November 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
CORROSION OF EQUIPMENT Corrosion of the commercial plant equipment (9, 9 ) waa much less severe than was anticipated when the plant waa designed. The dump trucks, dragline machines, and tray feeder on the melter s d e r e d most from corrosion attack because of their intimate contact with the highly acid, wet bin-bottom sulfur. Most of this attack waa confined to the easily repaired truck beds and dragline buckets. The reciprocating tray of the sulfur feeder waa lined with aluminum plate after its fmt failure and thereafter gave very good service. The %ton hopper on the sulfur feeder was lined with aluminum plate in its original installation and gave excellent service. The 3-iich steam coils in the melting pit showed very little, if any, indication of Corrosion attack. This is especially noteworthy because the original coils were fabricated of second-hand, galvanized steel pipe from which most of the galvanizing was gone. This experience indicates that aa long as the coils are hot, common black pipe will be satisfactory in this type of service. One equipment failure, although it was not especially serious, served to illustrate the conditions which are particularly contributive to corrosion attack on steel. A basket-type, coarse strainer, located in the discharge line of the melting pit pump, naa housed in a steam-jacketed pressure vessel. A small amount of air waa bled into the top of the vessel a t all times to facilitate the measurement of line pressure by an ordinary Bourdon gage. The air waa moist, and the sulfur flowing through the screen vessel contained some residual acid. Severe internal corrosion of the
2215
steel vessel occurred a t the air-sulfur interface near the top. Here, air, moisture, and sulfur in direct contact constituted extremely corrosive conditions. In the Kelly filter, steam-jacketed sulfur linea, and storage tanks, no detrimental corrosion occurred. The fact that there was no noticeable corrosion in the filter, which also had a small amount of air flowing into it through the pressure gaging system similar to that on the screen vessel, can best be explained by the fact that no air-sulfur interface was allowed to exist. The air flowing into the filter was continuously removed by a bleeder in the top of the filter through which.part of the sulfur feed was recirculated to the storage tank throughout the filtering cycle. By this means the filter was maintained full of sulfur at all times, and no static air-sulfur interface was ever formed.
LITERATURE CITED (1)Bacon, R. F.,and Fanelli, R., IND. ENO. CREM.,34, 1043-8 (1942). (2) Butterworth, C.E., and Sohwab, J. W., Ibid., 30, 746-51 (1938). (3) Chem. &Met. Eng., 39,392(1932). (4) Frasch, Herman, J. TND.
[email protected].,4, 134-40 (1912). (6)Lundy, W. T.,Trans. Am. Inst. Mining Met. Enprs., 109,354 (1934). (6) Mason, D. B., IND. ENG.CHEW,30,74C-6 (1938). (7) Pough, F. H., Ibid., 4, 143-7 (1912). (8) Schwab, J. W., and Duecker, W. W., Chem. & Met. Bng., 44, 441-2 (1937). (9) West, J. R., Ibid., 53,225 (October 1946). ,----I
REIC~IWD Msroh 27, 1050.
IMPROVEMENTS IN ROTARY SULFUR BURNER ALFRED LIPPMAN, JR. Bay Chemical Company, Weeks, La.
A description is given of four improvementa in a rotary sulfur burner which permit the burning of sulfur even with a high bituminous content, automatically and at much greater capacity, unirormity, and safety. Them improvements comprise: tilting the burner to establish a continuous overflow of sulfur to purge bituminous material;
fin^ within burner to increase interior surface area and to cause droplete of molten sulfur to fall through hot gas to ash bituminous matter; insulation on shell to utilise heat more effectively ; and a unique automatic level controller for smooth, economical, and safe operation. The combination has increased burner capacity over 2.5 times.
T
in the sulfur dioxide gas concentration: ( a ) the adjustable ports N , on Bend of burner; (b) the length of gap 1; ( c ) the adjustable damper, L; and ( d ) the draft applied to the combustion chamber. The burning rate can be controlled accurately by regulation of the draft at the discharge end of the burner, indicated by a diaphragm-type draft gage reading up to 0.30 inch of water in 0.01inch divisions, with its tube M placed through gap 1 and extending slightly into end B‘.
HE well-known rotary sulfur burner consists of a horizontal steel cylinder, A, with cast-iron fnrstro-conical ends, B, B’,
Figure 1. The burner rotates about 1 r.p.m. on trunnion rings, C, C’, to keep the interior surfaces wet with molten sulfur from the pool, 2, maintained by either liquid or solid feed. Air drawn through the burner by 0.25 to 2 inches of draft a p plied at the discharge end of combustion chamber D, sweeps out the sulfur vapors evolved from the hot molten sulfur and oxidizes some of the sulfur vapor within the burner to provide heat necessary for the melting and volatilization of the sulfur feed.
CONVENTIONAL CONTROLS OF BURNER Air paasing through the burner determines the rate of sulfur volatilization and consumption; while air entering gap 1and combustion chamber D through port L,provides for the combustion of sulfur vapor evolved from the burner, and determines the concentration of the sulfur dioxide gaa product. Therefore, the foibwing controls allow a considerable range in the sulfur burning rate and
BURNING OF SULFUR WHICH CONTAINS BITUMINOUS MATTER Sulfur virtually devoid of bituminous matter burns well in an ordinary rotary burner, but some sulfur indigenous to the Gulf Coast contains about O.&% bituminous matter which makes a tarry scum over the surface of the sulfur pool, thereby seriously reducing the capacity. In time, large carbonaceous lumps may be found floating on top of the sulfur pool; these lumps reduce the capacity still further by impeding gaa flow through the burner.
2216
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 42, No. 11
correct level of the sulfur pool instead of relying on costly and less dependable manual control. Conventional level controllers were found to be virtually inoperative because of separate or conjoint action of the following factors: the changing density and viscosity of the pool, the l u m p of melting sulfur, the carbonaceous matter, the limited space, the currents of hot gases and liquids, the rotation of the burner, and the nonconductivity of sulfur. The level and feed controllers that were developed made u8e of the fact that, over a wide range in burning rates, a difference of over 300 a F. exists between the temperature of the sulfur pool which averages about 500" F., and of the burning gaa which averages about 825 F. The temperature-sensitive element (thermobulb J ) of temperature controller K , Figure 1, is placed at the desired mean level of the pool. The bulb should be in a protective sheath of 1-inch galvaniaed or stainless steel pipe. The maximum temperature is set on a controller at 560' F. and the minimum at 540" F. Lowering of the sulfur level exposes more of the thermobulb to the hot gas so as to increase the temperature of the bulb to 560" F., whereupon the temperature controller, K , s t a t e motor G and feed screw F , to charge the burner with solid sulfur from hopper E at a rate in excess of the maximum burning rate. The level of the pool gradually rises, immersing the thermobulb until the temperature drops to the minimum setting of 540' F., whereupon the lowtemperature relay of K stops the feed. This simple Figure 1. Tilted Rotary Sulfur Burner with Automatic mechanism maintains the level constant to within 0.6 Level Control inch of mean level near the feed end, and to less than 0.25 inch at the discharge or overflow end. The level While the overflow eliminates accumulation and lump forcontroller could also operate a valve or pump in installations mation of bituminous matter, nevertheless some scum still forms feeding liquid sulfur. The optimum temperature settings can prior to reaching the discharge end, so that the second improve be readily determined for each burner installation. ment comprises the installation of four fins attached to brackets riveted to and extending radially from the burner shell. These are shown in longitudinal and sectional (4-4) views in Figure 2. As a fin rises through the surface of pool, it lifts scum and molten sulfur with it; as the fin ascends further, a shower of droplets falls from it into the hot gas stream, wherein the sulfur content is quickly vaporized and bituminous content turned into light ash which does not form scum or lump, but which is carried away by the gas stream. The fins not only reduce scum, but also very substantially increase the surface area of contact between molten sulfur and air, thus augmenting burner capacity. It is important to provide a space, 5 (Figure 2), between fin and burner shell to prevent excessive turbulence or wave action in the pool S EC T I 0N 8-4" F I N SY 4I which then would tend to mix scum with mass of sulfur and render 4 less effective the purging and oxidation of scum. Figure 2. fins and Insulation in a Rotary Sulfur Burner Thirdly, insulation as shown in Figure 2 is applied to the burner shell to provide a hotter gas for speedier evaporation of sulfur from the droplets and to increase the capacity of the burner SUMMARY by reduction of heat loss to the atmosphere. These three items-the tilting, the fins, and the insulationThe level controller costs about 100 to 125 dollars completed permitted smooth and satisfactory burning of sulfur containing and installed and should prove useful in any of the following bituminous matter, Maintenance and repair of the modified applications : burner averaged only 400 dollars annually, including semiannual cleaning out of accumulated gravel and other extraneous 1. It controls the sulfur feed automatically even with wide material. variation in the rate of burning, thereby eliminating operating labor. 2. It provides simple control of a solid feed, so that the more AUTOMATIC LEVEL AND FEED CONTROLLER costly equipment and expense of a sulfur melter and liquid feeder may be eliminated. The fourth and possibly most important improvement com3. The burning rate may be kept constant at any selected prises making the burner function automatically and safely. figure over a wide range simply by controlling the draft at gap 1, via gage tube M , in a burner equipped with the level controller; The pool must be kept at the correct depth, because an excesthe rate of burning is then determined by the draft and may be sively high level would result in copious overflow of molten sulfur readily kept) as constant as 2% of a desired figure within capacity while a low level would result in hazardous overheating of the range. With constant drafts, the sulfur dioxide concentration burner. Therefore, it waa necessary to have a controller in the also has been maintained within 2% of a desired figure in the 8 to 15% range. burner which would regulate the feed so as to maintain the The troublesome effect of bituminous matter was overcome by three modifications. First, by lowering the discharge end of the burner about 2 inches below the feed end as shown in Figure 1. This tilting along axis 3-3', a t angle a with the horizontal, permits a very small continuous purging overflow of scum from thesulfur pool, which drops into cart H provided with ports I , through which air is drawn by draft extant at gap 1, so that the sulfur carried over with bituminous material in the overflow, coritinues to burn in the cart until about 4.41pounds of residue per ton of sulfur charged to burner are left. About 62% of the residue is sulfur, representing an over-all sulfur loss of only 0.14%.
O
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1950
4. I n any sulfur burner, the draft or the sulfur feed may fail, as from mechanical causes, and such failure could result in haeardous overflow from the burner or in overheating of the burner. Safety demands installation of the level device on every rotary burner to warn of excessively high or low level even if not to control the feed. 5. The controller in conjunction with the three other items permits the automatic and safe burning of sulfur, even that containing bituminous matter. A 4 X 10 foot burner containing bituminous matter ran up to 2100 pounds per hour of sulfur with
2217
these four improvements in comparison to 800 paunds maximum originally. The four improvementa which were discussed have been in succeasful operation a t the Bay Chemical Company plant a t Weeks, La., for 6 years, but prosecution of the patent application on the controller (now U. S. Patent 2,498,089, February 21, 1950) has prevented an earlier presentation. The controller will be available for license. RECErVlD
November 5 , 1919.
POLYSULFIDE POLYMERS E. M. FETTES AND J. S. JORCZAK Thiokol Corporation, Trenton, N. J. T h e chemistry of the condensation polymerization is reviewed briefly. The structures of the polymeric products as well as the effects of copolymerization, mom-linking, end groups, and molecular weight upon polymer properties are discussed. The composition and properties of the commercial crude rubbers, water dispersions, and
liquid polymem are presented together with ideas on the mechanism of their vulcanization. The reason for the odor of polysulfide polymers is discussed. Some information i s presented on solvent resistance, stress relaxation, and other charaoteristics of the polymers. Applications for the different types of products are summarized.
S
Reaction of sodium sulfide with sulfur NrtlS S d N a & 3. Reaction of sodium hydroxide, sodium hydrosulfide, and sulfur NaOH NaSH S -+ N a A HIO 4. Reaction of sodium with sulfur 2 Na 2 S --+ Na&
INCE the commercial introduction in 1929 of the polysulfide polymers they have been utilized for many types of applications in many industries. Their use is not large in volume compared with general purpose synthetic rubbers, but their excellent oil and solvent resistance as well as good aging properties make them valuable for special applications. Although at one time Dow Chemical Company produced material for the Thiokol Corporation, all of the polysulfide polymers sold under the name Thiokol are now manufactured a t Trenton, N. J. During the war, Dunlop Rubber, Ltd., in England.and Naugatuck Chemicals, Ltd., of Canada manufactured Thiokol under license. Polysulfide polymers are manufactured in other countries also, but these products will not be discussed. Other reviews (16,LW,40,47,60,66,66,63,67) have been made on the polysulfide polymers, but these have covered only the older polymers. The purpose of this article is to describe all of the crude rubbers, latices, and liquid polymers now manufactured. The variations that can be introduced to provide new materials with almost any desired characteristics will also be stressed.
POLYMER PREPARATION The preparation of polysulfide polymers (47, 48,66) is an example of condensation polymerization consisting simply of the reaction of an alkaline polysulfide with a suitable dihalide as illustrated by the reaction of ethylene dichloride with aqueous sodium tetrasulfide: NaS,Na
+ CICHZCH~CI+(CHzCH&), + 2NaCI
By using a dispersing agent such as rnagncsium hydroxide the polymer is obtained in the form of a suspension of particles 3 to 15 microns in size. Becauae of this particle sire and the high specific gravity of the polymer, the dispersion settles readily to permit washing free of electrolytes by successive decantations. INORGANIC POLYSULFIDES
There are several methods of preparing solutions of sodium polysulfide of varying ranks. 1. Reaction of sodium hydroxide with sulfur
6NaOH
+ 6s +2N& + 3HgO + NaaSzOa
2.
+
+
+
+
+
Rank is defined as the number of sulfur atoms present in the anion, thus sodium disulfide has a rank of 2.0 and calcium tetra sulfide, a rank of 4.0. A sodium polysulfide of rank 1.5 represents an equimolar mixture of sodivm monosulfideand sodium disulfide. The reaction of sodium hydroxide with sulfur is at present the most economical method for producing sodium polysulfide solutions of rank 2.00to 4.50. The sodium thiosulfate produced by the reaction does not influence the polymerization reaction. I t is known that aliphatic halides will react with sodium thiosulfate to form sodium alkyl thiosulfates. These salts can be converted to disulfides by oxidizing agents such as iodine and hydrogen peroxide (66). If these types of compounds are formed in the reaction, they must be converted to disulfides by the alkaline sodium polysulfide solution present. Methods 2 and 3 are useful in preparing sodium polysulfide solutions of ranks between 1.00and 2.00 which are difficult to prepare from sodium hydroxide and sulfur without relatively long reaction times or without use of temperatures above the boiling point of the solution. Method 4 is useful in preparing pure sodium polysulfides for research investigations. The reaction can be conducted in a solvent such as alcohol or in liquid ammonia (68). DIHALIDES
A large number of organic dihalides can be used in the preparation of polysulfide polymers, although the number of halides commercially available is very small, The suitable halides which are industrially available are: methylene dichloride, ethylene dichloride, propylene dichloride, glycerol dichlorohydrin, dichloroethyl ether, dichloroethyl formal, and triglycol dichloride. Satisfactory polymem and copolymers can be prepared from all of these raw materials. Polymers have also been prepared from di-
