Q & A > Question Details
What will be the steam dew point at 93 degC and 1.1 kg/cm2g? Pl let me know whether dew point of stripping steam used in distillation column depends upon partial pressure of steam in the total vapour mass flow going out in the distillation overhead. Pl note total vapour mass flow in our distillation column ovhd is 274000 kg/hr and stripping steam flow (at 14 kg/cm2g & 220 degC) to column is 8490kg/hr. Overhead vapour is mixture of gaseous hydrocarbon (C1 to C5 range components) & stripping steam. Pl let me know if you need more data to answer my question. Our distillation column top operates at 1.1 kg/cm2g & 93 degC.
06/07/2010 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
Dew point of steam is derived via taking mole percent steam in overhead vapour time absolute pressure [not gage pressure] and looking up steam saturation temperature for this steam partial pressure.
Preferably, the reflux return temperature is warmer than the steam dew point temperature.
For the reboiler and lowest tray, steam and oil must be modeled using VLE thermodynamics and theoretical stage approach.
In many instances, VLE water concent in bottoms is 'overstated' [higher than actual].
05/07/2010 A: keith bowers, B and B Consulting, kebowers47@gmail.com
A fundamental behavior in immiscible systems such as water-hydrocarbons is that each fluid acts (thermodynamically) as if it were the only component present. Thus water will behave as if it were the only component present. If you inject saturated steam at 14 Kgm/cm2 and 220 C into a vessel whose pressure is a little more than 1.1Kgm/cm2 it will expand adiabatically to the pressure in the vessel at the point of injection, cooling as it expands until it reaches the new saturation temperature for that pressure. Then it will gain (or lose) heat from the environment to reach the stated temperature of 93C. You should refer to your Steam Tables and perform the expansion, which will then show the saturation (dew-point) temperature. You must also determine the quality of the steam being injected (superheated, saturated, sub cooled ) so you begin the expansion (on the water enthalpy graph) at the proper initial conditions
05/07/2010 A: Morgan Rodwell, Fluor Canada Limited, morgan.rodwell@fluor.com
This is easily modeled with a simulator, such as HYSYS or Pro/II.
The dewpoint of water in the column is based on the partial pressure. To do that calculation you need the mole fractions of water and hydrocarbon.