US3060723A - Means for determining dissolved gas concentrations in liquids - Google Patents

Description

Oct. 30, 1962 s. F. KAPFF ETAL 3,060,723

MEANS FOR DETERMINING DISSOLVED GAS CONCENTRATIONS IN LIQUIDS Filed July 2, 1959 Fig. I

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INVENTORS Sixf Freden'c/r Kapff BY Robert E. Jacobs ATTORNEY IVEANS FOR DETERMlNmG DISSQLVED GAS CONCENTRATIONS IN LIQUmS Sixt Frederick Kapff and Robert B. Jacobs, Homewood,

111., assignors to Standard Oil Company, Chicago, 111., a corporation of Indiana Filed July 2, 1959, Ser. No. 824,580

Claims. (Cl. 73-19) This invention relates to means for measuring the extent of saturation of a liquid with respect to a dissolved gas. The invention, in a particular embodiment, relates to the measurement of dissolved oxygen or air in liquid hydrocarbon streams.

Several well known processes for sweetening light hydrocarbons employ molecular oxygen, in the form of air or technically pure oxygen gas, to oxidize sour mercaptans to disulfides. In these processes oxygen or air is comingled with the sour hydrocarbon stream and, in the presence of special catalysts such as cuprous chloride, effects the oxidizing or sweetening reaction. However, in order to obtain adequate conversion to disulfides, it generally is necessary to employ a stoichiometric excess of oxygen, normally from twenty to seventy percent or so. Since the amount of excess oxygen is not subject to precise calculation, the resultant hydrocarbon stream may contain large amounts of dissolved molecular oxygen.

When sweetening is conducted at elevated pressure, and there is a pressure drop in downstream processing vessels or pipelines, oxygen in excess of saturation may be released from solution in the hydrocarbon. There may then be formed a separate gas phase composed of oxygen together with vapors of the hydrocarbon. This gas phase may have a composition which is flammable and at least one sweetening plant explosion has been definitely traced to the ignition of a flammable mixture formed in this way.

It is therefore a primary object of our invention to provide a system for determining the oxygen saturation of hydrocarbon streams in hydrocarbon sweetening plants. Another object is to provide a system which can be used in continuous plant monitoring and which is simply constructed and rugged and foolproof in use. A further object is to provide a detector suitable for such systems which is capable of determining whether the liquid is unsaturated, saturated, or supersaturated with respect to the dissolved gas, and also measure the extent or degree of saturation. Other and more particular objects will become apparent as the description of this invention proceeds.

Briefly, the inventive system comprises a chamber into which the hydrocarbon or other test liquid is pumped at constant volumetric flow-rate. As it enters the chamber, the liquid is dispersed to secure intimate contact between the liquid and the gaseous phase existing in the top of the chamber. A constant liquid level is maintained at the bottom of the chamber, for example by means of a float-controlled outlet valve. A connection or vent is provided in the gas space of the chamber and communicates with the atmosphere (or other source of oxygen), whereby oxygen or air may freely pass into or out of the chamber, depending upon whether the liquid is unsaturated with respect to oxygen (and hence absorbs oxygen) or is supersaturated (in which event it releases the oxygen in excess of saturation) A detector is placed in this vent or conduit; the detector is capable of determining the direction of flow and the rate of flow of gases flowing through the vent. This detector preferably is composed of a pair of thermistors in a balanced bridged circuit, with an electrically heated filament between the two. Thus as air flows in either direction, it becomes heated in flowing across the filament and then heats the thermistor 3,h6,723 Patented Get. so, 1962 which is downstream of the filament, and changes its electrical characteristics. This change is measured by means of the bridge circuit and affords an indication of the gas flow rate, and hence the extent of saturation or supersaturation of the liquid hydrocarbon. It is particularly noted that the indication is virtually instantaneous, which permits continuous and immediate alteration of sweetening plant operation in the event that either too much or too little oxygen is present in the treated hydrocarbon stream.

The invention will be described in more complete detail, and additional benefits thereof will become apparent, in the ensuing description read in conjunction with the accompanying drawing wherein:

FIGURE 1 is a schematic representation of the appa- Iatus used for determining the dissolved oxygen or air content of hydrocarbon streams;

FIGURE 2 is a schematic electrical circuit for use with the improved thermistor detector of FIGURE 1;

FIGURE 3 is a diagram of the improved thermistor detector employed in a pneumatic or hydraulic bridge system for conducting the continuous chemical or physical analysis of homogeneous fluids.

Referring to FIGURE 1, a sample of hydrocarbon containing an unknown amount of dissolved oxygen or air is obtained through line 1 and continuously pumped into the system by pump 2 which is powered by motor 3. Pump 2 is of the positive displacement type, such as for example a rotary gear pump, and expels the liquid through line 4 into chamber 5 at constant volumetric flowrate.

Chamber 5 is an enclosed gas-confining vessel which is preferably maintained at approximately constant temperature by suitable insulation or by thermostatic means. Chamber 5 contains means for intimately exposing the hydrocarbon liquid to the gas in the upper portion of chamber 5. As shown in FIGURE 1, this latter system may include nozzle 6, distributor plate 7, and a multilayered pack of screens or louvers which function to provide a large surface area for intimate liquid-gas contact. Thus the liquid is brought into equilibrium with the gas in chamber 5.

A liquid level 9 is maintained at a constant height in the bottom of chamber 5. Suitable control of this level may be obtained by a float valve arrangement comprising float 10, and float arm 11 attached to fulcrum 12. Arm 11 operates plunger 13, which is connected to needle valve 14. Needle valve 14 is thus opened and closed against v seat 15 to release more or less liquid as level 9 (and hence float 10) tends to rise or fall. The liquid released through line 16 is exhausted or discharged to either a sewer or a suitable recovery sump.

In the top or gas phase portion of chamber 5 there is provided a vent connection 17 which ultimately exhausts to atmosphere. It is through this vent that dissolved oxygen or air, over and above that necessary for saturation of the hydrocarbon stream at test conditions, is exhausted when it is desorbed from the stream. Similiarly, should the stream be unsaturated, air is absorbed by hydrocarbons descending through screens 8, and accordingly atmospheric air passes through vent 17 to equalize the pressure in chamber 5 with the atmosphere.

Detector 18 is disposed in vent 17. This detector, along with chamber 5, is desirably thermally insulated or subject to thermostatic temperature control. Detector 18 may be made of heat-conducting metal such as brass or bronze in the form of a block comprising matched segments 23 and 24, which are clamped together by means of bolts or clamps 25. Illustratively, segments 23 and 24 are 1" diameter by 1 long brass cylinders, and conduit 19 is about A" in diameter. A conduit 19 is bored through detector 18 and permits flow of gases therethrough.

Similarly disposed within detector 18 is an electrically heated filament 22 which desirably is arranged in the form of a grid or a screen, and which is equipped with suitable connections for supplying heating current, not shown.

On each side of filament 22 'are thermistor detectors, i.e. temperature-responsive resistors having negative temperature co-efficients of resistance. For most accurate detection of flowrate, thermistors 20 and 21 are as nearly matched as possible, and are placed symmetrically within conduit'19 and at equal distances on each side of filament 22. The thermistor beads may suitably be spaced A3" from filament 22.

In the region of thermistors 20 and 21 and filament 22, segments 23 and 24 are desirably insulated by means of thermally-insulating sleeves 28, which may be made of compressed asbestos, asbestos-containing organic plastics, nylon, etc. Filament 22 is also insulated, both elec- 'trically and. thermally, from segments 23 and 24 by means of insulating ring 27.

Thermistor detectors 20 and 21, together with'heated filament 22, constitute the portion of the inventive system which is'responsive both to the direction of gas flow and to the rate of flow. The arrangement, of two matched thermistors and a centrally located heating element, subjects both thermistors to the heat produced by the element when there is no flow whatever through conduit 19. Consequently, they then have substantially equal resistances. However, if there is a gas flow, as shown by the alternative arrow 26 the gas in passing over filament 22 becomes heated, and thereby heats up that ther- 'mistor which is positioned downstream of the element.

. the resistance of one thermistor is utilized in the bridge circuit schematically shown in FIGURE 2 to provide an indication of flowrate. By measuring the diflerential resistance of the two thermistors, it is possible to obtain a measure of the gas flowrate.

Turning now to FIGURE 2, a suitable bridge circuit .is shown for determining which of thermistors 20 or 21, at any particular instant is the downstream thermistor and also for measuring the resistance (corresponding to temperature differential) .between the two thermistors.

'Thermistors 20 and 21, together with their lead wires 20a and 21a respectively, are arranged in a Wheatstone bridge circuit. The circuit employs fixed or var- .iable resistors 30 and 31' to form a four armed bridge,

with a galvanometer or similar type indicator 32 connected between the juncture of the arms containing resistors 30 and 31 and the juncture of the arms containing thermistors 20 and 21. The bridge of FIGURE 2 is furnished with power from power source or battery 33, which may be equipped with a variable resistor 34 and ammeter 35 to regulate the current flow to the Wheatstone bridge network.

The pointer of indicator 32 will, if resistors 30 and 31 are matched accurately, show a null reading if thermistors 20 and 21 are simultaneously at the same temperature. However, should thermistor 20 be at a higher temperature than thermistor 21, more current will flow through thermistor 20 due to the decreased resistance thereof occasioned by its higher temperature, and accordingly the pointer of indicator or galvanometer indicator 32 will move in one direction. If the reverse situation should obtain and thermistor 21 becomes momentarily hotter than thermistor 20, the pointer of indicator 32 will move in the opposite direction. It is also contemplated, and is within the scope of the present invention, that a suitable potentiometer recorder maybe substituted for the indicator 32 to furnish a permanent record, or a controller may be introduced into the circuit to regulate oxygen or air input to the hydrocarbon sweetening process.

An advantageous feature of the bridge circuit shown in FIGURE 2 is that it is self-compensating for concurrent temperatures experienced by both of thermistors 20 and 21. Thus detector 18 need not necessarily be maintained at a constant temperature (provided only that it be suitably insulated so as to avoid temperature gradients), and therefore the entire apparatus can be made portable without'the need for temperature control thereof. Obviously, the freedom with which detector 18 may be permitted to undergo temperature variations depends largely upon the accuracy to which thermistors 20 and 21 are matched.

A special advantage of detector 18 is that it is exceedingly sensitive to very low flowrates, and delivers an electrical output that exhibits a considerable change with only minor changes in the rate. In calibration tests of a detector built in accordance with FIGURE 1, indicator 32 showed an indication of about plus and minus eighty millivolts at a flowrate of only ten cubic centimeters per minute in either direction. With matched thermistors, the detector response is nearly linear with fiowrate, and accordingly need only be calibrated at a few points at the extreme ends of its useable range.

The inventive system was tested on three samples of heater oil, the first being completely devoid of air, the

second air saturated at room temperature and atmospheric pressure, and the third air saturated at room temperature and 20 pounds per square inch gauge pressure. The instrument was zeroed with no heater oil flowing. Using a heater oil fiowrate of cc./min., a filament resistance of 15 ohms, thermistor resistance of 2000 ohms at 70 F., a bridge current of two milliamperes, and 6.0 volt 0.40 amp filament current, the detector reported 22 millivolts on the first sample (equivalent to 1.5 cc./min. air absorption); zero millivolts for zero gas flow on the second sample, and ;+30 millivolts (equivalent to 3 cc./min. air evolution) on the third sample.

The improved thermistor detector 18 of the present invention also finds wide applicability in a variety of other types of analytical systems. For example, hy-

'draulic or pneumatic bridges are conventionally emtively small in cross-sectional dimension, for example 0.1 to 1.0 millimeters in diameter, and provide a restriction to the flow of sample gas through the passages. One of the flow passages, 42 and 46, has a means for effecting a change in the sample gas which is related to the desired analyses; in the example herein described where the sample gas contains oxygen, flow passages 42 and 46 may include an oxygen absorber 47 wherein oxygen is selectively absorbed by an absorbent such as an alkaline pyro-gallol solution, shown as liquid body 48.

Thus the change produced by absorber 47 is a reduction in the quantity of gas flowing through flow passage 46 by moving oxygen from the stream flowing through flow passage 42.

Connecting the intersections of flow passages 41 and 45 with the intersections of flow passages 42 and 46 is a pressure-equalizing conduit '43 and 44, through which gas tends to flow so as to equalize the pressures at the respective intersections. Thus when oxygen is removed .5 46 decreases and consequently gas tends to flow from passages 41 and 45 into passages 42 and 46 via conduit 43 and 44. To measure this flow, thermistor detector 18 is positioned in flow conduit 43 and 44, and indicates the quantity of flow. This quantity, of course, is related to the amount of oxygen absorbed in absorber 48, and hence is related to the oxygen content of the sample gas.

Although prior-art pressure or flow measuring devices have heretofore been employed in pneumatic bridges, their use has not been satisfactory due to the virtual impossibility of manufacturing the bridges with passages 41 and 42, and passages 45 and 46 having exactly equal resistances. Thus there had usually been a significant null reading with no absorption (or other efiect) taking place. The present detector is uniquely able to correct for this null reading. Because of its ability to determine flow in either direction, and because of the linearity of its difierential resistance with flow rate, a pneumatic bridge using the improved thermistor detector can be zeroed simply by passing a gas through both flow passages either with no absorber present or with no absorbable component in the gas, and noting the reading of thermistor detector 18 at this time. This is taken as the null or zero-absorption reading. If galvanometer 32 is then adjusted to show zero, the apparatus is then exactly zeroed irrespective of gas flow through conduit 43 and 44 caused by nonuniform passage resistance.

Hydraulic and pneumatic bridges can, in addition to measuring oxygen content, be used to determine a variety of other physical or chemical properties of fluids. These properties include CO content (e.g., with an alkaline absorbent), ammonia (charcoal absorbent), humidity (silica gel or alumina), thermal conductivity, viscosity, etc.

While the duo-directional flow detector aspect of the invention has been described with special reference to thermistors as detector elements, it is evident that saturation measuring instruments may employ other forms of detectors operating on similar or analogous principles. For example, when a higher signal-to-noise ratio can be tolerated, or where exceptional sensitivity is not essential, heated filament detectors can be employed.

From the above description and performance, it is apparent that the objects of this invention have been attained. While the invention has been described with reference to preferred embodiments thereof, these are intended as exemplary and illustrative but neither exclusive nor limiting. Accordingly it is contemplated that alternatives, modifications and variations will be evident to those skilled in the art in light of the foregoing description,

6 and hence, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the ensuing claims.

We claim: 1. An apparatus for continuously determining the concentration of a dissolved gas in a liquid which comprises: a chamber having a fixed volume of liquid therein and defining a gas space of predetermined fixed volume, and wherein said liquid contacts said gas phase;

means for introducing said liquid into said chamber at a constant volumetric flowrate, said liquid being the only material deliberately introduced into said chamber;

means for maintaining a constant level of liquid in said chamber and for discharging excess liquid from said chamber whereby the volume of said gas phase remains constant;

a conduit communication between the gas space of said chamber and an external source of gas;

and means for determining the direction and rate of flow of gas flowing through said conduit due to absorption and desorption of gas by the liquid in said chamber as a measure of the concentration of gas initially dissolved in said liquid.

2. Apparatus of claim '1 wherein said dissolved gas is air and said external source of gas is the atmosphere.

3. Apparatus of claim 1 including means in said chamber for dispersing said liquid so as to bring said liquid into intimate contact with said gas phase.

4. Apparatus of claim 1 wherein said means for determining the direction and rate of flow of gas flowing through said conduit includes a pair of thermistors disposed on opposite sides of a heated filament and connected in a bridge circuit.

5. Apparatus of claim 1 including means for maintaining constant temperature in said chamber.

References Cited in the file of this patent UNITED STATES PATENTS 2,319,516 Phelps May 18, 1943 2,647,401 Hathaway Aug. 4, 1953 2,671,343 Jacobs et al. Mar. 9, 1954 2,741,911 Fitzpatrick et a1 Apr. 17, 1956 2,745,282 Rochon May 15, 1956 2,813,237 Fluegel et al Nov. 12, 1957 2,987,912 Jacobson June 13, 1961 FOREIGN PATENTS 602,561 Germany Sept. 12, 1934

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