Distillation columns structure

Exploitation of Homogeneous Azeotropes Homogeneous azeotropic distillation refers to a flowsheet structure in which azeotrope formation is exploited or avoided in order to accomplish the desired separation in one or more distillation columns. The azeotropes in the system either do not exhibit two-hquid-phase behavior or the hquid-phase behavior is not or cannot be exploited in the separation sequence. The structure of a particular sequence will depend on the geometry of the residue curve map or distillation region diagram for the feed mixture-entrainer system. Two approaches are possible ... 

No one steel exceeds the tensile modulus of mild steel. Therefore, in applications in which rigidity is a limiting factor for design (e.g., for storage tanks and distillation columns), high-strength steels have no advantage over mild steel. Stress concentrations in mild steel structures are relieved by plastic flow and are not as critical in other, less-ductile steels. 

For specific final performance sizing of a distillation column using Norton s Intalox structured packing the designer is referred to the manufacturer s technical representatives, and should not assume the preliminary results obtained from any manufacturer s bulletin included here will necessarily serve as a final design. As a preliminary examination of a design problem (used by permission of Norton Chemical Process Products) ... 

Hufton, J. R, J. L. Bravo, andj. R. Fair, Scale-up of Laboratory Data for Distillation Columns Containing Corrugated Metal-type Structured Packing, Ind. and Eng. Chem. Res., American Chem. Soc., V. 27, No. 11 (1988) p. 2096. 

Fair, J. R. and Bravo, J. L. (1990) Chem. Eng. Prog. 86, (1) 19. Distillation columns containing structured packing. 

To illustrate the concept, consider a single distillation column with distillate and bottoms products. To produce these products while using the minimum amount of energy, the compositions of both products should be controlled at their specifications. Figure 8.13u shows a dual composition control system. The disadvantages of this structure arc (1) two composition analyzers are required, (2) the instrumentation is more complex, and (3) there may be dynamic interaction problems since the two loops are interacting. This system may be difficult to design and to tune. 

For example, in a distillation column the manipulated variables could be the flow rates of reflux and vapor boilup R — V) to control distillate and bottoms compositions. This choice gives one possible control stmcture. Alternatively we could have chosen to manipulate the flow rates of distillate and vapor boilup D V). This yields another control structure for the same basic distillation process. 

The older tall oil distillation columns used bubble cap trays. In new columns, structured packing is preferred. Because of the low pressure drop of structured packing, steam injection is no longer necessary. The low liquid holdup of this packing minimizes the reactions of the fatty and resin acids. A specific distillation sequence for vacuum columns using structured packing of Sulzer has been described (26). Depitching is carried out at a vacuum of... 

J.L. Bravo, J.A. Rocha and J.R. Fair, Distillation columns containing structured packings- A comprehensive model for their performance mass transfer model, Lnd. Chem. Res., 35 (1996) 1660-1667. 

Fair. J.R. and J.L. Bravo Distillation Columns Containing Structured Packing, ... 

Note that the pressure (temperature) of each distillation column is not fixed, but it is treated as explicit optimization variable. As a result, structural optimization takes into account simultaneously the effect of operating the distillation columns at different pressures (temperatures) which provides the trade-off between the investment and operating cost. 

Fixed-bed catalytic reactors and reactive distillation columns are widely used in many industrial processes. Recently, structured packing (e.g., monoliths, katapak, mella-pak etc.) has been suggested for various chemical processes [1-4,14].One of the major challenges in the design and operation of reactors with structured packing is the prevention of liquid flow maldistribution, which could cause portions of the bed to be incompletely wetted. Such maldistribution, when it occurs, causes severe under-performance of reactors or catalytic distillation columns. It also can lead to hot spot formation, reactor runaway in exothermic reactions, decreased selectivity to desired products, in addition to the general underutilization of the catalyst bed. 

L. Spiegel, W. Meier, Distillation columns with structured packings in the next decade, Chem. Eng. Res. Des. 81 (Al) (2003) 39-47. 

Rocha JA, Bravo JL, Fair JR. Distillation columns containing structured packings— 2. Mass transfer model. Ind Eng Chem Res 1996 35 1660-1667. 

Ellenberger J, Krishna R. Countercurrent operation of structured catalytically packed distillation columns pressure drop, holdup, and mixing. Chem Eng Sci 1999 54 1339-1345. 

Distillation is the dominant separation process in the petroleum and chemical industries. It is carried out continuously more often than batchwise, in large, vertical, hollow cylindrical columns (or towers). Figure 1 shows a large distillation column with its associated piping, heat exchangers, vessels, ladders, platforms, and support structures. Figure 2 shows a simple schematic representation. 

The developments in this subsection have revealed that high-purity distillation columns exhibit a dynamic behavior with three time scales. Thus, according to the results in Sections 7.4 and 3.4, the design of a control system involves the synthesis of a tiered structure featuring three levels of control action. 

---