PSI Explained: Material and Energy Balances

Title 8 CCR §5189(d)(3)(A)(7) and Title 29 CFR §1910.119(d)(3)(i)(G) require that “information pertaining to the equipment in the process shall include: Material and energy balances for processes built after May 26, 1992”. The same requirement exists in the RMP and CalARP regulation except that the applicability date is June 21, 1999 (Title 40 CFR §68.65(d)(1)(vii) and Title 19 CCR §2760.1(d)(1)(G)).

Material Balance

A material balance is an assessment of the material (mass) input into a process compared to the material output. This is particularly important for processes that involve reactions in which one or more of the chemicals is consumed. In this type of process, a material balance must summarize the names and quantities of the chemicals that are entering the process compared to the names and quantities of the chemicals/products produced by (or leaving) the process.

Certain processes exist for the purpose of storing a hazardous chemical and then distributing that chemical as needed. For example, a fertilizer distribution business may have a large anhydrous ammonia storage tank which is filled by a vendor for storage onsite. The business uses the large storage vessel to fill smaller shipping and/or application containers. In this example, the material balance would clearly document the rate at which ammonia is added to and removed from the process storage tank.

Other processes use hazardous chemicals in a closed loop system where the chemical is not added to or removed from the process. Ammonia refrigeration is an example of this type of process. Here, the system material balance becomes trivial since the overall quantity of ammonia in the process is not changing over time. Material balances can be performed on select components (e.g. compressors, condensers, evaporators, vessels, etc.), but this type of analysis is limited to the assumptions made and represent a hypothetical flow of ammonia through the process at a given load pattern. The Industrial Refrigeration Consortium prepared a detailed explanation of this approach in Volume 15, No. 4, 2015 edition of The Cold Front.

Energy Balance

Whereas a material balance compares the material input to output for a given process, an energy balance summarizes all energy input vs. energy output for a given process. In an ammonia refrigeration system, this is accomplished by examining the sources of ammonia energy input and output:

Energy Input

  • Compressor – Heat of Compression
  • Evaporator – Refrigeration Load

Energy Output

  • Condenser – Heat of Rejection

If the energy input from the evaporators and compressors exceeds the energy output of the condensers, the result will be poor system performance, high discharge pressure, or both.

Typically, both material and energy balances are analyzed and documented by an engineer familiar with the chemical process. Spreadsheets are often employed to assist with calculations associated with the analysis.

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