V O L U M E 2 7 , N O . 1, J A N U A R Y 1 9 5 5 alcohol alone for washing the potassium fluoborate precipitate free of occluded salts; only when high amounts of sodium chloride or other salts were present was more than one washing found to be necessary. The precipitate was considered free of impurities when washing with 30 ml. of the 1 to 1 ice-cold solution caused no greater than a 0.5-mg. change in weight. When ethyl alcohol or the 1 to 1 mixture of ethyl alcoholmethanol is used for the determination of potassium fluoborate, the precipitate has a white, almost gelatinous appearance, but when methanol alone is used, the potassium fluoborate appears to be very fine, granular, and almost transparent. The precipitate from methanol requires more careful handling during transfer to the crucible, and technique difficulties may be the reason for the slightly lower results.
83 The necessity of having to work with ice-cold solutions is a slight disadvantage but when more is known about the solubility of potassium fluoborate in other organir solvents, it is conceivable that a method could be developed for precipitating potassium fluoborate quantitatively a t room temperature. ACKNOWLEDGMENT
The author gratefully acknowledges the encouragement and advice of William A. Dupraw during the course of this investigation. LITERATURE CITED
Booth, H. S., and Martin, D. It., “Boron Trifiuoride and Its Derivatives,” pp. 99-106, New York, John Wiley & Sons, 1949.
Flaschka, H., and Amin, A. XI., Chemist Analyst, 42, No. 4, DISCUSSION
78 (1953). I h i L ‘ 4 3 . N o. .. _ 1.4 _..__,._,_ , _(1954). ~
Except for the difficulty encountered when calcium and aluminum are present in the same solution, the determination of potassium as potassium fluoborate in many materials is simple by this rapid method in the presence of the common alkalies and other salts. The method has its advantages over the perchlorate and chloroplatinate methods in speed and costR, respectively, and Rhould he ideal for routine work.
Gloss, G. H., Ibid., 42, No.3,50 (1953). Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” New York, N. Y., John Wiley & Sons, Ino., 1929. Mathers. F. C.. Stewart, C. O., Housernann, H. V., and Lee. I, E..
J. Am. Chem. Soc.. 37. 1515-17 (1915). Robinson, J. W., C h a . Age (London), 66,’447-50,467-9, 507-10, 573-7 (1952). RbrmvED
for review April 19. 1954.
Arcepted September 17, 1954
Semiautomatic Gas Separation Equipment CHARLES W. HANCHER’ and KARL KAMMERMEYER Chemical Engineering, State University of lowa, lowa City, lowa
With manual operation of apparatus for gas separation, the chance of introducing errors is great. A simultaneous-sampling, double automatic gas buret apparatus was developed whereby one operator could handle the equipment with a greater degree of accuracy because the timing and pressure control are automatic. Experimental data with the described instrument agreed well with data obtained from the manually operated apparatus and less time was required per determination.
F
ROhf 1820, lvhen Graham ( 4 ) carried out his initial work 011 gas separation, which resulted in the statement of Graham’s la\v, until 1945, gaseous phase separation was only a laboratory phenomenon. The first and foremost important application of gas separation, using a porous membrane, was in the separation of uranium isotopes. When a membrane or barrier, Khcthcr it be plastic, porous glass or ceramic, or metal is considered for use as a separating membrane, two characterist’ics should be determined: the rate of gas flow for a given pressure drop across t,he membrane. and the amount of enrichment in one or more of tlrr componerit,sof the gas as it permeates through the membr:tne. The theory and the literat,ure of the gaseou~phase separation have been well covered in previous publicat,ions (1, 2. 6, 7-9). The membranes under consideration will permit one or more of the three types of Row which are normally encountered \Then gases flow through membranes: molccular streaming or Knudsen flow, viscous or Poiseuille flow, and a mixture of niolecular streaming and viscous flow. Recent publications ( 3 . 5) have emphasized t,he fact that another flow phenomenon must be considered when vapor flow through rnicroporous membranes is included-that is, the occurrence of adsorbed (or condensed or surface) flow, resulting from presumable formation of a sorbed liquid phase in the microporous structure of the membrane. I
Prpsent a d d r a n . Oah Ridge National Laboratory, Oak Ridge, Tenn.
TYPES OF M E M H R i N E S
Membranes available for wparat’i(iii are essentially oi t n-o types: plastic films and porous bodies. While a plastic rnernbrane undoubtedly possesses a porous structure, it has been found helpful to differentiate between plastic films on one hand antl microporous membranes on the othcr hand. In general, microporous membranes are considrrctl t,o h r t v ~pores wit.h diamcters of about, tmhemean free pat’h size of gases, while the much smaller porous structure in the plastic membranes is caused by the spacing between the molecules of tlic plastic. JIicroporous membranes are actually capillary systems with interconnected pores. Such membranes can be prepared by two inethotls--producing microporee bj- rcnioving an interdispersed phast. or component or by reducing lwge holes which already exist in the membrane. A good example of the removal of a dispersed phasc is the preparation of porous glass (6). The technique of reducing the size of esisting holes in a membrane would be represented by ceramic pract.ice antl powder metallurgy. I t was shonn (1) that the test, gas mixture hydrogen-carboii dioside may behave very different~lyn hen plastic membranes arc used than when microporous membranes are used. The carbon cliositl(~i~ often selectively rnriched n.hrri the gas permeates through mstiy of the plastic inembrarics. This selective enrirlirneiit ia c.onsit1ert.d to he causcd by solubility pheiiomena (1). Sepai,atiniis i i r rnicroporous menibrsnes essentially obey Graham’s diffusion h t v , which iii it,s simplest form states that the separation is R funct’ion of the square root of the inverse ratio of the molecular weights. Under cerbain conditions the phenomenon of condcnsed flow will be encountered even with such a gaslikc substnncc a.s c : t i h ) t i dioxide. THEORE1‘IC:&L (;OVSII)ERATIONS
F r o m the rate equations for gas diffusion and a material balance, Reller and Steiner (8, 9) developed the equations for the binary system, providing a method for predicting separation results for two somewhat different caws of flow conditions. Th(,
84
ANALYTICAL CHEMISTRY control are automatic. The need for such an apparatus is evident; therefore, 8 simultaneoussampling, double automatic gas buret apparatus was developed. This apparatus is designed so that any diffusion cell can be tested. Any system of gases or noncondensable vnpors can be used 8 s B test mixture. The measurements to be made with this apparatus are the determination of the flaw rates af the permeated and purged streams and the accurate composition analysis uf the two streams; the operating temperature and pressure must also be rerorded. The flow rate is determined by allowing the test gas to displace a liquid in calibrated gas burets. The time to displace a standard volume can he determined by two different timing methods-the electrode mercury contact method and the photoelectric cell method. DESCRIPTION OF APPARATUS
The si:nultaneou.s-saml-ing, double gas buret system consist6 of three main parts: t.he separation cell, the two different flow
Fignre 1.
Automatic Cas Rurct Separation .Apparatus
Le., the purged stream-and the permeated stream. T6e feed gas flow need not be measured, as it is represented by the sum of permeated and purged streams, provided i t is certain t h a t no eaks exist in .the system. The feed gas composition is determined by analyzing t h e feed gas reservoir before the membrane testing is started. Figure 2 presents a diagram of the apparatus. A se aration cell must have a t least three gas conneotionsone forfigh pressure gas inlet, one for low pressure gas outlet, and one for high pressure purge outlet. I n the high pressure purge outlet line, there must be a valve for controlling the F factor. There must be some means to seal t h e membrane so that none
& allow Rood mixing of the feed and purge gases. gas &ret a t different heights,-thus defining a known volume. Figure 2.
Diagram of Automatic Gas Buret Separation Apparatus
I
I/,
I
.....
,
Test gas cylinder Test cas regulator
1 (1118
aepamuur,
C e l l 18 "t."IIB"
its
I, litCU"l.
r e l l l l ~ a u l u u yUULLT.
sponds to an P factor of 1.00. The F factor may be calculated from cither flow rate data or composition data. EQUIPMENT
Most of the types of apparatus for gas separation experiments ire 3. Stop-Clock Starting and Stopping Circuit which have been reported to date require a lame number of manual operations. If a simultaneous-sampling unit for the permeated and the purged gas is 6x5 to be used, two operators are needed to handle the apparatus during the sampling operations. In collecting gas samples, two leveling bottles have to be lowered a t such a rate that constant pressure (usually atmospheric) is maintained in tho gas burets a t all times. l u addition, multiple timing operations have to he performed. While manual operation is possible and has been used extensively, the chance of introducing errom is great. With an automatic gas buret system one operator could easily operate the equipment with an imprwed degree of experim m t a l accuracy. because th? timing and pressure Figure 4,, Five-Watt Direct Curreht Power Supply
-
V O L U M E 27. N O . 1, J A N U A R Y 1 9 5 5
8s RELAY I
LAMP
CLOCK
Figure 5.
RELAY 2 NORMALLY C b Diagram of Photoelectric R e.-,
tat t h e gas bureta have t o : cleaned daily and refilled ith Clem mercury, and (2) le toxic danger from merr y vapor poisoning if m y of the mercury is spilled. A disadvantage of the photoelectric cell method is the possibility of different amounts of gas being absorbed in the saturated salt solution. The presuure-control system actuates thevalve a t the bottom of the gas buret, which opens and closes at ich a rate that the gas in le buret is collected under "actically atmospheric p r e s Lie conditions. The gas ,ream from the separation :I1 is split into two lines: one mnecting to the gas buret idoneconnecting tothepresireeontroller which is vented > theatmosphere. T o maini n atmospheric conditions 1 t h e collection gas buret, the sethods of electrical control Id pneumatic control were ,ied.
As i t takes B few seconds for the instrument to gain control, the timing electrodes are installed a t 25 and 75 cc. when a. 100-cc. gas buret is used. The principle of the electric timer system is confining t h a t the electrical resistance of the mercury (used liquid for the gas) is muoh less than t h a t of the relay circuit, Figures. Therelay circuit for the topelectrodeis normally open, while t h e lower relay circuit is normally closed. One side of t h e 110-volt alternating current line which operat.es t h e electric timer is eonnect,ed to two relay circuits; t h e other side is connected directly to t.he electric timer. A 5-watt 75-volt direct current power supply operates the coils of t h e refay, Figure 4. The device used to measure rate of flow by means of the photoelectric cell method consists of two independent units which operate on 110-volt alternating current. Each unit includes an accurately calibrated gas buret and two photocell assemblies spaced a t a standard distance apart on the tube, usually 50 cc. The photocell assemblies are connected to an amplifier unit (Figure 5 ) which in turn controls an electric timer. Dyed salt solution is used in t h e buret as confining liquid. Two disadvantages of t h e electrode mercury contact method are: (1) when a gas test mixture is used with a high oxygen content (about 50%) the mercury oxidiaes a t such a rapid rate
To B e M w r a d
-. ..
Figure 6 . Diagram of Pressure Control System
When electrical control, consisting of an off-on pressure cell and solenoid valve, was used, it resulted in too much fluctuating or cycling control. Therefore, a proportional hand premure controller and n pneumatic presmre motor valve were installed to take its place. The electrical control failed to give suitable service because it superimposed a pulsating pressure differential on t,hc low pressure side of the membrane when in operation. Therefom, the equilibrium in the separation cell was continually beinnunset. TKe pneumatic precssure control aystem (Figures F and 7) was therefore developpd. It is patterned after a liquid-level con.troller. The system consistsof two glass vessels which are partiallv filled with water and codneeted below the w d e r line. One vessel is closed to the atmosphere and connects with a three-way
ANALYTICAL CHEMISTRY
86 sensitivity the closed vessel may be larger than the vessel open to the atmosphere. The float arm is attached to a level controller (Model 2504 Fisher Controller Level-trol). The instrument works on reverse action. Connected to the float arm is a pen which records movement of the float. Thus a permanent record of the pressure variations of either the permeated or the purged stream is obtained. By adjusting the motor valve loading, the pressure during the collecting period is regulated within f l / a ~inch of water, above or below atmospheric pressure. TYPES OF SEP4RATION CELLS
There are many types of separation cells in use today. One type used for plastic or sheet materials is the flanged separation cell (Figure S), constructed from two disks of stainless steel with six bolts. Inside the ring of bolts is a Teflon gasket. The capillary holes for the feed and purge stream are spaced on one of t h e diameters of the high pressure side flange. The capillary outlet hole for the permeated stream is in the center of the low pressure flange. For efficient operation the flanged cell must be free of all grease, oil, or dirt. The ceil gasket must also be clean. The membrane is cut the size of the outside diameter of the flange gasket. The backing (filter paper or other suitable porous material) which is used to fill the space between the membrane and the flange should be clean, as a small piece or particle of dirt may cut the membrane and a leak can develop. When all of the parts of the separation cell have been correctly assembled and the nuts tightened with a torsion wrench, the cell is connected to the apparatus.
\
Figure 9. Thimble Type of Diffusion Cell
1/8 &m. S ' d pipe lap
y-
/
13-NC Equally
6 Bolla Lasad
A . Feed gas
B . Purge gas
\
C. Permeated gas D. Rubber gasket
E . Screw cap F . Packing nut Q. Areas blanked with nonporous material
Low P r e i ~ r eFlange
Figure 8.
High
Pressure Flange
Diagram of Flanged Separation Cell
The thimble-type diffusion cell (Figure 9) was designed for porous glass membranes which are shaped like a test tube. I t has been completely described (5, 7).
trol stopcock a t the top of the pressure vessel is set manually, open to the pressure vessel and closed to the atmosphere, and the liquid in the buret is allowed to fall to a reservoir a t such a rate that the gas in the buret is kept under atmospheric pressure =k11/$2 inch of water head. When the run is finished, the stopcock on top of the pressure vessel is opened manually to the atmosphere; thus the controller returns to its normally closed position which closes the valve a t the bottom of the gas buret and liquid stops draining from the buret. Simultaneously with the opening of the pressure vessel
OPERATION OF APPARATUS
Prior to the start of the testing period, the gas flow is regulated to give the desired flow rate through the membrane. After the flow rate has been determined, the purge needle valve is set, which determines the F factor, and the system is allowed to reach equilibrium conditions by letting the purged and permeated gases escape to the atmosphere for a sufficient period of time, usually a number of hours. Equilibrium conditions were determined by checking either the flow rates or the composition of permeated streams a t half-hour intervals. This was continued until three consecutive readings were constant and this condition was taken as flow equilibrium. The purge needle valve control setting determines the F factor. To maintain the equilibrium conditions during the testing period, the samples of permeated and purged gases must be collected under atmospheric conditions. When equilibrium has been established, the membrane is ready to be tested. Before readings are taken, all of the electrical and pneumatic equipment is started and tested. The metering liquid is raised manually to the top of the gas burets. The pneumatic controls are set correc+,ly to correpond to the flow rate. The ron-
Li
Figure 10.
Flow Diagram of Gases
A . Manometer B . Diffusion cell C. Pressure regulator D . Bourdon valve (Foxboio Part U-IOl-BC) E . Controlling stopcock F . Gas burets 1. Purge stream to pressure controller 2 Purge stream to oxygen analyzer 3 Permeated stream to pressure controller 4 Permeated stream to oxygen analyzer 5 Feed stream to oxygen analyzer
V O L U M E 27, NO. 1, J A N U A R Y 1 9 5 5 stopcock, the gas buret is disconnected from the separation apparatus and is connected to the analyzing unit by means of a bypass stopcock system. With noncorrosive oxygen gas mixtures the Beckman oxygen analyzer was used exclusively and thus stream compositions could be obtained without upsetting the flow of gases or causing back pressures or surges. Therefore, in this situation the composition was determined first and then flow rates were determined as described. ANALYSIS OF T H E GAS STREAMS
The correct analysis of the gas streams is as important as the determination of the flow rates of the various gas streams. Any mixture of gases or noncondensable vapors can be used in the separation apparatus. A helium-oxygen mixture was used because of the relatively large molecular weight difference of the gases and convenience in analyzing such a mixture. Also, the mixture was safe to handle, as the gases were not toxic, flammable, or explosive.
E
0.70
? cn
Feed: High Pressure: Low Pressure:
EXPERIMENTAL DATA
The experimenta data taken with this apparatus agreed very well with the data obtained from the manually operated apparatus. The data could be obtained with much greater ease of operation and with less time required per determination. The results also may be more accurate, because the equilibrium of the cell is not likely to be upset as much as with manual operation. Separation results with a mixture of helium and oxygen are presented in Figure 11. The membrane used was porous glass. The operating conditions were as follows: Composition of feed mixture, xH,. = 0.501 mole fraction .EO,? = 0.499 mole fraction Pressure on high side, II = 3.72 atmospheres absolutv p = 0.98 atmosphere absolute Pressure on low side, He 01 = 2.28 Permeability ratio, P~Po,' Calculated composition ( 8 ) in permeated gas stream a t F = 0 is :x = 0.648 mole fraction The solid curve corresponds to the experimental data and t h e dashed curve represents values calculated according to the Weller and Steiner equation, Case I (7-9). The slight deviation a t F = 0 has been observed in many cases but has not as yet been explained in a satisfactory manner. The progressive deviation with increasing F values is due to a cell efficiency effect which becomes more pronounced as the flow type changes from turbulent to laminar. Results a r ' shown only to an F factor of about 0.6 because t h e limited degree of enrichment a t higher F values is usually not of much interest. Furthermore, operation a t F = 0.5 would usual1.ibe preferred, as it permits easy balancing of cells in multistage units.
n ran
"'" I
lyzer operated very quickly and with greater accuracy than the chemical absorption systems.
X y e = 0.501 M. F. 0 X f 2 = 0.499 M. F. 3.72 Atrns. abs.
0.98 A t m . abs.
c
0.60 (3
c ._ W
I
0.50 ._ c V
ACKNOWLEDGMENT
e LL
The authors wish to express their appreciation to the Uniteti States Atomic Energy Commission for sponsoring the project which made this work possible. They are also indebted to the Corning Glass Works for supplying the porous glass membrane.
I" 0.40 0.30 0
0.2
F Figure 11.
-
0.4 0.6 0.8 Fraction Permeated
LITERATURE CITED
I
Separation of Helium-Oxygen Mixture with Porous Glass
Once the gas sample has been collected, it can be analyzed by chemical absorption or instrumental analysis. A Beckman Model E-2 oxygen analyzer was incorporated in the experimental equipment. When this instrument was used, it was connected directly to the three gas streams with a selective manifold-type connecting system so that a continuous analysis could be obtained. Figure 10 shows the flow of gases through the apparatus to the oxygen analyzer. The Beckman oxygen ana-
(1) Brubaker, D. W., and Kammermeyer, K., I d . Eng. Chem., 44.1465 ~ ~(1952). (2) I b i d , 46,733-9 (1954). (3) Carman, P.C.,and Malherbe, P. le It., Proc. R o y . Soc. ( L o n d o n ) , A203,165-78 (1950). (4) Graham, T., Trans. Roy. SOC.( L o n d o n ) , 153,385 (1863). (5) Hagerbaumer, D.H., and Kammermeyer, K., Chem. Eng. Progr., S y m p o s i u m Ser., N o . 10; Collected Research Papers, 50, 25-44 (1954). (6) Hood, H. P.,and Nordberg, M. E., U. S.Patent 2,106,744(1938). (7) Huckins, H. E.,and Kammermeyer, K., Chem. Eng. Prop.. 49, 1804,2948 (1953). ( 8 ) Weller, S.,and Steiner, W. A,, Ibid., 46, 585 (1950). (9) Weller, S.,and Steiner, W. A., J. Appl. Phys., 21, 279 (1950).
.
. - - - - I .
RECEIVED for review April 24, 1954. Accepted October 8, 1954.
