Hydrotreater Reactor Operation
During operation, the hydrotreating catalyst gradually loses some of its activity.
In order to maintain the desired quality of the products at the design feed rate, the
average bed temperature is gradually increased. The temperature increase in many
cases is very small, less than 2
F/month (1
C/month). When the average bed
temperature reaches a value close to the design maximum, the catalyst has to be
replaced or reactivated. Because the required temperature increase per unit time is
relatively small, the reactor can be operated with the same catalyst for several years
before regeneration or replacement of the deactivated catalyst becomes necessary.
Quite often, catalyst regeneration or replacement is dictated by a high reactor
pressure drop, due to catalyst fouling.
In hydrotreating, the key catalytic properties are initial activity, stability, and
product quality. The temperature required to obtain the desired product at the start
of the run measures the initial activity. Catalyst stability is a measure of change of
reaction rate over time. The product quality is a measure of the ability of the process
to yield products with the desired use specification such as pour point, smoke point,
or octane. Table 8 shows some of the important product quality measurements and
the chemical basis for these measurements.
Hydrotreating 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.
Reactor Temperature
Reactor temperature should be minimized while maintaining desired product quality. Increasing reactor temperature will accelerate the rate of coke formation and
reduce the length of the operating cycle. The required temperature is dependent
upon feed rate and quality and processing objectives. The reactor inlet temperature
is most easily and commonly controlled by the operator to adjust for obtaining the
desired product quality. The reactor outlet temperature is a function of the feed
quality and cannot be easily varied except by changing the reactor inlet temperature
or the temperature profile across the beds by the use of quench. The inlet temperature must always be controlled at the minimum required to achieve the desired
product properties. Temperatures above this minimum will only lead to higher rates
of coke formation and reduced processing periods. The weight average bed temperature (WABT) is the kinetic parameter typically used to compare the relative
catalyst activity. The WABT can be calculated as shown in Fig. 27.
If the reactor only has inlet and outlet thermometry (as is the case in perhaps as
many as two-thirds of hydrotreaters), the WABT represents the average of inlet and
outlet temperatures. The rate of increase in this temperature is referred to as the
deactivation rate expressed as
F per barrel of feed per pound of catalyst (
C per m
3
Hydrotreating in Petroleum Processing
427
During operation, the hydrotreating catalyst gradually loses some of its activity.
In order to maintain the desired quality of the products at the design feed rate, the
average bed temperature is gradually increased. The temperature increase in many
cases is very small, less than 2
F/month (1
C/month). When the average bed
temperature reaches a value close to the design maximum, the catalyst has to be
replaced or reactivated. Because the required temperature increase per unit time is
relatively small, the reactor can be operated with the same catalyst for several years
before regeneration or replacement of the deactivated catalyst becomes necessary.
Quite often, catalyst regeneration or replacement is dictated by a high reactor
pressure drop, due to catalyst fouling.
In hydrotreating, the key catalytic properties are initial activity, stability, and
product quality. The temperature required to obtain the desired product at the start
of the run measures the initial activity. Catalyst stability is a measure of change of
reaction rate over time. The product quality is a measure of the ability of the process
to yield products with the desired use specification such as pour point, smoke point,
or octane. Table 8 shows some of the important product quality measurements and
the chemical basis for these measurements.
Hydrotreating 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.
Reactor Temperature
Reactor temperature should be minimized while maintaining desired product quality. Increasing reactor temperature will accelerate the rate of coke formation and
reduce the length of the operating cycle. The required temperature is dependent
upon feed rate and quality and processing objectives. The reactor inlet temperature
is most easily and commonly controlled by the operator to adjust for obtaining the
desired product quality. The reactor outlet temperature is a function of the feed
quality and cannot be easily varied except by changing the reactor inlet temperature
or the temperature profile across the beds by the use of quench. The inlet temperature must always be controlled at the minimum required to achieve the desired
product properties. Temperatures above this minimum will only lead to higher rates
of coke formation and reduced processing periods. The weight average bed temperature (WABT) is the kinetic parameter typically used to compare the relative
catalyst activity. The WABT can be calculated as shown in Fig. 27.
If the reactor only has inlet and outlet thermometry (as is the case in perhaps as
many as two-thirds of hydrotreaters), the WABT represents the average of inlet and
outlet temperatures. The rate of increase in this temperature is referred to as the
deactivation rate expressed as
F per barrel of feed per pound of catalyst (
C per m
3
Hydrotreating in Petroleum Processing
427