Further Reading: The Concept of Balances Let’s explore this general concept in more detail as it underlies so much of fundamental chemical engineering analysis, thinking, and problem solving. Assuming that we have the appropriate understanding of the fundamentals of our reaction and the physical properties of our materials, we can begin to think about the design of a process to manufacture the materials of interest. There are two fundamental concepts that must be understood and taken into account into the design and operation of any process: mass or material balance and system boundary. 1) Mass (material) Balance Concepts First, (material) mass must be conserved. Put another way, what comes out of a process must be what we put into it plus whatever mass is accumulating within a system. Chemical Engineering Progress and the Beacon publication from AIChE’s Centre for Chemical Process Safety have reported many cases of major fire and environmental disasters via the simple mechanism of overflowing tanks. The most significant of these was the Flixborough disaster (Figure 1), summarized in a Chemical Engineering Progress “Beacon” article (9/2006, p. 17). Faulty instrumentation, an inadequate number of layers of protection, and insufficient communication have all been causes of major environmental and fire disasters.
Figure 1: Incident Summary: Flixborough Case History, AIChE, 2013. All of the process control instrumentation we use in actual product processes are designed to calculate balances but a number of things can happen that can cause surprises: 1) A reaction, with a very slow kinetic rate constant (more later), may be forming a material in the process that was unanticipated. Depending upon the downstream processing and how the anticipated products are handled (distillation, filtration, crystallization, etc.), this material may “bleed” out of the system after building up to a certain level. So eventually the mass balance will close, but in a short or intermediate time frame, it may not. 2) An unanticipated liquid reaction product, with a boiling point, which inter- feres with a distillation separation downstream, can cause a temporary build-up of material.