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Material Balances involving Unsteady-State Process

In previous posts, I shared how to calculate material balances of several cases. Those calculation involve steady-state condition. In steady-state process, the accumulation is zero. The streams flowrate and composition are also did not vary with time. The calculation are more complex if these conditions are not met. The unsteady-state behaviour of a process is important when considering the process start-up and shut-down, and the response to process upsets.

Unsteady-state material balances are solved by setting up balances over a small increment of time, which results in a set of differential equations that describe the process. These equations can be solved analytically for simple problems. Computer-based methods would be employed for more difficult problems.

Example below is an example of general approach used to solve un-steady state problem. Read More

Material Balances Calculation Involving Purge Process

Bleeding off a portion of the recycle stream is important in a production process to avoid the accumulation of undesirable material. For instance, if inert components from a reactor feed are not separated from the recycling stream in the separation units, they would build up in the recycle stream until they made up the whole stream. To keep the inert level within reasonable level, some of the stream would need to be purged. Normally, a continuous purge would be employed. Assuming steady-state circumstances:

Loss of inert in the purge = Rate of feed of inerts into the system

The concentration of any component in the purge stream will be the same as that in the recycle stream at the point where the purge is taken off. Consequently, the relationship shown below can be used to calculate the necessary purge rate:

[Feed stream flowrate] × [Feed stream inert concentration] =

[Purge stream flowrate] × [Specified (desired) recycle inert concentration]

Let’s see example below. Read More

Material Balance for Process Involving Recycle

Recycle process is process in which a flow stream is returned to an earlier stage in the processing stage. The process is frequently used in the production facility. If the conversion of valuable reactant is considerably less than 100%, the unreacted material is usually separated and recycled. An example of a recycle process in which there is no reaction is the return of the reflux to the top of distillation column.

What do you think about the existence of recycle in material balance calculation?

In my opinion, recycle makes material balance calculation is more difficult. In this post, we will learn material balance for process involving recycle. I hope it will give you a little insight.

Without recycle process, the material balances are simple. The material balance can be carried out in serial, in which the calculated flows out of one unit become the feeds to the next. But if a recycle stream is present, then at the point when the recycle is returned, the flow will not be known as it will depend on downstream flows not yet calculated. Without knowing the recycle flow, the sequence of calculations cannot be continued to the point where recycle flow can be determined. Read More

Difference between Conversion and Yield

It is important to differentiate between conversion and yield. Sometimes, these terms are mixed up. So, in this post, I want to share the difference between conversion and yield. I also will show you three examples of these terms.

Conversion is to do with reactants (reagents). On the other hand, yield is to do with products.


Conversion is a measure of the fraction of the reagent (reactant) that reacts. The conversion of a particular reagent is often less than 100% to optimize reactor design and to minimize formation of by-product.

If more than one reactant is used, the reagent on which the conversion is based must be specified.

Conversion is defined by the following equation: Read More

How to Calculate Percentage of Excess Air in Combustion Process

In this post, I want to share how to calculate percentage of excess air flow in combustion process. The known data are:

  1. Fuel composition
  2. Flue gas composition

Before we jump into the calculation, we need to understand about excess air and why we need it.

To ensure complete combustion of the fuel used combustion chambers are supplied with excess air. Excess air increases the amount of oxygen to the combustion and the combustion of fuel. When fuel and oxygen from the air are in perfect balance – the combustion is said to be stoichiometric.

The combustion efficiency increases with increased excess air – until the heat loss in the excess air is larger than the heat provided by more efficient combustion.

Let’s see example below. Read More

Material Balances Calculation Approach using System Boundary

In this post, I want to share how to calculate material balances using the system boundary approach. This method is especially useful when dealing with a process configuration that involves several types of equipment with a different function, such as reaction, filtration, drying, and recycling.

How to do it?

We need to divide the overall process into several process sub-division or system boundaries. The flows into and out of the system are those crossing the sub-division and must balance with material generated or consumed within the boundary.

There are no rigid rules to use this approach. Selection of the best sub-division for any process is from our judgement and by practice a lot. However, these general rules can be used as a guide to calculate material balance using the system boundary approach:

  1. For complex processes, first thing to do is to take overall complete process and if possible, calculate the flows in and out. Raw materials in, products, and by-products out.
  2. Select the boundaries to sub-divide the process into simple stages and make a balance over each stage separately.
  3. Select the boundary round any stage to reduce the number of unknown streams to as few as possible.
  4. Include any recycle streams within the system boundary.

Let’s see example below to practice. Read More

Basis of Material Balances

In this post, I want to share the basis of material balances. Material balances are the basis of design. The amount of raw materials needed and the amount of product produced will be determined by a material balance applied to the entire process. In each process unit, there will be specific stream flows and compositions.

Process design requires a solid understanding of material balance calculation.

As you may experience as a process engineer, material balances are always key to the process. In almost every project, material balance calculation is always required. It is because equipment design and sizing need material balances. Utility consumption calculation is also generated from material balance information. Read More