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The pressure drop across the catalyst beds will increase over a period of time due to the build up of dust, deterioration of the catalyst, operating mishaps, etc. Eventually the increase in pressure drop will reach a point where the plant must be shut down and the catalyst screened to restore the bed to its normal operating parameters. Screening of the first catalyst layer is required more frequently than the others because any dirt entering the converter tends to plug this catalyst layer. A crust may also form on the top layer of the first catalyst bed due to operating problems. In this case the pressure drop across the catalyst bed will increase rapidly over a short period of time. Catalyst may need to be removed in order to facilitate repair or inspection of the converter. This is common in the post and grid type converters where collapsing of the grid supports may occur. The most common method of removing catalyst from the converter is by vacuum extraction. Removal by hand can also be done but this is usually restricted to smaller converters and is more labour intensive. Screening of catalyst is usually done when the converter has cooled down permitting entry without special protective clothing. Hot entry can be done but requires special procedures and equipment. The following provides a general procedure for the unloading of catalyst from the converter. The user must adapt the following procedures to their specific situation.
Vanadium pentoxide catalyst dust is toxic and special safety precautions are required for all personnel involved in the screening operation. Material Safety Data Sheets (MSDS) should be reviewed prior to commencing the operation.
Personnel shall wear as a minimum dust respirators, protective goggles, gloves and disposable protective clothing. Externally supplied breathing air is highly recommended for those personnel working in the converter.
Standard vessel entry procedures should be followed at all times. This should include as a minimum:
The following equipment will be required:
Trailer mounted screening equipment such as supplied by Auburn can be used for catalyst screening.
Equipment shown in the 'down' position for transportation to and from site
Equipment shown in the up position for catalyst screening
1. Purge the converter and cool the catalyst beds to approximately 90oC (194oF) before shutting down the main acid plant blower(s) and the acid circulating systems.
2. Remove the necessary insulation and manhole covers. Remove instrumentation (i.e. thermocouples and thermowells) to prevent damage to them.
3. Ensure that the converter is safe to enter and all vessel entry procedures are in place and all required safety equipment is provided.
4. Begin by removing the quartz rock and catalyst in the immediate vicinity of the manway working down to the bottom of the bed. Work towards the opposite side of the converter.
5. As the temporary storage containers for the quartz rock and catalyst fill, remove them and store in a dry area protected from the elements.
6. Once the quartz and catalyst are completely removed, thoroughly clean the catalyst support grids/plate. Clean the bed separation plate located below the catalyst bed as well.
Perform an inspection of the converter and initiate any repairs as required.
Carefully check for mechanical damage and areas where gas bypassing can occur.