The present invention relates to a reactor system and process for the oxidative dehydrogenation of ethane.

Background

It is known to oxidatively dehydrogenate ethane resulting in ethylene, in an oxidative dehydrogenation (oxydehydrogenation; ODH) process. Examples of ethane ODH processes are for example disclosed in US7091377, WO2003064035, US20040147393, WO2010096909 and US20100256432. The oxidative dehydrogenation of ethane is an exothermic reaction wherein ethane is converted into ethylene. In this process, ethane is reacted with oxygen in the presence of an ODH catalyst to produce a product stream comprising predominately ethylene, along with unreacted reactants (such as ethane and oxygen), and typically other gases and/or by-products (such as carbon monoxide, carbon dioxide, water, acetic acid).

The ODH reaction can intrinsically proceed at very high productivity, which is accompanied by a very large local heat production. Thus, in the context of a commercial oxidative dehydrogenation process with high productivity, effective heat management is critical for process control. More specifically, effective heat management can have a significant impact on key process parameters, such as catalyst efficiency, reaction selectivity, catalyst life, reactor design, materials of construction, and also plant/reactor safety.

It is known that a multitubular fixed-bed reactor may be used to conduct gas-phase catalytic reactions, especially those where high levels of heat transfer are needed, with the reactor employing a plurality of tubes containing a fixed bed of catalyst particulates and a shell in which the tubes are contained. In operation, gas reactants are fed into the reactor tubes where they flow past the catalyst and react to form the desired product. The heat of reaction is quickly transferred from the site of the reaction to the outside walls of the tubes and a circulating or boiling coolant on the shell side removes the heat of reaction. One advantage of such fixed-bed reactors is that, for many reactions, it provides the highest conversion of reactant per weight of catalyst. However, a disadvantage is that it can be difficult to control the temperature within the reactor, and in the case of exothermic reactions, a so-called "hot spot" (a localized temperature peak) in the catalyst bed can occur, which may adversely affect reactor performance, in terms of maximum productivity, selectivity or catalyst stability.

In an attempt to suppress the undesirable formation of a detrimental hot-spot, one commonly proposed solution is to reduce the diameter of the tubes in order to increase the heat transfer rate per unit volume of the catalyst. However, this typically increases the cost associated with building the reactor and also increases the amount of time required to load and unload the catalyst into the tubes. Another commonly proposed solution is to increase the length of the reactor tubes at a fixed space velocity GHSV. In general, the reactor typically becomes more isothermal as the reactor tube length is increased at constant space GHSV; however, undesirable increases in pressure drop inevitably accompany an increase in tube length. Especially for low pressure processes (e.g., < 5 bar) such as ODH, this rapidly leads to unfeasibly high pressure drops. A third solution includes operating at a lower productivity per unit volume of catalyst, for example by diluting the catalyst with an inert substance. However, this also has the disadvantage of increased cost and typically increases the difficulty of later recovering the spent catalyst from the reactor for regeneration, if desired.

Accordingly, it is desirable in the manufacture of ethylene by the oxidative dehydrogenation of ethane to utilize a multitubular fixed-bed reactor comprising oxidative dehydrogenation catalyst, which produces a high space time yield of ethylene while maintaining sufficient isothermal performance and acceptable pressure drop across the catalyst bed during operation.

It is, thus, an object of this invention to provide a reactor system suitable for use in the oxidative dehydrogenation of ethane to ethylene that maintains sufficient isothermal performance and acceptable pressure drop across the catalyst bed during operation, but still produces a high space time yield of ethylene. Summary

The present invention generally relates to reactor systems for the oxidative dehydrogenation of ethane to ethylene, and to improved processes for the oxidative dehydrogenation of ethane.

In particular, the present inventors have sought to provide improved ODH processes and reactor systems that enable the production of ethylene (via the oxidative dehydrogenation of ethane) in a multitubular fixed-bed reactor at a high space time yield, while simultaneously maintaining sufficient isothermal performance and acceptable pressure drop across the catalyst bed.

In one aspect, a reactor system for the oxidative dehydrogenation of ethane to ethylene is provided. The reactor system comprises:

a multitubular fixed-bed reactor comprising a plurality of reactor tubes having a tube length of from 4 to 12 m and a tube diameter (D _T ) of from 15 to 25 mm, wherein the plurality of reactor tubes comprise a catalyst bed comprising a major portion of oxidative dehydrogenation catalyst having a shape selected from:

a cylindrical ring configuration having an outside diameter (Do) such that DT/DO is from 3 to 5, and a cylinder bore inner diameter (Di) such that Do/Di is from 2 to 4, and a cylinder length (Lc) such that Lc/Do is from 0.7 to 1.5;

a trilobe geometric configuration having a trilobe nominal diameter (DNO _M ) such that DT/DNOM is from 3 to 8, and a trilobe length (LT) such that LT/DNOM is from 0.5 to 2; or a combination thereof.

In another aspect, a process for the oxidative dehydrogenation of ethane to ethylene is provided, the process comprising:

providing a reactor system comprising a multitubular fixed-bed reactor comprising a plurality of reactor tubes having a tube length of from 4 to 12 m and a tube diameter (D _T ) of from 15 to 25 mm, wherein the plurality of reactor tubes comprise a catalyst bed comprising a major portion of oxidative dehydrogenation catalyst having a shape selected from:

- a cylindrical ring configuration having an outside diameter (Do) such that DT/DO is from 3 to 5, and a cylinder bore inner diameter (Di) such that Do/Di is from 2 to 4, and a cylinder length (Lc) such that Lc/Do is from 0.7 to 1.5;

a trilobe geometric configuration having a trilobe nominal diameter (DNO _M ) such that DT/DNOM is from 3 to 8, and a trilobe length (LT) such that LT/DNOM is from 0.5 to 2; or

- a combination thereof, and

supplying a feed gas comprising ethane and oxygen to an inlet of the multitubular fixed- bed reactor and allowing the ethane and oxygen to react in the presence of the oxidative dehydrogenation catalyst to yield a reactor effluent comprising ethylene.

Brief Description of the Drawings

Some specific example embodiments of the disclosure may be understood by referring, in part, to the following description and the accompanying drawings. FIG. 1 depicts an oxidative dehydrogenation catalyst suitable for use in the present disclosure and which has a cylindrical ring configuration.

FIGS. 2A and 2B depict an end view and a perspective view of an oxidative dehydrogenation catalyst suitable for use in the present disclosure and which has a trilobe geometric configuration.

FIGS. 2C and 2D depict an end view and a perspective view of an oxidative dehydrogenation catalyst suitable for use in the present disclosure and which has a trilobe geometric configuration.

While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments have been shown in the figures and are herein described in more detail. It should be understood, however, that the description of specific example embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, this disclosure is to cover all modifications and equivalents as illustrated, in part, by the appended claims.

Detailed Description

It is believed that the various advantages of the present disclosure may be realized, at least in part, by the use of a multitubular fixed-bed reactor comprising a plurality of reactor tubes of appropriate length and diameter, in combination with a catalyst bed comprising a major portion (i.e. at least 50%) of oxidative dehydrogenation catalyst having a certain geometric configuration, as further discussed herein.

More particularly, it is believed that the use of a multitubular fixed-bed reactor comprising a plurality of reactor tubes having a tube length of from 4 to 12 m and a tube diameter (DT) of from 15 to 25 mm, in combination with a catalyst bed that comprises a major portion of oxidative dehydrogenation catalyst having a specified cylindrical ring configuration, a specified trilobe geometric configuration, or a combination thereof will preferably provide an ODH process that is capable of achieving a high space time yield of ethylene while still maintaining sufficient isothermal performance and acceptable pressure drop across the catalyst bed during operation.

In general, reactor systems of the present disclosure comprise a multitubular fixed-bed reactor. Multitubular fixed-bed reactors suitable for use in the present disclosure are not particularly limited and may include any of a variety known in the art. Suitable multitubular fixed-bed reactors generally comprise a reactor inlet, a reactor outlet, an interior shell space, and a plurality of reactor tubes disposed within the interior shell space.

Within the reactor, the upper ends of the reactor tubes are typically fixed in place by an upper tube plate and are in fluid communication with the reactor inlet. Similarly, the lower ends of the reactor tubes are typically fixed in place by a lower tube plate and are in fluid communication with the reactor outlet. Preferably, the reactor tubes are arranged within the reactor in a substantially vertical manner such that they are no more than 5° from vertical, and the upper and lower tube plates are positioned within the reactor in a substantially horizontal manner such that they are no more than 3° from horizontal.

In accordance with the present disclosure, suitable reactor tubes generally have a tube length of from 4 to 12 meters (m), or from 4 to 8 m, and a tube diameter (DT) of from 15 to 25 millimeters (mm). Typically, the inside cross section of the reactor tube perpendicular to the tube axis (hereinafter "tube cross section") is circular, which means that the tube, internally, represents an elongated cylinder. Alternatively, the tube cross section may be non-circular (e.g. oval, etc.) and for such tubes, the internal tube diameter as specified is deemed to represent the equivalent circular diameter, which equivalent circular diameter represents the diameter of a circle which has a circumferential length the same as the circumferential length of the non- circular tube cross section. The number of reactor tubes in a suitable reactor can vary and may range in the thousands, for example up to 40,000.

Within a multitubular fixed-bed reactor, the portion of the reactor tubes that comprise catalyst is commonly referred to as the "catalyst bed". In accordance with the present disclosure, a plurality of reactor tubes comprise a catalyst bed that comprises a major portion (i.e. at least 50%) of oxidative dehydrogenation catalyst having a shape selected from: a cylindrical ring configuration having certain specified cylinder dimensions, a trilobe geometric configuration having certain specified trilobe dimensions, or a combination thereof.

As used herein, the term "a major portion" of the catalyst bed refers to at least 50% by volume of the oxidative dehydrogenation catalyst in the catalyst bed. More preferably, at least 60%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 98% or 100%, by volume, of the oxidative dehydrogenation catalyst in the catalyst bed has a shape selected from the specified cylindrical ring configuration, the specified trilobe geometric configuration, or a combination thereof. Optionally, in one embodiment, the catalyst bed may comprise an oxidative dehydrogenation catalyst having the specified cylindrical ring configuration positioned in an upstream portion of the catalyst bed and an oxidative dehydrogenation catalyst having the specified trilobe geometric configuration positioned in a downstream portion of the catalyst bed.

In reference to FIG. 1, a suitable oxidative dehydrogenation catalyst having a cylindrical ring configuration (10) has a cylinder outside diameter "(Do)" (14) such that DT/DO is from 3 to 5, a cylinder bore inner diameter "(Di)" (16) such that Do/Di is from 2 to 4, and a cylinder length "(Lc)" (12) such that L _c /D ₀ is from 0.7 to 1.5.

Similarly, in reference to FIGS. 2A-2D, a suitable oxidative dehydrogenation catalyst having a trilobe geometric configuration (20) has a nominal diameter (24) "(DNO _M )" such that DT/DNOM is from 3 to 8, or from 4 to 6, and a trilobe length (LT) (22) such that LT/DNOM is from 0.5 to 2, or from 0.7 to 1.5, or from 0.9 to 1.1.

In embodiments where the catalyst bed contains less than 100% of oxidative dehydrogenation catalyst having the specified cylindrical ring configuration, the specified trilobe geometric configuration, or a combination thereof, the remaining portion of the catalyst bed may comprise oxidative dehydrogenation catalyst of any suitable shape and/or size. Suitable shapes may include any of a wide variety of shapes known for catalyst, such as pills, chunks, tablets, pieces, pellets, rings, spheres, wagon wheels, trapezoidal bodies, doughnuts, amphora, rings, Raschig rings, honeycombs, monoliths, saddles, cylinders having a geometric configuration that falls outside the specified cylindrical ring configuration, multi-lobed cylinders having a geometric configuration that falls outside the specified trilobe geometric configuration, cross-partitioned hollow cylinders (e.g., cylinders having at least one partition extending between walls), etc. Optionally, in addition to oxidative dehydrogenation catalyst, the catalyst bed may further comprise non-catalytic or inert material (e.g., to dilute and/or reduce the activity of the catalyst bed).

In general, the total amount of oxidative dehydrogenation catalyst in the catalyst bed and the overall height of the catalyst bed may vary over a wide range, depending upon, for example, the size and number of tubes present within the multitubular fixed-bed and the particular size and shape of the oxidative dehydrogenation catalyst. However, typical ranges for catalyst bed height may be from 80% to 100% of the reactor tube length. In those embodiments where the catalyst bed height is less than 100% of the reactor tube length, the remaining portion of the tube may be empty or optionally comprise particles of a non-catalytic or inert material.

Oxidative dehydrogenation catalysts suitable for use in the present disclosure are not particularly limited and may include any ethane oxidative dehydrogenation catalyst. Examples of suitable oxidative dehydrogenation catalyst include, but are not necessarily limited to, one or more mixed metal oxide catalyst comprising molybdenum, vanadium, niobium and optionally tellurium as the metals and may have the following formula:

MoiVaTe _b NbcOn

wherein:

a, b, c and n represent the ratio of the molar amount of the element in question to the molar amount of molybdenum (Mo);

a (for V) is from 0.01 to 1, preferably 0.05 to 0.60, more preferably 0.10 to 0.40, more preferably 0.20 to 0.35, most preferably 0.25 to 0.30;

b (for Te) is 0 or from >0 to 1, preferably 0.01 to 0.40, more preferably 0.05 to 0.30, more preferably 0.05 to 0.20, most preferably 0.09 to 0.15;

c (for Nb) is from >0 to 1, preferably 0.01 to 0.40, more preferably 0.05 to 0.30, more preferably 0.10 to 0.25, most preferably 0.14 to 0.20; and

n (for O) is a number which is determined by the valency and frequency of elements other than oxygen.

Optionally, a catalyst bed may comprise more than one oxidative dehydrogenation catalyst. For example, in one embodiment, a catalyst bed may comprise a plurality of oxidative dehydrogenation catalysts having varied activity levels (e.g., so as to vary the activity level along the length of the reactor tube).

In accordance with the oxidative dehydrogenation processes of the present disclosure, a feed gas comprising ethane and oxygen is supplied to the inlet of a multitubular fixed-bed reactor. As used herein, the term "feed gas" is understood to refer to the totality of the gaseous stream(s) at the inlet(s) of the reactor. Thus, as will be appreciated by one skilled in the art, the feed gas is often comprised of a combination of one or more gaseous stream(s), such as an ethane stream, an oxygen-containing stream, a recycle gas stream, a diluent stream, a ballast gas stream, steam, etc. Optionally, in addition to ethane and oxygen, the feed gas may further comprise other alkanes (e.g. methane), carbon monoxide, carbon dioxide, hydrogen, steam, an inert gas (such as nitrogen, helium and/or argon), and/or various by-products of the ODH reaction (e.g., acetylene, acetic acid).

In the processes disclosed herein, ethane and oxygen may be added to the reactor as mixed feed, optionally comprising further components therein, at the same reactor inlet. Alternatively, the ethane and oxygen may be added in separate feeds, optionally comprising further components therein, to the reactor at the same reactor inlet or at separate reactor inlets. Further, the order and manner in which the components of the feed gas are supplied to the reactor inlet is not particularly limited, and therefore, the components may be combined simultaneously or sequentially. Further, the components of the feed gas may optionally be vaporized, preheated and mixed (if desired) prior to being supplied to the reactor inlet using means known to those skilled in the art. For example, preheat techniques may include, for example, heat exchange from steam, a heat transfer fluid (e.g., coolant), reactor effluent, and/or a furnace.

Ethane in the feed gas may be from any suitable source, including natural gas, provided that impurities are sufficiently removed therefrom and may include fresh ethane and optionally, a recycle of unreacted ethane from the reactor effluent. Similarly, the oxygen may originate from any suitable source, such as air or a high purity oxygen stream. Such high-purity oxygen may have a purity of greater than 90%, preferably greater than 95%, more preferably greater than 99%, and most preferably greater than 99.4%.

In general, the molar ratio of molecular oxygen to ethane in the feed gas at the reactor inlet may be in the range of from 0.01 to 1, more suitably 0.05 to 0.5. Preferably, the feed gas comprises from 5 to 35 vol.% of oxygen, relative to the total volume of the feed gas, more suitably 20 to 30 vol.% of oxygen, and 40 to 80 vol.% of ethane, more suitably 50 to 70 vol.% ethane, and less than 80 (0 to 80) vol.% of an inert gas, more suitably less than 50 (0 to 50) vol.% of an inert gas, more suitably 5 to 35 vol.% of an inert gas, most suitably 10 to 20 vol.% of an inert gas. Suitably, the oxygen concentration in the feed gas should be less than the concentration of oxygen that would form a flammable mixture at either the reactor inlet or the reactor outlet at the prevailing operating conditions.

Ethane and oxygen are allowed to react in the presence of an oxidative dehydrogenation catalyst to yield a reactor effluent comprising ethylene. In general, various ODH processes are known and described in the art and the ODH processes of the present disclosure are not limited in that regard. Thus, the person skilled in the art may conveniently employ any of such processes in accordance with the ODH processes of the present disclosure. For example, suitable ODH processes, including catalysts and other process conditions, include those described in above- mentioned US7091377, WO2003064035, US20040147393, WO2010096909 and US20100256432, which are herein incorporated by reference.

Suitably, the temperature in the plurality of reactor tubes is in the range of from 100 to

600 °C, preferably in the range of from 200 to 500 °C. Further, the pressure in the plurality of reactor tubes is in the range of from 2 to 20 bara (i.e. "bar absolute"), or from 3 to 15 bara, or from 4 to 10 bara.

As previously mentioned, the ODH processes and reactor systems of the present disclosure enable the production of ethylene (via the oxidative dehydrogenation of ethane) in a multitubular fixed-bed reactor at a high space time yield, while simultaneously maintaining sufficient isothermal performance and acceptable pressure drop across the catalyst bed. As used herein, space time yield (STY) refers to the amount of ethylene produced in grams per liter of catalyst per hour. Preferably, the space time yield of the ODH processes of the present disclosure is in the range of from 300 to 1200 g of ethylene per liter of catalyst per hour (g/l/hr), or from 400 to 1000 g/l/hr, or from 450 to 900 g/l/hr, or from 450 to 800 g/l/hr, or from 450 to 750 g/l/hr, or from 500 to 1000 g/l/hr, or from 500 to 750 g/l/hr, or from 500 to 700 g/l/hr.

Further, in the processes of the present disclosure, a coolant is generally supplied to the interior shell space of the multitubular fixed-bed reactor. The coolant may be any fluid suitable for heat transfer, for example, a molten salt or an organic material suitable for heat exchange (e.g., oil, kerosene, etc.). Preferably, the coolant is supplied to the interior shell space of the reactor via a coolant circuit, which preferably comprises a cooling apparatus (e.g., heat exchanger, steam drum, etc.) and a circulation pump. In accordance with the present disclosure, coolant may be supplied to, and removed from, the interior shell space of the reactor in any suitable manner.

In general, coolant may be supplied to the interior shell space of the multitubular fixed- bed reactor in any suitable manner so as to maintain the desired level of isothermal performance. For example, coolant may be supplied to the reactor via one or more coolant inlets in a flow pattern that is counter-current with respect to the flow of the feed gas. Alternatively, coolant may be supplied in a flow pattern that is co-current with respect to the flow of the feed gas. Optionally, if desired, the multitubular fixed-bed reactor may be divided by a perforated partition into an upstream region and a downstream region, with coolant independently circulated in the upstream and downstream shell spaces of the reactor, thus providing for the independent control of the temperature within the upstream and downstream regions. In such embodiments, coolant may be supplied to the upstream and downstream shell spaces of the reactor in any suitable manner so as to maintain the desired level of isothermal performance. In more detail, applicable processes of coolant circulation may include, but are not limited to, those as disclosed in co-pending applications EP16157537.8, EP16181303.5, and EP 16181294.6, which are incorporated herein by reference.

As will be appreciated by one skilled in the art, suitable coolant flow rates may vary widely depending, at least in part, on the specific configuration of the multitubular fixed-bed reactor (e.g., the exact length and internal diameter of the tubes within the reactor, number of tubes, coolant flow pattern, etc.), process conditions, the activity level of the ODH catalyst employed, the exact size and/or shape of the catalyst employed, as well as the particular heat capacity of the coolant. It is within the ability of one skilled in the art to select a suitable coolant flow rate, taking into consideration, for example, the above-mentioned parameters. Suitably, if desired, simulation models can be used to determine the appropriate coolant flow rate needed in order to achieve the desired coolant temperature differential. Reference is made to, for example, A. Soria Lopez, et al., "Parametric Sensitivity of a Fixed Bed Catalytic Reactor", Chemical Engineering Science, Volume 36 (1981), pp. 285-291 for further discussion relating to the effects of temperature variation in a concurrent coolant on the operation of a fixed bed reactor.

The present invention is also applicable to a reactor system and a process for the oxidative dehydrogenation of alkanes having a higher carbon number than ethane, in particular alkanes having a carbon number of from 3 to 6 carbon atoms, including propane, butane, pentane and hexane, more specifically propane and butane, most specifically propane.