Process Plant Design: Material and energy balance in process plant design

Once the literature survey in done and the specific objectives and the plant size has been determined from demand study, the next step is to do the mass and energy balance. This is a crutial step as it affects the sizing and thus the economics of the plant.

In this blog, I will revise some principles and state how to do balances on filters, distillation columns, kilns and so on. Soon I will add more posts on solved energy and material balances on some key processes. Please subscribe to get email updates on the coming posts which you shouldn´t miss.

The market demand study and raw material availability studies give us the proper size of the manufacturing plant. The size of the plant should balance the raw material availability and the market demand. Based on this, the material and energy balances are done.

Using the information form demand study determine the capacity of the plant in terms of total annual production or total raw material requirement. This leads to the following ways of making balances.

There are two approaches for performing material and energy balances; upstream and downstream ways as known informally. In the upstream the balance is started from the last

unit in the process. Final product amount, quality, yield and equipment efficiency are mainly used to do the balance. In the downstream way, the initial raw material amount is determined to arrive at desired production capacity. This could be possible if the overall yield of the process in tons of product per tons of raw materials is known.

While writing material balance, the best format is to first present the overall or general material balance and second the component balance. Show clearly the total flow rate and composition of the known streams and represent the unknown stream with appropriate labels. At the end of each section prepare summary tables with columns input, output and remark. In the remark write balanced, or state the accumulated, generated or lost amount.

Balances for units with no change in mass like storage tanks are not done and it is acceptable. Electrical energy required for running units are not usually included in the balances. This is partly because their rating in only known in the sizing and at the end it is included in calculation of utility costs as fraction of total costs.

Choose the reference temperature to be the ambient conditions for energy balances. Always study the detail process steps before calculating the total energy requirement. Spray driers, kilns, coolers and combustion devices are included in this category and are always challenging. The common mistake is considering only sensible heat changes. The above units combine sensible, latent heat and super heating. Besides the processes are of simultaneous mass and heat transfer.

If you are dealing with mixtures in energy balance stream the overall physical properties like density, specific heat capacity and so on may be not found in tables. In this case they should be calculated based on the methods discussed in Coluson and Richardson, chemical engineering, Vol 6.

Balances of size reduction machines

Crushers, grinders and milling machines need mass balances in principle. Usually the dust loss and the recycled coarse fraction are ignored. This is not accurate and the best way to do this balance is to consider the grinder alone then balances for dust recovery cyclones (high efficiency or high throughput). If the control volume include the dust recovery system, it is possible to neglect the dust loss lost but still there is some loss. Wet grinding avoids the need for dust recovery systems. Since water is added during the crushing process the material balance should include this amount.

Energy balance for this units determines the energy required for crushing( Bond´s Law). But this is not relevant from energy balance point of view and is needed if it is designed in detail. This is because the energy required is usually calculated indirectly as utility cost. For more details see references 1 and 3.

Balances on filters

The filtration objective, whether the cake or the liquid is the final product, determines the type of filtration to be used and operating pressure. Overall balance on filters requires the calculation of the accumulated cake amount. This calculation intern requires pre-hand determination of the filter area, cake properties and operating pressures. Further complexity arises when the rate of filtrate flow should be determined for subsequent balances. This means the filter design and operation should be known pre-hand.

The best way to approach this problem is first to seek for general guides on operations of filtration units. See reference 11, which contains design guides such as cake properties of some common materials and performance data of filters. available.

Balances on spray driers

In spray driers hot air at given humidity and temperature absorbs moisture from wet substance and at the same time the heat is transferred. Hence both mass balance and energy balance equations should be used. The heat transferred raised the temperature of the dry air and the moisture. One typical problem is the final humidity of the exit air. It should be assumed to calculate the outlet air mass flow and solve the energy balance equations. Therefore some iteration is required to arrive at final values. For more details see reference 7.

Balances on kilns

Cement kilns balances are complex. The feed materials contain surface water and inherent water. The surface water undergoes sensible heat change and latent heat change at lower temperature. The inherent water in clay for instance, is evaporated in the kiln zone where temperature around 600C. Sensible heat change of every feed material should be considered. Besides separate balances of the primary and secondary kiln air should be done. Reaction energy of clinker formation reactions should be calculated. This calculation requires the knowledge of heat of formation of the kiln reactions at standard temperature and then at the kiln temperature. There is also considerable kiln gasses released as loss.

Useful references

1. Nicolas et al,2003, Hand book of chemical engineering calculations, McGraw Hill

2. Reynolds et al, 2002, Handbook of chemical and environmental engineering calculations,

3. Mc Cabe, Unit operation of chemical engineering, 3rd ed, Mc Graw Hill

4. Fundamentals of chemical engineering text book

5. Seader et al, 2^nd ed, Separation process principles, Willey

6. Chrome balance in leather processing http://www.4shared.com/document/2CzAlvHr/Chrome_balance_in_Leather_Proc.htm

7. Spray drier balances, G V Barbosa et al, 1997, Food engineering lab manual,

8. Mass balances in leather processing