Page 324 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das
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326    Chapter 11 Distillation




                Minimum inner diameter of inlet nozzle, d i;nozzle is
                                                                          1=2
                                        p  ffiffiffi
                                                            mix    u max;nozzle  ; m     (11.41)
                             d i;nozzle ¼ 2 = p   ðm L þ m V Þ= r
                From the standard pipe size chart the nearest higher id pipe is chosen for use as nozzle and its
             corresponding outer diameter value d o;nozzle ðmmÞ is noted.
                Nozzle locations, for a vertical drum


                          h 1 ¼ 900 þ d o;nozzle ðmmÞ=2; mm or 1200 mm; whichever is higher
                          h 2 ¼ 300 þ d o;nozzle ðmmÞ=2; mm or 450 mm; whichever is higher.  (11.42)
                Time to fill the drum from minimum to the maximum liquid level is decided based on the emer-
             gency response of the operator and this is usually between 3 and 6 min. This is the surge time (t s , sec).
                Surge volume V s ¼ t s  ðm L =r Þ; m 3
                                          L
                                                          2
                                          h L ¼ð4=pÞ  V s =d ; m
                                                                                         (11.43)
                                          h Total ¼ h 1 þ h 2 þ h L
                                          h Total =d ¼ðh 1 þ h 2 þ h L Þ=d
                Desirable h Total /d is between 3 and 5. In case h Total /d exceeds 5, a horizontal drum is chosen.
                Outlet nozzles for vapour and liquid
                Sizing of liquid and vapour outlet nozzles are based on typical vapour line velocity around 15 m/s
             and the liquid line velocity between 2 and 3 m/s.
                Orientation and location of the remaining nozzles are done based on operability consideration, e.g.,
             the drain and the vent are located at the highest and lowest points in the drum, utility connections (plant
             air and water) are located at accessible positions, etc.
                Finally, a general arrangement drawing of the drum is prepared that shows all nozzles, fixtures and
             vessel support. This drawing becomes the basis for the fabrication drawing of the vessel.


             11.7 Design illustration e flash distillation
                                  3
             Design problem: A12 m /hr stream of ortho-xylene is contaminated with 1%w/w benzene and 1.45%
             w/w toluene. Design a flash distillation process to bring down benzene content in the stream to below
             0.5%w/w.
                Solution
                The problem states the composition limit on the liquid stream from the drum. This needs to be met
             while ensuring that the product yield should be as high as possible. This sets the benzene content in the
             liquid stream to 0.5% w/w. Component properties are tabulated in Table 11.11.
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