SAFE PROCESS MANAGEMENT IN THE CHEMICAL INDUSTRY 791 CHIMIA 2003, 57, No.12 Chimia 57 (2003) 791–798 © Schweizerische Chemische Gesellschaft ISSN 0009–4293 Catalytic Hydrogenation in the Liquid Phase 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-cat- alyzed 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,