Reboiler duty much higher than actual
Reboiler duty much higher than actual
(OP)
I have simulated both a demethanizer and deethanizer in HYSYS. Getting the towers to converge based on product specs (and actual GC data) is not a problem. However, the reboiler duty calculated by HYSYS is much higher than what is seen in the plant. For example the deethanizer duty is 90% higher than what is calculated by seperate heat balances on both the actual reboiler and the oil heater that feeds it. I do not doubt the product flows and compositions that I am using as they are used for accounting and are verified by 3rd parties in addition to internal QA/QC.
Has anyone experienced similar problems? Am I missing a very simple solution?
Has anyone experienced similar problems? Am I missing a very simple solution?
RE: Reboiler duty much higher than actual
RE: Reboiler duty much higher than actual
Column: 40 2-pass valve trays, total condenser (I have tried simulating with and without a vent), kettle reboiler
Measured values available: feed flow & temp, product flows & temp, reflux flow & temp, product compositions (by GC), hot oil to reboiler temp in & out and flow, hot oil to heater (temp in and out and flow per pass). Product flows are from accounting meters and heater and reboiler flows agree within 20 gpm (out of a total of about 600). (total heat input approx 10 MMBTU/h). My temperatures actually come out pretty close and of course flows match since I am specing out mole fractions in the feed and product.
Model: 40 stages (I have tried everthing from 15 to 80), Peng Robinson EOS, feed vap frac is 0.20 (based on measured temperatures, flows, & compositions and HYSYS' flash)
Comments: Increasing number of stages does nothing (anything above 30 and the duties flatten right out). HYSYS' duty is about 18 MMBTU/h (vs 10). If I use a fully vaporized feed, I get a believable number for the duty (close to 10). However the temperatures at operating pressure, and pressures at operating temperatures are not even close to believable.
Your comment about feed condition maybe an area for further investigation.