Practical Process Engineering

#2019

#Horizontal_Knock_out_drum #HKO Let's review HKO sizing equipped with no demister mat per Shell DEP. Advantages: - Bulk separation - Unlimited turndown - High slug handling capacity - Very low pressure drop - Insensitive to fouling - Separation efficiency typically 90% Disadvantages: - Not suitable to remove liquid from mist Applications: - Flare Knock-out drum - Fouling services - For foaming or very viscous services - Slug catcher - Bulk separator

Solution/ I copied some data from the previous example: Gas density= 25.576 Kg/m3 Liquid density= 990.082 Kg/m3 Actual flow rate in m3/s= ? Using HYSYS and Peng-Robinson as the EOS, actual flow rates would be: Stream 4: mass= 4.45 Kg/h Volume= 0.004481 m3/h Stream 5: mass= 72280 Kg/h Volume=2782 m3/h= 0.7727 m3/s Step 1) Volumetric gas load factor calculation: = (1.2* 0.7727) * sqrt (25.576/ (990.82-25.576))= 0.151 m3/s Step2) Vessel diameter calculation First, we two derating factors are computed: Liquid viscosity (From HYSYS)= 0.0005922 Pa.s « 0. 001 ====> viscosity derating factor= 1 Flow parameter= phi= (0.004481/2782) * sqrt(990.082/25.576)= 0.00001 For both half-open pipe and schopentoeter, derating factor of flow parameter would be 0.1. D>= 3.48 sqrt (0.151/0.1) ===> D= 4.3 m Shoepentoeter must be installed. Diameter of this vessel is much larger than the vessel without demister mat. Step3) Level controls are already determined: Minimum distance fro BTL= 0.15 m LZA(LL)= 0.15+ 0.1= 0.25 m LZA(L)= 0.1+ 0.25= 0.35 m LA(H)= 0.35+ 0.35= 0.7 m LZA(HH)= 0.7+0.1= 0.8 m Step 4) Nozzle sizing Feed nozzle diameter criterion: (mean density)* (Velocity^2) <= 8000 mean density was 25.9828 Kg/m3 ===> d1 >=0.2367 = 236.7 mm= 9.32 in. So d1= 10" or 0.254 m d2 & d3 are the same as previous example. Step 5) Vessel height h= 0.8 m X1=0.05 * 4.3= 0.215 m < 0.3 ===> X1= 0.3 m X2= 0.254 + 0.02= 0.274 m X3= 0.45* 4.3= 1.935 m Demister mat thickness= 0.1 m X4= 0.15 m Therefore, total height is: 0.8+ 0.3+ 0.274+ 1.935+ 0.1+ 0.15= 3.559 m ~ 3.6 m Final minimum dimensions are: D= 4.3 m H= 3.6 m d1= 10" d2= 12" d3= 2" LZA(LL)= 0.15+ 0.1= 0.25 m= 250 mm LZA(L)= 0.1+ 0.25= 0.35 m= 350 mm LA(H)= 0.35+ 0.35= 0.7 m= 700 mm LZA(HH)= 0.7+0.1= 0.8 m= 800 mm

Example/ Resize V-1503 of previous example but assume the vessel must be equipped with a demister mat.

This is the same formula used for VKO except one more term is added to consider pressure drop across the demister mat

Please note that all general crteria for the effect of foaming or viscosity, nozzle sizing and level controls discussed during VKO sizing are applicable here. To calculate pressure drop, use following relations, but add schopentoeter pressure drop finally. It was discused earlier.

Step 3) Determine level control (already explained) Step 4) Nozzle sizing (already expalined with details) Vessels with diameter less than 0.5 m are equipped with downward half-open pipe. Schopentoeter is used if diameter is equal or greater than 0.5 m (this criterion was 1.5 m when no demister mat is installed) and inlet nozzle size is equal or larger than 0.15 m (~6"). Step 5) Calculate vessel height: Height= h+ X1+ X2+ X3+ demister mat thickness+ X4 h= LZA (HH) X1= 0.3D with minimum 0.3 m for half-open pipe X1=0.05 D with minimum 0.15 m for schopentoeter X2= Feed nozzle diameter for half-open pipe X2= Feed nozzle diameter+0.02 X3=0.45 D with minimum 0.9 m for half-open pipe X3=Feed nozzle diameter with minimum 0.3 m for schopentoeter Demister mat thickness= 0.1 m X4= 0.15 D with minimum 0.15 m All above dimensions are depicted in following picture:

Schopentoeter

Flow parameter in the above derating formula will not exceed 0.1 practically. However, following magnitudes are recommended by Shell DEP: For vessel equiped with half-open pipe= 0.2 * feed flow parameter (half-open pipe efficiency= 80%) For vessel equiped with schoepentoeter= 0.05* feed flow parameter (shoepentator efficency= 95%)