Regeneration - Spent Hydroprocessing Catalysts - page 175

hydrocarbons. After the flushing stage, temperature and pressure are decreased and recycle gas
replaced by N 2 .IntheflowofN 2 , the reactor entrance temperature is adjusted to about 330 ◦ C
before the addition of about 0.5 vol.% of O 2 . The addition of O 2 resulted in the temperature
rise to about 385 ◦ C. However, the actual temperature rise may vary from catalyst to catalyst.
Subsequently, the O 2 concentration in N 2 is increased to 0.65% while the entrance temperature
was increased from 330 to 345 ◦ C to achieve the second burn-off stage. The temperature may
be increased from 345 to 380 ◦ C, if the second burning stage did not occur. However, at the
same time, the O 2 concentration must be decreased from 0.65 to 0.25%. After burning at
380 ◦ C is completed, the temperature is increased to 400 ◦ C to attain third burn. At 400 ◦ C, the
regeneration may be completed by increasing the O 2 concentration to 1%. During this stage,
all precautions must be taken to avoid temperature runaways. The progress of regeneration is
monitored by analyzing the gas exiting the regenerator. Thus, regeneration is completed once
the concentration of carbon oxides in the gas approached zero. After regeneration, the catalyst
is cooled to 80 ◦ C before it is exposed to air. Several days or even weeks may be required to
complete the regeneration according to this method. As part of the modified method, N 2 may
be replaced by steam (e.g., one ton of steam per one ton of spent catalyst per hour). To avoid
wetting the catalyst, the steam temperature must be higher than 200 ◦ C and steam pressure at
the regenerator outlet lower than 7 atm.
The success of the above described regeneration method depends on the availability of skilful
operators. Considering periodic regeneration requirements (e.g., once in a few years), it may
not be easy to maintain such skills. It therefore appears that it may be more advantageous for
petroleum refiner to have the catalyst regeneration performed by the off-site certified
regenerating companies having all necessary expertise in place.
6.4.2 Off-Site Regeneration
The off-site regeneration allows the size or density grading of spent catalyst prior to
regeneration. Then, the unusable and/or non-regenerable portion of spent catalyst, i.e., either
due to unsuitable particle size or excessive deactivation, can be eliminated from the
regeneration process. In this way, the efficiency of regeneration is improved. This is an
incomparable advantage of the off-site regeneration compared with the in-situ regeneration.
The off-site regeneration processes began to emerge in 1970s [451,452] . During the early
stages, the off-site regeneration was performed in muffle furnaces. However, the lacks of
uniform distribution of oxidizing medium and insufficient temperature control were the major
disadvantages of this method. Under such conditions, occasional temperature excursions could
not be prevented. The next stage of the off-site regeneration development involved a fixed bed
reactor. In this case, a more uniform distribution of oxidizing media and the control of
regeneration process associated with this was achieved particularly when the spent catalyst
was prescreened prior to regeneration. Apparently, rotary kilns, which have been used in