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

17th April, 2007

A NEW APPROACH TO I MPROVE A NEW APPROACH TO I MPROVE THE HYDROGEN YI ELD FOR HI THE HYDROGEN YI ELD FOR HI X

X

SYSTEM OF I SYSTEM OF I -

• S PROCESS

S PROCESS

by

• Dr. Sadhana Mohan
• Dr. Sadhana Mohan

Heavy Water Division Bhabha Atomic Research Center Department of Atomic Energy India

Sadhana Mohan, India

Sadhana Mohan, India

17th April, 2007

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

Sadhana Mohan, India

17th April, 2007

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

• verall 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

Sadhana Mohan, India

17th April, 2007

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 oC) and high pressure ( > 2 2 bar)

• peration 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

Sadhana Mohan, India

17th April, 2007

Experience in reactive and distillation field

Experience gained in m ultiphase H 2-H 2O 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

Sadhana Mohan, India

17th April, 2007

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

Sadhana Mohan, India

17th April, 2007

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

Reference Scheme for HI Reference Scheme for HIX

X Reactive Distillation

Reactive Distillation

237MJ/hr Col Pr. 22 bar No of plates 10

• Eq. Yield 7.9%
• Sat. Liq. feed
• Sat. side stream

as 60% of feed

• Con. Temp. 25 oC

Sadhana Mohan

17th April, 2007

5

5 8

λ = ~2500MJ/hr (for liq. Feed)

Feed plate (8) Feed plate (8) Reactive Plate (3) Plate (5) Side stream withdrawal Plate (5) Side stream withdrawal Reactive Plate (3) Ref.: ROTH M. et. al. Int. J. Hydrogen Energy. 1989

Sadhana Mohan, India

17th April, 2007

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

Sadhana Mohan, India

17th April, 2007

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

Sadhana Mohan, India

17th April, 2007 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

Sadhana Mohan, India

17th April, 2007

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

Sadhana Mohan, India

17th April, 2007

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

• f internal reflux to m eet the design objectives

Scrubber is added to m eet the requirem ent of product hydrogen purity

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

• f 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 Sadhana Mohan, India

17th April, 2007

Sadhana Mohan, India

17th April, 2007 Side stream W ithdraw al plate location has been changed from condenser reflux to 2 nd plate from top

Effect of side stream position

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 % Sadhana Mohan, India

17th April, 2007

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. Sadhana Mohan, India

17th April, 2007

Sadhana Mohan, India

17th April, 2007 Separate w ater scrubber rem oved Scrubbing w ater added directly in the reactive distillation section

Effect of water on iodine flushing

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 % Sadhana Mohan, India

17th April, 2007

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 Sadhana Mohan, India

17th April, 2007

10 9 8 7 6 5 4 3 2 1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

H2O HI I2 liquid phase mole fraction Column plate no. from condenser

10 9 8 7 6 5 4 3 2 1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

H2 H2O HI I2 Vapor phase mole fraction Column plate no. from condenser

Sadhana Mohan, India

17th April, 2007 Reactive distillation section clubbed directly w ith physical distillation section

Effect of high internal vapor-liquid flow rates

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 % Sadhana Mohan, India

17th April, 2007

Observations from schem e-4 A analysis

Hydrogen production rate is not only function of HI concentration in the vapor phase but also concentration of iodine & HI in the liquid phase Based on positive points of earlier schem es a new schem e is generated w ith the follow ing features

Higher internal vapor-liquid flow rates are m aintained I n place of separate side stream and bottom stream , a Single bottom product stream carrying low est HI content is rem oved from the system Scrubber w ater is recycled to recover HI Sadhana Mohan, India

17th April, 2007

HI x REACTI VE DI STI LLATI ON (STAGE I I I )

7 Feed at 9th stage 61 km ol/ hr H 7% water 48% HI 43% 4.16 km ol/ hr yield 33% 25 ° C 99.8% H 6 km ol/ hr water at 25 ° C 4.5 km ol/ hr H 91.8% Hl 8.2% Partial condenser 63 km ol/ hr 26.5 ° C water 56% Hl 42% l 2% 127.5 km ol/ hr 245 ° C water 55% HI 3.3% l 41.7%

(Total No. of Theoretical plates: 10) (Colum n pressure: 22 bar)

Catalyst Liquid Collector I nert packing Liquid Distributor Scrubber Condenser 25 ° C TO STAGE -I FROM STAGE -I Feed at & 22 bar 125.8 km ol/ hr water 51% HI 10% l2 39%

• Sat. Vap.

Pre heater Feed Evaporator 125 ° C 120 ° C

2

237 MJ/ hr 221 ° C

2

I 2%

2 2 2 2

10 9 8 7 6 5 4 3 2 1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

H2O HI I2 liquid phase mole fraction Column plate no. from condenser

10 9 8 7 6 5 4 3 2 1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

H2 H2O HI I2 Vapor phase mole fraction Plate no. from condenser

Sadhana Mohan, India

17th April, 2007 Proposed Scheme for HIX Reactive Distillation

Schem e-4

Sadhana Mohan, India

17th April, 2007

% yield of Hydrogen

Sum m ary

20 40 50

21.00 33.00 19.87 12.48 7.95

Hydrogen Yield with various Schemes

R A B D C

10 30

R Modified Reference Scheme A B C Effect of water scrubbing on iodine flushing Side stream withdrawal location shifted downwards Effect of high internal reflux D Proposed scheme

Sadhana Mohan, India

17th April, 2007

Conclusion

I n the absence of definite therm odynam ic m odel available in the literature hydrogen yield m ay vary from the actual one. How ever, relative gains in various schem es cannot be

• ignored. Nevertheless w e are initiating the efforts to conduct

experim ents and im prove the m odel to reduce the

• uncertainties. I t is concluded that increased hydrogen yield is
• btained at the cost of low er iodine enrichm ent. W e strongly

feel that efforts to im prove hydrogen yield are m ore desirable than iodine enrichm ent as iodine stream is any w ay going to m ix w ith aqueous stream before it enters the Bunsen reactor. I n the present study m ore focus is given to equilibrium yield. Total optim ization w ill require energy considerations also.

Sadhana Mohan, India

17th April, 2007

Thank You! Thank You!