point is the lowest temperature at which a Pour petroleum oil will pour when it is chilled under prescribed conditions, and defines the lowest temperature at which these products are sufficiently fluid for use. Pour point is specified (2) in terms of the ASTMD 97 Standard Method of Test for Cloud and Pour Points ( 7 ) . In Table I, the principal petroleum distillate fuels and their typical ranges of pour points are listed. Petroleum refineries require frequent and accurate measurements of pour point, not only to control the
quality of finished products, but also for most economic use of stocks available. Some incentives to optimize refinery operations are based on conserving low-pour blending stocks and protecting the quality of products with respect to flash point, cetane number, or heating value, all of which are adversely affected when pour point is unnecessarily low ( 3 ) . Other incentives stem from easing process constraints imposed by quality requirements. For instance, most profitable operations of catalytic cracking units are sometimes determined by the pour point of the furnace oil produced (8).
‘The time-consuming, empirical AS TM-D 97 test procedure has been systematized and adapted to contin u ous, onstream, inst rumental ana lysis
New Instruments to measure Pour Point W.V. CROPPER
Figure 7.
n
Jt
?neral view of automatic pour point tester
G.
L. HAMMOND
--
VOL. 5 7
NO. 2
FEBRUARY
1 0 6 5
17
Cooling rate exerts the greatest single effect on pour point T o use distillates most economically within the group comprising No. 1 Fuel, Diesel Fuel, and No. 2 Fuel, Kapff (7) devised an instrument for rapid, automatic measurement of pour point in the range from +30” to -40° F. Instruments of this type have been used to monitor distillate fuel production at crude oil distilling units (5),catalytic cracking units, and blending facilities (6). Experience with them indicated a need for an improved instrument capable of testing products of lower pour point. Fundamentals of the Pour Point Test
ASTM-D 97 is an empirical test. A sample of oil is cooled under arbitrary and nonuniform conditions until the waxy hydrocarbons dissolved in it are precipitated in a network that physically prevents bulk flow of the remaining liquid. The sample is observed at intervals of 5 O F. until no motion is seen when the test jar is removed from the cooling bath and held horizontally for five seconds. The pour point is defined as the temperature 5” F. warmer than the final observation. my definition, at the pour point, the sample exhibits motion to some degree and is not frozen or glassy.) Transitions from the fluid state through a semicongealed condition to a solid or waxy state are observed when small samples are cooled in a thin layer. These transitions are neither well defined, nor uniform for all products. Materials with low pour points go from fluid to solid within a few degrees. Products with pour points near 0’ F., and with a wide boiling range as well, undergo transitions over a wider temperature
span. Transition temperatures are higher when the sample is warmed from the solid state than when it is cooled from the liquid state. Table I1 presents some data from these observations. “Solidification point” designates the temperature at which a liquid sample appears to have become solid. “Melting point” is the temperature at which a frozen sample first shows the presence of a liquid phase. Solidification point corresponds more nearly to the pour point than does melting point, and it is more reproducible. The waxy network m a y be more or less well developed, depending primarily on the way the waxy crystals are precipitated and allowed to grow together. Slow cooling promotes a coherent network made up of relatively large crystals. Rapid cooling (shockchilling) may supercool the sample, and the crystals will tend to be small (4). The resulting waxy structure may not be coherent enough to prevent flow until continued cooling increases liquid viscosity enough to impede flow. Therefore, rapid cooling tends to give erroneously low values for pour point. Although distillate fuels may be rapidly cooled, in service, the usual circumstances involve slow cooling which favors large crystal formation. The resulting effect on flow can be critical. The cooling rate in ASTM-D 97 of a degree or two per minute seems to be slow enough to set up such a network of large crystals, since the test has, over many years, correlated fairly well with experience in the field. The ASTM-D 97 test is too slow and lacks the precision to guide efficient refinery operations, because :
-the intervals between observations are too large ; -the details of the empirical procedure are difficult; -operator judgment plays a large part in determining results. I n contrast to the ASTM-D 97 method, Kapff's instrumental method cools a small, thin layer of sample a t a fast, smooth rate with constant, gentle motion, and keeps the sample under continuous observation. Results agree with ASTM values: precision is 1 2 " F.; testing time is 20 minutes or less. However, samples having pour points below -40" cannot be tested in Kapff's instrument. The mechanical refrigeration that cools a thermoelectric module lacks capacity for rapidly repeating tests to -40" and requires an objectionable amount of maintenance.
TABLE
I. TYPICAL POUR POINT RANGES PETROLEUM DISTILLATE FUELS
Product
TABLE II.
The automatic pour point tester (Figure 1 ) is the result of developments to remove the major limitations of the Kapff instrument. Sample may be poured manually into the instrument or introduced automatically from a flowing stream. The mode of operation is selected by a switch. In either case, the sample flows to the sample cell via a flooded inlet leg. Simultaneously, a slow stream of air is introduced into the cell above the surface to sweep liquid over a small weir. About 2 ml. of sample are retained in the cell in a layer about 25 mm. in diameter and about 4 mm. thick. The sample cell is refrigerated by a thermoelectric module that is cooled by water or, in some cases, by the flowing sample stream. During sample introduction and throughout the test, the sample cell assembly is continuously rocked 10' from the horizontal and back. A thermocouple measures the sample temperatures continuously, producing a spike-like chart record. Light from a stationary source is reflected by the sample surface to a photocell. When the sample congeals, its surface tilts with the sample cell and light is reflected away from the photocell. Loss of reflected light disturbs the balanced photocell circuit which actuates appropriate electrical relays to discontinue cooling, initiate a brief heating period, and then terminate the cycle. I n the manual mode, the instrument maintains itself in readiness for another sample. I n the automatic mode, a new sample is introduced immediately, and the cycle is automatically repeated. Explosion-proof electrical construction permits installation of the tester in processing areas. The thermoelectric module, with two stages of refrigeration, can maintain - 64" F. when rejecting heat to water at 70" F. The cold side of the module is directly attached to the sample cell, and the hot side to a heat exchanger through which water or oil is circulated. The weight of the aluminum sample cell is minimized to give low heat capacity and good heat transfer. The sample cell and module are thermally insulated with polyurethane at least 1 inch thick.
I)
Pour Point, OF.
~
1
-40
Melting
Point, OF. 52; 43 28; 24; 18 -19; -21
EFFECTOFCOOLING ON INSTRUMENTAL R ESU LTS
Point
Cooling Rate, a F. lMin.
5
20' F.
0
2
3
Sol idijcatron Point, OF.
-40
D 97 Pour Samfile
,
16 - 2
Mineral Spirits No. 2 Fuel Kerosine
TABLE Ill.
-56 t o -80 -40 t o -55 -30 to -50 15 to -20 20 t o -45 35 t o 20 40 to 0
COMPARISON OF POUR, SOLIDIFICATION, AND MELTING POINTS
Sample The Automatic Pour Point Tester
Pour Point Range, OF.
~
Aviation Turbine (Jet) Fuels Kerosine No. 1 Fuel No. 2 Fuel Diesel Fuels Mineral Spirits industrial (Special) Distillate Fuels
FOR
-2
Instrument Indication
20' F. 17
9 4 10
-1
4
-2
10
-7
-5
At a given power input, the module cools the sample most rapidly when the temperature is a t or above that of the coolant. The rate of cooling diminishes as the temperature falls, until a n equilibrium temperature is finally reached. This ultimate temperature depends on the temperature of the coolant. Figure 3 shows the lowest sample temperature that can be reached when oil (No. 1 Fuel) or water at the indicated temperature is used as coolant. If power input is reduced, the cooling rate is decreased, and the ultimate temperature is raised. At maximum power input (70 watts), the module cools the sample from 80" to 50" F. in the first minute; at -60" F., the cooling rate is only 2" F. per minute. This module eliminates the troublesome and expensive mechanically refrigerated glycol reservoir and circulating system, makes it possible to use the sample stream itself as the cooling medium for test temperatures down to -50" F., and extends the range of the instrument. Eliminating the mechanical refrigeration system results in simpler construction, better reliability, and lower costs. Limiting the need for cooling water reduces VOL. 5 7
NO. 2
FEBRUARY 1965
39
installation costs to the extent that the expense of providing water and drain lines can be avoided, as can the need to protect them against freezing. Calibration
The extended range makes the instrument suitable for testing kerosine and most jet fuels, but makes it necessary to employ faster cooling so that testing cycles can be conveniently short. Because erroneous results may be obtained when cooling rate is too fast, samples were observed visually as they were cooled at various rates in a sample cell. Kerosine and jet fuels, having pour points ranging from -40" to -60" F., were not sensitive to rapid cooling between about 80" and 20" F. Some materials having pour points between 20" and - 10" F. are sensitive to the rate at which they are cooled from ambient temperature. Table I11 illustrates the effect of cooling rate on indicated pour point of samples that exhibit supercooling or erratic crystallization when cooled 20 to 30 degrees per minute. At a cooling rate of 10 degrees per minute, the indicated pour points were about 4" lower than the D 97 values. The indicated pour points agreed with the D 97 values when the samples were cooled 4 degrees per minute. Observing the samples as they are cooled under instrumental conditions reveals that, at the temperature corresponding to their ASTM pour points, they are not fully congealed although fluidity is plainly impaired. Thus, to bring the indicated pour points into closer agreement with D 97 values, the instrument must detect impaired flow early in the transition of the sample from liquid to semicongealed condition. The photocell circuit is made as sensitive to the loss of reflected light as is practicable; the limitation is interference due to sample color. The optical housing and
the interior of the sample cell are dull black to prevent reflections except from the sample surface. Some light, however, traverses the thin layer of sample, and slightly more light reaches the photocell when colorless samples are tested than when the sample is dark. The photocell circuit is, therefore, made insensitive to the difference in illumination between a colorless sample and a colored one. Rocking the sample cell more rapidly serves to accentuate hindrance to flow. The tilting rate of eight cycles per minute gives the required sensitivity even when cooling rate is increased to 15 degrees per minute. Flow hindrance due to viscosity increase is not a source of error when testing distillate samples of about 100 SSU viscosity at 100"F. T o avoid excessively fast cooling, the rate is controlled by a motor-driven variable transformer in the power supply for the thermoelectric module. At the beginning of the test, when the sample is warm, the variable transformer supplies only about 307, of full power. During the first ten minutes of the cycle, the power is steadily increased to 1 0 0 ~ ,and held at this level until the cycle ends. Thus, the initial cooling rate does not exceed 12 degrees per minute; at temperatures below 20" F., the cooling rate does not exceed 8 degrees per minute. To assess accuraq- of the automatic pour point tester, it is necessary to modify the ASTM-D 97 procedure and to use averages of multiple determinations. Data points in Figure 4 designated "Modified D 97" were obtained by making the test observations at intervals of 2" F. T o obtain finer resolution from the standard test, observations were made on some samples every 5" F. as specified, but the initial and all succeeding observations were made at temperatures that ivere not multiples of five. (Kapff also found these modifications
30
-30 Y
10
-40 Y
-10
-30
-50
-7 0
-70
40
60
80
100
120
COOLANT TEMPERATURE "F
Figure 3. Performance characteristics of thermoelectric sample cell with coolantjow rate of O.S/g.p.m. 40
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
-50
-30
-10
POUR POINT ASTM-0 97 "F
Figure 4. Accuracy of pour point tester
10
30
to D 97 were necessary in establishing the accuracy of the instrumental method.) Accuracy of the automatic pour point tester is 1 1 ' F. Repeatability of instrumental results depends on exactly reproducing sample surface and completely displacing the previous sample from the inlet leg and sample cell. The weir a t the outlet port of the sample cell positions the sample surface. Positive drainage with minimum interference from surface tension is promoted by slightly pressuring the cell with instrument-quality air during the filling cycle, which assures that all liquid above the weir flows to the drain connection. Tilting the cell continuously during the filling cycle promotes uniform drainage and prevents a retention of liquid in the cell connections. Excessive vibration causes the sample surface to become wavy, thus effecting the operation of the detection system. Because the automatic pour point tester is heavy and equipped with vibration-absorbing feet, this difficulty is avoided. Bubbles on the sample surface interfere with reflection of light and give a false indication of pour point. They are prevented by the vertical run of tubing through which sample is introduced to the cell. The tubing is completely filled with liquid, and its diameter is large enough to let air rise through the liquid and escape from the sample cup. The previously tested sample is completely displaced by reversing the polarity of the thermoelectric power when the pour point is reached; this warms the cell and renders the sample fluid again. About 120 ml. of new sample are introduced to rinse the inlet leg and cell; only the last to enter the cell is retained because the inlet port is always below the top of the exit weir. Samples of lower pour point are not affected by the preceding tests on samples of higher pour point. Repeatability of the automatic pour point tester run-to-run and day-to-day is summarized in Tables I V and V. During one day, 97 consecutive results averaged -10.0O F.; no result was more than 1' F. from the average, and standard deviation was 0.46' F. On another sample, 50 results over 15 hours ranged from -36' to -40' F., averaging -39.2' F., with a standard deviation of 0.94' F. The same low-pour sample, tested 15 times one day and 13 times another day, with standard deviations each day of about 0.3" F., gave negligible differences between average results for the two days. Another sample of about 5" F. pour point gave standard deviations of 0.5" and 0.7" on different days; again the average results for the two days are not significantly different. The repeatability of the automatic pour point tester is, therefore, approximately 1" F. A Simplified Instrument
Knowledge gained of the effects of instrument variables led to design of a simpler automatic analyzer for less demanding applications. No provisions were made
Result OF.
No. of Obseraations
-11
10 77 10 97
- 10 - 9
Sample B (15-hour run):
- 40
21 21 6 0 -2 50
-39 - 38 - 37 - 36
Average: -339.2' F.; Standard Deviation: 0.94' F.
TABLE V.
Samfile
1
Day
DAY-TO-DAY REPEATABILITY
1
B
1
Auerage
-39.2 -40.0 -40.2 4.6
C
~
1 ~
No. of Tests
~
1
50 15 13 13
S t d . Dev.
0.94 0.27 0.38 0.52
for introducing samples manually, for programming the power input, for warming the sample cell after the pour point is reached, or for exactly reproducing the sample surface. The simpler instrument is, therefore, less versatile, accurate, and precise than the automatic pour point tester, but it is faster, less costly, and easier to install. The test-cell assembly, with its d.c. power supply, is enclosed in a n explosion-proof housing for location near the sample supply point. Vibration-absorbing mountings should be used to fasten the analyzer housing to a support strong enough to resist bump. A separate electrical control chassis of Division 2 construction is intended for remote location-for example, in a control room. Sample from the process is fed to the instrument through a-flow-indicating and -controlling valve. If the circulating sample stream is cool enough, it may be used instead of water to remove waste heat from the thermoelectric module, thereby avoiding the expense of providing a water supply and drain. The test cycle is initiated when a solenoid valve is energized by relay action for as much as 2 minutes to VOL. 5 7
NO.
2
FEBRUARY
1965
41
allow sample to flow into the test cell and flush out the previous sample. The sample level is controlled by an overflow weir at the outlet port of the test cell, but air is not provided for promoting drainage. At the end of the filling step, relays close the sample solenoid valve and simultaneously energize the power supply for the thermoelectric module. Power input is governed by a manually variable transformer in the 120-volt a.c. primary of the power supply. Pour point is detected in the same way as in the automatic pour point tester, with temperature continuously sensed by a thermocouple. IYhen the pour point is reached, cooling is stopped and a new cycle is begun by energizing the sample solenoid valve. The spent sample is delivered to an atmospheric sump via an explosionproof drain. When the correct power input is selected, and after the instrument is calibrated against samples typical of the product to be monitored, accuracy is 1 2 " F. Repeatability is 2' F. Automatic Control Capability
Both instruments that have been described can be applied to onstream monitoring of distillate fuels having pour points between 70" and -70" F. The results are accurate in terms of ASTM-D 97. The automatic pour point tester is especially suited for controlling each of several streams that may have widely different pour points because it is free of sample carry-over and requires no adjustments for power input. For multistream applications, an automatic device is employed to present each of four sample streams to the instrument in sequence. The device is actuated by an electrical signal from the instrument so that each new sample is introduced at the proper point in the test cycle, regardless of variations in the total time. The simpler instrument is more advantageously used for controlling a single stream, although it may be employed in multistream service if the pour points do not differ widely. I t is the more rapid of the two instruments because the step of warming the sample cell after reaching the pour point is omitted, and because power input for cooling is constant. Test cycles as short as 5 minutes have been obtained on samples having pour points of about -15' F. Such short test cycles are particularly valuable for controlling continuous blending operations, which require faster testing than processes such as distillation. AUTHORS Walter V . Cropper isgeneral manager of Precision Scient@ Development Co., Chicago, Ill. Gerald L. Hammond is development engineer with the same firm, which was licensed by Standard Oil Co. (Ind.) to develob commercial instruments based on an invention by S. F. Kapff of Standard. T h e authors wish to express their appreciation to D r . K a p f and his colleagues for valuable assistance and to B. W . Thomas f o r many helpful suggestions. Samples to azd in instrument evaluation were supplied by Texaco, Sinclair, Imperial Oil, Humble Oil and R@ining, Atlantic Refining, and American Oil. 42
I N D U S T R I A L AND ENGINEERING C H E M I S T R Y
Control elements must be actuated by a signal that corresponds only to the test value. The coinplctc temperature trace of the test cycle is, therefore, unsuitable. For this reason, the cyclic temperature signal from either instrument must be modified to make it compatible with conventional elements. A relay network actuates the balancing motor of an electronic recorder for a few seconds when the pour point is reached ; because the balancing motor is not energizcd during the rest of the test cycle, the complete temperature record does not appear on the chart. The spike-like chart record is thus changed into a series of short plateaus. TVhen the electronic recorder is equipped with a transmitting slide wire, the continuous signal can be handled in conventional ways by controllers or automatic data logging units. Some control elements employ pneumatic signals; for these, the electronic recorder may be equipped Lvith an accessory that produces a 3- to 15p.s.i.g. signal proportional to pen position. An installation incorporating these features provides automatic control of a blending operation. Conclusion
Possibilities are being studied for further improvcnients to these instruments to measure pour points as low as -80' F., to test more viscous materials, and to obtain valid results on products that contain additives to depress pour point. The technology of Peltier effect devices is advancing rapidly. A three-stage module designed to cool to -90" F., with a "hot-side" temperature of 70" F., is already available, but its size and power requirements prevent its use in the instruments described. Improved two-stage modules, capable of reaching -80" F., inay soon be available. The viscosity of lubricating oils is relatively high at ordinary temperatures and increases rapidly as the material is cooled. For this reason, the transition from highly viscous to impaired flow is difficult to detect reproducibly. Sample carryover is hard to prevent when transporting viscous samples into and out of the sample cell. Pour point depressants modify or delay the prccipitation of waxy materials, and they are most effective when precipitation is slow. For this reason, the accelerated cooling necessary for conveniently short test cycles prevents additives from exerting their effect. Further study of the behavior of inhibited samples under instrumental test conditions is needed to determine how they can be tested reliably. REFERENCES ( I ) A m . Soc. Testing .44ater., A.5'T.W Sld., Part 17, "Standard Method of 'Test for Cloud and Pour Points, ASTM Designation D 97-57," p . 58, 1964. (2) Ihid., pages 153: 351, 354, and 547. (3) ,4772. SOC.Tesiinq A-iaier., S p c . Tech. Publ. KO. 7-B. "Significancc of hSrM Test for Petroleum Products," 1957. (4) Buchler, C. C., Graves, E. W., IND.GNC. C i f E M . 19,718 ( 1 9 2 7 ) . ( 5 ) Farrar, G . L.: Oii G o s J . 5 8 , 147 (Nov. 14, 1960). ( 6 ) Zhid., 59, 118-119 (Oct. 23, 1 9 6 1 ) . (7) Kapfl, S. F., Oil Gas J . 62, 173-177 (July 27, 1964). ( 8 ) Rowell, R. I,., Hydrocarbon Process. Pelrol. Rejiner 43, No. 4, 176 (1764).
