Recoverina chlorine from waste HCI
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Hoechst-Uhde Corp. has constructed electrolysis plants which produce 300,000 short tons of chlorine annually; other plants under design will bring the aggregate chlorine output by its direct electrolysis process to 425,000 short tons per year Essentially all chlorine is presently nrnrlmmraA ~ , y y y " ~
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avoids the cost of neutralization. Process-wise, we are quite pleased with the unit. Of course, there were some mechanical and construction material problems but these problems have now been overcome In fact, Mobay is expanding by 50% the electrolytic process unit. Completion date is expected by early 1976."
mdant that special care had to be taken to ensure that the isocyante and hydrochloric acid electrolysis plants were tailored to each others needs. Two of these plants were equipped with so-called chlorine recovery facilities, in which the chlorine is almost completely recovered from the bleaching liquor obtained when the plant is started up or shut down and in the hydrogen scrubbing unit (98% yield). In the production of isocyanates, the total amount of chlorine introduced into the reaction in the form of phosgene is recovered as gaseous HCi.
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such as common salt by the conventional electrolytic method. I n 1970, some 22 million tons of this chemical were produced. At the same time, considerable quantities of aqueous hydrochloric acid or hydrogen chloride gas result each year as by-product materials from chemical manufacturing activities. These industrial wastes are also being converted to chlorine by an electrolytic process. In this way. a valuable chemical material is recovered from an otherwise discarded industrial waste strea m. Existing and proposed regulatiorIS prohibit the
Mobay plant manager Elliott "first U . S. use will be expanded 50% by 1976" dumping of this hydrochloric acid into waterways, and underground injection of such acid has raised environmental objections. The most successful designer of hydrochloric acid electrolysis plants was Friedrich Uhde GmbH. The company gained sufficient valuable experience i n the two plants it constructed in Germany. Developed through the joint efforts of Farbwerke Hoechst AG and Friedrich Uhde GmbH, the first commercial unit was constructed in 1963. The first eiectroiysis unit in the U.S. was built in 1971-1972 at the Mobay Chemical Corp. plant at Baytown, Texas. It is the largest one built to date, averaging 180 metric tons per day of chlorine production. Mobay plant manager Fred B. Elliott explains that hydrochloric acid is not easily disposed of. "The electrolysis process puts the waste by-prod16
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Sources of industrial HCi The major portion of gaseous HCi obtained in the chemical industries originates from organic chlorination proces!jes, some 50% of the chlorine used in many chlorination processes. A numt,er of other industrial HCi-producing operations include the chemical manufacturing operations for the producl!ion of isocyanates-intermediates in the production of poiyurethane polymeric materials-as well as other (:hemicals of commerce-organic Ifluorine compounds used as refrigerants and aerosol propellents and siliizones. In IIts manufacturing operation, Mobay has two or three sources of hydrogen chloride gas, all stemming from isocyanate manufacture. The majority of the Hoechst-Uhde electrolysis units have been installed and are used today with isocyanate manufacturing operations. In the early 1970's Uhde secured contacts for four other plants with a daily aggregate capacity of 520 metric tons. It is worth noting that three of these plants, including the Mobay ~~~
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Other disposal techniques Electrolysis of alkaline chlorides is still the most common method used for the production of chlorine, and as a result considerable quantities of sodium hydroxide have risen also. However, the demand for sodium hydroxide has not risen to the extent of chlorine. Consequently, there is a definite need to produce chlorine without obtaining caustic alkali as co-product. So. recovery of valuable raw materials is a must. Other processes have alleviated to a certain degree the disposal problem of excess hydrochloric acid. These processes include the oxychlorination in the production of ethylene dichloride as starting material for vinyl chloride; the use of hydrochloric acid in the metallurgical acid for pickling purposes as well as its . .
Where i n d u s t r i a l hydrochloric acid is generated Reactions
froduction of chlorine bearing solvents Production of raw materials for detergents Production of chlorination products Production of Frigens Production of silicones
3.CIS-
RCI
+ HF+ 2Si
4RCI
R
ZRzSiCI1+ H.O+ COCI.
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Production of isocyanates
RCI 3.HCI
RH
/
R F + HCI 2R2SiCI2
Si..O..Si
/R R'
+ 2HCI
+ RNH%+ RNCO + 2HCi
Open air. Electrolysis plants such as this one in France are not enclosed
use for phosphate rock digestion. Nevertheless, these processes did not constitute a universal solution to the HCI disposal problem. At the same time, there are four known processes for the production of chlorine from hydrochloric acid. Three are based on indirect electrolysis via metal salt solutions but only the direct electrolysis has been wideiy adopted and used on a commercial scale since 1964, despite the fact that the decomposition potential is considerably higher than in the case of the indirect processes. One reason chemical producers have preferred the direct process is probably because the other processes each consist of two independent stages working at different rates and efficiencies and are more costly and require larger reactions vessels. Another aspect is the fact that the high current densities that are possible in the direct process cannot be Ichieved in the indirect one.
I4ow it works In the electrolytic decomposition aqueous hydrochloric ac id, effi$ i c +h m rI n~s l.l iu7 n A Q + +b Ll i n n r ~m ,,,"-.. ..le minimum power requirements for the two 2 e at the reactions-2 CI = C12 2 e = H2 at the anode and 2H icathode, the theoretical minimum electrolysis voltage between the two electrodes is the potential of the chlorine-hydrogen cell of about 1.36 volts. In practice, this voltage will increase in line with the polarization at the two electrodes at the commonly used current densities. To this voltage at the electrodes must be added the voltage drop in the electrolyte that depends on the cell construction and the operating conditions selected. The voltage will further be influenced bv factors such as the diaphracjm permeability and the specific condiuctance of the hydrochlorlc acid. .. . The cell voltage depends on the hydrochloric acid concentration. It rises sharply for hydrochloric concentrations under 20% because of the decrease in conductivity. Optimum concentration of hydrochloric Cif
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acid for best conductivity is between 20-22% at a temperature of 80'C. At concentrations above 22% the conductivity again decreases. Hydrochloric acid at any concentration above 20% can be used in the process. LIsually, in consideration of the rising \rapor tension of the acid, its concentration is kept below 26%. Tam, .,...$ erature also affects the cell voltage. This effect is very marked at temperatures between 4O-5O0C, but less noticeable at higher temperatures Although the voltage can be improved at temperatures higher than 80"C, this value should normally not be exceeded so as not to impair the durability of construction materials used in the cell. Development progress
In 1938-42, the I.G. Farbenindus~
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working at current densities from 4-5 kA/m2. A typical electroiyzer with 30 elements is about 3.5 m long, 2.2 m wide, and requires a floor space of 4 X 4.8 m. The electrolyzers are of simple and rugged construction. The cell itself consists of a frame of phenol formaldehyde or cresol formaldehyde-based plastics. containing ducts for the products and for the feed and effluent hydrochloric acid. Fixed into the frame is the bipolar graphite plate with slots on both sides for the purpose of withdrawing the gaseous products obtained. Normally, the entire unit is of the open-air type. The gases rise along the vertical electrodes and are distributed at the upper edge of the graphite! plate through a system of ducts ii?to the discharge openings. Togethe!r with the anolyte, the chlorine is d irected toward one end and the hy drogen ~ ~ . . * ~ . , .~ . ~ . .. ., wirn me carnolyre IO me olner end. The diaphragm is fixed to the side of the frame. The electrolyzer is closed at each end by a steel plate with rubber-lined inside surface. The end plates and cell frames are held together by eight spring-loaded tie-rods in a manner similar to a pressure filter. Electric power is fed to the graphite plates mounted in the end plates through graphite nipples with stuffing boxes. The electrodes of the Hoechst-Uhde electrolyzer have an effective surface of 2.5 mz and can withstand a current load of up to 12,000 amps. The diaphgram consists of a PVC fabric specially developed for this purpose and serves to separate the electrode chambers, and consequently to prevent the two products from mixing. The electrolyzers are supported by steel frames ~
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develop hydrochloric acid electrolysis in its central German works at Bitterfeld and to dispose of excess hydrochloric acid in this manner. The company used the process on a commercial scale at Wolfen from 1942-44 when Its operations were interrupted during World War Ii. In the 1950's Farbwerke Hoechst AG, one of the successor companies formed from I. G. Farbenindustrie, and Friederich GmbH continued to develop the Bitterfeid cell. The first experimental cell had all the characteristics of today's cells. It was, in fact, the first bipolar cell to be used in the chlorine industry. In the electrolysis, two gases are formed which must not mix, whereas the anolyte and catholyte may be mixed. Todav's Hoechst-Uhde electrolvzin tiirn ~. , ... . - . . . , . r_e_d . _ nn . . _ _c.oI .r.l.m . _i .r. .inqll. _ " -~ .~, which^ ers consist of 30-36 single elements lators. ~I~
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Status of HCI electrolysis units Ca acity
Client
Location
fl?
Chlorine"
Bayer Leverkusen Farbwerke Hoechst Mitsui
Germany Germany Japan
120
Bayer Dormagen Bayer Uerdingen
Germany Germany
89 89
Bayer Shell Mobay Chemical Corp.
Belgium USA
Eurane Asahi Glass Sud ltalia Resine S.p.A.
France Japan italy
146 180 90 106
89 33
106 106
status
Built 1952163 Built 1963164 Engineeringcompleted; Plant not built Built 1968 Engineering completed; Construction deferred Built 1971172 Built 1971/72 50% expansion planned Built 1971172 Under construction Under construction
Metric tonslday Source: Hoechst-Udhe Corp.
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Volume9, Number 1, January1975
17
