to use.


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Process Flowsheet Development


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Function Diagram


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Operation Diagram


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Process Flow sheet


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MB for Multiple Units Process


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Example on MB for Multi-units Process


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Recycle System

The stream containing the recycled material is kno

as a recycle stream.

Because of relatively high cost of industrial feerecycle of unused reactants/feed is a commpractice.

Recovery of catalyst, circulation of working fluidilution of process streams, control of procvariables, etc., are all reasons for applicationsrecycle system.


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Recycle with Chemical Reaction


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Bypass and Purging


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Bypassing wastewater to waste Lagoon


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Limiting reactant

In practice a reactant may be used in excess of thestoichiometric quantity for various reasons. In this casthe other reactant is limiting i.e. it will limit the yield oproduct(s)


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Excess reactant

A reactant is in excess if it is present in a quantity

greater than its stoichiometric proportion.% excess = [(moles supplied stoichiometric

moles)/stoichiometric moles] x 100


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Conversion

Fractional conversion = amount reactant

consumed/amount reactant supplied % conversion = fractional conversion x 100

Note: conversion may apply to single pass reactorconversion or overall process conversion


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Yield

Yield = (moles product/moles limiting reactant

supplied) x s.f. x 100

Where s.f. is the stoichiometric factor = stoichiometrimoles reactant required per mole product


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Multiple Reactions

There are four basic type of multiple reactions: series, parallel, complex, and indepe

Parallel reactions competing reaction)

A

k1

k2 C

B

Series reactions consecutive reaction)

Ak1 k2

CB

The reactant is consumed by two different reaction pathways to form different pro

The reactant forms an intermediate product, where reacts further to form another


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Complex reactions

ECA

DCBA

OHHCCHOCHHC

HCHOCHOHHC

OHHCOHHC

264342

2352

24252

Involving a combination of both series and parallel reactions

Independent reactions

FED

CBA

42143188

6324123215

HCHCHC

HCHCHC

Occurring at the same time but neither the products nor reactants react with thems

or one another.

formation of butadiene from ethanol

cracking of crude oil to form gasoline


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Selectivity tells us how one product is favored over another when we ha

multiple reactions. We can quantify the formation of Desired product (D

with respect to Undesired product (U) by defining the selectivity and yie

of the system.

The instantaneous selectivity of D with respective to U is the ratio of the rate

formation of D to the rate of formation of U.

UDS

/

~

Overall selectivity

Selectivity

/ =

=

=

=


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Selectivity

Selectivity = (moles product/moles reactant converted) x s.f. x

OR

Selectivity = moles desired product/moles byproduct

Where s.f. is the stoichiometric factor = stoichiometric moles reactant re

mole product


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Selectivity and YieldInstantaneous Overall

Selectivity:

Yield:

Example: desired product ,

undesired product ,

To keep the selectivity of the desired products high withrespect to the undesired products, carry out the reactioat high concentration of A and low concentration of B. Ahigh selectivity can easily be achieved in a semibatchreactor where B is fed slowly compared to A.

rD k

1CA

2CB

rU k

2CACB

2

material balance for multi-unit operations

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