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Diffusion of methane through a palladium membrane Somerton, Thomas W. 1933

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DIFFUSION OF METHANE THROUGH A PALLADIUM MEMBRANE. b y Thomas ¥• Somerton B.A.Sc. A Thesis submitted f o r the Degree of Master of Applied Science i n the Department of . . Chemistry THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1933, DIFFUSION OF METHANE THROUGH A PALLADIUM MEMBRANE. Though the d i f f u s i o n of gases through metals has been, studied by many, no investigators have, however, extended t h e i r work to the d i f f u s i o n of hydrocarbons through palladium. As hydrogen r e a d i l y diffuses through t h i s metal and as t h i s phenomenon i s made use of i n the o i l industry for the purpose of determining the percentage of hydrogen i n cracked gases, and also, as i t has been found recently that the Platinum metals r e a d i l y absorb hydrocarbons, i t was thought that an inv e s t i g a t i o n of the a b i l i t y of methane to di f f u s e through palladium would be of value. The phenomenon of d i f f u s i o n i s considered analagous to those of conduction of heat and e l e c t r i c i t y . That i s , the rate of d i f f u s i o n varies d i r e c t l y as the pressure gradient; and i n d i r e c t l y as the resistance to the flow* The resistance i s dependant on the character of the material and varies d i r e c t l y as the thickness of the material. The general equation governing d i f f u s i o n i s N s -D £ ^ dtdxdy where N - number of dz molecules d i f f u s i n g through a given area dxdy i n a time dt* In t h i s equation J^. i s the rate of change In the number of • • dz molecules of the gas ( i . e . the concentration) along the Z-axis normal to the xy plane. D Is the c o e f f i c i e n t of d i f f u s i o n . The concentration gradient ~ i s the cause of the process of d i f f u s i o n and i s proportional to the p a r t i a l pressure gradient ^ of the gas through the membrane* The sign i s a negative : dz sign since the transfer i s from higher values of ET to lower ones* As the thickness of the f o i l i s constant the expression — becomes 1 2 where (P-, - P P) i s pressure difference on the two sides of the f o i l and AZ. i s the thickness of the f o i l * Hence the rate of d i f f u s i o n i n t h i s case i s proportional to the pressure difference divided by the thickness of the membrane * Obviously the constant "D" depends on the rate at which molecules can move across the area dxdy as a r e s u l t of the pressure gradient of t h e i r p a r t i a l pressure p. This, as with a l l pressure phenomena, i s , obviously, caused by the heat motions, and the problem merely devolves i t s e l f into one of determining the net number of molecules moving across a given area under a concentration gradient due to t h e i r proper heat motions. Our aim i s , then, to f i n d the c o e f f i c i e n t of d i f f u s i o n nD" for a number of d i f f e r e n t temperatures and show the v a r i a t i o n by means of a graph. I t has been suggested that the mechanism of d i f f u s i o n of gases through a metal diaphragm i s such that the gas condenses on and forms a solution with the metal on the high pressure side and the gas evaporates from the metal gas s o l u t i o n on the low pressure side of the membrane* The method developed and described i n t h i s paper has for i t s object the determination of the rate of d i f f u s i o n of methane through palladium under varying conditions of pressure of the gas, temperature and thickness of palladium. Apparatus, The necessary apparatus required that the gas of which the d i f f u s i o n was to be measured should diffuse through a known area of the metal under a known pressure and at a known temperature. A l l tubing and vessels, except the d i f f u s i o n chamber, are of "Pyrex** laboratory glass. Apparatus f o r Measuring Gas Diffused. F i g , I represents the general hookup of the apparatus* "A" i s a metal d i f f u s i o n chamber of our own design, (to be described l a t e r ) , That part of the apparatus which i s attached to the r i g h t end of ^A" Is f o r the purpose of measuring the amount of gas which d i f f u s e s through the f o i l inside the chamber. The pure methane enters the chamber "A" at the l e f t end and any of the gas which has diff u s e d through the membrane comes out • at the r i g h t end where i t s pressure i s measured* This part of the apparatus was previously evacuated to a pressure of about 1 x IO""3 mm, of mercury. The small differences i n pressure due to the gas d i f f u s i n g through the f o i l are measur-ed on a McLeod Gauge n B M , W C M , a bulb of known volume and "D" a* manometer are both used for determining the volume of this part of the apparatus. The tube "E" goes to a "Genoa Megavac Suction Pump" i n series with a " P f e i f f e r Wetzlar" mercury d i f f u s i o n pump. Measuring Volume of Apparatus. The volume of bulb WC" was determined i n the same manner as volume of McLeod Gauge was found, (to be described l a t e r ) . The bulb was then sealed to the apparatus as shown i n the diagram. The manometer **D" was esp e c i a l l y constructed for t h i s job. A l e v e l l i n g bulb i s attached to the bottom of the manometer so that the mercury l e v e l i n the l e f t side of the manometer may be kept at the same height for a l l pressures i n the apparatus* Hence, i f the l e v e l of the mercury i n the McLeod gauge i s kept at a fixe d point the volume of the apparatus w i l l be f i x e d . To obtain the value of thi s volume dry a i r i s admitted leaving stopcock at "G" open. The next step i s to close t h i s stopcock and open stopcock "d" and evacuate. M d " i s then closed and "G" opened and the mercury l e v e l s i n the McLeod Gauge and l e f t side of manometer nD n are rai s e d to th e i r respective reference marks " a w and "b", the pressure i n the apparatus being noted on manometer "D". We can now calculate the volume by the application of the perfect gas law. Let Y • volume of apparatus Let C r volume of bulb Let P j = pressure i n bulb "C" = atomospheric pressure Let Pg * pressure i n bulb "C M and apparatus • Then .• . • Pg(Y-tC) * P XC PgV = CCPi- Pg) In every case when we measure the difference of pressure i n volume T, due to the methane d i f f u s i n g through the membrane, we must be sure that the mercury l e v e l i n the manometer i s at n b n . Hence knowing the volume of the apparatus and the change i n pressure at a known time, we may calculate the amount of methane which diffuses through unit area of metal, i n unit time* The volume of the bulb nC" i s excluded from the volume of the apparatus so as to increase the s e n s i t i v i t y . Apparatus for. Methane Supply, That part of the apparatus connected to the l e f t end of the d i f f u s i o n chamber "A** i s for the supply of pure methane to the high pressure side of the palladium f o i l . The gas was prepared by Scporlemmer fs method. (C, Schorlemmer, Chemical •News* Vol* 29, P, ?•), An. intimate mixture of anhydrous sodium acetate with more than twice i t s weight of dry calcium hydroxide and sodium carbonate i s heated i n florence f l a s k n F M • The gas thus obtained always contains some acetone which i s e a s i l y removed by passing i t through a s o l u t i o n of sodium acid sulphite. The methane i s then passed through sodium hydroxide solution to remove any carbon dioxide that might be evolved. Cuprous chloride solution was used to take out any carbon monoxide which might be present. The cuprous chloride (Mahin: Quantitative Analysis) was prepared by dissolving 17 grams of eupric oxide and 3 grams of f i n e l y divided copper i n 200 c.c. of a mixture of equal volumes of concentrated hydrochloric acid and water, s t i r r i n g u n t i l the s o l i d matter dissolved and placing i t i n the woulff bottle having bundles of copper wire reaching from top to bottom. The methane i s stored i n a gas reservoir "G" which was made by cuttin g the bottom o f f a "Winchester" with a hot wire, and placing i t upright i n a pneumatic trough containing d i s t i l l e d water. The "Winchester" i s f i t t e d with a two-hole rubber cork. The gas from the purifying t r a i n goes through a stopcock and enters the rese r v o i r through the one hole f o r c i n g the l e v e l of the water down and leaves by the other hole which i s connected by rubber tubing to stopcock " f " . The apparatus from n f " i s made completely of pyrex glass so that i t w i l l be a i r t i g h t . This consists of two sulphuric a c i d towers "H" to remove water and unsaturated hydrocarbons and a PgOtj tube to remove the l a s t traees of moisture. The PgO^ tube i s oonnected to the d i f f u s i o n chamber "A" and to the l e f t side of manometer "J"" which i s to.measure the pressure of the methane on the high pressure side of the diaphragm. The right side of the manometer i s connected to a vacuum pump through a stopcock. The l e f t side of the manometer i s also connected to the vacuum pump through a stopcock "g". To get the methane into chamber "A" we close stop-cock " f " , open "g" and evacuate. ttgn i s then closed and " f " opened to admit methane from the reservoir "G". This process i s repeated several times to remove l a s t traces of a i r which may remain i n the apparatus* The d i f f u s i o n chamber i s enclosed within an e l e c t r i c resistance furnace e s p e c i a l l y constructed for the purpose, the temperature being regulated by means of a variable resistance rheostat. McLebd Gauge: (see F i g . .I). The c a p i l l a r y tubing for the t i p was f i r s t picked. This was to be of uniform bore to cancel surface tension e f f e c t i n pressure readings* Lengths of about 80 cms. were ava i l a b l e . The tubing was cleaned and dried and a piece of rubber tubing attached to one end* A thread of mercury (about 5 cms.) was sucked i n at i t s length measured with a cathotometer at several points along the tube. A piece was found i n which the small variat i o n s i n length of thread indicated a uniform bore. This was given a more thorough examination and the most uniform 15 cm, portion was taken f o r the t i p leaving a 30 cm. length f o r the manometer. Into the 15 cm. portion was sucked a thread of mercury (about 1 cm.) i t s length from the centre of meniscus on either end being measured accurately with a cathotometer for several places along the tube. The mercury was then run into a watch glass and weighed. By calculations using data obtained from the International C r i t i c a l Tables i t was seen that i f the mercury was f l a t on the ends the same volume of mercury would occupy a thread length from the centre of the meniscus on each end* Then, from these values and knowing the temperature and weight of the thread,the average cross-sectional area of the c a p i l l a r y tube could be calculated. This value was then checked by a s i m i l a r consideration of a thread about 14 cms. long. One end of the t i p was sealed and a b e l l blown on the other end i n readiness for attachment to the gauge globe . To make the gauge globe a piece of pyrex tubing about 3 cms. i n diameter was obtained, the end of which was pulled down and sealed to the c a p i l l a r y t i p 9 The other end was then pulled down and pulled out to a tube smaller i n diameter than that to which i t was to be sealed and broken o f f . The mouth of the globe thus formed (represented by A i n Fig* ;i) was inserted into the pyrex tubing and sealed. The purpose of t h i s mouth was to afford an accurate cut-off, A hole was blown i n the tube i n such a way that the centre of the hole was at the end of the mouth and a piece of pyrex tubing sealed into the hole, to which i s l a t e r attached the manometer. The centre of the hole must be at the end of the mouth, otherwise i t w i l l not cut o f f properly* To obtain the volume of the globe, we clean, dry, and weigh. Boiled, d i s t i l l e d water i s used to calculate volume« The problem was to get the l i q u i d down into the c a p i l l a r y t i p i To accomplish t h i s the gauge was p a r t i a l l y f i l l e d with water. suction applied to pump the a i r out of the c a p i l l a r y and then released l e t t i n g the water flow down the c a p i l l a r y tube. The remainder of the gauge was f i l l e d with d i s t i l l e d water and immersed i n a constant temperature bath (40°C) f o r a couple of hours to ensure the water within the globe being 40°C, This was carried out at 40°C so that when the globe was removed the water inside would contract and none would be s p i l l e d . The gauge was dried-and: weighed. Our next step was to estimate the volume of the a i r cone at the sealed end of the c a p i l l a r y t i p . The height of the cone was measured with a cathotometer and knowing the height and the area o f i t s base we were able to calculate i t s volume. The next task was to put etchings on the c a p i l l a r y t i p such that the pressure readings at these marks mu l t i p l i e d by a simple factor would give us the true pressure. Multiply-ing factors of 10""4, .j? x 10 ~ 3 , and 10" 3 were chosen. The product of the gauge volume and t h i s factor gives the volume i n the c a p i l l a r y t i p above the etching. From t h i s value was subtracted the volume of the a i r cone. Knowing the cross-seCtional area of the tube the length of c a p i l l a r y correspond-ing to t h i s volume was calculated. Then s e t t i n g t h e , t r a v e l l i n g crosshair of the cathotometer at the base of the cone i t was moved the exact distance of the above calculated length. At t h i s point a piece of paper was glued around the tube, care being taken to have the edge of the paper come d i r e c t l y under the crosshair. This was done for each of the three compression ' - 10/ - ' • • r a t i o s . A t h i n layer of wax was spread around tube at each paper s t r i p . The gauge was then set i n a lathe and the wax scratched along the edge of the paper with a fine s t e e l point. Hydrofluoric acid was then applied to the scratch with a fine cotton thread and allowed to remain a few minutes, after which the wax, acid and paper were washed o f f leaving a very fine scratch aroundthe c a p i l l a r y . The rest of the McLeod gauge was made as per diagram. D i f f u s i o n Chamber* F i g . . I I i s a diagram of the diffusion'chamber which i s of our own design,although the o r i g i n a l idea was obtained from a consideration of one used by LaRose and Johnson, (Journ. Am. Ghem* S o c , V o l . 46, 'P. 1377, June 1924). The chamber i t s e l f i s constructed of st e e l and i s made i n two parts ^A" and "B" which screw together. The part "B", into which we place the palladium f o i l "E", i s well machined so that i t w i l l s i t snugly on the seat* A recess i s cut out of the seat and a porcelain plate of same width and depth i s placed i n i t . This porcelain plate serves to support the diaphragm against the pressure but i s so porous that i t w i l l not In h i b i t d i f f u s i o n . The e f f e c t i v e area through which the gas diffuses i s the area of the recess. This area i s found by measuring the average diameter with a cathotometer. The other part of the chamber "B" screws down into the f i r s t part and holds the membrane f i r m l y i n place* The end of ^B^, which screws down next to the f o i l , has a groove cut i n i t so that i t w i l l f i t down snugly over a soft copper r i n g which i s placed next to the f o i l and acts as a gasket, prevent-ing leaking at t h i s spot. This groove i s of a larger radius of curvature than that of the r i n g so that when the two parts are t i g h t l y screwed together the soft copper w i l l squeeze out and f i l l up the groove making an a i r tight j o i n t . The side of the r i n g next to the diaphragm i s f l a t . In the screwing up process the part "A" s l i p s on the copper r i n g and hence the r i n g also serves the purpose of preventing the f o i l from being cut, torn, or buckled by coming i n contact with the moving part "A"» Before each assembling the r i n g must be heated to red heat and quenched i n cold water to soften the copper. The inside of the chamber was copper plated i n order to prevent the methane at the high temperatures employed from reacting with the s t e e l , also to prevent gases and hydrocarbons dissolved i n the s t e e l from being evaporated into the chamber. Into the ends of the chamber were screwed copper pipes "C M and "D" fo r purposes of leading the methane to and from the chamber. Copper tubing was used here for the same reason that copper p l a t i n g was applied to the inside of the d i f f u s i o n chamber. Water cooling jackets were placed around each pipe to keep the outer ends from getting warm enough to melt the "Dekhotinsky" used for sealing the glass tubing to the metal. I t was necessary to have the tubes s i l v e r soldered to the chamber i n order to make these joints a i r tight* Ordinary soft solder would not be suitable here because at the temperatures employed the zinc i n the solder would oxidize and vaporize and the presence of these fumes within the chamber-would a l t e r the pressure and upset the readings. A si m i l a r reason applies for not using "Shellac", white lead or "Smooth-On'? which are o r d i n a r i l y used for making threaded joints a i r ti g h t * To the outer end of tube "G" i s soldered a "T" joint i n such a manner that one opening of the "T" i s i n l i n e with the copper pipe. Through t h i s a resistance thermometer "L" i s placed so that i t s bulb i s within a few millimeters of the diaphragm. The thermometer i s sealed i n with "Dekhotinsky" cement. To the other opening i n the "T" i s sealed the pyrex tube coming,from the methane supply. To the outer end of pipe "D" i s sealed the pyrex tube which goes to the McLeod Gauge, etc. The chamber i t s e l f was enclosed i n an e l e c t r i c resistance furnace e s p e c i a l l y designed f o r t h i s job. A great deal of d i f f i c u l t y was encountered i n t h i s part of th8 apparatus i n getting r i d of leaks. Much time and experimenting was necessary before the ingenious devices to prevent leaking, as described above j were.devised* I t was also very d i f f i c u l t to obtain a i r tight joins between the metal and the glass using "Dekhotinsky" cement for sealing* • • - 13 . C a l l b r a t i o n of App aratus. Volume of Bulb "C". Weight of bulb •+• water * 278.6122 Weight of bulb = . 57.4943 Weight of water 221.1179 Temperature of water - 40°G Volume of 1 gram water at 40°C - 1.00782 c o . Volume of 221.1179 grams of water at 40° G = (221.1179) (1.00782) = 222.847 cu.cms* « V o l . o f Bulb Area of Recess. Average diameter of recess = 2.898 cms. Area of recess • (5.1416)(2.898) 2 - 6*59611 sq.. cms* Thickness of Membrane* The average thickness of the palladium membrane measured with a micrometer = .0043 i n * - .0109 cms. C a l i b r a t i o n of McLeod Gauge. Volume of Bulb Weight of Bulb + water - 166.6760 Weight of Bulb 84.4618 Weight o f Water 82*2142 Temperatureof water i n bulb = 40°G Yolume of 1 gram of water at 40°C s (82*2142) (1.00782) « 82.8571 c . c - Vol* of gauge. Diameter of C a p i l l a r y T i p . Length of thread at 24.2°C (Corrected f o r meniscus) = 14,560 cms, 1 gram of mercury at 24,2°C - 73,8763 cu.mm. Weight of mercury thread * 1.1912 grams, Volume Of thread *» (1,1912) (73.8763) • 88 , 0 0 1 5 cu.mm, , But the volume of thread - (Area)(Length) " '(5*1416)D8 x 14.560 cms. Hence Diameter "D" = (88.0015)(4) (W^TTBTMe) m .8773 mm. Cross-sectional area = 88,0015 s ,60442 sq. mm, 145*6 Calculations f o r Etching on Tip, The t r a v e l l i n g crosshair of the cathotometer was set at a point up from the base of the a i r cone equivalent to the volume of the a i r cone. This point was used as zero point. To obtain a compression r a t i o of 10~ 3 the volume i n C a p i l l a r y must be: (82857.1 cu.mm.)(10~3) = 82,8571 cu mm* Length of c a p i l l a r y corresponding to 82.8571 cu* mm* = 82.8571. .60442" = 137.089 mm. = 13,7089 cms. Length of c a p i l l a r y for a compression r a t i o of .5 x 10" 3 * (82857»l)(.5 xlQ" 2') .60442 •= 6 • 85445 cms. Length of c a p i l l a r y for a compression r a t i o of 10""4 s (82857.1)(1Q- 4) - 1.37089 cms. .60442 

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