In the name of Allah the mostGracious and MercifulKnowledge of Ammonia CatalystsBy: Nasir HussainPak Arab Fertilizers Multan13-12-12
Training outline Introduction to catalysis Principal characteristics of a catalyst Ammonia plant catalysts Poisoning of catalysts Replacement/Reduction of catalysts Catalysts catastrophes
Introduction Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations. Catalysts that speed the reaction are called positive catalysts. Substances that slow a catalysts effect in a chemical reaction are called inhibitors. Substances that increase the activity of catalysts are called promoters, and substances that deactivate catalysts are called catalytic poisons.
Introduction cont.….Catalyst is a substance that increases the rate of thereaction at which a chemical system approaches equilibrium ,without being substantially consumed in the process.Catalyst affects only the rate of the reaction, i.e. Kinetics. Itchanges neither the thermodynamics of the reaction nor theequilibrium composition.
Introduction cont.…. Activation Energy can be defined as the minimum energy required to start a chemical reaction. The activation energy of a reaction is usually denoted by Ea, and given in units of kilojoules per mole. The Activation Energy (Ea.) determines how fast a reaction occurs, the higher Activation barrier, the slower the reaction rate. The lower the Activation barrier, the faster the reaction
Activation EnergyCatalyst lowers the activation energy for both forward and reversereactions.
Activation EnergyThis means , the catalyst changes the reaction path by lowering its activationenergy and consequently the catalyst increases the rate of reaction
Principal Characteristics The efficiency of the catalyst depends upon three components Good mechanical strength Activity Selectivity Life low pressure drop · high tolerance for contaminants
Activity It is the extent to which the catalyst influence the rate of change of a reaction as measured by the disappearance of the reactants i.e. conversion It is often expressed as a rate per unit volume. The activity per unit volume is of practical importance because process economics can depend critically upon the cost of packed reactor space. The bulk density of a catalyst must always be as small as possible , consistent with other requirements.
Selectivity The selectivity of a catalyst is the ability to promote a particular reaction whilst minimizing the production of unwanted compounds. Very often a catalyst can be made more specific for a particular reaction whilst the activity can be diminished by poisoning with some other metal.
Life of a catalyst The life of a catalyst is the period during which it produces the required product at the required degree of activity and selectivity. The life of a catalyst is usually ended because of loss of mechanical strength or because of unacceptable changes in activity and selectivity.
Hydrogenation Hydrogenation is the conversion of organic Sulphur compound into inorganic Sulphur compound. For example RSH + H2 = RH + H2S Exothermic R2S + 2H2 = 2RH + H2S
Hydrotreator 107-D Catalyst CoMo Catalyst Type ICI 41-6T Form Cylinders Size Dia 3.2 mm Length 10mm CoO 3.3 % MoO 14 % Alumina Balance Bulk Density 700 kg/m3 or 0.9 kg/lit Volume 2 Beds each of 6.35 m3 Loaded Jan 2008
2.0 H2S Removal H2S is removed by ZnO bed according to following reaction ZnO + H2S = ZnS + H2O There are Two vessels for H2S removal 101-D & 102-D
Desulphrisers 101/102-D 101-D Catalyst Type Mixture of Catalysts (ZnO) Volume 22.5 m3 (NCT 305 +10.5 Mixture) Loaded on 2010 Bulk density 1.1 kg/lit Shape Pellets Service Life 2 years 102-D Catalyst Type Mixture of Catalysts (ZnO) Volume 22.5 m3 (NCT 305 +10.5 Mixture) Loaded on 2010/2008 Bulk density 1.1 kg/lit Shape Pellets
Primary reformer catalyst Performance of reforming catalysts is periodically evaluated by considering following parameters and operating conditions are duly adjusted : Approach to equilibrium Methane slip Pressure drop Tube skin temperature 19
Secondary Reforming Air is introduced in secondary reformer. The combustion of a part of H2 & CH4 takes place at top portion. Heat of combustion is utilized for further reforming. CH4 + H2O = CO +2 H2 CH4 + O2 = CO + 3H2 2H2 + O2 = 2 H2O - 242 kJ / mole The gases leaves the secondary reformer at 996 OC.
Secondary Reformer 103-D Catalyst Type ICI 54-4 NiO 10.5 % Catalyst Volume 33.5 m3 Loaded on 1993
The purpose of HTS converter is to oxidize CO to CO2.HTSC Reaction CO + H2O = CO2 + H2 - 41.1 KJ / mole The CO slippage from HTS should be less than 3.0 %. The principle operating variables are steam to gas ratio & temperature. Increasing steam results in an increase to CO conversion. Under severe condition magnetite can be reduced to FeO or even metallic iron. Such change of phases and over reduction causes crystallite changes in the catalyst that lead to physical degradation, weakening and fracture of pellets, and increase in pressure drop. Metallic iron promotes side reaction such as Fisher Tropsh reaction. Such reactions can affect adversely the performance of down stream catalysts.
HTSC 110-D Catalyst Type C-12-4 (SUD CHEMIE) Catalyst Volume 53 m3 Loaded on 2006 Service Life 4 years Size Dia 9 * 5 ~ 7 mm Shape Tablet Fe2O3 88 % Cr2O3 09 % CuO 2.6 % Catalyst is supplied in the form of Fe2O3 Hematite Catalyst is reduced to active Fe3O4 Magnetite
LTSC 104-D Catalyst Type LK-821-2 & LSK-2 Catalyst Volume 60 m3 Loaded on 2012 Service Life 4 years Size 4.5*3.4 mm & 4.5*4.5 mm Shape Tablets CuO ZnO Al2O3
LTSC by product CO2+H20 CH3OH+H20 Methanol increase with• High temperature• High inlet CO levels• Low S:C ratio 27
Promotors Ammonia synthesis catalyst is based on metallic iron promoted with alkali (potash) and various metal oxides such as those of aluminum, calcium or magnesium. The main component of the catalyst is magnetite (Fe3O4). Various promoters in catalyst plays main role in catalyst activity. Such promoters are classified as structural or electronic depending on their accepted mode of action. Production and preservation of porous structure during reduction of ammonia synthesis catalyst is main role of structural promoters such as alumina, magnesia and chromium. Alkali metal such as calcium, potassium ,rubidium etc. are essential components of catalyst to attain high activity, are called electronic promoter. 34
Promoters Chromium promoted Iron catalyst is of HTSC.
Poisons for Ammonia CatalystsCatalysts PoisonsLTSC S power full poison( trapped by the catalyst as CU2S and ZNS) Cl, reacts with Cu and Zn to from chlorides. CuCl provides the mechanism for loss of activity by sintering. Water, excess water can damage the catalysts du to thermal shock101B S, can affect the catalyst by chemisorption on its surface. C105-D Main poisons to synthesis catalyst are oxygenated compounds such as water, CO, CO2.(reversible) chloride and sulphur affect the catalyst irreversibly.
Poisons for Ammonia Catalysts106-D S, Catacarb solution blocks pores of Ni catalyst by evaporating K2CO3 Arsenic107-D Carbon, this cab be avoided by operating at low temperatures. 37