Variable pressure process

Ruhrstahl-Hausen (RH) and vacuum oxygen decarburization (VOD) are secondary steel-making processes to produce specialty steels. The process occurs vmder vacuum, which enables reduction of the carbon content to very low levels (<0.01%) without vmdesirable oxidation of chromium. Special inlet systems have been designed to sample from these variable pressure processes and still maintain a constant flow of sample gas into the ion source. 

Three examples of simple multivariable control problems are shown in Fig. 8-40. The in-line blending system blends pure components A and B to produce a product stream with flow rate w and mass fraction of A, x. Adjusting either inlet flow rate or Wg affects both of the controlled variables andi. For the pH neutrahzation process in Figure 8-40(Z ), liquid level h and the pH of the exit stream are to be controlled by adjusting the acid and base flow rates and w>b. Each of the manipulated variables affects both of the controlled variables. Thus, both the blending system and the pH neutralization process are said to exhibit strong process interacHons. In contrast, the process interactions for the gas-liquid separator in Fig. 8-40(c) are not as strong because one manipulated variable, liquid flow rate L, has only a small and indirec t effect on one controlled variable, pressure P. 

The extent to which each of the above reactions occur is strongly influenced by feed quality and the levels selected for the major process variables pressure, temperature, recycle rate, and frequency of regeneration. From a process viewpoint, these variables affect catalyst requirement, gasoline yield, and coke make. 

Students often ask, What is enthalpy The answer is simple. Enthalpy is a mathematical function defined in terms of fundamental thermodynamic properties as H = U+pV. This combination occurs frequently in thermodynamic equations and it is convenient to write it as a single symbol. We will show later that it does have the useful property that in a constant pressure process in which only pressure-volume work is involved, the change in enthalpy AH is equal to the heat q that flows in or out of a system during a thermodynamic process. This equality is convenient since it provides a way to calculate q. Heat flow is not a state function and is often not easy to calculate. In the next chapter, we will make calculations that demonstrate this path dependence. On the other hand, since H is a function of extensive state variables it must also be an extensive state variable, and dH = 0. As a result, AH is the same regardless of the path or series of steps followed in getting from the initial to final state and... 

The combination of fundamental variables in equation (l.23) that leads to the variable we call G turns out to be very useful. We will see later that AG for a reversible constant temperature and pressure process is equal to any work other than pressure-volume work that occurs in the process. When only pressure-volume work occurs in a reversible process at constant temperature and pressure, AG = 0. Thus AG provides a criterion for determining if a process is reversible. Again, since G is a combination of extensive state functions... 

Control of the process. Prevention of hazardous deviations in process variables (pressure, temperature, flow), by provision of automatic control systems, interlocks, alarms, trips together with good operating practices and management. 

Controllers are the controlling element of a control loop. Their function is to maintain a process variable (pressure, temperature, level, etc.) at some desired value. This value may or may not be constant. 

Ultimately, we must realize that entropy is essentially a mathematical function. It is a concise function of the variables of experience, such as temperamre, pressure, and composition. Natural processes tend to occur only in certain directions that is, the variables pressure, temperature, and composition change only in certain—but very complicated—ways, which are described most concisely by the change in a single function, the entropy function (AS > 0). 

The primary variables influencing hydrotreating are hydrogen partial pressure, process temperature, and contact time. Higher hydrogen pressure gives a better removal of undesirable... 

Creation of a multi dimensional experimental space containing also process variables (pressure, temperature, pH, flow rate, etc.)... 

The total energy is calculated in terms of the ratio y of the thermal part of the pressure to the elastic part, and other variables. Isentropic processes are discussed, and the initial conditions of a Landau-Stanyukovich-Zel dovich-Kompaneets (LSZK) detonation are obtained in terms of y... 

The loads on a propellant charge are generally known and recorded as the process variables (pressures), or they can be calculated from temperature and thermal and mechanical properties of the system. Until recently, the required mechanical properties and failure criteria were... 

As with the energy, we are often interested in determining the change in the value of the enthalpy function of a closed system for some change of state without having to measure the heat absorbed and without being confined to constant-pressure processes. We choose the temperature and pressure as the two convenient independent variables to use, and write the differential of the enthalpy as... 

There being two components and two phases in the following streams at the bottom, feed and overhead, Gibbs s law states that only two of the three variables (pressure, temperature, and composition) are independent. Therefore, the number of independent variables is only 11. The number of defining equations is two (the conservation of mass and energy), and, therefore, the number of degrees of freedom for this process is 11 - 2 = 9. Consequently, not more than nine automatic controllers can be placed on this process. 

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