Designing for Safe Reactor Vent Systems - American Chemical Society

15 Designing for Safe Reactor Vent Systems L O U I S J. J A C O B S , JR. and F R A N C I S X . K R U P A

Downloaded by CORNELL UNIV on October 22, 2016 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch015

Monsanto Co., Corporate Engineering Dept., St. Louis, M O 63166

This paper i s p r i m a r i l y concerned with safe venting of polymerization reactors, though the same p r i n c i p l e s apply to almost any vessel containing v o l a t i l e , p o t e n t i a l l y hazardous substances. In polymerization vessels one usually deals with exothermic reactions of v o l a t i l e monomers. The reactions may occur i n either emulsion, suspension, mass or solution-type polymerizat i o n on a batch or continuous b a s i s . Other papers at t h i s conference have discussed each of these extensively and each has advantages and disadvantages regarding control of emergencies. The suspension and emulsion systems generally have a built-in heat sink with the water present, but exhibit higher vapor pressure due to the nearly additive e f f e c t of the immiscible monomer and water phases. I.

Defining the Venting Problem

The need for venting, or the cause of an emergency which r e s u l t s i n a runaway r e a c t i o n , can occur i n seve r a l ways: Cooling system f a i l u r e could occur due to f a i l u r e of pumps or controls supplying cooling media to the reactor vessel jacket, c o i l s , or overhead r e f l u x condensers. Piping to or from the condensers could become plugged or any of the heat exchange surfaces could become excessively fouled. Agitator f a i l u r e either due to e l e c t r i c a l or mechani c a l f a i l u r e could r e s u l t i n loss of system c o n t r o l and "hot spots" i n the reactor. In suspension systems loss of a g i t a t i o n could negate much of the "heat sink" effect as the immiscible phases separate and s t r a t i f y . 0-8412-0506-x/79/47-104-327$05.00/0 © 1979 American Chemical Society Henderson and Bouton; Polymerization Reactors and Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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Incorrect vessel charge either due to automatic cont r o l f a i l u r e or plant operator error could r e s u l t i n excess c a t a l y s t or reactant concentration, e t c . This could cause a r a p i d l y accelerating reaction rate or could i n i t i a t e unexpected side r e a c t i o n s , which could be more severe than the normal r e a c t i o n .


External f i r e could cause an emergency by overloading the normal reactor systems that are operating properly. Each of these cases involves an accumulation of heat i n the system which manifests higher temperature and pressure. The increased temperature accelerates the reactions further which subsequently adds even more heat to the system. II.

Strategies to Handle Emergencies

In the event that one or more of the cases c i t e d above w i l l occur at some point i n the l i f e of a process, we need to have a design strategy to cope with such emergencies. Selection of a strategy w i l l involve judgment of r i s k s and l i k e l i h o o d of occurrence, which w i l l not be discussed here. There are several design strategies that can be used to minimize the consequences of the emergency by a n t i c i p a t i n g system response. Elaborate, redundant reactor control systems could be i n s t a l l e d , such as multiple temperature sensing p o i n t s . On high temperature, these t r i g g e r actions such as feed shutdown, emergency c o o l i n g , or the a d d i t i o n of substances to deactivate the c a t a l y s t . Other control techniques could include a high pressure switch to a c t i v a t e automatically c o n t r o l l e d venting by allowing v o l a t i l e s to be vented from the r e a c t o r . The quantity of v o l a t i l e monomer present could be l i m i t e d by using smaller volume continuous r e a c t o r s , or using a semicontinuous monomer feed. Small q u a n t i t i e s of monomers present would quickly be consumed by an uncontrolled r e a c t i o n , and with the system deprived of further reactants pressure r i s e would be l i m i t e d . Another strategy would involve design of the reactor vessel for a pressure r a t i n g i n excess of any l i k e l y emergency system pressure. This assumes we can adequately predict a l l possible worst case s i t u a t i o n s , which i s doubtful. A more conventional approach i s to provide a safety r e l i e f valve or rupture disc to protect the vessel by venting material when pressure approaches c e r t a i n l i m i t s , such as the maximum allowable working pressure.

Henderson and Bouton; Polymerization Reactors and Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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This strategy may be used i n combination with the f i r s t two s t r a t e g i e s . An alternate approach to the above i s to provide p a r a l l e l r e l i e f valve-rupture disc systems. The valve w i l l have a setting s l i g h t l y above the normal operating pressure with the rupture disc at about a 10% higher setting. The r e l i e f valve should c o n t r o l minor pressure excursions, can vent material and then reseat to minimize process l o s s e s . The rupture disc would provide the ultimate safety p r o t e c t i o n . The remainder of t h i s paper w i l l discuss design of systems where venting of material i s necessary. III.

S i z i n g the Vent System

A. Available Design Methods for Vent S i z i n g . Several methods are a v a i l a b l e to size the vent with a wide range of s o p h i s t i c a t i o n . The FIA chart, F i g . 1 prepared by the Factory Insurance Association i n the mid 1 9 6 0 ' s i s a simple chart summarizing a wealth of experience. Reactions are classed by the degree of exothermic r e a c t i o n . With vessel size and a judgment of reaction type a vent s i z e range can be s e l e c t e d . This chart was prepared to be a guide to insurance inspectors and not a design technique. Experience i n d i c a t e s , however, i t i s often used by designers to estimate a reactor vent s i z e . In 1 9 6 7 a paper by Boyle ( 1 ) provided a more quantitative method for designing vents for polymer reactors. It was based on reaction r a t e , heat of r e a c t i o n , and vapor pressure data. Boyle assumed that the venting of a system can be approximated by s i z i n g to discharge the entire batch contents as a l i q u i d . The vent l i n e size i s determined so the time to vent the entire batch contents i s less than the time to go from r e l i e f set point to maximum allowable vessel pressure. A frequent s i z i n g technique, which i s u s e f u l when the reaction k i n e t i c s and heat of reaction are not known, i s to conduct small scale t e s t s . Then scale up to large equipment i s done by providing a vent with s i m i l a r vent area per mass of contents. In 1 9 7 2 a paper on venting by Huff (2J documented concerns that many designers suspected: that to t r u l y be safe the vent s i z i n g of many systems should be based on assuming two-phase flashing flow in the vent system. A two-phase flow vent method developed by Huff was compared with Boyle's a l l - l i q u i d method, and values from the FIA chart i n Figure 2 . It can be seen that under many conditions, previous methods were not




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15.

RELIEF LINE ID

(=) INCHES Chemical Engineering Progress

Figure

2.

Peak reactor pressure vs. relief line size

(2)



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providing conservative design. Monsanto and other companies are working independently on design methods to s i z e vents more r i g o r o u s l y using two-phase flow c a l c u l a t i o n s i n complex computer programs. Several assumptions have been made in an e f f o r t to allow a wide range of a p p l i c a t i o n . Most notable i s the use of the correlations of M a r t i n e l l i and co-workers for pressure drop (_3) and h o l d - u p ^ ) . The momentum and energy balances are developed for the separated flow regime by Hewlett and H a l l - T a y l o r (J5) . A homogeneous flow basis must be used when thermodynamic equilibrium i s assumed. For further s i m p l i f i c a t i o n i t i s assumed there w i l l be no reaction occurring i n the p i p e l i n e . The vapor and l i q u i d contents of the reactor are assumed to be a homogeneous mass as they enter the vent l i n e . The model assumes adiabatic cond i t i o n s i n the vent l i n e and maintains constant stagnat i o n enthalpy for the energy balance. The M a r t i n e l l i c o r r e l a t i o n s for void f r a c t i o n and pressure drop are used because of t h e i r s i m p l i c i t y and wide range of a p p l i c a b i l i t y . France and Stein (6J d i s cuss the method by which the M a r t i n e l l i gradient for two-phase flow can be incorporated into a choked flow model. Because the M a r t i n e l l i equation balances f r i c t i o n a l shear stresses and pressure drop, i t is important to provide a good v i s c o s i t y model, e s p e c i a l l y for high v i s c o s i t y and non-Newtonian f l u i d s . As the g a s - l i q u i d mixture t r a v e l s down the vent l i n e , the phases w i l l s l i p past each other and the f l u i d s w i l l accelerate. This contribution to the energy balance can be most s i g n i f i c a n t for high p r e s sure blowdown. Pressure increments are c a l c u l a t e d and when the pressure gradient becomes i n f i n i t e the flow i s choked. If t h i s occurs at the end of the pipe the assumed flowrate i s the converged choked flow s o l u t i o n . If choked flow does not occur and the end of the l i n e i s reached at the reservoir pressure, the non-choked flow s o l u t i o n i s obtained. B. Defining the Reaction K i n e t i c s and Component Physical P r o p e r t i e s . The rate expression needed for use i n a vent design model should represent the condit i o n that would e x i s t during the emergency. Kinetic data based on the normal reaction rate are only useful i n cases when loss of heat transfer can be experienced. A simple power law rate expression (usually f i r s t order) w i l l be s u f f i c i e n t i f Arrhenius constants can be f i t t e d . For complex r e a c t i o n s , involving competing and undesirable side reactions, the most conservative approach would be to s i z e the vent system for the one or two



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reactions that add the greatest amount of energy to the system over a given duration. Use of thermal s t a b i l i t y t e s t s (DTA's) to determine the heat s e n s i t i v i t y of a given process mixture is desirable. Recent advances i n a n a l y t i c a l methods permit good calorimetric determination of heat of reaction. Heat of reaction data are c r i t i c a l for exothermic reactor vent s i z i n g . Heat impact from f i r e i s usually small i n comparison, but should not be neglected. Any convenient model for l i q u i d phase a c t i v i t y c o e f f i c i e n t s can be used. In the absence of any data, the i d e a l solution model can permit adequate design. For multiple l i q u i d phases (e.g. suspension processes) or increasing concentrations of polymers, some more r e a l i s t i c models are desirable (van Laar, Flory-Huggins, Wilson). In design of emergency r e l i e f systems the i n t r i n s i c f l u i d properties can often make a d i f f e r e n c e . Usually a l i n e a r i n t e r p o l a t i o n of density, v i s c o s i t y (for Newtonian f l u i d s ) and heat capacity w i l l provide suitable f l u i d p r o p e r t i e s , i f the simulated temperatures f a l l within that range of data. C. W i l l Two Phase Venting Occur? One of the key decisions i n venting c a l c u l a t i o n s i s to determine whether two-phase vent flow w i l l a c t u a l l y occur. Assume a reactor geometry as in Figure 3 with a vapor space and r e l i e f device located in the vapor space. One way for two-phase vent flow to occur i s through gross entrainment of l i q u i d with the discharging vapor. Another mechanism that can develop two-phase flow i n volves swelling or expansion of the contents due to bubble nucleation throughout the l i q u i d volume. This f i l l s the vapor space and the entire vessel with something approximating a homogeneous v a p o r - l i q u i d mixture which w i l l discharge as a f r o t h . Before the onset of two-phase venting, there w i l l be a b r i e f period of a l l - v a p o r venting as i l l u s t r a t e d in Figure 3. Correlations are needed to predict whether twophase flow w i l l occur a f t e r vapor venting i s i n i t i a t e d by rupture disc f a i l u r e or r e l i e f valve opening. Research i s needed i n t h i s area, but for the present we recommend the following c o r r e l a t i o n s to predict batch swell. For systems with low v i s c o s i t y (less than 500 cp) an equation based on bubble column hold-up i s used to obtain a swell r a t i o :




334

F

=

y

^2.

+(^A,

+ £|V«) sinO

Figure 4. Forces on bends: (A) = Area f f ; (F) = Force lb ; (g) = Acceleration of gravity ft/sec ; (g ) = Conversion factor lb ft/lb sec ; (p) = Pressure lb//ft ; (Q) = Volume flow fi?/sec; (V) = Velocity ft/sec; (W) = Mass rate Ibjsec; ( ) = Density IbJfP. 2

2

c

f

m

f

2

P


2

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60 + 2V 60 + V

&

s V i s the s u p e r f i c i a l v e l o c i t y of the gas i n the r e actor body i n feet/minute. It conservatively assumes a l l of the vapor i s generated i n the bottom of the vessel. For f l u i d s with v i s c o s i t y greater than 500cp no good general r e l a t i o n s h i p i s a v a i l a b l e . Experimental work on one system allowed a swell r a t i o c o r r e l a t i o n of the following form: g

f


S

=

1

+

K R

v

2

/

y

3

1

/

2

where K i s a constant, R i s the volume rate of gas per l i q u i d volume, and y i s the v i s c o s i t y . When the swell r a t i o exceeds the r a t i o of vessel volume to l i q u i d volume, two-phase homogeneous venting i s assumed. v

IV.

Mechanical Design

S p e c i f i c a t i o n of r e l i e f valves and rupture d i s c s must be done with care because of the p o t e n t i a l l y t r a g i c consequences of haphazard s e l e c t i o n of s i z e and set or burst pressure. Disc burst conditions for example are very temperature s e n s i t i v e and should be selected for the temperature at which they w i l l r e l i e v e , not the normal operating temperature. Discs also have a normal manufacturing tolerance of ±5% of the set pressure. A 5% higher r e l i e v i n g pressure could be s i g n i f i c a n t i n s a f e l y c o n t r o l l i n g a r e a c t i o n . Rupture discs are also susceptible to fatigue f a i l u r e , e s p e c i a l l y i n pressure f l u c u a t i n g applications and require periodic replacement. R e l i e f valves have an open area much smaller than t h e i r stated size and t h i s must be considered on s e l e c t i o n , i . e . a 2" r e l i e f valve may have an open area of 0.7 i n ^ . Design of vessel and vent l i n e pipe supports i s very important because very large forces can be encountered as soon as venting begins. Figure 4 shows the equations and nomenclature to calculate forces on pipe bends. The authors have heard of s i t u a t i o n s where vent l i n e bends have been straightened, l i n e s broken o f f , or vent catch tanks knocked o f f t h e i r foundations by excessive forces. For bends, the transient e f f e c t s of the i n i t i a l shock wave, the t r a n s i t i o n from vapor flow to two-phase flow, and steady state conditions should be considered. Transient conditions, however, are l i k e l y to be so rapid as to not have enough dura-



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t i o n to cause problems.


V.

Containment of Vented Material

Many of the materials handled are either explosive or toxic to people and the environment. Careful design i s required to handle materials being vented. Since much of the vented material w i l l be l i q u i d , separators such as knockout pots or tangential entry separators can provide disengagement and possible r e covery. Figure 5 i s a t y p i c a l v a p o r - l i q u i d separator design found to be e f f e c t i v e for these a p p l i c a t i o n s . Inlet design s u p e r f i c i a l vapor v e l o c i t y i s about 100 f t / s e c , with s u f f i c i e n t volume provided to accumulate the e n t i r e reactor l i q u i d contents. The l i p on the outlet vapor l i n e and the h o r i z o n t a l plate to separate the accumulated l i q u i d are important features to p r e vent re-entrainment. Flammable or toxic vapors can be piped to a f l a r e a f t e r separation of l i q u i d i s obtained. An important design problem i n f l a r e use i s the very high vent rate experienced for a r e l a t i v e l y short time, i f an e x i s t i n g f l a r e i s used. Also back-pressure e f f e c t s on the l i q u i d separator vessel must be considered, e s p e c i a l l y i f choked flow of vapor occurs downstream of the separator. Another containment strategy for condensible or water-soluble emissions i s to use a water quench system with the discharge being sparged into a large volume of cool l i q u i d . In very extreme cases, t o t a l containment can be provided to prevent any atmospheric emission or to provide a surge volume for c o n t r o l l e d f l a r i n g , absorpt i o n , or other disposal methods. This approach, however, requires use of a very large pressure v e s s e l to provide the required volume, and i s usually only a l a s t choice a l t e r n a t i v e . VI.

PIERS Program

While venting technology and methods are improving, considerable uncertainty remains as to the v a l i d i ty of various assumptions and accuracy of the c o r r e l a tions. Nearly a l l of the experimental data to v e r i f y c a l c u l a t i o n s to-date are with air-water-steam systems. Several chemical, r e f i n i n g , and engineering companies are currently i n the process of forming a r e search i n s t i t u t e to obtain r e a l i s t i c , v e r i f i e d design methods for reactor venting. The group i s c a l l e d DIERS (Design I n s t i t u t e for Emergency R e l i e f Systems) and i s sponsored by AIChE. Funds w i l l be provided by the mem-



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LIQUID Figure 5.

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ber companies on a s c h e d u l e based on company s i z e f o r a f o u r y e a r program. Emphasis o f t h e program w i l l be on 1) e s t a b l i s h i n g c o r r e l a t i o n s f o r t h e b a t c h s w e l l i n low and h i g h v i s c o s i t y systems v e r i f i e d by e x p e r i m e n t s , 2) e s t a b l i s h i n g good two-phase flow c o r r e l a t i o n s v e r i f i e d by experiments f o r vent p i p i n g and r e l i e f v a l v e s , w i t h emphasis on v i s c o u s , two-phase flow, 3) d e v e l o p i n g an o v e r a l l two phase v e n t i n g d e s i g n method, and 4) e x p e r i m e n t a l v e r i f i c a t i o n o f t h e d e s i g n method on both s m a l l and l a r g e s c a l e w i t h r e a c t i n g and n o n - r e a c t i n g c h e m i c a l systems. Membership i s s t i l l a v a i l a b l e f o r companies i n t e r e s t e d i n p a r t i c i p a t i n g i n t h i s program. VII.

Summary

The r e a c t o r v e n t i n g problem c o n s i s t s o f s e v e r a l key p a r t s each o f which must be u n d e r s t o o d and c a r e f u l l y handled: 1) t h e heat i n p u t e i t h e r from exo t h e r m i c r e a c t i o n s o r o t h e r m i s c e l l a n e o u s heat s o u r c e s , 2) t h e b a t c h s w e l l mechanism, 3) t h e f l u i d mechanics o f t h e v e n t system, 4) t h e m e c h a n i c a l d e s i g n o f t h e system, and 5) v e n t e m i s s i o n s c o n t r o l . Literature

Cited

1.

Boyle, W. J . , Chem. Engr. Prog., 61-66.

(1967), 63,(8),

2.

Huff, J . Ε., Chem. Engr. Prog. Symp. Ser. - Loss Prevention, (1972), 7, 45-57.

3.

Lockhart, R. W. and M a r t i n e l l i , Prog., (1949), 45,(1), 39-48.

4.

M a r t i n e l l i , R. C., and Nelson, D. B., Trans. ASME, (1948), 70, 695-702.

5.

Hewlett, G. F. and H a l l - T a y l o r , N. S., "Annular Two-Phase Flow", 23-27, Pergammon Press, Oxford GB, (1970).

6.

France, D. M. and S t e i n , R. P., I n t . J. Heat and Mass T r a n s f e r , (1971), 14, 1407-1413.

R. C., Chem. Engr.

R E C E I V E D January 18, 1979.


Designing for Safe Reactor Vent Systems - American Chemical Society

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