Cycling Operation

The column is initially operated at total reflux until equilibrium is reached. The condensate is then totally diverted to a receiver for a short period of time, during which the column operates at zero reflux. Before the composition of the liquid in the receiver goes off specification, the condensate is diverted back to the column, and the unit is operated again at total reflux. The cycle is repeated for all the remaining cuts. 

Given an objective function to be maximized or minimized, it is possible, in principle, to compute an optimum profile of reflux ratio and distillate rate for the entire distillation cycle. The principles of such approaches are introduced in Section 17.2.4. 

At total reflux the reboiler duty is balanced by the condenser duty. If, at this point, a distillate product is drawn at a certain rate without changing the reflux rate, both reboiler and condenser duties must be increased to handle the higher rate of vaporization and condensation. In general, any set of reboiler and condenser duties determines the reflux and distillate rates. The reflux and product valve positions must be controlled to maintain reboiler and condenser liquid levels. The boilup rate is also determined from the other variables by material balance. Thus, of the five variables—reboiler duty, condenser duty, boilup rate, reflux rate, and product rate—two are independent and the system has two degrees of freedom. 

The overall control strategy depends on the reflux policy selected and can vary 

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In a combined power cycle operation, clean (sulfur- and particulate-free) gas is burned with air in the combustor at elevated pressure. The gas is either low or medium heat-value, depending on the method of gasification. 

Heat Pumps. Heat pumps involve the appHcation of external power to pump heat from a lower temperature to a higher temperature. Heat pumps are frequently used for space heating and are simply refrigeration cycles operated in reverse. The heat rejected in the condenser becomes the primary objective of operation. Consequently, refrigerants used for mechanical compression refrigeration have similar appHcation in heat pumps. 

N. R. Johanson and J. W. Muehlhauser, "MHD Bottoming Cycle Operations and Test Results at the CFFF," paper presented at Second International Workshop on Fossil Fuel Fired MHD, Bologna, Italy, 1989. 

Open-Cycle Operation. In many chemical processes, the product to be cooled can itself be used as the refrigerating Hquid. An example of this is in gathering plants for natural gas. Gas from the wells is cooled, usually after compression and after some of the heavier components are removed as hquid. This Hquid maybe expanded in a refrigeration cycle to further cool the compressed gas, which causes more of the heavier components to condense. Excess Hquid not used for refrigeration is drawn off as product. A typical open-cycle is shown in Figure 12. 

Determine the motor rating, for a 10-minute cycle operating... 

Figure 2.2 The indicator diagrams for the Carnot and the Otto engines. The Carnot cycle operates between the two temperatures Tj and T2 only, whereas the Otto cycle undergoes a temperature increase as a result of combustion.

Improving the economics of gas plant design, construction, and operations is essential to ensure the approval of future de-bottlenecking, capacity expansion, and new projects. The economics include not only capital investment, life cycle operations, and maintenance costs, but also the monetary equivalents of safety, reliability, and availability. 

The aim of life cycle cost calculations is to ensure that investment decisions are not made solely on the basis of a low purchase price, but that the life cycle operating costs are considered in the equation. 

Another way to utilize LCC calculations is in support of a purchase. The supplier can offer both the purchase price and an estimate of the present value of the life cycle operating costs At the same time the supplier guarantees the operating cost for some period, for example, 3 years. If these target costs are exceeded, the supplier pays a penalty, and if the operating costs were overestimated, the supplier and the customer share the bonus. 

The exergy equation (2.26) enables useful information on the irreversibilities and lost work to be obtained, in comparison with a Carnot cycle operating within the same temperature limits (T ,ax = Ey and T in = To). Note first that if the heat supplied is the same to each of the two cycles (Carnot and IJB), then the work output from the Carnot engine (Wcar) is greater than that of the IJB cycle (Wijg), and the heat rejected from the former is less than that rejected by the latter. 

No thermodynamic cycle can be more efficient than a reversible cycle operating between the same temperature limits. 

Diesels, gas turbines and steam turbines are the more commonly used prime movers for the generation of electrical power. Additionally, the steam turbine can be employed in combination with either the diesel or gas turbine for combined cycle operation. The following describes the basic operation of each of these prime movers in relation to its associated power-generating scheme and reviews the more significant factors affecting performance and efficiency. Further information on the actual plant and installation is given later in Section 15.6. 

Provide mid-season heating from condenser heat or heat pump (reverse-cycle) operation... 

An operational change because caustic concentration typically results when high-firing cycle operations permit departure from nucleate boiling (DNB) to occur... 

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