Practical Process Engineering

this diameter limits the liquid velocity

Foaming: The 3rd criterion is to limit liquid velocity according to following equation for foaming services:

Liquid velocity should satisfy this criterion. If bubbles are larger than 200 micro meter, carry-under is negligible.

The last parameter to size a VKD was h that is equal to LZA(HH). Now you are able to calculate the distance between BTL and TTL. So far only one criterion (gas load factor= 0.07) has been explained for VKD sizing to have enough gas handeling capacity and minimize liquid carry-over. There are still two more issues to be considered: Vapor carry-under: Gas carry-under is against liquid carry-over; The later means liquid droplets are carried by vapor phase and the earlier means liquid phase takes some gas buubles out. If the maximium allowable of carry-under is less than 1% (i.e. the ratio of gas volume in liquid phase to total volume gas is less than 1%), vessel diameter must satisfy another criterion called liquid de-gassing criterion:

Let's have a general discussion on recommended level spacings by Shell DEP: 1- LOW LOW LEVEL or LZA(LL): It is low level trip. Lower tapping of the sight glass is connected to 0.15 m above the vessel bottom (TBL for vertical drum or the vessel bottom for horizontal drum). Low level trip should not be less than 15% of the sight glass span (preferable 15%-85% or 20% to 80% for processes with no trip). LZA(LL) should be at least 0.1 m above the bottom connection of the sight glass. So, The minimum LZA (LL) or low level trip is 0.25 m above TBL for vertical drums or vessel bottom for horizontal drums. 2- LOW LEVEL or LA(L): At least 0.1 m above the LZA(LL). Generally, it depends upon the process requirement for operator intervention. It is recommended by Shell DEP to keep 1-2 min liquid hold-up for control room operator intervention and 5 min liquid hold-up for outside operator action. 3- HIGH LEVEL ALARM or LA(H): At least 0.35 m above the LA(L). Following hold-ups are recommended: - 3 min on total product outflow for separator trains - 5 min on total product outflow from a column to a furnace or to another column - 10 min on total product outflow for surge drums 4- HIGH HIGH LEVEL ALARM or LZA(HH) At least 0.1 m (or 0.35 m for liquid with foaming tendency) above the LA(H). Liquid hold-ups and time for operator intervention is the same as LA(L).

H= h+ X1+ X2+ X3 h: the height between BTL and maximum liquid holdup level X1: The height beween maximum liquid holdup level and the nozzle X2: See the pictures X3: The height between the nozzle and the TTL The recommended value for X1, X2, X3 are as below : (VKD with half-open pipe inlet device/ VKD with schoepentator inlet device) X1= 0.3D/ 0.05D with minimum 0.3 m/ 0.15 m X2= See details on the picture X3=0.9D/0.6D with minimum 0.9 m / 0.6 m The calculation for h is a little bit different and will be discussed in next part.

This is a typical layout of a VKD. We have the diameter and now we can calculate the distance between bottom tangent line (BTL) to top tangent line (TTL):

So, the minimum diameter is calculated by this simple formula

For a vertical vessel: 1- Without weirmesh, this minimum cross sectional area is the same as vessel cross sectional area 2- With weirmesh, it would be the cross sectional area of the weirmesh that is smaller than the vessel itself. The details for horizontal separators will be explained later. Based on the shell DEP, the maximum landa for a VKO is 0.7

Based on the maximum amount of Landa and volumetric gas load factor, minimum cross sectional area for gas flow is computed:

#Separator_sizing #Shell_DEP #Vertical_knock_out_drum #VKO Advantageous: 1- Unlimited turndown (maximum flow rate/minimum flow rate) 2- High capacity to handle slugs 3- High liquid removal efficiency (around 90%) 4- Good option for bulk separation 5-Very low pressure drop 6-Insensitive to fouling Disadvantages Poor removal efficiency of liquid from mist Typical applications 1-Vent and flare knock-out drum (which need minimum internals) 2-Bulk separator 3-Flash vessel First os all, another parameter called GAS LOAD FACTOR (K-Factor or Souders-Brown factor) should be defined:

VKO- Vertical knock-out drum SVS- Schoepentoeter-vane pack-swirldeck separator HKO- Horizontal knock-out drum SMSM- Schoepentoeter-mistmat-swirldeck-mistmat separator VW- Vertical wiremesh demister CT- Cyclone with tangential inlet HW- Horizontal wiremesh demister CS- Cyclone with straight inlet and swirler ("Gasunie" cyclone) VV1- Vertical in-line separator with vane pack VRMC- Vertical separator with reversed-flow multicyclone bundle ( conventional multicyclone) VV2- Vertical two-stage separator with vane pack FS- Filter separator HV- Horizontal vane-type demister SMS- Schoepentoeter-mistmat-swirldeck separator

Selection chart recommended by Shell

These parametrs should be evaluated to choose a separator. Gas load factor will be discussed later during the sizing procedure.

It is recommended by Shell if there are two immiscible liquids, use the lower density as the liquid phase density providing that volumetric flowrate of the lighter liquid is at least 5% of the total volumetric flowrate.

SEPARATION EFFICENCY

FLOW PARAMETER