Page 187 - Applied Process Design For Chemical And Petrochemical Plants Volume II
P. 187

176                      Applied Process Design for Chemical and Petrochemical Plants


            Liquid throw: The horizontal distance traveled by the liq-   Ffactor: The vapor kineticenergy parameter, often used
          uid after flowing over a weir.                         as a correlating term for flooding velocity, foam density, etc.
            DualJlow: Both liquid and vapor pass through the per-   Souder and  Brown equation: G/A  = K[dv (dL - dv)]1/2,
          forations on the tray; there are no downcomers.        where G/A  = superficial vapor flow, lb/ (hr) (ft2), and d,
            Radialflow: Liquid flowing radially from, or to, an inlet   and dL = vapor and liquid densities, lb/ft3. See Equation
           (or outlet) located at the center of the tray, to  (or from)   8-219.
          downcomers (or inlets) at the tray periphery.
            Reverseflow: Liquid flowing from the inlet on one side of   Tower Diameter
           the tray  (around a center baffle) reverses its direction at
          the other side of the tray, and flows back to the downcom-   The tower diameter may be calculated for first approxi-
           er on the same side of the tray where the inlet is.   mation by  the Souders-Brown method; however, this has
             SpZitflow: Liquid flow across the tray is split into two or   been found to be conservative, since it is based on no liq-
          more flow paths.                                       uid  entrainment  between  trays.  Actually, some  entrain-
             Double pass: A split-flow tray with two liquid flowpaths on   ment can be tolerated at negligible loss in efficiency or
           each tray. Each path handles half of the total liquid flow.   capacity.
             Blowing: A condition where  the  rising vapor  punches   There  are  several  approaches  to  column  diameter
          holes through the liquid layer on a tray and usually carries   design  [65,  741 as well as the proprietary techniques of
                                                                 major industrial and engineering designers. Some of these
          large drops and slugs of liquid to the next tray.      use the proprietary Fractionation Research Institute meth-
             Coning: A condition where the rising vapor pushes the   ods which are only available on a membership basis and
           liquid back from the top of  the hole, and passes upward   do not appear in the technical literature.
          with poor liquid contact.                                In  general, a better  first approximation and  often  a
            Dumping: A condition caused by low vapor rates where   more economical tower diameter is determined using Fig-
          all of the liquid falls through some holes (rather than over   ure 8-121 [33].
           the weir) to the tray below, and vapor rises through the   e,  = 0.22  (5)
          remaining holes.                                               (:)     3.2
             Raining: A condition similar to dumping (no liquid goes                                         (8 - 250)
           over the weir) except that, because of higher vapor rates,
           the liquid fall through the holes is more uniform.    S' = St - 2.5 h,                            (8 - 251)
             Weeping: A condition occurring when the vapor rate is
           not large enough to hold all the liquid on the tray, so that   where  e, = weight of liquid entrained/unit  weight of vapor
          part of the liquid flows over the outlet weir while the rest     flowing in sieve tray column
          falls through the holes.                                      (J = liquid surface tension, dynes/cm
             Hooding: A condition that gives rise to a sharp decline in   v,  = vapor velocity based on column cross-section,
                                                                           ft/sec
           tray  efficiency and  a  sharp  increase  in  pressure  drop.   S'  = effective tray spacing, distance between top of
           Flooding is commonly due to either an excessive carryover       foam and next plate above, in.
           of liquid to the next tray, or to an inability of the system to   h,  = height of clear liquid in bubbling zone, in.
           convey the liquid flow to the tray below.
             Oscillation: A wave-type motion of the liquid on the tray,   This is based on a frothed mixture density of 0.4 that of
           perpendicular to the normal direction of flow.        the clear liquid on the tray, and has been found to be a
             Seal  point:  The  point  at  which  a  weeping  condition   reasonable average for several mixtures.
           changes to raining.                                     Entrainment values of 0.05 lbs liquid/lb vapor are usu-
             Injection regime: A condition in which the liquid above   ally  acceptable,  with  0.001  and  0.5  lb/lb  being  the
           the plate is in the form of individual drops dispersed in   extremes. The specific design dictates the  tolerance on
           the vapor; thus, there is virtually no mixing in the main   entrainment. From  the calculated vapor velocity, v,,  the
           bulk of the liquid.                                   diameter of the column can be calculated using:
             Stable regime: The preferable hydrodynamic condition of
           the aerated liquid on a sieve tray. The aerated material
           exists as a stable froth; gas-liquid contact is good.                                             (8 - 252)
             Turndown ratio: A term used by designers to denote ratio
           of minimum-allowable to operating throughput.           Entrainment does not usually become a problem until
             Segmental  downcomer:  The  channel  for  liquid  flow   the tray is operating at 85-100% of the flooding condition.
           formed by an enclosed segmental tray section.         Figure 8-121 is convenient for solving for %.
   182   183   184   185   186   187   188   189   190   191   192