Restricted columns

Figure 14.4 Schematic diagram of the cliromatographic system used for the analysis of very low concentrations of sulfur compounds in ethene and propene CP, pressure regulator CF, flow regulator SL, sanrple loop R, restriction to replace column 2 VI, injection valve V2, tliree-way valve to direct the effluent of column 1 to either column 2 or the restriction column 1, non-polar- capillary column column 2, tliick-film capillary column SCD, sulfur chemiluminescence detector FID, flanre-ionization detector.

If the concentration and amount of sample are not restricted, columns of larger inner diameter are advantageous, because of decreasing contributions of wall effects and external peak broadening to the overall column dispersion, as has been shown by several workers . For analytical purposes, columns up to 10 mm I.D. can be used. 

The support ring width should not be excessive, or it may restrict the column capacity. Strigle (386) gives the maximum recommended support ring width as a function of column diameter for columns between 4 and 12 ft in diameter. This width, W, in inches, can be calculated from W = D/4 + 0.5, where D is the tower diameter in feet. For small columns (< 4 ft in diameter), support rings sometimes restrict column capacity (386). 

The employer must check the box that reflects the most severe outcome associated with a given injury or illness. The severity of any case decreases on the log from column G (Death) to column J (Other recordable case). Since days away from work is a more severe outcome than restricted work the employer is required to remove the check initially placed in the box for job transfer or restriction and enter a check in the box for days away from work (column H). Employers are allowed to cap the number of days away and r restricted work/job transfer when a case involves 180 calendar days. For purposes of recordability, the employer would enter 180 days in the Job transfer or restriction column and may also enter 1 day in the Days away from work column to prevent confusion or computer related problems. 

OPTIONAL DEVICE FOR RESTRICTING COLUMN OPENING (SEE NOTES)... 

Once you know your establishment s NAICS code, go to httpy/www.bls.gov/iLfioshsmn.htm. Find the Summary Table called Table 1 - Incidence rates - detailed industry level. Scroll through the BLS table and find the corresponding NAICS code in the left-hand colmnn. (NAICS data are presented in numeric order in the BLS Table). To compare your injury and dlness rates, use the number listed in the Total recordable cases column for your NAICS as the industry average incidence rate. Use the number listed in the total from the Cases with days away from work, job transfer, or restriction column as the industry average DART rate. Note At press time, the most current data available were for calendar year 2011 injuries and illnesses 2012 should be available soon. 

These heuristics are based on observations made in many practical applications. In addition to being restricted to simple columns, the observations are based on no heat integration (i.e., all reboilers and condensers are serviced by utilities). Difficulties can arise when the heuristics are in conflict with each other, as the following example illustrates. 

Whether heat integration is restricted to the separation system or allowed with the rest of the process, integration always benefits from colder reboiler streams and hotter condenser streams. This point is dealt with in more general terms in Chap. 12. In addition, when column pressures are allowed to vary, columns with smaller temperature differences are easier to integrate, since smaller changes in pressure are required to achieve suitable integration. This second point is explained in more detail in Chap. 14. 

The final restriction of simple columns stated earlier was that they should have a reboiler and a total condenser. It is possible to use materials fiow to provide some of the necessary heat transfer by direct contact. This transfer of heat via direct contact is known as thermal coupling. 

Unless there are constraints severely restricting heat integration, sequencing of simple distillation columns can be carried out in two steps (1) identify the best few nonintegrated sequences and (2) study... 

Establish simple sequences. Using methods described in Chap. 5, sequences of simple columns with low overall vapor load are established. Consideration should not be restricted to the single sequence with the lowest overall vapor load, since many factors need to be considered in finally arriving at the best design. 

When fluid flows past objects or through orifices or similar restrictions, vortices may periodically be shed downstream. Objects such as smokestacks, chemical-processing columns, suspended pipehnes, and electrical transmission lines can be sul ected to damaging vibrations and forces due to the vortices, especially if the shedding frequency is close to a natural vibration frequency of the objecl. The shedding can also produce sound. See Krzywoblocki (Appl. Mech. Rev., 6, 39 397 [1953]) and Marris (J. Basic Eng., 86, 185-196 [1964]). 

Unhke the RDC and Oldshue-Rusnton columns where the specific throughput of the scaled-up version is the same as the pilot column, it is the characteristic of the Scheibel column that the throughput of the scaled-up column is on the order of 3-5 times greater than that acheived on the pilot column. The reason for this is that the restricted geometry of the 0.075 m diameter column limits throughput these restrictions are removed in the scaled-up columns. 

Now, the maximum sample volume (Vi) that can be placed on the column that would restrict the increase to less than 5% has been shown to be. 

In fractionation columns for petroleum products, where the oxygen content is restricted, higher temperatures can be used without excessive waste of the metal. 

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