ENGINEERING, DESIGN, A N D PROCESS DEVELOPMENT The Fater recorder, during the same period, indicated that all of these batches of product Lwre dry except for one short period. It is doubtful if this small quantity of moisture lvould seriously affect the quality of a batch of product.
Applications The sediment recorder was developed t o study the movement of sediment in distilled product pipelines, and has been very useful for this purpose. The recorder helps t o locate the source of sediment or water contamination, as well as t o show the necessity for cleaning t'anks and pipelines. Several of these recorders have been installed a t key points along refined products distribut,ion pipelines and are performing satisfactorily. When a sediment recorder is installed, a quality iniprovcment often occurs which is not directly traceable to any information obtained by the instrument. The recorder probably makes the operating personnel more quality conscious, resulting in a better product and increased consumer satisfaction. Light products are frequently dried before they enter the pipeline, to remove any free moisture that would corrode and damage the h e . The recorder checks the perlormarice of these driers, and this informat,ion should permit keeping the pipeline dry a greater percentage of the time and in extending t'he life of t'he line. The recorder has also been employed t'o monitor performance tests on several brands of large commercial filters which are used to remove sediment from distilled products.
Conclusions This instrument provides information on the sediment and moisture levels in products t,ransported by pipeline and should lead t o an improvement, in the quality of product finally delivered to the consumer. The sediment record aids in tracing the source of the contamination while the moisture record shows the extent of water contamination which can be utilized both for the segregation and reclamation of moist products and for the protection of pipelines from corrosion. The instrument should have many other applications in the petroleum industry, such as recording the eediment level in settling tanks and st,udying the effectiveness of additives which are blended v i t h the products to improve quality or reduce corrosion. The usefulness of this instrument is not limited to pipelines or the petroleum industry since many other industries may find a need for a continuous liquid filter recorder.
Literature Cited Gas Engineers Handbook, 1st Ed., pp. 514-15. Xew York, ;1IcGrawP-Hil1,1934. ( 2 ) Instrunzenis, 25, 601 (May 1952). ( 3 ) Morgan, J. D. (to Power Patents Co.) U. S.Patent 2.228.216 (Jan. 4,1941). (4) Yon Brand. E. K.. M e c h . Eng., 72, 479-81 (1950). (1)
RECEITED f o r review September 7, 1953.
ACCEPTEDl i a y 4 1 9 5 4 .
tr HENRY LANDSBERG AND EDWARD E. ESCHER Consolidafed Engineering Corp., Pasadena,
Calif.
A n automatic instrument continuously records trace quantities of oxidizabie sulfur compounds in the range of 0.1 to several hundred p.p.m. by volume in gases and atmospheres. Coulometric titration with electrolytically generated bromine i s used. Titration is electronically controlled at a potentiometric balance point. Various applications in the natural gas, process, and air pollution fields are discussed.
T
HE problem of analyzing for sulfur compounds, especially in
trace quantities, has always been one of great importance in a large number of industries. This problem is rapidly increasing in mope a i t h the continuous strides indust,ry is making in processing materials, many containing sulfur compounds, even trace quant'ities of which often are detrimental to efficient and economic operation. Likewise, the emphasis on safety, such as the odorization with certain sulfur compounds of nonodorous natural gas for domestic use, and the monitoring of atmospheres for dangerous concentration of toxic sulfur compounds, has enlarged the problem of sulfur compound anal.&. The generally used conventional chemical methods of analyses present inadequacies not only because of the consumption of tmimebut also because of inaccuracies in the lower concentrations and the ever present doubts resulting from epot checks. It is, therefore, the purpose of this paper to discuss t,he operation of an automatic and continuously recording titrator, commercially known a8 the Titrilog, and its applications in some industrial sulfur problema.
1422
Develop rne nt During World War 11,t,he U. S.Army Chemical Wa.rfare Service needed a sensitive automatic inatrument for detecting and recording very l o v concentrations of mustard gas in the air. Analytical procedures \$-eretedious and time consuming because of the long periods necessary t o absorb a usable sample. Also, accuracy was poor because of the instability of the dilute reagents required. Eckfeldt ( 4 ) a t Leeds & Xorthrup advanced the theory that a potentiometric end point and electrolytic generation of reagent could be used to produce an automatic titrating instrument. At approximately the same time. Schaeffer ( 6 ) ,working independently of Eckfeldt, a t the California Institutc of Technology, then under contract with Xational Defense Research Committ,ee, advanced the theory thst such a systcm could be CIPCtronically coupled t o produce a fully automatic, continuously titrating instrument,. This m-as based on the principle that a feedback amplifying system could be used to control the rate of generation of reactive reagent so it would a t all times be equivaleriC to the rate of absorption of reactive gag i n a titriition cell.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. ?
PROCESS INSTRUMENTATION The well-known reaction of bromine with organic sulfur compounds prompted the use of generated bromine as the reactive agent. Sense and associates ( 7 ) , give the oxidation of organic sulfides with bromine as follows :
RSR
+ B T +~ H,O
RBO
+ 2 H + + 2B1-
The theoretical work done a t the California Institute of Technology led to a contract between the Chemical Warfare Service and Consolidated Engineering Corp. for the development and manufacture of a practical field instrument for detecting and recording minute concentrations of mustard gas in air. Under the direction of Austin an automatic titrimeter (1, 3) was produced which subsequently v a s further developed by Consolidated Engineering into a commercial instrument (2, 8). This chemical-electronic instrument, the Titrilog, has been designed to continuously record concentrations of oxidizable sulfur compounds such as hydrogen sulfide, sulfur dioxide, mercaptans, thiophene, and organic sulfides and disulfides in gas streams or atmospheres. The range of the instrument can be varied over wide limits by varying the rate of sample introduction. At maximum sensitivity the Titrilog is capable of recording concentrations as low as 0.1 p.p.m. by volume or 0.005 grains pel 100 cubic feet. A very interesting feature of the instrument is its speed of response, changes in concentration being recorded within 20 to 30 seconds. The measurement of the sulfur compounds is accomplished by a titration with bromine. The bromine is electrolytically generated in a solution in which the sulfur compounds are absorbed from the gas stream. The feedback amplifying system controls the bromine generating current so that the net rate of bromine generation is at all times equivalent to the rates of absorption of the sulfur compounds. A recording milliammeter records the generating current. The net current is proportional to the sulfur compound concentration in the incoming gas stream.
tion cell, the bromine concentration is reduced and momentarily the voltage from the bromine-sensitive electrode drops so that a signal is sent to the amplifier. The amplifier immediately responds, increasing the bromine generating current so that now the bromine requirements of the reaction are satisfied and the desired residual bromine concentration corresponding to the reference voltage is maintained. Thus the reaction is carried out at a point approaching the equivalence of the two reactants. A recording milliammeter in the amplifier output circuit continuously records the amplifier output which is the bromine generating current. Since, according to Faraday's law, the quantity of
Reference Bottery Voltage
+
-
Cell sensor e l e c t r o d e voltage
Amplifier
-
!I
,
generating current
f
I
Inner compar;men;. (titration)
r t e r carnpartment,8
Outer sensor electrode. 4
G a s dispersion tube Charcoal filter
Principle of Operation
a m i n e I g en erator electrode, I
Inner sensor e l e c t
Feedback Amplifying System. Referring to Figure 1, the feedback amplifying system is composed of a d.c. amplifier, a source of adjustable d.c. reference voltage, and the titration cell. A recording milliammeter is included to measure amplifier output current. There are two sources of voltage in the input circuit. The first is a reference voltage from a stable source such as battery. The second input voltage is that of the bromine-sensitive electrode and a reference electrode in the titration cell and is opposed to the first. The net voltage from these two sources acts as a control on the amplifier output which is the bromine generating current. The reference voltage is set to equal the voltage of the brominesensitive electrode a t a fixed bromine concentration. When the tm.0 are equal, the potentiometric balance point has been established, and no signal is given to the amplifier. If the bromine concentration falls below the desired level, the sensor voltage will fall below the stable reference voltage, producing an input voltage to the amplifier. The amplifier responds to increase the bromine generating current to the titration cell. Thus, the bromine concentration is increased and consequently the voltage from the bromine-sensitive electrode increases until the input approaches zero. I n this manner, the feedback system acts automatically to maintain a set concentration of bromine in the titration cell. Conventional circuits are used to damp oscillations originating either in the electrical system or physically in the titration cell. Because of the continuous flow of gas through the titration cell, a small amount of bromine is always being swept out. A small signal is, therefore, always being sent to the amplifier and the generating current never falls to zero. The amplifier is set a t a high gain, so that the output will reflect extremely small signals, resulting from very small changes in bromine concentration. When a gas which will react with the bromine enters the titra-
July 1954
Figure 1 .
Titrilog Cell and Amplifier
bromine generated is a function of the current, the recording of the current is a n indication of the extent of titration. I n an oxidation-reduction reaction, the voltage developed at the sensing electrode is a logarithmic function of the concentration. Therefore, as the point of equivalence is approached, the change In voltage is large. The feedback principle utilizing a high gain amplifier allows the reaction to take place near the point of equivalence, and the resulting instrument is extremely sensitive and rapid in response to changes in sulfur compound concentration. Titration Cell. The Titrilog titration cell and amplifier are shown in Figure 1. The titration cell is a glass vessel containing an electrolyte of dilute sulfuric acid, 7.5N, and potassium bromide, O.lN, from which the bromine is generated. The concentrations can be varied over wide limits. The titration reaction takes place in the inner cell chamber A , where the sample gas ia continuously introduced and the sulfur compounds are absorbed. Bromine is generated from electrode 1. Electrode 2 in the same compartment is a platinum electrode responsive to changes in bromine concentration. The outer chamber B acts as a reservoir for electrolyte and contains electrodes 3 and 4, nhich complete the electrical circuits. Electrode 3 completes the electrolysis reaction, generating hydrogen which is vented, taking no part in the titration reaction. Electrode 4 is a calomel half-cell type of reference electrode against which the voltage of the bromin++ sensitive electrode 2 is developed. Instead of the conventional mercurous chloride, mercurous bromide is used. The electrode is in contact with the electrolyte and is therefore 0.1N in respect to potassium bromide. The reference voltage obtained from
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1423
ENGINEERING. DESIGN, AND PROCESS DEVELOPMENT Because of the variation in degree of absorption of the many sulfur compounds and because of the various degrees to which these compounds are oxidized, a factor must be determined experimentally for each compound. Several additional features have been incorporated into this automatic titrating system in order to make possible continuous unattended operation for long periods of time. Among these, the following may be briefly considered: Figure 2.
Typical Titrilog Record
this electrode against the sensor electrode, a t zero bromine concentration, is of the order of 750 millivolts. The balancing reference voltage, from a dry cell battery, is connected in the circuit to oppose the voltage between the brominesensitive electrode 2 and the calomel type reference electrode 4. As previously mentioned, the voltage of the bromine-sensitive electrode vill, by design, a t sll times tend to approach that of the balancing reference voltage. Prior to the introduction of a sample gas containing any reactant, a t which time filtered air is passing through the cell, the balancing reference voltage is adjusted for some desired bromine concentration. Because the air passing through the cell continuously carries out a very emall amount of bromine, the generating current never drops to zero, the amplifier continuously operating to equalize the rate of generation to the rate of loss. This steady rate of generation is called the zero level, and in practice is set for a very low bromine concentration, Figure 2. Sample
orifice
C r i t i c a l Flow O r i f i c e
1. I n order to protect the cell electrolyte from a continuous buildup of reaction products, there is a charcoal filter bed in an annular space between the reaction chamber and the reservoir chamber. The geometry of titration cell causes the gas flow to slowly circulate the electrolyte from the reaction chamber through the charcoal into the reservoir chamber. Cleansed electrolyte Slowly flows from the reservoir back into the reaction chamber. I n field operation, c h a r c d , acid, and potassium bromide are changed about every 3 months where sulfur concentrations are in the normal operating range of the instrument. For example, from 0.05 p.p.m. to 120 p.p.m. by volume of hydrogen sulfide. 2. Zero-level generation is subject to variation with temperature in the titration cell because of temperat,ure characteristics of the sensing system. I n order to avoid this objectionable drift, a compensating temperature sensitive element has been placed in a well in the cell electrolyte and is connected in series with the balancing voltage, so that this voltage changes with temperature a t the same rate as the sensing voltage. 3. Amplifier drift, if present, vould also result in objectionable zero-level drift. An &.c. amplifier is therefore used, because of its inherent stability. The d.c. input voltage to the amplifier is converted to a s . by B stable synchronous converter. The a x . output from the amplifier is rectified and fed to the bromine generating system. Amplifier output is of the order of 1 milliampere per millivolt input. Maximum amplifier output is nhout 10 milliamperes, but usual operation need not be above 3 milliamperes.
Reactive Compounds
m b
Air f i l t e r
T i t r a t i o n Cell Airnospheric
Figure 3.
Air
Titratable Compounds. 9 majority of the oxidizable sulfur compounds mill titrate in the Titrilog. Notable compounds that do are hydrogen sulfide, Eulfur dioxide, mercaptans, thioethers, thiophenes, organic disulfides, and all the presently known commercial gas odorants. Carbonyl sulfide and carbon disulfide do not titrate.
Pressure Sampling System Three-Way V a l v e
The titration takes place on the introduction into the titration cell of a gas containing reactants. The bromine concentration in the inner chamber momentarily falls because of the reaction. The sensor-electrode voltage drops, giving rise to a larger net voltage input t o the amplifier. The amplifier responds by increasing the feedback current, and, hence the rate of bromine generation until this rate is sufficient to satisfy the bromine requirements of the reaction, as well as the prevlousljr set residual bromine concentration corresponding to the balancing reference voltage. The amplifier gain is sufficiently high, so that bromineconcentration changes required to drive it to maximum output are low enough in terms of bromine-generating current to be neglected in efficiency calculations. AB previously mentioned, the net bromine-generating current is directly proportional to the sulfur compound concentration of the introduced sample. The quantity of sulfur can then be computed from the Faraday equivalent amp. sec. X equivalent wt. of sulfur compound = 96,500 xt. of sulfur compound 1424
-
C r i t i c a l Flow O r i f i c e
'1
-
v
Sample
m
W Titration
Figure
4,
Cell
Vacuwrn Sampling System
Nontitratable Compounds. Among the gases which will not titrate or cause an interference in the Titrilog are saturated hydrocarbons, carbon monoxide, carbon dioxide, nitrogen, hydrogen, oxygen, and ammonia. Interfering Compounds. There are some compounds which titrate in the Titrilog only t o a very limited degree. These do not titrate t o a degree sufficiently large t o warrant classification as titratable, yet do titrate sufficiently so that relatively large concentrations will obscure the sillfur compound recording.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46,No. 7
PROCESS INSTRUMENTATION Most prominent of these are the olefins and diolefins. These compounds titrate a t an efficiency of only 1 to 2% of that of the eulfur compounds. Consequently, unless they are present in concentrations large as compared t o the sulfur compound, the interference is negligible. Phenolic compounds titrate even to a lesser degree.
sensitivity varies directly with the rate of sample flow and range varies inversely with this rate. The range and sensitivity of the pressure sampling Titrilog can, therefore, be varied over a wide range, whereas that of the vacuum sampling instrument is set a t the maximum sensitivity and minimum range. Table I shows the threshold sensitivities and ranges for hydrogen sulfide a t various rates of flow.
Table I.
Threshold Sensitivities and Ranges for Hydrogen Sulfide Threshold Sensitivity
Rate of Sample Flow, Cc./Min. 100 300 700 1000 (vacuum type)
a/o
Figure 5.
Change
Composite Data for Four Cells
There are also some compounds which will cause an interference because of their reactive similarity to the bromine titrant. Compounds such as chlorine or nitrogen dioxide will act siniilarly to the bromine in the titration cell and cause a decrease in the necessary bromine generation level. This will result in an erroneously low titration. I n fact, an investigation is anticipated, utillring this phenomena for determining chlorine with the Titrilog by negative titration.
P.p.m.. vol. 0.6 0.2 0.07
Grains/100 standard au. ft. (60° F., 1 atm.)
0.05
0.03 0.01 0.004 0.003
Range Grains/100 standard C J P.p.m., ft. (60° F , vol. 1atm.) 0-40 0-2 7 0-13 0-0.9 0-6 0-0, 'I 0-4
0-0.3
The preceding ranges and sensitivities for either type sampling system will vary with the various sulfur compounds titrated and, to a degree, between individual titration cells. An electrical switch is provided which shunts two thirds of the generating current around the recorder, thus providing an additional threefold extension of any of the above ranges. Concentrations in excess of the preceding limits can be analyzed with the Titrilog by use of an external sample dilution system. Concentrations as high as 100% hydrogen sulfide have been successfully tested.
Sample Handling System The Titrilog is provided with a manifold which can be modified to receive a sample under pressure or to draw the sample from the
atmosphere. The pressure system provides a means of varying the threshold sensitivity as well as the range of the instrument by changing the amount of sample input. The vacuum system offers the maximum sensitivity at the minimdm range. Pressure System (Figure 3). Filtered air is introduced at atmospheric pressure. Sample gas is introduced a t a regulated pressure of approximately 10 inches of water above atmospheric pressure. A vacuum pump continuously draws a mixture of filtered air and sample gas through the titration cell. The total flow is fixed at approximately 1000 cc. per minute by a critical flow orifice. This rate remains unchanged even though the rate of the sample flow can be varied from approximately 100 cc. to 700 cc. per minute, merely by changing the size of the sample orifice. During the period a t which there is no sample flow, the total flow to the cell is filtered air. At t h i a time, the titration zero level is established. Vacuum System (Figure 4). This system functions in the same manner as the pressure system. Homver, there is no need for a sample pressure regulator or a sample orifice. The sample is pulled from atmospheric pressure through the titration cell with the rate being that of the critical flow orifice. A three-way valve a t the junction of the sample line and the filtered air line allows one or the other to pass to the cell. During the flow of filtered air, the titration zero level is established.
Performance Range and Sensitivity. As previously mentioned, the range and sensitivity of the Titrilog can be varied by changing the amount of sample input. Since the rate of titration is a function of the rate at which the sample is introduced, it follows that
July1954
Net Bromine Generating Current, Milliamperes
Figure
6. Linearity for Hydrogen Sulfide for Four Different Titration Cells
Accuracy and Stability. Because of the variation in degree of absorption and reaction of the different oxidizable sulfur compounds, the accuracy of the Titrilog depends upon its calibration. To calibrate the instrument against some standard anal tical procedure would have been a relatively simple matter. However, presently known analytical procedures, in the low concentration range of the Titrilog have proved insufficiently accurate for calibrating the instrument. A gravimetric rocedure for calibrating has, therefore, been devised, whereiy a known weight of a sulfur compound is introduced into a Titrilog by means of diffusing into a gas stream. This is referred to as a primary calibration. A secondary calibration procedure for field use employs a prepared sample of gas of known sulfur concentration. This sample is prepared with carefully scrubbed methane and a metered volume of sulfur compound. Extreme care is taken to exclude any air and the sample is contained in a stainless steel tank with stainless steel fittings and a packlesstype valve. An additional check is made on the synthetic sample by anal zing with a Titrilog which has been calibrated immediately be&re by the rimary procedure. Several analyses are made over a period oPapproximately 3 weeks in order t o determine stability of the eample concentration. Generally, the concentration will drop approximately 5% after the first
INDUSTRIAL AND ENGINEERING CHEMISTRY
1425
-
ENGINEERING, DESIGN, A N D PROCESS DEVELOPMENT week and then tend to remain relatively stable for a period of several months. The accuracy of the instrument is dependent upon the technique of the primary calibration. A series of these calibrations have proved that reproducibilit'y is well within +5%.
Linearity. Since the Titrilog operates on a mass basis, linearity over t,he entire range of the instrument has been determined by simply varying the rate of introduction of a sample of fixed sulfur concent,ration. Figures 6 to 8 shorn linearity for hydrogen sulfide, sulfur dioxide, and n-butyl mermptan for four different titration cells. The figures show the titration efficiency to be quite linear for hydrogen sulfide and sulfur dioxide. For nbutyl mercaptan, the efficiency decreases somewhat with increasing titration levels.
' 7 ! I I I
Selective Filtration
I
I
I
01 0
0.2
0.4
0.6
Net Bromine G e n e r a t i n g C u r r e n t , Miiliomperes
Figure 7. Linearity for Sulfur Dioxide for Four Different Titration Cells
A great amount of data has,been gathered to determine the calibration stability of the Titrilog. Four instruments were connected to carefully calibrated synthetic samples. The known samples were intermittently and automatically introduced into the Titrilog for 15 minutes every 18 hours. The tests lasted for approximately 1 month and were repeated a second time. Field conditions were simulated by continuously operating the four instruments on the city gas which contained some hydrogen sulfide, mercaptans, and a cyclic sulfide-type odorant. During these experiments, the instruments were given no more than normal care and maintenance.
I n order to dist,inguisli between some of the different sulfur conipounds, liquid absorptive filters are used. These filters absorb one or more of the cornpounde of interest, enabling their concent,ration to be determined by difference. TKO types of absorptive filter solutions are available at present; the zinc sulfate-sodium dichromate solution ( 5 ) removes hydrogen sulfide, and the alkaline cadmium sulfate solution removes: both hydrogen sulfide and mercaptans. By use of these filters, hydrogen sulfide. mercaptan, and residual sulfur compounds can be separatelq- determined. Efficiencies of the absorptive filters are not 100%) and allow ance must be made for this TThen highest accuracy is desired. The dichromate filter absorhs about 9% of the total mercaptan and passes about 5% of the hydrogen sulfide. The alkaline cadmium sulfate absorbs over 95% of both. A programming system of 15-minute intervals is provided. This system d l route the sample through either of the filters, bypass the filters, and establish a zero level on an automatic repetitive cycle.
Calculation of Concentration Efficiency varies among individual cells. This efficiency is affected by flow pattern and dispersion of the sample gas, and perhaps by surface effects of a cell.
Table II. Days
E 1apsed 0 1 2
0.2
0.6
0.4
0 .e
I .O
Net Bromine G e n e r a t i n g Current, Milliamperes
Figure 8. Linearity for n-Butyl Mercaptan for Four Different Titration Cells Some of the data are given in Table 11. They illustrate the degree of variation among individual instruments. The composite data for the entire experiment have been plotted (Figure 5 ) with the following resulte:
.
Calibration Daily Weekly Biweekly Monthly
90% Changed
Maximum Change,
3 7 11 11
10 13 16 16
Not More Than, %
70
T h e data obviously indicate that the more frequent the calibration, the greater the degree of accuracy. However, they also seem to indicate that when the lapsed period between calibration is over approximately 2 weeks, the changes appear to have reached a ceiling.
1420
Variation of Titration Cell Efficiency with Time Equivalents Bromine/Mole Sulfur Compound Cell Cell Cell Cell A B C D 3.2 2.9 3.0 2.9 2.9 3.0 3.3 3.0 2.9 2.9 3.4 3.0
...
3 4
3
3.3 3.2 3.2
3.0 3.0 3.0
2.9 2.8 3.0
2.9 2.8
6 7 8
3.4 3.2 3.2
2.9 3.0 3.0
3.1 3.0 3.0
2.8 2.9 2.8
9 10 11
3.3 3.2 3.1
3.0 3.0 2.9
3.0 3.0 2.9
2.9 2.9 2.8
12 13 14
3.2 3.3 3.2
2.9 3.0 3.0
2.8 2.8 2.8
2.9 2.9 2.9
15 16 17
3.3 3.2 3.2
3.0 3.0 2.9
2.8 2.9 2.8
2.9 3.0 2.9
18 19 20
3.1 3.1 3.1
3.0 3.1 3.1
2.7 3.0 2.9
2.9 2.9 2.9
21 22 23
3.2 3.4 3.3
3.1 3.2 3.1
2 8 3.1 3.0
2.9 2.9 2.9
24 25 26
3.5 3.5 3.5
3.1 3.3 3.3
2.8 3.0 2.8
2 9 2.9 3.0
27 28 29
3.4 3.4 3.4
3.3 3.4 3.2
2.8 2.8 2.8
3.0
30
3.4
3.2
2.6
..*
INDUSTRIAL AND ENGINEERING CHEMISTRY
... ...
Vol. 46, No. 7
PROCESS INSTRUMENTATION In practice, a n individual cell efficiency constant, K , is determined by calibration with a gas of known concentration. For a cell of a certain known efficiency, the compound factor (C.F.) is the mole relation between the bromine and a specific sulfur compound. The cell constant, K , corrects the compound factor to the efficiency of the particular cell Concentration
-
C.F. X N T L Flow X K
where C.F. = compound factor for individual sulfur compound, experimentally determined mole relation, in desired units X T L = net bromine generating current, ma. Flow = sample feed rate, cc./min. K = cell constant
Application The potential uses of a continuous titrating instrument such as the Titrilog are many. .4t present, only a limited number of these have been proved by extensive field use. These are as follou~s: Monitoring of Natural Gas. The instrument is used to monitor natural gas hydrogen sulfide removal plants. A continuous record is established of the hydrogen sulfide concentration of the residual gas. An alarm is actuated when the concentration exceeds permissible limits due to operating difficulties or loss of plant efficiency. The instrument generally operates at a concentration of about 1.5 p.p.m. in these applications with the maximum permissible limit being 4 p.p.m. Likewise, the Titrilog is used by natural gas purchasers to determine whether hydrogen sulfide concentration is within contract limits. A similar application with the portable instrument is testing gas a t well heads. Natural Gas Odorization. Since natural gas is odorless, it has become a common practice to add a malodorant, generally a mercaptan or organic sulfide, as a warning agent for the detection of escaped gas. It is important that the gas a t all times contains a sufficient concentration of odorant for safe operation. Also, overodorization results in expensive nuisance complaints because of negligible escapes of gas. The Titrilog has been used effectively to continuously monitor the odorant concentration as well as to evaluate various odorants for particular gas systems. Air Pollution Studies. An application of increasing importance is the study of air pollution. Sulfur dioxide is one of the principal offending agents and is generally an index of the level of pollution. In some instances, hydrogen sulfide or mercaptans are also of interest. The Titrilog being capable of recording trace quantities of these compounds is being used effectively in air pollution studies. The concentration range generally encountered is in the order of less than 0.1 p p.m. to 3 or 4 p.p.m. The instrument in this type of application is limited to recording the total titratables. No successful method of selective separation has yet been devised. The instrument is being used to survey areas for sources of pollutants by correlating the record with meteorological data. Data gathered in this manner seem to indicate that damage by pollutants may not be due to low average concentrations but to the short periods of peak concentrations. An interesting recent application has been recording concentrations in the vicinity of industrial stacks a t upper levels from a cruising helicopter. Personnel Protection. A similar application is in the monitoring of atmospheres in plants or areas in which hydrogen sulfide or sulfur dioxide may reach toxic levels. The instrument is equipped to sound an alarm when a preset maximum concentration ie reached. Generally, a concentration of 10 p.p.m. is considered by toxicologists to be the maximum allowable for either gas. July 1954
Catalyst Protection. Quite often e x t r e m e l y small t r a c e s of s u l f u r compounds are detrimental to the efficiency of catslysis. In a plant synthesizing a m monia from natural gas, it was found that the hydrogennitrogen r e a c t i o n catalyst Frequently lost efficiencyContinuous monit o r i n g of t h i s stream indicated a e u l f u r concentration in the order of 4 p.p.m. The instrument was used to trace the source of this sulfur, and c o r r e c t i o n s were made toeliminateit. Process Monitoring. I n a plant extracting b r o m i n e from sea water, a factor of efficiency is the presence of B slight excess of sulfur dioxide in the reactor. The Titrilog is being used to monitor the tail gae Figure 9. Cabinet Titrilog Model for sulfur dioxide 26- 102 concentrationin the order of 40 p.p.m. Corrosion Studies. A natural gasoline compression and absorption plant was subject to high maintenance cost because of condenser tube corrosion in the third stage of compression. Chemical analyses were insufficiently sensitive t o indicate the presence of sulfur compounds. Monitoring of the incoming gas with the Titrilog indicated very small amounts of hydrogen sulfide. Recordings on the second and third stage of compression and absorption showed that the hydrogen sulfide was being 0011centrated successively. The maximum concentration wa8 reached in the third stage condenser where the corrosion was most evident. Odor Studies. Many natural substances are sulfur compounds. In cases so far studied, the titration correlates with the degree of odor. An interesting application of this type is one by a manufacturer of chlorophyl. A solution of garlic or onion oils was swept with gas to obtain a titration level. Decrease of titration correlated with the addition of various amounts of chlorophyl. Potential Applications. There are many other potential appltcations which are being studied or considered. An example is the determination of trace quantities of chlorine by negative titration. Chlorine, upon entering the titration cell, releases bromine, thus reducing the amount of generating current required to maintain the bromine content of the titration compartment. By setting the zero level a t a high value, chlorine may be determined by the decrease in generation level. Heretofore, all efforts have been to increase the threshold sensitivity of the instrument. Applications have proved that present sensitivity is ample and efforts are being made to increase the
INDUSTRIAL AND ENGINEERING CHEMISTRY
1427
ENGINEERING, DESIGN, A N D PROCESS DEVELOPMENT Physical Description and Appearance The cabinet model Titrilog (Figure 9 ) is housed in a floor mounted case 6G inches high, 22.5 inches wide, and 18.5 inchee deep. The strip chart recorder is mounted at eye level. Directly below it is the control panel. The portable model (Figure 10) consists of the Titrilog, 11 X 22 X 12 inches high and weighing about 50 pounds, the portable recorder, 10 X 13 X 17 inchep high, and the filter unit, 6 X 13 X 11 inches high. All units of the portable Titrilog can be transported easily in the trunk compartment of an automobile.
Figure 10.
Literature Cited
Portable Titrilog Model 26-1 03
(1) Austin. R. R., Am. Gas Assoc., Proc., 31, 505-16 (19491.
existing range. .4 laboratory dilution apparatus has indicated the feasibility of testing concentrations of hydrogen sulfide up to
100%. Preliminary work, with some promise, has been conducted toward using the Titrilog to determine sulfur in liquid samples by combustion of the sample and determining sulfur dioyide in the combustion gases. Gaseous samples have similarly been burned when interfering substances such as olefin are present. The interfering compound is burned to carbon dioxide and the sulfur to sulfur dioxide. .The principle of continuous electrolytic generation of titrating agent a t a potentiometric balance point may be applicable to the generation of other titrants, or t o the continuous titration of liquid samples. Keither of these applications has as yet been investigated.
(2) Austin, R. R., Percy, L. E., and Escher, E. E., Gas, 26, No. 6, 47-63, NO. 8, 33-8 (1950) (3) Austin, R.R., Turner, G. Ti., and Percy, L. E., Instrztments, 22, 588 (1949). (4) Eckfeldt, E. L. (to Leeds & Sorthrup Co.), U.S. Patent 2,821,671 (Dec. 16, 1952). (5) Reitmeier, R. E. (to The Girdler Corp.), Ibid., 2,405,872 (Aug. 13, 1946)). (e) Schaeffer, P. A., Briglio, A , , Jr., and Brookman, J. A,, Jr., Anal. C h a . , 20, 1008-14 (1948).
(7) Sease, J. W., Lee, T., Holzman, G., Swift, E. H., and Nieman, C., Ibid., pp. 431-4, (8) Washburn, R. W., and .lustin, R. R., Pioc. Natl. Air Pollution Symposium 1949, 1, 89-76 (1951). RECEIVED for reyiew September 7, 195.3.
~ C E P T E D February
17, 1954.
tomat ic Ilastrumentatio
enc
nits E. R. ROTH The
Aflanfic Refining Co., Philadelphia, fa.
Properly limited in its application, the bench scale unit i s an important and reliable laboratory tool. This i s particularly true when industrial control instrumentation is used to full advantage. The type of unit described finds wide use for cafalytic research studies, catalyst evaluations, catalyst life determinations and product yield, and distribution data for a variety of problems. A broad range of gas and liquid feed rates are controlled at system pressures up to 1000 pounds per square inch and reactor temperatures up to 1050' F. Gas rates of 1 to 16 standard cubic feet per hour of hydrogen are controlled and recorded, and liquid feed rates of 30 to 600 milliliters per hour are maintained. Liquid level control and recording is a feature that has been added with excellent results.
TYPICAL schemat,ic flow diagram is shown in Figure 1. The unit consists essentially of a tubular reactor and its furnace, a controlled gas feed system, a low flow proportioning pump for liquid feed, and a gas separator with liquid level and system pressure control. Gas sample train and gas metering devices are necessary adjuncts to the unit. Safety devices vary from unit to unit, and may only protect against excessive furnace
1428
temperature or be so complete as t o shut the unit down and keep a protective stream of hydrogen passing over the catalyst. The piping, fittings, valvee, and vessels are engineered for high pressure hydrogen application. A view of one of the unita i s shown in Figure 2. Actually this is one of two units operated by one operator per shift on a 24 hour continuous basie. All pneumatic control instruments are dual recorder-controllers.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 7
