Process for the separation of a product mixture

[0001] The present invention relates to the separation by distillation of a light component and a heavy component, which are normally difficult to separate.

[0002] Chemical synthesis of small molecules is often hampered by difficult separation. Chemical reactions often do not run to completion, which leads to the formation of product mixtures, which need to be separated to obtain valuable compounds. For example, in the direct dimethyl ether synthesis from syngas, the separation of dimethyl ether (the desired product) from CO₂ (one of the reactants/side products) is known in the art to be problematic.

[0003] WO 2017/121817 describes a process for the synthesis of dimethyl ether from a feedstock comprising H₂ and CO_x. A gaseous mixture comprising dimethyl ether is obtained, which is subjected to dimethyl ether/synthesis gas separation, wherein dimethyl ether is separated from the other components in the gaseous mixture, such as H₂, CO, and CO₂. This separation step may typically be performed by flash evaporation, distillation, or a combination thereof. The use of a flash separation is associated with some known problems. In case this is operated such that CO₂ is evaporated, significant amounts of dimethyl ether are lost in the off-gas of the flash evaporation. If the flash evaporation is operated such that CO₂ is not evaporated, significant amounts of CO₂ remain in the product mixture. Further separation of CO₂ and dimethyl ether using a distillation column may be needed, which is energy consuming and may provide near-complete separation only with a high number of stages and a large reflux ratio.

[0004] Alternative proposed solutions for this problematic separation include scrubbing (see U.S. Pat. No. 6,458,856) and absorption (see US 2015/0232401 and KR 100882726). U.S. Pat. No. 6,458,856 describes three approaches to separate CO₂ from dimethyl ether after the one-step production of dimethyl ether from synthesis gas: (1) by scrubbing out dimethyl ether (and methanol) in the dimethyl ether reactor; (2) by removing CO₂ from the syngas before dimethyl ether synthesis; and (3) downstream product separation, in particular by scrubbing dimethyl ether from unreacted syngas using a scrubbing solvent. US 2015/0232401 describes a process for the preparation of dimethyl ether by catalytic conversion of synthesis gas, comprising a step wherein the product mixture comprising dimethyl ether and CO₂ is contacted with a liquid absorbent capable of absorbing CO₂. KR 100882726 describes a method for separating dimethyl ether from a product mixture of dimethyl ether synthesis, by using an aqueous methanol solution as a dimethyl ether absorption solvent.

[0005] The present invention provides in the need for an efficient separation of mixtures that are difficult to separate, such as dimethyl ether and CO₂.

[0006] The inventors have developed an improved distillation process, wherein the separation of light and heavy components is improved. The distillation according to the invention reduces the reflux ratio within the distillation column, reduces the energy consumption of the distillation, increases the yield of the heavy component (less heavy component ends up in the top fraction) and reduces the amount of required equilibrium stages needed in the distillation column. In addition, the need for a flash evaporation upstream of the distillation column is avoided, and the distillation according to the invention has an increased flexibility in terms of the operating pressure, which enables more efficient integration with synthesis reactors. These effects are achieved by feeding a clearing gas into the distillation column, in addition to the mixture to be separated.

[0007] The invention is preferably defined as follows:

• • [0008] 1. A process for the separation of a product mixture, the process comprising:
    • [0009] (a) providing the product mixture containing at least a light component and a heavy component, wherein a binary system of the light component and the heavy component has a Henry's law constant of at least 0.001 mol m⁻³ Pa⁻¹, and the boiling point of the light component is at least 30° C. lower than the boiling point of the heavy component, determined at ambient pressure;
    • [0010] (b) subjecting the product mixture to a distillation step, wherein the product mixture is heated in a distillation column to a distillation temperature at a distillation pressure;
    • [0011] (c) feeding a clearing gas at a location below the feed point of the product mixture;
    • [0012] (d) collecting at the top of the distillation column a product gas containing the light component and the clearing gas;
    • [0013] (e) collecting at the bottom of the distillation column a product liquid containing the heavy component.
  • [0014] 2. The process according to embodiment 1, wherein the product gas obtained in step (d) is recycled to a reaction step that affords the product mixture or is used for a carbon capture and utilization process.
  • [0015] 3. The process according to embodiment 1 and 2, wherein the product mixture originates from reaction step selected from a dimethyl ether synthesis reaction, aqueous phase NH₃ synthesis, Fischer-Tropsch synthesis and biomass gasification.
  • [0016] 4. The process according to any one of the preceding embodiments, wherein the light component is selected from CO, CO₂, ethanol, NH₃, H₂ and H₂O and/or the heavy component is selected from di-C_1-4-ethers, C_1-6-alkanes, H₂O and I₂.
  • [0017] 5. The process according to any one of the preceding embodiments, wherein the clearing gas is selected from argon, helium, CO, N₂, air, natural gas, water, light hydrocarbons, H₂ and mixtures thereof.
  • [0018] 6. The process according to any one of the preceding embodiments, wherein:
    • [0019] (i) the light component is CO₂, the heavy component is dimethyl ether;
    • [0020] (ii) the light component is CO₂, the heavy component is n-butane;
    • [0021] (iii) the light component is water, the heavy component is I₂;
    • [0022] (iv) the light component is NH₃, the heavy component is water.
  • [0023] 7. The process according to embodiment 6, wherein the light component is CO₂, the heavy component is dimethyl ether and the clearing gas is H₂ or CH₄.
  • [0024] 8. The process according to any one of the preceding embodiments, wherein a binary system of the light component and the heavy component has a Henry's law constant of at least 0.002 mol m⁻³ Pa⁻¹.
  • [0025] 9. The process according to any one of the preceding embodiments, wherein the boiling point of the light component is at least 50° C., preferably at least 80° C., lower than the boiling point of the heavy component, determined at ambient pressure.
  • [0026] 10. The process according to any one of the preceding embodiments, wherein a binary system of the heavy component and the clearing gas has a Henry's law constant of at most 0.001 mol m⁻³ Pa⁻¹, determined at ambient pressure and temperature.
  • [0027] 11. The process according to any one of the preceding embodiments, wherein no flash evaporation step is performed upstream of the distillation column.
  • [0028] 12. Use of a clearing gas in the separation of a product mixture containing light component and a heavy component, wherein the separation is performed in a distillation column to a distillation temperature at a distillation pressure and affords a product gas containing the light component and the clearing gas; and a product liquid containing the heavy component.
  • [0029] 13. The use according to embodiment 12, which is for at least one of:
    • [0030] (i) reduction of the reflux ratio within the distillation column;
    • [0031] (ii) reduction of the energy consumption of the distillation column;
    • [0032] (iii) reduction of the loss of the second species in the product gas; and
    • [0033] (iv) reduction of the amount of equilibrium stages in the distillation column.

[0034] The inventors have developed an improved distillation process, wherein the separation of light and heavy components is improved. The present invention provides a process for the separation of a product mixture, and comprises the following steps:

• • [0035] (a) providing the product mixture containing at least a light component and a heavy component, wherein a binary system of the light component and the heavy component has a Henry's law constant of at least 0.001 mol m⁻³ Pa⁻¹, and the boiling point of the light component is at least 30° C. lower than the boiling point of the heavy component, determined at ambient pressure;
  • [0036] (b) subjecting the product mixture to a distillation step, wherein the product mixture is heated in a distillation column to a distillation temperature at a distillation pressure;
  • [0037] (c) feeding a clearing gas at a location below the feed point of the product mixture;
  • [0038] (d) collecting at the top of the distillation column a product gas containing the light component and the clearing gas;
  • [0039] (e) collecting at the bottom of the distillation column a product liquid containing the heavy component.
    The Product Mixture

[0040] Many industrial processes for the chemical synthesis of components include a distillation step downstream of a reaction step, to separate light and heavy components. The light component is collected as gaseous top fraction of the distillation column, and the heavy component as liquid bottom fraction of the distillation column. In the incoming mixture, the heavy component may be a gas or a liquid, typically a liquid, and the light component is gaseous or may be dissolved in the heavy component. The distillation step is often preceded by a flash evaporation to remove most of the volatile components of the reaction product mixture. The inventors found that such a flash evaporation step is not needed in case the distillation step according to the invention is used, and could be eliminated from the downstream processing of the reaction product mixture.

[0041] Any product mixture can be subjected to the separation of the present invention. The product mixture to be separated preferably originates from a reaction, in which case the product mixture may also be referred to as a reaction product mixture. Any reaction product mixture that contains two components defined above can efficiently be separated using the process according to the invention. It is irrelevant if the light component and the heavy component are the reactant(s) or the product(s) of the reaction. In one embodiment, one of the light component and the heavy component is a reactant and the other one is a product of the reaction, preferably the light component is a reactant of the reaction and the heavy component is a product of the reaction. In an alternative embodiment, both the light component and the heavy component are products of the reaction.

[0042] Most advantageously, the light component is a reactant of the reaction and the heavy component is a product of the reaction. In the context of this embodiment, it is preferred that the light component collected at the top of the distillation column (product gas) is recycled to the reaction step. Alternatively, the product gas may be used in a carbon capture and utilization (CCU) process, which is especially useful in case the light component is CO₂. The reaction is thus preferably a dimethyl ether synthesis reaction. In the context of the preferred mixtures of heavy and light component, alternative preferred reactions include aqueous phase NH₃ synthesis, Fischer-Tropsch synthesis and biomass gasification.

[0043] The light component is obtained as the gaseous product during the distillation step, obtained as top fraction. The light component may be selected from CO, CO₂, ethanol, NH₃, H₂ and H₂O. Preferred light components are selected from CO₂, H₂O and NH₃. Most preferably, the light component is CO₂.

[0044] The heavy component is collected as liquid bottom fraction during the distillation. During the distillation, the heavy component typically boils and condenses, and the condensates are collected as liquid bottom fraction. The heavy component is preferably selected from di-C_1-4-ethers, C_1-6-alkanes, H₂O and I₂, more preferably the heavy component is dimethyl ether, H₂O, I₂ or n-butane. Most preferably, the heavy component is dimethyl ether.

[0045] In order to being able to separate the light and heavy components, it is preferred that the boiling point of the light component is at least 30° C. lower than the boiling point of the heavy component, preferably this difference is boiling point is at least 50° C. or even at least 80° C. This difference in boiling point is determined at atmospheric pressure (1 bar). Without being bound to a theory, it is believed that such a difference in boiling point ensures that the light component and the heavy component can be readily separated, even if the light component would have a good interaction with the heavy component, as further explained below. With lower boiling point differences, the clearing gas may aide molecules of the heavy component to the top fraction of the distillation column, where heavy component would be lost in the gaseous product stream, thereby reducing the yield of the heavy component in the bottom fraction of the distillation column. From a process perspective, there is no real upper limit to the boiling point difference, but typically the difference in boiling point between the light component and the heavy component is at most 200° C.

[0046] The process according to the invention is especially useful for separating a light component and a heavy component that have a good interaction, meaning that they are normally difficult to separate. For example, the separation of dimethyl ether and CO₂ is known to be difficult in view of the good interaction between the light component (CO₂) and the heavy component (dimethyl ether). The good interaction between the heavy and the light component may be defined as a good solubility of the light component in the heavy component. A well-known measure of the solubility of a light component (gas) in a heavy component (liquid) is the Henry's law constant. Henry's law constant (HCP) is a proportionality factor that is defined as the concentration (c) of dissolved gas in a liquid divided by the partial pressure (p) of the gas above the liquid. High Henry's law constants are obtained when the concentration of dissolved gas is relatively high when the partial pressure is relatively low, meaning that the gas dissolves well in the liquid. Any binary system of a gaseous species and a liquid species has a Henry's law constant, which may be known (see e.g. www.henrys-law.org) or may be determined experimentally by methods known to the skilled person. In the context of the present invention, it is preferred that Henry's law constant of the binary system of the light and heavy component is at least 0.001 mol m⁻³ Pa⁻¹, preferably at least 0.002 mol m⁻³ Pa⁻¹, more preferably at least 0.01 mol m⁻³ Pa⁻¹. The upper limit of the Henry's law constant is only governed the binary systems available, but is not limited from a practical point of view. They may be as high as 100 mol m⁻³ Pa-1 or even higher. Preferably, the Henry's law constant is not higher than 1 mol m⁻³ Pa⁻¹. In the context of the present invention, the Henry's law constant (HCP) is normally determined at the (ambient or elevated) pressure and temperature which corresponds to the diluted region of the light component in the binary VLE system.

[0047] Preferred combinations of light and heavy components to be separated with the process according to the invention, that have such a difference in boiling point and a Henry constant in the desired range, include: (i) dimethyl ether (heavy) and CO₂ (light); (ii) n-butane (heavy) and CO₂ (light); (iii) water (light) and 12 (heavy); and (iv) water (heavy) and ammonia (light). Preferred are combinations (i), (ii) and (iii), more preferably the combination is selected from (i) and (ii). In an especially preferred embodiment, the light component is CO₂ and the heavy component is dimethyl ether (combination (i)).

Distillation

[0048] The mixture is subjected to a distillation step. Distillation of a product mixture containing a light and a heavy component to separate the light component from the heavy component is known in the art. Such a distillation step is performed in a distillation column, in which the mixture is heated to a distillation temperature at a distillation pressure. The skilled person is well aware of distillation and is able to select an appropriate distillation temperature and pressure for the specific mixture to be separated. For example, the distillation temperature may be in the range of −200° C. to 350° C., preferably in the range of −20 to 250° C. The distillation pressure may be in the range of 0.01 to 50 bar, preferably in the range of 15 to 40 bar. The inventors found that the beneficial effects of the clearing gas are greater at elevated pressure, such as 15 bar or higher.

[0049] Distillation columns are known in the art, and any type can be used in the present invention. Typically, the distillation column comprises a boiler wherein the feed mixture is heated towards the distillation temperature. Preferably, the distillation column comprises a reboiler, which is normally located at the bottom of the column and is used to evaporate the liquids collected at the bottom. The distillation column further typically contains a feeding point at or near the middle of the column. In other words, the distillation column typically contains a rectifying section and a stripping section The distillation column is operated as common in the art. The mixture is fed into the distillation column. During distillation the heavy component boils and condenses, and accumulates at the bottom of the distillation column. The bottom fraction of the distillation thus contains the heavy component. The bottom fraction may also contain further components that accumulate at the bottom of the distillation column and that were present in the incoming mixture. The light component is gaseous and is collected at the top of the distillation column. The top fraction of the distillation thus contains the light component. The top fraction may also contain further components that accumulate at the top of the distillation column and that were present in the incoming mixture. The top fraction also contains the clearing gas.

[0050] During operation of the process according to the invention, the product mixture is fed into the distillation column. The crux of the present invention is that additionally a clearing gas is fed into the distillation column during the distillation. The clearing gas is fed at a location in the column below the feed point of the product mixture. Preferably, the clearing gas is fed at the bottom of the column. Without being bound to a theory, the inventors believe the presence of gaseous bubbles in the distillation column replaces part of the gaseous molecules of the heavy component, thereby reducing the vapour loading within the distillation column and thus reducing the number of equilibrium stages required for optimal separation. Also, less of the reflux will flow back down in the distillation column, since the clearing gas that is collected in the top fraction of the distillation column takes up a substantial part of the reflux. The clearing gas molecules as well as the molecules of the light component flow upward and are collected in the top fraction, and only the molecules of the heavy component flow back down to the bottom fraction. Thus, the reflux ratio is reduced by virtue of the clearing gas, without jeopardizing the separation efficiency of the distillation process.

[0051] The clearing gas may be any gas that does not dissolve well in the heavy component. Such a clearing gas helps to clear the liquid heavy component from the liquid component. The clearing gas preferably does not have a good interaction with the heavy component. The clearing gas should not be the same species as the light component. In a preferred embodiment, the Henry's law constant of the binary system of the heavy component and the clearing gas is at most 0.001 mol m⁻³ Pa⁻¹. Suitable clearing gases include argon, helium, CO, N₂, air, natural gas, water, light hydrocarbons (such as C₁₂, preferably CH₄), H₂ and mixtures thereof. In a preferred embodiment, the clearing gas is CH₄ or H₂. In an especially preferred embodiment, the clearing gas is H₂.

[0052] The inventors found that H₂ is particularly suitable as clearing gas when the light component is CO₂. The efficiency of CO₂ removal during distillation is greatly improved, as further explained below. Also, the top fraction collected from the distillation column contains CO₂ and H₂, a mixture which is perfectly suitable for a carbon capture and utilization (CCU) process. Likewise, the use of CH₄ as clearing gas provides a top fraction which is ideally suited for dry reforming.

[0053] The bottom fraction of the distillation column is typically the main product of the reaction, and can be used as deemed fit. The top fraction can also be used as deemed fit. For example, it can be used for carbon capture processes, e.g. when the light component is CO₂. The top fraction may also be recycled to the reactor, especially in case the light component is a reactant of the reaction, such as CO₂ in a dimethyl ether reactor. The use of the clearing gas according to the invention enables the distillation to be performed at higher pressure. The beneficial effects of the clearing gas are greater at elevated pressure, such that a pressure that is closer to the pressure of a preceding reaction step can be used. As such, the pressure changes in the overall process can be reduced and the reactor, the distillation column and the recycle to the reactor can all be kept at the elevated pressure of the reactor.

[0054] The process according to the invention has several advantages over conventional distillation processes, which do not employ a clearing gas. As is shown in the example below, the separation according to the invention requires less equipment due to the omission of a flash evaporation step, overall requires less energy, but also the distillation itself requires less energy, due to reduced fluid load in the distillation column by addition of a non-condensable component of the clearing gas. Further advantages of the distillation process according to the invention include an enhanced yield of the heavy component, a reduction in reflux ratio and a reduction in equilibrium stages within the distillation column. The enhanced yield of the heavy component in the bottom fraction of the distillation is caused by a reduced loss of that component in the light fraction and/or during flash evaporation. In other words, more of the heavy component is collected at the bottom of the distillation column when a clearing gas according to the invention is used.

[0055] Thus, the present invention also concerns the use of a clearing gas in the separation of a product mixture containing at least a light component and a heavy component, wherein the separation is performed in a distillation column at a distillation temperature at a distillation pressure and affords a product gas containing the light component and the clearing gas; and a product liquid containing the heavy component. The use according to the present aspect is typically for at least one of the following:

• • [0056] (i) reduction of the reflux ratio within the distillation column;
  • [0057] (ii) reduction of the energy consumption of the distillation column;
  • [0058] (iii) reduction of the loss of the heavy component in the product gas; and
  • [0059] (iv) reduction of the amount of equilibrium stages in the distillation column.

[0060] In one embodiment, the use according to the present aspect is at least for reduction of the reflux ratio within the distillation column. In one embodiment, the use according to the present aspect is at least for reduction of the energy consumption of the distillation column. In one embodiment, the use according to the present aspect is at least for reduction of the loss of the heavy component in the product gas. In one embodiment, the use according to the present aspect is at least for reduction of the amount of equilibrium stages in the distillation column. In an especially preferred embodiment, the present use is for at least two, or at least three or most preferably all four of the above defined effects (i)-(iv).

[0061] The use of the clearing gas according to the invention also increases the flexibility of the distillation, as it can be performed at higher pressure when a clearing gas is employed. Thus, a pressure that is closer to the pressure of a preceding reaction step can be used, such that the pressure changes in the overall process can be reduced. As such, the reactor, the distillation column and the recycle to the reactor can all be kept at the elevated pressure of the reactor.

[0062] Two reaction mixtures of a dimethyl ether synthesis reaction (see Table 1 for composition) were fed to a distillation column. The reaction was performed at 50 bar and 250° C., and distillation of the reaction product mixture at 20 bar was simulated in Aspen Plus, using the PSRK (Predictive Soave-Redlich-Kwong) equation of state, with and without clearing by hydrogen gas. The separation without clearing gas involved a flash evaporation upstream of the distillation column, operating at 50 bar and −51.9° C. (to keep dimethyl ether losses below 1%). Total energy consumption (including heat exchangers, flash vessel and the reboiler and condenser), total product (dimethyl ether) loss, number of equilibrium staged and reflux ratio of each of the four scenarios was simulated, and the results are presented in Table 2.

[0063]

TABLE 1
Feed mixture compositions (kmol/h)
		High-CO2	Low-CO2
	Component	mixture A	mixture B
	CO2	36.07	4.66
	CH3OH	3.00	1.58
	CH3OCH3 (DME)	66.18	82.27
	H2	50.79	62.31
	CO	12.14	17.26
	H2O	0.27	0.35

[0064]

TABLE 2
Separation performance
				DME	no.	reflux
Entry	Mixture	Clearing gas	Energy	loss	stages	ratio
1	A	10.8 kmol/h H2	888.3	kW	0.59%	10	0.65
2	A	None	1269	kW	1.33%	11	1.7
3	B	13.1 kmol/h H2	911.6	kW	0.6%	8	0.5
4	B	None	1065	kW	0.9%	15	2.8

[0065] The separation process according to the invention requires less energy (ca 150-400 kW), which may be attributed to the more efficient distillation. The amount of dimethyl ether lost in the top fraction was markedly reduced using the clearing gas. Also, the number of equilibrium stages and the reflux ratio were significantly reduced, but not at the expense of separation efficiency as demonstrated by the dimethyl ether loss.

[0066] In a similar experiment, an equimolar mixture of 10 kmol/h I₂—H₂O was separated by distillation at 10 bar, which was simulated in Aspen Plus, using the NRTL (non-random two-liquid) equation of state. Distillation was performed with and without clearing by hydrogen gas, both using 10 equilibrium stages. The separation without clearing gas was operated at 10 bar and 100-184° C. with a total energy consumption of 707 kW, while use of 2 kmol/h of the clearing gas resulted in a temperature of 91-178° C. and total energy consumption of 246 kW.

[0067] The results show that the process according to the invention, employing a clearing gas, is able outperform the conventional distillation within a pre-defined distillation column (of 10 equilibrium stages), the process of the invention requires less energy.

[0068] In a similar experiment, product mixture C (Table 3) was separated by distillation at 10 bar using 7 and 8 equilibrium stages, which was simulated in Aspen Plus, using the NRTL (non-random two-liquid) equation of state. Distillation was performed with and without clearing by hydrogen gas, both using 8 equilibrium stages. The results are summarised in Table 4, entries 1 and 2. The separation without clearing gas was operating at −54-81° C. with a total energy consumption of 62 kW, while use of 2 kmol/h of the clearing gas resulted in a total energy consumption of 62 kW. Using 7 equilibrium stages (Table 4 entries 3 and 4), the separation without clearing gas operated at −54-81° C., with a total energy consumption of 157 kW, while stripping with 2 kmol/h resulted in a total energy consumption of 86 kW.

[0069]

TABLE 3
Feed mixture C (kmol/h)
		Molar
	Component	flowrate
	CO2	2.0
	CH3OH	1.0
	n-butane	5.0
	H2	0.5
	CO	1.0
	H2O	0.5

[0070]

TABLE 4
Separation performance
Entry	Mixture	Clearing gas	Energy	no. stages	reflux ratio
1	C	2 kmol/h H2	62 kW	8	0.67
2	C	None	62 kW	8	1.3
3	C	2 kmol/h H2	86 kW	7	1.5
4	C	None	157 kW	7	4.4

[0071] The separation process according to the invention requires a lower reflux ratio, which may be attributed to the more efficient distillation. Based on the results shown in Table 4, the cases with clearing gas (entries 1 and 3) show a marked reduction in reflux ratio when compared to the conventional cases (entries 2 and 4). In addition, shown when comparing entries 3 and 4, for a given column size (fixed number of equilibrium stages), the operation with a clearing gas leads to a marked reduction in total energy requirement.

[0072] In a similar experiment, product mixture D (Table 5) was separated by distillation at 30 bar using 5 equilibrium stages, which was simulated in Aspen Plus, using the NRTL (non-random two-liquid) equation of state. Distillation was performed with and without clearing by hydrogen gas, both using 5 equilibrium stages. The results are summarised in Table 4, entries 1 and 2. The separation without clearing gas was operating at 98° C. with a total energy consumption of 126 kW, while use of 1 kmol/h of the clearing gas operated at 95° C. and resulted in a total energy consumption of 111 kW. Ammonia losses were similar for both processes

[0073]

TABLE 5
Feed mixture D (kmol/h)
		Molar
	Component	flowrate
	CO2	1.0
	H2O	4.0
	NH3	5.0

[0074]

TABLE 6
Separation performance
		Clearing		no.	reflux
Entry	Mixture	gas	Energy	stages	ratio
1	D	1 kmol/h H2	111 kW	5	0.3
2	D	None	127 kW	5	0.5

[0075] The separation process according to the invention requires a lower reflux ratio, which may be attributed to the more efficient distillation. Based on the results shown in Table 6, the use of the clearing gas (entry 1) shows a marked reduction in reflux ratio when compared to the conventional process (entry 2).