What is basic chemical calculation?

A basic chemical calculation involves the application of the law of chemical proportions to various reactions and material and energy balance calculations on various chemical processes. Basic chemical calculation helps students analyze various chemical processes and develop problem-solving skills. The chemical manufacturing processes are divided into unit operations involving physical changes of material and unit processes involving the chemical transformation of materials. For instance, fluid flow, heat transfer, and mass transfer are unit operations, and oxidation, reduction, and pyrolysis are unit processes.

Processes

In chemical engineering, a process means an operation or series of operations where the input material will be undergoing physical, chemical, or biological changes and the output will be the product with desired characteristics.

Process variables

The variables that characterize the processes, their streams, and materials are called process variables. They give the amount, composition, and conditions of the process streams and materials entering and leaving the process units. Process variables are as follows:

Mass and volume

The mass of material is the amount of matter it contains. The volume of a substance is the amount of space it occupies. The mass per unit volume of a substance is called Density. The SI (International System of units) unit of density is kg/m³. The volume occupied by the unit mass of a substance is called Specific Volume. It is the inverse of density and its unit is m³/kg. The density of a pure substance varies with temperature but is independent of pressure.

The specific gravity (SG) of a substance is defined as the ratio of the density of substance (ρ) to the density of the reference substance (ρ_ref). The reference substance used for solid and liquid is water at 4^oC. The density of water at 4^oC is 1000 kg/m³ or 1g/cm³.

SG = ρ/ρ_ref

Mass and Volumetric flowrate

The flow of material in and out of the process is an integral part of the chemical process. The rate at which the material flow is referred to as the flow rate of the substance. The flow rate of a stream can be mainly of two types mass flow rate and volumetric flow rate. The mass flow rate is the amount of fluid passing through a given cross-section in unit time or mass/time (kg/s). The volumetric flow rate of fluid is the volume of fluid passing through a given cross-section in unit time (m³/s).

Chemical composition

Most of the materials we use in the chemical processes are a mixture of various substances. Hence the physical properties of such mixtures depend on the composition. The composition of mixtures is mainly expressed in mass and mole fractions

Mass Fraction: The mass fraction of a component in a mixture is defined as the ratio of the mass of that component to the total mass of the mixture. Hence, for a mixture containing components A and B the

Mass fraction of A= mass of A / (mass of A+ mass of B)

Mass percent is the mass fraction multiplied by 100.

Mole Fraction: The mole fraction of a component in a mixture is the ratio of moles of that component to the total moles in the mixture.

Mole fraction of A= moles of A / (moles of A+ moles of B)

Mole percent is the mole fraction multiplied by 100.

Pressure

Pressure is defined as the force acting per unit area of the surface. The pressure exerted by the atmosphere is called atmospheric pressure and at the sea level, it is 1.01325 ×10⁵ N/m² or 760 mmHg. The pressure measured by instruments gives the difference between the pressure in the system and atmospheric pressure which is referred to as gauge pressure. The absolute pressure in the system is thus given by -

Absolute pressure = Gauge pressure + atmospheric pressure

The pressure below atmospheric pressure is reported as vacuum pressure and the absolute pressure is -

Absolute pressure = atmospheric pressure - vacuum pressure

Temperature

Temperature is the measurement of the thermal energy of molecules that are at random motion in a substance. The different temperature scales are developed based on the freezing and boiling temperature of water at 1 atm. They are Fahrenheit Scale, Kelvin Scale, and Celsius scale, and the temperature unit in these scales are ⁰F, $K$, and  ⁰C respectively.

The following relations can be used to convert temperature on one scale to another.

T(K)=T(⁰C)+273.15

T( ⁰F)= 1.8 T(⁰C)+32

Processes

Chemical processes are the processes that convert the given material/feed to the desired product using a series of processes where the substance undergoes a physical and chemical change. The processes can be batch, continuous or semi-batch. Batch processes are used when less product is required. For large production capacity, continuous processes are used in industries.

Batch process

In the batch process, the reactants are fed to the reactor or processing unit initially and the product is withdrawn from the unit after the reaction or process is complete. Alcohol fermentation is an example of the batch process.

Continuous process

In a continuous process, the materials flow in continuously to the processing units or rectors, and the product is withdrawn continuously from the system. For example, the liquid mixture to be separated is continuously given to the distillation column and the pure distillate is continuously withdrawn.

Semi batch process

In the semi-batch process, the product is intermittently or continuously added or removed. For instance, reactants A and B are reacting in a semi-batch reactor where A is continuously fed to the reactor containing B, and the product is withdrawn at the end of the reaction.

Steady and Unsteady state processes

If the variables in a process like temperature, pressure, flowrate, etc. do not change with time, such processes are steady state processes. When any of the processes’ variables change with time, such processes are called unsteady state or transient processes. Both batch and semi-batch processes are transient processes.

Balances on steady state continuous processes

The general material and energy balances on steady state continuous processes are

Input + generation = output + consumption

The basic chemical calculations involve the material and energy balance of unit operations in chemical engineering. Some of the unit operations are: 

Evaporation

Evaporation is used to concentrate the aqueous solution by vaporizing water. Here the feed solution is passed to the evaporator and heated using steam in the steam chest. The thick liquor or concentrated product leaves from the bottom of the evaporator.

Crystallization

In this operation, solid crystals are produced by evaporating and cooling the saturated solution in a crystallizer. It is the main method of purification of materials like sugar. When the saturated solution is subjected to evaporation it becomes concentrated. This solution is cooled so that the concentration of solute exceeds solubility at that temperature and solute crystallizes.

Distillation

Distillation is used to separate the aqueous mixture with different volatilities or different boiling points. The continuous distillation is carried out using a fractionation column where feed is separated into distillate which is the top product and residue which is the bottom product. A part of the distillate is returned to the column as reflux to purify the distillate.

The material balance calculation helps to calculate the quantities or flow rate of various steams involved in the process.

Context and Applications

Understanding chemical processes and process variables are the basic steps in material and energy balance calculation in chemical engineering. While applying chemical engineering principles to design, operate and control various processes in the chemical industry, the knowledge of process variables and their effect on other processes is very important. This topic is useful in following graduate and post graduate courses:

• Bachelor in Chemical Engineering
• Master in Chemical Engineering

Practice Problems

1. The pressure inside an evaporator is maintained at 500 mmHg. Identify the absolute pressure in mmHg from the following:

1. 260 mmHg
2. 200 mmHg
3. 1260mmHg
4. 760 mmHg

Answer: Option a

Explanation: Absolute pressure = atmospheric pressure - vacuum pressure

Atmospheric pressure is 760 mmHg.

Absolute pressure = 760 - 500 = 260 mmHg

2. Classify the process of boiling water in an open vessel among the following:

1. Batch
2. Semi batch
3. Continuous
4. None of the above

Answer: Option b

Explanation: In a semi-batch process, a small amount of water evaporates when heated.

3. The mass flow rate of hexane (density = 0.65 g/cm³) in a pipe is 6.5 g/s. Calculate the volumetric flow rate.

1. 10 cm³/s
2. 1 cm³/s
3. 100 cm³/s
4. 5 cm³/s

Answer: Option a

Explanation: Volumetric flowrate = mass flow rate / density

Volumetric flowrate = 6.5/0.65

Volumetric flowrate = 10 cm³/s.

4. 10 moles of mixture of gases SO₂, CO₂ and N₂ contain 5 moles of N₂. Calculate the sum of mole fraction of  SO₂ and CO₂.

1. 1
2. 0.5
3. 4
4. 5

Answer: Option b

Solution: Total mole fraction of mixture=1

Mole fraction of nitrogen$=\frac{5}{10}=0.5$

The sum of mole fraction of SO₂ and CO₂= 1-0.5 =0.5

5. A mixture contain 10 g of NaCl and 10g sand. What is the mass fraction of NaCl in the mixture?

1. 1
2. 0.5
3. 4
4. 6

Answer: Option b

Solution: Mass fraction of NaCl$= \frac{Mass of NaCl}{Total mass}= \frac{10}{20}=0.5$

Related Concepts

• Material and energy balances
• Stoichiometry
• Unit operations and processes
• Reaction engineering