Packing maximum operational capacity

Select a design Q vapor rate as a percent of the MOC (maximum operating capacity) of the packing. 

The recommended design procedure for an approximate evaluation utilizes a final design vapour-rate, Cs, which is a percentage of the reduced maximum operational capacity factor (usually between 80% and 87%). Note that in high-pressure operations, the usable hydraulic capacity of the tower packing may be reached because of excessive liquid hold-up before the... 

Figure 6.2-2. Maximum operational capacity for IMTP Norton packings versus flow parameter [18].

The maximum operational capacity or throughput (often also referred to as maximum efficient capacity) is defined (Strigle, Packed Tower Design and Applications, 2d ed., Gulf Publishing, Houston, Tex., 1994) as the Maximum vapor rate that provides normal efficiency of a packing (i.e., point B in Fig. 14-54). The MOC is clear-cut in Fig. 14-54. On the other hand, locating the MOC in other cases is difficult and leaves a lot of room for subjectivity. 

Based on the above, some designers argued (5,15,57) that the traditional approach of using the flood point as the upper capacity limit of a packed column is best abandoned. They recommended shifting to alternate criteria such as maximum operational capacity or maximum permissible pressure drop. Others (3,37,41,51,58-60) stick with the flood-point criterion while recognizing its limitations. There are strong, practical reasons for retaining the flood point as the prime criterion for the upper capacity limit. 

Kister and Gill (60,60a) demonstrated that despite differences in definitions, flood-point data compared quite well to correlation predictions. Both Kister and Gill (60,60a) and MacDougall (53) show that flood data from various sources (using various definitions) can be correlated to with 10 to 15 percent accuracy. It was also demonstrated that the flood point can be predicted far more reliably than packing pressure drop (55,58) and maximum operational capacity (60). 

The maximum operational capacity or throughput is defined (15) as the Maximum vapor rate that provides normal efficiency of a packing (i.e., point F in Fig. 8.16a). 

MOC. Strigle et al. (15,57,97) recommended designing packed towers with a 10 to 20 percent margin from the maximum operational capacity (MOC). Since the MOC iB usually about 5 percent below the flood... 

In distillation, maximum operational capacity (sometimes called efficient capacity because of the nature of the definition) is determined by the amount of liquid entrainment required to reduce separation efficiency (see Chapter 7). Data of Strigle and Rukovena for IMTP, metal Pall Ring, and ceramic Intalox saddle packings indicate that the pressure drop at maximum operational capacity is [11] ... 

The use of maximum operational capacity produces a design that provides the desired efficiency, and the packed bed does not operate in an unstable region near its hydraulic capacity limit. 

This gas rate represents only 53% of the maximum operational capacity at a flow parameter of 0.0400. Therefore, the replacement of the 1-in. Pall Ring packing with Intalox structured packing 2T easily should permit an increase of 50% in the air rate through this scrubber. 

By experimentation with systems such as those listed in Table 7-1, the maximum operational capacity for Intalox Metal Tower Packing was determined. This capacity was expressed as a function of the flow parameter and system properties by Dolan and Strigle [18]. The flow parameter is a ratio of the square roots of the liquid kinetic energy to the vapor kinetic energy ... 

Figure 7-6 is a plot of the maximum operational capacity as a function of the flow parameter for IMTP packing. These curves indicate that the maximum capacity of this packing is reduced by approximately 0.2 times the log of the flow parameter as the liquid rate or operating pressure is... 

Equation 7-15 also is used to calculate the maximum operational capacity of this structured packing. The values of are taken from Fig-... 

From Figure 7-6, the Cq value for 40 IMTP packing is 0.369 ft/s. After adjustment of the maximum capacity for system properties, the trayed column operating at a Q of 0.231 ft/s is at only 65% of the maximum operational capacity of the 40 IMTP packing. 

Based on the previous calculations, the use of Intalox structured packing 3T will permit an increase of 45% in the feed rate for the present trayed column. With this packing, the column would operate at 75% of maximum operational capacity at the bottom of the rectifying section. The bottom of the stripping section would run at not more than 87% of maximum operational capacity in order to provide a 15% safety factor. 

The maximum operational is the greatest vapor flow rate attained before loss of normal separation efficiency of the packing (see Chapter 7). In vacuum distillations, massive entrainment of liquid upward in the vapor phase throughout the packed bed will limit the maximum operational capacity (maximum C ) because it reduces the separation efficiency. However, if liquid entrainment only occurs at the top of the bed, as long as the entrained liquid carried into the condenser is of the same composition as the reflux liquid, the separation is not impaired. Thus, some entrainment in the operation of the column is acceptable and simply constitutes a recycle of liquid in addition to the usual reflux. 

The maximum operational Cg value at the top of this column (from Figure 7-7) is 0.462 ft/s at a flow parameter of 0.010, after adjustment for physical properties. Thus, the present flow rates in the trayed column can be increased by as much as 65%. The column top, with Intalox Structured packing 2T at 165% rates, will operate at only 87% of the maximum capacity of this structured packing. 

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. 

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