Flow Diagrams and Material Balances

Different materials and procedures are used to execute each process; however, every operation offers an opportunity to track material flow. Figure 1 provides an extremely oversimplified material flow chart.

Before a waste reduction plan can be formulated, it is crucial to determine the amount of waste that the business is already producing. How? Perform a waste audit. What is a waste audit? It is the direct application of the law of conservation of matter taught in Physics or Chemistry 101: Matter is neither created nor destroyed. In other words, nothing ever disappears. Everything that was here on the first day this planet was created is still with us today, although it may be in a different form.

Since nothing ever disappears or goes "away," it must be somewhere. So to find out where the "matter" is going, a waste reduction team must perform a material balance. In this process, a quantitative relationship is established between raw materials, product, and waste. Material balances allow for quantifying many losses or emissions that were not previously recorded by any bookkeeping or inventory system.

In its simplest form, a material balance is represented by the equation

Mass in = mass out + mass accumulated








Figure 1 Simplified flow chart indicating process components to be included in a material balance study.

An independent material balance should be made for all components that enter and leave a process. When chemical reactions take place in a system, an "elemental balance" should be performed for specific chemical elements.

Material balances assist in determining concentrations of waste components where data are limited. They are particularly useful when there are points in a process where it is difficult (due to inaccessibility) or uneconomical to collect analytical data. A material balance can determine whether or not fugitive losses are occurring. For example, the evaporation of solvent from a cleaning tank can be estimated as the difference between solvent put into the tank and solvent removed from the tank.

Characterizing waste streams by material balance can require considerable effort; however, the result is a complete picture of a waste stream. This helps to establish a focus for waste reduction activities and provides a baseline for measuring performance.

1. Sources of Material Balance Information

Material balance includes materials entering and leaving a process. The following list exemplifies potential sources of material balance information:

Samples, analyses, and flow measurements of feedstocks, products, and waste streams Raw material purchase records Material inventories Emission inventories

Equipment cleaning and validation procedures

Batch make-up records

Product specifications

Design material balances

Production records

Operating logs

Standard operating procedures and operating manuals Waste manifests

Material balances are easier, more meaningful, and more accurate when done for individual units, operations, or processes. It is important to define a material balance envelope properly. An envelope should be drawn around a specific area of concern rather than a larger group of areas or an entire facility. An overall material balance for a facility can then be constructed from individual unit material balances. This highlights relationships between units and helps to point out areas for waste reduction.

Several factors must be considered when preparing material balances in order to avoid errors that could significantly overstate or understate the waste streams. The precision of analytical data flow measurements may not allow an accurate measure of a stream. In processes with very large inlet and outlet streams, the absolute error in measurement of these quantities may be greater in magnitude than the waste stream itself. Whenever a reliable estimate of a waste stream cannot be obtained by simply subtracting the quantity of material in a product from that in a feed, the team must be creative in developing a more advanced method of quantitative analysis.

Time span is important when constructing a material balance. Material balances performed over the duration of a complete production run are typically the easiest to construct and are reasonably accurate. Time duration also affects the use of raw material purchasing records and on-site inventories for calculating input material quantities. The quantities of materials purchased during a specific time period may not necessarily equal the quantity of materials used in production during the same time period. Purchased materials can accumulate in warehouses or stockyards.

Developing material balances around complex processes can be a complicated undertaking, especialy if recycle streams are present. Such tasks are generally performed by chemical engineers, often with the assistance of computerized process simulators.

Material balances will often be needed to comply with SARA Title III reporting requirements to establish emission inventories for specific toxic chemicals. EPA's Office of Toxic Substances has prepared a guidance manual entitled Estimating Releases and Waste Treatment Efficiencies for the Toxic Chemicals Inventory Form (EPA 560/4-88-02). This manual contains additional information on developing material balances for listed toxic chemicals [11].

2. Tracking Wastes

By tracking wastes on a regular basis, seasonal variations in waste flows or single-batch waste streams are distinguished from continual, constant flows. Changes in waste generation cannot be meaningfully measured unless the information is collected both before and after a waste reduction option is implemented. Fortunately, it is easier to do material balances the second time, and it continues to become easier as more are completed. It is relatively simple and inexpensive, with today's computer software, to use computerized database systems to track and monitor waste streams.

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