NITRIC ACID IN GREAT BRITAIN - Industrial & Engineering Chemistry

OXKDATIUN OF AMMONIA

GORDON C. INSKEEP Associate Editor in

collaboration with

T. H. HENRY Imperial Chemieol Industries, Ltd., Billingham Division, Billingham, England

-'

Absorption Section and Acid Storage Tank. at the Rillingham WorLs. Imwrial Chemical Industries, Ltd.

A StaffilndusdrgCollaborative Report.

N

ITRIC acid wae first produced in laboratory quantities in 1839 by the oxidation of ammonia with air, using a heated platinum catslyst. Development of the ammonia oxidation method Was slow, and it wm not used commercially until 1909, when a small plant wm built in Germany. Today, commercial nitric acid planta are located all over the world, and production has increased ~tesdily,particularly since 1941 (me Figure 1). In the United Kingdom the only nongovernment producer of nitric acid is Imperial Chemical Industries, Ltd. Six units are in operation at the B i l l i a m works of 101. The latest unit, built in 1949, produces 50% HNO, by the oxidation of ammonia at 1.2 atmospheres absolute preeaure. In the United States the first plant for producing nitric acid by the oxidation of ammonia was built in 1916 at Warnera, N. J., by the American Cyanamid Co. Studies of this technique for producing nitric acid, carried on during World War I by the U.8. Bureau of Mines, were deecribed in an early report by P a m n s (16). The expansion of the synthetic ammonia industry in America carried with it a development of the nitric acid industry. By 1927, the conditions had changed in such a manner that i t became apparent that future production of nitric acid would moat eaonomically come from the oxidation of ammonia (17). Today, io this country, nitric acid ia produced by most of the major chemical companies, as shown in Tuble I.

dation of Ammonia Is Most mportant M e t h o d of Production Nitric acid can be produced commercially by the reaction of ulfuric acid with S O ~ ~ U I I nitrate I or by the oxidation of ammonia

..

over a catalytic gause. Prior to World War I, most of the acid wae produced from Chilean nitrate, and for several years after the war the new oxidation p l a n a found it diflicult to compete with acid fromChilean nitrate. However, today the economics have been revered and it has been estimated that lem than 10% of the commercial production of nitric acid is now from the sodium nitrate-sulfuric acid reaction (14). The sodium nitrate proceae is camed out in a direct-iired cantiron retort. The eulfuric acid and eodium nitrate are held at 150" to 200" C. for about 12 hours, during which time nitric acid, nitrogen oxides, and water distill off. The acid vapors am condensed 88 concentrated HNOt (96 to 99%) and the uncondensed Vapors and oxidee of nitrogen paas through a water absorption .E Air is introduced into a bleacher and s t r i p the lower O ~ E to S yield water-white nitric acid. A by-product of thin procem ia niter cake, which is the sodium-bisulfate and aulfwic acid residue remaining in the retort. There are e v e d variations in the ammo& oxidation technique. Both oxidation and absorption are done commercially at atmospheric and at elevated premmes. Fmly pilot plant work on the Du Pont technique of p-e oxidation has been described (SO). The principal advantage in lower capitsl cost, due to d e r equipment r8quired. Principal disadvantage ia bigher operating ooets. The approximSte emice and power requirements for a standard Du Pont nitric acid unit are as follows: Per Ton 100% " 0 , Electricity, kw.-br. 270 Steam, Ib. 600 Water, gal. 14,000

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July IS53

INDUSTRIAL AND ENGINEERING CHEMISTRY

The cnnverhr (burner) and t h e absorption ayntam operate at a preaaure of approximately 100 pounds per quare inch gage. T h e standard D u Pont unit is normally built to produce 50 to 55 tons of 100% nitric acid per day. The actual yield dependn on operating rate, a y a h preeaure, and c o o l i i water temperature. Yieldn in excena of W% are normally realised. I n Holland, nitric acid plants, built by the Dutch State Mines, oxi&e ammonia at atmospheric preasure but absorb the gaeee at 3 a t m o s p h preaeure. The conventional platinum-rhodium catalytic gauze ia used, the hot gases me cooled in a Ia Mont type boiler and a hot gas cooler and then c o m p d by rotary compmm. The conventional ammonia oxidation plant hae two main emtiOns, oxidation and absorption. I n the oxidation phaee the important reaction is 4N&

+ 50, +4NO + 6H9O + 214,600 calories

Other reactions which will take place if not suppreesed me:

+ 3(x+ 2N2 + 6&0 + 302,800 calories 4NEa + 6NO +6N. + 6H20 + 476,400 calories 2 N K %NI + 3H2 - 2 2 , Wcalories

1. 4N& 2.

3.

In the abmnption section the important reactions me: 1. 2NO

+ 0, e2N01 + 27,100calories

+ 13,000 calories 3N0, + H20+ZHNO, + NO + %',SO0

3.

IN U N ~%rA1%8 D AW or J m m 1, TABLE I. NITRICACIDPLANTS 1953 E. I. du Pont de Nemoum & Co. Birmiwhern, Ah. Hemula, Powder Co. Beasemar 'Ala. Tsnnesua Valley Authorit Sh&Tdd. Ala. lion Oil Co. Eldorado A& A he P o d e r Co., C w t h Aria. €l%ulea Powder Go..Ken&. Calif. E. I. du Pont de Nemours & 00..lousiem. Colo. E . I. du Pont de Nemoum & Co. 8eneca. Ill. 8 aneer Chemical Co. tittabym' Kon. &mm€.mid 8OlVsr.ta 60rn..st.rIington. La. Lion oil co. Luli La. Mathieson dhem% Co Lake Cbarlaa. La. Momanto Chemioal Co. '&&mtt. M e . Sponwr Chemical Co., h l s b u m . Muu. (will be Kn operation 1st. 1 0 s or url

1954)

Atka h w d s r Co. Atls. Mo. Calm Chemloal Div%i& Amoriaan Cyanamid Co.. Bound Brook, N. J. E. I. du Pont de Nsm& & Co. Glhbatown. N. J. E S W I powder ~ CO. mii in N. i. K m d w Powder Co.' K e d N J General Chemical Di'Gion h&3'Chemicsl and Dye Corn. Bufialo. N. Y. Nitr BIL Division llllied dhsmioal and Dye Corp., 8outh €bird. Ohio Atls.%wder Co. 'Fa noIda Pa. Ametiosn Cyana&d N& h t l o Pa. Gensnl & m i d EivisiAn Allied Chbmid and Dye Corp.. Newell, Pa. ""pa" DiGion Allied & m i d and Dye Corn.. HoparaU. Va. E. I u Pont de ibmoum & Co. Du Pont W d . G e m ) C h a m i d Division. A&ed C h & i d and Dye Corn.. Hadord.

80.

Wash.

E. I. du Pont de Nemovrs 6 Co.. hrhdala, Wia.

p i n t . The gases ara further cooled and oxidized in a gas mokr and oxidation chamber. The remainder of the water ia m o v e d as nitric acid. The gas contains practically pure nitrogen tetroxide (N20,) and is condensed in a b r i n w o l e d liquefier. Liquid tetroxide is mixed with the proprtiou of aqueous nitric acid to furnish the water required by the reaction 2N.0,

2. 2N0, CN,O,

calories

I n Eumpe a s v d nitric acid plants me in operation using liquid nitrogen tetmxide (7,24). Ammonia ia oxidized with pure oxy-, Using &am ae a diluent to reduce combustion temperature and to avoid explosive mixtm. The gaa temperature ia reduced from 850' C. in a waste heat boiler. Water is condensed out and disaarded. The state of midation is d u l l y Wnboled to limit f o m t i o n of N(x and minimize 1om of "0, at this

1387

+ 0,+ HzO +4HNO.

The mixture ie treated with oxygen in an autoclave at M) atmob phms p m for 4 hours. The temperature riw to 70' C. The excees nitrogen tetroxide is then stripped offby heating at atmospheric pressure. By this technique, acid of 98% concentration can be produced. It has been m e n t l y reportad ( 8 )that this pmes8 hae been improved in such a manner that it can be used for either h i or low l designed concentration acid, using6the.r oxygmn or sir. It c ~ l be for either batch or continuous operation. Plante using this Fauser (or Barnag) process have beenbuilt in Switserland, France, England, Poland, Hungary, Rumania, I d y , Spain, Japao, and G?rmany. The procees has been developed principally by Barnag-Meguin ofBerlin. Wincansin Pmesss of Thermal Fixation of Nitrogen Is Recent Development

' ' figure 1.

pmductiou of Nitric Acid in the United Stat- and the United Kingdom

One of the most recent commercial devdopmeote has been based on experimental work done at the University of Wieoonsin (8, 9). The Food Machhery & Chemical Corp. took over the development and is building at Lawrence,h. a, plant expeoted to produce 40 tons of "01 per day. It is being built for the Ordnance D e p d m e n t of the United S t a m Army. Briefly, the Wiaoonaia process oonsiste of preheating air with hot pebbles of magnesium oxide, burning the hot air with fuel gas, and quickly c h i u i i the combustion gaaas with a eacond pebble bed of magnssium oxide. By meming the air flow through these two pebble beds, it is pasible to operate continuously and heat the air to 210O0 C. where something over 2% of nitric oxide is produced. C h i l l i the gases at the rate of aO,OOOo C. per second makes it p d b l e to obtain 2% nitric oxide in the exit gasae. The pebble beds =we to preheat the air, conm e the heat, and presenre the nitric oxide. The 2% nitric oxide is catalytically oxidized by silica gel and the resulting nitrcgem dioxide ia adsorbed by silica gel. Later the nitrogen dioxide

1388


is driven out of the silica gel in concentrated form by heating, and it reacts with water to form nitric acid. The idea of using pebble beds as a means of achieving high temperatures and chilling is based on a patent by Cottrell(5). The heat exchange between a gas stream and a pebble bed is extremely rapid. High Concentration of €IN03 K o t Possible by Fractional Distillation

Although the boiling point of 100% nitric acid is 86" C., it is not possible to concentrate the weak acids formed from the oxidation of ammonia and water absorption by fractional distillation. A constant boiling point mixture forms a t 122" C. and contains 68% HNOJ and 32% water. In the conventional concentration system, as described by Manning ( I @ , weak nitric acid (50 to 6OY0), mixed with 95 to 96% sulfuric acid enters the top of a distillation tower and meets an upward flow of steam. Concentrated nitric acid vapors evolve from the top, and dilute sulfuric acid runs off the bottom. A wocess for the direct manufacture of nitric acid from the oxidation of ammonia has been patented ( I O ) . The absorption medium instead of water is 60' BB. sulfuric acid, circulating in a closed cycle. The hot nitrous gases are cooled in a boiler which also concentrates the sulfuric acid from a denitration tower. The nitrous gases then pass through an absorption tower, countercurrent to the sulfuric acid. The nitrosyl sulfate is heated by live steam from the recovery boilers in denitration towers. Billingham Works-One of Eleven Alanufacturing Divisions of Imperial Chemical Industries, Ltd.

The World War I submarine blockade of Great Britain brought on a serious shortage of Chilean nitrate and led the government t o begin the construction of a synthetic nitrate plant a t Billingham. The war ended before completion of the plant and in 1919 Brunner, Mond & Co. acquired the site and assets and continued the development, operating the Billingham plant until 1926. Imperial Chemical Industries, Ltd., was formed in 1926 and, in the main, represented the fusion of Brunner, Mond & Co., British Dyestuffs Corp., Kobe1 Industries, and the United Alkali co. Since that time, IC1 has experienced a tremendous expansion. In the United Kingdom today ICI's manufacturing capacity is based on its eleven manufacturing units, called divisions. The divisions are, in a sense, separate industrial enterprises, with a wide measure of autonomy in all that pertains to the effectiveness of the unit-commercial policy, research, development, and technology. This is somewhat in contrast to the practice of centralizing research, engineering, development, and other such staff activities, as is more commonly done in America. The Billingham works is the largest chemical factory in the United Kingdom and one of the largest in the world. It occupies a site of about 1000 acres on the north bank of the River Tees, is 7 miles in circumference, and employs some 15,000 people. Gross annual output of the works is over 1,750,000 tons of products ranging from fertilizers to high octane gasoline. The heart of the plant is the ammonia synthesis unit. End products of the factory include fertilizers, solid carbon dioxide, urea, and alcohols, as well as nitric acid. ICI's first nitric acid plant was built a t Billingham in 1926. It incorporated the first all-stainless steel absorption unit in Great Britain. The original plant is still in operation and is now one of six producing units. One of these units, built in 1936, was a pressure oxidation unit using the D u Pont process. Nitric acid is also manufactured by the Nobel and Dyestuffs Divisions of ICI. Some is sold as nitric acid and some as nitrates. iit the present time, roughly two thirds of the Billingham production goes into the manufacture of a granulated fertilizer-a mixture of ammonium nitrate and by-product chalk

Vol. 45, No. 7

from the manufacture of ammonium sulfate. Other products from the dilute nitric acid include ammonium nitrate, sodium nitrate, and the concentrated form of nitric acid. Operating Pressure of 1.2 Atmospheres Selected for New Billingham Unit, Based on Economic and Technical Studies

Although completely designed and constructed nitric acid units are commercially available, IC1 decided to build their latest unit to their own design, particularly suited to the conditions a t Billingham. In the determination of the design, the effects of absorption pressure, heat recovery as steam and as power, and the operation of the burners a t atmospheric and a t absorption pressures were first examined. The pressures chosen for this survey were 1, 2.7, 4, and 9 atmospheres absolute. These pressures fitted in with normal engineering standards in Great Britain. This survey showed that the most economical pressure for the Billinghain conditions, which included the recovery of 5 to 10% of the oxides of nitrogen as sodium salts, lay between 1 and 2.7 atmospheres absolute, and that the mode of heat recovery did not change the economics a t higher pressures. The field near 1 atmosphere absolute was then explored in more detail and a pressure of 1.2 atmospheres absolute in the absorption system was chosen as being the most economic under Billingham conditions. Since at such a pressure power recovery by let dovn of exhaust gases is uneconomic because of the very small expansion ratio, recovery of heat as steam at 300 pounds per square inch gage was selected. The concentration of aqueous nitric acid produced was fixed a t soy0 HKOs for design purposes. Higher operating pressures would have permitted higher acid strengths, but the value of this (which was included in the economic survey) did not counterbalance the high compression costs a t the higher operating pressures. The economic pressure is very much influenced by the f a c t that in this plant 5 to 10% of the oxides of nitrogen are ahsoriled in caustic soda solution to give sodium nitrite. 2NaOH

+ NO + NO2 --+

2NaXOz

+ H20

This substantially reduces the total absorption volume necessary. For recovery of the heat evolved from the oxidation of ammonia a combined waste-heat boiler-preheater arrangement was selected, because of its convenience. suitability, and reliability. hdditional heat recovery w-as obtained by provision of a steam superheater and a boiler feed water preheater. Catalyst for the oxidation of ammonia has been the subject of considerable study both by IC1 and others. The platinumrhodium alloys were selected almost 20 years ago as the most advantageous and economical ( 1 2 ) . Loss of catalyst is considerable and increases both with temperature in the normal operating range (850" to 9.50" C.) and with the absolute pressure a t the gauze. An estimate of 0.01 to 0.02 ounce of platinum per ton of nitric acid produced has been made ( I C ) . However it is claimed that the net consumption of catalyst for the D u Pont system is about 0.01 ounce per ton 100% nitric acid produced. Metal turnings and other types of catalytic beds have been tried but without great success. Excessive contact must be avoided. It is believed that in the oxidation of ammonia only one contact of the molecule with the catalyst is desirable and that more frequent contact leads to the decomposition 2N0

--+

N,

+

0 2

The gases leaving the heat recovery apparatus must be cooled and the water produced by the oxidation of ammonia condensed

July 19%

INDUSTRIAL A N D ENGINEERING CHEMISTRY

1389

b

bc

i;

i.

',~.

::.....

Vol. 45, No. I


1390

system was designed for continuous operation under automatic control. T o obtain the full benefit of the design work, the whole plant was arranged for automatic control. This ensures t h a t conditions are kept at the optimum levels determined by design, during the whole period of plant operation. Central Header Supplies Ammonia to Acid Plants

The new plant has been located adjacent to the existing acid plants on the location. Design capacity is 35,000 tons equivalent 100% nitric acid per annum. This particular unit is supplied with ammonia gas from the same header furnishing the gas to the other atmospheric units. The ammonia is supplied through a 10-inch mild steel pipe a t roof level. Figure 2 is a flow sheet of the Billingham process, The ammonia pipe passes along the roof, and in this straight run have been placed instrument fittings and several important control Inlet Ammonia Control Svstem valves. For example, a solenoid trip valve, a Ammonia gas is automatically controlled to a steady pressure; a solenoid valve operates electrically to isolate the ammonia supply under any emergency Conditions control valve actuated bv a recording gressure controller, and the control valve operated by a recording flow ratio controller. Air for the oxidation reaction is filtered through a static bagout before t h e gases are suitable for absorption. T h e economics type suction filter. T o ensure t h a t the air is always dry when of various methods of cooling the gas were considered, and t h e it enters the bags, thus minimizing pressure drop, it is drawn engas cooling tower, in which the gas is cooled by direct contact tirely from inside the burner house. The bag filter is divided with a circulation of cold acid in a ring packed tower, was adopted into three identical compartments, any one of which can be isoas the most economical arrangement. lated for bag cleaning or replacement. I t s design capacity is T h e design of the absorption system was based on the funda14,290 cubic meters per hour. mental studies of Denbigh and Prince (6) and on experience in t h e design and operation of similar systems, Work on nitrous fume absorption has also been reported by other workers ( I , $1). Gas Blower Provides Mixing of Such variables as the size and number of individual absorption Ammonia and Primary Air towers, the amount of acid circulated over the packingj and the The filtered air and ammonia gas lines join on the roof, and the distribution throughout the system of the secondary air required mixed gases pass to the suction side of a Rateau turboblower 10for the oxidation of NO to XO2 were considered. As a result of rated on the ground floor of the burner house. The blower serves studies of the costs of various arrangements a six-tower system both as a mixer for the gases and a means of raising the system was chosen, all towers being of the same size and containing the pressure to the desired point. The rate of ammonia and primary same packing. T h e first, primarily an oxidation tower, was for air consumption, along with secondary air for oxidation of nithe purpose of removing the heat generated in the reaction trous gases, is shown in Table 11. 2N0 0 2 +2N02 I

I

+

while t h e subsequent five were primarily absorption towers. Circulation on all towers was provided by standard size pumps, but to carry away the considerable heat of reaction, the circulation rate over the oxidation tower was designed as three times t h a t of the absorption towers. Secondary air must be added before the oxidation tower, but careful investigation indicated that the absorption volume required was considerably reduced by adding only part of t h e secondary air a t the inlet of the oxidation tower and the remainder later in the system when the state of oxidation falls, rather than adding the total amount required for t h e oxidation of the nitric oxide a t the inlet of the oxidation tower. T h e gases leaving the water absorption system are a t too low a state of oxidation for satisfactory alkaline absolption. Provision was therefore made t o add sufficient secondary air a t this point t o ensure a satisfactory state of oxidation after passage through an unpacked toiver, before going t o the alkaline absorption system. This system consisted of two ring-packed, circulated towers in series, to the last of which caustic soda solution was added and from the first of which the nitrite liquor was withdrawn. I n the past such systems had been operated on a semibatch process, but to ensure a low and constant acidity in the final exhaust gases, the

TO PRODUCE 4.46 TONS TABLE IT. GASRATESREQUIRED EQUIVALENT 1 0 0 ~HNO, o PER HOUR

Gas

Rate, Cu. M. (Standard Temp. and Press.)/IIour

Ammonia Primary air Secondary air

1,860 13,900 6,600

The blower is designed to deliver 16,200 cubic meters per hour (at atmospheric pressure and 20" C.) and is supplied with a pressure oil circulation system and a small cooler. Waste Heat Boiler Is Used for Preheating and Heat Recovery

The mixed gas from the blower passes through tubes in the upper drum of a waete heat boiler. This is actually a combined unit serving both as a waste heat boiler and as the mixed gas preheater. The burner exit gases pass through tubes in the lower drum and some of the steam raised is used to preheat the gas in the upper drum. The upper drum, or preheater, is 16 feet long

July 1953

4


and 5 feet in diameter and the lower drum, or waste heat boiler, is 17 feet long and 6 feet in diameter. The two drums are coupled together by two 24-inch water circulation pipes. The upper drum is set horizontally and the lower drum a t a slight angle below horizontal. The designed working pressure of the boiler is 300 pounds per square inch. The mixed gas leaving the blower is a t approximately 50" C., and this is raised to 200" C., a t the exit of the preheater. The mixed gas leaves the preheater in two lines, each leading to two filters, each of which supplies two burners. The four mixed gas filters are identical and are constructed of */a-inchmild steel plate. The filters clean the gas, thus ensuring a high conversion efficiency and reducing the frequency of catalyst pickling required. The body of each filter is a shallow cylinder with truncated conical top and bottom, the top being removable. The cylinder is 10 feet in diameter by 2 feet 6 inches high. The filter is provided with a fixed bottom grid plate and an upper removable grid plate. Each grid is covered with a layer of metal gauze. On the lower grid is spread '/a-inch broken quartz and on the upper grid a layer of '/a-inch broken quartz with a layer of white asbestos on top. Each filter supplies two burners. The mixed gas leaves the filter in a stainless steel line, which branches into two header? leading to the burners. All piping after the preheater, and the filters are lagged. The gas flows downward through eight burners, which are arranged four on each side of the waste heat boiler. The upper part of the burner is of welded stainless steel and forms a cone opening out to a square section to accommodate the catalyst gauzes. The lower part of the converter is of cast iron, lined with refractory insulating brick. Each burner is fitted with shutoff valves so that it can be isolated without shutting down the plant. The catalyst consists of three or four layers of gauze made of platinum-rhodium alloy. The gauzes are supported on a chrome nickel mesh resting on bars of the same material slotted into the

t

ceramic lining of the catalyst carrying frame. The whole assembly is clamped between the upper and lower bodies of the convertor with asbestos joints. The joint between the upper and lower burner bodies may be easily broken for routine catalyst pad changing or inspection. I t is secured by 1-inch cotter bolts. For initiating the oxidation reaction in the burner, an electrical pilot heater is provided. The heating element is a length of nichrome wire, shaped in the form of a double hairpin bend. The heater is centrally located, slightly abovc the catalyst body. The hot gases leave the burner a t approximately 900' C. The main header for the hot gases is a 44-inch mild steel pipe, lined with insulating bricks. Bellows joints, pipe loops, and roller bearing assemblies are used to compensat,e for thermal expansion of the pipework and burner assembly. The hot gases pass through the tubes of the waste heat boiIer, where the temperature is lowered to approximately 260" C. The steam raised is superheated by passing the exit gases through an additional tube-and-shell exchanger. The superheater is mounted vertically arid has a total surface of 545 square feet. The temperature of the gas passing through the shell is reduced to 250" C. The gases then pass through a boiler feed water preheater, which is another exchanger of the tube-and-shell type. The over-all dimensions are approximately 12 feet by 4 feet diameter, and the heat exchange surface is 1700 square feet. The gases pass t,hrough the tubes and leave the exchanger a t 150" C. Absorption System Consists of Ten Towers

The gases leave the burner house through a 30-inch stainless steel overhead line and first enter thc gas cooling tower. The tower is of all-welded stainless steel construction, 7 feet in diameter and 35 feet high. The tower is lined in the lower section with brickwork and packed with 3 X 3 inch earthenware rings.

Secondary Air and iMixed Gas Blowers

'The mixed gas blower provides pressure for the plant; secondary air is injeoted into appropriate positions in absorption section to give necessary oxidation of NO to NO2

Mixed Gas Solenoid Valve

1391

*

Apart from solenoid valve on ammonia supply, further safeguard is provided by a mixed gas solenoid valve against mixed gas blower failure; in foreground is arrangement for lighting catalyst gauze in burner

1392


Acid is circulated by means of a vertically mounted centrifugal pump. It passes through four shell-and-tube heat exchangers in series and enters the top of the cooling tower through 3-inch drilled pipe sprays. The pumps for the gas cooling tower, the oxidation tower, and the five absorpt,ion towers which follow are identical. All parts in contact with the acid (excepting a special silicon-iron sleeve) are stainless steel. The piping to each pump is arranged in such a manner that the pumps are interchangeable. A cylindrical basket-type stainless steel strainer is located in the suction line to each pump These have been inst'alled as a safeguard against pump damage from possible broken packing materials. The acid passes through the shell of each cooler in series countercurrent to the cooling water in t.he tubes. Condensate is added to the tower for an initial start-up, but in operation there is a continual gain due to condensation of water formed in the oxidation of ammonia. Acid leaving the cooling tower is approximately 30% FIN03 and is piped to the KO.3 absorption tower. The gas leaves the top of the cooling tower a t 30' C. and passes through a 30-inch line to the top of the oxidation tower. The oxidation tower and the folloring five absorption towers are identical with the exception of minor differences in pipe connections. The towers are of all-welded stainless steel construction, 18 feet in diameter and 45 feet high, and are packed with earthenware rings. Acid is circulated through t'uhe-and-shell heat exchangers and enters the top of the oxidation tower through twelve sprays similar in design to those of the gap cooling tower. The design provides for three separate cirrulating systems, each with two coolers in series. The acid coolers on the oxidation and water absorption towers are smaller than those used on the gas cooling tower but are of similar design. Secondary air leaving a bleacher enters the gas stream just ahead of the oxidation tower. This c,ombined gas flox through the oxidation tower is cocurrcnt with the acid flow. The gas leaves the bottom of the oxidation t,ower and enters the bottom of the S o . 1 absorption tower. The gas flow then is from the top of No. I to the top of No. 2 ; bottom of KO. 2 to bottom of No. 3; top of No. 3 to top of KO. 4; bottom of No. 4 to bottom of No. 5 . -4dditional secondary air is added between No, 2 and No. 3 towers, between S o . 3 and No. 4 towers, and after No. 5 tower. The acid accumulated flows by gravity from No. 5 absorption tower through the intermediate towers to No. 1 tower, from which it is pumped to a bleacher and thence to storage. Each of the towers is provided with circulation pumps and coolers. The acid bleacher is a packed tower of stainless steel down which the acid is sprayed with secondary air blowing countercurrent through it. The bleacher serves to decolorize the acid and the liquid discharge is 50% nitric acid, ready for product storage. The air discharge (with entrained acid) joins the gas stream just ahead of the oxidation tower. The bleacher itself is 3 feet 6 inches in diameter, 18 feet long, and is provided with a conical bottom. It is packed with earthenware rings. The gas leaves the top of the KO.5 absorption tower and enters the top of a tail gas oxidation tower. ThiF tower is 12 feet in diameter and of similar construction to the absorption towers, but it is not packed. The tail gas oxidation tower is fitted with only four acid sprays in the top. Make-up in the form of condensate is added to the tail gas oxidation tower a t a rat,e of 790 gallons per hour The gas Ieaving the tower passes through a spray arrester of the Calder-Fox type. In the spray arrester the gas passes through two slotted plates which are close together, with their respective slots out of line. The plates are made of polymeth,ylmethacrylate. The casing is barrel-shaped, 5 feet in diameter and 2 feet long. It is provided with a I-inch drain. The gas leaving t h e spray arrester goes to the alkaline absorp-

Vol. 45, No. 7

tion system, which consists of two circulated towers in series. The first tower, called the neutralizing tower, is 7 feet in diameter and 32 feet high. It is fabricated from stainless steel plates and is packed with 3 X 3 inch earthenware rings. The second tower, called the alkaline absorption tower, is the same diameter but is 46 feet high. It is constructed of mild steel throughout and is packed with mild steel 3 X 3 inch rings. From elsewhere on the Billingham site, 25% sodium hydroxide solution is piped to a head tank with capacity of 12 tons. The caustic soda feed to the alkaline absorption tower is by gravity flow, governed by a liquid level controller. The liquor is circulated by centrifugal pumps through four spray heads located in the top of each tower. The pumps are identical with those used for acid circulation, except that all parts are either cast iron or mild steel. Three pumps are provided and piped in such a manner that the third pump can be used as a spare for either tower. The level in the base of the neutrahzing tower is maintained by bleeding forward liquid from the delivery of the pump circulating the alkaline tower. The bleed is controlled automatically by a level controller in the neutralizing tower. Discharge from the neutralizing tower is by gravity, controlled by a valve actuated by a pH controller. The neutral nitrite liquor is collected in a welded mild steel tank having a capacity of 10 cubic meters. From this tank the neutral nitrite liquor is pumped to various other units in the Billingham plant. The gas leaving the top of the neutralizing tower enters the bottom of the alkaline tower and leaves at the top. From the top of the alkaline tower the gas passes through a spray arrester, a back pressure regulating valve, and is then vented to the atmosphere.

TABLE 111.

SERVICE REQUIREYEXTS PER T O X EQUIVALEXT 100% HSOI

APPROXIMATE

OF

Commodity

Required

Electric power, ka.-hr. Untreated water, gal. Treated boiler feed water, gal. Condensate gal. Compressed air, cii. ft. Ammonia gas, ton Caustic soda, ton

178 356 304 178 3960 0.296 0.040

Extensive Use of Automatic Controls Ensures Optimum Operating Efficiency; Minimizes Personnel Required

The new Billingham nitric acid plant has been designed for automatic control in so far as possible. The instruments are assembled mainly on a large central panel, situated in the burner house control room, One operator can observe and control the entire operation of the plant from this particular panel. €10~7ever, on each shift there is an assistant operator to make routine checks of the equipment and take samples at various points in the process. The control panel itself is laid out in the form of a colored flow diagram of the plant that it serves; different colors are used to represent the various gases and liquids involved. Each instrument is fitted close behind the point on the diagram corresponding to the actual point in the plant to which i t is connected. The emphasis is on the flow diagram more than on the instruments; only the minimum essential parts of the instruments are visible from the front of the panel. There are no front title plates for the instruments, as their positions are self-explanatory. Identification plates have been fitted behind t,he panel to facilitate maintenance work. The instruments are accessible from the front for renewing the recording charts through a flush-mounted door. The position of

July 1953


the various control valves is indicated by 2-inch diameter air pressure gages-the pressure shown corresponding to the amount the valve is open or closed. The master control is the primary air flow, and this is used to control, by flow ratio controllers, the ammoniagasrate and secondary air rate. The secondary air is subdivided for the different points of addition by a further group of flow-ratio controllers. The absorption system pressure is also controlled automatically. Solenoid-operated trip valves have been installed in the ammonia and mixed gas lines. On the panel board these are rep-

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resented by a light-green when open, and red when closed For example, the ammonia valve is automatically closed when any one of a number of emergencies arises: 1. A low level or hydrogen alarm from the ammonia plant 2. High ressure on the ammonia gas line 3. Mixelgas blower failure 4. High temperature on the catalytic gauze

Each method of tripping has its own pair of light signals, once again red and green, which indicate at once the type of emergency involved. The operation of the waste heat boiler is also controlled from

c

UUU.

(Top)Mixed Gas Filters and Waste Heat Boiler Top Drum Air before mixin is filtered; further filtration by mixed gas filters gives longer period at economic gauze effioienoy%eforegauze pickling is required; mixed gas is preheated in top drum of waste heat boiler installation before going to burners

(Bottom) Burner Arrangement Design gives balanced layout of burners around waste heat boiler

2. The feed water is deaerated and softened for protection of the waste heat boiler and its auxiliary equipment. 3. Circulating water in the a b s o r p tion section is maintained in a slightly alkaline condition.

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Vol. 45, No. 7

4. Inspection plates are removed from absorption towers and the condition of the packing determined. 5 . Spray nozzles are removed for inspection. 6. H o t gas lines are rebricked as required and re asketed. The mixed gas filters are inspected and cleaned if necessary.

5.

Expanding Agricultural Uses for Nitric Acid Point to Promising Industrial Future

There is a myriad of uses for this versatile chemical. Faith, Keyes, and Clark (6) have estimated the end-use pattern for nitric acid as follows: Ammonium nitrate fertilizers, 55%; explosives, 25%; and miscellaneous, 20%. Of course the end-use pattern of any chemical, and particularly nitric acid, is subject to economic and political fluctuations. The general classes of use, with examples, are summarized in Table IV. According t o Turrentine (26) it is inevitable t h a t we shall have t o turn to nitric acid as a substitute for sulfuric acid in the production of Acid Distribution to Oxidation Tower agricultural phosphates. He goes on t o point out t h a t this substitution is fully justified by Oxidation space is provided in a packed tower prior to water absorption; acid circuOation with cooling removes heat of oxidation; a similar system is used on absorption the basic economy which it reDresents-namelv. towers t h e elimination of acid costs through the purchase of anhydrous ammonia and its resale as nitrates. H e suggests also t h a t acid absorption towers be filled T h e earthenware materials used for packing the absorpt'ion with pebble phosphate rock and t h a t t h e nitrous gases be passed towers has been shoa-n to have an extremely good life in nitric upward through t h e bed countercurrent t o a downward flow of acid and nitrous gases of all the concentrations encount'ered. The water. material is used for the.brick\vork and for t'he packing rings. Considerable research has been done at the Tennessee Valley Refractory brickwork is used for lining the bottom half of the Authority on the production of compound fertilizers from rock burner and the hot gas mains and inlet head of the bottom drum phosphate. A pilot plant development of a fertilizer process of t h e waste heat boiler. It' is therefore required t o wit'hstand in which rock phosphate is acidulated with nitric and phosphoric the action of oxides of nitrogen a t continuous temperatures of acid has been demibed (23). A 4-ton-per-day unit, involving approximately 900" C. T h e hriclrs are made to special shapes acidulating the rock with mixed acids, ammoniating the resultant for t h e job in order to reduce cutting and jointing to a minimum. solution, drying and granulating, was proved technically feasible. T h e correct setting of the bricks is important, in order to proT h e principal reactions involved were: tect the bricks and the cast iron or steel backing. After thorough cleaning, the metal surfaces are painted with a special cement Cald?dPO4)e 20HIL'03 ~ H ~ P O =I based on sodium silicate. The bricks are jointed with a high 10Ca(N0& f 10HZP04 -t 2IIF (1) temperahre aluminum silicate cement. I

+

Program of Preventive Maintenance Reduces Nonproductive Time to a Minimum

I n view of the highly coli osive nature of the materials handled, maintenance and repair would be expected to be a big factor in operating costs. However, this has been held to a minimum by means of a planned general overhaul period each year and routine inspection during operation. T h e catalytic gauze of the burners is changed a t regular intervals. The burner can be isolated by gate valves and the entire operation keeps the burner out of service less than one half day. During the change-over, the flow is increased t o the other burners to such an extent that no reduction in total throughput is required. T h e catalyst is reactivated by 17-ashing in hot dilute hydrochloric acid. Each year the plant is shut down for a general overhaul. This is planned well in advance so t h a t nonproductive time is kept to a minimum. During the general overhaul period any conditions which have developed during the year and which require a shutdown for repair are taken care of. I n addition to these jobs, other items are attended t o on a routine basis, for example:

1. The waste heat boiler is conlpletely inspected and hydraulically tested. 2 . Blowers are dismantled and bearings examined. 3. Heads are removed from acid coolers and tubes are inspected.

,

+

Several other processes of this general nature have been studied by TVA (16). I n place of t h e phosphoric acid, sulfuric acid may be used t o react with the excess calcium. The TVA development through the pilot plant stage has been described (19), and a process similar in many respects t o this one has been reported to be in commercial production in France ( 1 8 ) .

TABLE IV. Classification Production of nitrates (dilute or concentrated acid)

USES O F NITRIC ACID

Examples Ammonium nitrate Compound fertilizers Barium nitrate Strontium nitrate Cellulose nitrate Nitroglycerin Production of nitro compounds Explosives such as picric acid and T N T (concentrated acid) Dyest,+ intermediates Weedkillers such as dinitro-o-cresol Insecticides such as nitrostyrene Fungicides such as dinitrophenol Nitropara5ns Oxidizing action (usually di- Benzoic acid from toluene Iodic acid from iodine lute acid) Arsenates from arsenites Ferric salts from ferrous salts


July 1953

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Pioneer work on the production of nitrophosphates has been done by ICI. The British company has developed a process for which patent applications are pending in many countries, including the United States. In the first stage of the IC1 process ground phosphate rock is reacted with 50 t o 60% nitric acid. The amount of nitric acid used can be varied, depending on the analysis of the final product required. Actually a quantity somewhat under the theoretical amount required to convert all the tricalcium phosphate in the rock to monocalcium phosphate and calcium nitrate has been used. The reaction can be represented by the equation

The second stage is ammoniation with sufficient excess of ammonia liquor to ensure substantially complete conversion of the monocalcium phosphate t o dicalcium phosphate. I n this reaction theoretically half the calcium nitrate is converted into ammonium nitrate. The reaction proceeds according to the equation Ca(HzPO4)z

+ Ca(NO& + 2NHa +2CaHPOa + 2”4P\TO3

When the above reaction is substantially complete, sufficient ammonium sulfate is added t o the reaction slurry with continued agitation to convert the remaining calcium nitrate into calcium sulfate Acid Circulating Pumps

-_

I n practice the excess of ammonium sulfate added is dependent on the required nitrogen content of the product. To t h e slurry is then added recycled material, with continued mixing. The mixture passes to a rotary dryer system in which granulation is completed, and the granules are dried by a countercurrent stream of hot gas. The granulated material is screened, the oversize crushed, and the fines, with any additional graded material necessary to make up the required quantity, make up the recycle for controlling the moisture content of the feed t o the dryer system. T h e hot granules after drying are cooled in a rotary cooler before screening. Crittenden ( 4 ) also emphasizes that major possibilities for greater use of nitric acid exist in fertilizer technology. He predicts continued expansion in ammonium nitrate products, both of solids and liquids; production of potassium nitrates in limited quantities in the United States for fertilizer use, expansion of calcium nitrate production abroad; and growth in nitric acid processing of phosphate throughout the world. Anticipating expanding outlets not only in agriculture but also in other sections of the chemical industry, Imperial Chemical Industries, Ltd., a t the present time has under construction three additional units for producing nitric acid at Billingham. These will be located adjacent to the six existing plants and each will be almost identical with the latest unit which has just been described. There will be a centralization of controls for the new units, leading to sizable operating economies. T h e future of nitric acid is, of course, linked closely with developments in world politics. I n the event of another major war, nitric acid production would be increased and diverted to making explosives. Many plants now in standby condition would be recommissioned. Acknowledgment

The authors wish to acknowledge with appreciation the valuable technical assistance of A. W. Holmes and other members of t h e staff of Imperial Chemical Industries, Ltd., and Richard L.

Each tower of absorption section has individual acid circulation systcrns with coolers; a special siEcon iron sleeve is used i n the gland of the purnpa

Kenyon, of INDUSTRIAL AND ESGIKEERISG CHEMISTRY, in the preparation of this article. Literature Cited

Chambers, F. S.,Jr., and Sherwood, T. IC, IND.ENG.CHEM. 29,1415-22 (1937).

Chem. Eng., 59,238 (Januaiy 1952). Cottrell, F. G., U. S.Patent 2,121,733 (June 21, 1938). Crittenden, E. D., Chem. Eny., 59, 177-9 (June 1952). Denbigh, K. G., and Prince, A. J., J . Chem. SOC.1947, pp. 790801.

Faith, W. L., Keyes, D. B., and Clark, R. L., “Industrial Chemicals,” pp. 432-7, New York, John Wiley & Sons, 1950. Fauser, G., Chem. & Met. Eng., 39, No. 8 , 430-32 (1932). Foster, E. G., and Daniels, F., IND.ENG.CHEM.,43, 9 8 6 9 2 (1951).

Gilbert, N., and Daniels, F.,Ibzd., 40, 1719-21 (1948). Guareschi, P., Pettenati, C., and Maragliano-Busesti, G., Brit. Patent 35138 (1947). ENG.CHEM.,26, 1287Handforth, S.L., and Tilley, J. N., IND. 92 (1934).

Hignett, T. P., Chem. Eng., 58, 166-9 (May 1951). Houston, E. C., Hignett, T. P., and D u m , R. E., IND.ENO. CHEM.,43,2413-18 (1951). Kobe, K. A., “Inorganic Process Industries,” New York, Maomillan Co., 1948. Manning, A. H., Chemistry & Industry, 62, 98-104 (March 1943).

Parsons, C. L., IND.ENG.CHEM.,11, 541-52 (1919). Ibid., 19,789-94 (1927). Quanquin, M., Industrie Chimique, 34, 165-7 (1947). Striplin, M. iM.,McKnight, D., and Hignett, T. P., IND. ENG.

CHEM.,44,236-42 (1952). Taylor, G. B., Chilton, T. H., and Handforth, S. L., Ibid., 23, 860-5 (1931).

Toniolo, C., Chem. & Met. Eng., 34, 92-5 (1927). Turrentine, 6. W., Chem. Eng. News, 29, No. 34, 3454-7 (1951).

NITRIC ACID IN GREAT BRITAIN - Industrial & Engineering Chemistry

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