Industrial and Laboratory Nitrations

22 Safe Manufacture and Handling of Liquid Nitric Esters G. S. BIASUTTI

Downloaded by UNIV LAVAL on March 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0022.ch022

Dr. Ing. Mario Biazzi Soc. An., Quai Maria Belgia 18, Ch-1800 Vevey, Switzerland

The industrial manufacture of n i t r i c esters began with the discovery by Alfred Nobel that the enormous power released by the explosive decomposition of nitroglycerine could be harnessed and used. Inadequate knowledge of this blasting o i l ' s properties has been the primary cause in the past of many explosion disasters in all parts of the world. Slow but constant progress towards better safety was made during the following 40 years. Nevertheless, due to fast growing demand, the amount of nitroglycerine being manufactured increased manifold and, with it, the number of accidents and their destructiveness. The f i r s t important step towards safer manufacture was made when Arnold Schmid, i n 1927, designed and produced his f i r s t continuously operating nitration plant. Then, in 1935, Mario B i a z z i , a young Italian engineer, came out with a new process where great emphasis was placed on safe manufacture. Biazzi's ideas were based on the assumption, which was later proved not quite true, that nitroglycerine could not shoot when finely emulsified with another immiscible l i q u i d . Based on this belief, Biazzi b u i l t his nitrator so as to create within, by means of a turbine wheel, a powerful and turbulent liquid flow. The nitroglycerine formed an extremely fine emulsion with the nitrating acid. Even with lack of proof that this emulsion would not propagate a detonation, Biazzi obtained some interesting results from his nitrator design. The much improved mixing effect resulted in a greater reaction speed and better heat transfer, thus allowing a reduction in the size of the apparatus. Washing of the nitroglycerine was performed under the same powerful agitation. Under these conditions i t was possible to neutralise the acidic o i l directly with a soda solution, thus eliminating the need of a pre-wash. Separation of the o i l phase from the acid or wash water was carried out by exploiting the coalescing effect produced when a slow rotating movement is given to the two liquids.

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In Industrial and Laboratory Nitrations; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


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The r e s u l t of these improvements was a great r e d u c t i o n of the q u a n t i t y o f explosive being present a t any time i n the equipment. Over the y e a r s , the B i a z z i process has been f u r t h e r improved and remote c o n t r o l introduced but i t s e s s e n t i a l features have been l i t t l e changed from the o r i g i n a l design. Other continuous processes f o r the manufacture o f n i t r o g l y c e r i n e were developed i n the l a t e f i f t i e s ; the Swedish i n j e c t i o n process and the Hercules tube n i t r a t o r . Both these processes are based on the n i t r a t i o n of g l y c e r i n e i n a t u r b u l e n t flow of pre-cooled and d i l u t e d a c i d , coupled with c e n t r i f u g a l separation. I t i s not the scope o f t h i s paper t o describe the B i a z z i process i n i t s d e t a i l s or to compare i t with other processes. Rather, I wish t o emphasise those features of design and operation t h a t have c o n t r i b u t e d to an i n c r e a s e of s a f e t y i n the manufacture, t r a n s p o r t and f u r t h e r p r o c e s s i n g o f l i q u i d n i t r i c ester. Production

Control

Safety can be improved by i n s t a l l i n g automatic or remote alarm and c o n t r o l systems t h a t prevent, with or without the i n t e r v e n t i o n o f personnel, the occurrence o f abnormal c o n d i t i o n s . These c o n t r o l systems must be f a i l s a f e , i . e . i n case of f a i l u r e they must keep the p l a n t i n a safe c o n d i t i o n . F u r t h e r , they must not hide other f a i l u r e s which, otherwise, would be apparent. For i n s t a n c e , a thermostatic c o n t r o l of a r e a c t i o n would not make i t p o s s i b l e t o detect a f a i l u r e i n the c o o l i n g system u n t i l the l a s t moment, which could be too l a t e . B r i e f l y , the b a s i c automatic c o n t r o l s which are a p p l i e d to the B i a z z i i n s t a l l a t i o n s f o r the manufacture o f n i t r i c e s t e r s and many dangerous n i t r o b o d i e s are the f o l l o w i n g : Ratio c o n t r o l o f the reactants Temperature c o n t r o l Interface l e v e l control These c o n t r o l s have been developed along conventional l i n e s and need no f u r t h e r explanation. I t i s more i n t e r e s t i n g t o consider what measures have been taken t o prevent f a i l u r e s of the automatic c o n t r o l system. I t must a l s o be s t r e s s e d t h a t c a l c u l a t i o n of the n i t r a t i o n r a t i o ( r a t i o of p o l y o l to n i t r i c acid) i s o f t e n l e f t to the e m p i r i c a l judgement of the operator. I t i s known t h a t the spent a c i d r e s u l t i n g from the n i t r a t i o n o f p o l y - a l c o h o l s contains a v a r i a b l e amount of d i s s o l v e d n i t r i c e s t e r which, even at ambient temperature, can go through an exothermic process of h y d r o l y s i s . This r e a c t i o n , a c c e l e r a t e d by a temperature i n c r e a s e , can e a s i l y go out of c o n t r o l . I f t h i s happens when the a c i d i s s t i l l i n the stage of s e p a r a t i o n from the n i t r i c e s t e r , i t w i l l almost c e r t a i n l y end up i n an e x p l o s i o n .



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This problem was i n v e s t i g a t e d by B i a z z i and h i s co-workers and the c o n d i t i o n s were found under which the spent a c i d produced from the n i t r a t i o n can be considered s a f e . The method o f determining the s t a b i l i t y of the spent a c i d was based on the n i t r o u s a c i d formed from the o x i d a t i o n r e a c t i o n as a f u n c t i o n of time a t constant temperature. The determination was made on s y n t h e t i c spent acids s a t u r a t e d with the n i t r i c e s t e r . The amount o f NO2 formed was measured by e l e c t r o m e t r i c a n a l y s i s , keeping the sample f o r three hours a t 70°C. As only N O 2 i n the a c i d acts as a c a t a l y s t f o r f u r t h e r o x i d a t i o n of the n i t r i c e s t e r , i t i s easy to determine from the shape o f the asymptotic curve the l i m i t s o f i t s s t a b i l i t y . I t was found t h a t the s t a b i l i t y of the spent a c i d i s a f u n c t i o n of i t s n i t r i c a c i d content and o f the molar r a t i o H 2 O / H 2 S O 4 . Diagrams have been drawn up a f t e r e x t r a p o l a t i o n to a temperature of 35°C t o show what i s l i k e l y to occur i n an o p e r a t i n g p l a n t . The next q u e s t i o n i s how t o determine, i n the continuously o p e r a t i n g p l a n t , the s t a b i l i t y o f the spent a c i d . ôhman i n 1938 and B i a z z i i n 1952 i n v e s t i g a t e d the p o s s i b i l i t y of determining the s t a b i l i t y of the n i t r o g l y c e r i n e spent a c i d by measuring the Redox p o t e n t i a l . T h i s method proved to be an extremely p r e c i s e one f o r c o n t r o l of the n i t r a t i o n process. Since 1955 a l l B i a z z i n i t r a t o r s have been equipped with a Redox c o n t r o l system, through which any d e v i a t i o n from the normal n i t r a t i o n r a t i o d u r i n g o p e r a t i o n o f the p l a n t i s immediately s i g n a l l e d and c o r r e c t i v e steps can be taken manually or a u t o m a t i c a l l y . At s t a r t - u p , the Redox value i s lower than a t steady s t a t e due to the h i g h e r water and n i t r o u s a c i d contents o f the a c i d used f o r displacement on the previous run. A f t e r some time the Redox p o t e n t i a l becomes p e r f e c t l y constant. Knowing the exact composition o f a r e f e r e n c e s o l u t i o n , the absolute amount o f n i t r i c a c i d i n the spent a c i d can be determined. A high Redox value, corresponding t o a low n i t r i c a c i d content i n the spent a c i d , w i l l give an alarm o r p o s s i b l y shut down the p l a n t . N e u t r a l i s a t i o n o f the n i t r i c e s t e r f l o w i n g out from the separator i s u s u a l l y done i n B i a z z i p l a n t s by d i r e c t treatment with a concentrated sodium carbonate s o l u t i o n . Automatic c o n t r o l o f t h i s r e a c t i o n has been achieved by means o f a pH-meter which c o n t r o l s the soda water flow as a f u n c t i o n o f the pH o f the emulsion l e a v i n g the washer. In the event o f m a l f u n c t i o n i n g o f t h i s instrument, a second pH-meter, the e l e c t r o d e o f which i s p l a c e d a f t e r the second or t h i r d washer, shuts down the p l a n t . In order to i l l u s t r a t e the sequence of events i n an unusual s i t u a t i o n , let us d e s c r i b e what happens i n the case o f a t o t a l e l e c t r i c power f a i l u r e . This f a i l u r e w i l l be detected by any one o f the f o l l o w i n g failures :


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slowing down of n i t r a t o r or washer s t i r r e r s low b r i n e pressure low mixed a c i d flow

and the corresponding alarms w i l l appear on the p a n e l . The s l i d i n g arm feeding the g l y c e r i n e to the n i t r a t o r w i l l immediately r e t r a c t , p r e v e n t i n g any g l y c e r i n e from e n t e r i n g the reactor. The c o n t r o l and alarm systems remain energized by DC power and compressed a i r . The n i t r a t o r s t i r r e r g r a d u a l l y slows down. Nevertheless, due t o the heavy f l y w h e e l , the time i s s u f f i c i e n t t o complete the r e a c t i o n . The same happens t o the washers but, i n these v e s s e l s , a g i t a t i o n i s immediately r e s t o r e d by compressed a i r bubbling through the bottom. One minute l a t e r , unless the operator i n t e r v e n e s , the automatic displacement process begins: spent a c i d flows from an overhead tank through the n i t r a t o r and slowly d i s p l a c e s the organic phase t o the separator. The n i t r i c e s t e r flows f u r t h e r to the washers as i n normal operation u n t i l i t reaches the top overflow. When the washing time has elapsed, the bottom valves of the washers are opened and the emulsion flows to storage. In c o n c l u s i o n , i t can be s a i d t h a t the B i a z z i production p l a n t f o r n i t r i c e s t e r s operates i n the absence of personnel under very acceptable s a f e t y c o n d i t i o n s . Transport and

Storage

The s i t u a t i o n may not be the same f o r the f u r t h e r steps of t r a n s p o r t , storage and p r o c e s s i n g . From a survey we have made c o v e r i n g accidents r e l a t e d t o l i q u i d e x p l o s i v e s , there i s no doubt t h a t the g r e a t e s t frequencey and d e s t r u c t i v e n e s s i s to be found i n those o r i g i n a t e d during the storage, t r a n s f e r and h a n d l i n g o p e r a t i o n s . We have recorded a t l e a s t four cases of l i g h t n i n g s t r i k i n g the storage house and causing t o t a l d e s t r u c t i o n of the p l a n t . However, most of the accidents were undoubtedly due t o human f a i l u r e . The need t o s t o r e n i t r o g l y c e r i n e i s due t o the t r a n s i t i o n from a continuous to a batch process. I f the f u r t h e r operations of weighing and mixing the n i t r o g l y c e r i n e with other i n g r e d i e n t s were t o be continuous, no storage whatever would be r e q u i r e d . This has been p a r t i a l l y achieved i n Japan and Sweden f o r the manufacture o f commercial e x p l o s i v e s . In a l,2001bs/hr p l a n t , only 40kg of n i t r o g l y c e r i n e and 20kg o f dynamite are present at any time. Safety i n the storage o f l i q u i d e x p l o s i v e s has been improved by B i a z z i through the a p p l i c a t i o n of automation and remote c o n t r o l . The concentration o f a l a r g e q u a n t i t y o f e x p l o s i v e i n one p l a c e must s t i l l be avoided.



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T r a n s f e r of l i q u i d e x p l o s i v e s i n s i d e the f a c t o r y has been i n the p a s t the source o f a g r e a t number o f a c c i d e n t s . The o l d - f a s h i o n e d method o f c a r r y i n g the n i t r o g l y c e r i n e from one b u i l d i n g t o another i n hand-driven wheel c a r t s has not been abandoned and has even regained p o p u l a r i t y a f t e r s e v e r a l d i s a s t e r s caused by the t r a n s m i s s i o n of a detonation through the t r a n s f e r pipe or g u t t e r connecting two b u i l d i n g s . The method o f t r a n s f e r r i n g an e x p l o s i v e l i q u i d through a p i p e l i n e as a water emulsion was introduced by B i a z z i i n 1950 and l a t e r adopted i n many f a c t o r i e s before the mechanism by which an e x p l o s i o n can be i n i t i a t e d i n a l i q u i d had been i n v e s t i g a t e d by, amongst o t h e r s , Bowden and Y o f f e . They demonstrated (1952) t h a t a sudden change o f pressure i n the l i q u i d could cause, by a d i a b a t i c compression of a i r bubbles, a temperature i n c r e a s e s u f f i c i e n t t o t r i g g e r a detonation o f the surrounding e x p l o s i v e . In order to prevent an i n i t i a t i o n due t o a d i a b a t i c compression, the energy changes (pressure, temperature, v e l o c i t y ) w i t h i n the system must be as gradual as p o s s i b l e . These changes occur when the system goes from a s t a t e of r e s t to the steady s t a t e o f t r a n s f e r , and v i c e - v e r s a . A gradual change can be obtained by slowing down t h i s t r a n s i t i o n p e r i o d , i n other words by slowing opening the water valve t o the t r a n s f e r e j e c t o r . Another important energy change occurs w i t h i n the e j e c t o r i t s e l f where, due to the decreasing c r o s s - s e c t i o n o f the p i p e , the l i q u i d e x p l o s i v e i s subjected t o a considerable a c c e l e r a t i o n . A f t e r having measured the energy changes i n s i d e an experimental e j e c t o r provided with a g r e a t number of p i e z o e l e c t r i c elements, B i a z z i designed and constructed a new type of e j e c t o r where these changes are kept near the t h e o r e t i c a l minimum. This new e j e c t o r has been i n s t a l l e d i n s e v e r a l p l a n t s and no accidents have been r e p o r t e d with i t s use. Once i n i t i a t e d , a detonation w i l l propagate, under c e r t a i n circumstances, through a water emulsion, causing the e x p l o s i o n of the e n t i r e mass o f the n i t r i c e s t e r . In recent years much work has been done i n order t o f i n d out the c o n d i t i o n s under which the t r a n s m i s s i o n of an e x p l o s i o n through a water emulsion cannot take p l a c e . I t was found t h a t these c o n d i t i o n s are p r i m a r i l y d i c t a t e d by the c r i t i c a l diameter o f the mass, which i s a constant f o r each type o f e x p l o s i v e . For n i t r o g l y c e r i n e , the c r i t i c a l diameter i s a few m i l l i m e t e r s . However, i f the e x p l o s i v e o i l i s e m u l s i f i e d with water, the diameter i s of the order of 20 m i l l i m e t e r s , depending on the volume r a t i o between the two l i q u i d s . According to the above theory,which has been confirmed by experimental data, a detonation w i l l not propagate through a pipe o f a diameter of 20 m i l l i m e t e r s where an emulsion of three p a r t s i n volume of water and one p a r t of n i t r o g l y c e r i n e t r a v e l s a t such speed t h a t t u r b u l e n t flow i s maintained.


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Based on t h i s knowledge, B i a z z i has designed and produced an e n t i r e l y remote c o n t r o l l e d and automated system which handles the l i q u i d e x p l o s i v e from the o u t l e t of the n i t r a t i o n u n i t t o the d e l i v e r y i n t o the mixing p l a n t , without the presence o f personnel.


Future Developments In c o n c l u s i o n , i t can be s a i d t h a t the manufacture and h a n d l i n g of n i t r o g l y c e r i n e and other n i t r i c e s t e r s has reached an unprecedented stage of s a f e t y . N e v e r t h e l e s s , the chances o f an accident s t i l l e x i s t . Is there any way o f f u r t h e r improving the safety conditions? E v i d e n t l y , once the personnel have been removed from the area exposed t o a p o t e n t i a l e x p l o s i o n , the a c c i d e n t w i l l cause only m a t e r i a l damage. I f t h i s l i n e o f thought i s accepted, the next step i s t o reduce the chances and the d e s t r u c t i v e n e s s o f an accident. D e t e c t i n g a t the e a r l i e s t p o s s i b l e stage the onset of decomposition i n the manufacturing equipment i s , f o r i n s t a n c e , one o f the areas which c o u l d be f u r t h e r improved. The f i r s t s i g n of decomposition i n a n i t r i c e s t e r i s the formation o f n i t r o u s a c i d . In the presence of a i r , i t forms the c h a r a c t e r i s t i c reddish-brown fumes. A continuous monitoring system, based e i t h e r on p h o t o e l e c t r i c and c o l o r i m e t r i c d e t e c t i o n or on chemical a n a l y s i s , could be conceived, which would give an alarm and operate the s a f e t y devices upon d e t e c t i o n o f n i t r o g e n oxides i n the a i r above the l i q u i d being processed. Another approach would be determination of NOj ions i n the presence o f NOJ ions by e l e c t r o m e t r i c a l a n a l y s i s . S e l e c t i v e e l e c t r o d e s of t h i s type have been developed. Another problem open f o r research i s a continuous determination of chemical s t a b i l i t y o f the f i n i s h e d n i t r i c e s t e r . I t i s known t h a t n i t r i c e s t e r s are very s e n s i t i v e t o t r a c e s o f n i t r o u s oxides, which have a c a t a l y t i c e f f e c t on thermal decomposition r e a c t i o n s . Their chemical s t a b i l i t y i s u s u a l l y t e s t e d by means o f the Abel heat t e s t , based on q u a l i t a t i v e d e t e c t i o n of minute amounts o f n i t r o g e n oxides i n the a i r above a sample heated a t 180°F. This time-wasting and u n r e l i a b l e method cannot e a s i l y be adapted t o continuous monitoring. Ideas f o r b e t t e r methods have been put forward but, t o my knowledge, none has r e c e i v e d p r a c t i c a l a p p l i c a t i o n . A f u r t h e r need, which has been p o i n t e d out i n t h i s Symposium, i s completion and extension t o n i t r i c e s t e r s of knowledge on r e a c t i o n k i n e t i c s , mass t r a n s f e r and the formation o f s i d e products. I t i s our o p i n i o n t h a t the ways t o be followed i n f u t u r e t o improve s a f e t y , which g e n e r a l l y c o i n c i d e with economics, are:


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1. f u r t h e r d e v e l o p m e n t o f a u t o m a t i o n b y a p p l y i n g more r e f i n e d methods f o r d e t e r m i n a t i o n o f t h e r e a c t i o n c o n d i t i o n s a n d o f t h e chemical s t a b i l i t y ; 2. r e d u c t i o n i n t h e a c c u m u l a t i o n o f d a n g e r o u s p r o d u c t s b y u s i n g the continuous process, n o t only f o r t h e i r manufacture b u t a l s o for their further handling.


Industrial and Laboratory Nitrations

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