Bed Depth

Several considerations go into choosing the maximum allowable depth of a packed bed.

Maximum number of theoretical stages generated in a bed e.g., 15 theoretical stages per bed is a rule of thumb used often. But as Table I shows we have observed packed beds of Intalox Structured Packing 1T generating over 21 theoretical stages in a single bed, irrespective of the pour point density.
Table 1 also shows that a single bed of Intalox Structured Packing 2T can generate as many as 32 stages in a single bed.

Height-to-diameter ratio of the bed again, as can be seen from Table 1, the authors have observed that a bed of Intalox structured packing can operate well with a height-to-diameter ratio of up to 15.

Another consideration is the mechanical strength of the packing and the support system for a tall bed. A packed bed depth chosen based on the two criteria listed above can be supported without any problem.

Table 1: Intalox Structured Packing Performance

Chemical Systems to be Distilled

A large number of pilot plant tests are performed during the various stages in the development of a new packing. Since the primary purpose of these tests is to compare the performance of the new packing with other packings, all tests are performed with one or a few chemical systems. For example, Zuiderweg (Circa 1966) lists a number of test mixtures that can be used at atmospheric, vacuum and pressure distillation. FBI typically uses the cyclohexane-heptane system at 34.5 kPa (260 mm Hg Abs), atmospheric pressure and 165 kPa (24 psia), the p-xylene/o-xylene system at 13.3 kPa (100 mm Hg Abs) and lower, and the i-butane/n-butane system at high pressures ranging from 689 kPa (100 psia) to 2758 kPa (400 psia). The SRP typically uses the test system cyclohexane/n-heptane at 33.3 kPa (250mm Hg Abs), atmospheric pressure, 165 kPa (24 psia) and 413 kPa (60 psia).

Historically, NCPPC has used the iso-octane/toluene system at atmospheric pressure and 13.3 kPa (100 mm Hg Abs) as the standard test system. We also have used the cyclohexane-heptane system and p-xylene/o-xylene system as standard test systems.

In the carbon steel tower, NCPPC has tested our packings with numerous other chemical systems, e.g., methanol/water, cyclohexanone/cyclohexanol, acetone/water, water/MEG, to name a few. We also have tested numerous proprietary systems for our customers over the years. Using the data generated from our test columns, as well as commercial installations, NCPPC has developed, efficiency, capacity and pressure drop correlations, which have been used to design world-class high vacuum to high-pressure distillation columns using NCPPC packings.

In the future NCPPC will be adding, to its data bank, test data on high pressure and corrosive systems taken in our high-pressure stainless steel distillation column.

Sampling Techniques

Generation of HETP data from distillation tests requires drawing samples from the system after steady state has been attained. For example, during a distillation test in the rectification mode, liquid samples are drawn from the overhead vapor condensate, from below the packing and from the reboiler. The overhead sample is collected from the discharge side of the reflux pump. The sample from under the packing is collected using a trough type sampler in the shape of a cross; the liquid leaves the sampler from the center and is conducted through tubing to the outside. These samples are chilled, if necessary, to avoid loss due to vaporization and consequent change in composition. At a given boil-up rate, the onset of steady state is monitored by drawing samples periodically, say every l/2 hour, and analyzing the samples. For most binary organic systems, a gas chromatograph is the most convenient analytical tool. The difference between the compositions of the light component in the overhead sample minus that in the packing sample increases gradually until it reaches an asymptotic value at steady state. During the run, three consecutive samples are taken at half hour intervals. Typically, for a good run at steady state HETP values calculated using the three samples differ from one another by no more than 8 mm (0.3 in.). For example, in Figure 5, for every run (i.e., C_s) three HETP values and delta P measurements were obtained. It can be seen that for the majority of runs the three measurements coincide with one another. The time required to attain steady state, after changing the boil-up rate, is typically about two hours for common organic systems with relative volatilities above -1.1. But systems with relative volatility approaching 1, e.g. isotopes, it can take from 16 to 24 hours for the attainment of steady state composition profile in the packed bed. The reboiler sample, together with the sample drawn from under the packing, is used to calculate the number of stages generated in the reboiler - which is usually around 1; this procedure is used as a check on the accuracy of the sample drawn from under the packed bed.

Reproducibility of Test Results

Factors that affect the reproducibility of test results for a given packing, without considering the manufacturing tolerance of the packing are numerous.

Method of packing the distillation column. For structured packings, care should be taken to see that the wall wipers properly engage the tower wall so that bypassing of vapor and liquid through the gap between the packing and the tower wall are minimized, if not eliminated. It is important that consecutive layers of structured packing are rotated by a fixed angle, usually 70" with respect to each other, so that the seams of segmental bundles do not line up (for a 387 mm (15.25 in.) I.D. test tower each layer is made as a single piece). We make sure that no gap is allowed to exist between the packing and manways. A plug contoured in the shape of the tower wall is pressed against the packing. We have noticed that, in the absence of this plug, short-circuiting of liquid and vapor can result in poor packing efficiency.

Analysis of samples - In a typical distillation test in which gas chromatography is used for analyzing the samples, it is important that standards are run every day. It is necessary to make a sufficient number of standards to cover the anticipated range of composition bracketing that of the overhead sample and the re-boiler sample. In a binary mixture, for example, the response factors of the two components can vary over the range composition. The response factors can be drastically different when the composition approaches pure light component and pure heavy component compared to those of a 50-50 mixture.

Insulation of the tower walls- In both our distillation pilot plant, (150 mm (6 in.) thick fiberglass blanket insulation with aluminum wrap is used to cover the re-boiler and the tower wall to minimize condensation of the internal vapor traffic at the tower wall; this condensation would otherwise affect the internal reflux ratio. We try to limit the heat loss to about one percent of the heat input to the reboiler.

Accurate and reproducible pressure drop measurement requires careful design of pressure taps and lines leading to the pressure transducers. It is important to make sure that any vapor that condenses in the lines flows back to the tower without affecting the pressure measurement. The pressure taps are designed so that the opening faces downwards to prevent liquid from entering the tap. A baffle is provided in the opening to prevent vapor from impinging on the opening. This baffle ensures that only static pressure is measured. Typically 12 mm (0.5 in.) diameter tubing, which continuously pitches from the transducer to the pressure tap, assures that any vapor condensing in the line runs back to the tower.

The authors have found that it is possible to obtain reproducible HETP data on the same system-packing combination, after repacking the tower and recharging the system, with variation not exceeding 15 mm (0.6 in.).Download a legacy print-ready version of this article with high-resolution versions of Figures 1 and 2