{"id":2155,"date":"2015-09-01T00:00:00","date_gmt":"2015-09-01T05:00:00","guid":{"rendered":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/?p=2155"},"modified":"2015-09-01T10:54:06","modified_gmt":"2015-09-01T15:54:06","slug":"gas-liquid-separators-sizing-parameter","status":"publish","type":"post","link":"http:\/\/www.jmcampbell.com\/tip-of-the-month\/2015\/09\/gas-liquid-separators-sizing-parameter\/","title":{"rendered":"Gas-Liquid Separators Sizing Parameter"},"content":{"rendered":"

Download PDF Version<\/a><\/p>\n

In the December 2014 tip of the month<\/a> (TOTM) [1], we discussed troubleshooting of gas-liquid separators for removal of liquids from the gas stream leaving the separator. There are two methods for sizing gas-liquid separators: 1. Droplet settling theory method, 2. Souders-Brown approach. Historically the Souders-Brown equation has been employed as it can provide reasonable results and is easy to use, but has shortcomings in terms of quantifying separator performance.\u00a0 References [2-4] provide the details on the droplet settling theory methods which can be used to more accurately quantify separator performance.\u00a0 The Souders-Brown method is limited in that it is based on the average droplet size, but cannot quantify the amount of liquid droplets exiting the gas gravity section.<\/p>\n

In this TOTM, we will focus on the application of Souders-Brown approach in gas-liquid separators and present diagram, simple correlations and tables to estimate the Souders-Brown equation constant, KS<\/sub><\/em> (the so called sizing parameter). We will consider both vertical and horizontal gas-liquid separators. Knowing the actual gas flow rate through the vessel, one can use KS<\/sub><\/em> parameter to determine the maximum allowable gas velocity through the vessel and determine the required separator diameter. One can also use the appropriate value of KS<\/sub><\/em> to size the mist extractor in the vessel. The performance of a gas-liquid separator is highly dependent on the value of KS<\/sub><\/em>; therefore, the choice of appropriate \u00a0KS<\/sub><\/em> \u2013values is important.<\/p>\n

Gas Gravity Separation Section<\/strong><\/p>\n

The gas gravity separation section of a separator has two main functions:<\/p>\n

    \n
  1. Reduction of entrained liquid load not removed by the inlet device<\/li>\n
  2. Improvement \/ straightening of the gas velocity profile.<\/li>\n<\/ol>\n

    Most mist extractors have limitations on the amount of entrained liquid droplets that can be efficiently removed from the gas, thus the importance of the gas gravity section to remove the liquids to an acceptable level upstream of the mist extractor.\u00a0 This is particularly important for separators handling higher liquid loads. For scrubber applications with low liquid loadings, \u00a0the KS<\/sub><\/em> \u2013values will be primarily dependent on the mist extractor type, and the gas gravity separation section becomes less important. For the higher liquid load applications, i.e. conventional separators, there are two approaches for sizing the gravity separation section to remove liquid droplets from the gas:<\/p>\n

      \n
    1. The Souders-Brown approach (Ks <\/sub><\/em>Method)<\/li>\n
    2. Droplet settling theory<\/li>\n<\/ol>\n

      The Souders-Brown Approach<\/strong><\/p>\n

      If we consider a spherical liquid droplet with a diameter of DP<\/sub><\/em> in the gas phase two forces as shown in Figure 1 act on it. The drag force, FD<\/sub><\/em>, is exerted by flow of gas and gravity force, FG<\/sub><\/em>, is exerted by the weight of droplet. The drag force acts to entrain the liquid droplet while the gravity force acts to pull it down and separating it from the gas phase.<\/p>\n

      \"Figure
      Figure 1. Schematic of the forces acting on a liquid droplet in the gas phase [5]<\/figcaption><\/figure>Assuming plug flow with no eddies or disturbances, a single droplet and ignoring the end effect, at equilibrium (free fall or terminal velocity), these two forces are equal.<\/p>\n

      \"Fd=Fg\"\u00a0 \u00a0 \u00a0 \u00a0(1)<\/p>\n

      As presented in the Appendix, substitution of expressions for the drag and gravity forces in Equation 1, the maximum allowable gas velocity, VGmax<\/sub><\/em>, which prevents entrainment of liquid is obtained.<\/p>\n

      \"eq2\"<\/p>\n

      Equation 2 is called Souders-Brown [6] equation and KS<\/sub><\/em> is referred to as the design or sizing parameter. The terms \u03c1G<\/sub><\/em> and \u03c1L<\/sub><\/em> are the gas phase and liquid phase densities, respectively.<\/p>\n

      Once the maximum gas velocity, VGmax<\/sub><\/em>, through the vessel is determined by Equation 2, one can calculate the minimum vessel diameter, Dmin<\/sub><\/em> by Equation 3.<\/p>\n

      \"eq3\"<\/p>\n

      Where:<\/p>\n

      FG<\/sub><\/em>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 = Fraction of cross section area available for gas flow (FG<\/sub><\/em> = 1 for vertical separators and is a function of liquid height for horizontal separators)<\/p>\n

      qa<\/sub>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 <\/em>= Gas flow rate at the actual flowing condition<\/p>\n

      The Design Parameter, KS<\/sub><\/em><\/strong><\/p>\n

      The design parameter, KS<\/sub><\/em>, in the Souders-Brown equation is an empirical parameter and is a key factor for the sizing the gas-liquid separators vessel diameter as well as for determination of the mist extractor diameter. Its value depends on several factors including:<\/p>\n