Patent Application
20200190413
None.
This invention relates to refining hydrocarbons and particularly to operating catalytic hydrotreating units to reduce sulfur in fuel products and most particularly to removing sulfur in gasoline.
Sulfur in motor fuel causes tailpipe pollution and is therefore significantly limited by regulatory authorities. Since it is naturally occurring in crude oil, oil refineries must include process systems to remove sulfur from the fuel products before they are brought to market. Sulfur is typically bound up in liquid fuels in a variety of molecule structures primarily including mercaptans and thiophenes. Typical processes to remove sulfur focus on converting the sulfur containing compounds to hydrogen sulfide that is more easily separable from gasoline. Unfortunately, processes that convert the sulfur compounds to hydrogen sulfide also convert other highly valued components of motor fuels to much less desirable constituents. For gasoline, preserving high octane species is quite interesting to refinery operators as higher-octane gasoline blend stock is quite valuable owing to its capacity to be blended with very low-cost sub-spec hydrocarbon liquids and yield a larger combined volume of product to sell at gasoline prices. Losing octane to reduce sulfur content may be necessary but represents a lost profit opportunity. In other words, octane loss has substantial economic impact and, therefore, the goal for any sulfur management process focuses on the necessary job of converting the sulfur while trying to preserve as much of the desirable components as possible.
The problem is exacerbated by tightening sulfur specs as the process efforts to remove the last bits of sulfur have to be pretty aggressive and those aggressive process conditions really take their toll on the most valued components. In addition, more and more crude oil production is coming from higher sulfur formations. Low sulfur content crude oil is called “sweet crude” and high sulfur crude is called “sour crude” and it turns out that the world seems to have way more sour crude than sweet crude. So, as crude oils are produced with higher and higher sulfur contents and regulatory authorities are imposing ever more restrictive environmental regulations limiting sulfur content to a very low ppm range, crude oil refiners must undertake greater and more aggressive efforts to remove sulfur from fuel products.
For gasoline, a significant portion of the sulfur content comes from catalytic cracking of the heavier crude oil components where sulfur tends to concentrate itself in the heavier fractions from the initial distillation processes of the crude. As the heavier components of the crude are subjected to cracking to convert the heavier molecular weight species into gasoline range species, the sulfur compounds end up in gasoline streams. Before this cracked gasoline is blended with other gasoline, it is typically subjected to its own hydrodesulfurization treatment process to convert the heavier sulfur containing compounds into more easily separated lighter sulfur compounds such as hydrogen sulfide.
Current hydrodesulfurization treatment processes are capable of reducing the sulfur content sufficient to meet the newest specifications, but at considerable octane loss. At previous specifications that allowed more sulfur, the octane loss was seen, but was not as severe. As noted above, it appears that the most significant octane loss is sustained at the most aggressive conversion conditions for converting mercaptans and thiophenic compounds to hydrogen sulfide.
Improved sulfur removing technology is needed and desired for meeting gasoline demand for very low sulfur content specifications.
The invention more particularly relates to a process for desulfurizing a gasoline stream to or below a target sulfur content specification for finished gasoline that also minimizes concurrent octane loss. The process includes providing a sulfur containing gasoline stream to a first hydrodesulfurizing reactor with hydrogen and hydrodesulfurizing catalyst at catalytic conditions to convert hydrogen and sulfur compounds to hydrocarbons and hydrogen sulfide to create a first pass sulfur converted gasoline stream and then separating hydrogen sulfide from the first pass sulfur converted gasoline stream to create a first pass desulfurized gasoline stream. The first pass desulfurized gasoline stream is then provided to a one or more additional hydrodesulfurizing reactors each provided with hydrogen and hydrodesulfurizing catalyst at catalytic conditions to convert hydrogen and sulfur compounds to hydrocarbons and hydrogen sulfide to create a follow up pass sulfur converted gasoline stream where the hydrogen sulfide is separated from the follow up pass sulfur converted gasoline stream to create a follow up pass desulfurized gasoline stream. The follow up pass desulfurized gasoline stream is provided to a series of two polishing reactors, each targeting specific chemistries. The first is the thiophenic polishing reactor with a hydrodesulfurizing catalyst at catalytic conditions including at a temperature range of 480 to 500° F. where the catalyst is selected to have a first polishing catalytic activity to convert thiophenes and mercaptans to hydrogen sulfide and hydrocarbons and where the sulfur content in the thiophenes is thereby reduced to a level below the target specification for finished gasoline, but where the total sulfur content is still above the target specification for finished gasoline creating a sulfur converted semi polished gasoline stream. The hydrogen sulfide is then separated from the sulfur converted semi polished gasoline stream to create a degassed semi polished gasoline stream and then the degassed semi-polished gasoline stream is heated to a higher temperature and provided to a mercaptan polishing reactor provided with a hydrodesulfurizing catalyst at catalytic conditions including at a temperature range of 500 to 570° F. where the catalyst is selected to have a second polishing catalytic activity but where the second polishing catalytic activity is selected to be less active for thiophene conversion such that within the mercaptan polishing reactor the mercaptans are converted to hydrogen sulfide and hydrocarbons where the sulfur content in the mercaptans becomes less than the sulfur content in the thiophenes and wherein the total sulfur content of the gasoline is reduced to a level equal to or below the target specification for finished gasoline to create a sulfur converted fully polished gasoline stream. The hydrogen sulfide is then separated from the sulfur converted fully polished gasoline stream to create a degassed fully polished gasoline stream.
A more complete understanding of the present invention and benefits thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings in which:
Turning now to the detailed description of the preferred arrangement or arrangements of the present invention, it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow.
Turning to
In the system 10, a first hydrodesulfurization reactor 20 uses a hydrodesulfurization catalyst in a fixed bed 21 at catalytic conditions (about 100 to 200° C. and 1 to 4 atmospheres pressure) to use hydrogen to convert sulfur compounds to form hydrocarbons and hydrogen sulfide, the second of which is more easily separated from liquid fuel. The hydrogen sulfide produced in reactor 20 is separated from the liquid gasoline cut either in the reactor itself or in a separator downstream of the reactor. In
The sulfur remaining in the gasoline stream is typically in forms that are less reactive at the conditions in the first hydrodesulfurization reactor 20 and, those sulfur bearing compounds may be subjected to more aggressive hydrotreating conditions in a second hydrodesulfurization reactor 40 with less concern about recombination reactions occurring because of the diminished hydrogen sulfide content. The conditions will still not be so aggressive to cause many of the olefins to become saturated. As olefins may comprise a significant portion of the gasoline product (up to about 35%), the conversion of olefins to alkanes would substantially reduce the octane rating and, therefore, would significantly compromise market value of the gasoline product.
Like with reactor 20, hydrogen is fed via line 37 and the hydrodesulfurization catalyst bed 41 in reactor 40 uses the hydrogen to convert more sulfur containing compounds to hydrocarbons and hydrogen sulfide. The conditions in reactor 40 may be similar to the conditions in reactor 20 but are preferably more aggressive to remove the more resistant sulfur compounds from the gasoline stream. Again, reactor 40 is shown as catalytic fractionator with a light end top outlet 45 to direct hydrogen sulfide along with other light ends to amine gas treating or other processing. It is noted that there are many optimizing processes that are known in the art that may be employed for reactors 20 and 40 and that are not shown but may be included with the system of the present invention.
Within reactors 20 and 40 a number of reactions occur concurrently, and some are desirable, and some are not. One of the additional desirable reactions is the conversion of di-olefins to mono-olefins. The sulfur bearing species that are more reactive to the hydrogen and hydrodesulfurization catalysts are most likely converted in one of these two reactors 20 and 40. The undesirable reactions include the saturation of olefins (which reduces octane), the saturation of aromatics (which reduces octane) and any olefin recombination with hydrogen sulfide to recreate a mercaptan.
The gasoline stream at outlet 49 contains about 30-300 ppm sulfur, which is still too high for current specifications. So, focusing now on the more key aspects of the invention, the next two reactors are polishing reactors to clean up the sulfur content of the gasoline stream to a very small constituent amount that is preferably less than or equal to about 10 ppm. To get the sulfur content down to such a low constituent, the inventors have observed that the conversion of mercaptans to hydrogen sulfide strongly correlates to the temperature of the conversion reaction in an equilibrium relationship and can be performed with a less aggressive catalyst formulation that has little activity for hydrogen conversion of thiophenes. Therefore, the strategy for reducing sulfur content can be different for thiophenes than for mercaptans. It is also observed that thiophene conversion is relatively highly correlated to aromatic saturation. With these observations, the inventors have come up with a way to reduce sulfur content down to the ultra-low levels that the fuel sulfur specifications require but preserves a higher-octane number for the fuel or more of the existing high-octane species in the gasoline stream as practical. The process essentially focuses on removing as much sulfur containing mercaptans as possible or practical while removing simply a sufficient amount sulfur containing thiophenes to meet the specification. So, more thiophenes will be present in the final gasoline product than mercaptans and with more thiophenes, higher-octane aromatic content will remain in the gasoline.
Turning back to
The gasoline stream is delivered into the mercaptan reactor 80 where the sulfur conversion is focused on the mercaptan compounds. The catalyst in catalyst bed 81 is a less chemically active hydrodesulfurization catalyst, but the temperature is notably higher, around 260 to 300° C. or about 500 to 570° F. where the temperature, in comparison, in the thiophenic polishing reactor 60 is about 250 to 260 or about 480 to 500° F. A such, mercaptan based sulfur content is driven down to about 3 ppm as seen in the chart shown in
The invention may be accomplished by having two polishing reactors with piping and valves to direct partially desulfurized gasoline in to one polishing reactor, the other polishing reactor, the two in series with either physical reactor being upstream of the other. This affords considerable operational flexibility for the refinery in that when lower sulfur gasoline is produced by the hydrodesulfurizing reactors upstream of the polishing, only one polishing reactor would be in use to meet specification. During that operation, the polishing catalyst would age. Then that reactor may be operated to be second in the series of the two polishing reactors under higher temperature conditions to reduce mercaptans. And the catalyst may be deactivated or further deactivated using known catalyst poisons.
The desulfurized gasoline stream product is delivered at outlet 99 from separator 90 that separates off any remaining light ends
Having 70 percent of the sulfur present in the gasoline being in the form of thiophenes, the aromatic content may preserve 1 to 2 octane numbers which translates into considerable value. If the octane rating of a volume of gasoline becomes too diminished, expensive octane enhancing materials must be added. On the other hand, excess octane number in the gasoline product makes that product itself an octane enhancing material for low octane gasoline feedstock. The value differences between octane excess materials and octane deficient materials can be quite substantial.
Catalysts for hydrodesulfurization are commonly based on molybdenum sulfide containing smaller amounts of cobalt or nickel and are formulated such that some catalysts have higher catalytic activity and others have lower activity. Understanding that the mercaptan polishing reactor must have a catalyst that is much less active for thiophenic conversion to finalize the sulfur polishing of the gasoline product and especially as compared to the catalyst selected for the thiophenic polishing reactor is an important distinction to operating the present invention. Using a deactivated catalyst of the same type as in the first reactor is one way of arranging the refinery in accordance with the present invention, however, a different catalyst such as nickel-alumina catalyst would like cause much more conversion of mercaptans and minimal conversion of thiophenes and higher-octane gasoline species. The two polishing reactors are not simply more of the same reaction but are targeted differently to get an octane advantage while meeting sulfur specification.
To provide an example of the invention, representative feed gasoline was provided through four arrangements. The first arrangement is a single polishing reactor with fresh catalyst. The second arrangement directs the product through two successive polishing reactors both with fresh catalyst in each. The third is where there are dual polishing reactors, but the second uses an aged or deactivated catalyst and the temperature is increased by 25° F. over the first reactor. The last is the same except that the temperature difference is 50° F. above the first reactor. The inputs, conditions and results are all shown in Table 1 below. The key point is that in the second to last arrangement, the octane loss was 0.57 RON and the last arrangement the RON lose is 0.48 while the other two arrangements included a greater octane loss. The lower octane loss is confirmed by the relative content of thiophenes to other sulfur molecular species in that conversion of thiophenes correlates to conversion of higher octane species in gasoline.
In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as additional embodiments of the present invention.
Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.
This application is a non-provisional application which claims benefit under 35 USC § 119(e) to U.S. Provisional Application Ser. No. 62/780,600 filed Dec. 17, 2018, entitled “Maximizing Octane Savings in a Catalytic Distillation Unit via a Dual-Reactor Polishing System”, and to U.S. Provisional Application Ser. No. 62/780,638 filed Dec. 17, 2018, entitled “Maximizing Octane Savings in a Catalytic Distillation Unit via a Dual-Reactor Polishing System”, both of which are incorporated herein in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62780600 | Dec 2018 | US | |
| 62780638 | Dec 2018 | US |
