TECHNICAL FIELD

The present disclosure relates generally to separation of oxygenates from lower alkanes using caustic waste.

BACKGROUND

Olefins like ethylene, propylene, and butylene, can be basic building blocks for a variety of commercially valuable polymers. Since naturally occurring sources of olefins may not exist in commercial quantities, polymer producers may rely on methods for converting the more abundant lower alkanes into olefins. Typically, a polymer producer can utilize steam cracking to produce alkenes from the lower alkanes. Steam cracking is a highly endothermic process where steam-diluted lower alkanes are subjected very briefly to a high temperature of at least 700° C. which requires a high energy demand. Additionally, steam cracking can cause coke formation in the reactor which can lead to increased maintenance costs and decreased profitability.

Oxidative dehydrogenation (ODH) is an alternative to steam cracking that can be exothermic, can have a low energy demand, and can produce little or no coke. In ODH, a lower alkane is mixed with oxygen in the presence of a catalyst and optionally an inert diluent at low temperatures such as, for example 300° C., to produce the corresponding alkene. In some examples, various other by-products such as, for example, carbon monoxide, carbon dioxide, and an oxygenate may also be produced in the ODH process. The by-products may be subject to further processing prior to being a marketable product or may be disposed of. The additional processing for separation of by-products from the marketable product can increase the complexity of a chemical complex and associated energy demands.

Additional processing downstream of the ODH process includes removal of oxygenates, such as acetic acid, using a quench tower, followed by removal of carbon oxides, particularly carbon dioxide, using an amine tower or caustic wash, or both. Oxygenates removed from the quench tower using a quench tower results in dilute solutions of the oxygenate that may require further processing to be marketable. Use of a caustic wash to remove carbon oxides produces spent caustics, or metal salts such as sodium carbonate or sodium hydrogen sulfide, which need to be disposed of using deep well injection, wet air oxidation, or incineration. This disclosure relates to use of the metal salts to simplify separation and concentration of oxygenates present in and removed from a gaseous stream.

SUMMARY

In one aspect, a method for separation of an oxygenate from a stream is provided. More specifically, the stream comprising the oxygenate is introduced to a quench tower along with a caustic outlet stream comprising a metal salt. The streams are quenched with addition of water and contact between the oxygenate and the metal salt during quenching facilitates conversion of the oxygenate into a derivative salt. An oxygenate outlet stream comprising a substantial portion of the derivative salt and at least a substantial portion of unconverted oxygenate is removed from the quench tower. A quench outlet stream, comprising gaseous components present in the stream, is also removed from the quench tower.

In yet another aspect, an apparatus for separation of an oxygenate from a stream is provided. More specifically, the apparatus comprises a quench tower comprising a quench inlet, a quench outlet, a metal salt inlet, and an oxygenate outlet. The quench inlet is configured to receive the stream comprising the oxygenate. The metal salt inlet is configured to receive into the quench tower a caustic outlet stream comprising a metal salt, allowing contact of the caustic outlet stream with the stream. The quench outlet is suitable for removing a quench outlet stream and the oxygenate outlet is suitable for removing an oxygenate outlet stream comprising at least a substantial portion of a derivative salt formed by contact of the oxygenate with the metal salt and at least a substantial portion of the unconverted oxygenate.

In yet another aspect, a system for separation of an oxygenate from a stream is provided. More specifically, the system comprises a quench tower configured to receive a stream comprising the oxygenate and a caustic outlet stream comprising a metal salt resulting in contact of the oxygenate with the metal salt and conversion of a portion of the oxygenate into a derivative salt, quench the stream and the caustic outlet stream, remove at least a substantial portion of a derivative salt and at least a substantial portion of the unconverted oxygenate, and remove an quench outlet stream comprising gaseous components of the stream.

In one aspect, a method is provided to convert a lower alkane to an alkene. More specifically, an input stream comprising oxygen and the lower alkane is introduced to an oxidative dehydrogenation (ODH) reactor. At least a portion of the lower alkane is converted to the alkene in the ODH reactor and an ODH outlet stream comprising the alkene, an oxygenate, and a carbon-based oxide is produced. The ODH outlet stream and a caustic outlet stream comprising a metal salt are in introduced to a quench tower and quenched. Contact in the quench tower between the oxygenate and the metal salt facilitates conversion of a portion of the oxygenate into a derivative salt. A quench outlet stream comprising at least a substantial portion of the alkene and at least a substantial portion of the carbon-based oxide is removed from the quench tower, as is an oxygenate outlet stream comprising at least a substantial portion of the unconverted oxygenate and at least a substantial portion of the derivative salt. The quench outlet stream is introduced to a caustic wash tower and contacted with a caustic agent in the caustic wash tower to form a metal salt that is removed from the caustic tower and may recycled and used as part of the caustic outlet stream introduced into the quench tower with the ODH outlet stream.

In another aspect, an apparatus is provided for oxidative dehydrogenation (ODH) of a lower alkane to an alkene. More specifically, the apparatus comprises an ODH reactor, a quench tower, a caustic wash tower, and a return line. The ODH reactor comprises an ODH inlet and an ODH outlet. The ODH inlet is suitable for transporting an ODH inlet stream comprising the lower alkane into the ODH reactor. The ODH outlet is suitable for transporting an ODH outlet stream comprising the alkene, an oxygenate, and a carbon-based oxide. The quench tower comprises a quench inlet, a quench outlet, a metal salt inlet, and an oxygenate outlet. The quench inlet is in fluid communication with the ODH outlet to receive the ODH outlet stream. The quench outlet is suitable for transporting a quench outlet stream comprising at least a substantial portion of the alkene and at least a substantial portion of the carbon-based oxide. The oxygenate outlet is suitable for transporting an oxygenate outlet stream comprising at least a substantial portion of the oxygenate and a derivative salt. The caustic wash tower comprises a wash inlet, a wash outlet, a caustic inlet, and a caustic outlet. The wash inlet is in fluid communication with the quench outlet to receive the quench outlet stream. The caustic outlet is suitable for transporting a caustic outlet stream comprising a metal salt. The return line is in fluid communication with the caustic outlet to receive the caustic outlet stream and output the caustic outlet stream into the metal salt inlet of the quench tower.

In another aspect, a system is provided for oxidative dehydrogenation (ODH) of a lower alkane. More specifically, the system comprises an ODH reactor, a quench tower, a caustic wash tower, and a return line. The ODH reactor is configured to receive an input stream comprising oxygen and the lower alkane. The ODH reactor is configured to produce an ODH outlet stream comprising an alkene, an oxygenate, and a carbon-based oxide. The quench tower is configured to receive and quench the ODH outlet stream and a caustic outlet stream comprising a metal salt, contact the oxygenate with the metal salt to convert a portion of the oxygenate to a derivative salt, remove an oxygenate outlet stream comprising at least a substantial portion of the unconverted oxygenate and at least a substantial portion of the derivative salt, and produce a quench outlet stream comprising at least a substantial portion of the alkene and at least a substantial portion of the carbon-based oxide. The caustic wash tower is configured to receive the quench outlet stream and contact a substantial portion of the carbon-based oxide from the quench outlet stream with a caustic agent to form a caustic outlet stream comprising a metal salt. The return line is configured to direct the caustic outlet stream into the quench tower and contact the caustic outlet stream with the ODH outlet stream to form a derivative salt from the metal salt and the oxygenate. The oxygenate outlet stream comprises a substantial portion of the derivative salt.

It is understood that the inventions described in this specification are not limited to the examples summarized in this Summary. Various other aspects are described and exemplified herein.

