Packing pressure distillation

Many of the packings of the various manufacturers are essentially identical in shape, size, and performance factors. Some packing manufacturers suggest adjusting packing factors for vacuum and pressure distillations however, this should only be done after consultation. 

For high pressure distillation of light hydrocarbons, industrial performance indicates that the pressure drop actually obtained is about two times that predicted by the use of the GPDC charts. Figure 9-21F and 9-21G (for Norton s IMTP packing only). When the vapor density is at least 6% of the liquid density, the actual pressure drop is expressed [82] ... 

Zuiderweg, F. J. and D. E. Nutter, On the Evidence of Vapor Backmixing in Packed Columns in the Case of High Pressure Distillation, copyright by Institution of Chemical Engineers. Note undated and publication not given. 

Reactors with a packed bed of catalyst are identical to those for gas-liquid reactions filled with inert packing. Trickle-bed reactors are probably the most commonly used reactors with a fixed bed of catalyst. A draft-tube reactor (loop reactor) can contain a catalytic packing (see Fig. 5.4-9) inside the central tube. Stmctured catalysts similar to structural packings in distillation and absorption columns or in static mixers, which are characterized by a low pressure drop, can also be inserted into the draft tube. Recently, a monolithic reactor (Fig. 5.4-11) has been developed, which is an alternative to the trickle-bed reactor. The monolith catalyst has the shape of a block with straight narrow channels on the walls of which catalytic species are deposited. The already extremely low pressure drop by friction is compensated by gravity forces. Consequently, the pressure in the gas phase is constant over the whole height of the reactor. If needed, the gas can be recirculated internally without the necessity of using an external pump. 

Packed fractional distillation columns run in the batch mode are often used for low-pressure drop vacuum separation. With a trayed column, the liquid holdup on the trays contributes directly to the hydraulic head required to pass through the column, and with twenty theoretical stages that static pressure drop is very high, e.g., as much as 100-200 mm Hg. 

Tray columns are the best choice for high-pressure distillation, especially above 20 bar. An important factor in trays favour is that they have been used for many years in high-pressure distillation, while it is only in the last 10-15 years that packings have been used significantly in such duties, and especially for structured packings above about 15 bar. 

Practical guidance on how to get the best choice between trays and packings in high-pressure distillations is very difficult [33],... 

Strigle (15) and Kister and Gill (60) compared predictions from the latest version of the Eckert correlation (Fig. 8.19a and b) to thousands of random packing pressure drop measurements. The Eckert correlation was shown to give good predictions for most pressure drop data (15,60). It generally works well for the alr-wator system for flow parameters as low as 0.01 and as high as 1 (60). For nonaqueous systems, it works well for flow parameters of 0.08 to 0.3 (typical of atmospheric distillation). 

Structured packings have replaced trays and random packings as their cost has decreased and more is known of their performance behavior. Initially thought to be appropriate only for high vacuum distillations, they are now used for absorbers, strippers, and pressure distillations. Because of their open structure (over 90% voids) and large specific surface areas, their mass transfer efficiency is high when proper distribution of liquid and gas over the cross section can be maintained. Table 13.15 shows a comparison of features of several commercial makes of structured packings. 

A packed-bed distillation column is used to adiabatically separate a mixture of methanol and water at a total pressure of 1 atm. Methanol—the more volatile of the two components—diffuses from the liquid phase toward the vapor phase, while water diffuses in the opposite direction. Assuming that the molar latent heat of vaporization is similar for the two components, this process is usually modeled as one of equimolar counterdiffusion. At a point in the column, the mass-transfer coefficient is estimated as 1.62 x 10-5 kmol/m2-s-kPa. The gas-phase methanol mole fraction at the interface is 0.707, while at the bulk of the gas it is 0.656. Estimate the methanol flux at that point. 

With the availability of economical and efficient packings, packed towers are finding increasing use in new distillation processes and for retrofitting existing trayed towers. They are particularly useful in applications where pressure drop must be low, as in low-pressure distillation, and where liquid holdup must be small, such as when distilling heat-sensitive materials whose exposure to high temperatures must be minimized. 

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