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Htir - Convergence Problems To A Thermosiphon Reboiler Design

thermosiphon reboiler

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#1 Geovando


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Posted 18 November 2017 - 01:05 PM

Hello experts,
I am engaged in a revamp of a styrene process plant. My actual challenge is to evaluate the possibility to use a existing exchanger as reboiler of a ethybenzene recovery destillation column. To do it I've used HTRI.
I before shall to say: 
- the botton pressure of my column is 0,415 kgf/cm²A.
- exchenger in vertical position
- the height betwee LLL level liquid and lowest tubesheet is 3455 mm.
- the styrene flows through tube side and vapor flows through shell side.
- styrene design flow is 735 t/h
Primaryly I evaluated it setting the "inlet pressure" (at 0,415 kgf/cm²A) and reboiler type as "No peping specified". Until here HTRI presented me 7,34% in overdesign and all process variables (cold and hot flow, temperatures, % vapor, etc ...) converged properly.
The second step was to set reboiler type as "thermosiphon reboiler" and the pressure location at column botton (0,415 kgf/cm²A). As expected the styrene flow increased as a result of pressure balance. However the HTRI converges the styrene flow to a negative overdesign even in "simulation" mode. 
The question is: which will the styrene flow be for a null overdesign ? I already put in blank the steam and styrene flow, steam temperature, % vapor and set only outlet styrene temperature and even this the HTRI converges the results to a styrene flow higher the styrene design flow negative overdesign.
Could you help me by showing me where I'm going wrong or the best way to make the HTRI converges the results to a thermosiphon reboiler ?
Thanks in advance

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