Vacuum distillation structured packing

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... 

At deep vacuum (< 1 to 2 psia) there are data to suggest that efficiency decreases as pressure is lowered for random packings (117,118), but most of these data can also be explained by poor wetting or maldistribution. For distillation at high pressures (>200 to 300 psia), there is evidence to suggest that structured-packing efficiency diminishes as pressure is raised (24,31a). 

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. 

Gorak et al. (1991) and Wozny et al. (1991) presented a brief description of their use of a nonequilibrium state model to simulate vacuum distillation of fatty alcohols in columns fitted with structured packing. They found that the compositions predicted by the nonequilibrium model were closer to the experimental data than were the results of an equilibrium stage-HETP calculation. 

A12. Structured packings work very well in vacuum and atmospheric pressure distillation columns, but sometimes structured packings do not work well in high-pressure columns. What is different about high-pressure columns that may make the structured packings not work as well ... 

Subsequently, other wire-mesh structured packings, such as Intalox Wire Gauze packing (trademark Norton Chemical Process Products) have been developed. Figure 2-1 shows HETP values for Intalox Wire Gauze packing in two vacuum distillations ortho/para xylene at 50mm... 

For atmospheric pressure or vacuum distillations, the vapor flow rate is limited by the loss of normal separation efficiency due to entrainment of liquid upward in the vapor phase. The amount of entrainment increases rapidly above a threshold value for the particular system. As the vapor flow rate is increased further, the mass of entrained liquid becomes sufficient to reduce the concentration profile established in the column. The maximum operational Cs has been defined as the greatest vapor flow rate attained before loss of normal separation efficiency. Figures 7-6 and 7-7 give a prediction of the maximum capacity for IMTP random packings and Intalox structured packings, respectively, as limited by liquid entrainment. 

Initially, structured packings were applied in vacuum distillation services because their very low pressure drop characteristics and low HETP values provided reduced pressure gradients per theoretical stage. Later, these packings were used to increase capacity in atmospheric distillations, since replacement... 

Distillation in a packed tower is preferred over a plate tower for small diameter towers having a diameter less than 2 ft. For corrosive liquids, ceramic/plastic packings are preferred along with a packed tower over plate towers. For low pressure drops or vacuum distillation applications, structured packings are preferred. The details provided in Section 8.1.2.1 for gas absorption/stripping processes are equally valid for distillation processes. Calculation methods for flooding based design illustrated there may also be used here. 

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