Catalyst Bed Maintenance

In some liquid-phase processes, catalyst components are slowly leached from the catalyst bed and eventually the catalyst must be replaced. The feasibility of this type of process involves economics, ie, the costs of catalyst maintenance and keeping a unit out of service for catalyst replacement, and product quality and safety, ie, the effects of having catalyst components in the product and their ease of removal. 

Design considerations and costs of the catalyst, hardware, and a fume control system are direcdy proportional to the oven exhaust volume. The size of the catalyst bed often ranges from 1.0 m at 0°C and 101 kPa per 1000 m /min of exhaust, to 2 m for 1000 m /min of exhaust. Catalyst performance at a number of can plant installations has been enhanced by proper maintenance. Annual analytical measurements show reduction of solvent hydrocarbons to be in excess of 90% for 3—6 years, the equivalent of 12,000 to 30,000 operating hours. When propane was the only available fuel, the catalyst cost was recovered by fuel savings (vs thermal incineration prior to the catalyst retrofit) in two to three months. In numerous cases the fuel savings paid for the catalyst in 6 to 12 months. 

The main catalytic converter control objective is maintenance of constant, specified catalyst bed input gas temperatures. 

Keeping in mind these insights, we turn to a second application of the model, the estimation of the lifetime of a catalyst bed. This is an important consideration in the design of any reactor and is particularly critical in transportation applications where maintenance intervals must be well known. As an initial approach to developing metrics for this analysis we define Ae time in service or service life, at constant wall temperature, as the duration when the overall conversion in the reactor (for full power output) is greater than 85%. This conversion was chosen because it is close to the autothermal point of operation where the burning of unreacted methanol will just balance the endothermic heat of reaction. A reference catalyst mass was determined by requiring a methanol conversion of 85% at the wall temperature of 240"C for fresh catalyst. 

Level 3 Maintenance has been completed the facility has been buttoned up, with all blinds and lockouts removed solid catalyst beds have been recharged. However, the equipment contains air and possibly small amounts of water from the clean-out following the turnaround work. Process equipment is at ambient temperature and pressure. 

During a sulfur plant outage, quantities of SO2 may be observed emanating from open manways. Also, a blue fire can sometimes be seen in cold process vessels and lines. Pyrophoric iron is the cause of these phenomena. When dry, it spontaneously ignites after exposure to air. The heat evolved from pyrophoric iron combustion in catalyst beds can lead to high reactor temperatures. Also, irritating SO2 vapors interfere with maintenance personnel working to repair the unit. 

Temperature instruments are usually required at reactors they are u.sed to measure the temperature at different levels of the catalyst bed. These iastruments can be individual nozzles located at various levels on the shell of the reactor or, to minimize multiple a>n-nectioas, immersed in a well from the top tif the reactor, either on an individual nozzle or through the maintenance access. lasirument requirements for re-... 

Some investigations of the effect of pressure on activity for a given activation temperature indicate that the maintenance of a very low pressure over the catalyst bed during activation is necessary for attainment of maximum catalytic activity. 

Figure 17.9 represents a much more hazardous situation. Vessel A is in use while Vessel B is blocked in and positively isolated for maintenance work—say to change out a catalyst bed. Valve 4 is closed in order to protect workers from a release from Vessel A. If an external fire occurs near Vessel B, it may be overpressured because it is isolated from the relief system. In such situations, Vessel B must be provided with another pressure relief mechanism or the vessel should be open to the atmosphere. 

The maintenance of uniform flow distribution in fixed bed reactors is frequently a problem. Maldistribution leads to an excessive spread in the distribution of residence times with adverse effects on the reactor performance, particularly when consecutive reactions are involved. It may aggravate problems of hot-spot formation and lead to regions of the reactor where undesired reactions predominate. Disintegration or attrition of the catalyst may lead to or may aggravate flow distribution problems. 

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