Wax Precipitation from A. P.ANDERSON AND 5.K. TALIEY Shell Petroleum Corporation, Wood River, 111.
HE purposes in studying the of .nreciuitation . wax from propanesolution were to determine the most suitable conditions for conimercial continuous filtration and to gain an insight into the mechanism of wax structure formation f r o m r e s i d u a l stocks. Because of the smaller filtering surface in continuous filters,
greater care must he taken in precipitating the wax than with batch filters in order to ensure rapid filtration and clean discharge of the wax cake. The effect of wax structure on filter rate wax3 &st determined by correlating photomicrcgraphs with filter rate. The effect of the various precipitating conditions on the wax structure was then determined to establish the optimum conditions. A Midcontinent stock with a Sayholt Universal viscosity of 100 a t 210' F. was used in most of this work.
T
FIOWRE 1.
EXPERIMENTAL FII-
TERING
APPARATUS
Apparatus Figure 1 shows the experimental chilling bomb, B, and filter, A . They vere designed to correspond to the conditions of time, pressure, and temperature existing in the continuous Oliver-United type of rotary filter used in the dewaxing plant. This simple apparatus has given excellent results and has proved to be a sensitive indicator of the filterability of the various types of wax that can he precipitated from propane solutions. The filter consists of a cylindrical filtering element contained in a Cylindrical shell. The latter in fitted with a flanged cover to which the element is permanently attached. The shell is contained in a lagged kerosene bath which can he chilled with solid carhon dioxide to -40' F. or below. Connections for6lling and emptying the shell are made through the w a b of the bath. The filtering element is made of perforated 4inch pipe around which is wrspped a quarter-inch wire screen and, over that, the filtering canvas. The wire screen provides channels under the cloth, thereby making all the filterin area effective. The canvas is bound at each end with wire &atnres. The ends of the cylinder element are solid. The discharge pi through which the filtered oil passes, goes through the top filtor element and, inside the element, extends down within a quarter-inch of the bottom. The filtering ares. is 48 square inches. Air pressm for filtering is a plied through the chilling bomb and charging line. This procexure chills the air before it entera the filter shell. The pressure is adjusted to exactly 3 pounds per square inch as indicated on a mercury manometer and the filtering started by opening the valve on the discharge line of the filter element. The filtering is sto ped at the end of 3 minutes by opening the drain valve on the gottom of the shell End drop $ng any remaining unfiltered solution out of the apparatus. hese conditions of time and nlpdsure annmximate those in the
The conditions that affect the wax structures, produced in propane dewaxing of Midcontinent residuum, have been studied by means of the polarizing microscope. With this information and a knowledge of the relation between wax structure and filter rate, it has been possible to set up optimum conditions for mixing, chilling, and handling propane dewaxing solutions. Dewaxing solutions of residuum in propane are best mixed at temperatures considerably above the temperature of complete solution. Chilling by spontaneous evaporation of propane produces a more filterable wax than does indirect refrigeration. The most easily filtered wax is made up of clusters which consist of small wax grains cemented together by naturally occurring aromatic resins.
OExi
____ 1
25 t o 15
I=>. EN^. Caar., And. Ed., 6 , 387 (1934).
15 t o 10
10 to 5
ON FILTER RATE( X 120) FIGWRE 2. EPPECTOF WAX STRVCITRE
(Filter rate in sallona per
432
SQUBIB
foot per hour)
5 to 0
Propane Solution
acetone, used for a cooling m e d i u m , flows from an elevated reservoir through a copper coil, which is chilled in an alcohol and solid carbon dioxide bath, and then goes through an insulated cold stage which is thereby chilled. The alcohol goes to an air lift which carries it back to the elevated reservoir. The air for the air lift is dried in a calcium chloride tower. Under ordinary conditions with alcohol, optimum chilling conditions are obtained when the c h i l l i n g bath is m a i n t a i n e d at -40" F: lower temperatures congeai the fluid. If other liquids are used, they s h o u l d be miscible with traces of water; otherwise the system soon becomes clogged by ice and frost unless c o n t a c t w i t h open air is avoided. It has
PUMP ROOM (ABOVE) AND CONTROL BOARD(LEFT)IN THE PROPANE:DEWAXINQ PLANT
6 representative fields. This angle of attack did not seem p r o m i s i n g and no f u r t h e r work was done. On the other hand, a study of wax particle size distribution and a g g l o m e r a tion of small wax particles can be made at magnifications of the order of 100 times with the advantages that there is a larger field of view for selecting and studying representative specimens and there is a simplification in observing and photographing because of greater depths of focus, greater working distances, and less critical adjustments of the lighting and optical equipment. All observations were made in polarized light with crossed Nicols. Other methods of illumination failed to distinguish clearly between oil and wax. The contrast obtained with polarized light is a distinct aid in making photomicrographic records. Ordinary Hughenian oculars are used for visual observation with or without a green color filter. Photographic work de-
also been found feasible to chill the cold stage directly by simple expansion of liquid propane.
Microscopical Examination The selection of the magnifying power to be used in microscopical examination of the wax precipitate depends upon what is sought in the final picture. During the early part of the work preliminary observations of wax precipitated in propane showed a possibility of obtaining an indication either of the form of the individual wax particles or of the relative distribution of the sizes of the particles. For observation of the wax structure or wax form it is necessary to use magnifications of about four hundred times or higher, increasing thereby the difficulties of focusing and of seIecting 433
INDUSTRIAL AND ENGINEERING CHEMISTRY
434
VOL. 29, NO. 4
FIGURE 3. V18n.k~ OBSERDAM ON INITIAL WAX SEPARATION IN PnoPANE SOLVIION OF DEVATION
A~PRALTTIZEDMIDCONTINENT
REBIDUUM
mands a field lens that projects a flat field. Spencer planoscopic oculars FIGURE 4. INFRARED T ~ ~ satisfy this requirement and CURVES SIIOWIXG 'IRE QEPIRATION OP WAX ON COOLING SOLUTIONS OP DEwere used in making ASPBALTIZED MIDCONTINEN? REsIDonnr all p h o t o m i c r o g r a p h s . IN PROPANE These oculars are used with a color filter. Green affords the greatest contrast with wax and, because of the lightsensitivity characteristics of the eye, makes for easy focusing.
Y
Photographing Technic
~
M
~
~
~
~
~
~
5. APPARATUS FOR DETERMININQ INIr n Wax ~ SEPARATION IN PROPANE SOLVTION~ FIQWE
The following procedure applies to photographing wax that is contained in a propane solution at -40" F.:
% g l ~ stirrhng s md and the &ioro&co& slides. After'the rod I's chilled, it is used to stir the sample. IDthis and in other operations preCautiOD8 are taken to prevent the evaporation of propane
BYMEANSO~INFRAREDLIQHT
Temperature oi oil solution is eontrolled by means oi s oooling tube snd B" eleotriosl heating coil in a Dewar flask.
d
mm o?' ,&w.4
M , "
0 OAT8 OBTAINIO bY
0'4
i
i
,N,m*Ea P H O r D M m Y/.'VBL 06SPAllON
i
3 YOLUNf fRTM P r n C / &
FIGURE 6.
2
h
AS#@%
IHITIAL WAXSEPARATION
FRDX PROPSOLUTIONS OF DEABPEALTIZED MIDCONTINENT RESIDUUM
'116'
F.
125' F.
130'
F.
140"
FXG~RE 7. E m m
F.
M n x i w u ~TEMPERATURE TO W ~ I C H WAX-BEARING PROPANE ~ O L W r I O ~ASR E HEATED IN TAE PLANT ( X 120) OF
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INDUSTRIAL AND ENGlNEERING CHEMISTRY
435
tures can be correlated with filter rates to the extent that certain forms are always associated with low ftlter rates and others with high rates. It is possible to have all the various W& forms present in one chilled solution, and then the actua,l filter rate depends more upon the proportions of the poorer wax forms. Figure 2 shows effect of structure on filter rate as determined on an experimental scale. The best filtering wax structure has the appearance of popcorn. The wax is gathered into durable, well-formed clusters which are made up of microscopic grains of wax. The nature of the cementing material holding the grains together is discussed later on. The ex75" F. 50' F. perimental filter rates with this wax form are 15 to 25 gallons of dewaxed stock per square foot per hour which would represent about 10 gallons per square foot per hour or less under plant conditions, since only 60 per cent immersion and used cloth are employed. The less desirable wax presents various appearances. Very fine wax particles often appear in quantities sufficient to form clouds. In such cases there is no tendency for the particles to cohere, and the filter rate is reduced to zero because such particles then plug on the filter cloth. Another undesirable wax form has the appearance of needles, free or interlaced. A third wax form, that has not been distinctly photographed, shows up as an indistinct c o a t ing on other wax forms. These last two forms 90' F. 1 w o F. are obtained only under special experimental FIQURE 8. EFFECT OF ;RAPIDLYWAmR-CooLme TUE WARM WAX-BEARING conditions. The large crystal groups naturally 8OLUTIONB TO DIFFERENT TEMPERATURES BEFORE TRANSFERRING THEM TO tend to give the best rates but size alone CHILLING VESBEM(X120) cannot be used as R criterion of filterability since the distribution of sizes or uniformity is To revent accumuletion of frost on the objective, it is adof equal or greater importance. Seldom are NBX particles vi3fIile to keen it well inside the cold staee, raisinr it only in a given chilling batch perfectly uniform in size. The presence or absence of fines in wax precipitates produced in the plant is an important item in predicting filter rate. The correlation of experimental and plant filterability for a given type of wax structure or microscopic appearance is excellent.
Effect of Precipitating Conditions on Structure The important variables in precipitating wax from propane solution by internal or direct refrigeration are: mixing temperature, propane-stock ratio. chilling conditions, and crystallizing agents such as asphaltic compounds.
~
..
~ . ~ -
ples of the wax itself, 6Jtered relhtivelv free of oil. have also been taken from & n a r m d wax cake: The wax from this s o m e was suspended in liquid monsne and examined acenrdincr' , tn the nhove ...~ ~~. . $moedure. The appesrenceof suck,WBX was substantially the same as of wax taken from the ~ f i l tered slurry. It was also found that'moderate additions of propane to a wax suspensiop in propaneoil solution at - N oF. produce only Smignifieant changes in the appearance of the w s s structure. ~
~
~
,
Effect of Wax Structure +n FilterRate
A number of different wax structudes can be obtained in propane dewaxing, and thkse struc-
with interns1 agitation
FrannE 9. EFFECT
Without inteinsl adtation
O F AUITATION OF SOLDTIolVs DURING CHILLING
(X120)
lNDUSTRIAL AND ENGINEERING CHEMISTRY
436
Initial ratio: 1.6
to 1
VOL. 29, NO.4
above or below the clouding curve, it is important to raise the temperature considerably above the cloud point before starting to chill. Apparently the higher temperature decreases some molecular aggregation or cyhotactic effect. Thia aame conclusion has been reached in plant operation as is shown by the photomicrog&phs of 7. The rate of chillins?. within rather wide ranges, does n?t appear to he an important factor. Gooa filter rates can apparently be obtained &a long as instantaneous or flash chilling is avaided over any extent of the chilling curve. In oite experiment a bomb of mixed propantwil solution a t 140' was placed in a bath a t -60a F., and propane was vented from i t as rapidly as pomible without spraying out the liquid aontents. Ten minutes were required to go fro 100' to -40' F., making an average rak of F. per minute. Actually the rate varied from about HIoF. per minute in the high-pressure stages to about 5" per minute ab the lowest pressure. Both the wax structure and filter rite were good. In the plant i t has been fopid that the last IOo of chiing can be obtained by transferring the incompletely chdled batch directly into the
1.8 t o 1
3'
Initial ratio: 1.9 to 1 2.0 t o 1 EFFECT OF INITIAL PROPI\NE-QTOCK RATIO(X120)
FIRURE 10.
It was noticed early in this work that complete solution of the waxy oil in the propane was conducive to good filtration. However, the temperature a t which complete solution occurred seemed to vary with successive hatches; consequently the cloud point of solutions with different propane-stock ratios was investigated. The visual curve shown in Figure 3 was the result. The unusual shape of the curve for the residual stock, showing a minimum beyond which an increase in ratio decreases solubility, plus the dficulty in observing cloud points in dark oils made the relation open to question. It was therefore subsequently checked by making up solutions of waxy deasphaltiaed Midcontinent residuum of appropriate concentrations in glass ampoules and determining their infrared transmission during cooling. Corrections were made for the propane in the gas phase, and all volume ratios were calculated, as in the rest of this paper, on a 60" F. basis. The photometer used was a baUistically operated galvanometer-thermopile system. A break in the transmission-cooling curves indicates the initiation of wax precipitation (Figure 4). Figure 5 shows the infrared photometer apparatus used. The visual and infrmed cloud-point curve8 are shown in Figure 8; the curves are the same shape, hut the infrared method is more sensitive. In order to determine the signiiicance of this cloud-point curve, mixing temperatures were chosen both above and below (but near) the curve, and chilling was started from these points. The 6nal chiUed mix was brought to a 2 to 1 ratio. Little dBerence could be found. However, when point$ such as 2 and 3 on Figure 6 were taken as initial mixing temperatures, a considerable ditference was obtained in wax structure. On chilling and filtering, a filter rate of 19 gallons per square foot per hour for Doint 2 was obtained as compared to 10 gallons for point 3. Although it apparently makes little difierence whether the midng temperature is either slightly
peratnresskowr,. Precooling beyond 100' F. is harmful. Oddly, however, waterwoling to 75" F. before chilling by expansion gives as filterable a wax as did cooling to 100" F., although intermediate temperatures do not give very filterable wax. The type and amount of agitatiofl during chillilog have an effect on wax structure. indirect refrigeration, us continually agitated whil
Violent churning of pected, is harmful. For example! agitation of a cold bomb c o n t a i ~ n gchilled mix and scrap b n in 8 ahakmg machine showed considerable breaking u$ of wax crystals and a 20 per cent reduction in filter rak Ciinlation of an otherwise excellent batch of chilled solution for 48 hours
occur under normal conditions. The propane-stock ratio does not appear to he important, provided it is not too loy. For the stock studied an initial propane-stock ratio of 1.6 to 1was about the lower limit for good wax structure. * e 10 shows the effects of thie variable in plant p r a c t b . A ratio below 1 t o 1 gives gel formations which are p r y difficult t o transport and impossible to filter. M d e d no gel is formed, a low propnne ratio can he increasedby the addition of cold propane before filtering to eliminate viaaoalty diirerences and the filtering rate is quite unharmk. Figure 11 summ& photographically the effects of the
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INDUSTRIAL AND ENGINEERING CHEMISTRY
important variables discussed above as determined experimentally. The effect of three variables upon wax structure under adverse conditions and four combinations of these variables are shown for Midcontinent residual stock. Similar results were obtained for raflinate made from the residuum by propane-cresylic acid extraction except that the filter rates were somewhat lower. The photomicrographs for low propane ratio do not correspond exactly with the filter
FIGURE
11.
437
peratures. The crystals Dray be examined either in solution, under propane pressure, or out of solution, under atmospheric pressure. For obvious reasom it would he preferable to examine the crystals while suspended in solutions of propane and oil; but early work indicated that, although visual microscopical observations of wax crystals in propane offered possibilities, photographic records were virtually impassible because of the easy and rapid movement of the suspended
PHOToMlCHOoRAPHS OF WAX FROM DEASPHALTIZED MIDCONTINENT LONQREsInunm, WITH ASSOCIATEDFILTER FATES
Normal conditione: 1. Oil end propane mired at 140- F. 2. Propans-oil ratio 2 t o 1 3. Solution chilled & propme ew.Poi*tion. internal reiriseration
rates shown but, as explained, this is due to the increased viscosities a t lower propane ratio. The effect of a fourth variable, crystallizina.aaents, . . is discussed later.
Wax Structure In order to study the formation of the clusters which give the best filtering conditions, photomicrographs were taken at intervals during the chilling cycle. The conditions under which the 6rst wax is thrown out of solution can idnence the form of the wax a t filtration tem-
Adverse cooditions (existing only QB ahown): 1. Oil and proy~nemired at 190' F. 2. Prapsna-oil ratio. 1.3 to 1 (low) 3. Solution chilled in closed bomb, external refrigerstion
particles. The present work was limited to isolatimg the initially precipitated wax particles with the least distortion and ohservine and nhotoeraohine them. The apparzttus used consists of two homhs 8 inches long made of l'//rinch pipe and hell reducers, and 3/rinch valves. The bombs are joined by a flat-faced union; hetween the faces of the union is placed a disk of a t e r paper. A small circular filter screen is also inserted to hack up the filter paper and provide necessary strength. In operation one bomb is charged with waxy stock and -
I
Y
IlLUUSTKIAL AND ENGINEERING CHEMISTRY
VOI,. 29, KO. 4
microscope, and hence, with slight reservations, the method can be used for isolating the crystals for the present purpose at v a r i o u s stages of the chilling cycle. . I F. The Midcontinent residuum used for these experiments was obtained a t a different time from the residuum used in the rest of the work described in this article. This will account for the slight discrepancies in the clouding points shown in Figure 12 as c o m p a r e d to tlie curves on Figure 6. Propane solutions of waxy Mid.2 continent long residuum were made F. up at 140' F., chilled, and filtered in the described apparatus at various temperatures. The crystals were removed and observed under the microscope. Cold propane was found to be the most s u i t a b l e medium for transferring wax crystals from the filter paper to the microscope slide. It does not dissolve the wax and does not of itself .a F. cause clustering. The crystals removed at 88" F. showed a fine needle structure. As nienlioned previously, this needle appearance can result from plates viewed edgewise as well as from actual needles. IIigher magnifications fail to distinguish clearly between the two forms, and the evidence seems to point to the 4 presence of both. The wax crys1" F. tals did not show the s l i g h t e s t tendency to cluster as is shown by p h o t o g r a p h 1, Figure 12. At 85" F. the needles become larger and at 50°F. a very fine grained structure is obtained. Although this precipitate obviously contains the earlier precipitated needle apW a x irom cake aitei sixth w%h at -50' pearing forms, it shows little evi: F dusters de&wed, el! free . 5 dence of them. There is only a \>-ax partir1es ok, slight t.endency for the crystals to P. cluster and the clusten that do form are easily broken. The grains are much smaller than are obtained in normal aterings at -40'F. The vield of wax at F. is FrccnE 12. PAnTidi. CUYSTALLIZAF~~~~~ 18. paoaREasrvE D~~~~~~~~~~~~~ pRo. slight and in some runs was too 'WON or WAX FROM PROPANE-OIL PANE-PUECIPITATED WAX CLUSTERS DURING REsmall to give enough wax for obQOI.UTIONS (X120) PEATED PENTANE WA0ElNO AT L O W TEMPERATURESservation. However, the actual (X120) separation of wax from dewaxing solutions cliilled to only 88" F. shows something concerning the necessity of prelleating depropane and brought to the desired conditions and the empty one brought to tlie full vapor pressure of propane at the - waxing solutions to about 140" F. in spite of the fact that pronounced wax precipitation does not. take place unt,il temworking temperature. The bombs are joined nrith the peratures as low 8s 75" F. are reached. filter paper in position and the pressure in the whole apparatus A wax obtained in the same way as described above, except equaliaed without wetting the filter paper. The filtration that the filtration was carried out at 70" F., shows an indisis started by inverting the apparatus. Gravity and a slight tinct eombine,tion of the needle and microgranular structures. chilling applied to the Iower bomb supply the filtcring presIn order to check the influence of the first wax forms, a solusure. When the filtration is complete, the valves are shut tion was filtered at 88" F. and again at 50". Little differoff (thc upper one first), and the union broken. The crystals ence was observed between the wax obtained in this way at obtained with this apparatus axe a,pparentlythe same aa those 50" F. as compared with the wax obtained at 50' F. after that can be visually observed in propane solution under the
APRIL, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
439
chillingdirectlyfrom 140" F. Figure 12, photoWith Cemenang Matma1 \\ Ithout cementing M a t e n d graph -5, shows the appearance the '
