Packing disadvantages

Use of the peUetted converter, developed and used by General Motors starting in 1975, has declined since 1980. The advantage of the peUetted converter, which consists of a packed bed of small spherical beads about 3 mm in diameter, is that the pellets were less cosdy to manufacture than the monolithic honeycomb. Disadvantages were the peUetted converter had 2 to 3 times more weight and volume, took longer to heat up, and was more susceptible to attrition and loss of catalyst in use. The monolithic honeycomb can be mounted in any orientation, whereas the peUetted converter had to be downflow. AdditionaUy, the pressure drop of the monolithic honeycomb is one-half to one-quarter that of a similar function peUetted converter. 

The disadvantage is that volumetric efficiency is usually much less than conventional trays or packed contactors. Applications are usually limited to cases when only a few transfer units or a single eqiiihbriiim stage is required. Since many of these applications tend to be in heat-transfer sei vice, the following discussion will be in terms of thermal properties and thermal measures of performance. 

Disadvantages of these continuous countercurrent systems are associated primarily with the complexity of the equipment required and with the attrition resulting from the transpoiT of the ion exchanger. An effective alternative for intermediate scale processes is the use of merry-go-round systems and SMB units employing only packed-beds with no movement of the ion-exchanger. 

TABLE 25-18 Advantages and Disadvantages of Absorption Systems (Packed and Plate Columns)... 

B. Advantages and Disadvantages of Porous Silica Packings for HPSEC... 

Moisture-cured urethanes These are the only paints in this class of materials that are single pack. The curing is provided by moisture from the atmosphere. This has the advantage that the material can tolerate a degree of dampness in the atmosphere and on the surface to be coated. Its disadvantage is that the film thickness per coat is limited and that, once opened, the entire container must be used immediately. Conversely, if the humidity is very low the cure may be lengthened or stopped. 

Excellent open tubular columns may now be purchased, providing a number of stationary phases of differing polarity on WCOT and SCOT columns, and whose efficiency, greatly improved sample detectability, and thermal stability surpass those exhibited by packed columns their chief disadvantage is that they have a lower sample capacity than packed columns.65,66... 

Unfortunately, exclusion chromatography has some inherent disadvantages that make its selection as the separation method of choice a little difficult. Although the separation is based on molecular size, which might be considered an ideal rationale, the total separation must be contained in the pore volume of the stationary phase. That is to say all the solutes must be eluted between the excluded volume and the dead volume, which is approximately half the column dead volume. In a 25 cm long, 4.6 mm i.d. column packed with silica gel, this means that all the solutes must be eluted in about 2 ml of mobile phase. It follows, that to achieve a reasonable separation of a multi-component mixture, the peaks must be very narrow and each occupy only a few microliters of mobile phase. Scott and Kucera (9) constructed a column 14 meters long and 1 mm i.d. packed with 5ja... 

As a further disadvantage, it is known concerning operation in many parallel micro channels that mixed flow patterns and even drying of the channels can occur [9, 10]. This comes from phase maldistribution to the channels. To overcome this problem, first solutions for phase equipartition have been proposed recently, but so far have not been applied for the mixers described here, but instead for mini-packed reactors, having feed sections similar to the mixers [11,12]. Nevertheless, numbering-up of dispersive-acting micro devices generally seems to be more complicated than for two-phase contactors (see Section 5.1.1). 

Frozen reference materials have been produced by NIST (Wise et al. 1993). These materials do not have the disadvantages of the oils or freeze-dried materials, but are more difficult to transport. Obviously they have to be kept deep-frozen during transport, which makes their use rather expensive. Since the early 1990 s a new approach in this field has been introduced. This concerned the use of wet, sterilized fish and shellfish samples. These samples, packed in glass jars or in tins, were firstly used in the QUASIMEME program as reference materials for inter-laboratory studies (de Boer 1997). Later, when it appeared that the stability was maintained for longer periods, tests for organic contaminants based on this principle were also prepared. 

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