Maximum operational capacity

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

Pressure drop should not exceed at maximum operating capacity [94] [139] ... 

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].

MOC At maximum operational capacity VWe Weber number = (UlpLS)/(Ggc)... 

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. 

First and foremost, the alternative capacity criteria—namely the maximum operational capacity and the maximum pressure drop— have been demonstrated to be leBS reliable than the flood point (see Secs. 8.2.4 and 8.2.5). Bolles and Fair (55), MacOougall (58), and... 

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). 

Second, flood point data are easy to find, while maximum operational capacity data are less abundant. Generating maximum operational capacity data requires expensive efficiency measurements, while flood-point determination requires far less expensive tests. 

Third, maximum operational capacity data are practically nonaxist-ent at high liquid rates. Efficiency measurements are usually performed at or close to total reflux (liquid to vapor mass ratios of about unity) in order to prevent pinching from impairing data accuracy. In order to obtain data at high liquid rates, liquid to vapor mass ratios of the order of 2 to 3 or more are usually required. 

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). 

Flood and maximum operational capacity (MOC) data are also plotted on the GPDC interpolation charts, and the charts can be used for interpolating these. Where flood and MOC data are absent, Eqs. (8.1) and (8.10) can be used for inferring Good points and MOCs from pressure drop data on the charts. 

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... 

Interpolation of experimental flood, pressure drop, and maximum operational capacity (MOC) data is the most reliable and accurate method available for predicting flood, pressure drop, and MOC. As pointed out in Secs. 8.2.6 to 8.2.9, use of correlations to predict these parameters can lead to poor and dangerously optimistic results in many situations frequently encountered in commercial practice. 

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