I
A. J. DEROSSET Universal Oil Products
Co., Des
Plaines, 111.
Processing Industrial Gas S t r e a m s a t High Pressures
...
Diffusion of Hydrogen Through Palladium Membranes Permeability of palladium to hydrogen suggests interesting techniques for large-scale separation of process gas streams
SEVERAL
USES of palladium membranes for processing industrial gas streams have been proposed (4)-e.g., separating hydrogen isotopes, enriching hydrogen in recycle gases from catalytic reformers, and purifying gas feed to fuel cells. When evaluating palladium membranes for processing hydrogen-containing streams, the possibilities of superatmospheric pressure should be considered. Its principal advantage lies in the high diffusion rates obtainable at high pressure drops. Furthermore, some of the proposed uses, including purification of reformer recycle streams, involve high process pressures. In any case, for large scale application, superatmospheric presS u m are useful, if not mandatory, to take care of messure droD through " heaters and to avoid bulky equipment. There is a notable lack of data on the diffusion of hydrogen through palladium at high pressures. The usual experiment
( 7 ) measures the rate of leakage of hydrogen through a palladium plug or membrane from subatmospheric pressure into a high vacuum. Some data are reported for upstream pressures above atmospheric (6, 9, 77) but none seem to be available for such downstream pressure although, in principle, Watson's stream hydrogen analyzer (74) used this type of flow.
Diffusion Rates of Hydrogen Through Palladium W e r e Studied in New Ranges of Process Variables Temperature, O F. 650-850 Upstream press,, p.s,i.g. 1-700 Downstream pressure, p.5.i.g. 0-300 1-400 Press. drop, p.s.i. Hydrogen purity, mole % 66; 9 9 + Nondiffusible diluents No; CHI 0.0094 Membrane area, sq, ft. Membrane thickness, in. 0.0008
I n the work described here, it was found that hydrogen diffuses selectively through an 0.8-mil thick supported palladium membrane at rates over 250 standard cubic feet per hour per square foot a t 850" F. and a pressure drop of 400 p.s.i. Presence of up to 35y0 nitrogen or methane does not interfere with the process. Diffusion rate is proportional to pressure drop at low pressures and linear with the difference between the 0.8 power of pressure at high pressures. These relationships are associated with surface and bulk rate-controlling processes, respectively. Deviation from the normal square root of pressure law in the latter case correlates with high pressure solubility isotherms for the palladium-hydrogen system. Experimental
Apparatus. Palladium membranes, inches in diameter, were cut from a single sheet of rolled palladium foil, 0.0008 inch thick, supplied by Baker and Co., Newark, N. J. The membrane was laid on a support consisting of a 16/16-inch disk of porous stainless steel (Type X, '/le-inch plate, Micro Metallic Corp., Glen Cove, N. Y . ) . The porous plate was press-fitted into a stainlesssteel plate holder and backed by a drilled plug with a serrated face. This assembly was bolted between a pair of stainless steel flanges. Copper gaskets provided seals on both the palladium membrane and base of the plate holder. The upstream gas chamber formed by the upstream flange, the palladium membrane, and the upstream copper gasket was 15/16 inches in diameter and approximately inch deep. Gas was admitted to the periphery of the chamber through four equally spaced holes. A gas exit was provided in the center of the upstream 11/2
Upstream Flange Upstream G a s k e t 1
1
Palladium D i a p h r a g m Porous S t a i n l e s s Steel Backup P l a t e
1L
Downstream Flange Downstream Gasket Backup Plug
Backup P l a t e Holder
Palladium membrane assembly. A backup plate of porous steel permits large pressure drops to be taken across an 0.8-mil membrane
VOL. 52, NO. 6
JUNE 1960
525
X new palladium membrane was installed prior to obtaining these data. The temperature was kept a t 850' F. throughout. Downstream pressure was set successively a i atmospheric, 115, 21 5, and 315 p.s.i.a. The upstream pressure in each case was varied to give a pressure drop across the membrane of 30 to 400 p.s.i. in increments of 50. Constants corresponding to the four curves of Figure 2 are
B teed
Downstream Press P B , P.s.I.A.
Diffusate Ga S
Go s
15 115
Nondiffusable components in the feed must b e bled from the membrane assembly to maintain diffusion
flange so that a continuous stream of gas could be passed across the face of the palladium membrane. Gas, delivered to the upstream chamber at the desired pressure via the Grove downstream pressure regulator, R1, was withdrawn at any desired bleed rate through the needle valve, V1, and metered by a wet test meter. Pressure on the downstream side of the diaphragm was maintained by a Grove upstream pressure regulator, R2. Rate of gas flow from the downstream side of the membrane (diffusion rate) was measured by a wet test meter. Pressures were measured with Bourdon gages, P1 and P2. A bypass valve, V2, around the membrane permitted the upstream and downstream sections of the apparatus to be pressure tested independently Tvith a nondiffusible gas such as nitrogen. The gas leads directly to and from the flanges were of 3/16-inch stainless steel tubing connected with high pressure fittings made by Universal Oil Products, Inc. The gas inlet lead together Lvith the diaphragm assembly was heated in a split electric furnace, electronically controlled to maintain the desired temperature which was measured at the inner face of the upstream flange. Diffusion Experiments. The first series of measurements was made to determine the rate of diffusion of cylinder hydrogen through the membrane at 650') 750°, and 850" F. with upstream pressures ranging from 1 to 400 p.s.i.g. and the downstream side at atmospheric pressure. T h e hydrogen contained 0.770 nitrogen as a principal impurity. Presence of this nondiffusible component made the diffusion rate somewhat dependent on the bleed rate. However, operation at a bleed rate at least as great as the diffusion rate was sufficient to virtually eliminate this dependency. At low upstream pressures the diffusion rate was proportional to pressure drop. Above 25 p.s.i.a. upstream pressure the diffusion rate fell away from this
526
linear relation (Figure 1). For the high pressure range, a function involving the difference between the 0.8 power of the absolute pressures was selected 10 correlate the data : (1 1
D = Ki(P1 - Pz) 15.7 D
5 Pi < 24.7; Pa T
+
KZ(P10.8
PI >_ 2 5 ; Pz
= 14.7
( 21
- P20.8) 15
=
Values of Constants for the Three Temperatures Studied Temp., F.
rcI
1'
Iil
650 750 850
0.0075 0.0105 0.0140
0.02 0.05 0.15
0.0135 0.0173 0.0184
-4 second series of diffusion experiments was made with emphasis on the high pressure relationship (Figure 2) using Equation 2 to correlare the results.
i
0.8 LO1
&
0.6
Figure 1 . Two do0.4 mains of flow corre- 2 spond to surface and ;0,2 bulkcontrolof the diffusion process
5 E
0.0008 b y 0.0094 square foot palladium membrane; atmospheric pressure
INDUSTRIAL AND ENGINEERING CHEMISTRY
Olt
'i 1
215 315
r
ri '
0.25 0.13 - 0.02 0
0.0186 0.0190 0.0201 0.0196
The residuals, 7, became insignificant at Pz > 200. The coefficients K? were all abour: the same, and the same as found for 850' F. in the first series of experiments. For Pz = 15, the residual term was larger than found previously. This difference may be associated with the change in membranes. Separation Experiments. A third series of measurements demonstrated the separation of hydrogen from methane and from nitrogen. The hydrogennitrogen mixture made for this purpose contained (mass spectrometer analyses) 64.3 and 35.7 mole yo of hydrogen and nitrogen, respectively. The hydrogenmethane mixture contained 65.9, 33.9, and 0.2 mole yo of hydrogen, methane, and nitrogen, respectively. ,4new membrane was installed for the separation experiments. Upstream pressure was maintained at 700 p.s.i.g. and downstream pressure at 300. The temperature was 850' F. The principal process variable studied was rate of the bleed stream taken from the upstream gas chamber. Diffusion of hydrogen from the mixture was slow at
HYDROGEN D I F F U S I O N
Figure 2. Diffusion rate is an 0.8 power function of pressure 0,0008 b y 0.0094 square foot palladium membrane a t 850'F. 20
40
60
80
100
120
140
160
180
( Upstream Pressure, p s i 0
10
50
100
200
300
400
500
600
700
Upstream pressure, p s i a
low bleed rates, increased as the bleed rate increased, and approached a limiting value a t high bleed rates. The increase followed a hyperbolic low. This is shown in Figure 3 where the plot yields straight lines of the form
B
n
-
np
(5)
as B approaches 0. Substituting this value ofUin B Equation 3 andsettingB = 0 , Kd ---
K3
which is a method of expressing the equation of a rectangular hyperbola. Observed constants for the hydrogen-nitrogen mixture were Ks = 0.55 and Kd = -0.97. For the hydrogen-methane mixture the observed constants were Ks = 0.51 and Kq = -0.97. The diffusate gases in all samples taken analyzed 99+% of hydrogen. Traces of impurities were assumed to have been introduced during sampling or during transfer to the mass spectrometer. By hydrogen balance, the mole fraction of hydrogen in the bleed stream, n B , is related to that in the feed (nF) and to the bleed-diffusate ratio by Equation 4 :
1 - nF - P2/PI
-
1 nF nF - P ~ P I
(6)
For the hydrogen-methane mixtures) calcula'ed --4/K3 was ''56 compared to found) and for the hydrogen-nitrogen mixtures, it was .76 to 1.75 found. Equation was to the separafrom gas mixtures tion of substituting for 1' the hydrogen partial pressure in the upstream chamber' At high Of the rate this is approximately equal to the partial pressure of hydrogen in the feed. Equation 2 can then be rewritten : K~ = D m = K2[(npP1)0,8- pzo.8] ( 7 )
As noted before, the residual term is small at sufficiently high values of PI and Pz. Equation 7 was tested by evaluating the membrane constant, K z . Cylinder hydrogen was diffused a t the conditions used in the separation experiment. T h e value of Kz found, 0.0155, was substituted into Equation 7 together with the appropriate value of nF, PI,and Pz. D m calculated for the hydrogen-methane mixture was 0.57 standard cubic feet per hour compared to 0.51 found. For the hydrogen-nitrogen mixturc, it was 0.54 compared to 0.55 found.
Discussion The passage of hydrogen through palladium has been reviewed by several authors including Smithells (73)) Dushman ( 7 ) , and Barrer (2). There has been much variation reported ( 7 7) in the exponential dependency of the diffusion rate on pressure, ranging from P0.S to Pl.0. Proposed mechanisms of diffusion generally assume a rate-determining step, which may be either a surface process involving adsorption and dissociation of molecular hydrogen, or a bulk process involving transfer of protons through the palladium lattice. Salmon (72) and others conclude that rate should be surface controlled and proportional to upstream pressure at sufficiently low pressures; that the pressure range Over which the proportion~lityholds should increase for thin membranes, for membranes with smooth surfaces, so that the true surface area is small, and for membranes whose surfaces are poisoned; and that at sufficiently high pressures the rate should be bulk controlled and linear in p , s . The last conclusion is derived from Fick's law, that rate of diffusion through the bulk is proportional to concentration
8
For the gas mixtures used, the right side of this equation is equal to 0.35. Average value found for the left side was 0.40. This is in reasonable agreement, considering that the difference in mass spectrometer hydrogen analyses was only 0.05 to 0.15 mole fraction, and that this difference can be subject to 20% error. Constants Ks and Kq of Equation 3 are identified by considering the boundary conditions at high and low bleed rates. The slope, KQ,represents a limiting maximum value of the diffusion rate, D m , as the bleed rate is increased without limit. At very small values of the bleed rate, hydrogen content of the bleed stream approaches that imposed by static equilibrium, the ratio of the downstream to upstream pressure. I n the limit
