Mineral oils: processes and products – Chemical conversion of hydrocarbons – With subsequent treatment of products
Reexamination Certificate
2002-01-16
2004-05-25
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
With subsequent treatment of products
C208S103000, C208S107000, C208S109000, C208S25400R, C208S264000, C062S617000, C423S210000, C423S242100, C423S248000, C095S158000, C095S172000, C095S177000, C095S187000
Reexamination Certificate
active
06740226
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to improvements in hydroprocesses, such as hydrocracking and hydrotreating processes, used in refinery operations to produce middle distillates, including jet and diesel fuels.
BACKGROUND OF THE INVENTION
In processes for the catalytic conversion of a hydrocarbon feed stock, the step of recycling a hydrogen-rich vapor or gas phase separated from the reaction zone effluent is common. Practical reasons for utilizing this step reside in maintaining both the activity and operational stability of the catalyst used in the process. In hydrogen producing processes, such as catalytic reforming, hydrogen in excess of that required for the recycle feed stream is recovered and utilized in other processes integrated into the overall refinery. For example, the excess hydrogen from a catalytic reforming unit is often employed as the makeup hydrogen in a hydrocracking process, where the reaction principally is hydrogen-consuming.
Regardless of the particular process, the recycled hydrogen is generally obtained by cooling the total reaction product effluent to a temperature in the range of from about 60° F. (15.6° C.) to about 140° F. (60° C.), and introducing the cooled effluent into a vapor-liquid separation zone. The recovered vapor phase required to satisfy the hydrogen requirement of the reaction zone is recycled and combined with the hydrocarbon feed stock upstream of the reaction zone.
The art has long recognized the importance of improving the purity of the hydrogen in the recycle stream of hydroprocessors, such as hydrocracking and hydrotreating units. Thus, it has been the goal of the art to provide enhanced efficiencies of hydrogen utilization with little additional energy consumption and without undue deleterious effects on the maintenance or operation of the hydrocracking equipment. It has also been recognized that by increasing the efficient use of hydrogen, existing equipment could be employed to increase the throughput of the feed stock and also increase the yield of C
5
and higher hydrocarbons. A further obvious advantage to the more efficient utilization of hydrogen is the reduction in the amount of hydrogen that must be produced by, for example, a hydrogen plant to ensure that the hydroprocessing zone has sufficient hydrogen of adequate purity to enable the hydroprocessing to proceed in the most advantageous manner. See, for example U.S. Pat. No. 4,362,613 issued Dec. 7, 1982 to Monsanto Company.
In a conventional hydrocracking process, a heavy vacuum gas oil (VGO), which may be additionally mixed with demetalized oil (DMO) or coke-gas oil, is mixed with hydrogen gas to form a feed stream that is introduced under pressure into the top of a catalytic reactor. The VGO liquid and gaseous hydrogen mixture passes downwardly through one or more catalyst beds. The higher the partial pressure of hydrogen in the feedstream to the reactor, the greater will be the efficiency with which the heavier hydrocarbon feedstock is converted to the desired lighter middle distillate products, such as jet fuel and diesel fuel.
After passing through the catalyst, the hot reactor effluent is cooled and passed to a high pressure separator from which the liquid product stream is removed and, if desired, can be subjected to further fractionation.
The flash gases from the HP separator contain hydrogen and C
1
to C
5
hydrocarbons. These flash gases can contain for example from about 78 to up to 82 mole-percent (mol %) of hydrogen. In the conventional processes of the prior art, the flash gases are combined with a makeup hydrogen stream that is typically available at 96 to 99.99 mol % purity. The recycle gas and, if necessary, the makeup hydrogen streams are compressed and combined with the liquid feed stock at the inlet of the hydrocracking reactor. A portion of up to 2% of the flash gases from the high pressure separator are purged to the refinery fuel gas system to prevent the build-up of the light hydrocarbon products in the reactor gas recirculation loop.
If the heavy VGO feed is sour, i.e., it contains sulfur, the separated effluent gas stream will contain H
2
S in addition to the hydrogen and C
1
to C
5
hydrocarbons. In order to prevent build-up of H
2
S in the reactor gas recirculation loop, the flash gases are contacted with an amine solution to remove the H
2
S and to sweeten the gas stream. A portion of the sweetened low pressure flash gases are purged to the refinery fuel gas system to prevent build-up of C
1
to C
5
hydrocarbons in the reactor gas recirculation loop. The remaining sweetened recirculated gases are combined with makeup hydrogen, compressed and passed to the reactor inlet as part of the hydroprocessor feed. Depending upon the H
2
S content of the sour gas exiting the reactor, the hydrogen concentration of the sweetened recirculation gas stream can be increased to for example from 80 to 84 mol % hydrogen.
The type of feedstock to be processed, product quality requirements, and the amount of conversion for a specific catalyst cycle life determine the hydrogen partial pressure required for the operation of both types of hydroprocessing units, i.e., hydrocracker and hydrotreater units. The unit's operating pressure and the recycle gas purity determine the hydrogen partial pressure of the hydroprocessing unit. Since there is limited control over the composition of the flashed gas from the downstream HP separator, the hydrogen composition of the recycle flash gas limits the hydrogen partial pressure ultimately delivered to the hydroprocessing reactor. A relatively lower hydrogen partial pressure in the recycle gas stream effectively lowers the partial pressure of the hydrogen gas input component to the reactor and thereby adversely affects the operating performance with respect to distillate quantity and quality, catalyst cycle life, heavier feed processing capability, conversion capability and coke formation. To offset the lower performance, the operating pressure of the hydroprocessing reactor has to be increased. Conversely, by increasing the efficiency of hydrogen gas recovery and hydrogen composition, the hydrogen partial pressure of the recycle gas stream improves thereby improving the overall performance of the hydroprocessing reactor as measured by these parameters.
In the practice of the prior art processes, there are only four ways known to improve the hydrogen partial pressure in the hydrocracker or hydrotreater unit. These are as follows:
1. increasing the hydrogen purity of the makeup stream from the hydrogen unit;
2. purging or venting gas from the high-pressure separator;
3. reducing the temperature at the high-pressure separator to decrease the entrainment of light hydrocarbons in the recycle gas, and
4. improving the hydrogen purity of recycle gas.
All of foregoing methods have a very limited capability for improving the performance of an existing unit. If the hydrogen plant optimizes the purity of the makeup hydrogen, it will be in the 96 to 99 mol % range. Since the high purity H
2
makeup flow rate is typically only about one-third or less of the total combined hydrogen fed to the hydroprocessing reactor through recirculation of flashed recycle gas, the overall improvement in the H
2
purity or concentration of the combined recycle and makeup gas streams is limited.
Purging or venting gases from the HP separator will result in the loss of some of the hydrogen in the circuit which must eventually be replaced, thereby putting a greater demand on the hydrogen production unit. The extent to which the separator temperature can be lowered is limited by nature of the process and this change has, in any event, a relatively minor effect on recycle gas H
2
purity.
It has been recognized that unlike most hydrocarbon gases, hydrogen has the unique property of increasing its solubility in hydrocarbon liquids with increasing temperature. For example, the solubility of hydrogen in a particular oil at 900° F. (482.2° C.) can be five times as great as its solubility in the same oil at 100° F. (37.8° C.). This characteristic
Al-Abdulal Ali Hassan
Mehra Yuv Raj
Abelman ,Frayne & Schwab
Griffin Walter D.
Saudi Arabian Oil Company
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