Catalytic Hydrogenation in the Liquid Phase

SAFE PROCESS MANAGEMENT IN THE CHEMICAL INDUSTRY 791 CHIMIA 2003, 57, No.12

Chimia 57 (2003) 791–798 © Schweizerische Chemische Gesellschaft ISSN 0009–4293

Felix Roessler*

Abstract: Catalytic hydrogenation in the liquid phase is a very common reaction type in the production of fine chemicals and pharmaceuticals. Among the various reaction techniques, it is the slurry technique (stirred tank or venturi type loop reactor) in a semi-batch mode which is most frequently used. General safety aspects of catalytic hydrogenations will be discussed und exemplified for a typical three-phase semi-batch catalytic hydrogenation. Keywords: Catalytic hydrogenation · Hydrogen · Metal catalyst · Safety

1. Introduction Incident no. 1 due to lower quality of nitroaromatic com- (Decomposition in the hydrogenation pound and/or catalyst than normal), which Three-phase catalytic hydrogenations are of a nitroaromatic compound) upon heating led to disproportionation and reactions that can generally be carried out In 1976 a violent explosion occurred at coupling reactions with temperature in- with low risk provided that proper precau- Du Pont de Nemours Co., Deepwater, NJ crease which triggered further autodecom- tions are followed. Within the Roche com- with destruction of plant and plant building position of these intermediates and run- pany about 10% of all chemical steps in the [1]. The explosion occurred during the hy- away reaction with pressure build up in the synthesis of pharmaceuticals, vitamins and drogenation of 3,4-dichloro-nitrobenzene. closed system which finally led to destruc- fine chemicals are catalytic hydrogena- The analysis of the process of explosion tion of the hydrogenation vessel and plant tions. Despite this high proportion of cat- gave the following picture: (1) Hydrogen building. alytic hydrogenations, no noteworthy inci- uptake ceased before complete conversion; Lesson: Never produce chemicals with- dents occurred in the last few decades. (2) in order to complete the conversion, the out detailed information on the underlying Nevertheless, incidents with catalytic hy- temperature in the reactor was increased by chemistry of the process (kinetics, thermo- drogenations occasionally take place, as is heating; (3) the consequence was a violent dynamics, pathways). shown in the following examples. explosion. Analysis of the chemistry showed: (1) Incident no. 2 Reaction path of desired reaction: ArNO2 (Explosion during steaming of → → → ArNO ArNHOH ArNH2 (Ar = aro- Raney nickel matic ring system); (2) possible side reac- containing residual ethylacetate) tions: Disproportionation of ArNHOH (2 In 1998 an explosion took place at one → ArNHOH ArNO + ArNH2 + H2O), cou- of Roche’s contract manufacturers. 250 kg pling reactions with formation of azoxy-, of spent Raney nickel containing residual azo-, and hydrazo compounds, autodecom- ethylacetate was treated with hot water position reactions of nitroaromatic com- steam at 3.5 barg steam (148 °C) in a 650 l pounds and arylhydroxylamines (potential backflush filter. During this operation, the explosives). pressure in the filter increased to 1.5 barg. Kinetics of the reaction: First and sec- Because of this pressure increase, all valves → → ond step (ArNO2 ArNO ArNHOH) were closed. Nevertheless the pressure fur- are fast reactions, last step (ArNHOH → ther increased to 2.5 barg within 5 min and ArNH2) is slow and has a high activation after another 20 sec to 15 barg. The safety energy, i.e. is more temperature sensitive valve opened at 10 barg, but the enormous- than the first two steps. ly fast pressure build up could not be com- Thermodynamics of the reaction: (1) pensated quickly enough. Therefore the Main reaction is strongly exothermic (ca. cover of the filter housing was lifted and the *Correspondence: F. Roessler 170 kJ/mol H2); (2) disproportionation re- pressure was released via the opened Roche Vitamins AG action, coupling reaction and autodecom- flange. As a consequence, it came to igni- CH–4303 Kaiseraugst position also strongly exothermic. tion of combustible material (mainly hy- Tel.: + 41 61 687 27 13 Fax: +41 61 687 22 01 Explanation of incident: Accumulation drogen and ethylacetate). Retrospective E-Mail: felix.roessler@roche com of arylhydroxylamine ArNHOH (probably analysis showed that the incident was SAFE PROCESS MANAGEMENT IN THE CHEMICAL INDUSTRY 792 CHIMIA 2003, 57, No.12 caused by liberation of adsorbed hydrogen and reaction of water with residual alu- minium in the Raney nickel (and possibly Chemistry and Physics (Origin of Hazards) nickel itself) with formation of the metal oxides and hydrogen. Further metal-catalyzed reactions such as hydrogenolysis of ethylacetate with hydrogen in the gas phase

(initiated by hot spots) resulting in the for- Materials (chemical and Reactions (changes of volume, mation of methane and other volatile mate- physical properties, health temperature, pressure): rials were also discussed. data): • Hydrogenations with Lesson: Never treat Raney catalysts • Substrates formation of: with hot water or even steam in closed sys- • Intermediates • Intermediates tems. • Products • Desired product • Hydrogen • Side products by Incident no. 3 • (Oxyhydrogen explosion in a filter Solvents consecutive hydrogenation • housing containing catalyst) Catalysts and parallel hydrogenation • In the process of separating used Raney Acids/Bases • Side reactions: nickel, it came to an explosion in a filter • Heating-/cooling liquids • Isomerization housing. Accidentally it came to a subat- • Water • Decomposition mospheric pressure and then suction of air • Oxygen (air) • Disproportionation into the filter housing containing solvent- • Construction materials • Rearrangement wet Raney nickel. The reaction of oxygen • Polymerisationen with adsorbed hydrogen and solvent vapor • Oxidation led to a pressure increase which damaged • etc. the filter housing. Lesson: Never allow subatmospheric pressure in vessels containing combustible Methods, Machinery and Humans (Reasons of Hazards) materials and/or catalysts before careful to- tal inertization.

Systemic reasons: Deviations from design data: 2. Hazards of Catalytic • Improper design of process • Technical error Hydrogenation Processes • Improper design of • Human error machines and reactors As in every chemical process, hazards in catalytic hydrogenations originate from the materials used in the process and from the reaction of these materials, whereas the reasons for an incident taking place may be of systemic nature (incorrect process de- Fig. 1. Origins and reasons of hazards upon performing catalytic hydrogenation sign etc.) or a deviation from the design conditions (technical or human error) (Fig 1).

Unlike ‘normal chemical reactions’, 4. Hydrogen and 3. Prevention of Incidents catalytic hydrogenations require special ad- Catalyst Specific Properties ditional attention due to hydrogen being a and Safety Relevant Data As is common practice with chemical very energetic and easily ignitable com- reactions, precaution and measures for the bustible gas and metal catalysts being very As mentioned previously, general mate- safe execution of catalytic hydrogenations potent ignition sources. rial data and safety relevant material data are subdivided into: (1) Measures for the Evaluation of hazards of catalytic hy- are a very important basis for hazard evalu- prevention of explosive mixtures (com- drogenations can be made on the basis of a ation and risk assessment of catalytic hy- bustible material, particularly hydrogen/air standard procedure [2]. Such a procedure drogenations. Hydrogen and metal cata- mixtures) as well as prevention of condi- consists of: (1) Basic information such as lysts need special attention. tions of reaction runaway (primary explo- data on the process (material and reaction sion protection); (2) measures for the pre- data), (2) data on the plant equipment and 4.1. Hydrogen vention of ignitions by exclusion of ignition (3) data on methods of plant operation. A Hydrogen is a very potent, highly ener- sources (secondary explosion protection); risk assessment can then be made on the ba- getic combustible material. Some proper- (3) measures for the limitation of the con- sis of the hazard evaluation. ties of hydrogen are given in Table 1. When sequences of explosions (constructive ex- Both evaluation of hazard data and risk working with hydrogen, inertization is a plosion protection); (4) organizational assessment are very process specific. More must because of the broad explosion range measures, training of employees and emer- details on prevention of incidents and risk of hydrogen/air mixtures on one hand and gency concept. assessment can be found elsewhere [3]. the low ignition energy needed to ignite hy- SAFE PROCESS MANAGEMENT IN THE CHEMICAL INDUSTRY 793 CHIMIA 2003, 57, No.12 drogen/air mixtures on the other hand. Table 1. Properties of hydrogen Table 2 gives threshold values for partial and complete inertization. Properties of hydrogen Precautions, comments More safety relevant key data of hydro- Colourless and odourless Use of hydrogen sensors recommended gen and hydrogen/air mixtures can be Flame almost invisible; flame temperature of found elsewhere [3]. stoichiometric mixture of hydrogen in air (29.6% H2) has flame temperature of 2110 °C Highly flammable and forms explosive mix- Avoid air, proper inertization, be careful 4.2. Catalysts tures with air (4-75 vol% hydrogen), oxygen with vacuum, good ventilation Raney catalysts in particular, but also (4-96 vol% hydrogen), chlorine (3-92.5 vol% hydrogen) and many other gases noble metal catalysts that contain adsorbed hydrogen are pyrophoric and therefore ig- Deflagration: Deflagrative burning velocity of hydrogen in air is in the order of 3 m/s; nite in the presence of air. Spent catalysts max. pressure ratio in deflagration is are therefore preferably wetted with water ca. 7 (final pressure/initial pressure) before transport for recovery of the metals. Detonation: Detonative burning velocity Hydrogenation catalysts are very potent ig- of hydrogen in air is on the order of 2000 m/s nition sources (lowering of activation ener- Autoignition temperature of hydrogen/air gy for reaction of combustible materials mixture 580 °C with oxygen). Döbereiner made practical High diffusion rate of hydrogen Hydrogen spill in open area diffuses use of platinum as an ignition source in rapidly to nonexplosive mixture 1829 with his invention of the catalytic fire (2000 liter hydrogen spill in unconfined lighter (Fig. 2). area will diffuse to an nonexplosive Base metal catalysts (particularly mixture within about one minute). Raney catalysts) can react with water to Low ignition energy (0.019 mJ): ignites without Ignition even by catalytic effect form metal oxides and hydrogen (pressure apparent ignition source (for comparison: of surfaces, catalysts increase in closed systems!). Contact of hydrocarbon vapors 0.25 mJ) combustible materials in the gas phase with Negative Joule Thomson Heat generation upon expansion catalysts, particularly Raney catalysts) can of pressurized hydrogen lead to hot spots (reaction of adsorbed hy- Small size of hydrogen molecule Proper design of piping according drogen with organic solvents at elevated makes hydrogen prone to escape via leaks NFPA pamphlet 50 H: Standards for gaseous hydrogen systems at consumer temperature in the gas phase with formation sites, National Fire Protection Association, of hydrogenolysis products and pressure in- Quincy MA crease in closed systems). It is good prac- Storage in gas cylinders Gas cylinder must be properly secured tice not to completely dry spent catalysts or (grounded and fixed) and should be stored preferably to wet them with water. in a cool and dry and well vented area; It should also be borne in mind that handling of compressed gases compare powder catalysts can cause dust explosions. (www.airproducts.com) Solubility of hydrogen in solvents Rule of thumb: 0.1 normliter hydrogen per liter of solvent and bar 5. Design of Inherently Safe Compatibility of hydrogen with materials Compatible with almost all materials; Hydrogenation Processes embrittlement of materials that can form solutions with hydrogen (palladium) In order to ensure safe hydrogenation Specific weight low Hydrogen has tendency to escape to top, processes, the basic data of the reaction but be careful about this rule of thumb as diffusion rates and draught may dominate must be well established, i.e. the reaction this gravity driven escape direction network (main and side reactions including individual steps of multiple reactions) as well as kinetic (particularly temperature de- pendence) and thermodynamic data of all Table 2. Threshhold values for inertization these processes and reaction steps must be known. Threshhold values at partial inertization with N2:

Maximum oxygen concentration below 5 mol% O2 5.1. Reaction Network which no explosive mixtures exist Frequent side reactions of catalytic hy- whatsoever the hydrogen concentration drogenations and hydrogenolyses are met- will be (LOC) al-catalyzed isomerizations, disproportion- Minimum ratio of mol N2/mol air above 3.0 Mol N2/mol air ations and coupling reactions, acid/base which no explosive mixtures exist catalyzed reactions (due to acid or base whatsoever the hydrogen concentration is (MAI) traces or properties of the catalyst) as well Threshhold values at complete inertization with N : as consecutive (primary hydrogenation 2 product can be further hydrogenated) and Minimum ratio of mol N2/mol hydrogen, 17 Mol N2/mol hydrogen parallel hydrogenations (one particular above which no explosive mixtures exist whatsoever the air concentration will be molecule can undergo different reductions (MXC) leading to different products). Besides SAFE PROCESS MANAGEMENT IN THE CHEMICAL INDUSTRY 794 CHIMIA 2003, 57, No.12

Fig. 2. Döbereiner fire lighter (picture taken from A. Baiker, lecture 1997) these metal-catalyzed reactions, any other Table 3. Average reaction enthalpies of hydrogenations type of thermal reaction can be part of the reaction network. Reaction Reaction heat [kJ/mol H2] 5.2. Thermodynamics Carbon-carbon double bond to single bond 125 Hydrogenations are generally exother- Carbon-carbon triple bond to single bond 150 mic (Table 3). Aromatic ring saturation 70 Ketone to alcohol 65 5.3. Kinetics Nitro to amine 170 The rate of catalytic hydrogenation is Carbon-halogen hydrogenolysis 65 influenced by temperature, amount of catalyst, type of solvent, concentration of substrate (and product) and partial pressure of hydrogen. The rate of the surface reaction found out by stirring speed and catalyst can normally best be described by a Lang- loading experiments in a stirred tank reac- muir Hinshelwood rate law, which inte- tor in semi-batch mode. grates both adsorption as well as reaction For a simple power-law rate equation, rate of adsorbed species into the rate equa- the hydrogenation rate under equilibrium tion. conditions can be described as represented in Eqn 1: 5.4. Mass- and Heat Transport, Fluid Dynamics and its Relevance r = k a (c – c ) for Safe Hydrogenation Processes obs gl * gl * H2 g/l-interface H2 liquidbulk n m (1) Heterogeneous catalytic hydrogena- = kreact * ccatalyst * (cH2 liquidbulk) * (csubstrat) tions in the liquid phase are three-phase processes. Mass transport, heat transport where: robs = observed reaction rate, kgl = On the other hand, the hydrogenation and fluid dynamics have therefore to be gas/liquid transport coefficient, agl = sur- rate can be influenced via the rate of the sur- considered in addition to the chemistry. In face area gas/liquid, cH2 g/l-interface = partial face reaction per unit volume of reaction practice it is the gas/liquid transport (i.e. pressure of hydrogen at gas liquid interface, solution: transport of hydrogen from the gas to the cH2 liquidbulk = partial pressure of hydrogen • Temperature (kreaction is dependent on liquid phase) which is of most importance. in bulk liquid, kreact = reaction rate constant, the temperature) Depending on the individual conditions, we ccatalyst = amount of catalyst, csubstrat = con- • Amount of catalyst speak of a reaction in the transport regime centration of substrate. • Hydrogen partial pressure if n > 0 (reaction rate is limited by gas/liquid trans- A hydrogenation can therefore always • Substrate concentration if m > 0 port rate, i.e. slowest step is the transport of be slowed down (or even turned off) by: In practice, it is therefore always possi- hydrogen from the gas to the liquid phase) • Minimization of agl (decrease stirring ble to slow down a rapid heat production in or reaction in the kinetic regime (the sur- rate or even switching off stirrer) a catalytic hydrogenation by measures such face reaction is the slowest step). Whether • Minimization of (cH2 g/l-interface – cH2 as reduction of the stirring speed, reduction a hydrogenation takes place in the kinetic or liquid bulk) by decreasing pressure or of the pressure, feeding inert gas or switch- transport limited region can be quickly feeding inert gas ing off the hydrogen supply. Nevertheless SAFE PROCESS MANAGEMENT IN THE CHEMICAL INDUSTRY 795 CHIMIA 2003, 57, No.12 switching off the hydrogen supply is only a means to stop heat production which is due conversion (Hydrogen) to reaction of hydrogen. Kinetic regime: rate is controlled by 5.5. Safety Aspects of Various kinetics of surface reaction, i.e. slowest Hydrogenation Techniques step is the surface reaction

Each of the subsequently described hydrogenation techniques has its specific ad- Transport regime: rate is controlled by gas/liquid vantages and disadvantages regarding safety. i.e. transport rate, slowest step is the transport of hydrogen from the gas to the liquid phase 5.5.1. Semi-batch This is the most frequently used technique in the small to medium scale produc- time tion of fine chemicals and pharmaceuticals. The substrate is loaded into the reactor as Fig. 3. Time/conversion plot of semi-batch hydrogenation one batch and the hydrogen is fed continuously; the pressure is frequently held constant over the whole cycle time. Due to the limited availability of hydrogen in the reac- Isomers of Diene 1 tor (maximum amount of hydrogen in the reactor = hydrogen in the gas phase and hy- + 1 H drogen dissolved in the liquid phase), a high 2 degree of safety is given. An additional + 1 H2 + 1 H2 R2 R2 safety aspect is the possibility to slow down R1 n Monoenes R1 n the reaction by slowing down or even switching off the hydrogen supply. Diene 1 (n = 2) Alkane (desired product)

5.5.2. Continuous Hydrogenation side reaction in a Continuous Stirred Tank Reactor (above defined and allowed temperature of process) In case of an instable starting material, it is possible to run the hydrogenation at a high conversion degree, thus keeping the byproducts (polymers and decomposition products) steady state concentration of instable (and therefore safety relevant) starting material Scheme. Hydrogenation of diene to saturated alkane in the reactor low.

5.5.3. Continuous Hydrogenation amount of catalyst) the hydrogenation is main side reactions: (1) Isomerization of di- in a Catalytic Fixed Bed Reactor very fast and therefore is mainly gas/liquid ene (isomerization products are finally con- Due to the generally high catalyst transport limited as long as dienes are pres- verted to desired alkane). The rate as well holdup and therefore high timespace yield ent in the solution, i.e. the rate of transport as reaction enthalpy of this reaction is mi- of tubular fixed bed reactors, the reaction of hydrogen from the gas to the liquid phase nor compared with the rate and enthalpy of volume can be held small, which is an ad- is the slowest step (slower than the maxi- the hydrogenation reaction, but the activa- vantage in the case of an incident. mum possible surface reaction rate of tion energy is slightly higher, i.e. more tem- chemisorbed hydrogen with chemisorbed perature sensitive. (2) Polymerization and diene). As soon as all dienes have been con- decomposition of diene; as long as the hy- 6. Case Study verted (mixture then consists of mono-enes drogenation is run within the design tem- of a Semi-batch Hydrogenation and alkanes), the reaction gets slower and perature range of the process, the rate of transforms to kinetic control (reaction rate polymerization and decomposition reaction 6.1. Chemistry determined by rate of surface reaction) can be neglected. Nevertheless the activa- 6.1.1. Reactions and Reaction (Fig. 3). tion energy for both side reactions is much Pathways The temperature sensitivity of the de- higher than for the hydrogenation, i.e. a The process to be dealt with is the cat- sired reaction was determined to lie in a temperature increase favors the polymer- alytic hydrogenation of a diene to the cor- normal range (activation energy for con- ization/decomposition reaction and could responding saturated alkane. The overall version of diene to monoene and monoene lead to a runaway situation. Exceeding the reaction consists of a set of multiple reac- to alkane in the order of 60 kJ/mol). The re- upper limit of the design temperature range tions (equilibrium reactions, consecutive action enthalpy ∆H for diene to alkane was must therefore be avoided by all means. and parallel reactions) (Scheme). determined with 257 kJ/mol diene. The adiabatic temperature increase is very high 6.2. Reaction Technique 6.1.2. Chemical Kinetics with 550 °C, as the reaction is carried out in The hydrogenation is carried out as a and Thermodynamics the absence of solvent (diene is a liquid). slurry three-phase process in a stirred tank 6.1.2.1. Hydrogenation reactor in a semi-batch mode, i.e. hydrogen (Desired Main Reaction) 6.1.2.2. Side Reactions is fed continuously to the batch-fed diene. It Under the chosen reaction conditions Besides hydrogenation reactions, the re- will later be explained why this technique (pressure, temperature, stirring frequency, action network is characterized by two was chosen (section 6.4.3). SAFE PROCESS MANAGEMENT IN THE CHEMICAL INDUSTRY 796 CHIMIA 2003, 57, No.12

6.3. Results from Hazard Evaluation

Inertization of Loading of catalyst A hazard analysis of the hydrogenation hydrogenation reactor : 3 receiver with diene resulted in the following potential times 1.3 bar N2/20 mbar (under air) hazard/risk potentials: (1) Hydrogen and air Inertization of substrate (vacuum) form explosive mixtures; (2) catalyst and receive r transfer of catalyst from air form explosive mixtures (dust explo- Loading of catalyst container to sion); (3) very high adiabatic temperature hydrogenation reactor: catalyst receiver with increase of hydrogenation; (4) exothermic - diene Ystrahl (suction of dry Loading of substrate autodecomposition of diene at high temper- receiver with diene - catalyst suspension catalyst into diene solution below liquid ature (upper temperature limit in process is niveau), (under air) 140 °C); (5) spent catalyst is pyrophoric; Inertization of (6) ignition sources: catalyst is potent igni- Transfer of substrate to hydrogenation reactor : 3 Inertization of catalyst tion source to ignite hydrogen/air-mixtures, reactor (with pump) times 1.3 bar N2 / 20 receiver electrostatics; (7) spill from reactor to plant mbar (vacuum) building.

Transfer of catalyst Loading with hydrogen slurry from catalyst 6.4. Measures Taken to

(break vacuum with receiver to reactor (by Minimize Risks hydrogen) gravity) The process operations are schematical- ly represented in Fig. 4 and described in Reaction: conditions p max 1.5 barg, temperature 80 – 100 °C more detail in the following sections. Control of reaction rate by adjusting pressure If temperature cannot be held by means of pressure decrease, then hydrogen supply is shut off. When pressure in reactor gets subatmospheric (by consumption of residual 6.4.1. Measures to Prevent Dust hydrogen with closed hydrogen valve, then nitrogen is automatically fed to avoid Explosion (Fresh Catalyst/Air Mixtures) subatmospheric pressure) During Loading of Fresh Catalyst The dry catalyst is transferred by means Liquid sampling for analysis und decontrol of a special device (‘Ystrahl-stirrer’, Fig. 5) Liquid sampling (top of reactor with immersed tube) with special device that from the catalyst barrel to a catalyst-slurry minimizes risk of gas and liquid spill vessel previously loaded with diene. The catalyst is wetted as soon as it enters the cat- Release of hydrogen overpressure into waste gas collection (closed inertized system, alyst receiver. These operations are carried partial inerization); waste gas finally burnt off out under air.

Inertization of hydrogenation reactor : 3 times 1.3 bar N2/20 mbar (vacuum), all 6.4.2. Measures to Prevent waste gas goes into inertized waste gas collection Explosive Hydrogen/Air Mixtures The following measures were taken to Transfer of slurry from hydrogenation reactor to inertized filter feed tank, volume of avoid explosive hydrogen/air mixtures in liquid in hydrogenation reactor is replaced by nitrogen the reactor, receivers, catalyst filter and tubes on one hand as well as the plant hall Transfer of slurry to inertized fundabac filter by pump on the other side (Fig. 6): (1) Complete inertization of vessels before loading with hy- Filtration drogen; (2) gases from inertization (including gas from evacuation) are released only

Catalyst is “dried” with nitrogen (50 – 60 °C) Filtrate for further into inertized waste gas collector; (3) dou- operations ble mechanical seals (sealing liquid under

pressure, pressure of sealing liquid higher than reaction pressure, in case of sealing Transfer of spent catalyst to disposal bags resp liquid pressure below set threshold pres- conainers (under nitrogen or carbon dioxide) sure, then alarm); (4) good ventilation in plant; (5) sensors for combustible gases Fig. 4. Hydrogenation process operation steps (combustible gases, non specific) in plant on top of stirred tank reactors; (6) appropri- ate definition of Ex-zone: EExdeIICT6; (7) to avoid suction of air into reactors, subatmospheric pressure is avoided during process, except for inertization under controlled conditions; (8) in order to enable opening of all system parts without risks, a sufficient number of open/close valves and flush connections are installed. SAFE PROCESS MANAGEMENT IN THE CHEMICAL INDUSTRY 797 CHIMIA 2003, 57, No.12

6.4.3. Measures to Prevent High Reaction Temperatures During the Process (Control of Reaction Rate) In order to avoid exceeding the upper temperature limit of the process and runaway situation, the following actions were taken: (1) The reaction is run in a semi- batch mode: by this technique, one of the reaction partners (hydrogen) is fed continuously, i.e. its concentration in the reactor can always be kept at a low value, and if desired the hydrogen flow can be turned off to stop heat production by hydrogenation; (2) the reaction is run non-isothermally until the design temperature (80–100 °C) is reached; (3) when the design temperature is reached, the hydrogen pressure is set to a lower value. This results in a lower gas/liquid transport rate (reaction rate is gas/liquid transport limited) which allows to keep the Fig. 5. Separate cabin for preparation of catalyst/diene slurry (can be temperature constant by means of the avail- closed and easily cleaned in case of catalyst spills); A = suction stirrer (Ystrahl), B = catalyst slurry preparation and feed tank, C = tube for able cooling (Fig. 7); (4) finally when the transfer of catalyst slurry to reactor, D = container with catalyst from hydrogenation slows down, the pressure is supplier, E = nozzle for suction of catalyst increased to 1.3 barg.

a) b)

P I T I

N H2 2 M liquid sample

Qreaction Qcooling

∆Hreaction = 128 kJ/Mol hydrogen c) d) Hydrogen consumption per unit time is a fairly good calorimetric signal:

Qreaction = ∆H per mol hydrogen feed/hydrogen feed rate

Rate of hydrogenation (equals hydrogen feed rate at constant pressure) is controlled by the pressure difference between feed pressure and partial pressure of hydrogen in bulk solution (for agl = constant):

Rhydrogen feed (hydrogen feed rate) = kgl * agl * (pH2gas – pH2bulk liquid)

Heat removal by cooling is given by:

Qcooling = A * (Tr - Tc) * K

where: Q = heat removed in W (1 W = 1J/s) A = heat exchange area in m2 Tr = Temperature on reaction side in K Tc = Temperature on cooling side in K K = heat trensfer coefficient in W/m2*K Fig. 6a–d. Safety devices: A = mechanical seal with sealing liquid under pressure (pressure sealing liquid > pressure in reactor), B = flexible tube Under equilibrium conditions the heat production by chemical reaction equals heat removal by cooling, i.e.: for hydrogen feed (to avoid rupture by material fatigue), C = flame barri- er for all gases leaving reactor, D = pressure reduction valve (to adjust Maximum H2 feed [mol H2 / s] = Qcooling [Watt] / ∆H per mol H2 fed [J / mol H2] pressure in reactor), E = sensor on top of reactor, F = leak proof magnet driven pump for pumping liquid to filter feed tank, G = no bottom sam- Fig. 7. Semi-batch reaction mode, heat balance and maximum reac- pling, H = cooling to keep reaction temperature between 80–100 °C tion rate SAFE PROCESS MANAGEMENT IN THE CHEMICAL INDUSTRY 798 CHIMIA 2003, 57, No.12

Fig. 8. Unloading spent catalyst; A = backflush filter housing, B = lock, C = tubes for inertization, D = permanent inertization, E = plastic bag with spent catalyst (new version: steel container)

6.4.4. Prevention of Ignition 6.4.7. Prevention of Spills 6.4.8. Emergency Measures of Pyrophoric Catalyst After Filtration In order to avoid spills, the following In case of emergency (spillage, fire etc.) In order to avoid contact of spent cata- measures are taken: (1) To avoid leakage by local and sector emergency switch off but- lyst with air, the transfer of spent catalyst is squeezed sealings, tongue and groove type tons are installed. carried out under nitrogen (Fig. 8). flanges are used; (2) in order to prevent rupture of tubings by vibrations, hydrogen feed Received: October 3, 2003 6.4.5. Ignition Sources to reactor is via flexible tubes; (3) regular In order to avoid ignition by catalyst, control of tightness and stability against [1] W.R. Tong, R L. Seagrave, R. Wiederhorn, the following actions were taken: (1) The pressure (reactor 10 barg); (4) in case of Loss Prevention Bulletin 1977, 11, 71-75. [ ] plant, particularly the zones where the pres- overpressure in reactor due to overfilling: 2 Schriftenreihe der Expertenkommission für Sicherheit in der chemischen Industrie ence of hydrogen is most probable, is kept safety valve and expansion to blow down der Schweiz (ESCIS), 1986, Heft 4, 2. Au- essentially free of catalyst dust; (2) catalyst vessel; (5) in case of overpressure in reac- flage. addition device is placed in a separate com- tor due to chemistry (explosion of hydro- [3] ‘Handbuch des Explosionsschutzes’, Ed. partment, and even there catalyst spill is gen/air mixture, decomposition of diene): H. Sten, Wiley-VCH-Verlag, Weinheim, avoided by a special device for the transfer design pressure of reactor (7 barg) together 2000. of catalyst from the catalyst container to the with safety valve sufficient to survive ex- catalyst/slurry vessel; (3) catalyst filtration plosion of hydrogen/air mixture; safety units and devices to unload spent catalyst valve with expansion to blow down tank; are placed in compartments far away from (6) safe liquid sampling device; (7) hydro- the hydrogenation reactors and operations gen comes in with a maximum pressure of are carried out under nitrogen. Special at- 2.0 barg (reaction pressure 1.3 barg); safety tention is given to keep the catalyst unload- valve with 2.0 barg (control by external ex- ing area clean and free of catalyst. pert not compulsory); (8) bottom valve is fire proof; (9) provisions to stop liquids en- 6.4.6. Prevention of Flame tering gas feed tubes in backward direction; Propagations (10) use of resistant and compatible materi- In order to prevent propagation of als. flames, flame barriers are installed in all gas tube connections with the reactor.