Process for improved carbon dioxide recovery

Abstract

A process for purifying a carbon dioxide stream containing water and hydrocarbons by which purification is improved by a close coordination of successive stages so that succeeding stages both remove impurities from prior stages and prepare the stream for impurity removal in the next stage.

Description

44680PCT_ApplicationPROCESS FOR IMPROVED CARBON DIOXIDE RECOVERYFIELD OF THE INVENTION

[0001] The present invention relates to the purification of a carbon dioxide stream.BACKGROUND OF THE INVENTION

[0002] Found in trace amounts amidst the vast cosmic dust clouds, the molecule of ethylene oxide has long been a part of our universe's natural composition. It was not until 1859, however, that this molecule was synthesized on Earth, a feat achieved by Charles-Adolphe Wurtz through the pioneering chlorohydrin process. The full industrial potential of this compound remained latent until the rise of the automotive industry, which increased demand for ethylene glycol, an antifreeze derived from ethylene oxide. This demand catalyzed further developments, leading to Thèodore Lefort's 1931 discovery of a more economically viable production method - the direct catalytic epoxidation of ethylene over a silver-based catalyst.

[0003] Since then, ethylene oxide production has soared, so that today it is among the most produced chemicals, with worldwide production reaching 34 billion tons by 2020. (Most of this ethylene oxide is further processed into derivatives such as ethylene glycol.) Concurrent with the rise in production, research into ethylene oxide catalysis and processing has flourished. A particular area of interest lies in developing more efficient and sustainable production processes. The epoxidation reaction, forming ethylene oxide from ethylene and oxygen, is a partial oxidation reaction paired with the full oxidation reaction that produces carbon dioxide and water from the same reactants. While higher selectivity catalysts have reduced the byproduct carbon dioxide, the amount can nonetheless be considerable. This not only contributes to greenhouse gas emissions, reducing the sustainability of the process; it also reduces economic efficiency as it converts valuable feedstock into waste product.

[0004] One way to address this problem is to simply make less carbon dioxide by using catalysts that are more selective to the desired epoxidation / partial oxidation reaction, resulting in less carbon dioxide. And in fact, after improving little since Lefort’ s original silver-based catalysts for much of the 20th century, selectivity of ethylene oxide catalyst have seen a gradual, but sustained increase over the last few decades to approach, or even exceed, 90%. However, even at these selectivity levels, a significant amount of carbon dioxide is still generated - as much as a ton of carbon dioxide for every three tons of ethylene oxide equivalent.

[0005] This supply of carbon dioxide is both a challenge and an opportunity. This presence of carbon dioxide as a major byproduct of ethylene epoxidation reduces the sustainability of the process and imposes the additional cost of disposal. However, instead of viewing carbon dioxide as a waste product, it can be seen as a valuable component of many product supply chains. For example, carbon dioxide has long been recovered at the point of production for the synthesis of chemicals of commercial importance, such as urea and methanol. However, these applications are limited to specific industries and often require specialized infrastructure, making them impractical for most ethylene oxide plant operators.

[0006] Other end-use applications for the carbon dioxide from ethylene oxide plants may be available that not only increase demand but possibly provide higher economic values for the carbon dioxide. Examples of such applications include supplying the carbon dioxide for the manufacture of food, beverage, and pharmaceutical products. Or industrial scale uses such as enhanced oil recovery. In enhanced oil recovery, carbon dioxide is injected into aging oil wells to providing a driving force to boost oil production. In new wells, this driving force is provided by naturally dissolved gases. As the well ages, other energy sources are necessary. Water is frequently used, followed by a miscible gas like carbon dioxide, which reduces the surface tension of trapped oil droplets, facilitating their mobilization.

[0007] Alternatively, rather than being repurposed for further downstream use, the carbon dioxide can be directed to a carbon capture and storage (“CCS”) system, which is either colocated with the ethylene oxide plant, or to which the carbon dioxide can be transported bypipeline or other means. This removes the carbon dioxide from potentially being emitted into the atmosphere and contributing to the stock of greenhouse gases. This increases the sustainability of the ethylene oxide process in which it was produced.

[0008] While these latter uses of carbon dioxide present opportunities for deriving more value from the carbon dioxide byproduct, and improving the sustainability of an ethylene oxide process, they also present challenges compared to simply using the carbon dioxide as a precursor for further chemical products. Prior attempts have struggled to recover an industrially sourced stream of carbon dioxide to the higher levels of purification necessary for e.g., food or beverage production or CCS. Or the recovery of carbon dioxide has been impractical or non-economic to be applied to an ethylene oxide plant.

[0009] U. S. Patent No. 4,990,168 describes a process for separating out carbon dioxide from an industrial waste stream using a separation membrane. While this technique is passive and offers possible energy savings, it produces a carbon dioxide stream of minimal purity, and its economic efficiency depends on having sufficient pressure driving force available for membrane separation. Moreover, the ‘168 patent does not address potential challenges related to membrane fouling or degradation over time. It would be quite impractical to apply this within an ethylene oxide production process, and, in fact, the ‘168 patent does not contemplate such an application. A patent more closely related to an ethylene oxide context, is U. S. Patent No. 6,224,843. The ‘843 patent gets closer to purifying a recovered carbon dioxide stream by use of a catalytic oxidizer targeting chlorohydrocarbons. However, the ‘843 patent is focused almost exclusively on combusting chlorohydrocarbons (and addressing coproduct oxygen and byproduct HC1) and fails to adequately address non-condensable impurities such as methane and argon. Moreover, the process scheme is impractical: requiring the use of two different catalysts and imports of both hydrogen and oxygen.

[0010] Accordingly, there is a continuing need for a carbon dioxide recovery process that can be economically and practically incorporated into ethylene oxide production, and that effectively removes the impurities from a recovered carbon dioxide stream allowing it to be used in a varietyof food, beverage, and pharmaceutical applications as well as energy industry applications such as enhanced oil recovery and CCS. This not only potentially increases the sustainability of ethylene oxide production but also converts byproduct carbon dioxide from a waste stream to a product stream having economic value.BRIEF SUMMARY OF THE INVENTION

[0011] The present invention is directed to a process for purifying a carbon dioxide stream comprising providing the carbon dioxide stream containing water and hydrocarbons; compressing the carbon dioxide stream to remove water from the carbon dioxide stream to form a compressed stream; combining the compressed stream with oxygen to form a combined stream; reacting the hydrocarbons and the oxygen in the combined stream in the presence of an oxidation catalyst to form an oxidized stream, the oxidized stream containing carbon dioxide, methane, HC1, and oxygen; washing the oxidized stream to produce a washed gas effluent; and separating in a cryogenic distillation column, oxygen and methane from the washed gas effluent to form a purified carbon dioxide product.

[0012] In some aspects of the present application, the process further comprises drying the washed gas effluent.

[0013] In some aspects of the process of the present application, the oxidation catalyst includes platinum deposited on a high-surface area support.

[0014] In some aspects of the process of the present application, and during the providing the carbon dioxide stream, the carbon dioxide passes through a regenerator vent.

[0015] In some aspects of the process of the present application, and during the reacting of the hydrocarbons and the oxygen, the hydrocarbons in the combined stream are reduced by greater than 99%, excepting methane.

[0016] In some aspects of the process of the present application, the combined stream contains 0.5 mol% to 2 mol% oxygen.

[0017] In some aspects of the process of the present application, the hydrocarbons and oxygen are reacted at a temperature of from 250°C to 380°C.

[0018] In some aspects of the process of the present application, the carbon dioxide stream is provided as a stream from an ethylene oxide manufacturing process.

[0019] In some aspects of the process of the present application, and during the reacting of the hydrocarbons and the oxygen in the combined stream, greater than 99% of non-methane hydrocarbons are removed by oxidation.

[0020] In some aspects of the process of the present application, the washed gas effluent is substantially free of hydrochloric acid.

[0021] In some aspects of the process of the present application, the process further comprises, after the washing of the oxidized stream, drying the washed gas effluent to a moisture concentration of less than 10 ppm (by volume) water.

[0022] In some aspects of the process of the present application, and when drying is employed, the process further comprises, after the drying of the washed gas effluent, cooling the washed gas effluent to liquify the washed gas effluent prior to the washed gas effluent being supplied to the cryogenic distillation column.

[0023] In some aspects of the process of the present application, and when drying is employed, the purified carbon dioxide product contains less than 1 mol ppm ethylene and less than 50 ppm oxygen.

[0024] In some aspects of the process of the present application, the carbon dioxide stream is provided by: providing scrubber overheads; in a carbon dioxide absorber, contacting the scrubber overheads with a carbon dioxide-absorbing solvent to form a remaining gas stream and a rich carbonate solution; and dividing, in a regenerator, the rich carbonate solution into a lean solvent and a discharge stream; and providing the discharge stream as the carbon dioxide stream.

[0025] In some aspects of the process of the present application, and after providing the discharge stream, the carbon dioxide stream is compressed to a pressure of 20 to 30 bar.

[0026] In some aspects of the process of the present application, the cryogenic distillation column contains at least 20 stages.

[0027] In some aspects of the process of the present application, the scrubber overheads comprises 30 vol% to 45 vol% methane, 25 vol% to 35 vol% ethylene, and 0.5 vol% to 5 vol% carbon dioxide.

[0028] In some aspects of the process of the present application, the scrubber overheads are provided by: producing an ethylene oxide reactor effluent by reacting ethylene, oxygen, and a chloride moderator in the presence of an ethylene oxide catalyst in an ethylene oxide reactor at a temperature of 225ºC to 280ºC; and contacting the ethylene oxide reactor effluent with lean cycle water to prepare a rich cycle water stream and scrubber overheads.

[0029] In some aspects of the process of the present application, the chloride moderator is present in concentrations (by volume) of from 0.5 ppm to 7 ppm.

[0030] In some aspects of the process of the present application, the ethylene oxide catalyst contains a rhenium promoter.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0031] The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:

[0032] Figure 1 is a schematic flow sheet showing a part of the process for making ethylene oxide; namely a carbon dioxide removal system;

[0033] Figure 2 is a schematic flow sheet showing a process for carbon dioxide purification according to the present invention; and

[0034] Figure 3 is a schematic flow sheet showing a process for carbon dioxide purification according to the present invention.DETAILED DESCRIPTION OF THE INVENTION

[0035] All parts, percentages and ratios used herein are expressed by volume unless otherwise specified. All documents cited herein are incorporated by reference.

[0036] By the present invention, an improved carbon dioxide recovery and purification scheme is provided to increase the purification of recovered carbon dioxide and thereby the number of potential applications to which it may be supplied. This is accomplished by a close coordination of successive stages so that succeeding stages both remove impurities from prior stages and prepare the stream for impurity removal in the next stage. Such coordination is not only the close matching of streams, but also the heat integration between. The result is a process that is highly effective, practical to implement and economical in operation.

[0037] The carbon dioxide recovery of the present invention is preferably incorporated into an ethylene oxide production process. Ethylene oxide is produced by continuously contacting an oxygen-containing gas with an olefin, preferably ethylene, in the presence of an ethylene oxide (“epoxidation”) catalyst (described in greater detail below). Oxygen is supplied to the reaction in substantially pure molecular form. The term “substantially pure molecular form” is used throughout the present application to denote that the supplied oxygen is greater than 99% molecular oxygen, i.e., O2. By way of example, typical reactant feed mixtures (after the completion of start-up and during normal operation) typically comprise from about 0.5% to about 45%, preferably about 5% to about 30% of ethylene, from about 3% to about 15% oxygen, and from about 0.3 % to about 10%, preferably from about 0.1% to about 1%, carbon dioxide with the balance comprising comparatively inert materials, including such substances as water, inert gases, other hydrocarbons, and the chlorohydrocarbon reaction moderators described herein. Inert hydrocarbons include, but are not limited to, methane and ethane; the reactant feed mixtures may contain from about 0.5% to about 45% methane and up to 3% ethane. Nonlimiting examples of inert gases include nitrogen, argon, helium and mixtures thereof. Nonlimiting examples of the other hydrocarbons include methane, ethane, propane and mixtures thereof. Carbon dioxide and water are byproducts of the epoxidation process as well as common contaminants in the feed gases. Both have adverse effects on the catalyst, so the concentrations of these components are usually kept at a minimum. (All percentage compositions listed in the present paragraph are in volume %.)

[0038] Also present in the reaction, as previously mentioned, are one or more chlorohydrocarbon reaction moderators, non-limiting examples of which include organic halogen-containing compounds such as C1to C8halohydrocarbons; especially preferred are chloride-containing moderators such as methyl chloride, ethyl chloride, ethylene dichloride, vinyl chloride or mixtures thereof. Controlling chloride concentration level is particularly important with rhenium-containing catalysts. Chlorides may be present in amounts of from (by volume) about 0.5 ppm to 7 ppm.

[0039] As mentioned above, a usual method for the ethylene epoxidation process comprises the continuous vapor-phase oxidation of ethylene with molecular oxygen, in the presence of a silverbased ethylene oxide catalyst, in a fixed-bed tubular reactor. Conventional, commercial fixed-bed ethylene-oxide reactors are typically in the form of a plurality of parallel elongated tubes (housed within a suitable shell). Typically, the outer diameter of the tubes is 20 - 70 mm, the inner diameter is 15 - 65 mm, and the length is 5 - 16 meters. The reaction feed mixture (described above) is introduced into these tubes, and the resulting reactor effluent gas contains ethylene oxide, unused reactants, and byproducts. The heat generated during the reaction is significant and heat management is typically employed. This is achieved by circulating water through the reactor, which absorbs the heat, and is converted to steam. This steam, a valuable byproduct of the reaction, can then be utilized within the plant for various heating purposes or exported for use in other processes. This efficient heat-recovery system contributes to the overall energy efficiency of the ethylene oxide process.

[0040] Typically, in the ethylene oxide process, the typical work rate for the reactor is between 130 and 300 kg / m3 / h, while the AEO is between 1.0% and 4.0%. The work rate is the production rate and is represented herein by the units kg / m3 / h. The AEO is defined as the moles of EO formed in the reactor per 100 moles of reactor feed and essentially represents the concentration of ethylene oxide in the reactor effluent, since the concentration of ethylene oxide in reactor feed is preferably maintained at very close to zero, indeed typically only a few ppm.

[0041] The specific configuration of the reactor is not critical, as the process of recovering ethylene oxide from the reactor effluent can be applied to various reactor designs. However, a particularly preferred design is the integrated reactor cooler described in U. S. Patent No.7,294,317. In this design, the reactor is a conventional shell and tube type, commonly used for ethylene oxide production, except that it contains an exit reactor head with an opening that connects directly to the cooler. The cooler is directly attached to, and integral with, the exit reactor head. The working principle of this integrated design is that the hot gases pass through the exit reactor head directly into the cooler / heat exchanger where the water on the shell side provides cooling, rapidly reducing the temperature of the reaction gases flowing through thetubes of the cooler and thereby minimizing the potential for further oxidation or undesirable side reactions. The immediate proximity of the cooler to the reactor also eliminates additional residence time at high temperatures in a separate conduit.

[0042] Typical operating temperatures for the reactor (as measured in the shell side coolant of the reactor) are in the range from about 180°C to about 330°C, and preferably from about 200°C to about 325°C, and more preferably from about 225°C to about 280°C. The operating pressure may vary from about 1 atmosphere to about 30 atmospheres, depending on the mass velocity and productivity desired. Higher pressures may be employed within the scope of the invention. Residence times in commercial-scale reactors are generally on the order of about 2 to about 20 seconds.

[0043] In operation, reactor effluent from the reactor outlet (not shown) flows to a scrubber (not shown) for the recovery of the ethylene oxide product. The reactor effluent contains from about 1.0 mol% to about 2.5 mol% ethylene oxide, preferably about 1.8 mol% to about 2.2.mol%.

[0044] In addition to ethylene oxide, the reactor effluent may also contain inert and unreacted gases supplied as components of the reactor feed such as argon, methane, ethylene, oxygen, and chlorohydrocarbons; reaction byproducts, in particular, carbon dioxide, but also ppm levels of formaldehyde, formic acid, acetic acid, and product isomers such as acetaldehyde; and additionally sulfur and other impurities, again typically at ppm or ppb levels. The byproducts and impurities intermixed with ethylene oxide can undermine the quality of downstream products such as ethylene glycol. Feedstock materials such as methane, ethylene and oxygen are typically recovered and recycled back to the reactor inlet feed in order increase the economic efficiency of the ethylene oxide process, and because if not recovered, for environmental reasons the hydrocarbons can be vented at only the most minute quantities.

[0045] The scrubber is the first step in the separation and recovery process and is where most of the ethylene oxide and light gases are removed from the reactor effluent. In the scrubber, the reactor effluent is contacted with lean cycle water. The temperature of the lean cycle water andpressure in the scrubber is managed to maximize the amount of ethylene oxide absorbed into the water and minimize the absorption of other components, especially carbon dioxide. This takes advantage of the very different degrees to which ethylene oxide and carbon dioxide are soluble in water; for example, at ambient temperature, ethylene oxide is essentially infinitely soluble in water, while carbon dioxide is only moderately soluble.

[0046] The liquid stream, which is now ethylene oxide-enriched cycle water, flows to the stripper, a distillation column that operates by applying steam to separate the ethylene oxide from the water. The heat input provides the energy required to vaporize the ethylene oxide out of the water, which then rises to form the stripping column overheads, leaving behind the water.

[0047] Those gases that are not absorbed in the cycle water, but instead rise upward to the top of the scrubber from the scrubber overheads. The gases in the scrubber overhead are not just carbon dioxide, but also the aforementioned feed gases such as methane, ethylene and oxygen. While gases such as methane have even lower solubility in water than carbon dioxide, they are present in much greater quantities in the reactor effluent than carbon dioxide and so are present in much higher quantities in the scrubber overhead.

[0048] Specifically, the scrubber overhead comprises about 30 vol% to about 45 vol% methane and 25 vol% to about 35 vol% ethylene (which has passed through the reactor without reacting with oxygen); and about 0.5 vol% to 5 vol% carbon dioxide. As mentioned above, in essence these valuable hydrocarbons in the reactor overhead are “contaminated” with carbon dioxide. which needs to be separated so that the ethylene and methane in the scrubber overhead stream can be recycled back to the reactor. Given this high amount of ethylene seen in the scrubber overheads, it would be economically deleterious (not only regulatorily impermissible) to simply vent this stream. Rather, the ethylene is typically recovered and recycled back to the reactor. A similar principle applies in the present invention to the carbon dioxide - it too can be recovered and recycled for further use.

[0049] As shown in Figure 1, in various embodiments, the recovery and recycling of carbon dioxide begins with the supply of the scrubber overheads 1 to the carbon dioxide absorber 15. The carbon dioxide absorber 15 has the role of preferentially removing carbon dioxide from the scrubber overheads 1 to allow recycling of the scrubber overheads to the reactor, and thus, the recovery of ethylene contained in the scrubber overheads and separation of the carbon dioxide.

[0050] Conventional equipment and techniques known to the person of ordinary skill for removing carbon dioxide from a gaseous feed may be used in constructing and operating the carbon dioxide absorber 15. Regardless of its specific construction, in the carbon dioxide absorber 15, the (carbon dioxide-containing) scrubber overhead stream (e.g., scrubber overhead 1) is contacted with a carbon dioxide- absorbing solvent so that carbon dioxide is preferentially and readily solubilized and absorbed into the solvent. Specifically, as shown in Figure 1, the (carbon dioxide-containing) scrubber overheads 1 is contacted with a carbon dioxide-absorbing solvent (which has been recycled from the regenerator 20) in the carbon dioxide absorber 15 to form a carbon dioxide-rich solvent phase and a carbon dioxide-depleted gas phase. The carbon dioxide-absorbing solvent preferentially absorbs carbon dioxide so that the hydrocarbons and oxygen in the scrubber overheads 1 are not absorbed, and these form the carbon dioxide-depleted gas phase in the carbon dioxide absorber 15 overhead. Preferably, the carbon dioxide-absorbing solvent is potassium carbonate, in which case the carbon dioxide reacts with the potassium carbonate to form potassium bicarbonate, thereby removing the carbon dioxide and forming a potassium bicarbonate-rich solvent phase.

[0051] In one embodiment the steps in the carbon dioxide absorber are carried out by reactive distillation.

[0052] The quantity of the scrubber overheads 1 sent to the carbon dioxide absorber 15 for carbon dioxide removal is determined by the plant operator and can be affected by various factors: some scrubber overheads may be bypassed around the carbon dioxide absorber is recycled back to the reactor inlet. Regardless of the exact percentage of scrubber overheads 1 that is sent to the carbon dioxide absorber 15, the percentage is sufficiently high in order toreduce the amount of carbon dioxide to relatively low levels. As mentioned above, carbon dioxide is a byproduct of the epoxidation process and carbon dioxide has an adverse effect on the performance of high selectivity catalysts, and so the total amount of carbon dioxide in the streams recycled back to the reactor should be minimized. For lower carbon dioxide concentrations, for example, to maintain the 1 vol% carbon dioxide in the inlet feed to a reactor containing high selectivity catalyst, close to 100% of the scrubber overheads 1 is fed to the carbon dioxide absorber 15. Any stream that “bypasses” the carbon dioxide absorber 15 is preferably recycled back to the reactor inlet because, while it would be possible to simply vent the untreated bypass stream to the atmosphere, this is typically not intentionally performed given the loss of valuable hydrocarbons in the bypass stream and the requirement for extensive emissions treatment prior to venting. Nonetheless, it is a possible under the operational constraints of emergencies that the bypass stream could be partially vented.

[0053] At the bottom of the carbon dioxide absorber 15, the carbon dioxide-rich solvent phase exits the carbon dioxide absorber 15 as the rich carbonate solution 3. The rich carbonate solution 3 is rich in carbon dioxide stored in the form of a compound like the potassium bicarbonate described above. The temperature of the rich carbonate solution 3 is about 65°C to about 85°C (additionally the pressure of this stream is generally high because, as mentioned above, the pressure in the carbon dioxide absorber can be as high as 20 atm). While the carbon dioxide absorber 15 is operated in the present invention so that the carbon dioxide-absorbing solvent preferentially absorbs only carbon dioxide and minimize the absorption of other gases, nonetheless other gases, in particular argon, residual oxygen, hydrocarbons (especially methane, ethylene, and ethylene oxide) as well as chlorohydrocarbons (such as ethyl-, vinyl, and allylchloride), may be absorbed in the carbon dioxide-absorbing solvent. To recover this carbon dioxide for its beneficial use elsewhere these impurities are removed. Separating these impurities not only prevents their emission into the atmosphere but allows the recovery of the carbon dioxide itself. This increases the environmental sustainability of the process.

[0054] This separation and impurity removal happens in the regenerator 20 where the carbon dioxide, oxygen, methane, other hydrocarbons, and chlorohydrocarbons are separated from therich carbonate solution 4 by steam-stripping to yield a gaseous regenerator overhead and a lean solvent 11. The lean solvent 11 is returned to the carbon dioxide absorber 15 ready to absorb carbon dioxide. This steam that is used in the regenerator section is a combination of both live steam imported through line 10 and also reboiler steam from the regenerator reboiler 21 that is introduced via line 13. The regenerator 20 preferably contains column internals positioned at the top of the regenerator to minimize carbonate solution entrainment into the vapor and reduce the carbonate in the overhead. The regenerator 20 preferably contains additional column internals that promote liquid vapor contact to facilitate the stripping of carbon dioxide and the aforementioned impurities from the rich carbonate solution by contacting it with steam.

[0055] The discharge stream 45 is the carbon dioxide stream which will be subject to the following purification steps. This discharge stream 45 contains from about 90 mol% to about 99 mol% of carbon dioxide. The balance of the discharge stream 45 is mostly water but may also contain methane (less than 500 ppm), ethylene (less than 1500 ppm), ethylene oxide (less than 75 ppm), argon, and mixtures of chlorohydrocarbons and other possible impurities that will need to be removed to render the stream suitable for more economical uses. Emissions of the overhead stream is generally strictly controlled and monitored. Moreover, as carbon dioxide is, of course, a greenhouse gas, its venting reduces the sustainability of the EO / EG process. Additionally, venting to the atmosphere represents an economic loss as the carbon dioxide has a value that is lost upon venting.

[0056] However, before the economic value of the carbon dioxide can be realized for applications like food or beverage production or in the energy industry, the above impurities such as hydrocarbons, chlorohydrocarbons and others are removed. This process starts with the discharge stream 45 passing from the regenerator overhead and through compressor 53 (as shown in Figure 2) where it is compressed from approximately atmospheric pressure to a pressure of about 20 to about 30 bar and a temperature of about 150°C to about 175°C. (Most of the moisture that was in the discharge stream 45 condenses and can be removed from the process at this point).

[0057] The compressed discharge stream 55 is then combined with an oxygen stream 50 (see Figure 2) so that the combined stream 60 contains about 0.5 mol% to about 2 mol% oxygen. The amount of oxygen added, and thus, the proportion of carbon dioxide to oxygen, can be varied to optimize the oxidation of impurities in the catalytic oxidizer. The combined stream 60 is directed through a pre-oxidizing heat exchanger 65 (described in greater detail later), where it is heated to a pre-oxidizing temperature of about 250°C to about 350°C or about 250°C to about 380°C. This latter temperature range is the approximate range in which the catalyst is active to promote the catalyst oxidation process for the impurities targeted by the oxidizer, especially ethylene oxide and chlorohydrocarbons and to ensure at least 99.99% oxidation of these hydrocarbons are removed.

[0058] Thus, the combined stream 60 flows through the pre-oxidizing heat exchanger 65 and into the catalytic oxidizing reactor 70 within the aforementioned pre-oxidizing temperature range.

[0059] This approach utilizes catalytic beds containing precious metal oxidizing catalysts like platinum or palladium to facilitate the oxidation reactions at the relatively low temperatures in the aforementioned pre-oxidizing temperature range. The catalytic oxidation process is designed to achieve high destruction efficiencies (greater than 99%) for ethylene oxide, chlorohydrocarbons, and other organic compounds present in the vent stream. It is also more energy efficient and environmentally friendly than conventional thermal oxidation methods.

[0060] In the reactor the ethylene oxide and the oxygen react with each other in the presence of the catalyst to remove the ethylene oxide as follows:2C2H4O + 5O2-> 4CO2+ 4H2O

[0061] While chlorohydrocarbons (e.g., ethyl chloride) are oxidized to make carbon dioxide, water, and HC1:C2H5Cl + 3O2→ 2CO2+ 2H2O + HClWhile HC1 adds another impurity into the carbon dioxide recovery process, it is easier to remove than the chlorohydrocarbon from which it was produced. (The catalytic oxidizer is not effective at removing methane because methane’s strong carbon-hydrogen bonds make a formidable activation barrier, thus requires a much higher activation temperatures - in excess of 500°C.)

[0062] The oxidizing step is tremendously effective for its target impurities - removing greater than 99%, and as much as 99.99% of non-methane hydrocarbons are removed by oxidizing / combustion in this step. (As explained above, the temperatures in the oxidizing reactor are not high enough to oxidize methane, which is removed later in the process in the cryogenic distillation column.)

[0063] The oxidizing or oxidation catalyst is preferably a platinum-based oxidizing catalyst. The platinum-based catalyst includes a high-surface area support, such as alumina, upon which is deposited a catalytically-effective amount of platinum. It is preferred that the catalyst be prepared so that it is resistant to deactivation by chlorohydrocarbons and the HC1 produced by their combustion in the oxidizer. Examples of suitable catalysts include Clariant’s EnviCat (R) 2532 catalyst (Clariant Corporation, Louisville, KY, USA). Also suitable is the Advocat VOC DEOXO Catalyst from Advanced Catalyst Systems (Maryville, TN, USA). Further suitable examples may be found in U. S. Patent No. 8,475,755, such as the catalysts disclosed in the ‘755 patent consisting of two or more platinum group metals, such as ruthenium or platinum, supported on refractory oxides like Al2O3, solid solutions of CeO2and ZrO2; and silica and / or tin oxide.

[0064] Catalytic oxidation is effective at about 20 to about 30 bar, which utilizes a special configuration of steps, avoids operating the oxidizer at these high temperatures and is thus, both more effective and more economical than prior art processes which rely solely on catalytic oxidation such as U. S. Patent No. 10,005,673. Like the present invention, the ‘673 patent is focused on purifying the byproduct carbon dioxide from an ethylene oxide process. Unlike the present invention, the ‘673 patent relies almost solely on a manganese oxide catalyst to separate impurities by the simultaneous oxidation of hydrocarbons like methane and chemisorption oforganic chlorides. The ‘673 patent's reliance on a chloride-absorbing metal oxide catalyst for removal of organic chlorides necessitates a regeneration step, adding complexity to the process and making it difficult to implement in a commercial process. Moreover, since it is very difficult to measure chlorides at these low concentrations, it is often impossible to detect when the chlorine-adsorbing metal oxide has been saturated, which can lead to levels of undetected chloride breakthrough that can cause corrosion even when such levels are present for only a short time. Indeed, possibly as a result of this factor the ‘673 patent mentions the long-term stability of the catalyst, a critical consideration for determining the economic efficiency of the process. Complicating this further is the high temperatures necessary for oxidizing hydrocarbons like methane (as mentioned above). The ‘673 patent discloses oxidizing reactor temperatures in the range of 400°C to 500°C, but it is likely that the temperatures are even higher than that to ensure full oxidation of the hydrocarbon species. Such temperatures are highly impractical for processes such as these, given that they accelerate catalyst aging, degrade the materials used in construction and equipment, such as seals, gaskets, and even reactor vessels, and require substantial amounts of energy to maintain these high temperatures.

[0065] In contrast to the chemisorption in the ‘673 patent, the present invention makes use of oxidizing catalysts that are resistant to deactivation through chemisorption. These catalysts are extremely long-lived, and rather than fouling the catalyst with chloride ions, the present invention makes use of a two-step process for removing chlorides (oxidizing and then washing) that leaves the oxidizing catalyst fully active.

[0066] The oxidized stream 73 (see Figure 2) leaves the catalytic oxidizer at a temperature of about 275°C to about 375°C or about 275°C to about 380°C (the outlet temperature being greater than the inlet temperature as a result of the highly exothermic reactions inside the catalytic oxidizer). The oxidized stream 73 is then subjected to a counter-current heat exchange process within the pre-oxidizing heat exchanger 65. Concurrently, the combined stream 60 (as described above) is introduced to the opposing side of the pre-oxidizing heat exchanger 65, facilitating indirect thermal contact with oxidized stream 73. This configuration makes use of the heating in the catalytic oxidizer reactor 70 to preheat the combined stream 60, while simultaneouslyreducing the temperature of the oxidized stream 73 to a temperature of between 30°C and 60°C, preferably between 40°C and 50°C. This heat integration provides considerable savings by reducing the temperature of the oxidized stream 73 prior to the washing stage. The temperature in the washing stage is generally as low as possible to enhance the solubilization of the oxidized compounds, especially hydrochloric acid.

[0067] In the washing stage, the cooled oxidized stream is contacted and mixed with demineralized wash water in a washing column 75 to produce a washed gas effluent 82 in the overhead and wastewater in the bottoms. The washing column 75 used in this stage is equipped with internals such as structured packings, plates, or other gas-liquid contact technologies well-known to those skilled in the art. The washing stage dilutes and mixes the hydrochloric acid in the cooled oxidized stream with water causing the hydrochloric acid to become solubilized and partitioned into the wastewater in the bottoms of the washing column 75. (This is the hydrochloric acid that was formed from the oxidizing of chlorinated hydrocarbons in the oxidizing stage.) This solubilization and separation into the wastewater is so complete or so nearly complete that the washed gas effluent 82 is substantially free of hydrochloric acid so that the level of hydrochloric acid is below detectable levels. By “substantially free of hydrochloric acid” it is meant that the washed gas effluent 82 contains 0.01 ppm or less hydrochloric acid. In addition to being an impurity in the carbon dioxide stream, hydrochloric acid can cause corrosion and can affect the performance and lifespan of the components and equipment in the process. See for example US Patent No.10,464,008, which is directed to removal of HC1 from a carbon dioxide by-product gas stream produced during the oxidation of ethylene to prevent corrosion in process equipment. However, in this case the ‘008 patent uses only a zeolite adsorbent to remove HC1, which is far less effective than the washing step of the present invention and moreover requires energy-intensive regeneration of the adsorbent. Moreover, the ‘008 patent fails to address any of the other impurities likely present in byproduct carbon dioxide. Finally, the effectiveness of this approach depends on the zeolite's selectivity and capacity for the specific organic chlorides present.

[0068] From the bottom of the washing column, the wastewater stream 80 is conveyed to a flash drum 85, maintained at an absolute pressure of 1 to 1.5 bar. This drop in pressure, relative to the elevated pressure within the preceding washing column, induces a rapid vaporization of dissolved carbon dioxide upon the wastewater stream’s entry into flash drum 85. The liberated carbon dioxide gas stream 90 is subsequently routed to the discharge stream 45 for recirculation within the system.

[0069] After the preceding steps, the final product and impurity separation occurs in the cryogenic distillation column. Preferably prior to entering the cryogenic distillation column 101 the washed gas effluent 82 flows from the overhead of the washing column 75 through several pre-distillation steps. First, the washed gas effluent is subjected to a drying step in a drying section 93 to remove as much of the water as possible, preferably after passing through the drying section the dried washed gas effluent has less than 10 ppm (by volume) water. Drying the washed gas effluent prior to cryogenic distillation is employed not only to improve the economic and separation efficiency of the column but also to prevent the formation of unwanted compounds in the column such as dry ice or carbonic acid.

[0070] Any suitable drying technique may be used in the drying section 93. In a preferred embodiment, the drying section uses molecular sieve beds to dry the washed gas. In a typical setup, there are two adsorbing vessels, each containing molecular sieve beds. The carbon dioxide gas flows through the beds in one of the absorbing vessels, while the other is being regenerated. During regeneration, a heated regeneration gas is forced through the molecular sieve bed to remove the adsorbed water from the offline molecular sieve absorber.

[0071] Next, this dried stream 95 is preferably readied for cryogenic distillation by a cooling and liquefaction step. In the present invention, this step is optimized by integrating the hot and cold streams of the cryogenic distillation column 101 as is illustrated in Figure 3. Specifically, the dried stream 95 first passes through the carbon dioxide vaporizer 98 which uses the liquid split stream 110 from the bottom of the cryogenic distillation column 101 as a cold source. The liquid split stream 110 divides from the liquid carbon dioxide product 104, and in the carbon dioxidevaporizer 98 the liquid split stream 110 vaporizes as it absorbs sensible heat from the incoming dried stream, thus cooling it. This heat integration not only vaporizes the liquid carbon dioxide product (thus, providing the product in gaseous form), but by cooling the dried stream 95 helps reduce the energy demand for the subsequent (liquification) stage. This adds considerably to the cost-efficiency and sustainability of the inventive process.

[0072] The cooled dried stream 95 having been pre-chilled and cooled in the carbon dioxide vaporizer 98 then flows into a chiller 109, which further cools and liquifies the stream at a temperature suitable for supply to the cryogenic distillation column. This pre-distillation temperature is from about -30°C to about -15°C. The chiller 109 is cooled via a refrigerant stream 100 supplied by conventional refrigeration and chilling technology known to the skilled person. Liquefying the carbon dioxide prior to its supply to the cryogenic distillation process is, of course, necessary to effectively separate components by distillation.

[0073] Cryogenic distillation, and its configuration with respect to the preceding stages, is a particularly integral part of the present invention because it effectively separates impurities in the carbon dioxide stream that previous stages have been unable to affect, especially noncondensable gases like methane, as well as the oxygen that was introduced for the oxidizing step. Other impurities like argon are also easily removed in the cryogenic distillation column. Thus, the configuration of purification steps in the present invention, ending with the cryogenic distillation column, provides a level of purification that is not obtained by the prior art. The cryogenic distillation column 101 in the present invention preferably includes at least 20 stages.

[0074] This separation occurs by the heavier carbon dioxide accumulating in liquid phase in the bottoms of the column and which is collected as the high-purity liquid carbon dioxide product. (A small draw of the bottoms liquid may also be collected for reflux back to the distillation column. This draw is cooled using the same refrigeration source as for cooling the carbon dioxide vaporizer.)

[0075] Meanwhile, lighter and non-condensable gases, especially methane and oxygen, but also other impurities like argon, rise to the top of the column. These more volatile components remain in the gaseous phase under the same conditions and are passed out as the distillation column overheads 107.

[0076] The liquid carbon dioxide product 104 (the final product of the process) is a liquid stream of high purity carbon dioxide that meets the standards as an in ingredient for food or beverage production. As desired by the plant operator, liquid split stream 110 may be taken from the liquid carbon dioxide product 104 and sent to the carbon dioxide vaporizer 98 as a cold source to cool the dried stream 95 (as described above) and simultaneously vaporize the liquid split stream 110. The liquid split stream 110 gaseous carbon dioxide has the same purity and composition as the liquid carbon dioxide product 104 and is also a final product of the process. Thus, the plant operator has the option to produce liquid and gaseous high purity carbon dioxide in whatever portions she or he prefers.

[0077] The final composition of the carbon dioxide product, whether in gaseous or liquid state, may meet the requirements of the application for which it is intended. For example, CGA G-6.2-2011 (Commodity specification for carbon dioxide) specifies the CO2purity requirements for food, beverage, and pharmaceutical applications. With the oxygen and methane removed, the carbon dioxide may also be used in the sustainable fuels area such as for Enhanced Oil Recovery (“EOR”) or Carbon Capture and Storage (“CCS”). For EOR and CCS, purification guidelines are based on the pipe and destination parameters necessary for safe, efficient and cost-effective handling of CO2. For example, oxygen concentration should be maintained well below 50 ppm due to potential exothermic reactions with hydrocarbons. Additionally, ethylene oxide and other hydrocarbons should be maintained at a level of less than 1 mol ppm in the event of an inadvertent release at an energy or storage site due to equipment upset, facility startup, or facility shut down. In some embodiments, the carbon dioxide purity within the carbon dioxide product is greater than 99.0 %, 99.5 %, 99.8%, 99.9 % or 99.99 %.

[0078] Thus, the present invention provides an integrated multifunctional system that combines multiple stages like compression, catalytic oxidation, washing, and cryogenic distillation to treat the discharge through the regenerator vent streams more effectively. Such systems can leverage the strengths of the different technologies in each stage to achieve higher overall removal efficiencies, selective recovery of valuable compounds, and minimized emissions. For instance, the compression step separates water out by condensation, followed by catalytic oxidation of the non-methane organic compounds, then the washing of HC1 created by the combustion of chlorinated hydrocarbons, and finally cryogenic distillation to remove methane and providing a “polishing” of the product carbon dioxide to a high purity. Thus, unlike the prior art, the present invention allows the plant operator hitherto unattained flexibility and versatility in operation. By operating different stages in the process at different intensities, then the same carbon dioxide purification train may produce carbon dioxide meeting different specifications. This provides the operator a versatile and flexible process for producing carbon dioxide of different purity levels adjusted by the operator to meet demand (as well as alter the ratio of liquid to gaseous carbon dioxide on demand as described above). So for example, the same unit may make both food-grade and CCS-grade carbon dioxide in sequential and almost simultaneous batches, all at the complete control of the plant operator.Silver-Based Ethylene Catalyst

[0079] As mentioned above, in a preferred embodiment the inventive carbon dioxide recovery or purification process is part of an ethylene oxide manufacturing process. This process makes use of a silver-based ethylene oxide (or “epoxidation”) catalyst. The silver-based ethylene oxide catalyst includes a support, and at least a catalytically effective amount of silver or a silver-containing compound; also optionally present is a promoting amount of rhenium or a rhenium-containing compound; also optionally present is a promoting amount of one or more alkali metals or alkali-metal-containing compounds. The support employed in this invention may be selected from a large number of solid, refractory supports that may be porous and may provide the preferred pore structure. Alumina is well known to be useful as a catalyst support for theepoxidation of an olefin and is the preferred support.

[0080] Regardless of the character of the support used, it is usually shaped into particles, chunks, pieces, pellets, rings, spheres, wagon wheels, cross-partitioned hollow cylinders, and the like, of a size suitable for employment in a fixed-bed epoxidation reactor. The support particles will preferably have equivalent diameters in the range from about 3 mm to about 12 mm, and more preferably in the range from about 5 mm to about 10 mm. (Equivalent diameter is the diameter of a sphere having the same external surface (i.e., neglecting surface within the pores of the particle) to volume ratio as the support particles being employed.) Suitable supports are available from Saint-Gobain Norpro Co., Sud Chemie AG, Noritake Co., CeramTec AG, and Industrie Bitossi S.p. A. Without being limited to the specific compositions and formulations contained therein, further information on support compositions and methods for making supports may be found in U. S. Patent Publication No. 2007 / 0037991.

[0081] In order to produce a catalyst for the oxidation of an olefin to an olefin oxide, a support having the above characteristics is then provided with a catalytically effective amount of silver on its surface. In one embodiment, the catalytic effective amount of silver is from 10% by weight to 45% by weight. The catalyst is prepared by impregnating the support with a silver compound, complex or salt dissolved in a suitable solvent sufficient to cause deposition of a silver-precursor compound onto the support. Preferably, an aqueous silver solution is used.

[0082] A promoting amount of a rhenium component, which may be a rhenium-containing compound or a rhenium-containing complex may also be deposited on the support, either prior to. coincidentally with, or subsequent to the deposition of the silver. The rhenium promoter may be present in an amount from about 0.001 wt. % to about 1 wt. %. preferably from about 0.005 wt. % to about 0.5 wt. %, and more preferably from about 0.01 wt. % to about 0.1 wt. % based on the weight of the total catalyst including the support, expressed as the rhenium metal.

[0083] Other components which may also be deposited on the support either prior to, coincidentally with, or subsequent to the deposition of the silver and rhenium are promotingamounts of an alkali metal or mixtures of two or more alkali metals, as well as optional promoting amounts of a Group IIA alkaline earth metal component or mixtures of two or more Group IIA alkaline earth metal components, and / or a transition metal component or mixtures of two or more transition metal components invention will provide an improvement in one or more catalytic properties over the same combination of silver and support and none, or only one of the promoters.

[0084] As used herein the term “promoting amount” of a certain component of the catalyst refers to an amount of that component that works effectively to improve the catalytic performance of the catalyst when compared to a catalyst that does not contain that component. The exact concentrations employed, of course, will depend on, among other factors, the desired silver content, the nature of the support, the viscosity of the liquid, and solubility of the particular compound used to deliver the promoter into the impregnating solution. Examples of catalytic properties include, inter alia, operability (resistance to runaway), selectivity, activity, conversion, stability and yield. It is understood by one skilled in the art that one or more of the individual catalytic properties may be enhanced by the “promoting amount” while other catalytic properties may or may not be enhanced or may even be diminished.

[0085] Suitable alkali metal promoters may be selected from lithium, sodium, potassium, rubidium, cesium or combinations thereof, with cesium being preferred, and combinations of cesium with other alkali metals being especially preferred. The amount of alkali metal deposited or present on the support is to be a promoting amount. Preferably, the amount ranges from about 10 ppm to about 3000 ppm, more preferably from about 15 ppm to about 2000 ppm, and even more preferably from about 20 ppm to about 1500 ppm, and as especially preferred from about 50 ppm to about 1000 ppm by weight of the total catalyst, measured as the metal.

[0086] Suitable alkaline earth metal promoters comprise elements from Group IIA of the Periodic Table of the Elements, which may be beryllium, magnesium, calcium, strontium, and barium or combinations thereof. Suitable transition metal promoters may comprise elements from Groups IVA, VA, VIA, VIIA and VIIIA of the Periodic Table of the Elements, andcombinations thereof.

[0087] The amount of alkaline earth metal promoter(s) and / or transition metal promoter(s) deposited on the support is a promoting amount. The transition metal promoter may typically be present in an amount from about 0.1 micromoles per gram to about 10 micromoles per gram, preferably from about 0.2 micromoles per gram to about 5 micromoles per gram.

[0088] The silver solution used to impregnate the support may also comprise an optional solvent or a complexing / solubilizing agent such as are known in the art. A wide variety of solvents or complexing / solubilizing agents may be employed to solubilize silver to the desired concentration in the impregnating medium. Useful complexing / solubilizing agents include amines, ammonia, oxalic acid, lactic acid and combinations thereof. Amines include an alkylene diamine having from 1 to 5 carbon atoms. In one preferred embodiment, the solution comprises an aqueous solution of silver oxalate and ethylene diamine. The complexing / solubilizing agent may be present in the impregnating solution in an amount from about 0.1 to about 5.0 moles per mole of silver, preferably from about 0.2 to about 4.0 moles, and more preferably from about 0.3 to about 3.0 moles for each mole of silver.

[0089] When a solvent is used, it may be an organic solvent or water, and may be polar or nonpolar. In general, the solvent should have sufficient solvating power to solubilize the solution components. At the same time, it is preferred that the solvent be chosen to avoid having an undue influence on or interaction with the solvated promoters. Organic-based solvents which have 1 to about 8 carbon atoms per molecule are preferred. Mixtures of several organic solvents or mixtures of organic solvent(s) with water may be used, provided that such mixed solvents function as desired herein.

[0090] The concentration of silver in the impregnating solution is typically in the range from about 0.1% by weight up to the maximum solubility afforded by the particular solvent / solubilizing agent combination employed. It is generally very suitable to employ solutions containing from 0.5% to about 45% by weight of silver, with concentrations from 5 to35% by weight of silver being preferred.

[0091] Impregnation of the selected support is achieved using any of the conventional methods, for example, excess solution impregnation, incipient wetness impregnation, spray coating, etc. Typically, the support material is placed in contact with the silver-containing solution until a sufficient amount of the solution is absorbed by the support. Preferably the quantity of the silver-containing solution used to impregnate the porous support is no more than is necessary to fill the pores of the support. A single impregnation or a series of impregnations, with or without intermediate drying, may be used, depending, in part, on the concentration of the silver component in the solution. Impregnation procedures are described, for example, in U. S. Patent Nos. 4,761,394, 4,766,105, 4,908,343, 5,057,481, 5,187,140, 5,102,848, 5,011,807, 5,099,041 and 5,407,888. Known prior procedures of pre-deposition, co-deposition and post-deposition of various the promoters can be employed.

[0092] After impregnation of the support with the silver-containing compound, i.e., a silver precursor, a rhenium component, an alkali metal component, and the optional other promoters, the impregnated support is calcined for a time sufficient to convert the silver containing compound to an active silver species and to remove the volatile components from the impregnated support to result in a catalyst precursor. The calcination may be accomplished by heating the impregnated support, preferably at a gradual rate, to a temperature in the range from about 200°C to about 600°C at a pressure in the range from about 0.5 to about 35 bar. In general, the higher the temperature, the shorter the required heating period. A wide range of heating periods have been suggested in the art; e.g., U. S. Patent No. 3,563,914 discloses heating for less than 300 seconds, and U. S. Patent No. 3,702,259 discloses heating from 2 to 8 hours at a temperature of from 100°C to 375°C, usually for duration of from about 0.5 to about 8 hours. However, it is only important that the heating time be correlated with the temperature such that substantially all of the contained silver is converted to the active silver species. Continuous or step-wise heating may be used for this purpose.

[0093] During calcination, the impregnated support may be exposed to a gas atmosphere comprising an inert gas or a mixture of an inert gas with from about 10 ppm to 21% by volume of an oxygen-containing oxidizing component. For purposes of this invention, an inert gas is defined as a gas that does not substantially react with the catalyst or catalyst precursor under the conditions chosen for the calcination. Further information on catalyst manufacture may be found in the aforementioned U. S. Patent Publication No. 2007 / 0037991.

[0094] While the present disclosure has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present disclosure. It is therefore intended that the present disclosure not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.

Claims

44680PCT_ApplicationCLAIMSWe claim:

1. A process for purifying a carbon dioxide stream comprising:providing a carbon dioxide stream containing water and hydrocarbons;compressing the carbon dioxide stream to remove water from the carbon dioxide stream to form a compressed stream;combining the compressed stream with oxygen to form a combined stream;reacting the hydrocarbons and the oxygen in the combined stream in the presence of an oxidation catalyst to form an oxidized stream, the oxidized stream containing carbon dioxide, methane, HC1, and oxygen;washing the oxidized stream to produce a washed gas effluent; andseparating in a cryogenic distillation column, oxygen and methane from the washed gas effluent to form a purified carbon dioxide product.

2. The process of claim 1, further comprising drying the washed gas effluent.

3. The process of claim 1, wherein the oxidation catalyst includes platinum deposited on a high- surface area support.

4. The process as in any one of the preceding claims, where, during the providing the carbon dioxide stream, the carbon dioxide passes through a regenerator vent.

5. The process as in any one of the preceding claims, wherein, during the reacting of the hydrocarbons and the oxygen, the hydrocarbons in the combined stream are reduced by greater than 99%, excepting methane.

6. The process as in any one of the preceding claims, wherein the combined stream contains 0.5 mol% to 2 mol% oxygen.

7. The process as in any one of the preceding claims, wherein the hydrocarbons and oxygen are reacted at a temperature of from 250°C to 380°C.

8. The process as in any one of the preceding claims, wherein the carbon dioxide stream is provided as a stream from an ethylene oxide manufacturing process.

9. The process as in any one of the preceding claims, wherein, during the reacting of the hydrocarbons and the oxygen in the combined stream, greater than 99% of non-methane hydrocarbons are removed by oxidation.

10. The process as in any one of the preceding claims, wherein the washed gas effluent is substantially free of hydrochloric acid.

11. The process as in any one of the preceding claims, further comprising, after the washing of the oxidized stream, drying the washed gas effluent to a moisture concentration of less than 10 ppm (by volume) water.

12. The process according to claim 11, further comprising, after the drying of the washed gas effluent, cooling the washed gas effluent to liquify the washed gas effluent prior to the washed gas effluent being supplied to the cryogenic distillation column.

13. The process according to claim 11, wherein the purified carbon dioxide product contains less than 1 mol ppm ethylene and less than 50 ppm oxygen.

14. The process according to claim 1, wherein the carbon dioxide stream is provided by: providing scrubber overheads;in a carbon dioxide absorber, contacting the scrubber overheads with a carbon dioxideabsorbing solvent to form a remaining gas stream and a rich carbonate solution;dividing, in a regenerator, the rich carbonate solution into a lean solvent and a discharge stream; andproviding the discharge stream as the carbon dioxide stream.

15. The process according to claim 14, wherein, after providing the discharge stream, the carbon dioxide stream is compressed to a pressure of 20 to 30 bar.

16. The process according to claim 1, wherein the cryogenic distillation column contains at least 20 stages.

17. The process according to claim 14, wherein the scrubber overheads comprises 30 vol% to 45 vol% methane, 25 vol% to 35 vol% ethylene, and 0.5 vol% to 5 vol% carbon dioxide.

18. The process as in claims 14, 15, or 17, wherein the scrubber overheads are provided by:producing an ethylene oxide reactor effluent by reacting ethylene, oxygen, and a chloride moderator in the presence of an ethylene oxide catalyst in an ethylene oxide reactor at a temperature of 225ºC to 280ºC; andcontacting the ethylene oxide reactor effluent with lean cycle water to prepare a rich cycle water stream and scrubber overheads.

19. The process according to claim 18. wherein the chloride moderator is present in concentrations (by volume) of from 0.5 ppm to 7 ppm.

20. The process according to claim 18. wherein the ethylene oxide catalyst contains a rhenium promoter.

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