Wednesday, May 14, 2008

Properly Simulate a Separator with Demister in HYSYS

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Vapor Liquid separation can be simulated in HYSYS using 2-phase separator. Following is an example with below feed gas at 32 barg @ -30 degC feeding a vapor liquid separator. Feed gas composition are N2 (0.38%), CO2 (0.68%), C1 (90.4%), C2 (4.1%), C3 (2.6%) and nC4 (1.84%), all in mole percent. Demister is provided in the vapor phase and maximum allowable pressure of 15 kPa is assumed. Those would like to understanding details of demister, read more in "Quick Understanding & Estimation of Mist Eliminator in Gas-Liquid Separator".

Why liquid in Vapor outlet ?
Above has been simulated HYSYS. It is pretty simple using 2-phase separator module in HYSYS. One of observation is the vapor outlet stream contains liquid. Vapor outlet from 2 phases separator should be in vapor phase. Why HYSYS predict present of liquid in vapor outlet ?



When pressure drop is allocated on the demister by entering pressure drop at "Vapour outlet", HYSYS view it as pressure drop on the vapor stream, outlet of 2-pases separator module. Vapor from separator is saturated vapor and pressure drop on the saturated vapor would result condensation due to Joule-Thompson (JT) effect. A liquid generated will be carried into the Vapor stream.

How to properly simulate a separator ?
In real separation, liquid generated in the demister (due to JT effect) will coalesce, formed larger droplet, trap by demister, droplet is separated from vapor leaving the separator and drop into liquid phase by gravity force. Thus, above straight forward simulation does not really represent a true separation. A proper simulation may include a "dummy separator" with zero pressure drop, downstream of the vapor outlet (2 phases). Separated liquid is mixed with feed stream using a "dummy pump". With this method, the vapor outlet from "dummy separator" will represent the true vapor from the separator. In this method, the "dummy separator" represent the demister and "dummy pump" represent the gravity form.



Above is just a simple trick for simulation and many may ignore it or do not even aware of this phenomenon. However, it is pretty important in presenting a true & correct Heat & Material Balance.

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