In conventional hydroprocessing of petroleum products it is necessary to transfer hydrogen from a vapor phase into the liquid phase where it will be available to react with a petroleum molecule at the surface of the catalyst. This is accomplished by circulating very large volumes of hydrogen gas and the petroleum oil through a catalyst bed. The petroleum oil feed and the hydrogen flow through the catalyst bed and the hydrogen is absorbed into a thin film of oil that is distributed over the catalyst. Because the amount of hydrogen required can be large, e.g. 1000 to 5000 SCF/bbl of liquid, the reactors are very large and can operate at severe conditions, from a few hundred psi to as much as 5000 psi, and temperatures from around 250° F.-900° F.
The temperature inside the reactor is difficult to control in conventional systems. While the temperature of the oil and hydrogen feed introduced into the reaction zone can be controlled, once the feed/hydrogen mixture is inside the reaction zone no adjustments to the system can be made to raise or lower the temperature of the oil/hydrogen mixture. Any changes in the reaction zone temperature must be accomplished through an outside source. As a result, conventional systems often inject cold hydrogen gas into the reaction zone if it becomes too hot. This method of cooling a reactor is expensive and is a potential safety risk.
While controlling the temperature of the reaction zone is often a difficult task in conventional systems, controlling the pressure of the hydroprocessing system is a much easier task. Pressure control systems are used to monitor the pressure of the system. The controls are used to release pressure through a valve or valves if the pressure becomes too great, and to increase the pressure of the system if the pressure becomes too low. A pressure control system cannot be used to control the pressure on a single hydroprocessing reactor, however. This is of no serious consequence, however, because pressure may be maintained on the entire system, but not on individual reactors.
One of the biggest problems with hydroprocessing is catalyst coking. Coking occurs when hydrocarbon molecules become too hot in an environment where the amount of hydrogen available for reaction is insufficient. The hydrocarbon molecules within the reactor crack to the point where coke, a carbonaceous residue, is formed. Cracking can take place on the surface of the catalyst, leading to coke formation and deactivation of the catalyst.
High-contaminant and/or high-olefinic feedstocks further complicate the hydroprocessing process. High-contaminant and/or high-olefinic feedstocks may include petroleum materials but primarily include non-petroleum products, such as renewable feedstocks derived from biological sources. These may be based on vegetable- or animal-derived materials, such as vegetable and animal oils. Such high-contaminant and/or high-olefinic feedstocks may also include pyrolysis oils derived from biomass materials, such as cellulosic biomass materials, or coal. These non-petroleum feedstocks may be highly olefinic and/or contain high levels of heteroatom contaminants, such as oxygen, nitrogen, sulfur, etc. Such olefinic compounds and heteroatom contaminants may be at levels of from about 10% by weight or more of the feed.
In order to produce a valuable product from such highly olefinic and highly contaminated feedstocks, a large amount of hydrogen is required, roughly 1500-4000 scf/bbl. Furthermore, these reactions are highly exothermic. They generate a great deal of heat, significantly more than what is found in a typical hydroprocessing process of petroleum products. The excessive amounts of heat generated put the entire process at great risk. One concern is the effect of such large quantities of heat on the catalyst. It is widely known that overheating, and subsequent coking, is one of the most common causes of catalyst deactivation. In a process that generates significantly more heat than the typical hydroprocessing process, temperature control in order to maintain catalyst activity is crucial. The most serious threat involved in the hydroprocessing of high-contaminant and/or high-olefinic feedstocks, however, is the risk of creating a runaway reaction, a reaction that generates so much heat that the process can no longer be brought under control. Despite turning off heaters and maximizing all cooling efforts, a runaway reaction can continue to heat and has the potential to cause serious damage to the reactor and process equipment. Runaway reactions contribute to a significant number of refinery explosions, damaging equipment, slowing or stopping production, and endangering workers. Consequently, the heat generated by the hydroprocessing of high-contaminant and/or high-olefinic feedstocks is one the greatest problems that must be surmounted.
An additional concern that is unique for high-contaminant feedstocks is the effect of hydroprocessing byproducts on the system. Water, hydrogen sulfide, ammonia, sulfur, carbon, and nitrogen oxides are all common byproducts created during hydroprocessing reactions. While these byproducts are undesirable in the finished product and must eventually be removed from the finished product, when using conventional petroleum feedstocks, these byproducts are not generally present in amounts significant enough to pose any real threat to the integrity of the process. This is not true for high-contaminant feedstocks. The hydroprocessing of these feedstocks results in much larger quantities of these byproducts being present in the system. These byproducts, particularly water, can be especially harmful to the catalyst. If water is allowed to build up in the catalyst bed, a separate aqueous phase can form. This aqueous phase is extremely harmful to the catalyst, essentially causing it to dissolve inside the reactor. In addition, hydrogen sulfide and ammonia, in large quantities, are widely known to inhibit catalyst activity. Therefore, it is of great importance that the quantities of these byproducts created during the hydroprocessing of high-contaminant feedstocks be controlled to prevent or minimize any damage they may cause to the system.
Because prior art methods do not adequately address the problems associated with hydroprocessing highly contaminated and/or highly olefinic feedstocks, improvements are needed.