Catalyst poisoning reactor operation

In the feed preparation section, those materials are removed from the reactor feed which would either poison the catalyst or which would give rise to compounds detrimental to product quality. Hydrogen sulfide is removed in the DBA tower, and mercaptans are taken out in the caustic wash. The water wash removes traces of caustic and DBA, both of which are serious catalyst poisons. Also, the water wash is used to control the water content of the reactor feed (which has to be kept at a predetermined level to keep the polymerization catalyst properly hydrated) and remove NH3, which would poison the catalyst. Diolefins and oxygen should also be kept out of poly feed for good operation. 

It has been suggested that a pilot plant operation to determine the feasibility of developing this process be carried out in a tubular flow reactor with a volume of 0.15 m3. It is suggested that the reactor operate at 450 °C and 1 atm with a feed flow rate of 41.7 moles of pure tetra-chloroethane per kilosecond. Will the catalyst be susceptible to poisoning under these operating conditions ... 

Expanded-bed reactors operate in such a way that the catalyst remains loosely packed and is less susceptible to plugging and they are therefore more suitable for the heavier feedstocks as well as for feedstocks that may contain considerable amounts of suspended solid material. Because of the nature of the catalyst bed, such suspended material will pass through the bed without causing frequent plugging problems. Furthermore, the expanded state of motion of the catalyst allows frequent withdrawal from, or addition to, the catalyst bed during operation of the reactor without the necessity of shutdown of the unit for catalyst replacement. This property alone makes the ebullated reactor ideally suited for the high-metal feedstocks (i.e., residua and heavy oils) which rapidly poison a catalyst with the ever-present catalyst replacement issues (Figure 5-8). 

Awareness of the importance of air quality has triggered several home appliances of catalysts, whereby monoliths play a role. Table 6 gives several applications in the consumer sector. The limited lifetime of such applications overcomes a disadvantage of monolithic systems, in that they contain usually a relatively small amoimt of catalyst, and as a consequence, the buffer capacity against poisoning is less than that of conventional packed-bed reactors. In industrial operation, either robust catalysts or pure feeds are needed, but for home appliances, these criteria may be less of an issue. Similarly, for the use of N2O in hospitals, ambulances, etc., catalyst poisoning is less important. N2O decomposition can be achieved easily, and the use of a monolith structure is highly attractive for mobile applications. 

In many large-scale reactors, such as those used for hydrotreating, and reaction. system.s where deactivation by poisoning occurs, the catalyst decay is relatively slow. In these continuous flow systems, constant conversion is usually necessary in order that subsequent processing steps (e.g.. separation) are not upset. One way to maintain a constant conversion with a decaying catalyst in a packed or fluidized bed is to increase the reaction rate by steadily increasing the feed temperature to the reactor. Operation of a "fluidized bed in this manner is shown in Figure 10-30. 

Metal catalysts are poisoned by a wide variety of compounds, as is evidenced by Fig. 5.2.a-l. The sensitivity of Pt-refonning catalysts and of Ni-steam reforming catalysts is well known. To protect the catalyst, guard reactors are installed in industrial operation. They contain Co-Mo-catalysts that transform the sulfur... 

Recent catalysts can be operated at medium temperatures around 300°C. Copper is more sensitive to catalyst thermal sintering and should not be operated at higher temperatures. Sulfur is also a poison to LTS reactors. Typical exit concentration is of 0,1% of CO. 

T5q)ical reaction conditions are 90- 110°C and 3-4.5 MPa. The diluent used is normally isobutane, which facilitates the separation by vaporisation in a low pressure flash tank and permits higher operating temperatures than longer chained organic solvents. After treatment in purification beds to remove catalyst poisons, the ethylene monomer, comonomer and recycled diluent are fed to the loop reactor. The catalyst is flushed out of the feed container into the loop reactor by the diluent. The polymer concentration in the reactor slurry may range from 30 to 50 wt-%. The slurry from the reactor is passed through a sedimentation zone, where the polymer concentration increases up to 55 - 65 wt-%, and the slurry is then fed to the flash tank, where the hydrocarbons evaporate at a low pressure of about 0.15 MPa. 

The shift reaction is exothermic and thus the equilibrium is favored by low temperatures (Figure 6.2.4). Thus, the reaction temperature should be kept as low as possible, but is limited by the activity of the catalyst. The Fe-Cr shift catalyst is sufficiently active only above about 300 °C. Catalysts based on copper and zinc are active enough at about 200 °C but these catalysts are very sensitive to poisoning and require extremely pure gases, typically with less than Ippm H2S. In practice, the water-gas shift reaction is carried out in two adiabatic fixed-bed reactors with intermediate cooling between both converters. The first high-temperature shift reactor operates with a Fe-Cr catalyst, and the second low-temperature shift reactor contains the more active Cu-Zn system. At the exit of the second shift reactor, the CO2 present in the converted syngas is removed in a gas scrubber, usually by chemical absorption in aqueous amine solutions, for example, mono- or diethanolamine (Section 3.3.3). 

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