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A new method for the determination of adsorption isotherms Bulman, Norman 1947

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A NEW METHOD FOR THE DETERMINATION OF ADSORPTION ISOTHERMS Thesis submitted i n p a r t i a l f u l f i l m e n t o f the requirements f o r the degree of Master of A r t s i n the Department of Chemistry' by Norman Bulman THE UNIVERSITY OF BRITISH COLUMBIA May 1947 ACO"0WIiEI)GEMEM? Sincere thanks i s expressed to Dr. M.J. M a r s h a l l of the Department of Chemistry u n d e r whose d i r e c t i o n t h i s i n v e s t i g a t i o n took p l a c e . ABSTRACT A new apparatus for the determination of adsorption iso-therms i s described. The chief feature of this apparatus i s that the gas under investigation i s allowed to leak slowly, at a known rate, into the adsorbent. The rate at which the pressure increases i n the system i s recorded. From this data the amount of gas adsorbed by the solid can be calculated and with the measured pressure oan be plotted as an adsorption isotherm. The adsorption of carbon dioxide on eharooal has been studied by the use of this apparatus. The reported presence of discontinuities i n this adsorption isotherm has not been confirmed. I n t r o d u c t i o n The u s u a l methods f o r the determination of a d s o r p t i o n isotherms of a gas on a s o l i d adsorbent are ve r y time consuming and exaoting. The other methods of t e s t i n g s o l i d adsorbents, suoh as the " s e r v i c e time" method f o r t e s t i n g charooals (1) , are not completely s a t i s f a c t o r y ( 2 ) . A r a p i d method f o r the determination of a d s o r p t i o n isotherms was sought t h a t would overcome the d i f f i c u l t i e s of the o r d i n a r y isotherm determination and yet would y i e l d data of g r e a t e r s i g n i f i c a n c e than t h a t o f the " s e r v i c e time" methods. The f o l l o w i n g apparatus was de-? veloped w i t h t h i s end i n view. Apparatus The apparatus i s o u t l i n e d i n F i g . I . The gas generating apparatus (F) was maintained at one atmosphere by the use o f the mercury overflow ( A ) . B i s a f i n e l y drawn c a p i l l a r y t h a t o o n t r o l l e d the r a t e of f l o w of the gas. C i s the c a l i b r a t i o n bulb and D the oharooal b u l b . The oharooal bulb was arranged so t h a t the gas was admitted t o one side of the oharooal and the pressure was read on the other. E i s a m o d i f i e d MeLeod gauge of the type desoribed by McMahon and M a r s h a l l ( 3 ) . This gauge gave r a t i o s of 1.06 to 9.62, 1.06 t o 17.87 and 9.62 t o 17.87. The apparatus was evacuated by a mercury d i f f u s i o n pump backed by a r o t a r y pump. The furnace was c o n t r o l l e d by a Wheelco C a p a o i t r o l . An o r d i n a r y MeLeod gauge capable of reading pres-sures down to 1 x 10"*-* mm. was used to measure very low pres-sures and the other gauge was used during the ad s o r p t i o n det e r m i n a t i ons. <  :  > p<*ya w »CK> A I Experimental Prooedure A 14.4 gram sample of "Columbia* a c t i v a t e d carbon (Grade 6G, s i z e 8/14 mesh) was degassed by h e a t i n g a t 1000 C. w i t h the pumps on f o r 8 t o 10 hours. The same sample was used f o r a l l the experiments and the same degassing prooess was used each time. A f t e r c o o l i n g t o 500 C. w i t h the pumps on, the oharooal was allowed t o oool completely. The oharooal was kept a t low pressures and before an experiment, was pumped down w h i l e oold t o l e s s than 1 x 10"* mm. of mercury pressure. The gas, earbon d i o x i d e was d i s t i l l e d i n t o the apparatus by the use of l i q u i d a i r from s o l i d oarbon d i o x i d e , i'he generating apparatus (F i n F i g . I) was f l u s h e d out once or twice w i t h oarbon d i o x i d e t o remove any r e s i d u a l a i r and was pumped down again u n t i l i t was t o be used. The s o l i d earbon d i o x i d e tube (G) was surrounded by l i q u i d a i r . Water vapour was removed by a l l o w i n g the oarbon d i o x i d e to pass over phosphorous pentoxide. When the apparatus was i n use the oarbon d i o x i d e was maintained at one atmosphere. The r a t e of f l o w through the o a p i l l a r y was determined by shut-t i n g o f f the oharooal and ad m i t t i n g the oarbon d i o x i d e to the l e a k ( B ) . The r a t e of increase of pressure i n the c a l i b r a t i o n system was measured. Readings were taken every two minutes u n t i l a pressure of ten centimeters was reached. The gas was then shut o f f and the system was again degassed. This took about two hours. The prooedure was then repeated, t h i s time w i t h the c a l i b r a t i o n system c l o s e d and the gas running through o the c h a r c o a l . I t was found t h a t surrounding the s o l i d oarbon d i o x i d e supply with, a Dewar f l a s k c o n t a i n i n g a l i t t l e s o l i d c a r t o n d i o x i d e decreased the r a t e of s u b l i m a t i o n of oarbon d i o x i d e so t h a t l e s s than 10 grams was s u f f i c i e n t f o r a complete deter-mination. The r e s u l t i n g time-pressure ourves were p l o t t e d . R e s u l t s The r e s u l t s of e i g h t determinations as d e s c r i b e d are p l o t t e d on graphs 1-8. As oan be seen these are a l l q u i t e s i m i -l a r . From graph 1 an a d s o r p t i o n isotherm was p l o t t e d . Knowing the volume of the o a l i b r a t i o n system i t was p o s s i b l e to c a l c u -l a t e the r a t e o f f l o w o f the gas through the c a p i l l a r y . From the pressure of the gas over the oharooal the amount of gas not adsorbed by the c h a r c o a l oould be c a l c u l a t e d . From the above, the amount of gas adsorbed by the oharooal oould be found. Thus the ads o r p t i o n isotherm could be p l o t t e d . This i s shown i n graph 9. The complete data f o r curve 1 i s gi v e n i n Table 1. D i s c u s s i o n The above method seems to o f f e r a r a t h e r simple method f o r t e s t i n g c h a r c o a l s . The o r i t i o i s m may be made tha t the pressure recorded does not represent a tr u e e q u i l i b r i u m pressure. This i s undoubtedly t r u e , however,the percentage d e v i a t i o n should be q u i t e s m a l l . As shown the pressure decrease from the f i n a l measured pressure was not very great on standing o v e r n i g h t , xhe c o n t i n u a l change of pressure i s a souroe of p o s s i b l e e r r o r even i n the s t a t i c methods f o r the determination of isotherms* TABLE I Time Gas Admitted Pressure Gas i n gas Gas Adsorbed (Min,) (co. N.T.P.) Phase (oo. N.T.P.) . (oo. K.T.P.)  1 .468x10 .178 .025x 10 .445 2 .937 .208 •026 .911 4 1.87 .238 .030 1.84 6 2.81 .582 .074 2.74 8 3.74 1.13 .143 3.60 10 4.68 1.93 .245 4.43 12 5.62 3.06 .388 5.23 14 6.55 4.10 .520 6.03 16 7.50 5.41 .686 6.81 18 8.44 6.81 .862 7.58 20 9.37 8.42 1.07 8.30 22 10.30 9.97 1.26 9.04 24 11.23 11.9 1.51 9.72 26 12.18 13.9 1.76 10.42 28 13.11 15.9 2.02 11.09 30 14.05 18.0 2.28 11.77 32 14.99 20.3 2.58 12.41 34 15.91 22.5 2.86 13.05 36 16.87 25.0 3.17 13.70 38 17.80 27.2 3.45 14.35 40 18.71 29.9 3.79 14.92 42 19.67 32.5 4.12 15.55 44 20.6 35.4 4.49 16.1 TABLE I (Cont.) Time (Min.; Gas Admitted (oo. ET.T.P.) Pressure Gas i n gas Phase ( c c . ET.T.P.) Gas Adsorbed ( c c . I.T.P.) 46 21 . 5 5x 10 38.7 4.90 x 10 16 . 6 5x 10 48 22.45 41 .2 5.22 17.23 50 23 .4 43 .8 5.55 17.85 52 24.36 46 .2 5.85 18.51 54 25.28 49 .8 6.31 18.97 56 26.22 52.4 6.64 19.58 58 27 .2 55.1 6.99 20.21 60 28.1 58.0 7.35 20.75 62 29.03 61 .0 7.74 21.29 64 29.98 63.9 8.10 21.88 66 30.90 66.5 8.43 22.47 68 31.83 69 .7 8.84 22.99 70 32 .80 73 . 3 9.29 23.51 72 33.74 76.5 9.70 24.04 74 34.64 79.4 10 .07 24.57 76 35 .60 82.5 10.47 25.13 78 36 .55 85.5 10.84 25 .71 80 37.44 88.5 11.22 26.22 Volume Volume of c a l i b r a t i o n system of c h a r c o a l system 2042 CO. 1041 c.c. 5 The time allowed for equilibrium to set up should be constant. In this new method the approach to equilibrium should be rea-sonably constant over the whole range. As physioal adsorption i s considered to be a rapid pro-cess ( 4 ) , the curve plotted may give quite a reasonable picture of adsorption and the time lag may represent some process suoh as diffusion. It w i l l be noticed in the diagram that an ionization gauge i s included in the apparatus. This was not used but i t was l e f t i n the system to see i f the presence of metal parts would cause any d i f f i c u l t i e s . It l s hoped eventually to obtain some type of gauge or gauges that w i l l give readings over the range 0-10 om. by generating some small ourrent. It i s hoped that this ourrent may be used to run a recording instrument and thus a oomplete curve w i l l be obtained. This method w i l l be limited of course by the effect of the gases on the gauges used. One' of the ohief d i f f i c u l t i e s i n making the apparatus was that the rate of leak had to be of the oorreot order of magnitude and had to be constant. With the arrangement shown i t i s a simple matter to change capillaries or to shorten the capillary to increase the rate of flow. The rate of leak oan be checked before eaoh determination as was done. Care must be taken i n the use of the apparatus as varied pressures are formed in different parts of the apparatus. This oan lead to d i f f i c u l t i e s i n opening up one part of the apparatus to another. Many workers have report e d d i s c o n t i n u i t i e s i n a d s o r p t i o n isotherms (5 t o 7 ) . In p a r t i c u l a r Burrage (j>) i n an i n v e s t i -g a t i o n of the a d s o r p t i o n isotherm of carbon d i o x i d e on oharooal has observed t h i s phenomena. The l a o k of a t h e o r e t i c a l explana-t i o n f o r these d i s c o n t i n u i t i e s makes t h i s e s p e c i a l l y i n t e r e s t i n g . I t was expected that i f these d i s o o n t i n u i t i e s were r e a l they would be r e f l e c t e d i n some manner i n the curves of i n -crease i n pressure. The only curves showing marked d i s o o n t i n u -i t i e s were curves 4(b) and 5» I n curve 5 the range showing the d i s o o n t i n u i t i e s was covered by readings taken every minute. This s i b e l i e v e d to be the source of these d i s o o n t i n u i t i e s . Readings oould not be conveniently made at t h i s r a t e and as the gauge oould only be opened t o the system f o r about 5-10 seconds these readings are b e l i e v e d t o be i n ' e r r o r . I n curve 4(b) the l a s t few readings are considered i n e r r o r as the apparatus was beginning t o admit a i r through a loosened stop-cock. The other curves do not show d i s o o n t i n u i t i e s that are very d i f f e r e n t from the i r r e g u l a r i t i e s i n the c a l i b r a t i o n curves. There i s no evidence o f the r e g u l a r d i s o o n t i n u i t i e s observed by Burrage* One advantage of t h i s apparatus i s t h a t w i t h the reoord-' i n g mechanism i t would be p o s s i b l e to o b t a i n complete curves. These curves would answer the o r i t l o i s m t h a t the reason f o r not o b t a i n i n g d i s o o n t i n u i t i e s i s that not enough readings are taken. However, the present curves c o n s i s t of about the same number of readings as the curves of Burrage. By u s i n g a much slower l e a k a longer time may be taken t o reaoh a g i v e n pressure and a curve c l o s e r t o the t r u e isotherm would be obtained. I t i s b e l i e v e d that an adsorption isotherm obtained by a des o r p t i o n prooess i s a more accurate Isotherm than one obtained from measurements of p u t t i n g a gas onto the oharooal ( 4 ) , However Burrage has been able to reproduce the observed d i s o o n t i n u i t i e s on both a d s o r p t i o n and d e s o r p t i o n runs, Thus one would s t i l l expect t h a t the curves obtained by t h i s new prooess would r e f l e c t these d i s o o n t i n u i t i e s * In c o n c l u s i o n i t may be s a i d that t h i s apparatus o f f e r s a r a t h e r convenient method f o r t e s t i n g c h a r c o a l s , A f u r t h e r development of i t w i l l probably l e a d to an apparatus t h a t would be q u i t e u s e f u l f o r the r o u t i n e t e s t i n g of c h a r c o a l s . Furthermore, o b t a i n i n g complete curves w i l l throw more l i g h t upon the subject of d i s o o n t i n u i t i e s i n a d s o r p t i o n isotherms. REFERENCES (1) Lamb, Wilson, and Chaney. J.Ind.Eng.Chem. 11,430,(1919). (2) H.H.Lowry, and G.A.Hulett. J.Amer.Chem.Soc. 42,1393»(1920)• (3) H.O.McMahon and M. J . H a r s h a l l . Trans.Eleotrochemioal Soo. 84,109,(1943). (4) S.J.Gregg, The Adsorption of Gases by S o l i d s . Methuen and Company, L i m i t e d . (5) Burrage, L . J . J.Phys.Chem.-36,2272, (1932). (6) Allmand, A.J., Burrage, L . J . , and C h a p l i n , R. Trans. Faraday Soo. 218,(1932). (7) Ubbelohde, A.R. and Egerton, A. Proo.Roy.Soo.(London) 512,(1931). 

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