A NEW APPROACH TO I MPROVE A NEW APPROACH TO I MPROVE THE HYDROGEN - PowerPoint PPT Presentation

17 th April, 2007 Sadhana Mohan, India A NEW APPROACH TO I MPROVE A NEW APPROACH TO I MPROVE THE HYDROGEN YI ELD FOR HI X THE HYDROGEN YI ELD FOR HI X SYSTEM OF I - - S PROCESS S PROCESS SYSTEM OF I by Dr. Sadhana Mohan Dr. Sadhana Mohan Heavy Water Division Bhabha Atomic Research Center Department of Atomic Energy India

17 th April, 2007 Sadhana Mohan, India Objectives • I ndia is planning to study the I -S process for production of hydrogen in conjunction w ith high tem perature nuclear reactor. This requires optim ization of param eters for im proved efficiency. • I -S process involves three m ajor steps Bunsen reaction Sulfuric acid decom position Hydrogen production by HI X reactive distillation

17 th April, 2007 Sadhana Mohan, India Overview of I -S ( I odine-Sulfur) process Overall efficiency of the process is dictated by individual Stage perform ance I m provem ent in the yield of hydrogen production by various design options of HI X system w ill lead to overall perform ance enhancem ent of I -S process No conclusive design data from literature is available for HI X system All theoretical param etric estim ations are subjected to uncertainty due to lack of experim ental data

17 th April, 2007 Sadhana Mohan, India Com plexities involved in HI X system HI X system is m ulti-com ponent, m ulti-phase system form ing hetro-azeotrope at norm al tem perature and pressure To have reasonable driving force for distillation High tem perature ( > 3 0 0 o C) and high pressure ( > 2 2 bar) operation is necessary HI X System is highly corrosive at this tem perature thus sealing m aterial and system com ponent fabrication require special care Direct m easurem ent of equilibrium vapor and liquid com positions is an analytical challenge

17 th April, 2007 Sadhana Mohan, India Experience in reactive and distillation field Experience gained in m ultiphase H 2 -H 2 O isotopic exchange is utilized w hile designing HI X system I n both the cases though overall reaction is betw een liquid and gas phase actual reaction takes place in the vapor phase alone and requires Pt/ Pd loaded catalyst Total isotopic transfer rate enhancem ent in liquid phase catalytic exchange m ode com pared to vapor phase catalytic exchange m ode is due to replenishm ent of gas phase reactant from liquid phase Catalyst gets poisoned w ith excessive liquid loading. I ntroduction of segregated bed addresses this problem and reduces HETP

17 th April, 2007 Sadhana Mohan, India Design basis of the study Hydrogen is produced by direct decom position of HI in the gas phase Equilibrium tray concept is used for reactive as w ell as physical distillation stages based on standard free energy change Equilibrium yield is taken as the ratio of hydrogen production to feed HI content Equilibrium yield is estim ated by param etric variation having fixed re-boiler load NRTL three param eter m odel is taken for vapor-liquid equilibrium

17 th April, 2007 Sadhana Mohan, India Param etric study for equilibrium yield enhancem ent Analysis of already published case studies Effect of the num ber of theoretical plates Effect of side stream location I nfluence of in-situ flushing of iodine from vapor phase I nfluence of scrubbed liquid recycling to replenish decom posed HI Colum n concentration profile in the absence of reaction

17 th April, 2007 Reference Scheme for HI X Reactive Distillation Reference Scheme for HI X Reactive Distillation Sadhana Mohan Reactive Con. Temp. 25 o C Plate (3) Plate (5) Side stream 5 withdrawal Feed plate (8) 5 8 Sat. side stream as 60% of feed Sat. Liq. feed Reactive Plate (3) 237MJ/hr λ = ~2500MJ/hr (for liq. Feed) Plate (5) Side stream withdrawal Col Pr. 22 bar Feed plate (8) No of plates 10 Eq. Yield 7.9% Ref.: ROTH M. et. al. Int. J. Hydrogen Energy. 1989

17 th April, 2007 Sadhana Mohan, India Analysis of m ost elaborate published schem e as Reference Schem e This is the only published schem e depicting com plete colum n vapor-liquid com position profile Colum n pressure is based on driving force i.e. difference betw een the feed and azeotropic com position Azeotropic com position of HI decreases from 1 0 % to 1 % for iodine concentration of 3 9 % to 9 5 % Hydrogen production is significant only for iodine depleted vapors Stripping section plates of the colum n follow s azeotropic com positions corresponding to the iodine concentration

17 th April, 2007 Sadhana Mohan, India Problem s associated w ith the Reference Schem e Overall and individual com ponent m aterial balance are not m atching I nternal reflux for the colum n is not clearly brought out Re-boiler load is insufficient for saturated liquid feed Based on this analysis Schem e-1 is proposed for sam e design objectives w ith m inor m odifications

17 th April, 2007 Sadhana Mohan, India Colum n has been split into tw o distinct sections w ith a partial Condenser in betw een W ater scrubber has been added to get Product hydrogen purity Extension of reference scheme

17 th April, 2007 Sadhana Mohan, India Schem e-1 : Extension of Reference Schem e Com m on features as of reference Schem e Total num ber of plates and reaction zone plates are kept sam e Feed and side stream plate locations are unchanged Re-boiler load and Colum n operating pressure are sam e All feed and product stream s flow and com positions are unchanged Equilibrium hydrogen yield and product hydrogen purity is kept sam e

17 th April, 2007 Sadhana Mohan, India Modifications to reference Schem e Feed quality is changed to saturated vapor as against saturated liquid Reference colum n is split into tw o colum ns to take care of internal reflux to m eet the design objectives Scrubber is added to m eet the requirem ent of product hydrogen purity

17 th April, 2007 Sadhana Mohan, India Observations from schem e-1 analysis Bottom product is enriched to m ore than 9 0 % of iodine by rem oving w ater content of the feed as side stream Rem oval of w ater as side stream from enrichm ent section of the colum n leads to significant loss of HI I m provem ent in equilibrium yield is possible by shifting the side stream dow nw ards I n view of the above side stream plate location is shifted dow nw ards in Schem e-2

17 th April, 2007 Sadhana Mohan, India Side stream W ithdraw al plate location has been changed from condenser reflux to 2 nd plate from top Effect of side stream position

17 th April, 2007 Sadhana Mohan, India Salient features of side stream location: Schem e-2 Except side stream location all design param eters are kept sam e Side stream w ithdraw al location has been changed from condenser reflux stream to 2 nd plate from the top I t reduces the HI content in the side stream w hich increases the total am ount of HI going to reactive distillation colum n This m odification led to increased yield of hydrogen in the product to 1 2 .5 %

17 th April, 2007 Sadhana Mohan, India Observations from schem e-2 analysis Dow nw ard shifting of side stream increases w ater and HI content going to the reactive distillation section for a given iodine enrichm ent Low er iodine concentration im proves equilibrium yield. Thus requires proper com bination of w ater and HI Scrubber w ater is directly added to reactive distillation section in schem e-3 to see the effect of iodine flushing.

17 th April, 2007 Sadhana Mohan, India Separate w ater scrubber rem oved Scrubbing w ater added directly in the reactive distillation section Effect of water on iodine flushing

17 th April, 2007 Sadhana Mohan, India Salient features of w ater flushing: Schem e-3 Separate w ater scrubber is rem oved from the system W ater is added as an additional reflux to the reactive distillation colum n This utilizes the scrubber w aste stream HI content available for the reaction This m odification led to increase in yield of hydrogen to 2 0 %

17 th April, 2007 Sadhana Mohan, India Observations from schem e-3 analysis I odine com position in the bottom product stream reduces due to additional w ater in the system Higher HI content in the reactive distillation stages results in better yield of hydrogen in product stream Based on this observations schem e-4 A is prepared to check the effect of high internal vapor-liquid flow rate

17 th April, 2007 Sadhana Mohan, India 1 2 Column plate no. from condenser 3 H2O HI 4 I2 5 6 7 8 9 Reactive distillation 10 section clubbed 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 directly w ith physical liquid phase mole fraction distillation section 1 2 Column plate no. from condenser 3 H2 4 H2O 5 HI I2 6 7 8 9 10 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Vapor phase mole fraction Effect of high internal vapor-liquid flow rates

17 th April, 2007 Sadhana Mohan, India Salient features of high internal reflux: Schem e-4 A Reactive and physical distillation colum ns are clubbed together so that the w hole colum n observes increased internal vapor and liquid flow rates HI recovered by scrubber w ater is utilized by recycling Scrubbing w ater flow is reduced This m odification led to increase in yield of hydrogen to 2 1 %