580
The Chemistry and Technology of Petroleum
together as subcategories of one process) are described here with the remainder appearing elsewhere
(Chapter 22) since these other processes are more amenable to hydrocracking than to hydrotreating.
21.4.1 resIduum desulFurIzAtIon And vACuum resIduum desulFurIzAtIon ProCess
Residuum hydrotreating processes have two definite roles: (1) desulfurization to supply low-sulfur
fuel oils and (2) pretreatment of feed residua for residuum fluid catalytic cracking processes. The
main goal is to remove sulfur, metal, and asphaltene contents from residua and other heavy feedstocks to a desired level. On the other hand, the major goal of residuum hydroconversion is cracking of residua with desulfurization, metal removal, denitrogenation, and asphaltene conversion.
Residuum hydroconversion process offers production of kerosene and gas oil, and production of
feedstocks for hydrocracking, fluid catalytic cracking, and petrochemical applications.
The RDS/VRDS process is (like the residfining process, q.v.) a hydrotreating process that is
designed to hydrotreat vacuum gas oil, atmospheric residuum or vacuum residuum to remove sulfur
metallic constituents while part of the feedstock is converted to lower-boiling products. In the case
of residua, the asphaltene content is reduced.
The process consists of a once-through operation of hydrocarbon feed contacting graded catalyst
systems designed to maintain activity and selectivity in the presence of deposited metals. Process
conditions are designed for a 6-month to 1-year operating cycle between catalyst replacements. The
process is ideally suited to produce feedstocks for residuum fluid catalytic crackers or delayed coking units to achieve minimal production of residual products in a refinery.
The major product of the processes is a low-sulfur fuel oil and the amount of gasoline and middle
distillates is maintained at a minimum to conserve hydrogen. The basic elements of each process are
similar and consist of a once-through operation of the feedstock coming into contact with hydrogen
and the catalyst in a downflow reactor that is designed to maintain activity and selectivity in the
presence of deposited metals. Moderate temperatures and pressures are employed to reduce the
incidence of hydrocracking and, hence, minimize production of low-boiling distillates (Table 16.11).
The combination of a desulfurization step and a vacuum residuum desulfurizer (VRDS) is often
seen as an attractive alternate to the atmospheric residuum desulfurizer (RDS). In addition, either
RDS option or the VRDS option can be coupled with other processes (such as delayed coking, fluid
catalytic cracking, and solvent deasphalting) to achieve the most optimum refining performance.
21.4.2 resIdFInIng ProCess
The residfining process is a catalytic fixed-bed process for the desulfurization and demetallization of residua. The process can also be used to pretreat residua to suitably low contaminant levels
prior to catalytic cracking. In the process, liquid feed to the unit is filtered, pumped to pressure,
preheated and combined with treat gas prior to entering the reactors. A small guard reactor would
typically be employed to prevent plugging/fouling of the main reactors. Provisions are employed to
periodically remove the guard while keeping the main reactors on-line. The temperature rise associated with the exothermic reactions is controlled utilizing either a gas- or liquid-quench. A train
of separators is employed to separate the gas and liquid products. The recycle gas is scrubbed to
remove ammonia and H 2 S. It is then combined with fresh makeup hydrogen before being reheated
and recombined with fresh feed. The liquid product is sent to a fractionator where the product is
fractionated.
The different catalysts allow other minor differences in operating conditions and peripheral
equipment. Primary differences include the use of higher purity hydrogen makeup gas (usually
95% or greater), inclusion of filtration equipment in most cases, and facilities to upgrade the offgases to maintain higher concentration of hydrogen in the recycle gas. Most of the processes utilize downflow operation over fixed-bed catalyst systems but exceptions to this are the H-Oil and
LC-Fining processes (which are predominantly conversion processes) that employ upflow designs
The Chemistry and Technology of Petroleum
together as subcategories of one process) are described here with the remainder appearing elsewhere
(Chapter 22) since these other processes are more amenable to hydrocracking than to hydrotreating.
21.4.1 resIduum desulFurIzAtIon And vACuum resIduum desulFurIzAtIon ProCess
Residuum hydrotreating processes have two definite roles: (1) desulfurization to supply low-sulfur
fuel oils and (2) pretreatment of feed residua for residuum fluid catalytic cracking processes. The
main goal is to remove sulfur, metal, and asphaltene contents from residua and other heavy feedstocks to a desired level. On the other hand, the major goal of residuum hydroconversion is cracking of residua with desulfurization, metal removal, denitrogenation, and asphaltene conversion.
Residuum hydroconversion process offers production of kerosene and gas oil, and production of
feedstocks for hydrocracking, fluid catalytic cracking, and petrochemical applications.
The RDS/VRDS process is (like the residfining process, q.v.) a hydrotreating process that is
designed to hydrotreat vacuum gas oil, atmospheric residuum or vacuum residuum to remove sulfur
metallic constituents while part of the feedstock is converted to lower-boiling products. In the case
of residua, the asphaltene content is reduced.
The process consists of a once-through operation of hydrocarbon feed contacting graded catalyst
systems designed to maintain activity and selectivity in the presence of deposited metals. Process
conditions are designed for a 6-month to 1-year operating cycle between catalyst replacements. The
process is ideally suited to produce feedstocks for residuum fluid catalytic crackers or delayed coking units to achieve minimal production of residual products in a refinery.
The major product of the processes is a low-sulfur fuel oil and the amount of gasoline and middle
distillates is maintained at a minimum to conserve hydrogen. The basic elements of each process are
similar and consist of a once-through operation of the feedstock coming into contact with hydrogen
and the catalyst in a downflow reactor that is designed to maintain activity and selectivity in the
presence of deposited metals. Moderate temperatures and pressures are employed to reduce the
incidence of hydrocracking and, hence, minimize production of low-boiling distillates (Table 16.11).
The combination of a desulfurization step and a vacuum residuum desulfurizer (VRDS) is often
seen as an attractive alternate to the atmospheric residuum desulfurizer (RDS). In addition, either
RDS option or the VRDS option can be coupled with other processes (such as delayed coking, fluid
catalytic cracking, and solvent deasphalting) to achieve the most optimum refining performance.
21.4.2 resIdFInIng ProCess
The residfining process is a catalytic fixed-bed process for the desulfurization and demetallization of residua. The process can also be used to pretreat residua to suitably low contaminant levels
prior to catalytic cracking. In the process, liquid feed to the unit is filtered, pumped to pressure,
preheated and combined with treat gas prior to entering the reactors. A small guard reactor would
typically be employed to prevent plugging/fouling of the main reactors. Provisions are employed to
periodically remove the guard while keeping the main reactors on-line. The temperature rise associated with the exothermic reactions is controlled utilizing either a gas- or liquid-quench. A train
of separators is employed to separate the gas and liquid products. The recycle gas is scrubbed to
remove ammonia and H 2 S. It is then combined with fresh makeup hydrogen before being reheated
and recombined with fresh feed. The liquid product is sent to a fractionator where the product is
fractionated.
The different catalysts allow other minor differences in operating conditions and peripheral
equipment. Primary differences include the use of higher purity hydrogen makeup gas (usually
95% or greater), inclusion of filtration equipment in most cases, and facilities to upgrade the offgases to maintain higher concentration of hydrogen in the recycle gas. Most of the processes utilize downflow operation over fixed-bed catalyst systems but exceptions to this are the H-Oil and
LC-Fining processes (which are predominantly conversion processes) that employ upflow designs