Complex system distillation

A modem petroleum refinery is a complex system of chemical and physical operations. The cmde oil is first separated by distillahon into fractions such as gasoline, kerosene, and fuel oil. Some of the distillate fractions are converted to more valuable products by cracking, polymerization, or reforming. The products are treated to remove undesirable components, such as sulfur, and then blended to meet the final product specifications. A detailed analysis of the entire petroleum production process, including emissions and controls, is obviously well beyond the scope of this text. 

Example 12.6. Let us consider a much more complex system where the advantages of frequencynlomain solution will be apparent. Rippin and Lamb showed how a frequency-domain stepping technique could be used to find the frequency response of a binary, equimolal-overflow distillation column. The column has many trays and therefore the system is of very high order. 

A simple distillation tower, like that shown in Fig. 3.2, also must obey its own phase rule. Here, because the distillation tower is a more complex system than the reflux drum, there are three independent... 

During molecular distillation, small variations in process conditions can lead to considerable changes in the characteristics of product streams (5-7). Thus, it is very important to have robust simulation results in order to evaluate the process and even to guide the experimental work. The objective of the present work was to incorporate in the Dismol simulator the data to simulate the process. The complex systems typical of the application of molecular distillation must be well characterized. 

Describing the behavior of undefined mixtures, whether from natural or synthetic sources, often begins with the separation of these complex systems into effective pseudocomponents by distillation (1 ). Each pseudocomponent is then characterized as if it were a pure compound, and its characterization data are used in appropriate correlations. The presence of nonvolatile residuum poses a serious limitation to such methodology. For coal-derived liquids, heavy crude oils, tar sands, and shale oil, more than 50 percent of the fluid may not be distillable (JL). Since this nonvolatile residue cannot be separated using conventional techniques, new methods of separation and characterization must be developed to provide the necessary information for design and operation of plants utilizing the fossil fuels mentioned above (2). 

Summing up, the influence of the kinetics of a chemical reaction on the vapor-liquid equilibrium is very complex. Physical distillation boundaries may disappear, while new kinetic stable and unstable nodes may appear. As result, the residue curve map with chemical reaction could look very different from the physical plots. As a consequence, evaluating the kinetic effects on residue curve maps is of great importance for conceptual design of reactive distillation systems. However, if the reaction rate is high enough such that the chemical equilibrium is reached quickly, the RCM simplifies considerably. But even in this case the analysis may be complicated by the occurrence of reactive azeotropes. 

However we choose to look at it, a basic distillation column has two control degrees of freedom. When we turn to more complex column configurations with sidestreams, side strippers, side rectifiers, intermediate reboilers and condensers, and the like, we add additional control degrees of freedom. These more complex systems are discussed in Sec. 6.8. 

The atmospheric distillation column in a refinery is highly complex system because of the interactions between the main column with different side strippers and draw streams where the study of this complex system will be more difficult. However, the decomposition of the complex column into a series of simple columns ease and simplify its study. There are a number of advantages of decomposing a complex tower, namely ... 

A complex system is one containing so many components that they cannot be separated into discreet pure components by the distillation process. An example of such a system is naturally occurring petroleum, which contains hundreds of chemical constituents. Crude tall oil from paper pulping is another example of a complex system. 

Figure 12.18 shows how a complex system differs from a multicomponent system. The complex system exhibits a smooth TBP curve, because even the high separating power of the TBP apparams cannot resolve all the many close-boiling constiments. The multicomponent system, on the other hand, is easily resolved by the powerful TBP apparams, although any azeotropes will be distilled over as though they were pure (pseudo) components. 

A more complex system is shown in Figures 1.30 and 1.31. Ethanol, water, and benzene display three binary azeotropes and one ternary azeotrope. The resulting ternary diagram (Fig. 1.31) has three distillation boundaries that separate the ternary space into three regions. 

The complex nonideal distillation columns considered in this chapter provide good examples of the difficulties and capabilities of using simulation in distillation systems for steady-state design. Now we are ready to move to an equally important phase of design in which the dynamics and control of the column or systems of columns and other units are considered. Simultaneous design involves both steady-state and dynamic aspects of the process. 

This con letes the introduction to residue curves. Residue curves will be used in the explanation of extractive distillation (Section 8.61 and azeotropic distillation (Section 8.7T In Section 11 -6 we will use residue curves to help synthesize distillation sequences for complex systems. Doherty and Malone (20011 develop the properties and applications of residue curves in much more detail than can be done in this introduction. 

Aromatics/aliphatics separation is accomplished by solvent extraction. A number of solvents have been used (Bailes, 1983). Although these separation processes are efficient, they are energy intensive, and more importantly, the solvents (such as sulfolane) increasingly pose as environmental hazards. Another possible separation technique is fractional distillation. It is, however, difficult because of the close relative volatilities. For benzene/cyclohexane, the mixture has a minimum azeotrope at about 53%. Therefore, acetone is added as an entrainer and a complex hybrid system (distillation combined with extraction in this case) can be used for separation (Stichlmair and Fair, 1998). 

The holdup effects can be neglected in a number of cases where this model approximates the column behavior accmately. This model provides a close approximation to the Rayleigh equation, and for complex systems (e.g., azeotropic systems) the synthesis procedures can be easily derived based on the simple distillation residue curve maps (trajectories of composition). However, note that this model involves an iterative solution of nonlinear plate-to-plate algebraic equations, which can be computationally less efficient than the rigorous model. 

Well designed distillation columns are close to electronic filters and while complex their performance can be well predicted from thermodynamic data. The complexity of chemical reactors varies from case to case. The performance of a well designed methanol or ammonia reactor can be predicted as well as that of a distillation column, whereas in more complex systems the risks of predictions is greater. In some sense identification of chemical reaction models has some features of a purely statistical correlation. Even in a simple well defined system such as isomerization of xylene where we can measure all reaction... 

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