A New Plant Detective
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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 invented 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 depolarization by oxygen of an inert cathode of a galvanic couple. The current is proportional to the oxygen concentration of the gas in contact with the electrode. For the preferred system of silver cathode, lead anode, and 24y0 potassium hydroxide electrolyte, the reactions are ( 2 ):
+ 2 H20 + 4e 4 0 H + H2O + 2e OH- + OzH-
At cathode: 0 2
--c
0 3
--c
+
+
At anode: Pb 20H--,Pb(OH)2 2e 2Pb f OHOzHH2O -c 2Pb(OH)z 4e Finally: Pb(OH)2 KOH + KHPbOz H2O 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 cathode reaction must be only depolarized by oxygen with the anode supplying the necessary electrons for oxygen reduction. The oxygen apparently reacts at the electrode and electrolyte interface; some dryness of the cathode is essential. Oxygen absorbed on the cathode surface migrates to the inter-
+ +
+
+
+
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 diffusion 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 sample flow of 200 cc./min. 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 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
METER (100~1 INTERNAL RESISTANCE)
J
SCRUBBER
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 completely 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
INCHES
2
3
4
5
6
7
8
3
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
728
GALVANIC CELL
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 finding 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 Chrome1were 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 advantage, 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 hydroxide solution and draining in the presence of oxygen-free gas, and much more convenient for plant applications.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
This simple galvanic cell oxygen analyzer has a sensitivity better than 1 p.p.m.
Some dryness of the cathode was essential. This was determined on a cell similar to galvanic cell (above) with the pool of potassium hydroxide solution 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 drysample operation. The cell was modified 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 hydroxide 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 K O H C3NCEhKRLTION WEIGqT PER CENT
11-
LI
Figure 2. Optimum sensitivity occurred a t 24y0 potassium hydroxide
G A L V A N I C 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 modification of the original cell and has been adopted as standard construction. The prescrubber was designed to provide 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 experiments with the original wickless cell the fritted bubbler provided more stability indicating incomplete saturation with the open tube. No difference between 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 sensitivity 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. Whatman No. 50 hardened filter paper was chosen for the %yopotassium hydroxide service and satisfactory results were obtained. 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% additional 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 exhibited high sensitivity and excellent response, but deteriorated in the 24% potassium hydroxide. The Whatman No. 50 paper was accepted as a practical solution to the problem. Sensitivity us. potassium hydroxide concentration was determined (Figure 2), and 24% confirmed as optimum, with the added information that concentration is not particularly critical. A few other electrolytes were briefly examined. The sensitivities in microvolts per 1 p.p.m. of oxygen were: for acetic acid, 0.3; ammonium hydroxide, 0.6; and sodium chloride 0.2, referred to 4.0 for potassium hydroxide. The galvanic cell is essentially a current generator, and only a simple meter circuit is necessary with recorder being optional (page 728).
0- I "2-
4.5 3.0 1.5 0.3 0-100 F Z R M 7 ,
I.
.'
