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I am operating Pre-topping column at 2.4 kg/cm2 and 118 C . M.P. steam at 12 kg/cm2 at 550 kg/hr is fed as stripping steam. Gasoline reflux at 28 m3/hr and 13m3/hr of gasoline is taken out of overhead condenser. How do I calculate the dew point of overhead vapors?
11/02/2012 A: Abdulkarim Aldabass, Saudi Aramco, dabassaa@yahoo.com
You can develop your own online calculation tag using the formulae to calculate the dew point of the water [T= (3896.5/11.813-Ln (paritial pressure water) -230.2). Where, T in Deg.c and Paritial pressure in bara. Paritial Pressure= (Column Pressure*Molar of Water)/(HC molar flow+Molar flow Water).
29/12/2011 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Tower overhead should be at the hydrocarbon dewpoint, so a calculation is unnecessary. The hydrocarbon dewpoint is simply the overhead temperature. If you are looking for the water dew point you can use Antoine's equation or you can calculate the partial pressure of the steam in the overhead system and use a steam table to look up the saturation temperature that corresponds to your steam partial pressure. If your calculated water dew point is significantly less than the overhead temperature you may need to go through an interative process to reduce the moles of hydrocarbon vapor to account for hydrocarbon condensation in front of the water dew point.
An alternative is to use a process simulator like hysys or Aspen and do a dew point calculation that way. Your input should include the steam, liquid product rate, reflux and any offgas. You can calculate both hydrocarbon and water dew points with the process simulation package.
27/12/2011 A: sam lordo, Becht Engineering, salordo@comcast.net
First you need to get the distillation data for the hydrocarbon stream and then use a process simulator such as SimSci, Aspen, etc. that is the most accurate way... for a back of the envelope number you can look at the steam table for the saturated steam temperature at the column ovhd pressure pressure