Distillation optimization

During the global transition from the present oil-based economy to a clean and inexhaustible energy economy, the importance of distillation will increase because biofuel production also involves distillation and because 

Distillation separates the components of a mixture on the basis of their boiling points and on the difference in the compositions of the liquids and their vapors. The product purity of a distillation process is maintained by the manipulation of the material and energy balances. Difficulties in maintaining that purity arise because of dead times, nonlinearities, and variable interactions. 

The main distillation equipment is the column (also called tower or fractionator). It has two purposes First, it separates a feed into a vapor portion that ascends the column and a liquid portion that descends. Second, it achieves intimate mixing between the two phases. The purpose of the mixing is to get an effective transfer of the more volatile components into the ascending 

Post-Oil Energy Technology After the Age of Fossil Fuels 

The contact between liquid and vapor is made intimate as the vapors ascend through the liquids which are held on each tray, as the liquid descends (left). The dynamics of a multiple tray column can be approximated as a second-order lag plus dead time. 

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Spreadsheet Applications. The types of appHcations handled with spreadsheets are a microcosm of the types of problems and situations handled with fuU-blown appHcation programs that are mn on microcomputers, minis, and mainframes and include engineering computations, process simulation, equipment design and rating, process optimization, reactor kinetics—design, cost estimation, feedback control, data analysis, and unsteady-state simulation (eg, batch distillation optimization). 

The problem of extensive computing time typical of batch distillation is compounded with the superimposition of an optimization routine. For this reason batch distillation optimization algorithms are usually built around shortcut batch distillation methods. Possible approaches to formulating the optimization problem are presented, although a detailed mathematical discussion is outside the scope of this book. 

Several types of distillation optimizations have been considered in this chapter. The approaches presented are simple and practical. There are many advanced techniques in the optimization area that are beyond the scope of this book. 

Lima, R. M., Salcedo, R. L., Barbosa, D., 2006. SIMOP Efficient reactive distillation optimization using stochastic optimizers. Chem. Eng. Sci. 61(5), 1718-1739. 

The indices of performance used in batch distillation optimal control problems... 

The following example of batch distillation optimal control (optimal reflux policy problem illustrates this. 

As with distillation, no attempt should be made to carry out any optimization of liquid flow rate, temperature, or pressure at this stage in the design. The separation in absorption is sometimes enhanced by adding a component to the liquid which reacts with the solute. 

No attempt should be made to optimize pressure, reflux ratio, or feed condition of distillation in the early stages of design. The optimal values almost certainly will change later once heat integration with the overall process is considered. 

The most common alternative to distillation for the separation of low-molecular-weight materials is absorption. Liquid flow rate, temperature, and pressure are important variables to be set, but no attempts should be made to carry out any optimization at this stage. 

Triantafyllou, C., and Smith, R., The Design and Optimization of Fully Thermally Coupled Distillation Columns, Trans. IChemE, Part A, 70 118, 1992. 

Glinos, K., and Malone, M. F., Optimality Regions for Complex Column Alternatives in Distillation Columns, Trans. IChei lE ChERD, 66 229, 1988. 

Distillation capital costs. The classic optimization in distillation is to tradeoff capital cost of the column against energy cost for the distillation, as shown in Fig. 3.7. This wpuld be carried out with distillation columns operating on utilities and not integrated with the rest of the process. Typically, the optimal ratio of actual to minimum reflux ratio lies in the range 1.05 to 1.1. Practical considerations often prevent a ratio of less than 1.1 being used, as discussed in Chap. 3. 

Thus the optimal reflux ratio for an appropriately integrated distillation column will be problem-specific and is likely to be quite different from that for a stand-alone column. 

Sensitivity Sensitivity in flame atomic emission is strongly influenced by the temperature of the excitation source and the composition of the sample matrix. Normally, sensitivity is optimized by aspirating a standard solution and adjusting the flame s composition and the height from which emission is monitored until the emission intensity is maximized. Chemical interferences, when present, decrease the sensitivity of the analysis. With plasma emission, sensitivity is less influenced by the sample matrix. In some cases, for example, a plasma calibration curve prepared using standards in a matrix of distilled water can be used for samples with more complex matrices. 

Most aroma chemicals are relatively high boiling (80—160°C at 0.4 kPa = 3 mm Hg) Hquids and therefore are subject to purification by vacuum distillation. Because small amounts of decomposition may lead to unacceptable odor contamination, thermal stabiUty of products and by-products is an issue. Important advances have been made in distillation techniques and equipment to allow routine production of 5000 kg or larger batches of various products. In order to make optimal use of equipment and to standardize conditions for distillations and reactions, computer control has been instituted. This is particulady well suited to the multipurpose batch operations encountered in most aroma chemical plants. In some instances, on-line analytical capabihty is being developed to work in conjunction with computer controls. 

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