Role and Chemistry of Catalyst in Hydrogen Based Heavy Water Plants

6-2 ROLE AND CHEMISTRY OF CATALYST IN HYDROGEN BASED HEAVY WATER PLANTS

DR. D.G. PRADHAN HHP(TALCHER)

Heqvy Water plays an important role in the peaceful use of atomic energy which Is increasingly contributing to the total energy requirement of mankind. It is used as moderator and coolant in the reactors utilising natural uranium as fuel* Isotopes of hydrogen i.e. deuterium and tritium are being considered to be the most likely mote- rial for fusion reaction which may ultimately be th« source of unlimi- ted power for mankind as such energy will be environmentally benign. Major portion of energy in the universe is released by such processes. Heavy Water and tritium recovery, enrichment and production plants are therefore going to play significant role in future.

The vlabtlity of producing heavy water depends on the isotopic mass transfer rate in a chemical exchange process involving

hydrogen containing compounds such as VL, H^O, HO5, WI-, CH^ etc.For an acceptable isotopic mass transfer rate, the chemical bonds betwsen H-H, O-H,K-H, S-H8C-H etc. should be made labile, "ne bonds 0 - H, S - H and N -' H are so much labile that the bond breaking and making processes occur in the isotopic exchange reaction without

any catalytic agent at temperatures of interest in the II,, 5 - MOC and ?.'H_ - hUO exchange processes. However, equilibrium constant/separation factors at these temperatures and hazard as v/all as corrossive prpper- ties of KUS has loft scope for development of alternative methods for

producing heavy water based on HH_ - h"2, CH-NfU - H,,, H^C - H2 and CH. - H, - fUO exchange processes. All these alternative processes depend on successful breaking/making of covalent H-H and C - H bonds at an acceptable rate in the temporatirre range of interest which is possible only with the aid of a suitable catalyst.

Immediately after the existence of riiuterium was proved hy Urey, Prickwertde and Murphy from the difference in vapour pressures

of Ho & HD, attention was focused on the development of suitable catalysts for chemical exchange processes for large scale production of deuterium and heavy water. Both hetrogenous and homogenous catalysts were investigated during the days of the Manhattan Project and the first heavy water production plant of the '.'or Id was established at b.2

Trail, Canada and operated from 1943 to 1956 utilising Pt on active charcoal as the catalytic agent in the water vapour hydrogen oxch?n

Oxygen which will otherwise create problem at llould hydrogen tempor.iT turc, deoxo catalyst, which is noble metal on alumina/active charcoal, is used. The same catalyst is used to purify Oxyrjen used for burninn pure D_ to get pure heavy water. In the enrichment column, there is a necessity for the equilibrium

2HD =5r D2 + Hg to be achieved rapidly for which a catalyst is required as this involves breaking of HD bond and making of H - H £ D - D bonds.

The exchange reaction

NH3 + HD SN^D* H2 having potential for hydrogen isotope production was studied to 2 obtain the equilibrium data in" the gaseous phnso above room terap, and in the liquid phase below room temperature . Both heterogenous catalyst such as Pt. on active charcoal and homogeneous catalyst such as potassium anide in ammonia were A Investigated thoroughly and heavy water production plants based on ammonia-hydrogen exchange process have been conceived, designed constructed and operated successfully utilising the catalytic action of potassium amide in liquid phase at an acceptable rate even at -30 C. This process has emerged as an alternative process for tha well established GS process based on H»S - H,O exchange process without employing any catalyst. The exchange reaction

H20 +HD ^ HDO + Rj having equally Increasing significance in the production of hydrogen isotopes was studied in gas phase utilising heterogeneous catalyst such as Pt, on active charcoal/alumina and in liquid phase utilising waterproofed noble metal catalyst as well as alkali solution under

Contd..3/- high pressure. Based on this exchange reaction. Heavy Water Plants at Trail, Canada & Norway operated for many years and the plant in Norway (t.'ater-Hydrogen exchange units added in 1943 during the German Occupation of Norvay were destroyed by Allied raids) is operating after it was rebuilt and expanded after the War. An improved version of this process using exchange with liquid water has been developed to a stage for commitment of prototype plants and is the basis for several detriation plants including one at the Institute Max Von LAUE, Langevin in Grenoble , and one at Mound Laboratory?. Advantage of the exchange reactions D_0 + DT D-0 + HD is taken to get rid of both H £ T from the system leading to recovery of valuahle Tritium. Combined Electrolysis and Chenicnl Exchange (CECE) is the relevant process where catalyst plays a nn.ior role. The Chemical Exchange reaction

CI!3f,'H2 + HD —-^ CHgNHD + Ho is analogus to the ammonia-hydrogen exchange system R necessitates the use of a mixture of Potassium ft Lithium Methyl /.nude in view of the catalyst distintegration of Potasrium Jmide at high temperature of the bi-thermal process. The chemistry of catalyst and exchange process has been thoroughly investigated . The chemistry of heterogeneous catalysts such as Pt. or Pc*. on active surfaces such as finely divided charcoal or alumina is similar to that of normal chemical reactions. The isotopic exchange reactions HD + HgO (Vap) —-^ Hg + HDO (Vap) N S. HD + NH3 (Vap) ^ H, + Niy) (Vap) occur on the active catalyst sites followed by vapour liquid transfer reactions

HDO (Vap) + H20 (Liq) ^ ^ HDO (Liq) + HjO (v?p)

8 NH2D(Vap) + NH3 (Liq) ^zr^ HH^Ulq) + NH3 (Vap) which occur on any s'jrfaco. It is therefore necessary that active cntalyr.t sites should bo available without being blocked by poisons such as Carbon I'.onoxlde.

3 Con t-1. ,-i /- 6.2

The chemistry of homogenous catalyst particularly of iZ\Un in ammonia, based on which a number of plants are oooi-Jtiny, is discussed below in some detail considering its importance and complexity. Robert Dell las in his report CEA - R - 3377 entitled n Isotopic Exchange between Molecular Hydrogen and Liquid Ammonia Catalysed by Alkali Amides " has discussed the chemistry of amides with particular reference to hydrogen isotope exchange in liquid ammonia. He has i~omo to the conclusion that ionlsaticn of the alkali amides is the controlling factor and catalysing spncie is This is formed by the reversible reaction, K . MtJ **» intM . 4 try M

The reversible and exothermic nature of the above reactions shows that with increase in hydrogen pressure, temperature & amide concentration there will be an increase in the concentration of solvated electron (e~) (Figs. 1,2). The solvated electron plays a crucial role in the catalyst stability and imposes a limitation on the higher operating temperature. Potassium Methyl amirle () Is not stable beyond about 50 C due to which Lithium addition has been suggested to overcome the problem. The rich chemistry on the system Me£hylmine-hydrogen-potassium-lithium amide has been dealt 9 with in detail . It was observed that potassium hydride was forming by the reaction, CHgNHK + Hg ^ "^ CHgNHg + KH which is analogous to reaction of solvated electron i.e.

2K + H? ^ ~~T 2 KH (K+ + e") Methyl group, being an electron donating group due to its hypercon- jugative effect,facilitates hydride formation at a relatively lower temperature compared to KHtU system,

V.'ith the increases in catalyst concentration, in addition to the formation of ION pair (K* + NH^") there is appreciable increase in the formation of triplet ion

Contd..r) which is similar to NHt i.e. ammonium ion (R Delmas CEA-R-3377). This cation along with the solvated electron may give rise to the formation of I^NH & Hg under favourable condition. It is thus clear from the above discussion that solvated electron, KNhU concentration, temperature & hydrogen pressure play an important role for catalyst stability in the Anunine/Ammonia-hydrogen exchange processes for heavy water production. A study of temper- 10 • \

ature dependance of Cp for KNHg and KND2 VFig.3; shows a transi- tion betv.-een 335 to 351 K beyond which co-ordination under six similar to NaCl arrangement alone exists.

This probably determines the upper limit of temperature for heating the amide solution in ammonia hydrogen exchange processes. The accepted mechanism for ammonia-hydrogen exchange process vdth amide anion as catalyst is based on the formation of a transi- tion complex in which the catalyst anion " takes part " in the exchange reaction and is again generated from the reactnnt as follows (Fig.A)

N~" + H - D + WU ^^ JA H .... N ^ NH^D + Ho + \\\\ H H . . H - *

: ..... n Finally, catalyst decomposition by reaction with oxygenated impurities such as H20 & 02, C0o & CO etc. should be eliminated by strict quality control of the process fluids foiling --hich following reactions generating insoluble solids shall occur

i) KNHj + H20 *- KOH + NH3

li)4K}iH2 + 3O2 »- 2K0H +02KN

iii) KNH2 + C02 »- H2N - C - OK (Potassium Carbamate) o

iv) KKH2 + CO »- H - C - NHK (Potasrium formanide) Cperatlng Exoorlunce v.'ith Potassium Ar.ido Hontinq In the Monothermol hydrocjon-aiDr.onia exchc,n;io pl;ints, potassium amide solution is required to be heated for separating the enriched armonia from the catalyst prior to feeding it to the cracker for either (i) proviriir. the reflux or (ii) burning v.'ith oxynen for obtaining heavy v.'ater as product.

5 "ofi'..-! In case (i) amide solution is depres«-urisod and heated by means of subatmospheric 6team using an ejector vacuum system thereby controlling the temperature so that the problem described above is not encountered. If the temperature is not controlled within the limits, catalyst deposition occurs rapidly which requires washing by water ultimately as the deposits are not dissolved by ammonia. However If the heating is stopped Immediately after noticing deposition trends, the deposits can be dissolved with ammonia without reso- rting to water washing. It is possible that there are two competetive processes i.e. formation of deposits of unstable compounds like KH, KJIH which can lead to formation of stable compounds like K_H with !

At HWP,Talcher, in the catalyst separation section of 3rd stage of enrichment, a problem was encountered when hard deposition over the steam coil was formed to such an extent that heat transfer practically stopped when the heating steam temperature was 137°C and amide solution was stagnant. The problem was solved by installing a thermosyphon heating system using subatmospheric steam.

In case (il), deuterium enriched ammonia is picked up cs vapour by means of a stream of unsaturated syngas and subsequently condensed end separated. •• _ Here also, a thermosyphonic heating system with subatmospheric steam is utilised for avoiding the catalyst deposition even though the hydrogen/nitrogen partial pressure is relatively low.

At HWP,Talcher, the problem of catalyst deposition at the bottom of hot stripper hot enrichment towers surfaces out £ becomes severe whenever the temperature of amide solution exceeds 65°C and when due to contact of large quantity of unsaturated syngas, ammonia pick up increases in a localised manner increasing the cata- | lyst concentration in these areas substantially (Fig.5). The problem fv is further aggravated due to large hydrogen/nitronon partiJI Fg

6 Con:ci..7/- 6.2

pressures* High temperature and low pressure operation is harmful due to excessive pick up of ammonia from the amide solution by the unsa'turated gas at the bottom of hot stripper and hot exchange toners. Gas chromatographlc analysis of gases liberated by reaction of the solid deposits with water shows the presence of hydrogen which can indicate presence of KH.Similarly the percentage of K in some solid deposits was found to be about OOfj shoeing t-.hereby the presence of compounds like KH, KgNH and KgK (?'. of K in KHHj, is only about 71%). Finally, catalyst also nlaye a roll in thp deuterium content of feed gas to IRVP from the mother fertilizer plants. In the CO shift conversion section where the reaction

CO + H20 —*- C02 + H2 occurs in presence of a catalyst at temperature of about 340°c, the isotopic exchange reaction,

H2O + HD ^=i HDO + 1^ also occurs and enriched condensate containing HDO is lost. Arran- gement at K'iP.Talcher has been made to recycle the er.rlched condensate to the CO shift converter via one HDO exchange unit for increasing the D-content of feed syngas. In fact the D - content in feed synnas to Hl.P,Talcher is the highest in the country*

In the ammonia convertor of FCI, where there is circulation of a large quantity of syngas with ammonia vapour at a temperature of about 450°C over a catalyst bed, the isotopic exchange reaction

NH3 + HD v ** NHjD + Hj occurs, for which equilibrium constant is about 2 and hence a separation factor of about 1.33 is possible* This indicates that with ammonia having about 150 ppm. Deuterium, it Is possible to increase the depleted return syngas D-content to about 115 ppm thereby making HWPs independent of the Fertilizer Plants. Here, advantage can be taken of the rapid isotonic exchange possible betivoen ammonia and water. The same is true with the CO-shift convertor catalyst also where water can be used as feed directly. Acknowledgement t My thanks are due to the General Manager, HV.P.Talchcr for encouragement ar.: support.

v • • >• REFERENCES 1) H.C.Urey, F.G.Brickwedde, G.M.Murphy Phys. Rev.40.1,1932

2) A.Farks, Tram, Farad. Soc. 32,416 (1936), K.Mrtz, Z.Physik Chem. B30, 289 (1935).

3) Y.Claeys, J.C.Dayton, W.K.V.ilmarth J.Chem. Phys.18, 759 (1950).

4) Jacob Blgeleiserv, Report BNL-116 (1951) and Robert Delmas, Report CEA-R-3377 (1966).

5) J.P.Butler, Separation Science & Technology, 15 (3), 371-369 (1980).

6) Ph. Pautrof & M.Damiani, Separation of Hydrogen Isotopes, ACS, Symposlous series 68, p 163-170,(1978).

7) M.L.Rogers, P.H.Lamberger, R.E.Ellis, T.K.Mills, Separation of Hydrogen isotopes, ACS, Symposious series 68, p-171-176,(1978).

8) V/.J.Holtslander & W.E.Lockerby, Separation of Hydrogen isotopes, ACS, Symposlous series 68, p-40-52,( 1978).

9) W.J.Holtslander and W.E.Lockerby (Hydrorjen-Amlne process for Heavy Water Production, separation of Hydrogen Isotopes Ed. by H.K.Rae A.C.S. symposium series 68, 1978, p-40).

10) H.Jacob, M.Nagib, E.Von Osten, Atom Kernerciie, 29 (1977) 41 & 303 and unpublished data. 6-2

ALKALI METAL-LIQUID AMMONIA RBACTIOM

Figure 1.—Electron concentration (as determined by optical measurements) vs. the «quare root of the hydrogen pressure at several amide concentrations.

I "II V- ->, \ 6-2

'JO 15 20 Wavelength The absorption gprarum or wlu»ie,J tlrcfroni In * 0 23 molf/l, KNH.-NKj lujaicJ UIHIIT 100 aim with hpiro**n «l v«rio.u pnwurti i \

6-2

c'l

K>\ 151 Ql»

KHH;

*o\

208 a>\

*oi liO XX) HO ».»/

. for KNHj. KNOt. \ 6.2

KIM3+HD

ACTIVATION ENCRGY DIAGRAfVl FOR THC EQUILIBRIUM RCACTIONI

rig /* ittttw

HOT STRIPPER SATURATION SSCTION

GAS FROM COLV STRIPPER, SATURA- -T£D AT -3O' c AT SCO

TO COLO $rHIPP£R POr#SS!U#l AMIDS SOt-UT/ON ef TRANSFOK CIRCULATING CAHN£D MOTOR P

FIG.5 . AMIDE' SOLUTION H£ATfA/G €j SYN GAS SATURATION SECTION IN TUB HOT STRIPP£& OF 1ST CD STkSE . t-O