aromatic yield. The greater the space velocity, the higher the temperature required
to achieve a given product target. It is important to note that as the conversion of the
naphtha is increased producing higher octane reformate, the yield of C 5 + reformate
declines due to paraffin cracking reactions forming C 4 À products. However, the
yields of aromatics and hydrogen will typically continue to rise as the conversion
level is increased.
Hydrogen-to-Hydrocarbon Ratio
The hydrogen-to-hydrocarbon (H 2 /HC) mole ratio is the ratio of moles of hydrogen
in the recycle gas to moles of naphtha charged to the unit. The recycle gas is a
mixture of hydrogen and light hydrocarbon gases, typically 75–92 mol% hydrogen.
The ratio of total recycle gas to hydrocarbon is sometimes called the gas-to-oil
ratio. Recycle hydrogen is necessary to maintain catalyst life stability by removing
coke precursors from the catalyst. A proposed mechanism is that the hydrogen
allows hydrogenation of the coke precursors and inhibits polymerization to coke.
Because the rate of coke formation on the catalyst is a function of the hydrogen
partial pressure, increasing the H 2 /HC ratio reduces the coking rate and increases
stability with only a small effect on product quality or yields. Lower H 2 /HC ratios
can provide higher C 5 + and hydrogen yields although this benefit is difficult to
measure in commercial units. Since SR units do not regenerate continuously, the
cycle length between regenerations can be lengthened by operating under milder
conditions which include a higher H 2 /HC ratio (i.e., a higher hydrogen partial
pressure) and lower reactor temperatures. Continuous units can operate under
more severe conditions including lower H 2 /HC ratio because catalyst is being
continuously removed from the last reactor and sent to the regenerator while
fresh catalyst is continuously being fed back to the first reactor.
Process Flow Schemes
Fixed-Bed SR Reforming Units
A simplified SR Platforming flow diagram is shown in Fig. 9. The feed to the unit is
mixed with recycled hydrogen gas and heated to reaction conditions by passing
through a combined feed-effluent heat exchanger followed by a fired heater.
Because most of the reactions that occur in a reforming process are endothermic,
the reactor section is separated into several stages or reactors. Inter-heaters are
installed between these stages to maintain the desired temperature range across the
catalyst in the reactor section. Effluent from the last reactor is cooled by the feedeffluent heat exchanger for maximum heat recovery. Air or water cooling provides
additional cooling to near-ambient temperature.
The effluent is then charged to the separation section, where the liquid and gas
products are separated. A portion of the gas from the separator is compressed and
Catalytic Reforming in Petroleum Processing
253
to achieve a given product target. It is important to note that as the conversion of the
naphtha is increased producing higher octane reformate, the yield of C 5 + reformate
declines due to paraffin cracking reactions forming C 4 À products. However, the
yields of aromatics and hydrogen will typically continue to rise as the conversion
level is increased.
Hydrogen-to-Hydrocarbon Ratio
The hydrogen-to-hydrocarbon (H 2 /HC) mole ratio is the ratio of moles of hydrogen
in the recycle gas to moles of naphtha charged to the unit. The recycle gas is a
mixture of hydrogen and light hydrocarbon gases, typically 75–92 mol% hydrogen.
The ratio of total recycle gas to hydrocarbon is sometimes called the gas-to-oil
ratio. Recycle hydrogen is necessary to maintain catalyst life stability by removing
coke precursors from the catalyst. A proposed mechanism is that the hydrogen
allows hydrogenation of the coke precursors and inhibits polymerization to coke.
Because the rate of coke formation on the catalyst is a function of the hydrogen
partial pressure, increasing the H 2 /HC ratio reduces the coking rate and increases
stability with only a small effect on product quality or yields. Lower H 2 /HC ratios
can provide higher C 5 + and hydrogen yields although this benefit is difficult to
measure in commercial units. Since SR units do not regenerate continuously, the
cycle length between regenerations can be lengthened by operating under milder
conditions which include a higher H 2 /HC ratio (i.e., a higher hydrogen partial
pressure) and lower reactor temperatures. Continuous units can operate under
more severe conditions including lower H 2 /HC ratio because catalyst is being
continuously removed from the last reactor and sent to the regenerator while
fresh catalyst is continuously being fed back to the first reactor.
Process Flow Schemes
Fixed-Bed SR Reforming Units
A simplified SR Platforming flow diagram is shown in Fig. 9. The feed to the unit is
mixed with recycled hydrogen gas and heated to reaction conditions by passing
through a combined feed-effluent heat exchanger followed by a fired heater.
Because most of the reactions that occur in a reforming process are endothermic,
the reactor section is separated into several stages or reactors. Inter-heaters are
installed between these stages to maintain the desired temperature range across the
catalyst in the reactor section. Effluent from the last reactor is cooled by the feedeffluent heat exchanger for maximum heat recovery. Air or water cooling provides
additional cooling to near-ambient temperature.
The effluent is then charged to the separation section, where the liquid and gas
products are separated. A portion of the gas from the separator is compressed and
Catalytic Reforming in Petroleum Processing
253