A New Plant Detective . . .

The Galvanic Cell Oxygen Analyzer

P R m I s E ANALYSIS for 0 to 1000 p.p.m. of oxygen in hydrocarbon streams is important a t Monsanto's Texas City plant. A device for measuring small concentrations of oxygen had been in- vented by Hersch (7-4) from which this instrument was built for evaluation.

This instrument was rapid, highly sensitive, accurate, and an excellent laboratory analyzer for ranges as low as 100 p.p.m. of oxygen. However, it lacked the ruggedness and long term stability required for a continuous plant analyzer. Therefore, a practical plant analyzer based on the Hersch principle was developed for a specific application. However, emphasis was placed on time, and this developmental work, neither critical nor thorough, does not limit the potential performance of the analyzer.

The analyzer is based on depolari- zation by oxygen of an inert cathode of a galvanic couple. The current is pro- portional to the oxygen concentration of the gas in contact with the electrode. For the preferred system of silver cathode, lead anode, and 24y0 potassium hy- droxide electrolyte, the reactions are ( 2 ) :

At cathode: 0 3 + 2 H20 + 4e --c 40H- 0 2 + H2O + 2e --c OH- + OzH-

At anode: Pb + 20H--,Pb(OH)2 + 2e

H2O -c 2Pb(OH)z + 4e 2Pb f OH- + OzH- +

Finally: Pb(OH)2 + KOH +

The formation of hydroperoxyl ion is a minor reaction.

Other electrode systems may be better for samples which react with the above electrodes. The electrodes must not react with the electrolyte and the cath- ode reaction must be only depolarized by oxygen with the anode supplying the necessary electrons for oxygen reduc- tion. The oxygen apparently reacts at the electrode and electrolyte inter- face; some dryness of the cathode is essential. Oxygen absorbed on the cathode surface migrates to the inter-

KHPbOz + H2O

face at a rate apparently much greater than oxygen diffusion through the liquid. The depolarization current is proportional to oxygen concentration and seems to be limited by the diffusion of oxygen from gas to silver surface. Above a minimum sample flow the dif- fusion rate appears constant and the reading is nearly independent of flow.

Hersch's original analyzer used a platinum foil cathode partially immersed in potassium hydroxide solution with a variety of anodes. The latest consists of a lead anode separated from a silver screen cathode by a porous membrane saturated with potassium hydroxide solution. Sensitivity and stability are much improved. This later type of cell was developed into a process analyzer.

Sensitivity Over-all precision Stability output Response time

Air exposure Life Range Interference

~

Charactetistics of the Analyzer Better than 1 p.p.m. of oxygen Oxygen analysis consistently within f 2 p.p.m. Better than 1 p.p.m./4 hours Approximately 4 pa./p.p.m. oxygen with 100-ohm load resistor 90% response in 1.5 minutes, full response in 3.5 minutes with a

No permanent effect Approximately 3 months in field service between reconditionings 0 to 100 p.p.m. essentially linear; usable to 1000 to 2000 p.p.m. No response to hydrocarbons. Materials reacting with silver or

24y0 KOH affect sensitivity

sample flow of 200 cc./min.

VOL. 51, NO. 6 JUNE 1959 727

.RUBBER Sl 'OPPER

GLASS TUBE

1/64" LEAD SHEET WHATMAN #50 FILTER PAPER

20-MESH, SILVER NO. 27 WIRE

OFF

J SCRUBBER GALVANIC CELL

METER (100~1 INTERNAL RESISTANCE)

This simple galvanic cell oxygen analyzer has a sensitivity better than 1 p.p.m.

Galvanic Cell Oxygen Analyzer

A simple analyzer was first constructed and examined in the laboratory, using ethylene with air added as samples. Standards were analyzed by a method which was precise to better than 1 p.p.m. Pure ethylene contained less than 2 p.p.m. oxygen and was used as zero gas. This simple laboratory analyzer had a sensitivity better than 1 p.p.m. oxygen and permitted these observarions.

Soldering to the silver screen com- pletely destroyed any output. Solder on the lead in the active part of the cell caused loss of sensitivity after several hours. Hence, there should be no soldering on the active electrode system. Soldering the connecting wire to the

I 0 1 2 3 4 5 6 7 8 3 INCHES FROM LIQUID TO CENTER OF SILVER SCREEN

Figure 1 . Maximum sensitivity with the silver screen occurs about 4 to 5 inches above the liquid surface

lead anode is desirable as mechanical contacts become noisy from corrosion by potassium hydroxide. The solder connection to the lead is made above the active surface.

The silver-lead electrode system was very good with little advantage to find- ing an alternate. The lead anode was accepted and a few brief tests were made on various available cathode materials. Although the results were not directly comparable they did indicate silver was as good as any material tested for these purposes. Copper had a slightly lower sensitivity and was noisy. Nchrome V and Chrome1 were very low in sensitivity and noisy. Gold had good sensitivity and has an advantage if materials reacting with silver are present in the sample stream. Platinum has a similar advan- tage, although it was not available and not tested. I t was impossible to estimate the long range stability from these screening tests. The more noble metals may also have an advantage.

The silver screen cathode is readily available, low in cost, high in sensitivity, and stable. The 20-mesh, No. 27 wire silver screen was on hand and therefore used. Finer wire and/or mesh might improve characteristics.

Flushing the cell with oxygen-free ethylene was as effective for placing a cell in operation as Hersch's method of evacuating, filling with potassium hy- droxide solution and draining in the presence of oxygen-free gas, and much more convenient for plant applications.

Some dryness of the cathode was es- sential. This was determined on a cell similar to galvanic cell (above) with the pool of potassium hydroxide solu- tion to keep the membrane moist by capillary action omitted. The electrode assembly was initially wet and exhibited little or no sensitivity and a noisy output. .4s the cathode dried in a sample stream the sensitivity gradually increased to a maximum and then decreased. An optimum wetness was apparent and the life definitely limited for continuous dry- sample operation. The cell was modi- fied to the wick-action type and a life of one week or more obtained with dry sample. For a sample saturated by bubbling through 249;', potassium hy- droxide solution, the cell life is very long. The wick action is essential for appreciable life with dry sample but for very thoroughly saturated sample it is probably of little or no significance. The wick action also provides an in-

.- 0 3 6 9 I2 15 18 Zl 24 27 30 33 36 39 4 2 45 11- L I

KOH C3NCEhKRLTION WEIGqT PER CENT

Figure 2. Optimum sensitivity occurred at 24y0 potassium hydroxide

7 2 8 INDUSTRIAL AND ENGINEERING CHEMISTRY

GALVANIC CELL O X Y G E N ANALYZER

surance factor in case sample saturation is incomplete and probably contributes something to the general operational stability. The wick is a simple modifi- cation of the original cell and has been adopted as standard construction.

The prescrubber was designed to pro- vide a minimum sample flushing time with a large liquid reservoir, and was used with a open tube bubbler and also with a fritted glass bubbler. In experi- ments with the original wickless cell the fritted bubbler provided more sta- bility indicating incomplete saturation with the open tube. No difference be- tween the two types of bubblers was noted with the wick.

Results Sensitivity as a function of cathode

dryness was determined on a long cell having a series of separate 1-inch silver screen cathodes on a common core (Figure 1). Three separate cells were run to obtain results independent of differences in assembly. Maximum sen- sitivity occurred with the silver cathode about 4 to 5 inches above the surface of the liquid. A cathode of silver screen, 3 inches wide, located between 2 and 5 inches above the potassium hydroxide solution surface was accepted as standard.

The membrane material used by Hersch was not readily available and several alternates were examined. What- man No. 50 hardened filter paper was chosen for the %yo potassium hydroxide service and satisfactory results were ob- tained. Membrane thickness affected cell response time. One layer of filter paper and sample flow at 200 cc. per minute gave IOOyo response to 110 p.p.m. oxygen in 2.5 minutes. Two layers of filter paper required 40% ad- ditional time, with three layers, 90%. A faster response would be advantageous and other membranes were briefly examined. Porous Teflon and fiber glass paper proved to be too flimsy. Gelled cellophane was very thin and ex- hibited high sensitivity and excellent response, but deteriorated in the 24% potassium hydroxide. The Whatman No. 50 paper was accepted as a prac- tical solution to the problem.

Sensitivity us. potassium hydroxide concentration was determined (Figure 2), and 24% confirmed as optimum, with the added information that con- centration is not particularly critical. A few other electrolytes were briefly examined. The sensitivities in micro- volts per 1 p.p.m. of oxygen were: for acetic acid, 0.3; ammonium hydrox- ide, 0.6; and sodium chloride 0.2, referred to 4.0 for potassium hydroxide.

The galvanic cell is essentially a cur- rent generator, and only a simple meter circuit is necessary with recorder being optional (page 728).

4.5 3.0 1.5 0.3

3.0 2.0 1.0 0.2 UJ W

I-- (L 2 W

g!k 2 2

z 1.5 1.0 0.5 0.1

0- I "2- 0-100 F Z R M 7 , I. --;'.I,

0 / / / , < /-*0-2000 W M . .'

0-20

/

IO 20 30 40 50 60 70 80 90 1 0 0 W M . 200 400 600 800 1000 1200 1400 1600 1800 2000FRM. 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 09 1.0 % 2.0 4.0 6.0 8.0 IO 12 14 16 18 20

OXYGEN CONCENTRATION

A Figure 3. Re- sponse is linear to about 70 p.p.m. of oxygen

b Figure 4. Linear- ity of response is i n c r e a s e d t o about 600 cc. per minute and then decreases w i t h further increase in flow

90

ao

70

f 6 C a i- 2 0

w 50

$ 40

IL

30

20

I O

400 CC./ M I N . 9 ,

180 CC./ M I N . 7 // 4"/L85 CC./ MIN.

;' /' L800 CC./MIN.

0 50 IO0 150 200 250 300 OXYGEN CONCENTRATION IN P.P.M.

The temperature sensitivity of the cell was briefly studied. No temperature effect was observed in the air-condi- tioned laboratory by changing the ther- mostat. The cell was then heated to 25' C. above room temperature, 4'0- ducing a 2.5 p.p.m. increase in reading on zero gas, equivalent to 0.1 p.p.m. per ' C. This could be temperature- induced outgassing rather than a real effect. The change in reading of an oxygen containing sample was equiva- lent to the thermal coefficient of expan-

sion of the gas, about -l/&o of reading per ' C. Rapid heating caused large thermocouple effects. Thermostating is not necessary if the cell is used in an area of reasonably constant temperature. Where temperature extremes are ex- pected rmgh thermostating is probably desirable. The thermocouple effects in- dicate rapid changes in temperature should be avoided.

The linearity of response and a gen- eral calibration from zero oxygen to air was obtained (Figure 3). Response

VOL. 51, NO. 6 JUNE 1959 729

TAMPLE INLET

SAMPLE OUTLET A

LEVEL OF KOH SOLN. IN PRESCRUBBER

LEVEL OF #I KOH SOLN. IN CELL

1/4" TUBING

- I / 2 " P I PE

L T U B U L A R VESSEL TO SEPARATE CELL FROM PRESCRUBBER CAUSTIC

Figure 5. An all-metal cell with

is linear to about 70 p.p.m. of oxygen and decreases slightly to 400 p.p.m. Above 400 p.p.m. the response decreases markedly with concentration, apparently in a logarithmic manner.

Linearity is also a function of sample flow rate. Response increased in lin- earity to about 600 cc. per minute and then decreased with a further increase of flow (Figure 4). A sample flow of 200 cc. per minute was selected as the best compromise of response linearity and speed, use of zero and reference gases, and evaporation of moisture from the presaturator. Within the linear range the cell output is independent of flow so precise control of sample flow rate is not necessary.

Change in cell response with change in flow within the linear range and a high zero reading almost invariably in- dicate oxygen leakage or evolution from components. Release of oxygen from components of a sample system is a continuing and difficult problem. A kinetic system assembled with care from selected components permits the 2cl p.p.m. analytical precision required. Plant use requires conventional com- mercial components, pressure regulators, valves, etc., to be used. Carefully selected components with outgassing volumes at a minimum and proved to provide no leakage are necessary. The gaskets in a particular rotameter gave oxygen equivalent to 5 p.p.m. at a sample flow rate of 180 cc. per minute. Surgical rubber tubing is very porous to oxygen. Tygon tubing is slightly porous. The precision of z t 2 p.p.m. obtained in plant practice is not only the limit of the cell itself, but also the

prescrubber for process analyses

limit of the over-all system, and the range and precision of the Hersch gal- vanic cell could probably be extended to significantly lower oxygen values.

It is desirable from a process instru- ment maintenance standpoint to have an analyzer as simple as possible. An effort was made to eliminate the sepa- rate prescrubber. The sample con- tained traces of unsaturated carbonyls, believed to increase considerably at times although the actual concentra- tions have not been studied. The effect of acrolein on the galvanic cell was tested as a limiting case. The cell sensitivity was almost completely de- stroyed after a 2-hour exposure to ethylene bubbled through acrolein at about 25" C. The need for a prescrub- ber is dependent on the concentration of contaminants in the gas sample to be analyzed.

Glass cells were placed in operation in the plant almost immediately and served as a guide for subsequent de- velopment. Plant and laboratory ex- periences with these glass cells were used in designing the final plant in- strument. Glass equipment is unde- sirable in plant installations and an all metal cell was the final development. hTo effect on cell output was found with stainless steel construction, and a plant cell based on laboratory studies and plant experience with varioiis glass cells was designed (Figure 5).

In this cell the prescrubbing caustic solution is separated from the caustic solution in contact with cell wick and electrodes. Contaminants are removed in the outer prescrubbing solution and the inner solution remains clean. The inner

solution level varies slightly due to pre- saturation of the gas sample. A large volume of prescrubbing caustic solution eliminates frequent refilling due to evaporation. Cells of this type have operated satisfactorily for 3 months. The only maintenance required is to add caustic solution to the prescrubber about every 3 weeks.

Cell life is difficult to determine except by actual service, and long time operat- ing experience is required to determine precision in actual plant applications. A cell operated in the laboratory for 1 week on air had one half of the initial sensi- tivity and a rather high zero level, but was still usable. The practical life of a cell is not limited by the effects of a sus- tained oxygen reaction. Principal indi- cations of cell decay are loss of sensitivity and excessively slow response to sample changes. When the cell is no longer usable it may be reconditioned by thoroughly washing electrodes with dis- tilled water, resaturating the membrane with caustic solution, reassembling the cell, and flushing with an oxygen-free gas.

Several factors make the galvanic cell oxygen analyzer well adapted to process analysis. Simplicity and rugged- ness facilitate installation and mainte- nance, and insensitivity to variations in environment makes precise tempera- ture and flow control unnecessary. The output is low voltage and low impedance, and does not constitute an explosion hazard. These factors are very impor- tant in a field instrument.

The accuracy and stability of the an- alyzer has been repeatedly checked at f l p.p.m. over 4-hour intervals by reference analyses. The accuracy is limited by the oxygen evolution from and diffusion through surfaces in the sample system. With care in selection and assembly a complete sample system can be made by conventional methods. The practical advantages of this are very great and no further attempt was made to determine the limiting precision of the cell. The general performance indicates the cell itself is capable of sub- stantially better precision if carefully applied in an oxygen-free system. Re- liability is excellent, with essentially 100% continuity of analyses provided. Maintenance is normal for this type of process analysis equipment.

Literature Cited (I) Hersch, Paul (to Mond Nickel Go.,

Ltd.), Brit. Patent 707,323 (April 14, 1954).

(2) Hersch, Paul (to Mond Nickel Co., Ltd.), Brit. Patent of Addition 750,254.

(3) Hersch, Paul (to International Nickel Co.), U. S. Patent 2,805,191 (Sept. 3, 1957).

(4) Wise, E. M., International Nickel Co., New York, N. Y., private com- munication.

RECEIVED for review March 5, 1958 ACCEPTED March 3, 1959

730 INDUSTRIAL AND ENGINEERING CHEMISTRY