The System and Its Environment

In thermodynamics, we distinguish between the system and its environment. The system is that part of the whole that takes our special interest and that we wish to study. This may be the contents of a reactor or a separation column or a certain amount of mass in a closed vessel. We define what is included in the system. The space outside the chosen system or, more often, a relevant selected part of it with defined properties, is defined as the environment. 

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Process Systems. Because of the large number of variables required to characterize the state, a process is often conceptually broken down into a number of subsystems which may or may not be based on the physical boundaries of equipment. Generally, the definition of a system requires both definition of the system s boundaries, ie, what is part of the system and what is part of the system s surroundings and knowledge of the interactions between the system and its environment, including other systems and subsystems. The system s state is governed by a set of appHcable laws supplemented by empirical relationships. These laws and relationships characterize how the system s state is affected by external and internal conditions. Because conditions vary with time, the control of a process system involves the consideration of the system s transient behavior. 

A system is a set of elements inter-relating in a structured way. The elements are perceived as a whole with a common purpose. A system s behavior cannot be predicted simply by analysis of its individual elements. The properties of a system emerge from the interaction of its elements and are distinct from their properties as separate pieces. The behavior of the system results from the interaction of the elements and between the system and its environment (system + environment = a larger system). The definition of the elements and the setting of the system boundaries are subjective actions. 

If one may consider the environment of a system as a stmcturally plastic system, the system and its environment must then be located in the intricate history of their stnictural transformations, where each one selects the trajectory of the other one. 

We thus see that nonequilibrium may be a source of order. Two kinds of situations and two kinds of order must then be distinguished. In the first place, we have situations that correspond to slight deviations from an equilibrium state between the system and its environment. In that case... 

When Amundson taught the graduate course in mathematics for chemical engineering, he always insisted that all boundary conditions arise from nature. He meant, I think, that a lot of simplification and imagination goes into the model itself, but the boundary conditions have to mirror the links between the system and its environment very faithfully. Thus if we have no doubt that the feed does get into the reactor, then we must have a condition that ensures this in the model. We probably do not wish to model the hydrodynamics of the entrance region, but the inlet must be an inlet. One merit of the wave model we have looked at briefly is that both boundary conditions apply to the inlet. 

Generally, a chemical/biological system has a boundary that distinguishes it from the environment. The interaction between the system and its environment determines the type of the system and its main characteristics as will be detailed later. 

Notice that the direction of the process and time have been determined This has been called the arrow of time [2], Time proceeds in the direction of entropy generation, that is, toward a state of greater probability for the total of the system and its environment, which, in the widest sense, makes up the universe. Finally, we wish to point out that an interesting implication of Equation 2.10 is that for substances in the perfect crystalline state at T = 0 K, the thermodynamic probability Q = 1 and thus S = 0. 

Earlier we mentioned that the first law deals with the quantity of energy, and the second law deals with the quality of energy. Looking at the system and its environment at the same time, we see that the first law expresses that for a real process the total number of Joules involved remains unchanged. The second law expresses that their quality declines. The total amount of... 

Containment A physical system in which no reactants or products are exchanged between the system and its environment under any conditions. 

Adiabatic process (thermodynamics) — In - thermodynamics a process is called adiabatic (or isocaloric) if no exchange (gain or loss) of heat occurs between the system and its environment. The word was first used by W.J.M. Rankine in 1859 as a synonym for non-crossing being derived from the classical Greek word adiabatos, meaning something like (it is) forbidden to cross [i]. 

From the state equation of A, and from the general law of energy conservation [for a (substitute) reversible exchange of heat 5q between the system and its environment] we formulate the I. Principle of Thermodynamics, 6q=dll + pdV... 

The quality (or state) of system safety and efficiency is determined by all the elements of the system and its environment converging. For instance, if your hydrogen unit will be in a submarine, different considerations come into play than if it will be in a factory setting. The environments are different, and certain space, safety and other factors are very important in one setting while they may not be relevant another. Designing a hydrogen system requires a holistic approach. You should always design a system with careful consideration of the conditions of its external environmental. 

This is a Langevin type equation that describes strong coupling between the system and its environment. Obviously, the limit y 0 of deterministic motion cannot be identified here. 

Second, we should keep in mind that between the two extreme limits discussed above there exists a regime of intermediate behavior, where dephasing/decoherence and molecular response occur on comparable timescales. In this case the scattering process may exhibit partial coherence. Detailed description of such situations requires treatment of optical response within a formalism that explicitly includes thermal interactions between the system and its environment. In Section 18.5 we will address these issues using the Bloch-Redfield theory of Section 10.5.2. 

So far nothing specific has been said about the nature of tfie work exchanged between the system and its environment. In this book we shall restrict ourselves to two types of work. The first is chemical work... 

Temperature differences in heat exchangers and between the system and its environment and in fluid mixing... 

A second procedure, using the methods of thermodynamics applied to Irreversible processes, offers another new approach for understanding the failure of materials. For example, the equilibrium thermodynamics of closed systems predicts that a system will evolve In a manner that minimizes Its energy (or maximizes Its entropy). The thermodynamics of Irreversible processes In open systems predicts that the system will evolve In a manner that minimizes the dissipation of energy under the constraint that a balance of power Is maintained between the system and Its environment. Application of these principles of nonlinear Irreversible thermodynamics has made possible a formal relationship between thermodynamics, molecular and morphological structural parameters. 

Bifurcations are the manifestation of an intrinsic differentiation between parts of the system and its environment...The temporal description of such systems involves both deterministic processes [between bifurcations]... 

For illustration we may think that the system consists of an ideal gas. The system and its environment is presented schematically in Fig. 2.7. 

Several types of assumptions are relevant. One is the assumptions under which the system will be used and the environment in which the system will operate. Not only will these assumptions play an important role in system development, but they also provide part of the basis for creating the operational safety control structure and other operational safety controls such as creating feedback loops to ensure the assumptions underlying the system design and the safety analyses are not violated during operations as the system and its environment change over time. 

It is noted that for heating (or cooling) a system at constant p, the heat exchange between the system and its environment is equal to the enthalpy exchange. Hence, for the heat capacity, at constant p... 

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