LXXXVIII. Hazardous Reaction Cells S. A. HEIDER, National Science Foundation, Hazardous reaction cells are built to protect against the accidental bursting of high pressure chemical test apparatus. Relatively little information based on experience is available so design data, is spane and opinions differ. The purpose of this article is to summarize some of the published information (see Biblio~ra~hv). and to discuss recom-
Washington, D. C. 2 0 5 5 0
sarily apply to industrial research (especially explosives research) nor to large batch chemical processes.
Description of Cells Hazardous reaction cells far university chemistry research usually consist of heavilv-reinforced structural enclosures whichire built-in as a part of the chemistry research complex. The cells are designed to enclose experimental apparatus for
S. A. Heider holds the title of Engineer with the Architectural andEngineering Services Staff of the National Science Foundation. He evaluates university proposals requesting Foundation grants for science research facilities, and monitors the grants. He has served on the technical staffs of the Navy Department, Federal Housing Administration, National Academy of Siences, and industry groups. Mr. Heider is active with the National Society of Professional Engineers, the American Society of Heating Refrigerating and Air Conditioning Engineers, and the American Association far the Advancement of Science. He is a registered engineer, holds a BS degree in electrical engineering from the University of Wisconsin, and has prepared a number of technical papen relating to the design of science research facilities.
conducting chemical reactions where there is a rik of nunexpected and hazardous reactiom, espwislly with possible failure of equipment. An unexpected chemical reaction may occur slowly with creeping build-UD and temnerature, or it . of Dressure . may detonate (supersonic wlocity,. Equipment failwm may occur ~s a result of such build-up of ~ P C S S U Tand ~ temperature, especially where the equipment may have been weakened because of a fault in the high pressure reactor or in the connecting lines, or because of fatigue from previous use. Since a gas or vapor explosion may develop pressures up to 100 psi, and ordinarymasonryconst~ctionwillbede8tmyed a t pressures of about 2 psi, explosion venting must function a t 0.2 psi. A shock wave is frequently produced which may bounce within the cell area creating greater ~hvsicaldamaee than the orieinal shock. k i k e cells are Tntended to protkct personnel and property, they are designed to:
(1) Withstand and relieve a sudden build-up of pressure (2) Containricochettingparticlw (3) Minimize secondary explosions (4) Contain snd relieve released gases (5) Minimize fires Although high pressure chemiral experiments arc constantly changing, .itme of the processes requiring the use of hrtzsrdous reaction cells include hydrogenolysis, oxosynthesis, high pressure hydrogenation, per-compound catslysis, telomerization, and oolvmerimtion. The most daneerous
Figure 1.
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Hamrdour reaction unit d l .
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temperature and pressure. This would place sudden stress on experimental equipment, which could result in failure a t a point already weakened from earlier
strains. Although many of the above chemical processes are known not to be extremely dangerous under normal conditions, accidental e m r s in proportioning or procedure could result in an unexpected and abrupt pressure build;up. For this resson hazardous reaction cells should always be used for dangerous experiments, and should be constructed for the maximum hazard likely to occur regardless of the batch size or whether the chemical procedures have been safely performed previously. The hazardous reaction test area is designed to accommodate the high pressure test equipment within an enclosed structural cell constructed of heavily reinforced materials, while the experimenter, the control equipment, and the instruments are located in a control room immediately adjoining, but protected from the cell (Fig, 1 ) The cell must be vented directly to the outdoom to relieve a. sudden build-up of pressure within the cell. I n addition, it must he ventilated with large volumes of air to quickly remove a sudden release of combustible or noxious gases, and should be equipped with scuppers leading to an isolated holding reservoir outside the building which can safely receive and store hazardous liquids. These matters are discussed below.
Location The safest location is a t the edge of the campus, adjoining an unpopulated piece of land such as a wooded area or a river. However, because usen usually desire
close communications between the cells and their departments, most hazardous reaction cells me built into the chemistry building, but situated in an isolated location such as the roof or a corner of the top floor of thechemistry building, andoriented so that the vent opening faces a n area not frequented by students and the public (Fig. 2). Cells should not belocated below ground, nor at or near ground level unless completely isolated. If the arrangement of the campus is such as to preclude isolation or appropriate orientation, the best solution is to locate the cell on the roof, and to provide a heavy impact wall oppmite the vent opening or to hang a blasting mat, nesr the opening to catch flying debris (Fig. 3). The blasting mat is not recammended because it must be hung away from the vent ooenine (to nermit instant
Materials Various materials have been used for construction of test cell walls, ceilings, and floors. The most common is reinforced concrete. Other materials which have been used include wood or steel, usually in a sandwich type construction, with the void sometimes filled with sand. Wood is less frequently used because of high costs and because of potential splintering. Steel, in the thicknesses required, is both expensive and difficult to fabricate, whereas reinforced concrete has the ad-
Figure 2.
Roof top cell locotion.
vantage of economy, physical mass, flexibility of fabrication, ready availability, and easy maintenance. (Continued a page A490)
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Safety
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Because ordinary reinforced concrete may not hind well a t the joints between walls and floor and ceilings, it is essential to imbed hemy reinforcing rods in the conorete at frequent intervals bath horizontally and vertically, and to tie these rod5 together securely st the intersections,
preferably by welding. Five-eighths inch diameter steel rods spaced 6 in. on centers both horizontally and vertically are recommended for reinforcing. Where steel plates are used instead of concrete cell walls, all joints and s e a m should be welded. Bolts and rivets are generally considered unsatisfactory because a sudden build-up of pressure or the impact of s. flying missile within the cell may cause a failure, with the possibility that a loose
bolt head or rivet may become a secondary missile. Because the impact of missiles may cause the outside surface of a concrete wall to spall, thereby creating the possibility of a secondary missile, steel apall plates are recommended to cover the concrete an the side exposed to personnel.
Thickness of Cell Wall Most hazardous chemical reactions performed a t universities involve very small quantities of materials, but it is reasonable to expect that at some time, either by chance or through ignorance, a, relatively large autoclave nearly filled with materials, could he used, thns presenting the maximum hazard. This could occur a t any university, especially during a time when supervision is minimal. Since a 3-1 autoclave (Fig. 4). is the largest test device likely to be used s t a university, such a container, filled with TNT is believed to represent the most hazardous condition. Although materials other than T N T could be used which are far less dangerous, T N T is believed to represent an extreme condition suitable for design purposes. Also, the U.S. Army Ordnance has developed a nomograph (Fig. 5) based on tests with TNT, which provides a reasonable relstionship between the pounds of TNT detonated, the nominal distance of a test device from a barricade or protective wall, and the thickness of this wall when constructed of reinforced concrete. Assuming that the T N T contained in a. 3-1 autoclave weighs approximately 11 lh and assuming that the barricade is 3'1% ft away (in an 8 X 10 ft room), according to the Army nomograph the reinforced concrete for the barricade should be about 8 in. thick. However, because the nomograph is applicable to a single wall barricade only, a '/a increase in wall thickness is recommended hy the Army for a threesided open cell (open on top and a t one end), thns requiring a 101/~in. reinforced concrete wall. For a closed cell with an open vent such as is being discussed here, this figure should be increased further. Because we are dealing with gross assumptions, and because recommendations of the Army for this condition are not available, a nominal 12 in. thickness wall is believed to be acceptable. This thickness should be used illso for the ceiling and floor of a cell (but is not needed for a slab poured on grade). An alternate method for calculating wall thickness is discussed s t the conclusion of this article.
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