biochemical engineering lec 14

8/6/2019 Biochemical Engineering Lec 14

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Material Balances

Engr. Abdul Waheed Bhutto


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Material Balances Objective:- How the

law of conservation of

mass is applied toatoms, molecularspecies and total mass,and sets up formaltechniques for solvingmaterial-balanceproblems with andwithout reaction.


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Thermodynamic Preliminaries

In thermodynamics, a system consists of any matteridentified for investigation. As indicated in Figure 4.1, thesystem is set apart from the surroundings, which are the

remainder of the universe, by a system boundary. Thesystem boundary may bereal and tangible, such as thewalls of a beaker or fermenter, or imaginary.

If the boundary does not allow mass to pass from system tosurroundings and vice versa, the system is a closedsystemwith constant mass. Conversely, a system able to exchangemass with its surroundings is an open system.


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Process A process causes changes in the system or surroundings.

Several terms are commonly used to describe processes.

(i) A batch process operates in a closed system. All materials areadded to the system at the start of the process; the system is thenclosed and products removed only when

the process is complete.

(ii) Asemi-batch process allows either input or output of mass,but not both.

(iii)feed-batch process allows input of material to the systembut not output.

(iv) A continuous process allows matter to flow in and out of thesystem. If rates of mass input and output are equal, continuous

processes can be operated indefinitely.


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Steady State If all properties of a system, such as temperature,

pressure, concentration, volume, mass, etc. do not

vary with time, the process is said to be at steadystate.

According to this definition of steady state, batch,

fedbatch and semi-batch processes cannot operate

under steady-state conditions.

Such processes are called transientorunsteady-

stateprocesses.


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Equilibrium A system at equilibrium is one in which all opposing

forces are exactly counter-balanced so that the

properties of the system do not change with time. From experience we know that systems tend to

approach an equilibrium condition when they areisolated from their surroundings.

At equilibrium there is no net change in either thesystem or the universe.

Equilibrium implies that there is no net driving forcefor change


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Strategy is to avoid equilibrium The best strategy is to avoid equilibrium by

continuously disturbing the system in such a way

that raw material will always be undergoingtransformation into the desired product.

In continuous processes at steady state, mass is

constantly exchanged with the surroundings; this

disturbance drives the system away from

equilibrium so that a net change in both the system

and the universe can occur.


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Law of Conservation of Mass


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Material Balance Problem

Solution


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Material Balance Problem

Solutionflow sheetshowing all

data with units.


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Mass-balance table (kg)

Completed mass-balance table (kg)


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Answers

biochemical engineering lec 14

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