Patent Application
20120083634
The field of the invention relates to the regeneration of adsorbents. In particular, the invention relates to the regeneration of adsorbents in the methanol to olefins process.
The MTO unit includes three separate adsorbent services for the removal of product impurities, primarily water and oxygenates. The adsorbents must undergo periodic, in-situ regeneration. There is insufficient net gas produced to regenerate the adsorbent without importing additional gases. Thus, regeneration requires large quantities of either nitrogen or clean natural gas, which is ultimately vented to the flare or fuel gas header. These external regenerant streams increase utility costs and can potentially introduce new impurities to the process. As an example, an extra purge step is required to remove residual CO2, introduced via natural gas, from the product driers following regeneration.
After the regeneration of the adsorbents, the imported gases are vented to a fuel gas header, or to a flare. The subsequent venting and flaring of these gases is a wasteful expense that can be eliminated.
A process is presented for the regeneration of adsorbent beds in the methanol to olefins conversion process. The process includes passing an oxygenate stream to a methanol to olefins reactor, wherein an intermediate stream is generated having olefins. The intermediate stream is first quenched to remove water and oxygenates, and then compressed, and separated into a stream comprising heavier hydrocarbons having 4 or more carbons and a lighter stream comprising light olefins. The light olefin stream will have residual oxygenates and CO2 in the stream. The oxygenates and CO2 are removed from the compressed stream, and a recycle oxygenate stream is returned to the methanol to olefins reactor. The compressed light olefins stream is passed through a drying adsorbent bed to remove water that has been generated in the MTO process or the quenching process, before cooling and separating the light olefin products. The dried light olefin stream is passed to a deethanizer to create an overhead stream having ethane, ethylene and lighter components, such as methane, and a bottoms stream comprising primarily propane, propylene and some C4 and heavier hydrocarbons.
The overhead stream is passed to a demethanizer to generate a demethanizer overhead stream and an ethane/ethylene bottoms stream. The demethanizer overhead (DMO) stream is used to regenerate the adsorbent beds used in drying the light olefins stream. The adsorbent beds comprise at least two adsorbent beds where one bed is on-line during operation, and the other adsorbent beds are off-line and regenerated for use when the on-line adsorbent bed is spent. The ethane/ethylene stream is then passed to an ethane/ethylene splitter to recover an ethylene product stream.
The bottoms stream from the deethanizer is passed to a depropanizer where a bottoms stream comprising C4 and higher hydrocarbons is recovered. The depropanizer overhead stream comprises propane and propylene, and is passed to a second adsorbent bed for further removal of residual moisture in the propane/propylene stream. The dried propane/propylene stream is passed to a propane/propylene splitter and the propylene product stream is recovered. The second adsorbent beds comprise at least two adsorbent beds where one bed is on-line during operation, and the other adsorbent beds are off-line and regenerated for use when the on-line adsorbent bed is spent. The DMO stream is passed to the off-line adsorbent beds and the off-line adsorbent beds are regenerated with the moisture removed from the adsorbent beds.
Additional objects, embodiments and details of this invention can be obtained from the following drawing and detailed description of the invention.
The methanol to olefin (MTO) process uses three separate adsorbent services. The adsorbents are for the removal of product impurities that include, primarily, water and either residual oxygenates or oxygenates that are generated in the MTO process. One adsorbent unit is for the removal of water from the product stream; a second adsorbent unit is for the removal of CO2 from the methane stream, which is used to regenerate the drier; and a third adsorbent unit is for drying the feed to the olefin cracking unit (OCU) reactor during catalyst regeneration. The adsorbents are periodically regenerated in situ. With the current process there is insufficient net gas produced to regenerate the adsorbents, and therefore, additional gases are imported for use. The regeneration requires large quantities of gas, and therefore large quantities of imported gases are used. The imported gases are either nitrogen or clean natural gas which are expensive, and can include the need to process for the removal of residual sulfur compounds in the gas.
This is an example of a wasteful expense that should be reduced or eliminated. The external regenerant streams increase utility costs and can potentially introduce new impurities to the process. As an example, an extra purge step is required to remove residual CO2, introduced via natural gas, from the product driers following regeneration.
The invention is the use of recycled MTO reactor purge gas for adsorbent regeneration. The total demethanizer overhead satisfies 80% of the total regenerant requirement for all three services combined. The use of recycled purge gas will significantly reduce the need for external regenerant gases. Moreover, it may be possible to eliminate the need for external regenerant gases for certain services by adjusting the adsorbent cycle lengths. Using the demethanizer overhead (DMO) as the sole regenerant for the product driers would eliminate the extra purge step required for residual CO2 removal from the MTO reactor, as the DMO regenerant has only ppb levels of CO2.
The present invention utilizes a gas stream that is generally vented to the atmosphere, or to a combustion unit. The process is shown in
The regenerant gases in the DMO stream 62 can also be used to regenerate guard beds used to protect the catalyst in the MTO reaction zone 12.
One specific embodiment of the present invention of regenerating adsorbents in a methanol to olefins conversion process is illustrated in
The dry light olefins stream 42 is passed to a light olefins recovery unit. The light olefins recovery unit includes a deethanizer 50 which generates a C2 and lighter stream 52 and a C3 and heavier stream 54. The C2 and lighter stream 52 is passed to a demethanizer 60 and creates a demethanizer overhead (DMO) 62 comprising hydrogen and methane. The demethanizer 60 also creates a C2 stream 64 which is passed to an ethane/ethylene splitter 66 creating an ethylene stream 68 and an ethane stream 70. The ethane/ethylene splitter 66 is operated under refrigeration conditions to facilitate the separation, and to reduce the size of the column. The operating conditions for the splitter 66 are such that the ethylene product exits the splitter 66 at a temperature between −20° C. and −35° C., and at a pressure between 1.9 MPa and 2.1 MPa. The feed to the ethylene splitter 66 is cooled and compressed from the demethanizer 60. The DMO is a useful stream for regenerating adsorbents, rather than passing the DMO to a flare, or for disposal with other flue gases. The demethanizer is operated at conditions such that the overhead gases exit the demethanizer at a temperature between 10° C. and 20° C. at a pressure between 700 kPa to 900 kPa.
The first adsorbent bed 40 can comprise a multibed system with one bed on-stream while the other beds are regenerated. When the on-stream bed is saturated, the system is switched such that the on-stream bed is taken off-line, and an off-line bed that has been regenerated is switched to on-stream. At least a portion of the DMO is passed through the off-line first adsorbent beds 40 to regenerate the beds and remove the adsorbed water, creating purge gas streams 44, 94. The purge gas streams 44, 94 are passed to the MTO reaction zone 12 to purge the MTO reactor beds. The purge gas stream exiting the MTO reaction zone 12 is included in the intermediate olefins stream 14 and ultimately recycled to the adsorbent beds 40, 86 as a dry DMO stream for regeneration of the beds. This cycle includes a bleed line for passing excess DMO gas generated as the DMO is cycled through the adsorbent beds 40, 86 and the MTO reaction unit 12.
In one embodiment, the process includes passing the C3 and heavier stream to a depropanizer 80. The depropanizer 80 separates the stream into a propane/propylene stream 82 and a C4+ stream 84. The propane/propylene stream 82 is passed to a second adsorbent bed 86 to dry and remove oxygenates from the propane/propylene stream 82. The purified and dried propane/propylene stream 88 is passed to a propane/propylene splitter 90, where the propylene product stream 92 is recovered. The operating conditions for the splitter 90 are such that the propylene product exits the splitter 90 at a temperature between 40° C. and 60° C., and at a pressure between 2.1 MPa and 2.6 MPa.
The ethane and propane recovered from the ethane/ethylene splitter 66 and the propane/propylene splitter 90 can be either kept as separate streams, or mixed as a combined stream. The ethane and propane products are generally recovered at temperatures between 40° C. and 50° C., and at pressures between 500 and 700 kPa. The C4+, or butane, product stream is generally recovered at temperatures between 40° C. and 50° C., and at pressures between 1.1 MPa and 1.2 MPa.
The second adsorbent bed 86 can comprise a multibed system with one bed on-stream while the other beds are regenerated. When the on-stream bed is saturated, the system is switched such that the on-stream bed is taken off-line, and an off-line bed that has been regenerated is switched to on-stream. At least a portion of the DMO 62 is passed through the off-line second adsorbent beds 86 to regenerate the beds and remove the adsorbed water and other impurities such as oxygenates absorbed by the adsorbent beds 86.
A portion of the DMO stream 62 is passed to the off-line second adsorbent beds 86 to regenerate the adsorbent and removed the water in the adsorbent bed 86.
In an optional arrangement, a portion of the C4+ stream 84 can be mixed with a portion of the DMO stream 62 to regenerate the first and/or second adsorbent beds 40, 86.
In another embodiment, the DMO stream 62 can be used to purge the MTO reactor 12. A portion of the DMO stream 62 is passed to the MTO reactor to purge residual CO2 adsorbed in the reactor. The DMO stream 62 is ideal for this, as residual CO2 has already been removed from the DMO stream, and only very small amount might remain in the DMO stream, or on the order of a few ppm.
In another embodiment, the process can include passing the C4+ stream 84 to an olefin cracking unit 100. The olefin cracking unit 100 cracks the larger olefins, which are separated into a light olefins stream 102, an effluent stream comprising butanes and butylenes 104, and a stream 106 comprising C6 and heavier hydrocarbons. A portion of the butane and butylene stream 104 can be mixed with the DMO stream 62 and passed to the first or second adsorbers 40, 86 during regeneration.
Lower ethylene content is desirable during the purge step of the MTO reactor 12. The mixing of butanes and butylenes can be adjusted to ensure the ethylene content in the DMO stream 62 is sufficiently low so as to prevent any potential coking problems that can arise from a too high level of ethylene in the purge gas. Additionally, the demethanizer 60 can be operated to ensure a sufficiently low amount of ethylene is allowed in the DMO stream 62.
The process can further include separation equipment for separating out heavy materials that are generated by the MTO reactor, prior to compression. An intermediate by-products stream having unreacted materials and heavy materials can be recycled to the MTO reactor, or can be further separated with unreacted oxygenates recycled to the MTO reactor.
The present invention significantly reduces the need for external regenerant or purge gases. By adjusting the regeneration cycles of the adsorbers, the demethanizer overhead can supply the needs without additional external regenerant gases. In addition, this process saves on the clean-up of external regenerant gases. A common regenerant gas is natural gas, and natural gas comes with residual amounts of CO2. These residual amounts of CO2 require treating the natural gas to remove the CO2 prior to using the gas as either a regenerant for the adsorbers, or as a purge gas for the MTO 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.
