āļøWe have already given detailed information about introduction to Process Design, project steps, nature of design and etch. in previous posts. Now, time to talk š¤©about Fundamentals of Material Balances and their roles in Process Design. Here we go šš«
What is Material Balance?
Material balances are the basis of process design. A material balance taken over the complete process will determine the quantities of raw materials required and products produced. Balances over individual process units set the process stream flows and compositions. A good understanding of material balance calculations is essential in process design. Material balances are also useful tools for the study of plant operation and trouble shooting. They can be used to check performance against design; to extend the often limited data available from the plant instrumentation; to check instrument calibrations; and to locate sources of material loss.
āµļøHere we will divide this topic into subtopics š to talk about the properties with details.
Conservation of Massš§©
The general conservation equation for any process system can be written as:
For a steady-state process the accumulation term will be zero. If a chemical reaction takes place a particular chemical species may be formed or consumed in the process. If there is no chemical reaction the steady-state balance reduces to :
Material out =Material in
Let's give example and solve it by yourself:
Example 1. 2000 kg of a 5 per cent slurry of calcium hydroxide in water is to be prepared by diluting a 20 per cent slurry. Calculate the quantities required. The percentages are by weight.
Expressing Compositions
When specifying a composition as a percentage it is important to state clearly the basis: weight, molar or volume. The abbreviations w/w and v/v are used to designate weight basis and volume basis.
Example 2. Technical grade hydrochloric acid has a strength of 28 per cent w/w, express this as a mole fraction.
Choice of Basis for Calculations
The correct choice of the basis for a calculation will often determine whether the calculation proves to be simple or complex. As with the choice of system boundaries, no all-embracing rules or procedures can be given for the selection of the right basis for any problem. The selection depends on judgement gained by experience. Some guide rules that will help in the choice are:
Time: choose the time basis in which the results are to be presented; for example kg/h, tonne/y.
For batch processes use one batch.
Choose as the mass basis the stream flow for which most information is given.
It is often easier to work in moles, rather than weight, even when no reaction is involved.
For gases, if the compositions are given by volume, use a volume basis, remembering that volume fractions are equivalent to mole fractions up to moderate pressures
š½For much better explanation with problems, you should check this link out: https://www.youtube.com/watch?v=m-pTbduSYJg
Number of Independent Variables š¼
A balance equation can be written for each independent component. Not all the components in a material balance will be independent.
Physical systems, if no reaction occurs š
If there is no chemical reaction the number of independent components is equal to the number of distinct chemical species present. Consider the production of a nitration acid by mixing 70 per cent nitric and 98 per cent sulfuric acid. The number of distinct chemical species is 3; water, sulfuric acid, nitric acid.
Chemical systems, reaction occursš
If the process involves chemical reaction the number of independent components will not necessarily be equal to the number of chemical species, as some may be related by the chemical equation. In this situation the number of independent components can be calculated by the following relationship:
Number of independent components =Number of chemical species-Number of independent chemical equations
Constraints on Flows and Compositions š§°
It is obvious, but worth to emphasize that the sum of the individual component flows in any stream cannot exceed the total stream flow. Also, that the sum of the individual molar or weight fractions must equal 1. Hence, the composition of a stream is completely defined if all but one of the component concentrations are given.
š The component flows in a stream (or the quantities in a batch) are completely defined by any of the following:
Specifying the flow (or quantity) of each component.
Specifying the total flow (or quantity) and the compositions.
Specifying the flow (or quantity) of one component and the composition.
While solving this problem, you will understand it fully:
šExample 3:
The feed stream to a reactor contains: ethylene 16%, oxygen 9%, nitrogen 31 %, and hydrogen chloride. If the ethylene flow is 5000 kg/h, calculate the individual component flows and the total stream flow. All percentages are by weight.
šSolution:
Percentage HCl= 100-(16+9+31)=44
Percentage ethylene=(5000/total)*100=16
hence total flow =5000*( 100/16)= 31,250 kg/h
so, oxygen flow = (9/100)*31,250=2813 kg/h
nitrogen=31,250*(31/100)=9687 kg/h
hydrogen chloride= 31,250*(44/100)=13,750 kg/h
General rule š:
The ratio of the flow of any component to the flow of any other component is the same as the ratio of the compositions of the two components. The flow of any component in Example 3 could have been calculated directly from the ratio of the percentage to that of ethylene, and the ethylene flow.
Flow of hydrogen chloride= (44/16)*5000=13,750 kg/h
āļøWe stop here for today, tomorrow we will learn about Excess reagent/Limiting reagent, Purge, By-pass, General procedure for Material balance problems. That's why do not miss tomorrow's blogpost šš§®!
References š:
Coulson&Richardson, Chemical Engineering Design Volume 6
CHOPEY, N. P. (ed.)Handbook of Chemical Engineering Calculations (McGraw-Hill, 1984).
FELDER, R. M. and ROUSSEAU, R. W.Elementary Principles of Chemical Processes, 6th edn (Pearson, 1995).
HIMMELBLAU, D. M.Basic Principles and Calculations in Chemical Engineering (Prentice-Hall, 1982).
Additional Resources š§Ŗ
šYou can get deep insight about Process/Chemical Engineering from these sourcesš:
https://www.youtube.com/channel/UCqioh32NOJc8P7cPo3jHrbg- Piping Analysis
https://www.youtube.com/channel/UCQfMyugsjrVUWU0v_ZxQs2Q -Mechanics of engineered devices
http://chemicalengineeringguy.com/- suggests a wide range of courses in Chemical engineering (you can find free courses on topic of Aspen HYSYS, Aspen Plus)
https://www.youtube.com/user/LearnEngineeringTeam- suggests working principles of every engineered devices, equipment and etch.
https://www.youtube.com/channel/UCR0EfsRZIwA5TVDaQbTqwEQ- suggests great information about pumps, compressors with animation.
šToday we have already begun and learned about Fundamentals of Material Balances,compositions, number of independent and dependent variables, choice for basis of calculation, now time to say goodbyešš» until tomorrow and Stay tuned for more content ššāØ!
š¤Note: Example 1 and 2 are easy, thatās why, I have not included here, but you can easily google it. If you need one of those books or links, you can contact me via my email or LinkedIn profile.
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