Patent Application
20160083660
The field of this invention relates to hydrocarbon cracking processes, and in particular the production of light olefins from cracking a heavy hydrocarbon feedstock
The production of light olefins, ethylene and propylene, are used in the production of polyethylene and polypropylene. These are among the most commonly manufactured plastics today. Other uses for ethylene and propylene include the production of other chemicals. Examples include vinyl monomer, vinyl chloride, ethylene oxide, ethylbenzene, cumene, and alcohols. This list is by no means exhaustive, but is representative of the versatility of ethylene and propylene. The production of ethylene and propylene is chiefly performed through the cracking of heavier hydrocarbons. The cracking process includes stream cracking and catalytic cracking of hydrocarbon feedstocks, such as naphtha, gas oils, and other hydrocarbon streams, as well as other sources of carbonaceous materials, such as recycled plastics and organic materials.
A light olefins plant involves a very complex combination of reaction and gas recovery systems. Feedstock is charged to a thermal cracking zone in the presence of steam at effective conditions to produce a pyrolysis reactor effluent gas mixture. The mixture is then stabilized and separated into purified components through a sequence of cryogenic and conventional fractionation steps. Ethylene and propylene yields from steam cracking and other processes may be improved using known methods for the metathesis or disproportionation of C4 and heavier olefins, in combination with a cracking step in the presence of a zeolitic catalyst, as described, for example, in U.S. Pat. No. 5,026,935 and U.S. Pat. No. 5,026,936. The cracking of olefins in hydrocarbon feedstocks comprising C4 mixtures from refineries and steam cracking units is described in U.S. Pat. No. 6,858,133; U.S. Pat. No. 7,087,155; and U.S. Pat. No. 7,375,257.
Currently, the majority of light olefins production is from steam cracking and fluid catalytic cracking (FCC). However, the demand for light olefins is growing and other means of increasing the amount of light olefins have been sought. Other means include paraffin dehydrogenation, which represents an alternative route to light olefins and is described in U.S. Pat. No. 3,978,150 and elsewhere. More recently, the desire for alternative, non-petroleum based feeds for light olefin production has led to the use of oxygenates such as alcohols and, more particularly, methanol, ethanol, and higher alcohols or their derivatives. Methanol, in particular, is useful in a methanol-to-olefin (MTO) conversion process described, for example, in U.S. Pat. No. 5,914,433. The yield of light olefins from such a process may be improved using olefin cracking to convert some or all of the C4+ product of MTO in an olefin cracking reactor, as described in U.S. Pat. No. 7,268,265. Other processes for the generation of light olefins involve high severity catalytic cracking of naphtha and other hydrocarbon fractions. A catalytic naphtha cracking process of commercial importance is described in U.S. Pat. No. 6,867,341.
Another process for enhancing propylene yield is disclosed in U.S. Pat. No. 4,980,053, where a deep catalytic cracking process is disclosed. The process requires 5-10 seconds of contact time, and uses a mixture of Y-type zeolite and a pentasil, shape-selective zeolite. However, the process reports relatively high yields of dry gas.
Other patents disclose short catalyst contact times, but do not recognize significant light olefin yields, such as in U.S. Pat. No. 5,965,012 which discloses an FCC process. The process has a catalyst recycle arrangement with a very short contact time of the feed with the catalyst. However, further cracking takes place in a contacting conduit where regenerated and carbonized catalyst contacts the feed, and not in the riser. Another FCC process is disclosed in U.S. Pat. No. 6,010,618 where there is a very short catalyst and feed contact time in the riser, and the cracked product is quickly removed below the outlet of the riser. Other patents, such as U.S. Pat. No. 5,296,131 disclose very short FCC catalyst contact times, but these processes are operated to improve gasoline production rather than production of light olefins.
Other patents, U.S. Pat. Nos. 4,787,967, 4,871,446, and 4,990,314, disclose the use of two component catalysts used in FCC processes. The two component catalyst systems use a large-pore catalyst for cracking large hydrocarbon molecules and a small-pore catalyst for cracking smaller hydrocarbon molecules.
To enhance propylene yields, shape selective additives are used in conjunction with conventional FCC catalysts containing Y-zeolites. The additives all have essentially the same selectivity characteristics. The problem with current catalysts is that selectivity is limited, and the amount of propylene produced is only a function of the amount of additive used in the catalyst mixture. The propylene yield reaches a maximum at a crystalline shape selective zeolite content in the catalyst blend of approximately 10-12%.
To overcome this, the FCC operation severity (temperature, catalyst/oil ratio, etc.) is increased to increase light olefin yield, but at the cost of increased undesirable yields of coke, dry gas, or methane and ethane, as well as C4 and C5 olefins. The final olefin yields are limited by the equilibrium distribution even at high severity.
Despite the variety of methods for generating light olefins industrially, the demand for ethylene and propylene is still increasing faster than new processes can provide. Moreover, further demand growth for light olefins is expected. A need therefore exists for new methods that can economically increase light olefin yields from existing sources of both straight-run and processed hydrocarbon streams.
There is an increase in demand for light olefins, and in particular propylene. The present invention provides for a process to increase the yields of light olefins from a hydrocarbon feedstock.
A first embodiment of the invention is a process for improving light olefin yields, comprising passing a hydrocarbon stream to an FCC reactor to generate an FCC effluent stream comprising light olefins; passing the FCC effluent stream to a product recovery unit to generate a first stream comprising light components, a second stream comprising C4 and C5 hydrocarbons, and a third stream comprising C6+ compounds; passing the second stream to an extractive distillation unit to generate a fourth stream comprising C4 and C5 olefins, and a fifth stream comprising paraffins; passing the fourth stream to a secondary reactor to generate a sixth stream comprising light olefins; and passing the sixth stream to the light olefins separation unit. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the secondary reactor is a bubbling bed reactor, a slow fluidized bed reactor or a fast fluidized bed reactor with partial regeneration. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising passing a catalyst to the secondary reactor, thereby generating a catalyst effluent stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising passing the catalyst effluent stream to the FCC reactor. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the FCC reactor comprises a riser section, a catalyst separation section and a stripper section, and wherein the catalyst effluent stream is passed to the stripper section. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the catalyst comprises a cracking catalyst selected from the group consisting of Y-zeolite, ZSM-5, ST-5, ZSM-11, ZSM-22, beta, erionite, ZSM-34, SAPO-11, non-zeolitic amorphous silica-alumina, faujasite, chabazite, modernite, and mixtures thereof. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the catalyst comprises ZSM-5. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising passing a regenerated catalyst stream to the FCC reactor, to generate an intermediate stream of catalyst and reactants; passing the intermediate stream to a reactor separation stage to generate the FCC effluent stream and an intermediate catalyst stream; passing the intermediate catalyst stream to a stripping section to generate a stripped catalyst stream; and passing the stripped catalyst stream to a regenerator to generate the regenerated catalyst stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the hydrocarbon stream is a VGO stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the extractive distillation unit comprises a selective olefin absorption process utilizing a solvent to generate the fourth stream comprising olefins and the fifth stream comprising paraffins. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the solvent is selected from the group consisting of NMP (n-methyl-2-pyrrolidone), DMF (dimethylformamide), THF (tetrahydrofuran), ACN (acetonitrile), and mixtures thereof.
A second embodiment of the invention is a process for improving light olefin yields, comprising passing a hydrocarbon stream to a cracking reactor, wherein the reactor includes a cracking catalyst, to generate a cracking effluent stream comprising light olefins; passing the cracking effluent stream to a separation unit to generate a first stream comprising C3 and lighter compounds, a second stream comprising C4 and C5 hydrocarbons, and a third stream comprising C6+ compounds; passing the second stream to an extractive distillation unit to generate a fourth stream comprising C4 and C5 olefins, and a fifth stream comprising paraffins; and passing the fourth stream to a secondary reactor, wherein the secondary reactor includes a cracking catalyst, to generate a sixth stream comprising light olefins. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the cracking reactor is a fluidized catalytic cracking reactor comprising a riser reactor. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising passing a regenerated catalyst stream to the cracking reactor. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising passing the sixth stream to the light olefins separation unit. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising passing a cracking catalyst to the secondary reactor to generate a secondary catalyst effluent stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising passing the catalyst effluent stream to the cracking reactor, thereby generating a cracking reactor catalyst effluent stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising passing a regenerated catalyst stream to the cracking reactor thereby generating a spent catalyst effluent stream; and passing the spent catalyst effluent stream to a regenerator thereby generating the regenerated catalyst stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising passing a fresh catalyst stream to the secondary reactor thereby generating a spent secondary catalyst stream; and passing the spent secondary catalyst stream to the cracking reactor stripping zone. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the secondary reactor can comprise a bubbling bed reactor, a slow fluidized bed reactor, or a fast fluidized bed reactor, and wherein the secondary reactor utilizes the same catalyst, or a different catalyst, as the FCC reactor.
Other objects, advantages and applications of the present invention will become apparent to those skilled in the art from the following detailed description and drawings.
The demand for light olefins, ethylene and propylene, continues to increase. Methods to increase the production include trying new catalysts, and other flow processes, but the primary source of light olefins is the cracking of a hydrocarbon stream through either steam cracking or fluidized catalytic cracking (FCC) reactor. The principal hydrocarbon stream is naphtha, but heavier hydrocarbon streams, such as a vacuum gas oil (VGO) can also be used. However, the yields are not as great as with naphtha, and also cracking a heavier hydrocarbon stream also produces heavier by product streams.
Other processes for increasing propylene yields can include operating at higher severity, but these also need substantial amounts of ZSM-5 additive. Due to equilibrium constraints, the FCC reactor generates a substantial amount of other olefins, such as butenes and pentenes. This is in particular true for a typical Arabian Light VGO feedstock. While the current technology generates about 18 wt % of propylene, the process also generates about 20 wt % or more of butenes and pentenes. By recovering and passing the butenes and pentenes to a separate, but smaller reactor, the yields of propylene can be increased.
The present invention allows for the use of a heavier hydrocarbon stream, and is a new method that adds a smaller secondary reactor, wherein the catalyst for the reactor flows through the reactor and then into the FCC reactor. The integration will increase the propylene yield and is one of the objects of this invention.
The process of the present invention is shown in
The secondary reactor 20 can comprise a bubbling bed reactor, a slow fluidized bed reactor, or a fast fluidized bed reactor with regeneration. With a fast fluidized bed reactor, the regeneration of the catalyst can be partial of total.
The process further includes passing a catalyst stream 32 of fresh catalyst from the fresh catalyst feed hopper 30 to the secondary reactor 20. A catalyst effluent stream 24 is generated during the movement of catalyst through the secondary reactor 20. The catalyst effluent stream 24 is passed to the FCC reactor 10, and enters the cycle of catalyst in the FCC system. The catalyst cycle is well known to those in the FCC arts, and comprises flowing a regenerated catalyst stream 42 through the FCC reactor 10. The catalyst is separated from the product stream 12 and a spent catalyst stream 14 and passed to a regenerator 40.
The FCC reactor 10 comprises a riser section 52, a catalyst separation section 54, and a stripper section 56. The spent catalyst from the separation section is passed to the stripper section to collect in a moving bed, where a gas is passed through the moving bed to remove residual hydrocarbons and other adsorbed materials that reduce the efficiency of the regenerator 40. The regenerator 40 generates a regenerated catalyst stream 42 and passed the stream to the FCC reactor. The FCC reactor generates an intermediate stream leaving the FCC riser section 52. The stream leaving the riser section 52 enters the separation stage 54 wherein an intermediate catalyst and an FCC effluent stream are separated. The intermediate catalyst stream enters the stripping section 56 to generate a stripped catalyst stream, and the stripped catalyst stream 14 is passed to the regenerator.
The FCC reactor uses a catalyst, and the present invention uses the same catalyst for performing the cracking function. Suitable cracking catalyst are selected from one or more of Y-zeolite, ZSM-5, ST-5, ZSM-11, ZSM-22, beta, erionite, ZSM-34, SAPO-11, non-zeolitic amorphous silica-alumina, faujasite, chabazite and modernite. A combination catalyst can comprise two or more zeolites mixed into a common catalyst pellet, or can comprise a mixture of catalyst pellets of different types of catalytic materials. A preferred catalyst is ZSM-5.
The product recovery unit 100, as shown in
The second compressed stream 152 is passed to a third separation vessel 170 to generate a third vapor stream 172 and a third liquid stream 174. The third liquid stream 174 is passed to a deethanizer 180 to generate a deethanizer overhead 182 comprising C2 and lighter gases, and a deethanizer bottoms stream 184 comprising C3 and heavier hydrocarbons. The deethanizer bottoms stream 184 is passed to a depentanizer 190 to generate a depentanizer overhead stream 192 comprising C3 to C5 hydrocarbons, and a depentanizer bottoms stream 194 comprising heavier hydrocarbons. The depentanizer overhead stream 192 is passed to a depropanizer 200 to generate a depropanizer overhead stream 202 and a depropanizer bottoms stream 204 The depropanizer overhead stream 202 comprises C3s and is passed to a C3 splitter to recover propylene. The depropanizer bottoms stream 204 comprises C4s and C5s. and is passed to an extractive distillation unit 210. The extractive distillation unit 210 generates a paraffins stream 214 and an olefin stream 212. The olefin stream 212 comprises C4 and C5 olefins and is passed to the secondary reactor 20.
The naphtha overhead stream 162 is passed to a light gas stripper 220. The third vapor stream 172 is passed to the light gas stripper 220. The depentanizer bottoms stream 194 can be passed to other process units in a refinery, or a portion, can be passed to the light gas stripper 220. The bottom stream 222 of the light gas stripper 220 is recycled to the third separation vessel 170. The light gas stripper overhead 224 can be passed to a sponge absorber 230 for removing contaminants to generate a lean gas stream 232.
The extractive distillation unit 210 comprises a selective olefin absorption process utilizing a solvent to generate the olefin stream 212, and the paraffin stream 214. Suitable absorbents for the extractive distillation unit can include one or more absorbents selected from n-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), tetrahydrofuran (THF), and acetonitrile (ACN).
The flow configuration of the product recover unit 100 lends itself to the heat exchange of several streams leaving or entering the different separation columns. This is well known and not elaborated further here.
This novel reactor configuration does not need additional catalyst for high propylene operation except the fresh makeup ZSM-5 catalyst for the FCC system. The ZSM-5 makeup catalyst is due to attrition losses in the FCC system during operation. However, this is a relatively small amount added per day (on the basis of total catalyst in the system) to maintain a constant level of activity. The makeup catalyst is first passed through the secondary reactor before passing into the FCC reactor. Since the spent catalyst will be regenerated in the FCC regenerator, catalyst regeneration process for this additional catalytic cracking process is optional. The separated reactor will allow the reaction condition to be optimized independently, so ethylene and propylene concentrations will not be constrained by the FCC riser condition. As a result, high ethylene and propylene yields of single pass can be achieved from this reactor.
Unlike an FCC riser, the catalyst density in this secondary reactor is much higher, and can be at least 10 times higher. Hence, the reactor size is much smaller than a second riser for the same purpose. And, unlike a fixed bed reactor such as the olefin cracking process, where dual reactors loaded with special catalyst are needed to maintain a continuous operation during catalyst regeneration. The secondary bed reactor also allows a catalyst heat exchanger to work. The reactor, like the FCC reactor, will be operated at low pressure, 170 to 210 kPa (absolute) and high temperature 580 C or above. Therefore, total high C3=yield (26+ wt % on VGO) and C2=yield (10+ wt % on VGO) can be achieved in integrated system with typical VGO feedstock. Although the secondary bed reactor is integrated with the FCC unit, the FCC unit itself is a conventional FCC system. It can be operated with other modes such as gasoline mode by shutdown down this additional reactor.
While the invention has been described with what are presently considered the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.
