Preliminary material balance

Now you can reconsider the material balance equations by adding those additional factors identified in the previous step. If necessary, estimates of unaccountable losses will have to be calculated. Note that, in the case of a relatively simple manufacturing plant, preparation of a preliminary material-balance system and its refinement (Steps 14 and 15) can usefully be combined. For more-complex P2 assessments, however, two separate steps are likely to be more appropriate. An important rule to remember is that the inputs should ideally equal the outputs - but in practice this will rarely be the case. Some judgment will be required to determine what level of accuracy is acceptable, and we should have an idea as to what the unlikely sources of errors are (e.g., evaporative losses from outside holding ponds may be a materials loss we cannot accurately account for). In the case of high concentrations of hazardous wastes, accurate measurements are needed to develop cost-effective waste-reduction options. It is possible that the material balance for a number of unit operations will need to be repeated. Again, continue to review, refine, and, where necessary, expand your database. The compilation of accurate and comprehensive data is essential for a successful P2 audit and subsequent waste-reduction action plan. Remember - you can t reduce what you don t know is therel... 

Note Problems 2.5 to 2.8 can be taken together as an exercise in the calculation of a preliminary material balance for the manufacture of aniline by the process described in detail in Appendix F, Problem F.8. 

It is not necessary to specify the reflux when calculating a preliminary material balance the system boundary can be drawn to include the reflux condenser. 

The preliminary material balance sets the limits for the variables of significance for process performance, namely the minimum material consumption and the maximum yield in products, and from this viewpoint is a kind of ideal material balance. 

Gather all the available information on one of the ethylene processes for which a flow sheet was prepared in the preceding problem and make a preliminary material balance for the production of 50 million lb/yr of ethylene. Assume an operating factor of 90 percent. 

Preliminary material balances and engineering designs were used to analyze each potential option. For each option the following items were determined ... 

Prepare a preliminary material balance. A material balance is designed to provide better understanding of the inputs and outputs, especially waste, of a unit operahon such that areas where informahon is inaccurate or lacking can be identilied. The inihal balance should be considered as a rough assessment that must be further refined and improved. 

Instrumentation Calibration may be required for the instruments installed in the field. This is typically the job of an instrument mechanic. Orifice plates should be inspected for physical condition and suitability. Where necessary, they should be replaced. Pressure and flow instruments should be zeroed. A preliminary material balance developed as part of the preliminary test will assist in identifying flow meters that provide erroneous measurements and indicating missing flow-measurement points. 

The Shortcut Method. The shortcut calculation is carried out as follows First, a preliminary material balance is set up on the assumption that no methane appears in the bottoms and no propone or heavier components appear in the distillate. The balance, in Ib-moie/h is as follows ... 

Occasionally, a situation will arise where the key components are not adjacent when ranked according to volatility. In this case, components lying between the keys will appear in substantial quantities in both product streams and, for this reason, are called distributed components. In setting up the preliminary material balance, the designer must estimate the quantitative presence of these components in the products. This preliminary assumption will later be checked by the techniques described in a later portion of this procedure. 

The design of most natural gas processing plants is based on the recovery of a certain component, usually either propane or ethane. Since both the absorber exit gas and the ROD overhead gas contain the base component as the heavy key, the design material balance of those two systems must be interdependent. For example, in setting up the preliminary material balance for a gas plant based on 70 percent propane recovery, the absorber must overabsorb so that there is enough extra propane in the ROD feed to permit a reasonably low lean oil rate to the latter tower. The optimum absorption will result in the lowest total lean oil rate to the absorber and the ROD combined. As a rule-of-thumb, the absorption section of the ROD will have to absorb a minimum of 95 percent of the heavy key vapor feed, including the internal vapor. 

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