Fractionating Column Design

The liquid gradients and the pressure drops encountered by the liquid and the vapor are important for several reasons  

The contact between the vapor and liquid is a function of the liquid gradients and the gas pressure drops. 

The pressure drop per plate is of vital importance in vacuum rectification. 

Pressure Drop for Vapor Flow. The vapor meets its main resistance in passing through the bubble cap and the liquid on the plate. Consider the section of a cap shown in Fig. 16-2. The vapor from the plate 

Pressure Drop through Risers and Cap, This loss is chiefly a kinetic velocity effect due to the changing cross-sectional areas. The pressure drop in inches of the liquid equivalent to the kinetic head is 

---

Greatly to our surprise we have discovered that some readers of Chemical Abstracts do not take advantage of the fractional column designation in C.A. references, both in abstracts and in the indexes. Chemical Abstracts pioneered in the numbering of columns instead of pages and in the designation of fractions of a column. The large amount of time to be saved by the use of these devices is obvious. 

The oil industry in general owes more to Warren K. Lewis than to any other individual for the quick and successful application of the scientific principles of fractionating column design to the oil industry. We have continued to develop the chemical engineering technology for fractional distillation and Exxon now has continuous distillation units capable of handling up to 275,000 bbl/day (40,000 tons/day) of crude oil. 

Consider the following changes in the operating conditions of the fractionation column designed in Illustration 7.8. 

The model of theoretical equiHbrium trays with entrainment is readily treated by computer with methods analogous to those used for the design of fractionating columns. 

Several descriptions have been pubUshed of the continuous tar stills used in the CIS (9—11). These appear to be of the single-pass, atmospheric-pressure type, but are noteworthy in three respects the stills do not employ heat exchange and they incorporate a column having a bubble-cap fractionating section and a baffled enrichment section instead of the simple baffled-pitch flash chamber used in other designs. Both this column and the fractionation column, from which light oil and water overhead distillates, carboHc and naphthalene oil side streams, and a wash oil-base product are taken, are equipped with reboilers. 

The simplest unit employing vacuum fractionation is that designed by Canadian Badger for Dominion Tar and Chemical Company (now Rttgers VFT Inc.) at Hamilton, Ontario (13). In this plant, the tar is dehydrated in the usual manner by heat exchange and injection into a dehydrator. The dry tar is then heated under pressure in an oil-fired hehcal-tube heater and injected directly into the vacuum fractionating column from which a benzole fraction, overhead fraction, various oil fractions as side streams, and a pitch base product are taken. Some alterations were made to the plant in 1991, which allows some pitch properties to be controlled because pitch is the only product the distillate oils are used as fuel. 

The fix for the erratic reflux drum pressure problem was to provide for separate pressure control of the fractionator column and the reflux drum. A new pressure control valve was installed upstream of the condenser and the old condenser outlet control valve was removed. A hot gas bypass, designed for 20% vapor flow, was installed around the pressure control valve and condenser. A control valve was installed in the hot gas bypass line. The column pressure was then maintained by throttling the new control valve upstream of the condenser. The reflux drum pressure w as controlled by the hot gas bypass control valve and the psv saver working in split range. The new system is shown in the figure below. 

A more quantitative and lengthy method, but still very useful for checking of the type required here is the Smith-Brinkley method (Reference 5). It uses two sets of separation factors for the top and bottom parts of the column for a fractionator or reboiled absorber and one overall separation factor for a simple absorber. The method is tailor-made for analysis of a column design or a field installed column. The Smith-Brinkley method starts with the column parameters and calculates the resulting product compositions unlike other methods that require knowing the compositions to determine the required reflux. 

Another arrangement which provides for expansion involves the use of hairpin tubes, as shown in Figure 9,64. This design is very commonly used for the reboilers on large fractionating columns where steam is condensed inside the tubes. 

In extraction column design, the model equations are normally expressed in terms of superficial phase velocities, L and G, based on unit cross-sectional area. The volume of any stage in the column is then A H, where A is the cross-sectional area of the column. Thus the volume occupied by the total dispersed phase is h A H, where h is the fractional holdup of dispersed phase, i.e., the droplet volume in the stage, divided by the total volume of the stage and the volume occupied by the continuous phase, in the stage, is (1-h) A H. 

---