abstract efce final corrected

8/13/2019 Abstract EFCE Final Corrected

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Comparative Risk Evaluation of Batch and Intensified Continuous Processes

13 th International Symposium on Loss Prevention

S. Machefer a

, S. Bahrounb

, N. Di Miceli c

, N. Gabasc

, M. Cabassud c

, C. Gourdonc

, S.Li b

,C. Valentin b , C. Jallut b , L. Falk a , J. Jenck d , F. de Panthou e , J.-M. Franois f

a Laboratoire des Sciences du Gnie Chimique (LSGC)-CNRS, Nancy Universit, 1 rue Grandville, BP20451, F-54001 Nancy, France

b Universit de Lyon ; Universit Lyon 1 ; LAGEP, UMR CNRS 5007 ; ESCPE, 43 Bd du 11 novembre1918, 69622 Villeurbanne Cedex, France

c Universit de Toulouse; INPT, UPS; Laboratoire de Gnie Chimique CNRS UMR 5503; 5 rue PaulinTalabot, BP1301, F- 31106 Toulouse cedex 01, France

d ENKI Innovation, 3 chemin des Balmes, 69110 Sainte-Foy-ls-Lyon, France e AET GROUP SAS , 6 monte du Coteau, 06800 Cagnes-sur-mer, France

f Chilworth Sarl, 6 Htel dentreprises Pierre Blanche, Alle des Lilas, 01150 Saint Vulbas, France

1. Introduction

Small (continuous) reactors are inherently safe. This is more or less the uniform tenor ofthose dealing with reactor intensification that leads to a significantly lower reactor hold-up.This safety benefit has especially established as characteristic of micro-reactors, whichhave shown a booming tendency since one decade with nowadays more than 250 newpatents each year [1]. On micro scale, the term inherently safe may indeed be justified forsome applications. Tubular reactors for gas phase reactions, as an example, can bedesigned explosion-resistant on a micro scale and potential explosions would have asignificantly reduced blast [2].However, on a production scale, the (parallelized) micro scale must normally be abandonedbecause of economical and practical reasons [3,4,5]. This results in reactor sizes on a milli-or centi scale in best case (some intermediates, consumer products and polymers). Reactorsof several litres are more realistic for large scale productions (>100 t/a) or reactions that arelimited by mass transfer and require longer residence times. Thus, inherent safety in terms ofvolume gets somewhat questionable for production scale. But there are first of all still otheraspects to consider: Besides specific surface, other conditions are usually also intensified (high pressure,

temperature, concentration) which can fortify other hazards or create new risks. Intensification also permits reaction routes which were infeasible in batch mode. This

means that extremely hazardous reactions such as direct fluorinations [6] can be carriedout, which implies major safety concerns already on a very small scale.

Smaller reactors lead to certain control obstacles. Small holdups mostly leave less timefor the operator and the control system to respond to disturbances [7].

Additional risk potential from up- or downstream operations and units may arise incontinuous operation. Risk-scenarios such as no reaction or backflow can dislocatethe risk for example to separation units or storage facilities. The corresponding vesseldesigns are normally less safe and their volume may not differ from batch processes(e.g. reagent storage) .

Consequently a more differentiated discussion about safety of small intensified reactors isneeded. We would like to contribute to this discussion by comparing an ortho-cresolhydrogenation process carried out in batch and intensified continuous mode. Process safety

is assessed by means of a HAZOP study which has been carried out in a team of industrial


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and academic contributors. Selected scenarios were investigated in detail by means of adynamic process model.

2. Process Characteristics

A catalyzed gas-liquid reaction is chosen as test case for the present study. Main reason forthis choice is that these reactions are mostly limited by mass transfer. Consequently thesafety study does not depend that much on the kinetics of the chosen system. Resultsobtained in the present study may be easily transferred to other gas-liquid reactions.

1.1 Continuous intensified minireactor

Small size, intensified gas-liquid contactors which made it to production scale are rather rare.However, one such minireactor that permits considerable throughputs (ca. 100-1000 t/a) isthe RAPTOR developed by the AETGroup [5]. This reactor has amongst others been usedfor the hydrogenation of o-cresol which is also taken as test case for this study. Figure 1illustrates the continuous o-cresol hydrogenation process using this minireactor. Molten o-cresol and suspended catalyst (Pd/C) are continuously fed to the minireactor after havingbeen preheated to reaction temperature. Intensified mass transfer performance permitssmall residence times (a couple of minutes) which in turns permits a small reactor volume(


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solvent evaporation. The purified product is finally filled into drums. The flow sheet of thevirtual batch process is illustrated in Figure 2.The reactor size has been determined based on the same productivity as for the continuousprocess described above. The batch process requires many preparative and changeover

operations (see above). Moreover the exothermic reaction requires strong dilution (ethanolsolvent) in order to be operated safely. Finally a reactor volume of 6m 3 (2/3 filled) has shownto be necessary. The relevant process parameters are compared to those of the intensifiedcontinuous process in Table 1.

Figure 2: Simplified flow sheet of the batch process for a hydrogenation reaction.

Table 1: Safety relevant process propertiesIntensified minireactor Virtual batch process

Reactor volume 0.0007 m 3 6 m 3 Hydrogen pressure 200 bar 10 barOperation temperature 170 C 100CCatalyst concentration 0.4% 4%Solvent (ethanol) none 75 vol.-%Residence/operation time


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The hazards severity and its probability of occurrence were classified as shown in Table 2. After having identified a scenario with a potential risk, measures of loss prevention arediscussed and the residual risk (safety measure considered) is re-evaluated. Herein differentmeasures are by convention treated differently: Correctly designed rupture discs or safety

valves reduce the probability of occurrence by three and two classes respectively. Safetymeasures which are implied by a central control system lead to a reduction by just one class.The HAZOP study requires a heterogeneous team of safety experts. For the present case ateam of 13 chemical engineers and chemists was formed, there under specialists for processsafety, process control and chemical reaction engineering. Members from academia as wellas (process users) from industries participated. The study has been leaded and recorded bya safety expert (Chilworth Technology) using a computer assisted HAZOP software.

Table 2: Definition of severity and probability classes for the HAZOP studySeverity of the scenario Probablilty of occurrenceClassification Definition Classification Definition (frequency)

1(moderate)

Minor injuries Minor environmental impact Process unit marginally affected Production not interrupted

1

(quasi excluded)1e-7 1e-6 1/y

2(improbable) 1e-6 1e-5 1/y

2(serious)

Violation of emission standards Process unit seriously damaged Production interrupted

3(very rare) 1e-5 1e-4 1/y

3(fatal)

Multiple severe injuries One possible dead Pollution remains on site Process unit not operable Considerable financial damage

4(rare) 1e-4 1e-3 1/y

5(probable) 1e-3 1e-2 1/y

4(major)

Multiple deaths on site Lethal effects off site Off site pollution Impact on brand image possible Process unit destroyed

6(occasional) 1e-2 1e-1 1/y

7(frequent) 1e-1 1e0 1/y5

(disastrous)

Multiple deaths on and off site Serious environmental damage Whole process closed

3.2 Results and Discussion

For the batch process, 43 Scenarios have been identified and 17 were classified to be of

relevant potential risk as illustrated in Figure 3. The continuous process, on the other hand,revealed 22 Scenarios of which 14 form a potential hazard. Considering also the residualrisks we conclude from these quantitative results (Figure 3) of the HAZOP study that:

The batch reactor constitutes a higher potential hazard (17 hazardous scenarios, 5unacceptable risks) when compared to the continuous minireactor (14, 2)

One scenario of the batch process remains of unacceptable residual risk. Apart from this the allocation of the residual risks shows no fundamental difference

for the batch and intensified continuous process. It is worth mentioning, that most ofthe batch scenarios with a residual risk in the ALARP region (gray shaded) areassociated to human failure (5 out of 6). Residual ALARP risks for the intensifiedcontinuous process have their origin predominantly in the principle of continuous

operation (leakages, control failures etc.) rather than intensification (6 out of 7).


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Hence these risks are unspecific with respect to the reactor and would as well bepresent in any other continuously operated process.

Batch scenarios of unacceptable risk (black region) are rather characterized by ahigh severity whereas they are of high probability in the continuous minireactor.

The last aspect is of special importance since it must not be forgotten, that each severityand probability class represents some kind of order of magnitude (Table 2). A closerlook at the black scenarios is provided in Table 3. The impact of the solvent turns out tobe the key difference between both processes. For the batch process the presence of asolvent leads to scenarios in which vessel rupture is expected because of the solventsvapour pressure or in which solvent vapours form an explosive mixture in theevaporation unit. In three out of five black scenarios the solvent is the activator of anincident. In the continuous minireactor, on the other side, the absence of solvent meansa much higher adiabatic temperature rise, hence one scenario in which a thermalexplosion would lead to an unacceptable potential risk. Another important difference isthe role of scenarios based on operator failures which are obviously more frequent forthe batch process and lead to an elevated number of corresponding scenarios of whichtwo lead to severe hazards, e.g. forgotten inertisation.

Figure 3: Risk allocation of the HAZOP analysis for the hydrogenation of o-cresol

Vessel size and respective quantities which are processed do not only have an impact on theseverity of an incident: In contrast to the batch vessel, the minireactor could be installed in ablockhouse as ultimate safety measure. Another advantage of the minireactor is thepossibility of instant drainage (low volume, high pressure). Loss prevention for the batchprocess is somewhat limited to process control and standard relief devices.The impact of working pressure is more difficult to evaluate. Within the HAZOP analysis thisaspect has been considered indirectly by applying higher probabilities for correspondingscenarios. Main reason is the fact that the risk of leakage is elevated and radius of exposuremay be larger. The latter is especially important to consider for the emitted hydrogen whichcan create flammable mixtures already at low volumetric fractions. However, adecomposition of the entire content of the minireactor would approximately correspond to anequivalent of 5g TNT, a pressure rise of 140mbar and broken windows in worst case.


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Table 3: Description of those scenarios with high potential risk (s:severity, p:probability)

Batch reactor

Scenario Consequences

pot.

risk(s,p)

Safetymeasures

res.

risk(s,p)

B1Reactor:

Temperature (T)control failure

High vapour pressure (solvent) causespartial vessel rupture / Hydrogen jet

emission and explosion (self ignition).(4,6) Safety valve +rupture disc (4,2)

B2Reactor : T-probedefect (Tmax not

detected)

Continuous hydrogen feed (normallystops at Tmax) / slow pressure in-

crease / Hydrogen jet emission andexplosion (see B1)

(4,6)Safety valve +rupture disc +Pressure alarm

(4,1)

B3Reactor:

Hydrogen pressurereducer fails

Reactor design pressure is exceeded.Hydrogen jet emission and explosion

(see B1)(4,6) Safety valve +rupture disc (4,2)

B4Reactor:

No Inertisation afterreaction (operator

failure)

Hydrogen is not removed / contactwith air in separator which is notdesigned for high pressures (e.g.condenser) / Explosion if ignition.

(3,7) Safety valve +rupture disc (3,4)

B5

Separator:solvent evaporation:

No inertisation(operator failure)

Solvent vapours in contact with air /Explosion if ignition (e.g. electrostatic

source)(4,5)

Safety valve /rupture disc arenormally NOT

designed for thisscenario

(4,5)

Continuous minireactor

C1

Reactor jacket:No circulation of

service liquid (pumpfailure)

Adiabatic temperature rise,decomposition reaction, thermal

explosion(3,7)

Temperaturecontrol (stop

agitation, stophydrogen feed),

rupture disc

(3,3)

C2 o-cresol reservoir(melt): Empty Aspiration of air, hydrogen moves

upstream, mixture with air, explosion (3,7) Flow rate control (3,6)

Process scenarios which are particularly difficult to evaluate are those which involve failure inthe cooling system. Two factors complicate the evaluation of these scenarios which areparticularly crucial for the undiluted minireactor with its adiabatic temperature rise of almost1000K: Firstly the reaction (like most gas-liquid reactions) is believed to be limited by masstransfer. Hence standard procedures for thermal runaway calculations, assuming a kineticregime, are not applicable [9]. Secondly the massive pressure-proof construction of the

minireactor exerts certainly a pronounced effect on the dynamics of the reactor. A processmodel has therefore been developed which should give insight into the special dynamicbehaviour of the intensified minireactor.

4. Dynamic simulation

The intensified continuous minireactor has been modelled as reactor cascade. A kinetic andmass transfer model has been applied which includes adsorption of the reagents on thecatalyst [10]. Possible decomposition reactions have not been implemented. Model detailshave been published elsewhere [11]Scenario B1, about which the HAZOP team felt most unsure, shall be taken as exemplary

simulation. The simulation results as shown in Figure 4. After start up and maintenance ofsteady-state, the circulation of service liquid is stopped (hydrogen is fed further). After a


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latent period of a few minutes, a critical wall (metal) temperature is exceeded which couldpossibly initiate consecutive decomposition reactions. However, thanks to the massivereactor construction the latent period provides enough time for countermeasures such ashydrogen feed cut, agitation stop and/or emergency drainage of the reactor. Hence the initial

evaluation of Scenario B1 in the HAZOP analysis was confirmed by the reactor model.The strong impact of reactors metal mass on the latent period is illustrated in Figure 5. Aconstruction where the metal mass is small when compared to process liquid mass wouldhave allowed a significantly shorter time for countermeasures. Hence intensification in termsof pressure does not only constitute an additional hazard but indirectly contributes also tocertain safety benefits.

Figure 4: Dynamic simulation of the HAZOP scenario B1 (intensified continuous minireactor)

Figure 5: Hypothetical impact of the intensified reactors metal mass on the latent period

5. Conclusions

We compared a large scale batch reactor with an intensified continuous minireactor(RAPTOR) for a highly exothermic hydrogenation reaction in terms of safety. A model


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assisted HAZOP analysis has been performed which highlighted critical operation scenarios. Against mainstream we cannot entirely confirm the statement that safety is inherent insmaller reactor designs. The qualitative difference of corresponding risks should rather bepointed out. In our example, the classical batch process is associated with potential risk

scenarios of high severity. The intensified continuous minireactor reveals scenarios of similarrisk which are of much lower severity, but significantly higher probability. However, for thespecific application described here, the intensified process turned out to be safer butbecause of other reasons which are mostly just indirectly related to reactor volume:

Intensification leads to a solvent-free operation, which avoids risk scenariosassociated with a flammable solvent of high vapour pressure. These scenarios haveshown to be most problematic for the batch reactor or following (separation) units.

Intensification in terms of high pressure leads to a massive reactor construction whichdelays thermal runaway and provides more time for intervention.

Smaller reactor size allows for additional measures of loss prevention such as instantdrainage and blockhouse installation, which are infeasible or too expensive for largescale processing.

Due to very different designs the safety of intensified processes has to be evaluatedtechnology specific. In best case results obtained for one technology can be generalized fora reaction class. The results of this study, for example, should be transferrable to other gas-liquid reactions. In many other cases, however, the question whether or not intensificationleads to safer processes will be not only technology but also highly application specific.

AcknowledgementsThe authors would like to thank the ANR (Agence Nationale de la Recherche, France) forfinancial support in connection with the project PropreSur (french acronym for clean andsecure processes towards future chemical plants) with grant number ANR-06-BLAN-0381 .

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[6] T. Obein Caractrisation dun microracteur film tombant et tude de la fluorationdirecte de lAnisole dans ce microracteur. 2006, phD-thesis INPL/Rhodia.

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[9] J. Steinbach Safety Assessment for Chemical Processes Wiley-VCH, Weinheim, 1999[10] H. Hichri, A. Accary and J. Andrieu Kinetics and Slurry-type Reactor Modelling during

Catalytic Modelling of o-Cresol on Ni/SiO 2 Chem. Eng. Process. 1991, 30, 133-140.[11] S. Bahroun, C. Jallut, C. Valentin et al. Dynamic modelling of a three-phase catalytic

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