Column design methods Packed columns

Use the HETP method to design a packed column. Determine the HETP from data... 

For many years now, there has been a constant demand for low pressure drop column internals in rectification, absorption, stripping and liquid/hquid extraction. The prevailing trend in the chemical industry is to replace tray columns with those containing modem structured and random packings. When planning the design of packed columns, it therefore particularly important to use reliable methods for predicting the mass transfer and hydrodynamic behaviour of the two-phase flow. 

Packed columns are used for distillation, gas absorption, and liquid-liquid extraction only distillation and absorption will be considered in this section. Stripping (desorption) is the reverse of absorption and the same design methods will apply. 

For the absorption of soluble gases or particulates from the gas streams, scrubbers can be used. These are generally simple packed columns, spray towers, or Venturi units. Design methods are well established [236]. Attention must be paid to the prevention of sprays, mists, and carryover when wet scrubbers are used. 

In this chapter consideration is given to the theory of the process, methods of distillation and calculation of the number of stages required for both binary and multicomponent systems, and discussion on design methods is included for plate and packed columns incorporating a variety of column internals. 

The curves represent a plot of Log.(/V),(Reduced Plate height)against Log.(v), (Reduced Velocity). The lower the Log.(/7) curve versus the Log.(v) curve the better the column is packed. At low velocities the (B) term dominates and at high velocities the (C) term dominates as in the Van Deemter equation. The best column efficiency is achieved when the minimum is about 2 particle diameters and thus, Log (.ft) Is about 0.35. The minimum value for (H) as predicted by the Van Deemter equation has also been shown to be about two particle diameters. The optimum reduced velocity is in the range of 3 to 5 that is Log.(v ) takes values between 0.3 and 0.5. The Knox equation is a simple and effective method of examining the quality of a given column but, as stated before, is not nearly so useful In column design due to the empirical nature of the constants. 

Another feature of the breakthrough method is that the uniform geometry of the packed column permits fairly straightforward analysis of the thermal waves produced due to heats of adsorption, and of their effect on the shape of the sorption fronts. The most obvious advantage of this method, however, is the fact that the results of the breakthrough experiments can be applied rather directly to the design of commercial adsorbers, with relatively little analysis of the data. 

Column design and preparation incorporated previously described methods reported in the literature (39). Two different adsorbents were employed a 100/120 mesh crosslinked styrene/ divinylbenzene resin (Polypak P-Waters Associates) and a Woelm aniontropic activity grade alumina. These adsorbents were packed in 300 and 94 cm. stainless steel columns having a 1 mm. internal diameter. Pressure drop across the adsorbent bed was kept to a minimum (<0.02 atm.) by using a heated pressure reduction valve at the end of the column. Typical linear flow velocities through the columns were in the range of 0.27-2.17 cm/sec. 

When an estimator costs pressure vessels such as reactors and distillation columns, care must be taken to ensure that the wall thickness is adequate. The default method in IPE calculates the wall thickness required based on the ASME Boiler and Pressure Vessel Code Section VIII Division 1 method for the case where the wall thickness is governed by containment of internal pressure (see Chapter 13 for details of this method). If other loads govern the design, then the IPE software can significantly underestimate the vessel cost. This is particularly important for vessels that operate at pressures below 5 bara, where the required wall thickness is likely to be influenced by dead weight loads and bending moments from the vessel supports, and for tall vessels such as distillation columns and large packed-bed reactors, where wind loads may... 

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