Material and Energy Balance Calculations Guide

Process Mass and Energy Balances

Once a flow diagram has been prepared, the next step is to calculate the material and energy balances. The following worksheets provide a convenient means for displaying the results of mass and energy balance calculations for each unit operation. Also the basic assumptions and idealizations use in developing the balances can be recorded and displayed.

Material and energy balance calculations effectively summarize and present basic design data.

Supporting Calculations

Calculation sets are prepared to clarify the basis of values given in the material and energy balance data tables. The following is a guide to the preparation of material and energy balance calculations and supporting documentation. Depending upon the number of assumptions, the effect of company policies, and the amount of empirical process development, calculations may require more or less detail.

Procedure

The desired product and/or intermediate quantities are first set down as the goals for the overall plant material and energy balance calculations. Research, development, and operating data are employed in working backward from the desired quantity of product to the raw material input to be considered as yields, conversions, equipment efficiencies, recycle streams, etc. Apparently inconsequential side streams, such as drain lines, waste streams, rejected charges, and spills should not be overlooked. After establishing preliminary figures for the plant balance, area material balance flow diagrams are developed. Adjustments are made later to the plant balance flow diagram as required.

The utility tables are typically not included for conceptual or initial process flow diagrams. The heat loads and the utility quantities are completed later by the process design engineers and specialist design engineers as part of the preliminary design.

By so developing the balances, advantage is taken of each succeeding degree of specialization. Each person is then given the greatest latitude within their field for economical equipment selection, consistent with process requirements.

Supporting Calculations

Generally, only those calculations need be included in the reported material and energy balance reports which involve assumptions or which cannot be readily reproduced by an experienced engineer. Thus, routine calculations may be omitted. The following shows the character of desired calculations and, for the sake of brevity, they cover only portions of the reference flow diagrams.

Overall Plant Balance Calculations

No sample calculations need be shown for the overall plant balance flow diagram since the representative calculations for the other flow diagrams will illustrate nearly all the important points. Nevertheless, it is of prime importance that the results of the overall plant balance calculations be included.

Continuous Process Calculations

An important part of supporting calculations on continuous processes is the development of the flow diagram basis.

Batch Process Calculations

Calculations for batch processes need to cover unusual conditions which are not evident from the flow diagram. Filter-press cleaning cycles, filtering rates, etc., as they affect the specification of equipment, are good examples of information useful to the design engineer in providing adequate operating facilities.

Computer Flow Diagram Analysis

Computer based material and energy balance procedures such as HYSYS, ASPEN and FLOWTRAN should be used where available. They can greatly reduce the amount of detail calculations necessary in preparing the material and energy balances. However, particular care must be exercised in checking that all results are correct.

Discussion

Complicated qualifications should be expanded in a process description or the supporting calculations. For processes that are in the bench to pilot plant scale of development, a great number of processing and equipment performance assumptions will necessarily be used. Some assumptions are easily obtained from similar unit operations of commercial processes. Other assumptions must be regarded as speculative and subject to engineering judgments. All assumptions and design practices should be fully documented. This will allow an assessment of their impact on the overall economics.

A discussion of how rigorous or how inflexible the assumption might be and which factors are limiting permits the design engineer to take advantage of research and operating know-how.