Patent Application
20160362613
The present invention relates to the fluid catalytic cracking (FCC) process for converting high boiling point petroleum oils to lower boiling products.
The fluid catalytic cracking (FCC) process has become the pre-eminent source for motor gasoline in the USA and also serves the petrochemical industry with light olefins as petrochemical feedstock. In the FCC process, a pre-heated high boiling point petroleum feed such as a vacuum gas oil or residual fraction is subjected to a bulk boiling range conversion by contact with a hot, acidic-function catalyst in a specialized process unit in which the feed comes into contact with the hot catalyst at the bottom of a tall vertical pipe or “riser” in which the essential cracking reactions take place to produce a range of cracked hydrocarbon fragments in the vapor phase. The mixture of catalyst, vaporous cracking products and unconverted residues then enters a disengaging zone in which the catalyst is separated from the hydrocarbons, usually by cyclones or other inertial devices; for reasons arising from the early history of the process, the disengaging zone is usually referred to as the “reactor” although the majority of the cracking reactions take place, as intended, in the riser and the intention is that cracking in the reactor itself should be minimized. The separated, spent catalyst is then stripped of occluded hydrocarbons with steam in a stripping zone at the bottom of the reactor and the stripped catalyst is sent to a regenerator in which the carbon (‘coke) which accumulates on the catalyst as a result of the carbon rejection reactions taking place during the cracking process is oxidatively combusted to reactivate the catalyst and to supply the heat for the endothermic cracking reactions. The hot catalyst from the regenerator is then recirculated to the riser to participate in another round of cracking.
Many types of FCC unit exist with variations too numerous to described here but all rest upon a few simple principles; namely, that the cracking reactions which effectively reduce the boiling point range and molecular weight of the hydrocarbon species in the feed, are endothermic in nature are mediated by the acidic functioning catalyst which becomes deactivated as a result of the endothermic cracking by the deposition of rejected carbon onto the catalyst. This carbon (“coke”) is then burned off in the regenerator to supply the heat required for the cracking reactions to take place and this heat is carried from the regenerator to the cracking riser by the circulating catalyst. In this way, an overall heat balance for the unit is maintained with only a minor portion of the total heat requirement being provided by the feed pre-heat. Thus, the operation of the unit depends on the balance between the heat consumed by the endothermic cracking reactions and the exothermic combustion of the coke on the spent catalyst.
For FCCs processing all but the heaviest feed (highest Conradson Carbon content), some of the coke which is burned in the regenerator is ‘discretionary’, to the extent that it is burned in the regenerator to supply the heat required for stable, heat-balanced operation. With lighter (lower boiling) feeds and hydroprocessed feeds, stable unit operation becomes problematical without operating under conditions which result in a greater amount of coke being generated during the cracking portion of the FCC cycle simply in order to supply the heat demands of the unit. Thus, economically valuable hydrocarbon liquids are turned to low value coke merely to sustain operations.
U.S. Pat. No. 8,753,502 (Sexton) describes a way to maintain the overall unit enthalpy balance by combusting a low carbon fuel in a FCC catalyst heater-fuel gas/catalyst combustion chamber of specialized design through which the catalyst is circulated. While noting that most conventional FCC feedstocks contain enough coke precursors in the form of multi-ring aromatics to deposit sufficient “catalytic coke” on the circulating catalyst to satisfy the overall unit enthalpy balance while achieving the desired level of conversion, it is also noted that FCC processes have continued to evolve with unit designs that offer greater processing flexibility with enhanced product yields via improved coke selectivity, i.e. less coke relative to liquid product volume. Catalyst improvements have also enabled coke yields to be reduced but in all cases, the unit enthalpy balance must be met via a certain amount of coke or coke yield on fresh feed regardless of the feedstock's quality. Thus, changes in catalyst, unit design and/or operations could be made to decrease the in-unit coke yield with consequent increases in liquid product yield but the heat balance then registers a deficit.
U.S. Pat. No. 8,354,065 (Sexton) presents a related conceptual approach using a combination of the FCC catalyst heater-fuel gas/catalyst combustion chamber with a catalyst cooler.
While the proposals in U.S. Pat. Nos. 8,753,502 and 8,354,065 appear in principle of maintaining the unit enthalpy balance, they fail to make the most effective use of the existing unit, requiring specialized unit modifications to provide the combustion chambers in which light hydrocarbon fuels are burned to supply the additional heat with this heat being transferred to the main body of circulating catalyst by means of a catalyst stream which is passed into or through the combustion chamber. With combustion chambers such as those described, necessarily operating at a high temperature, there exists the potential for catalyst hold-up in the chamber, excessive erosion by fast circulating catalyst flows, as well as inefficient heat transfer by a limited stream of catalyst.
We have now devised an improved scheme for redressing an inadequate heat supply to the cracking reactions in the FCC unit by the substitution of methane (natural gas) (or other light fuels such as fuel gas) for this ‘discretionary coke’. The combustion of this supplementary fuel in the regenerator itself will result in increased regenerator heat release for the same amount of air/oxygen consumption while decreasing CO2 emissions. The supplementary fuel is injected into a dense bed of the catalyst in the middle or upper region of the dense bed or even or just above the bed. Operation of the regenerator in this way will maximize the oxygen concentration at the bottom of regenerator for coke burning purposes which is a slower reaction while essentially eliminating excess oxygen in the dilute phase and the flue gas resulting in a more reducing atmosphere for minimizing NOx formation. The added heat of combustion is directly added to the body of catalyst being regenerated so avoiding problems of heat transfer and catalyst hang-ups in separate combustion chambers.
According to the present invention, a potentially heat-deficient fluid catalytic cracking process is modified to maintain an overall enthalpy balance between the endothermic cracking and exothermic regeneration by combustion of a supplemental fuel in the middle or upper region of the dense bed in the regenerator (including the region immediately above the dense phase bed).
The present fluid catalytic cracking process for effecting a bulk boiling point conversion of a high boiling point petroleum feed to lower boiling products contacts the feed with a hot cracking catalyst to effect endothermic cracking of the feed after which the spent catalyst is exothermically regenerated by oxidative combustion of coke deposited on the catalyst during the cracking in a dense bed of catalyst in a regeneration step; in this process, the overall enthalpy balance between the endothermic cracking and exothermic regeneration is maintained by combustion of a supplemental fuel in the middle or upper region of the regeneration dense bed (including the region immediately above the dense phase bed). There is a direct economic benefit from operation of the cracking process in this way since the preferred supplemental fuel is methane (natural gas) which is currently a low cost fuel in the USA while liquid products are higher value. Use of natural gas as a supplemental fuel (in a manner similar to torch oil in the regenerator) will allow re-optimization of the catalyst and operations separately from the heat balance demand.
The single FIGURE of the accompanying drawings is a simplified diagram of an FCC regenerator modified for the supply of supplemental heat by combustion of a light fuel in the regenerator.
With the current economic conditions (primarily in the USA) natural gas is a significantly economically advantaged fuel, while incremental ‘discretionary coke’ generated and burned in an FCC regenerator is potential liquid product of higher value. In addition, increased political pressure on decreasing greenhouse gas emissions drives toward using higher hydrogen content fuels; methane is the highest hydrogen content fuel. Methane releases about 7% more heat per unit of air consumed than does a typical FCC coke (at ˜7% hydrogen in coke); it does that while producing about 40% less CO2 per unit quantity of heat released. Burning methane to heat balance the unit, instead of the normal practice of burning incremental ‘discretionary coke’, allows recovery of that ‘discretionary coke’ as liquid products while reducing unit emissions not only of CO2 but also of NOx and SOx from the regenerator. The substitution of higher hydrogen content material for indigenous coke increases the heat release per unit of air, hence increasing the FCCs processing capacity.
While methane is the preferred fuel, similar although less marked benefits may be secured by using a light (low carbon) fuel such as refinery fuel gas, (mostly carbon monoxide), hydrogen, syngas or even light hydrocarbons such as ethane or propane if available in sufficient quantity and economically justifiable.
For full burn FCCs operating on light or hydrotreated feed, it is estimated that 20-50% of the coke currently burned in the regenerator may be ‘discretionary coke’, for which natural gas could be substituted. In a 100 KBD (about 16,000 m3/day) FCC this could result in an additional 1-2+ KBD (about 160-320+m3/day) of liquid product yield and 350-700 ktons/yr (315-630 ktonnes/yr) (˜10-20%) reduction in CO2 emissions, at the expense of an equivalent (heating value basis) amount of methane.
The FIGURE is a simplified fragmentary section of an FCC regenerator of the dense bed type. The regenerator comprises a body 10 linked to the cracking section (not shown) of the FCC unit in the normal way by means of a catalyst standpipe 11 for the transfer of spent, stripped catalyst from the stripper section of the reactor to the base of the regenerator as well as a returned catalyst standpipe linked to the foot of the cracking riser (not shown) via the conventional slide valve for controlling catalyst flow rate. The spend, coked catalyst is regenerated in a bed 12 in the regenerator by oxidative combustion of the coke on the catalyst in the presence of air or oxygen-enriched air from distributor 15. The bed has the characteristics of a dense bed 16 in the base region of the regenerator body immediately above the distributor and in this region the oxygen concentration relative to coke is high as a result of being in the vicinity of the oxygen-bearing gas entering the bed from distributor 15. The average bed density progressively decreases with ascending height in the bed which eventually becomes a dilute phase 17 higher up in the regenerator. The gases from the regeneration process leave the body of the regenerator by way of cyclone system 18 comprising both primary and secondary cyclones which return separated catalyst particles to the catalyst bed through the catalyst diplegs in the normal way. The off-gases exit the regenerator through plenum 19 and pass as stack gases to precipitators, filters and baghouse as is conventional.
Natural gas or other light (low carbon) supplemental fuel is injected into the middle region of the bed by means of a series of fuel injectors in the middle level (one only shown) 22 of the catalyst bed or at the upper level 22 (one only shown) either into the dense bed or where the dense phase catalyst particles enter the dilute phase; the amount of oxygen at this point is still adequate to ensure combustion of the supplemental fuel so that heat is generated for heat balance to maintain operation of the unit with an adequately high returned catalyst temperature. The injectors will normally be arranged uniformly around the periphery of the reactor to promote even heating or, if the catalyst circulation pattern in the regenerator is known to be non-uniform, in conformity with the established pattern to promote uniform heat transfer to the mass of catalyst according to the local bed density in the regenerator at the level at which the injectors are located. The injectors will be selected to provide a flow rate according to the supplemental fuel in use and its heating value.
The catalyst particles absorb heat from the combustion of the fuel as well as from the combustion of the coke on the particles themselves and carry it to the cracking reactions via the returned catalyst standpipe. The relative amounts of the fuel and the oxygen content of the catalyst bed at the level of fuel injection should be adjusted to ensure that there is sufficient oxygen in the regeneration gas for complete combustion of the added fuel and that the amount of added fuel is sufficient to supply the required amount of supplemental heat. As noted above, addition of the supplementary into the dense bed in the middle or upper region of the dense bed or even or just above the bed maximizes the oxygen concentration at the bottom of the regenerator so that combustion of the coke on the catalyst is maximized to form a regenerated catalyst with a desirably low residual carbon content; the carbon burning process is a slower reaction and therefore has time to be essentially complete by the time the catalyst has passed through the depth of the bed; at the same time, injection of the supplemental fuel at a higher level in the bed essentially eliminates excess oxygen in the dilute phase and the flue gas resulting in a more reducing atmosphere for minimizing NOx formation.
The present process is primarily applicable in those cases where cracking of the selected feed could be carried out to better advantage with regard to liquid yield by changes in operation or selection of catalyst but where such changes have so far been implemented because of the need to maintain unit heat balance. The addition of the heat from the supplemental fuel enables this to be done with a consequential improvement in liquid product yield and a reduction in CO2 and other gaseous emissions. Feeds to the unit will therefore tend to be lighter (lower boiling) distillate feeds such as gas oils with an end point typically below 550° C. (about 1020° F.), hydroprocessed feeds or mixed feeds in which feeds of this kind will predominate. Cracking conditions will be as appropriate for the selected feed and catalyst and the unit under consideration.
This application claims priority to U.S. Provisional Application Ser. No. 62/172,917 filed Jun. 9, 2015, which is herein incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62172917 | Jun 2015 | US |
