Chemical engineering: systems, processes & devices

Similar to the mechanical engineering textbook, the chemical engineering author (Sandler, 2006) addresses both open and closed systems (although the term control volume for open systems is not used by Sandler).

Sandler (2006) recognises the use of open systems as peculiar to engineering and one of the fundamental differences between thermodynamics as a subject in science and in engineering. He expresses a concern that engineering students are often exposed to thermodynamics in different disciplines (he mentions physics and chemistry) in undergraduate curricula. He believes the resulting redundancy is not beneficial, and that the different disciplinary emphases result in confusion for students. He is so concerned about this that he addresses engineering students specifically in the Preface to his textbook, and suggests that they should “forget what they have been taught about thermodynamics elsewhere” (p. ix), because the non-engineering courses only consider closed systems, whereas engineering applications need to also deal with open systems where mass flows into and out of systems. Typical examples in chemical

97 engineering are the work obtained from a fluid (steam) that flows across the large pressure drop in a hydroelectric power generation system, or a compressed-air tank being repressurised by connecting it to a high pressure air line.

Sandler points out that there are two broad types of problems of interest in chemical

engineering. The first of these, energy flow type problems, are similar to problems in mechanical engineering: calculating the energy changes associated with the flow of heat, work or mass across system boundaries. Mechanical and chemical engineering specialise towards different particulars though, and this is evident in the textbook. As can be expected, chemical reactions, the production (and rate of production) of chemical species are more important in chemical engineering than in mechanical engineering. Mass balance equations are prominent in Sandler’s chemical engineering textbook, also in the “rate-of-change” and “difference” forms of the

equations. The rate at which mass changes in a system, and integration of these changes over a period of time to calculate the total change in mass (or preferably the change in the number of moles) of chemical entities is important in chemical engineering.

The second type of problem Sandler regards as important in chemical engineering is equilibrium type problems, and these are not emphasised in mechanical engineering. The equilibrium state is described as the inevitable result when “a system is not subjected to the continual forced flow of mass, heat or work” (Sandler, 2006, p. 5). Equilibrium is a “time-invariant state in which there are no internal or external flows of heat or mass and no change in composition as a result of chemical or biochemical reactions” (p.5). These fairly general definitions and descriptions are contextualised in the typical problems under consideration in chemical engineering practice: predicting the nature of the equilibrium conditions that a system evolves to from non-

equilibrium (an example is predicting the final temperatures and pressures in gas cylinders if the valve between an empty and a filled cylinder is opened). Here the filling, mixing and leaking of gas in and from cylinders are examples of typical engineering processes and devices that chemical engineers use.

Another example of the kind of specialisation to particulars present in the chemical engineering textbook can be seen in Sandler’s distinction between a state of equilibrium and a steady state. Sandler distinguishes between “natural flow” and “forced flow” (p. 8), or “pressure induced energy flow” (p.49). It is possible for an open system with spontaneous mass, heat or work flows to reach an equilibrium state. Equilibrium states are defined by the fact that the characteristics of the system do not vary with time, that there are no internal temperature, pressure, velocity or concentration gradients in the system, that there are no net transfers of heat, mass or work between the system and surroundings, and that the net rate of chemical

98 FIGURE 2.4

Two subsystems, I and II, are enclosed in a rigid adiabatic enclosure. System I consists solely of the liquid in the beaker for each case. System II consists of everything else in the enclosure, and is the surroundings for system 1. (a) The liquid is heated using a flame. (b) The liquid is heated using a resistive coil, through which an electric current flows.

reactions is zero. However, if the surroundings impose a mass flow on the system by the action of a pump, or a temperature flow by exposing different parts of the system to different

temperatures, it is still possible that the system could reach a time-invariant state. Since this results in a temperature or pressure gradient in the system (even one that remains constant over time), this would not be equilibrium, but a “steady state” (p.8). This differentiation in the chemical engineering text between equilibrium and steady state speaks very clearly to the particulars that are characteristic in chemical engineering practice.

The emphasis on open systems in addition to closed systems in the chemical engineering textbook, together with the type of processes and devices discussed in relation to the systems, indicate the commitment to the particulars of the field of practice in which chemical engineers work.

The knowledge is neither idealised nor normative, and therefore no secondary modality is present.