Practical Process Engineering

Sizing procedure for VM separator based on Shell DEP: Step 1) Calculate volumetric gas load factor (already explained) Step 2) Calculate vessel diameter based on following gas handling criteron.:

Lower efficiency due to lower gas flow rate is offset by larger droplets.

#Vertical_Wiremesh_Demister #VW #Sizing_Shell_DEP I give you a summary of sizing a vertical wiremesh demister. It is a vertical knock-out drum equipped with a wiremesh demister pad. -Advanteges: High turndown ratio (factor 4) High slug handeling capacity High removal efficiency greater than 98% Low pressure drop -Disadvantages: Sensitive to fouling (wax, asphaltenes, sand, hydrates) Not recommended for viscous fluids where de-gassing criterion must be satisfied Applications: Compressor suction scrubber Non-fouling services Production/test separators (non-fouling with moderate GOR) Inlet/outlet scrubber for glycol contactor ... Mist mat specofocations Free volume of at least 97% (epsilon= 0.97) 0.23 mm <Wire Thickness < 0.28 mm Shell DEP enforces that perforated plates not to be installed upstream of a demister since accumulated water between the perforated plate and the demister mat, detriorates its performance. Lower flow rate than the design results in larger droplets which are easier to separate, but if the droplet size is constant, lower flow rate than 40% leads to lower separation efficency. See following picture:

Solution: Assumption: 1- To simplify the sizing process, I do not consider the demister pad. 2- The inlet device is a half-open pipe 3- Surge margin is 20% (it means multiply gas flowrate by 1.2 to have the maximum flowrate) 4- C7+ is C7 Step 1) What data we have? 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 2) Volumetric gas load factor calculation: = (1.2* 0.7727) * sqrt (25.576/ (990.82-25.576))= 0.151 m3/s Step 3) Minimum vessel diameter calculation: D>= 4.26* sqrt( volumetric gas load factor)= 4.26* sqrt (0.151)= 1.655 m ~ 1.7 m Step 4) Level control calculation: I use all minimum level controls as no pump is installed on liquid outlet and the liquid outlet flow rate is small. So: 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 LZA(HH) is called h and we use it to calculate vessel height later. Step 5) Nozzle sizing: Inlet device is a half-open pipe. Mean density= (4.45+72280)/(0.004481+2782)= 25.9828 Kg/m3 Feed nozzle (d1): Criterion: (feed density)* (feed velocity^2)<= 2100 ===> (25.9828)*(feed velocity^2)<= 2100 Feed velocity= Volumetric flowrate (m3/s)/ Area= 0.7727/ (pi/4 * (d1 ^2))= 0.9838/ (d1 ^2) ===>(25.9828)* (0.9838/ (d1^2))^2<=2100 ===>d1>= 0.3306 m or d1>= 13.01" (velocity<= 9 m/s) So: d1= 14" or 0.3556 m Gas outlet criterion: gas density * (gas velocity)^2 <= 4500 ===> 25.576 * (0.7727/(pi/4 *(d2 ^2))^2 <= 4500 ===> d2>= 0.2723 m or 10.72" So: d2= 12" Liquid outlet criterion: Max velocity= 1 m/s and minimum size= 2" Volumertic flow rate= 0.004481 m3/h= 0.00000124 m3/s Q=V.A & V= 1m/s ===> A= 0.00000124 m2 ===> d3= 0.00125 So d3= 2" Result: d1= 14" d2= 12" d3= 2" Step 6) Vessel height: h= 0.8 m X1=0.3* 1.7= 0.51 m X2= d1= 14"=0.3556 m X3= 0.9* 1.7= 1.53 m ===> height= 0.8+0.51+0.3556+1.53=3.1956 m~ 3.2 m

Example/ Size V-1503 in folllowing PFD. Stream 2 enters V-1503 and streams 4 and 5 are outlet liquid and gas respectively.

The schoepentoetor pressure drop should be added to previos pressure drop calculation.

Pressure drop calculation.

Please note that Shell DEP limits feed enterance velocity such that errosion, vibration and chokings are prevented. 1- The maximum gas velosity should be limited to maximum 70 m/s. While 2- It is less than or equal to 80% of the sonic velocity:

Liquid outlet nozzle is sized to limit liquid velocity to 1 m/s and its minimium size is 2". However, higher velocity up to 2.5 m/s is acceptable if minimum liquid static head can provide this higher velocity per follwing formula: h= 100 * (V)^2 h: in mm V: Liquid outlet velocity h is already calculated based on level control criteria. For example: if h is 500 mm, V can be up to 2.236 m/s.

Gas outlet nozzle sizing criterion. If it leads to high velocity and unacceptable pressure drop in vacuum system, you can enlarge the nozzle to meet pressure drop criterion.

density times the (velocity^2) gives the fluid momentum. For fluids with no liquids, momentum can be up to 8000 Pa. Larger momentum than what is recommended by Shell DEP leads to smaller droplets and lower separation efficiency.

Feed nozzle sizing per Shell DEP

#Nozzle_Sizizng I missed nozzle sizing. Here we have another general discussion not limited to VKD's. For a VKD, Shell DEP recommends half-open pipe inlet device with downward opening and limits shoepentator when diameter is larger than 1.5 m. Also, if gas velocity tends to be high because of low density of the gas phase, schoepentoeter is mandatory. There would be a trade-off between cost and separation efficiency. Schoepentoeter has better separation efficincy but it is more expensive. The feed nozzle diameter could have the same size the feed pipe but following criteria must be satisfied:

VKD sizing per Shell DEP is done and I take one example tomorrow night.

As discussed, three criteria are adopted to determine the VKD diameter. Here, Shell DEP has some recommendations: 1- For low gas concentration (<5%) systems, degassing criterion dominates foaming criterion.The foaming is more effective if the gas concentration exceeds 5%. 2- For foamy or viscous liquids, the degassing and foaming sriteria can overrule gas handling criterionto determine minimum VKD diameter. Following diagram gives minimum diameter based on flow parametr and liquid viscosity and you see 2nd and 3rd criteria overrule the first criterion in high flow parameter areas.