I
J. R. DONOVAN and B. J. BARNETT’ Inorganic Chemicals Division, Monsanto Chemical Co., St. Louis 24, Mo.
Filtration of Molten Sulfur These recommendations provide quantitative information which will be valuable in design and operation of molten sulfur filters
ELEMENTAL
sulfur is used chiefly in making sulfuric acid. I n both chamber and contact acid processes, sulfur is burned in air to produce a mixture of sulfur dioxide, oxygen, and nitrogen. Any solid matter or ash in this gas stream will ultimately foul the acid plant and require removal. Consequently, sulfur filtration has been adopted in many plants to replace earlier use of less efficient settling pits and gas filters. For a typical large contact plant, payout time for filtration equipment and accessories is estimated to be about 2 to 3 years. The work described here was undertaken because of operating difficulties with porous tube filters installed a t the Monsanto, Ill., plant. Several test filtration runs were made with this equipment to check its performance, and subsequently a laboratory study was made in a vacuum, nutsche-type filter.
amounts, ranging up to O.?yo carbon or more. T h e viscosity of liquid sulfur decreases normally as temperature is increased from the melting point (about 115’ C.) to 157’ C., then rises very sharply with a few degrees further heating to a high value which practically prevents flow. At 190’ C. the viscosity begins to decrease again, and a t 250” C. the liquid is once more reasonably mobile. Normal boiling point is 444.6’ C. Density, too, decreases with increasing temperature, the density of “dark” sulfur decreasing at the same rate as that of pure sulfur but at a slightly lower level. T o maintain viscosity at reasonably low values, it is necessary to control molten sulfur temperature within narrow limits. This is usually accomplished in the field by use of jacketed or traced pipe and equipment, with steam a t regulated pressure in the jackets.
Plant Tests with Porous Tube Filters
The original installation of carbon tube filters consisted of several units in parallel, each unit comprising nine closed end vertical tubes suspended from a top filtrate chamber within a jacketed shell. Specifications of Individual Carbon Tube Filter Filtration pressure, p.s.i. Average design on-stream capacity Tons S/hour Gal./ min. Filtration area, sq. ft. Tube size, inches O.D. Length Wall thickness Av. pore diameter of tubes, inch Diameter of minimum particle retained by tube, inch
50-60
3.2 7.2 31.5 4.5
36 13/18
0.0027
0.00098
Characteristics of Molten Sulfur
Molten sulfur is a difficult material to handle because of its relatively high freezing point and its peculiar viscosity characteristics. Also, much commercial sulfur today contains organic matter which tends to form gummy or solid deposits upon heating. This type of sulfur is designated as “dark,” in contrast to the purer grade known as “bright.” Typically, bright sulfur contains organics equivalent to a carbon content of about 0.05y0 or less; dark sulfur contains appreciably greater Literature Background Subject
Ref.
Filtration patents Porous tube filter Plant filtration experiments Kelly pressure leaf filters Filter patents Pressure filters for removing hydrocarbons from Mexican sulfur Modern sulfur mining
(8,10, 11) (1) (2718)
(7) (6,6) (3) (9)
Present address, Shell Oil Co., Wood
River. Ill.
Horizontal leaf-type sulfur filter is cleaned by rapping with rubber mallet VOL. 51, NO. 2
FEBRUARY 1959
165
THERMOMETER
0
II
STEEL PIPE 8 NICHROME WINDING i
FILTER SCREEN PRESSURE CONTROL
!
I 1
I--
PUMP
"AIR BLEED
-
GRADUATED 3" PYREX , PIPE
MANOMETER
Figure 1. To expedite laboratory work, this simple nutsche-type filter was constructed. Although pressure range was lower than plant pressure, tests provided a survey of important variables
T h e filters were precoated with diatomaceous earth by dropping the dry filter aid into a funnel leading to the suction side of a submerged centrifugal sulfur pump. Cake was removed from the filters by backwashing with low pressure steam, with discharge through a bottom nozzle in the shell. This nozzle was also used to admit unfiltered sulfur. Initial operation of the filters was disappointing, in that average capacity was about half the design rate, even with constant attention. Ash content of the filtrate was considerably higher than the desired 0.001 to 0.005%. Frequent backwashing was required, with resulting heavy losses of sulfur. Tubes blinded or cracked at frequent intervals, and sulfur feed pumps experienced excessive wear from erosion by filter aid. To determine exact capacity of this type of filter, one of the units was fitted with a complete set of new tubes (National Carbon Co. No. 40 Porocarbon). A separate tank was provided for mixing filter aid with a recycle stream of sulfur overflowing from the top vent of the filter shell, a continuous dry feeder being positioned over the tank to add known amounts of filter aid continuously. Five consecutive plant tests were then made with this single filter, three of them at essentially constant pressure of 65 p.s.i. (Table I). Data from the two runs at variable pressure are not presented, as attempts to throttle flow resulted in very erratic rates. Evaluation of Plant Tests. Poiseuille's formula as integrated for constant pressure conditions was used to correlate the results ( 4 ) :
Here 8 is time; V, volume of filtrate collected; A, area of filtering surface; p, viscosity of filtrate (pounds/foot hour) ; a, average specific cake resistance; w ,
Table I.
Plant Filtration Tests of Louisiana Dark Sulfur"
Amount of organic matter in the filtrate was about the same as in the feed
Run
Continuous Filter Aidb PrecoatSb Addition.
Feed Oreanics Ash, % as-c, %
Ash in Filtrate. ElaDsed Filtrate Time, Hr. Val., Cu. Ft. %
No.
Lb.
Lb./Hr.
1
6
2.2
0.029
0.25
0.007
0.17 0.33 0.67 1.00 1.50 1.83 2.17 2.83 3.25
18.4 33.1 57.7 78.0 102.4 116.8 129.9 153.4 167.8
2
12
1.5
0.028
0.27
0.006
0.17 0.33 0.50 0.67 0.83 1.17 1.50 1.83 2.17 2.67 3.17 3.50
49.4 86.6 114.6 137.0 155.2 182.5 204.7 223.7 240.5 263.3 284.1 297.7
0.23 0.40 0.56 0.73 1.06 1.39 1.89 2.22 2.89 3.56 4.23 4.90
42.6 65.0 83.7 99.9 126.9 149.1 176.7 193.6 224.6 253.5 280.2 304.8
12
4
(I
166
weight of dry cake solids per unit volume of filtrate; P, pressure drop through the filter medium and cake; r, resistance of unit area of filter cloth or medium plus pressure drop in lines. T o use the equation, e/( V / A ) is plotted as ordinate, with V I A as abscissa. The intercept of the resulting straight line represents pr/gcP while the slope represents pcuw/gC2P. Using the data in Table I, excellent straight lines were obtained for plots of B / ( V / A ) us. V I A for runs 1 and 4, but the data of run 2 gave a curved initial section. T h e straight portion of this plot had a negative intercept. Table I1 contains values of specific cake resistance and initial resistance calculated from these graphs. Conclusions. Filtration rate at constant pressure follows the normal pattern of a high initial rate, decreasing as filtration proceeds. To obtain design filtration rate, the filter must be cleaned at least once per 8-hour shift. Filtration rate can possibly be increased above the design value by cleaning twice per shift. Within the ranges tried, the amount of filter aid used for precoating is the most important variable affecting rate. A precoat of a t least 0.4 pound per square foot should be used for this filter, and
INDUSTRIAL AND ENGINEERING CHEMISTRY
2.5
...
Temp. of molten sulfur 146-150" C.
0.27
Hyflo Supercel.
0.002
M O L T E N SULFUR F I L T R A T I O N continuous filter aid addition is beneficial in maintaining high rates. Because the tests were run with new tubes, fouling was not a factor. T o minimize tube blinding only filtered sulfur should be used for precoating.
c:
Laboratory Tests
Apparatus. T o expedite laboratory work, a relatively simple nutsche-type filter was constructed (Figure 1). A vacuum pump and automatic pressure controller were connected to the filtrate chamber to provide pressure differentials ranging up to 14 p.s.i. Although this pressure range is considerably less than that encountered in plant practice, it was felt that the tests would provide a rapid survey of important variables. The filter element consisted of a piece of 24 X 110 mesh, Dutch weave, stainless steel wire cloth, sandwiched between two 2-inch No. 920 Garlock flat pipe gaskets. The wire and gasket "sandM ich" was glued together with Permatex Form-A-Gasket Compound No. 1 and placed in a press until the glue hardened. I n operation, the filter element was clamped between the upper sulfur reservoir and filtrate receiver flanges. Actual filtration area was equivalent to a 2-inch-diameter circle (3.14 sq. inches). T o precoat, 2.65 grams of filter aid were added to 1800 grams of molten sulfur filtrate a t 144' to 155' C. collected from a previous run. This gave a precoat approximateIy '/*-inch thick. T h e precoat mix was stirred thoroughly while Nichrome heating strips on the equipment were adjusted to give proper temperatures; it was then poured into the upper reservoir and slowly agitated while vacuum was applied to the filtrate receiver. When only 0.5 inch of sulfur remained above the filter element, the vacuum was carefully released and sulfur filtrate drained from the receiver. After precoating, a test charge of 5400 grams of molten sulfur was carefully added to the upper reservoir. During addition, a 4-inch deflection disk of wire cloth equipped with a pull wire was placed above the filter element and its precoat, to minimize disturbance. The reservoir agitation was started at a speed
Table II.
F 0.1
-
-.
n
lir
I
W 0-3
Figure 2. In this typical correlation of laboratory sulfur filtration data, straight lines were obtained with an occasional curvature near the origin. From these plots, specific cake resistances and subsequently cake compressibilities were calculated
of about 300 r.p.m., which was just enough to keep the sulfur moving slo~.ly. The deflection disk was removed, and vacuum was applied to the filtrate receiver. Filtrate volume and time readings were started hvhen filtrate began to flow into the receiver. I n runs simulating continuous addition of filter aid, the calculated quantity to be added was mixed with the test charge before pouring into the filter. Filter aid was obtained from the JohnsManville Corp. Comparable grades can be furnished by the Dicalite Division, Great Lakes Carbon Corp. Results. Raw data were correlated by plotting e / ( V / A ) as ordinate us. V I A as abscissa. Excellent straight lines were obtained, although occasionally a slight curvature was noted near the origin (Figure 2). Specific cake resistances (a)obtained from comparable runs at different pressures were used to calculate cake compressibility (S) by the relationship
S varies from zero for rigid incompressible
Specific Cake Resistance and Initial Resistance, Plant Tests
Values were calculated from graphs plotted from data in Table I POW
-9
cy,
Run No.
Hr./Sq. Ft.
Ft./Lb. x 10-11
1
0.0654 0.0415 0.0396
8.20 5.55 3.89
O
2
4
P
w
- 9
0 3
Hr./Ft.
l/Ft.
r,
x
10-lO
0.2
5.4
0.1
2.70
...
e . .
cakes to 1 for very highly compressible cakes (4). Table I11 summarizes the test conditions and results. Hydrated lime was added in runs 16, 17, and 18 because this material is frequently used to neutralize acidity in molten sulfur. Application of laboratory Data
Continuous addition of filter aid to dark sulfur drastically reduces cake resistance, which otherwise is excessivelv high. Continuous addition is also beneficial in the case of bright sulfur, but its relative importance is less. Specific cake resistance and compressibility vary inversely with filter aid particle size. A regular decrease in cake resistance occurs as filter aid is changed from Celite 512 (a relatively fine grade) through Hyflo Supercel to Celite 503 (a coarse grade). O n the other hand, filtration efficiency decreases slightly. Filtration efficiency is poor in the absence of continuous filter aid addition. However, with continuous filter aid addition, filtrate ash content will normally range 0.0005 to 0.005%, which is satisfactory. Addition of hydrated lime to sulfur adversely affects filtration rate and cake compressibility. There is less effect when lime is added subsequent to precoating. These conclusions were verified in the plant by operation of a vertical leaf-type pressure filter which was installed to replace the former carbon tube filters. Continuous filter aid addition and a coarse grade of filter aid led to a significant increase in filtration rate in this VOL. 51, NO. 2
FEBRUARY 1959
167
unit, without appreciable sacrifice in efficiency. T h e large filtration area provided by the leaf filter was also of considerable benefit. Average on-stream filtration rate in this equipment normally varies between 0.04 and 0.06 ton of sulfur per square foot per hour, when filtering dark sulfur, which is equivalent to about 0.09 to 0.135 gallon per minute per square foot.
Conclusions
Increased rates can be obtained from existing equipment by increasing both the amount of filter aid used to precoat and that added continuously, by cleaning filters more frequently, and by using relatively coarse filter aids (Table IV). These measures give a more open, porous cake, which is a slightly less efficient barrier for small ash particles. Con-
Table I l l .
sequently, if extremely high filtration efficiency is desired, some sacrifice in rate must be made. T h e future trend will possibly be toward the use of combustible filter aids based on cellulose or carbon in place of diatomaceous earth. Should a break in a filter screen or tube occur, these materials will be presumably destroyed in the sulfur burner without fouling subsequent equipment. However, these materials are more expensive than diatomaceous earth, and their use may not be justified when filter elements are carefully watched and/or protected from acidity. There is already evidence of a trend toward shipment and use of molten sulfur. Where the economics are favorable, this course has much to recommend it. One benefit is that amount of solid contamination is frequently reduced to a level low enough that filtration is not needed.
Laboratory Filtration Tests
All tests were made with simulated continuous addition of filter aid
Run
Type of Filter Aid
Xo.
AP,
P.S.I.
Final .4sh in Sulfur, %
Specific Resistance a ,Ft./Lb. X 10-0
except as noted
Initial Resistance, T , l/Ft. X 10-g
Cake Com-
pressibility S
Texas Bright Sulfur“ Hyflo Supercel Hyflo Supercel Celite 503 Celite 503 Celite 512 Celite 512 Hyflo Supercelb
2 3 4 5 6 7 8
11.8 3.9 11.7 3.8 11.8 3.9 11.8
0.0007 0.0007 0.0013 0.0005 0.0005 0.0002 0.002
1.6 0.75 0.75 0.12 1.4 1.6 1.9
5.62 3.37 2.87 2.21 12.3 6.87 14.0
0.5 0.5 0.2 0.2 0.5 0.5
...
Louisiana Dark Sulfur“ Celite 503 Celite 503 Celite 503 Celite 503 Hyflo Supercele Celite 503e Celite 503e Celite 50Jr Celite 5030 Celite 503g
9d 10d 11 12 13 14 15 16 17 18
2.1 11.8 3.9 11.8 11.8 11.8 3.9 11.8 11.8 3.9
0.008 0.003 0.001 0.003 0.013 0.007 0.012 0.003 0.012 0.008
7.60 16.0 27.0 29.8 1880 1190 500 32.0 32.9 14.2
1.3 2.8 1.3 3.6 -5.7 4.9 1.8 21 4.9 4.3
0.4 0.4 0.1 0.1
Type of Sulfur Bright Bright Dark
desired, some sacrifice in rate
must b e m a d e
Continuous Filter Aid Feed, % of Charge
Filter Area/ Ton S/ 24 Hr., Sq. Ft.
Av. Rate, Gal./Min./ Sq. Ft.
Precoat, Lb./Sq. Ft.
Time On-Stream Between Cleaning, Hr.
0 0.03-0.05 0.05
0.5-1.0 0.15-0.30 1.0-2.0
0.1-0.2 0.33-0.67 0.05-0.1
0.3-0.4 0.2 0.4
24-72 24-48 7-23
INDUSTRIAL AND ENGINEERING CHEMISTRY
literature Cited (1) Adams, R . P., Co., Buffalo, N. Y . , Tech. Data Sheet TD-2074 (1948). (21 American Machine & Metals. Inc.. Niagara Filters Division, East Moline; Ill., Bull. NS-2-53 (1953). (3) Bird Machine Co., South Walpole, Mass., Bird Extracts, No. 44 (February 1956). (4) “Chemical Engineers’ Handbook,” J. H. Perry, ed., 3rd ed., p. 965, McGrawHill, New York, 1950. (5, Houston, G. N. (to Mathieson Chemical Co.), U. S. Patent 2,637,630 (May 5, 1953). (6) Hurt, D. M., Stafford, G. S., Tauch, E. J. (to E. I. du Pont de Nemours & Co.), Zbzd.,2,402,553 (June 25, 1946). (7) Lee, J. A,, Chem. Eng. 55, 119-21 (April 1948). (8) Ridler, E. S. (to E. I. d u Pont de Nemours & Co.). U. S.Patent 2,295,605 (Sept. 15,1942). ( 9 ) Shearon, W. H., Jr., Pollard, J. H., IND.ENG.CHEM.42,2188-98 (1950). (10) Swem, H . A. (to Texas Gulf Sulphur Co.), U. S. Patent 2,249,063 (July 15, 1941). (11) Tauch. E. J. (to E. I . du Pont de Nemours & Co.),Zbzd., 2,300,849 (Nov. 3, 1942). (12) Yeiser, F. M. (to Niagara Research Corp.), Zbid., 2,459,764 (Jan. 18, 1949). I
,
RECEIVED for review March 24, 1958 ACCEPTED September 15, 1958 Division of Industrial and Engineering Chemistry, 133rd Meeting, ACS, San Francisco, Calif., -4pril 1958.
...
0.8 0.8
Pressure, 50-60 p.s.i.; diatomaceous filter aid; filtrate ash content, 0.0005-0.005%.
168
H. MacDonald for help in taking data, W. J. Gresham and his laboratory staff for sulfur analyses, and the staffs of Freeport Sulphur Co. and Texas Gulf Sulphur Co. for information on sulfur properties and analysis.
...
Recommended Conditions for Plant Filtration of Sulfur“
If high filtration efficiency i s
Thanks are due H. L. Furman and M. P. Lux for advice and assistance, D.
0.8 0.8
a Organics (as C) 0.01 to 0.08%; initial ash content 0.03 to 0.05%: precoat 2.65grains; addiOrganics (as C) 0.30 to 0.33%; No additional filter aid used. tional filter aid 0.1% of charge. initial ash content 0.01 to 0.04% ; precoat 2.65 grams, additional filter aid 0.1% of charge. These runs made with Texas dark sulfur containing 0.56% carbon. e T o additional filter aid added 0.1% afterprecoat. f Precoat contained 7.14 grams hydrated lime in addition to filter aid. hydrated lime added to charge in addition t o filter aid.
Table IV.
Acknowledgment
Correction Influence of Power Input on Efficiency of Dust Scrubbers I n the article on “Influence of Power Input on Efficiency of Dust Scrubbers” [K. T. Semrau, C. W. Marynowski, K. E. Lunde, C. E. Lapple, IND. EKG. CHEM.50, 1615 (1958)l the fourth paragraph, second column, on page 1616 should read: “ I n most tests a t the recovery furnace, two scrubber runs were made consecutively. Separate sampling runs were made a t the scrubber outlet, but only a single run a t the inlet.” In Figure 8 the solid symbols refer to tests on raw gas, and the open symbols to those on prewashed gas. I n Figure 1 there is an error; the inlet pipe of the cyclone, which is shown as V4‘’I.P.S., should actually be 3 ’*” I.P.S.
