Cyclical processes

Steam is injected into a reservoir to reduce oil viscosity and make it flow more easily. This technique is used in reservoirs containing high viscosity crudes where conventional methods only yield very low recoveries. Steam can be injected in a cyclic process in which the same well is used for injection and production, and the steam is allowed to soak prior to back production (sometimes known as Huff and Puff). Alternatively steam is injected to create a steam flood, sweeping oil from injectors to producers much as in a conventional waterflood. In such cases it is still found beneficial to increase the residence (or relaxation) time of the steam to heat treat a greater volume of reservoir. 

Flere the subscripts and/refer to the initial and final states of the system and the work is defined as the work perfomied on the system (the opposite sign convention—with as work done by the system on the surroundings—is also in connnon use). Note that a cyclic process (one in which the system is returned to its initial state) is not introduced as will be seen later, a cyclic adiabatic process is possible only if every step is reversible. Equation (A2.1.9), i.e. the mtroduction of t/ as a state fiinction, is an expression of the law of conservation of energy. 

Because a good catalyst is not consumed to a significant degree as it functions, catalysis is a cyclic process, and compact representations of catalysis are cycles tliat show tire various intennediate species, illustrated by the following simple example, where C is tire catalyst, R tire reactant, P tire product and RC tire intennediate ... 

Solving an indoor air quality problem is a cyclical process of data collection and hypothesis testing. Deeper and more detailed investigation is needed to suggest new hypotheses after any unsuccessful or partially-successful control attempt. Even the best planned investigations and mitigation actions may not produce a resolution to the problem. You may have made a careful investigation, found one or more apparent causes for the problem, and implemented a control system. Nonetheless,... 

Kreis-laufgas, n. recycle gas. -linie,/. circular line, circumference, -ordnung, /. district regulation. -prozess, m. cyclic process, cycle, -pumpe, /. circulating pump, kreisrund, a. drcular. 

Plots of the properties of various substances as well as tables and charts are extremely useful in solving engineering thermodynamic problems. Two-dimensional representations of processes on P-V, T-S, or H-S diagrams are especially useful in analyzing cyclical processes. The use of the P-V diagram was illustrated earlier. A typical T-S diagram for a Rankine vapor power cycle is depicted in Figure 2-36. 

Mechanism The mechanism of cracking has not been established. Even the corrosion reaction that is responsible for the initial attack has not been determined. Early work led to the suggestion that chlorine gas was generated and could cause fracture by a cyclic process requiring the formation and decomposition of TiCl2 ... 

We ve seen that the Diels-Alder cycloaddition reaction is a one-step, peri-cyclic process that occurs through a cyclic transition state. Propose a mechanism for the following reaction ... 

A pericyclic reaction is one that takes place in a single step through a cyclic transition state without intermediates. There are three major classes of peri-cyclic processes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. The stereochemistry of these reactions is controlled by the symmetry of the orbitals involved in bond reorganization. 

If, after any process, or series of processes, a system returns to its initial state, it is said to have undergone a cycle of changes, and the process is called a cyclic process (S. Carnot, 1824). 

In any cyclic process, let a quantity of heat Q be absorbed, and a quantity of work A be done, by the system. Heat emitted, or work done on the system, is to be reckoned with a negative sign. Then the heat absorbed is equivalent to the work done, in the sense explained ... 

We now suppose that a system undergoes any change, and that no energy changes occur outside the system except the disappearance of a quantity of heat Q and the performance of a quantity of work A. If the final state is the same as the initial state, the process must, by definition, have been a cyclic process, and hence... 

A particular case is a cyclic process an example of a non-cyclic aschistic change is afforded by the expansion of an ideal gas at constant temperature ( 71). 

The production of heat by friction, the passage of heat from one body to another at a lower temperature by conduction or radiation, and the diffusion of material substances, are intrinsically irreversible processes. A process (Carnot s cyclic process) will be described in the next chapter by means of which heat generated by friction may be withdrawn and reconverted into work. 

Equation (a) (being a definition) is generally applicable equation (b) only to aschistic, in particular to cyclic, processes. 

The cyclic process being aschistic, we have, as a consequence of the first law ... 

Theorem.—The work done in any isothermal reversible cyclic process is zero (J. Moutier, 1875). 

For if a cyclic process could be performed in a heat reservoir of uniform temperature so as to give out work, it would constitute a perpetanm mobile of the second kind, the existence of which is denied by the second law. And if the cyclic process absorbed work when performed at a uniform temperature, it would, by reason of its reversibility, give out an equal amount of work when reversed this would, however, be the case first considered. Hence the production of work in either cycle is impossible, which establishes the theorem. 

Let us now fix our attention on the working substance, i.e. on the material system undergoing the cyclic process. If Qb Q2 are the quantities of heat absorbed by the system from the source and refrigerator respectively ... 

If any cyclic process is performed with a given material system, the entropy of all the surrounding bodies which have in any way been involved in the process, either as emitters or absorbers of heat, either remains unchanged, if the cycle is reversible, or else increases, if the cycle is performed irreversibly. 

Now let us consider any process which is not a cyclic process, and in which the system is taken from an initial state [1] to a final state [2]. We shall prove that if Si, S2 are the entropies of... 

For this purpose we suppose the following reversible cyclic process executed. This process, it will be seen, is not a Carnot s cyclic process, but is of another type. 

Such a cyclic process, consisting of two isotherms and two adynamics, was thoroughly studied by Rankine, and may therefore be called a liankine s cycle. 

This conclusion was arrived at, from considerations based on Carnot s principle alone, by James Thomson in 1849. He also calculated the magnitude of the effect, in the case of ice, by means of a cyclic process. Since the reasoning is the same for both cases, we shall deal with both together, giving appropriate diagrams. 

The Clausius statement of the Second Law involves cyclic processes and engines. It can be stated as m... 

Since the cyclic process returns the system to the initial state, AU = 0, and we can write... 

Figure 2.10 (a) A schematic Carnot cycle in which isotherms at empirical temperatures 6 and 62 alternate with adiabatics in a reversible closed path. The enclosed area gives the net work produced in the cycle, (b) The area enclosed by a reversible cyclic process can be approximated by the zig-zag closed path of the isothermal and adiabatic lines of many small Carnot cycles. 

Consider any reversible cyclic process that involves the exchange of heat and work. Again, the net area enclosed by the cycle on a p-V plot gives the work. This work can be approximated by taking the areas enclosed within a series of Carnot cycles that overlap the area enclosed by the cycle as closely as possible as shown in Figure 2.10b. For each of the Carnot cycles, the sum of the q/T terms... 

Finally, (c) is represented by any cyclic process occurring within an isolated system. With the system returning to its initial state, and with no opportunity for transfer of energy between system and surroundings, all energy changes in the process must add to zero. 

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