In the reactor tubes filled with catalyst the developed heat has to be transferred to the thermostatic fluid.
Near the gas inlet, where the methanol concentration is at the maximum, also the heat to be transferred is the highest, and, in order to reduce the catalyst temperature increase, it is advisable to dilute in this entry layer the catalyst with inert, slowing there the reaction.
In this way, the temperature peak in the whole catalyst layer is reduced and its working life is improved.
ALDER suggests diluting about 28% of the catalyst layer (at the gas entry side) with 40% by weight of its special diluent (patent pending).
The diluent is made by special steel rings, of similar size and shape as the catalyst pills.
The diluent has to be thoroughly mixed with the catalyst before loading the reactor.
When the catalyst charge ends its life and is discharged for being sent to molybdenum recovery (but for some partial reuse, if some unspoiled catalyst can be separated), the diluent may be easily recovered and used for diluting the next load.
Part of the catalyst MOC5I supplied to new clients, (approximately the amount considered as reserve quantity) is delivered in the standard drums but unmixed, with the catalyst and the corresponding diluent (40%) in separate plastic bags: the mixing operation has to be made at the reactor loading moment only for the quantity really needed at the end of the operation.
For obtaining an even mixture, it is possible to split in half each of the two bags of the drum, to put about 6 kg of catalyst and 4 kg of diluent in an empty drum, to close it and to let it slowly roll for an adequate time.
In this way, the catalyst supplied as a reserve but not employed, can be loaded in the future in every one of the two layers, according to the needs.
When loading the reactor, the lower layer has to be loaded tube by tube, measuring exactly by volume the amount of catalyst in each tube, by means of the cups of known volume and slowly dropping the pills in the tube by means of a funnel. Instead, the upper layer has to be slowly loaded pouring the catalyst in the tubes by means of a shovel (up to about 1-2 cm down from the tube sheet surface).
High attention has to be paid to the feed point of the gas in the reactor:
if the gas flows downward (as in the old reactor types of ALDER, and in most the non ALDER plants) the pure catalyst layer (72% of the total height) has to be in the lower part, and has to be loaded tube by tube;
if the gas flows upward (as in the most modern reactors of ALDER) the diluted catalyst layer (28% of the total height) has to be in the lower part and has to be loaded tube by tube.
The apparent specific weight in tubes of internal diameter 22,1 mm is about 0,8 kg/lt for the diluted catalyst and about 0,69 kg/lt for the undiluted pure catalyst. So in the diluted catalyst layer of 28% of the height, about 31% of the total catalyst charge weight is loaded.
In such standard reactors, each tube should be loaded with about 95cc, equal to 76 g of diluted catalyst and about 242cc, equal to 167g of pure catalyst. The spent catalyst has to be discharged at the end of its life: it is recommended to unload it by sucking it from the top of the reactor, by introducing in each tube a copper pipe connected by a rubber hose to collecting drums, which are kept under vacuum by an adequate fan protected by a sleeves filter.
If the unloading operation is made in this way, it is recommended to unload separately the different layers (by inserting the copper pipes more than once, each time up to a different level), in order to collect separately the different qualities: diluted layer, undiluted to send to molybdenum recovery, undiluted (exit layer) for possible reuse.
For the diluent recovery, it has to be separated from the spent catalyst, which will be sent to molybdenum recovery. Such separation can be made by sieving the catalyst (the spent one is generally already broken, and anyway is fragile) and by collecting the steel diluent, which is not fragile and does not pass through a 3 – 4 mm mesh, by means of a magnetic device.
After separation from the spent catalyst, and elimination of the possible deformed rings, the diluent has to be washed with diluted caustic soda (5%), then with demineralized water and finally dried. In this way, the diluent can be reused in the next load, by mixing it with the pure catalyst in the above-mentioned ratio (4 to 6 by weight).