Processes isobaric

Both separation possibilities described above utilize an isobaric process allowing processing under optimized conditions. 

From the above, it is obvious that the design engineer has to evaluate optimized extraction conditions and to select the best way of separation in order to obtain a viable process. In the following sections, various design possibiUties are described in more detail. 

Single or Cascade Operation with Multistep Separation 

Most multipurpose plants are equipped with two or more extractors switched in cascade mode, which enables to come as dose as possible to phase equilibrium and are equipped with a multiple separation system, operating at different reduced pressures for fractionation. Such design is mainly used for the extraction of spices and herbs, allowing concentration of pungent substances like piperine from pepper or capsaicin from chili in a first step and the corresponding volatile oil afterward in one additional separator. In case that the valuables are highly concentrated in the raw material and also relatively highly soluble in the dense gas, installation of only two 

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Thus for isobaric processes a new fimction, the enthalpy H, has been introduced and its change A// is more directly related to the heat that must have been absorbed than is the energy change At/. The same reservations about the meanmg of heat absorbed apply in this process as in the constant-volume process. 

D) CONSTANT-TEMPERATURE CONSTANT-PRESSURE (ISOTHERMAL-ISOBARIC) PROCESSES... 

Although the T-s diagram is veiy useful for thermodynamic analysis, the pressure enthalpy diagram is used much more in refrigeration practice due to the fact that both evaporation and condensation are isobaric processes so that heat exchanged is equal to enthalpy difference A( = Ah. For the ideal, isentropic compression, the work could be also presented as enthalpy difference AW = Ah. The vapor compression cycle (Ranldne) is presented in Fig. H-73 in p-h coordinates. 

For apphcation to distiUation (a nearly isobaric process), as shown in Figs. 13-8 to 13-13, binary-mixture data are frequently plotted, for a fixed pressure, as y versus x, with a line of 45° slope included for reference, and as T versus y and x. In most binary systems, one of the components is more volatile than the other over the entire composition range. This is the case in Figs. 13-8 and 13-9 for the benzene-toluene system at pressures of both 101.3 and 202.6 kPa (1 and 2 atm), where benzene is more volatile than toluene. 

The Brayton cycle in its ideal form consists of two isobaric processes and two isentropic processes. The two isobaric processes consist of the combustor system of the gas turbine and the gas side of the HRSG. The two isentropic processes represent the compression (Compressor) and the expansion (Turbine Expander) processes in the gas turbine. Figure 2-1 shows the Ideal Brayton Cycle. 

The heat rejected per unit mass of carbon in the isobaric process 4-1 q4, ) is similarly analogous to process 2-3 ... 

The Isobaric Process Figure 2.3 shows the relationship between pexl and V during an isobaric (constant pressure) process. In this expansion, peM is constant and usually equal to p, the pressure of the fluid.6 When this is true, equation (2.11) becomes... 

Figure 2.3 Work of expansion for an isobaric process. In the expansion. pcxl is constant and equal to p, the pressure of the fluid, unless a mechanical constraint prevents the two pressures from being equal.

Combining equations (2.20) and (2.22) and equating peM to p. the usual case for an isobaric process,k gives... 

We have seen how to calculate q for the isochoric and isobaric processes. We indicated in Chapter 1 that q = 0 for an adiabatic process (by definition). For an isothermal process, the calculation of q requires the application of other thermodynamic equations. For example, q can be obtained from equation (2.3) if AC and w can be calculated. The result is... 

Both the compression and expansion are isobaric processes hence, the total work is given by... 

Besides the reversible and irreversible processes, there are other processes. Changes implemented at constant pressure are called isobaric process, while those occurring at constant temperature are known as isothermal processes. When a process is carried out under such conditions that heat can neither leave the system nor enter it, one has what is called an adiabatic process. A vacuum flask provides an excellent example a practical adiabatic wall. When a system, after going through a number of changes, reverts to its initial state, it is said to have passed through a cyclic process. 

The enthalpy is useful in considering isentropic and isobaric processes, but often it becomes necessary to rather deal with isothermal and isobaric processes. In such case one needs a thermodynamic function of T and P alone, defining the Gibbs potential G = U(T, P, Nj) as the Legendre transform of U that replaces entropy by temperature and volume by pressure. This transform is equivalent to a partial Legendre transform of the enthalpy,... 

The enthalpy released or absorbed in an isobaric process can be described in a manner similar to Equation (2-3) for constant volume conditions ... 

The enthalpy released or absorbed in a process can be described by Equation 6 for constant volume conditions and an isobaric process. While determining the safety subindex Irm the heat release of the main reaction is calculated for the total reaction mass (i.e. both the reactants and diluents are included) to take account the heat capacity of the system which absorbs part of the energy released ... 

Assume a process each for the eight devices (1) turbines as adiabatic with 80% efficiency, (2) splitters as nonisopar-ametric devices, and (3) condensers as isobaric processes. Notice that throttling devices are automatically constant enthalpy processes. 

The gas Brayton cycle adds heat in a isobaric process over a large temperature range. The temperature level is independent of the pressure level. No blade erosion occurs in the gas turbine. However, the compression process of the gas Brayton cycle requires large work input. The back-work ratio is small. 

A cycle called the Atkinson cycle is similar to the Otto cycle except that the isochoric exhaust and intake process at the end of the Otto cycle power stroke is replaced by an isobaric process. The schematic diagram of the cycle is shown in Fig. 3.18. The cycle consists of the following four processes ... 

The schematic Ericsson cycle is shown in Fig. 4.27. The p-v and T-s diagrams of the cycle are shown in Fig. 4.28. The cycle consists of two isothermal processes and two isobaric processes. The four processes of the Ericsson cycle are isothermal compression process 1-2 (compressor), isobaric compression heating process 2-3 (heater), isothermal expansion process 3-4 (turbine), and isobaric expansion cooling process 4-1 (cooler). 

Then, assume that the reaction takes place in a fixed bed of 1.61 m diameter and 16.1 m height, under contact time of 5 min, and the inlet temperature of gas being 50 °C, for different CO inlet concentration (several runs). Estimate the conversion of CO in an isothermal and adiabatic fixed-bed reactor and under the following assumptions isobaric process, negligible external mass transfer resistance, and approximately constant heat capacity of air (cp = 1 kJ/kg K) and heat of reaction (AH = -67,636 cal/mol). The inlet temperature of the reaction mixture is 50 °C and its composition is 79% N2 and approximately 21% 02, while the inlet CO concentration varies from 180-4000 ppm (mg/kgair) (for each individual ran). 

Processes with high mass-flow rates, for example more than 40 t/h, have energy demands of a very high level. Especially for the decaffeination processes, in which several hundred - up to a thousand tons of CO2 are in circulation, isobaric processes were developed. In these processes, the extraction step and the separation step have nearly the same pressure and temperature. The separation of the dissolved substance from the CO2 in circulation is maintained by adsorption on activated charcoal, with an ion-exchanger, or by absorption in a washing column. 

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