Modern Production of Chlorine and Caustic Soda WILL H. SHEARON, JH. Asoociate Editor
#D.
FRANK CHRENCIK ANI) C. L. DICKINSON
In collaboration with
NE of the favorite discussion questions in freshman chen-
lstry ooums over s long period of time has run something like this: “If you were in a location where there were plenty of water’and an adequate supply of electric power, and no raw materials hut common salt could he obtained, what kind of a basic ohemieal industry could you build up, and how would you go ahout it?’ T o the student, the solution to this problem has always proved breath-taking in its possibilities and scope, once he has done some thinking on it; t o alkali men, i t is an old and relatively straightforward matter, but out of it, with constant additions Bind improvements, has come the alkali industry as we know it today. Yet the undertaking has grown in the last decade t o such proportions as to astonish even those closest to it. Ten years ago thc daily production of chlorine by all methods was less than 1400 tuns, that. of caustic 3300. It is estimated (10)that by the end of this year the total United St,ates production n d l be 5000 tons daily or 1,800,000 tons annually of chlorine, and 5500 tons daily of caustic sods. Of that national production, the Southwest, which has made the most remarkable expansion record among t,he country’s alkali-producing areas, will average 20%; yet it. was not until 1934 that caustic soda (by the lime-soda process) was produced in the Southwest (6) and the first chlorine plant in that area went into opefation in 1937. The reason for this remarkahle growth is relatively simplefor chlorine, the unprecedented growth of synthetic organic ohemicds; for caustic soda, the tremendous growth of the rayon industry (from 35,000,000 to 700,000,000 pounds in 25 years). It has been stated (13) by Machlullin that the demand far chlorine is roughly proportional to the demand for synthetic organics. To show this he lists the current major applioations of ohlorine as GOY’ of the total produotion in chlorinated products, 21% in the paper industry, 5% in textiles, and 6% in sanitation. Of the chlorinated products, 20% goes into honrene chlorination, 30% into cleaning Huids and refrigerant,s, and IG% into ethylene chlorination (ot,herthan cleaning Huids). Chlorine production in America is by some adaptation of either the diaphragm or the mercury cell; in Germany there has also been some production of chlorine by electrolysis of by-product hydrochloric acid (92). A process such as this was advocated by the editors of this journal in 1941 (8), as a method of reducing the waste of the by-product hydrochloric acid from ohlorination processes and increasing chlorine output. Although the first, commercial mercury cell plant in America WBS built in 1895, only ahout 5% of the total production capacity today in this country is by mercury cell. I n comparing the two American methods (121, it is found that, capital invqtment and production costs are about the same, and that the diaphragm process is favored where oheap salt is available, the mercury cell where electric power is cheap. The other romvarison point, land it is an important o m ) is in regard to the
Houston Plant, Diamond Alkali Company, Houston, Tex.
camtic produced. Where regulsr grade caustic is desired, tla‘ diaphragm cell is slizht,ly favored; where rayon gcade caustic is to he produced, it is desirable t o use the mercury cell. Th*, mercury cell does not require as extensive pretreatment of briiw to remove impurities as does the diaphragm cell. nor is evaporation of mercury cell caustic neoassary for a 50 to 70% caustic, hut it does require dechlorination of the brine hefore resaturation, a higher degree of skill, more labor, and more electric current. The time may come when most of the country’s caustic soda requirements will be met by electrolytic methods. In that event, plants using the diaphragm cell will have to inst,aU purification units to produce a higher grade oaustie. The horizontal cell developed by Mathieson Alkali Company (3’) is a good example of modern mercury cells. It is rated at 15,000 amperes, has 20 anodes, and should operate a t a current density of 2.5 to 3.0 amperes per square inch, with a 95% currenl effioiency. Krebs mercury cells, rated at. 24,000~amperes, itre being installed by Dominion Chemicals, Beauharnois, Que., and me scheduled t o go into operation in January 1949. The latest switoh to mercury cells is by the Dow Chemioal Company, which is reported to be installing the Krebs cell and a modified I. G. Farhen cell a t its Ssmia, Ontario, plant. Those are the first uses in North America of the German cell design. Another method for producing chlorine has been investigated. I n 1941, when there was not tho short,age of caustic whioh we find today, Hixson (5)stated that “even under normal conditions present electrolytic chlorine processes are not economically suited for expansion to meet requirements because of the necessity for disposing of by-product soda,” and reported experiments showing the possibilities of producing chlorine without caustio by the reaction of sulfur trioxide with sodium chloride. anhydrous sodium sulfate being the only other final product Earlier still, the editors of this iournd (7) had greeted with considerable satisfaction the announcement that far the first time chlorine without caustic was to bo produced on a commercial scale by reaction of nitric acid with sodium chloride; the end product was sodium nitrate. Total production of chlorine in t,his country ,in 1947, by processes not involving production of caustic (Q), amounted to 282 tons (16). DIAMOND ALKALI AT HOUSTON
When the Diamond Alkali Company picked Houston, Teu., as the site for its newest and largest chlorine plant (220 tons per day), i t did so after having examined sites all over the country, nnd finding that the Houston area bad almost everything in its favor. In addition to a readily accessible source of cheap brine, es rates in tho nationHouston has the lowest over-all ut it has the cheapest rates for natural gas for fuel and power, and is one of the cheapest electric power area8 in the country. Product outlet considerations weigh heavily in the determination of plant, 2002
November 1948
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
both products of electrolysis-chlorine in the groiving petrochemical industry, caustic soda in petroleum refining. I n this particular case there was a chlorine demand of unusual size, this coupled with the export possibilities (Houston is the nation’s fourth largest port and Diamond has frontage on the Ship Channel where a dock may be built at some future date), was one of the determining factors in the decision. Diamond’s plant is not the largest chlorine installation in the Southwest-those of Don, at Velasco and Freeport, Tex., and Southern Alkali at Lake Cha-les, La., a t least, are somewhat larger. It does employ the late5t in design and in engineering techniques, however, and is considered an outstanding example of modern chlorine and caustic protliiction. R 4 W MATERIALS
Diamond gets its brine supply from a salt dome a t Barber ’ Hill, Tex., 17 miles from the plant. Two water wells adjacent to the dome supply a maximum of 400,000 gallons per day eaah, one in full operation and one as a spare. Water from thew \\-ells is forced down into the salt dome and the resulting brine is pumped into a 1,000,000-gallon brine reservoir a t the well site. From this reservoir a 10-inch cast iron pipe line carries the bline to raw brine storage at the Houston Works. Because the plant is on the opposite side of the Houston Ship Channel from Barbers’ Hill, i t was necessary to run the brine pipe line under the Shir) Channel, a neat engineering trick in itself. Flexible joint pipe was used, the len t h to bridge the channel was made up and tested on shore, a cabfe was put through it, and it was pulled across the channel and buried 6 feet deep in the bottom. h spare pipe was also laid, l e ~ sthan 2 hours being required to put this second line across the channel. Before reaching the channel an old lake bed, quagnurelike in nature, had to be crossed i\s the lake bed consisted of about 1 foot of water and 12 feet of mud a t high tide, it was necessary t o use mud-boats in laying the line. These boats were regular flat-bottomed skiffs with motors on which airplane propellers had been mounted. The line was prefabricated, a cable operating from a large drum was put through it, and the line was pulled across on the cable and pushed from behind by bulldozers.
down in entirety. Normally, however, some one or more units in a particular system might have to be shut down for cleaning, repair, etc. I n such a case i t is not, necessary t o shut down the entire system. On the contrary, the problem resolves itself into simply applying a n additional load on other operable units in the systems. This means, of course, that the total output ~vouldbe lowered somewhat or that the efficiency of some of the operations might be less than optimum, but as a temporary expedient i t has proved very satisfactory. Only the equipment and process in one system are described here; there are actually double the number of units mentioned in each case. The raw brine is pumped from the st,oritge reservoir to a “hot process” treat,mentsystem for removal of impurities. Convent,ional brine-treating systems employ the so-called “cold process,” which requires more reaction time and greater equipment spacc for the same treatment. The hot process also has a n additional advantage over the cold process in t>hati t increases the effectivcness of impurity removal. This hot, process system was designed by Diamond’s engineers on the basis of experimental data obtained by the development division over a period of several years, which brought to light some new and interesting information regarding the scaling characteristics of calcium sulfate and brine. It was this information which made the hot prooes possible. a l l equipment in the plant is of carbon st,eel, with the except,iori of nickel-clad equipment for handling hot caustic, chemical st,nne-
The ran brine has a gravity range of 1.185 1.200 a t 60”/60” F., which coriesponds to a salt content of 295 to 305 grains per liter, roJghly a 257, solution of sodium chloride. 4 tvpicalanalysis of this raw brine is as follows. lo
STORAGI: AhD PREPARATIOX. The raw brine is stored at the plant in a 1,000,000gallon open reservoir. I n order to be used in the diaphragm cells it must be free of im purities, notably calcium, iron, aluminum, and magnesium. I t therefore must be treated prior to use. Beginning with the brine preparation system and throughout the entire plant there are two systems of units for each chlorine operation: literally the chlorine plant consists of two separate and identical plants operating in parallel but completely interconnected. Such an arrangement represents advanced thinking along lines of engineering design. It is only in an extreme emergency that one of the two systems would have to be shut
2003
Figure 1.
Brine Resaturation T a n k
INDUSTRIAL AND ENGINEERING CHEMISTRY
2004
Vol. 40, No. 11
sulting in optimum salt decornpositioii arid therefore optimum effluent concentration. From the resaturation t,ank the brine p a s e s through a superheater in which the temperature is raised to approximately 180' F. 30 that there will be no sett'ling out of salt, and into the cells through Pyrex headers. An interesting addition here to usual practice is an "in-line" mixer (a mixer installed in the main brine line) between the superheater and the cells, to which dilute hydrochloric acid is fed from a head tank. This mixer serves to give a iieutral brine and helps to keep the chlorine gas strength in the cells at the proper value, an important point since a 1yo decrease in chlorine gasstrength results in approximately 1%loss in liquefaction efficiency later. Up to the point of addition of acid, a mdium carbonate concentration of 0.2 to 0.76 gram per liter and a sodium hydroxide concentration of 0.006 to 0.12 gram per liter are maintained in the brine t o ensure complete precipitation of cnalcium arid magnesium, respectively. CHLORIh-E PRODUCTION
Figure 2.
Chlorine Drj-ing Towers
ware for handling met chlorine, and glass for brine headers into the cell and chlorine exit piping. These materials, from a corrosion standpoint, were adopted from lists of recommended materials ( 1 ) as the best compromise on the basis of corrosion resistance and cost of materials. Precipit,ation of the salts of metal impurities, which would rapidly clog up ccll diaphragms in a short time unless removed, is accomplished by introducing hot brine, containing hydroxyl and carbonate ions, from the treat liquor and storage t'anks into the hot process system. Calcium is prccipitated as calcium carbonate, magnesium as magnesium hydroxide, and iron and aluminuni as the hydroxides. Traces of silica are removed by filtering. It is absolutely essential that. all calcium and magnesium be removed before the brine goes to the cells; precipitation on the cell diaphragms of the hydroxides of these metals by contact with sodium hydroxide in the cell would render the cell inoperative in a short time. The purified brine is drawn off near the top of the hot. process tank to two anthracite filters placed in parallel. These filters consist of tot,ally enclosed tanks, 78 inqhes in diameter and 60 inches high, containing a bed of anthracite coal of varying mesh. There are actually five, instead of four, in t'he two systems, so that one can be backwashed at any time without disturbing the sequence of operations, Each filter contains four sizes of anthracite sold under the name of Anthrafill. Eleven cubic feet of No. 6, 11 cubic feet of KO.4, 22 cubic feet of No. 2, and 66.5 cubic feet of 50.1 constitute the anthracite cont.ent of the entire set of filters. From the filters the treated brine may go to four storage tanks, total capacity 750,000 gallons, or directly to the brine prehcater, where i t is heated to 180" F. before passing to the 50,000-gallon brine resaturation tank (Figure 1). Salt slurry recovered from caustic evaporation is added to this tank and a continuous recycling of slurry takes place here, resulting in a situation comparable to that of the brine being passed through a bed of solid salt: optimum salt concentration (and therefore longer anode life). Concentration of salt in the entering brine, and caustic concentration in the effluent cell liquor, are two extremely iniportant points to be controlled in cell operation. Hammond and Johnson (4), in a study of factors influencing anode consumption, show that increase of salt, concentration in the feed brine d6creases anode consumption and that increased cell loads can he oompensated for by increasing feed brine concent,rations re-
The chlorine cells are of the diaphragm type ( I S ) . Of the [iuinerous types of diaphragm cells which are commonly used today, Dianiond enjoys the distinction of having developed its own exclusive diaphragm cell, and this is the first commercial installation. The principal objective in designing this cell in preference to using commercia,lly available types was to provide increased capacity per square foot of building space occupied. In comparison with this type of cell, conventional cells are generally smaller, but require morc housing spacc, morc attention, and more repair labor, Because the cell installation in any chlorine plant involves more building space than any other unit process in the plant, such a consideration means appreciable economy in structural and electrical materials. Large chlorine capacity also makes it important to pay attention t o compactness from the standpoint of avoiding operating unwieldiness. An unusual material of construction feature is the provision of Pyrex glass headers for distributing the brine to the cells. This feature, along wiih Pyrex for exit piping, was adopted for a number of reasons. Resides the fact, that glass pipe neighs less per foot of length than either chemical stoneware pipe or rubberlined iron pipe, the ability to observe pipe-line content, gas or liquid flow, and obst,ructive material is a distinct advant,age. I n additional point is that a much better joint, can he made between lengths as compared with conventional lead bands on ceramic stoneware joints, and it is difficult to foresee failure in a rubber-lined iron pipe until itj has actually occurred. Further, any irregularity of chlorine gas emission from the individual cells can be easily observed through the glass. Ot'her advantages of glass piping lie in such factors as reduced head losses, and reduced heat losses due to the lower heat transfer of glass as compared to steel. The flow of brine entering the anode compartments is controlled in proportion t o the ampere load on the cell unit by a special feed device. This may be expressed in another way by saying that the concentration of caustic in the efflucnt is maintained between 120 and 140 grams per liter. Chlorine gas from the anodes collects in the head of the cell and is piped to the liquefaction system. Hydrogen gas collecting at the steel cathode passes through a water seal and then either t o the atmosphere or to the hydrogen plant. Caustic soda, the other product of the reaction a t the cathode between sodium and water, is drawn off as an effluent or cell liquor, composed of approximately 10% caustic soda, 1670 undecomposed salt, and small quantities of inorganic salts. The cathode is separated from the anode by means of a submerged asbestos paper diaphragm instead of the usual deposited diaphragm. Use of the diaphragm to keep the cell products apart, is necessary t o prevent the formation of hypochlorous acid followcd by sodium chlorate, which is very corrosive to the materials of which the ccll is constructed. The cell operates at' atmospheric pressure and produces chlorine with very low hydrogen contamination. Since 4y0hydrogen in the noncondensables from chlorine liquefaction is considered the maximum for safety, the back-pressure is controlled very carefully within the range 0.25- t,o 0-50-inch positive pressure by man-
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IN D,V S T R I A L A N D E N G IN E E R IN G C H E M I S T R Y
ual adjustment of the main valve on the hydrogen stack. As long as positive pressure is held, air cannot enter the system and form an explosive mixture with hydrogen. Over-all cell characteristics are such that the operation is more favorable at amperage in the 10,000-ampere range than at low ranges.
COOLING.Chlorine is taken off the cells through individual 2inch Pyrex pipes to a 6-inch glass header which ties into a 14-inch ceramic main header. It is then cooled to 105" F. by water a t atmospheric temperature in a primary cooler consisting of three sections in parallel, followed by cooling with water a t 55' to 65' F. in a secondary cooler. These coolers consist of 3-inch Pyrex pipes with water cascading over the exterior surfaces. The primary coolers have a total heat transfer area of 3000 square feet and the area of the secondar cooler is 800. The gas is thus cooled within the range 60 O to $0 O F., which is considered to be as close as practicable (allowing a factor of safety) to the lower limit of 54" F., where chlorine hydrate is formed. Condensed water in considerable quantities is trapped off to waste. DRYING.The chlorine from the cooling system is still wet (dew point 70" F.) and is consequently passed through a series of three conventional ceramic plate-type surface drying towers in parallel followed by three cast-iron drying towers packed with 2 X 2 inch Raschig rings t o a depth of 20 feet (Figure 2). These towers are 4 feet in diamekwand 80 feet in height. Approximately 98% sulfurio acid is circulated continuously through each of the cast-iron towers, and the bleed-off from these towers goes to the ceramic towers, down the plates of which it drips slowly. The rate of flow in the iron-packed towers is maintained just below the flooding velocity for the towers, and the rate of drip through the ceramic towers is controlled by handoperated valves according to the results of specific gravity determinations, so as to give a sulfuric acid concentration of 80%. This acid is reclaimed, sold as by-product acid, or dropped to the sewer as current economics dictate. The inherently high pH (approximately 8.0+) of the Ship Channel water is conducive to an acid demand and therefore such dumping would not involve a stream pollution problem. From the dryer system the gas is pumped by two Nash Hytor pump units, operating under 40 to 45 pounds per square inch gage discharge pressure, t o two scrubbers in which entrained acid and organic material are removed by passing the gas through liquid
Figure 3.
2005
chlorine in an especially designed scrubber. This is a simpler means of accomplishing results similar t o those ordinarily obtained through fractional distillation (16). There are actually five Hytor pumps and three scrubbers for the two systems, which allows a spare in each case. Acid entrainment is removed in the chlorine-acid separator on the discharge side of the pump. From the scrubber the gas passes to the liquefaction system (Figure 3). Diamond's chlorine liquefaction setup is the LIQUEFACTION. outgrowth of an idea developed a t the Rocky Mountain Arsenal during World War I1 operation. Normally, chlorine liquefaction in industry has involved the use of a large number of reciprocating compressors, a coolant, a refrigerant, and the necessary equipment for handling these. The refrigerant (usually ammonia) is discharged by the compressors into a condenser where it is liquefied by water cooling. The liquid refrigerant is then admitted to a shell and tube heat exchanger in which it is on the shell side and calcium chloride brine is on the tube side; the brine is cooled by evaporation of the refrigerant, which is recycled t o the compressors. The cooled brine then passes from the exchanger t o another shell and tube vessel, in which it is now on the shell side, and chlorine is condensed on the tube side. The noncondensables are purged and liquid chlorine is drawn off. The brine leaves the chlorine condenser and passes into storage tanks, from which it is recycled by pumps to the refrigerant-calcium chloride exchanger. I n this type of refrigerating system the discharge pressure of the Nash compressors and the liquefying temperature of calcium chloride brine from the refrigerating system are the limiting factors. Further liquefaction can be obtained by the use of carbon ring compressors, compressing the sniff gas to a pressure limited by the explosive limit of the hydrogen content of the noncondensables. The original installation a t the Rocky Mountain Arsenal was a 100-ton-per-day chlorine plant with only 25 tons of chlorine liquefaction capacity, as it was anticipated that the major portion of the chlorine gas would be used directly as a gas at the arsenal in the production of toxic war gases. When the country's chlorine supply situation became critical, the War Production Board was anxious to ship liquid chlorine from the arsenal
Chlorine Liquefaction System
INDUSTRIAL AND ENGINEERING CHEMISTRY
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INDUSTRIAL AND ENGINEERING CHEMISTRY
and immediate increase of liquefaction capacity was desired. Conventional equipment was difficult to procure; therefore a Carrier 25O-ton-pe&day refrigeration unit, which 'wm s+rplus equipment at the arsend, was transferred to the chlorine plant, and a system was devised far the liquefaction of chlorine by direct heat exchange in a shell and tube condenser between Freon in the shell and chlorine gas in the tube. Because the Rocky Mountain Arsenal liquefaction system proved 80 satisfactory, is much less involved as a heat transfer problem, and contains con5iderabb less equipment than the usual industrial system,'Diamond adopted it for the new Houston plant. Figure 4 is a comparison of this system with a conventional industrial system. Three 100-ton Carrier refrigerating units, each containing a three-stage centrifugal compressor, form the nucleus of the liquefaction system. As a result, the capital investment coat has been materially reduced, approximately 10% less peweris required per ton of chlorine liquefied, and less opera& ing and maintenance attention is necessary, since trained refrigeration personnel are not required. The centrifugal compressor transfers Freon, as a vapor, from its receiver to a oondenser cooled by water a t atmospheric temperature. The liquid Freon then passes back to its receiver, which in reality is a shell and tube condenser with the Freon on the shell side. Chlorine is admitted to the tubes and is liquefied, and the liquid oklorine passes to the chlorine weigh tank. The only problem of 'any significance which Diamond 'hax in connection with this liquefaction system is in the cooling water temperature. In the summertime this water, drawn from the Houston Ship Channel, is at 95 F., requiring the qaintenance of a pressure of 6 pounds per square inch on the discharge side of the compressor and the use of a relatively high boiling Freon (F-11, boiling point -20" F.). On the entrance side of the compressor a 28-inch vacuum is always maintained, and in the winter, when eooling water temperature is around 80," F., a Freon can be used which has a boiling point of -40' F. Under these conditions the chlorine liquefaction efficiency is increased approximately 2%. From 94 to 97% of the chlorine is liquefied, and the rest, together with the 2% content of the nonpondensahles (carbon dioxide, nitrogen, hydrogen, oxygen), is passed through a twostage reciprocating compressor system with condensers. The noncondensables, together with some chlorine (the mixture is termed sniff gas), go into the hydrogen chloride furnace. Chlorine gas vaporizing from the liquid chlorine weigh tank also passes through this condenser. Liquid chlorine from the main chlorine condenser goes by gravity feed t o the chlorine weigh tank and from there may go to liquid chlorine tank cars, to small containers, or by pipe line directly to the consumer. CAUSTIC PREPARAhON
The cell liquor, which contains about 16% unreacted salt and 10% caustic (120 to 140 grams per liter), flows by gravity through individual collecting pipes for each cell into a sump, from which it is pumped to a cell liquor storage tank farm comprised of six tauks. From this tank farm it goes through a heater (heated to 180'to 190' F. by Qondensatefrom the evaporators) and is then concentrated to 50% caustic in nickel-lined double- and singleeffect evaporators. There are three sets of each of these evaporators, with barometric condensers. Salt'is removed in the second4ect and singleeffect evaporators. The caustic from the second-effect evaporator is pumped first through a conebottomed salt settler from which liquor is continuously recycled to the second effect. The settled salt flows by gravity t o a slurry tank where it is slurried by h e a m of a motor-driven agitator. Salt from the caustic coolers is also inFoduced to the slukry tank. From this tank the sluny is pumped to rotary vacuum filters. Clear liquor relatively free of salt (38% caustic, 8% salt) is drawn offthe filter or salt wheel and passes to the feed tank just ahead of the singleeffect evaporator.
m 7
Figure 4. Comparison of Diamond and Conventional Industrial Systems for Chlorine Liquefaction Such an arrangement, although consuming relatively larg? quantities of steam, is due to the fact that the equipment wax avadahle as surplus from the Defense Plant Corporation magnesium plant operated by Diamond during the war, and that an abundance of low-pressure (and otherwise waste) steam is available to tbe evaporators from the power turbines. After passing through the single effect the liquor is pumped to another salt settler (which discharges to the slurry tank) before going to the primary cooler feed tank. At this point the ooneentration of the solution is 50% caustic and 2.7% salt. I n the prima cooler the caustic is cooled to 105' F. It is then passed t h r o u g one of four 50% caustic cone-bottomed tanks where i t 1s bleached with chlorine in order to improve color, and through a secondary cooler where the temperature is further lowered to 65" to 75' F. to crystallize out additional residual salt. It is then cycled back to the 50% caustic tanks, from which it is passed throu h a Vallez fdter, which has slowl rotating circular leaves, End w%ere the last of the crystallized s a g (down to approximately 1%) is removed. If 50% is desired, the Vallez filter 19 the 6nal ste and the product IS merely put mto storage or tank cars. If 7 3 g caustic is desired, the50% solution is transferred to a feed tank for the single-effeeot evaporator in which 73% caustic is produced (Figure 5).
tau+
A t the present time solid and flake caustic ace not produced but these products will be made in the near future. Special sizeg of flake caustic will he available for the compounding trade. The salt from the salt wheel passes to the salt wheel slurry tank, where sulfates are removed bv concentration in a transfer liauor (saturated halt iolution wntiituoualy irrirrulatrd to arrive ai equilibrium solutiw of salt and sullares) which is sent M disposal. The slurrv ir rhrn numwd 1 0 x salr %trbr. ~ ~ .cone-hotmmd ..~. . ~ . ~ , wirh II slowly re;olving scraper, from which a screw conveyer (Figure 6) carries the salt to a second slurry tank. This tank has two ~ u r pores: T h r d t mag berecydnl i t , the solid 41 storagrrewldoir (cnnrrrrr, capacity 1,000,000 gallulls of brine) for urposes of Iriur rrraturalion. or it may be 3cnt throwli B Bird cLsjrfirr wd a rotary dryer before being prepared for saL. I
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This part of the plant is not yet completed, hut is scheduled for starting at a future date. ' I
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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VOl. 40, No. 11
Maude (14) describes a [ i e ~type of hydrochloric acid burner developed by ViTyandotte Chemicals Corporation. This process involves the use of a submerged burner, with absorption of the hydrogen chloridc gas in rvat,er directly in the burner itself. FILLING AND STORAGE SYSTEMS
CHLORINE.Liquid chlorine is transported by
Figure 3.
Single-Effect Evaporator For 73vo Cauetir Produrtian
air pressure from the four 50-ton weigh t,anks to tank cars, 2000-pound drums, or 150-pound cylinders. I n the past, industry was accustomed to thinking of chlorine transportation in terms of 16-ton tank cars. Diamond fills few 16-ton cars, preferring t o ship in 30- and %-ton cars, particularly the latter. Much of Diamond's present production does not go by car, nor does i t go very far. The longest liquid chlorine pipe line in the world (10,300 feet) trarisports chlorine to the nearby plant of the Shell Chemical Corporation to be uscd in the manufacture of a varied line of chlorine derivatives, such as allyl chloride, dichlorohydrin, epichlorohydrin, arid trichloropropane. This pipe is made of 3-inch schedule 80 seamless steel and is supported above thc ground on conventional steel pipe supports. Because Shell Chemical utilizes the chlorine as a gas, t,here is no necessity for insulation of the line. The liquid chlorine is forced into the pipe line a t 175 pounds per square inch pressure by air padding or forcing sniff gas into the weigh tanks; partial vaporization takes place in the process of t'ransfer to the Shell plant. CAUSTIC.Both 50 and 73Yc caustic are transported by tank car. Tank cars for the 73y6 material are insulated cars cont'aining steam coils; those for the 507;, caustic normally contain steam coils but are not necessarily insulated. For storage purposes a t the plant there are six tanks for 50Yc caustic, and three nickel-lined tanks for 73% causbic. Caustic is also delivered .to Shell Chemical Corporation b y a pipe line paralleling the chlorine pipc line.
Liquor from the salt Yettling tank which folLows the salt wheel slurry tank is pumped either back t o the slurry tank or to the three treat liquor surge and storage tanks, where the liquor for treating the raw brine to remove impurities is made up. From these tanks it is cycled continuously through a tower through which flue gas is passed, thus carbonating the hydroxide in the liquor and making the plant independent for outside sources of carbonate. Normally the treat liquor is started a t 2% caustic and carbonated down to low caustic concentration HYDROGEN CHLORIDE
The hydrogen chloride plant ail1 be put into operation in the near future to produce 33% hydrogen chloride. Capacity of this plant is 10 tons per day, calculated as 33% acid. Hydrogen from the eel!, after passing through the condensate trap, is cooled, compressed, and mised with sniff gas from the chlorine liquefaction system, and the mixture is passed into a furnace where the hydrogen and chlorine are burned to form hydrogen chloride. The resulting gas is cooled, absorbed in water, and stored as 33% hydrochloric acid.
As an indication of development in industry of types of burners used by other companies,
Figure 6.
Screw Conveyer and Salt Settling
November 1948
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Figure 7.
2009
Rectifier Building
UTILITIES
NATURAL GAS. The Houston area has the cheapest natural gas rates in the country. Plant requirements are in the neighborhood of 14,000,000 cubic feet per day, used entirely as fuel in the boiler house. WATER. In addition to the two 400,000-gallon-per-day wells for brine production a t Barbers' Hill, two wells a t the plant site produce both potable and process water (temperature 75O F.) a t a rate of 2,000,000 gallons per day each. Although it has been repeatedly stated that there is no immediate cause for alarm, it is still well recognized that the influx of industries into the Houston area in the last decade has caused and is causing a marked lowering of the water table, which may lead to a serious situation. The projected damming of the San Jacinto River is expected to ease this situation considerably. Diamond contributes to the conservation program by virtue of the fact that 58,000,000 gallons of cooling water per day are pumped from the Houston Ship Channel, and are circulated through the plant and back to the channel without contamination and with the loss of relatively small quantities of water. Diamond's method for producing refrigerated water where such is necessary for cooling caustic and chlorine t o temperatures not obtainable by use of straight channel water is to be noted particularly. Use is made of the Chilled-Vactor equipment, manufactured by the Croll-Reynolds Manufacturing Company, to give 720 gallons per minute of water a t 60" to 70" F., from 95' F. water. Steam ejection is the principle, with flashing of water under vacuum, similar to refrigeration systems in use in railway passenger cars. POWER.Except for natural gas, which is obtained from a pipe-line company, Diamond is totally independent of outside utilities. The power plant contains four boilers (900 pounds per square inch gage) each rated a t 100,000 pounds steam per hour, and 30,000-kw. total generating capacity. Alternating current at 13,800 volts is stepped doRn to the voltage required for the cells, then fed to mercury arc rectifiers and converted to direct current. Especially notable in the Diamond plant is the absence of the power poles which grace the conventional chlorine plant. Diamond prefers to place the armored power cables on the same trestles which carry pipe lines throughout the plant. In a conventional chlorine plant electrical load distribution is done from the power house by one man and the dispatching of the
load on the rectifiers is done in the cell room by another operator. At the Diamond plant one man distributes the electrical load, handles generator operation, dispatches the load on the rectifiers, and coordinates the rectifiers, all from a central dispatching station. This station is in a continuation of the rectifier building, but separated from the rectifiers by a concrete wall. Figure 7 shows the rectifier building and Figure 8 shows the instrument panel with the rectifiers visible in the room beyond. INSTRUMENTATION AND SAFETY
Throughout the plant automatic controls are used wherever possible. One of the heaviest uses of instruments is that of conductivity cells to control the quality of condensate to the boiler house. Whenever this quality is lowered by caustic or brine contamination so that the resistivity of the condensate is 7500 ohms per ml., the contaminated material is automatically dumped. The greater portion of the process instrumentation consists of pressure and flow regulators. These are generally of the diaphragm type equipped with stainless steel, Monel, or rubber valve mechanisms and coupled to recording instruments. Important examples of pressure variables that must be controlled are the pressure of chlorine to the Nash pump units and the liquefaction back pressure. In the latter case, use is made of Askania regulators controlled by an impulse line from the ceramic chlorine header leaving the cell room. Actual control of the processes is concentrated in the hands of the operators, and small plant laboratories are distributed throughout the departments. As it is necessary to have immediate results in order properly to control process operations, the operators have been trained to check by short-cut methods which indicate immediate conditions. These methods consist essentially of gravity determinations, simple titrations, and the use of graduates for measuring volumetrically such values as the salt content of slurry. Accurate confirming reports are given later by the main control laboratory, which serves as a center for control of product quality. Like many other modern chemical plants operated on as near an automatic basis as possible, the scarcity of operating personnel is striking. The duties of operating personnel are more a check and integration of automatic equipment than actual operation. The entire liquefaction system, including tank-car
2010
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Figure 8.
Vol. 40, No L I
lristrument Panel, Central Dispatching S ~ a t i o i i
and pipe-line' traiisiers, is lianciltd by o ~ i i :openitor and a h e l p ~ r per shift, and tho entire bririri tri~atnirntarea hy one operator pt'r shift. One man also handliv the brine area at Barbers' Hill. I.e.pj than 200 peoplo. including administrative and technical personnel, are requircd to opcirutc! t,hc entire 818,500,000 220-tori plant.
I n addition to UYP of corrosion-r u t rriatrrials of coIistruction for thc safety of plant eyuipnient, a i d use of a special causiicresistant paint for outsitlr tanks (IXainoiid's entire plant is v e r , ~ . striking in attractivr niultieolors. much ol which is robin's-egg are spotred a t interva 11 over the plant, arid there arc adequett,e firc extinguishers arid indoor. safety s l i o ~ . All pipe lines are banded at, i n t e n d s wit,h s t a n d a d color designations. Employees knon- that. any pipe line banded with yellow or orange is dangerous: they map carry hydrogen, caust,ic, or chlorine. Black, green, blue, or gray lines, which niay contain water, brine, air, etr., are reg:irdi:d as safe. Red indicates t k vices for fire protection: hright blue is a safety shoTvrr. I,OOKINC: A H K A L )
Uittiiiond is looking ttliead to iricxmed activity iri the Houston area. S o t only dors it crpect eventually to put, irito operat,ion its hydrogen chloride, solid caustic, and salt iniltallations, but it beliews that a guaranteed source of pure hydrogen in substantial quantities (2,000,000 wbic feet per day) may he very important t o some indust,ry in the area in t,hc not too distant future. A pan-dered metals plant is located near one of the Diamond plants in another part of the country for this reason. Diamond does not intend t o let slide its opporluni chlorinated derivatives (11)field. Kith plenty of chlorine on hand, and natural gas and cracked refinery gases available, Diamond iiiay in the next fen- yews he B figure t o be reckoned with iri thia field. Already under way are plans for tvi- and pwchloroethylene units, and other ideas are ill i h : making.
Efficiencies, 7,; Chlorine liquefaction Caustic erauoration (50 arid i U % c a l l a t i , , r C ' w t t ~ l froiii cell iiii:lor) Natural gas consumyrion, t:ioi:;uida of c ~ b i oi w t Per 1000 Bounds s t e a m r r n e r a t c d 1\Iaximu:n deily Power consumption, t i ~ m ~ a n adiskllowtt-hmv. D. C. per ton of chlorine produced Maximum dmiy povei. generittioii (,%.('.) S t e a m coilsumpiion, thousands of uoiind-. hlaximum daily (porrer and protest;) Cooling uiitcr recircnlated. thourands oi pallonr; lIaXiinuin daily
