Hydrocracking Process Variables
The proper operation of the unit will depend on the careful selection and control of
the processing conditions. By careful monitoring of these process variables, the unit
can operate to its full potential.
Catalyst Temperature
The amount of conversion which takes place in the reactors is going to be determined by several variables: the type of feedstock, the amount of time the feed is in
the presence of the catalyst, the partial pressure of hydrogen in the catalyst bed, and,
most importantly, the temperature of the catalyst and reactants. A high temperature
in the reactor causes a fast rate of reaction and, therefore, the high the conversion.
Since hydrocracking is exothermic, overall, the temperature increases as the feed
and recycle gas proceed through any particular catalyst bed. Maintaining temperature control at all times is very important. More heat can be generated from the
reactions than the flowing streams can remove from the reactors. If this happens, the
temperature may increase very rapidly. This condition is called a temperature
excursion or a temperature runaway. A temperature runaway is a very serious
situation since extremely high temperatures can be generated within a short period
of time. These high temperatures can cause damage to the catalyst and/or to the
reactors. To avoid these situations, temperature guidelines have to be observed.
These guidelines are dependent on the type of feedstock, and the type of catalyst,
and vary from catalyst supplier to catalyst supplier. But by and large, the temperature rise of catalyst beds loaded with noble metal catalyst is limited to about 30
F
(17
C). The temperature rise of catalyst beds loaded with high-activity base metal
catalysts (for naphtha production) is limited to about 25
F (14
C), and of those
loaded with low-zeolite content catalyst (for middle distillate production), the
temperature rise is limited to 40
F (22
C). Finally, maximum bed temperature
rises of about 50
F (28
C) are recommended for amorphous catalysts. The
maximum bed temperature rise of 50
F (28
C) is also recommended for most
pretreating reactors. To properly monitor the reactions as the reactants pass through
the catalyst bed, measuring the temperature of the flowing stream at the inlet and
outlet of each bed and/or the reactor is not sufficient. The temperature at the inlet,
outlet, and radially throughout the catalyst bed must be observed. A temperature
profile plot is a useful tool for evaluating performance of catalyst, effectiveness of
quench, and reactor flow patterns. A temperature profile can be constructed by
plotting the catalyst temperature versus distance into the catalyst bed or more
accurately plotting catalyst temperature against the weight percent of catalyst.
The hydrocracking reactor should be operated with equal catalyst peak temperatures for attaining the maximum catalyst operating cycle length. In this manner the
total catalyst volume is utilized during the entire cycle. The weight average bed
temperature (WABT) is typically used to compare the catalyst activity. Figure 24
gives a general description of how the WABT is calculated for a reactor.
Hydrocracking in Petroleum Processing
349
The proper operation of the unit will depend on the careful selection and control of
the processing conditions. By careful monitoring of these process variables, the unit
can operate to its full potential.
Catalyst Temperature
The amount of conversion which takes place in the reactors is going to be determined by several variables: the type of feedstock, the amount of time the feed is in
the presence of the catalyst, the partial pressure of hydrogen in the catalyst bed, and,
most importantly, the temperature of the catalyst and reactants. A high temperature
in the reactor causes a fast rate of reaction and, therefore, the high the conversion.
Since hydrocracking is exothermic, overall, the temperature increases as the feed
and recycle gas proceed through any particular catalyst bed. Maintaining temperature control at all times is very important. More heat can be generated from the
reactions than the flowing streams can remove from the reactors. If this happens, the
temperature may increase very rapidly. This condition is called a temperature
excursion or a temperature runaway. A temperature runaway is a very serious
situation since extremely high temperatures can be generated within a short period
of time. These high temperatures can cause damage to the catalyst and/or to the
reactors. To avoid these situations, temperature guidelines have to be observed.
These guidelines are dependent on the type of feedstock, and the type of catalyst,
and vary from catalyst supplier to catalyst supplier. But by and large, the temperature rise of catalyst beds loaded with noble metal catalyst is limited to about 30
F
(17
C). The temperature rise of catalyst beds loaded with high-activity base metal
catalysts (for naphtha production) is limited to about 25
F (14
C), and of those
loaded with low-zeolite content catalyst (for middle distillate production), the
temperature rise is limited to 40
F (22
C). Finally, maximum bed temperature
rises of about 50
F (28
C) are recommended for amorphous catalysts. The
maximum bed temperature rise of 50
F (28
C) is also recommended for most
pretreating reactors. To properly monitor the reactions as the reactants pass through
the catalyst bed, measuring the temperature of the flowing stream at the inlet and
outlet of each bed and/or the reactor is not sufficient. The temperature at the inlet,
outlet, and radially throughout the catalyst bed must be observed. A temperature
profile plot is a useful tool for evaluating performance of catalyst, effectiveness of
quench, and reactor flow patterns. A temperature profile can be constructed by
plotting the catalyst temperature versus distance into the catalyst bed or more
accurately plotting catalyst temperature against the weight percent of catalyst.
The hydrocracking reactor should be operated with equal catalyst peak temperatures for attaining the maximum catalyst operating cycle length. In this manner the
total catalyst volume is utilized during the entire cycle. The weight average bed
temperature (WABT) is typically used to compare the catalyst activity. Figure 24
gives a general description of how the WABT is calculated for a reactor.
Hydrocracking in Petroleum Processing
349