Chemical Engineering: The Two Types Of Balances

Balances are there are basic physical entities that can be conserved in a given process: material and energy. The conservation law states that these quantity can neither be created nor destroyed except in nuclear reactions. Hence, these quantities can be accounted for.

4.2 Balances

A Balance is a method of accounting or taking inventory of something (material or energy). In this chapter emphasis will be given to material. Example, the general material balance simply accounts for where things come and go, and how their total number (or amount) changes. This led to the general balance equation described in 4.2.1.

4.2.1 General balance equation

The general balance equation is a fundamental concept of chemical engineering which is based

The GENERATION and CONSUMPTION accounted for changes within the system as a result of chemical reactions. Equation (4.1) is used to describe and design almost every process unit in chemical engineering.

4.2.2 Types of balances

Basically there are two types of balances: (i) Differential balances and (ii) Integral balances.

4.2.2a Differential balance

A differential balance is the type of balance that describe what is happening in a system at one particular instant of time. It deals with rates (for mass balances: mass per time [kg/s]). This type of balance is best suited for continuous processes and may be written more precisely in mathematical form as:

dM/dt=M ̇_in-M ̇_out+G-C … (4.2)

where: dM/dt is the rate of change of material or rate of accumulation of material, M; M ̇_in and M ̇_out are the flow rates of materials into and out of the system respectively; G=rate of generation of material M; C= rate of consumption of material M. Note that the over-dots denote flow rates and all terms are in units of mass/time.

Table 4.1: Special cases of Equation (4.2)

Condition Modified form of Eq.

For example, mass balance theory is used to design unit processes such as chemical reactors, distillation columns, absorbers, heat exchangers, to analyze alternative processes to produce chemicals, as well as to model pollution dispersion and other physical processes or physical systems. Closely related and complementary analysis techniques include the population balance, energy balance and the somewhat more complex entropy balance. These techniques are required for thorough design and analysis of systems such as process plants, the refrigeration cycle