JP7586647B2 - Method and apparatus for concentrating alkenes and/or alkanes - Google Patents

Description

The present invention relates to a method and apparatus for concentrating at least one of an alkene and an alkane having the same carbon number from a mixture containing the alkene and the alkane by combining membrane separation and distillation.

Conventionally, propylene and ethylene have been produced by steam cracking naphtha or liquefied petroleum gas, fluid catalytic cracking (FCC) at oil refineries, dehydrogenation of propane in natural gas, and the MTO (Methanol To Olefin) and MTP (Methanol To Propylene) processes using methanol as a raw material.
It is produced by the (propylene) process, etc.
In such a production process, propylene needs to be separated from propane and concentrated, and ethylene needs to be separated from ethane and concentrated. Conventionally, a distillation column has been used to separate and concentrate alkenes from a mixture containing alkenes (propylene, ethylene, etc.) and alkanes (propane, ethane, etc.) with the same carbon number, but it is known that a large amount of energy is consumed in the process.

Energy-saving methods have been studied for the concentration process, which requires such a large amount of energy, and the use of membrane separation devices to separate and concentrate alkenes from a mixture of alkenes and alkanes is also being considered.

As forms of use of membrane separation equipment in concentrating propylene, those using one or two stages of membrane separation equipment without using a distillation tower, and those hybridizing one or two stages of membrane separation equipment with a distillation tower are being considered (see Non-Patent Document 1).

In addition, in a concentration process that combines a hybrid membrane separation device and a distillation column, it has been considered to have two membrane separation devices, supplying the membrane permeate of the first membrane separation device to the second membrane separation device, and circulating the retentate of the second membrane separation device to the first membrane separation device (see Patent Document 1).

JP 2018-154591 A

2016 NEDO "TSC Foresight" Seminar (2nd) Masahiko Matsukata "Latest Trends and Future Outlook of High-Performance Membrane Separation Processes" pp. 8-10

A hybrid of a one- or two-stage membrane separation apparatus and a distillation column is said to be capable of saving energy not only when the alkene concentration in the raw gas is high but also when it is relatively low. However, the inventors' investigations revealed that there is still room for further improvement from the viewpoint of energy saving.
An object of the present invention is to provide a novel concentration method and apparatus that combines a membrane separation apparatus with a distillation column to concentrate at least one of an alkene and an alkane having a carbon number in the range of 2 to 4 from a mixture containing the alkene and the alkane having the same carbon number.

The inventors continued to study concentration using a hybrid combination of a membrane separation device and a distillation column, and discovered that further improvements were possible from the perspective of energy conservation, which led to the completion of this invention.

The present invention includes the following gist.
(1) A method for concentrating an alkene and/or an alkane from a raw material mixture containing an alkene and an alkane having a carbon number of 2 to 4, by combining a membrane separation apparatus and a distillation column, comprising:
an alkene concentration step of supplying the raw material mixture to a membrane separation device and concentrating the alkene by permeating the alkene through a membrane; and an alkene recovery step of supplying the membrane-permeated mixture from the concentration step to a distillation column, distilling off low boiling fractions from the top of the column, and recovering alkenes from the bottom of the column.
The method for concentrating comprising the steps of:
(2) The membrane separation apparatus is a two-stage membrane separation apparatus;
The alkene concentration step includes supplying the raw material mixture to a first-stage membrane separation apparatus, supplying the membrane-permeated mixture to a distillation column, and supplying the membrane-retardant mixture to a second-stage membrane separation apparatus, and circulating the membrane-permeated mixture to the supply path of the first-stage membrane separation apparatus.
(3) The method for concentrating according to (1) or (2), wherein the ideal separation factor of the membrane separation device is 5 or more.
(4) a membrane separation device that concentrates alkenes by passing them through a membrane;
a distillation column for distilling out light boiling fractions by distillation;
a raw material supply line connected to the membrane separation device and supplying a raw material mixture containing an alkene and an alkane having a carbon number of 2 to 4 in the range;
an alkane recovery line connected to the membrane separation device for recovering alkane that has not permeated through the membrane;
A membrane permeation mixture supply line connected to the membrane separation device and the distillation column and supplying the membrane permeation mixture to the distillation column;
A light fraction recovery line connected to the distillation column for recovering a distillate from the top of the distillation column;
an alkene recovery line connected to the distillation column for recovering alkene from the bottom of the distillation column;
A concentrating apparatus comprising:
(5) a first-stage membrane separation device and a second-stage membrane separation device for concentrating alkenes by passing them through a membrane;
a distillation column for distilling out light boiling fractions by distillation;
a raw material supply line connected to the first-stage membrane separation device for supplying a raw material mixture containing an alkene and an alkane having a carbon number of 2 to 4 in the range of 2 to 4;
a membrane retentate mixture supply passage connected to the first-stage membrane separation device and the second-stage membrane separation device, for supplying the membrane retentate mixture of the first-stage membrane separation device to the second-stage membrane separation device;
A membrane permeation mixture circulation supply line connected to the first-stage membrane separation device and the second-stage membrane separation device, which circulates and supplies the membrane permeation mixture of the second-stage membrane separation device to the first-stage membrane separation device;
an alkane recovery line connected to the second-stage membrane separation device for recovering alkane that has not permeated through the membrane;
A membrane permeation mixture supply line connected to the first stage membrane separation device and the distillation column and supplying the membrane permeation mixture to the distillation column;
A light fraction recovery line connected to the distillation column for recovering a distillate from the top of the distillation column;
an alkene recovery line connected to the distillation column for recovering alkene from the bottom of the distillation column;
A concentrating apparatus comprising:
(6) The concentrating apparatus according to (4) or (5), wherein the membrane separation device has an ideal separation factor of 5 or more.

The concentration method and concentration device of the present invention can effectively concentrate at least one of the alkenes and alkanes from a mixture containing alkenes and alkanes with the same carbon number within the range of 2 to 4, with even less energy.

1 is a schematic diagram of a concentrator according to one embodiment of the present invention. 4 is a diagram showing the relationship between the membrane area of a membrane separation device 10 and the membrane area of a membrane separation device 9 in Example 1. FIG. FIG. 2 is a diagram showing the membrane area of the membrane separation device 10 and the propane flow rate in the membrane permeate (permeate flow) O in Example 1. FIG. 1 is a schematic diagram of Comparative Example 1. FIG. 1 is a diagram comparing the input energy between Example 1 and Comparative Example 1. FIG. 13 is a schematic diagram of Comparative Example 2. FIG. 11 is a schematic diagram of Comparative Example 3. FIG. 11 is a schematic diagram of Comparative Example 4. FIG. 1 is a diagram comparing the input energy of Example 1 and Comparative Examples 2, 3, and 4. FIG. 1 is a graph showing the change in energy consumption depending on the separation factor of the membrane.

One embodiment of the present invention is a method for concentrating alkenes and/or alkanes from a raw material mixture containing alkenes and alkanes having a carbon number in the range of 2 to 4, by combining a membrane separation apparatus with a distillation column. Specifically, the method includes an alkene concentration step in which the raw material mixture is supplied to a membrane separation apparatus and the alkene is concentrated by passing it through the membrane, and an alkene recovery step in which the membrane-permeated mixture from the concentration step is supplied to a distillation column, low boiling fractions are distilled from the top of the column, and the alkene is recovered from the bottom of the column.

Examples of combinations of alkenes and alkanes with carbon numbers in the range of 2 to 4 include ethylene and ethane, propylene and propane, and butene and butane. The raw material mixture may contain any of these combinations and may also contain other components.

The concentration of alkene required as a product varies depending on the intended use. For example, in the case of propylene, there are polymer grade (99.5% or more), chemical grade (93% or more), refinery grade (60% or more), etc., and the higher the concentration, the greater the energy load required for refining.

The alkene concentration step is a step in which the raw mixture is fed to a membrane separation device and the alkene is permeated through the membrane to concentrate the alkene concentration. The separation membrane used in the membrane separation device is not particularly limited as long as it can permeate the alkene from the raw mixture containing an alkene and an alkane having the same carbon number, and typically, a zeolite membrane, an MOF membrane, or a silica membrane is used. Examples of the zeolite membrane include an FAU type zeolite separation membrane and a BEA type zeolite membrane.
The type of membrane separation apparatus includes a cross-current type, a countercurrent type, a parallel current type, etc., and any type can be adopted. In the membrane separation apparatus, since alkenes and alkanes are separated by membrane in a gaseous state, a heater for heating the raw material mixture can be provided in the supply path of the raw material mixture depending on the state of the raw material mixture, such as the state where the raw material mixture contains liquid.

The separation membrane of the membrane separation device usually has an ideal separation factor of 5 or more, preferably 8 or more, and more preferably 15 or more. If the ideal separation factor is small, sufficient energy saving effect cannot be obtained. There is no need to set an upper limit for the ideal separation factor of the separation membrane, and any membrane within the feasible range (currently, up to about 100) can be used.

The membrane separation device may be a single-stage membrane separation device or a two-stage membrane separation device.
In the case of a single-stage membrane separation apparatus, the membrane-permeated mixture enriched in alkene is fed to a distillation column, and the membrane-refracted mixture is recovered as an alkane-rich mixture.
In the case of a two-stage membrane separation apparatus, the membrane-permeated mixture in which alkenes are concentrated in the first-stage membrane separation apparatus is supplied to a distillation column, and the membrane-refracted mixture, which is a mixture with a high concentration of alkanes, is supplied to the second-stage membrane separation apparatus. The membrane-permeated mixture in the second-stage membrane separation apparatus is again concentrated in alkenes, so it is circulated and supplied to the supply line of the first-stage membrane separation apparatus. The membrane-refracted mixture in the second-stage membrane separation apparatus is then recovered as a mixture with a high concentration of alkanes.

The distillation column may be any of the known plate columns and packed columns. The number of plates in the distillation column is not limited, but may usually be 10 to 300.
The bottom of the distillation column is heated in a reboiler, and a part of the effluent is refluxed to the lowest tray through a bottom reflux line, and the remainder is recovered as an alkene through an alkene recovery line. When the alkene is recovered as a gas, a heater can be provided downstream of the reboiler in the alkene recovery line. When the alkene is recovered as a liquid, a cooler can be provided downstream of the reboiler in the alkene recovery line.

Light boiling fractions are distilled from the top of the distillation tower, and some of them are cooled in a condenser installed in the top reflux line and then refluxed to the topmost tray of the distillation tower.

Hereinafter, a specific embodiment of the present invention will be described with reference to FIG. 1, which is a schematic diagram of a propylene concentrating apparatus.
The concentrating apparatus includes a first distillation column 1, a second distillation column 5, a third distillation column 15, a first-stage membrane separation membrane device 9, and a second-stage membrane separation membrane device 10. The concentrating apparatus also includes a raw material supply passage connected to the first-stage membrane separation device 9 and supplying a mixture to the first-stage membrane separation device 9, a membrane retentate mixture supply passage connected to the first-stage membrane separation device 9 and the second-stage membrane separation device 10 and supplying the membrane retentate mixture of the first-stage membrane separation device 9 to the second-stage membrane separation device 10, and a membrane permeate mixture circulating supply passage connected to the first-stage membrane separation device 9 and the second-stage membrane separation device 10 and circulating the membrane permeate mixture of the second-stage membrane separation device 10 to the first-stage membrane separation device 9. a feed line, an alkane recovery line connected to the second-stage membrane separation device 10 and recovering the alkane that has not permeated through the membrane, a membrane-permeated mixture supply line connected to the first-stage membrane separation device 9 and the third distillation column 15 and supplying the membrane-permeated mixture to the third distillation column 15, a light fraction recovery line connected to the third distillation column 15 and recovering a distillate from the top of the third distillation column 15, and a propylene recovery line connected to the third distillation column 15 and recovering propylene from the bottom of the third distillation column 15.
Equipped with.

Mixture A containing ethylene, ethane, propylene, propane, butenes, and butanes is supplied to the first distillation column 1. In the first distillation column 1, the raw material mixture A is separated into mixture C containing mainly ethylene and other light boiling components, and mixture F containing propylene, propane, butenes, and butanes, which have higher boiling points than ethylene. At this time, a portion of the ethane may be recovered in mixture C, and the remainder may be contained in mixture F. The number of stages in the first distillation column 1 is, for example, 100 stages, and the feed stage of the mixture is any stage in the distillation column.

Mixture F is supplied to the second distillation column 5. In the second distillation column 5, the raw material mixture F is separated into mixture K mainly containing the aforementioned remaining ethane, propylene, and propane, and mixture J mainly containing butenes and butanes. The number of stages in the second distillation column 5 is, for example, 100 stages, and the mixture is supplied to any stage of the distillation column.

The mixture K is supplied to a first-stage membrane separation device 9. In the first-stage membrane separation device, the mixture K is separated into a membrane permeate O (also called a permeate flow) that has permeated the membrane and a membrane retentate L (also called a retentate flow).
The ratio of propylene in the C3 components contained in the membrane permeate O (propylene/(propylene+propane)) is equal to or higher than the concentration required for the product, and is supplied to the third distillation column 15. On the other hand, the membrane retentate L is a concentrated version of the membrane retentate L, which contains the remainder of the propylene that could not be separated in the first-stage membrane separation, and is supplied to the second-stage membrane separation device 10.

In the second-stage membrane separation device 10, the membrane retentate L is separated into membrane permeate N and membrane retentate M. The membrane permeate N is concentrated with propylene that could not be separated in the first stage, and is fed back to the first-stage membrane separation device 9. On the other hand, the membrane retentate M contains very little propylene, the target product, and is discharged outside the system.

The membrane permeate O supplied to the third distillation tower 15 is further concentrated in propylene in the third distillation tower, so that the propylene concentration in the effluent T from the bottom of the tower can meet the product standard. In the conventional method, propylene is extracted from the top of the tower and propane is extracted from the bottom of the tower. In this method, the difference in boiling points between propylene and propane is small, so energy consumption in the distillation tower is large. In this embodiment, ethane, which has a different boiling point from propylene, is separated in the distillation tower, so energy consumption in the distillation tower can be reduced. On the other hand, the effluent Q distilled from the top of the third distillation tower 15 contains ethane. The number of stages in the third distillation tower 15 is, for example, 100 stages, and the membrane permeate O is supplied to any stage near the center of the distillation tower.

The membrane permeate O from the first membrane separation device is supplied to the third distillation column 15 at the tray where the difference between its alkene concentration and the alkene concentration on the tray is the smallest. It is desirable to adjust the heater in the raw material mixture supply path so that the temperature of the membrane permeate O is not significantly different from the temperature in the tray to which it is supplied. A temperature regulator such as a heater and/or cooler that adjusts the temperature of the membrane permeate may be provided in the membrane permeate supply path so that the temperature of the membrane permeate O supplied to the tray of the third distillation column 15 approaches the temperature in the tray.

In the above-mentioned concentrating apparatus, the target of concentration by the membrane separation units 9 and 10 and the third distillation column 15 is a mixture containing propylene and propane, but is not limited thereto. Even a mixture containing ethylene and ethane, or a mixture containing 1-butene and n-butane can be concentrated.
In this case, for example, the concentration can be achieved by combining a known separation membrane with a distillation column. In the upstream stage of the separation membrane, a mixture of the target alkene, an alkane with the same carbon number, and an alkane with a carbon number smaller than that of the target alkene is separated from the other components, the separation membrane separates the alkane with the same carbon number as the target alkene, and the downstream distillation column separates the target alkene and the alkane with a carbon number smaller than that of the target alkene, thereby reducing energy consumption.

The alkene concentration of the mixture supplied to the first-stage membrane separation device is not particularly limited, but if it is 60 mol% or more, concentration with relatively low energy is possible. There is no particular upper limit, but it is usually 90% or less. It is desirable for the raw material mixture containing alkenes and alkanes to contain no other components or impurities, but it is acceptable for other components and impurities to be contained in a predetermined amount or less (for example, 10 mol% or less, preferably 5 mol% or less, more preferably 1 mol% or less) as long as it does not interfere with concentration.

For the examples and comparative examples, a general-purpose process simulator Pro/II v9.4 was used to evaluate by simulation the process of separating propylene from a mixture of alkanes and alkenes having carbon numbers of 2 to 4. In the simulation, the Peng-Robinson equation was applied to the thermodynamic properties of the mixture, and the data required for property estimation was the data built into Pro/II v9.4.

(Feed conditions)
The feed was assumed to be a mixture obtained from an MTO reactor, with a feed flow rate of 2360 kmol/h, ethylene, ethane, propylene, propane, butene and butane concentrations of 18.7, 25.6, 46.2, 3.4, 4.2 and 1.9 mol%, respectively, a temperature of 39.8° C. and a pressure of 3873.6 kPa.

(Product Specifications)
The production of polymer-grade propylene was assumed, with a flow rate of 1082 kmol/h, a propylene concentration in the product of 99.5 mol % or more, and a propylene recovery rate of 99.5% or more.

(Membrane performance)
The membrane was assumed to be an Ag-FAU type zeolite membrane, with the propylene permeability set to 1×10 ⁻⁷ mol/(m ² ·Pa·s), the propylene to paraffin permeability ratio set to 25, and the propylene to ethylene permeability ratio set to 3.

Example 1
1 shows an outline of the hybrid process of membrane separation and distillation in Example 1. It includes three distillation columns 1, 5, and 15, reboilers 3, 7, and 17, condensers 2, 6, and 16, two membrane separation devices 9 and 10, compressors 11 and 13, heat exchangers 8, 12, and 14, and one valve 4.

In the first distillation column 1, mainly ethylene and light boiling components with boiling points lower than ethylene are recovered from the top of the column. The number of stages in the first distillation column 1 was 50. The pressure at the top of the distillation column was 3873 kPa. The feed stage for the mixture was the 28th stage, where the heat quantity of the reboiler 3 of the first distillation column 1 was the smallest. The flow rate of the bottoms F of the first distillation column 1 was set to 1333 kmol/h, and the flow rate of propylene contained in this bottoms was set to 1089 kmol/h. The flow rate of ethane contained in the bottoms was set to 20 kmol/h.

In the second distillation column 5, propylene/propane and butene/butane are mainly separated. The number of stages in the second distillation column 5 was 50. The pressure at the top of the distillation column was 1667 kPa. The feed stage for the mixture was the 8th stage, where the heat quantity of the reboiler 7 of the second distillation column 5 was the smallest. The flow rate of the distillate vapor K from the second distillation column 5 was set to 1164 kmol/h, and the flow rate of propylene contained in this distillate vapor was set to 1082 kmol/h.

The distillate vapor from the second column is heated to 120°C in heat exchanger 8 and then supplied to membrane separation unit 9, where propylene and propane are mainly separated. The retentate stream from membrane separation unit 9 is supplied to membrane separation unit 10, where propylene and propane are mainly separated. The permeate stream N from membrane separation unit 10 is pressurized and then supplied to membrane separation unit 9. The permeate stream O from membrane separation unit 9 is pressurized and then supplied to the third distillation column 15.

In the third distillation column 15, propylene and a small amount of ethane are mainly separated. The number of stages in the third distillation column 15 was 150. The pressure at the top of the distillation column was 1618 kPa. The feed stage of the permeate stream O of the membrane separation device 9 was the 27th stage, where the heat quantity of the reboiler 17 of the third distillation column 15 was the smallest. The flow rate of the bottoms T of the third distillation column 15 was 1082 kmol/h, and the propylene concentration in this bottoms T was separated to be 99.5 mol% or more.

In the membrane separation devices 9 and 10 of the hybrid process of membrane separation and distillation shown in Fig. 1, the separation performance (permeability ratio) of the membrane was the same. The pressure on the permeation side of the membrane separation was 200 kPa, and the pressure on the supply side was 1667 kPa.

(Analysis example)
In order to satisfy the product specifications in the hybrid process of membrane separation and distillation in Example 1, the flow rate of propylene contained in the permeate stream O must satisfy the recovery rate, and further, the propane concentration in the total amount of propylene and propane must be 0.5 mol% or less.
Therefore, the membrane areas of the membrane separation devices 9 and 10 that can satisfy the above conditions were analyzed using a process simulator. As a calculation method, the membrane area of the membrane separation device 9 was calculated so that the flow rate of propylene contained in the permeate flow O becomes 1080 kmol/h when the membrane area of the membrane separation device 10 is changed.

Figure 2 shows the membrane area of the membrane separation device 9 when the membrane area of the membrane separation device 10 is changed. By increasing the membrane area of the membrane separation device 10, the membrane area of the membrane separation device 9 decreases. An increase in the membrane area of the membrane separation device 10 means that the flow rate of the permeate flow N increases, leading to an increase in the recycle flow rate between the membrane separation devices 9, 10. As a result, more paraffin is removed by the membrane separation device 10, and the concentration of propylene supplied to the membrane separation device 9 increases. As a result, the membrane area of the membrane separation device 9 decreases.

FIG. 3 shows the flow rate of propane in the permeate stream O versus the membrane area of the membrane separation device 10.
By removing paraffins (particularly propane) by the membrane separation apparatus 10, the flow rate of propane contained in the permeate stream O of the membrane separation apparatus 9 decreases. In Example 1, in order to recover propylene from the bottoms T of the third distillation column 15 and to satisfy the product standard, the flow rate of propane contained in the permeate stream O needs to be less than 5.41 kmol/h. In other words, it can be seen that the membrane area of the membrane separation apparatus 10 needs to be greater than 2656 ^m2 .
On the other hand, setting the membrane area of the membrane separation device 10 to be excessively large increases the load on the compressor 11, and increases the input energy. Therefore, by adopting a small membrane area for the membrane separation device 10 within the feasible range of the embodiment, the input energy can be reduced.

(Comparative Example 1)
The input energy of the hybrid process of membrane separation and distillation in Example 1 and the separation process of distillation only (Comparative Example 1) was analyzed by a process simulator. Figure 4 shows an outline of the distillation process.

In the first distillation column 18, ethylene and propylene are mainly separated from propylene, propane, butene, and butane. The number of stages in the first distillation column 18 was 50. The pressure at the top of the distillation column was 3,873 kPa. The feed stage for the mixture was the 21st stage, which is the stage at which the load on the reboiler 20 of the first distillation column 18 is the smallest. The flow rate of the bottoms of the first distillation column 18 was set to 1,313 kmol/h, and the flow rate of propylene contained in this bottoms was set to 1,089 kmol/h.

In the second distillation column 22, propylene/propane and butene/butane are mainly separated. The number of stages in the second distillation column 22 was 50. The distillation column top pressure was 1667 kPa. The feed stage for the mixture was the 18th stage, which is the stage where the load on the reboiler 24 of the second distillation column 22 is the smallest. The distillate flow rate of the second distillation column 22 was set to 1164 kmol/h, and the flow rate of propylene contained in this distillate was set to 1082 kmol/h.

In the third distillation column 25, propylene and propane are mainly separated. The number of stages in the third distillation column 25 was 150. The pressure at the top of the distillation column was 1618 kPa. The feed stage for the mixture was the 89th stage, which is the stage at which the load on the reboiler 27 of the third distillation column 25 is the smallest. The distillate flow rate of the third distillation column 25 was set to 1082 kmol/h, and propylene was separated so that the concentration of propylene contained in the distillate was 99.5 mol%.

The results of comparing the input energy of the hybrid process of membrane separation and distillation in Example 1 and the distillation process in Comparative Example 1 are shown in Figure 5. The input energy values of the reboiler, heat exchanger, and compressor were compared. Here, the input energy of the compressor was calculated from the required power and the conversion coefficient to primary energy (9.76 x ^10-3 GJ/h/kW).

Comparing the energy input of the hybrid process of membrane separation and distillation in Example 1 with the distillation process, the introduction of membrane separation reduced the energy input of reboilers 3 and 17, but increased the energy input of reboiler 7. In the hybrid process of membrane separation and distillation in Example 1, energy input is required for the compressor and heat exchanger, but the energy input for the entire process is reduced by approximately 57.9% in the hybrid process of membrane separation and distillation in Example 1 compared to the distillation process.

The input energy of the hybrid process of membrane separation and distillation in Example 1 and the hybrid process of membrane separation and distillation in which only one membrane separation device was installed (Comparative Examples 2, 3, and 4) was analyzed using a process simulator.

(Comparative Example 2)
FIG. 6 shows an outline of a hybrid process of membrane separation and distillation in which a membrane separation device 36 is introduced immediately before the third distillation column 39. In the first distillation column 28, ethylene and propylene and propylene, propane, butene and butane are mainly separated. The number of stages in the first distillation column 28 was 50. The top pressure of the distillation column was 3873 kPa. The feed stage for the mixture was the 21st stage, which is the minimum heat amount of the reboiler 30 of the first distillation column 28. The bottoms flow rate of the first distillation column 28 was set to 1313 kmol/h, and the propylene flow rate contained in the bottoms was set to 1089 kmol/h.

In the second distillation column 32, propylene/propane and butene/butane are mainly separated. The number of stages in the second distillation column 32 was 50. The pressure at the top of the distillation column was 1667 kPa. The feed stage for the mixture was the 19th stage, where the heat quantity of the reboiler 34 of the second distillation column 32 was the smallest. The flow rate of the distillate steam from the second distillation column 32 was set to 1164 kmol/h, and the flow rate of propylene contained in this distillate steam was set to 1082 kmol/h.

The distillate vapor from the second column is heated by a heat exchanger 35 and supplied to a membrane separation device 36, where propylene and propane are mainly separated. The area of the membrane separation device was set to 1991 ^m2 , which is the minimum load on the reboiler 40 of the third distillation column 39. The pressure on the permeation side of the membrane separation was set to 200 kPa.

The permeate stream from the membrane separation is pressurized to 1700 kPa by a compressor 37 and supplied to the third distillation tower 39. The retentate stream from the membrane separation is also supplied to the third distillation tower 39. In the third distillation tower 39, propylene and propane are mainly separated. The number of stages in the third distillation tower 39 was 150. The top pressure of the distillation tower was 1618 kPa. The feed stages for the permeate stream and the retentate stream of the membrane separation device 35 were the 8th and 116th stages, respectively, at which the load on the reboiler 41 of the third distillation tower 39 was minimized. The flow rate of the bottoms from the third distillation tower 39 was set to 1082 kmol/h, and the concentration of propylene contained in the bottoms was separated to 99.5 mol%.

(Comparative Example 3)
FIG. 7 shows an outline of a hybrid process of membrane separation and distillation in which a membrane separation device 47 is introduced immediately before the second distillation column 50. In the first distillation column 42, ethylene and propylene and propylene, propane, butene and butane are mainly separated. The number of stages of the first distillation column 42 was 50. The top pressure of the distillation column was 3873 kPa. The feed stage of the mixture was the 21st stage, which is the minimum load on the reboiler 43 of the first distillation column 42. The bottoms flow rate of the first distillation column 42 was set to 1313 kmol/h, and the propylene flow rate contained in the bottoms was set to 1089 kmol/h.

The bottoms of the first distillation column 42 are heated and evaporated by a heat exchanger 46 and supplied to a membrane separation device 47 to separate olefins and paraffins. The membrane area of the membrane separation device 47 was set to 2705 ^m2 , which minimizes the reboiler load of the second distillation column 50. The pressure on the permeation side of the membrane separation was set to 200 kPa.

The permeate stream from the membrane separation is pressurized to 1700 kPa by the compressor 48 and fed to the distillation tower, and the retentate stream from the membrane separation is also fed to the distillation tower. In the second distillation tower 50, propylene/propane and butene/butane are mainly separated. The number of stages in the second distillation tower 50 was 50. The top pressure of the distillation tower was 1667 kPa. The feed stages for the permeate stream and the retentate stream of the membrane separation device 47 were the 15th and 29th stages, respectively, at which the load on the reboiler 52 of the second distillation tower 50 was minimized. The distillate flow rate of the second distillation tower 50 was set to 1164 kmol/h, and the flow rate of propylene contained in this distillate was set to 1082 kmol/h.

In the third distillation column 53, mainly propylene and propane are separated. The number of stages in the third distillation column 53 was 150. The pressure at the top of the distillation column was 1618 kPa. The feed stage for the mixture was the 87th stage, which was the minimum load on the reboiler 55 of the third distillation column 53. The distillate flow rate of the third distillation column 53 was 1082 kmol/h, and the concentration of propylene contained in the bottoms was separated to be 99.5 mol%.

(Comparative Example 4)
8 shows an outline of a hybrid process of membrane separation and distillation in which a membrane separation device 57 is introduced immediately before the first distillation column 60. The mixture is evaporated by a heat exchanger 56 and supplied to the membrane separation device 57 to separate olefins and paraffins. The membrane area of the membrane separation device 57 was set to 4123 ^m2 , which minimizes the load on the reboiler 62 of the first distillation column 60. The pressure on the permeation side of the membrane separation was set to 200 kPa.

The permeate stream from the membrane separation is pressurized to 3900 kPa by a compressor 58 and supplied to the first distillation tower 59, and the retentate stream from the membrane separation is also supplied to the first distillation tower 60. The first distillation tower 60 mainly separates ethylene and propylene and propylene, propane, butene, and butane. The number of stages in the first distillation tower 60 is 50. The top pressure of the distillation tower is 3873 kPa. The permeate stream and the retentate stream from the membrane separation device 57 are respectively set to the 10th and 28th stages where the load on the reboiler 62 of the first distillation tower 60 is the minimum. The bottoms flow rate of the first distillation tower 60 is set to 1313 kmol/h, and the propylene flow rate contained in this bottoms is set to 1089 kmol/h.

In the second distillation column 64, propylene/propane and butene/butane are mainly separated. The number of stages in the second distillation column 64 was 50. The distillation column top pressure was 1667 kPa. The feed stage for the mixture was the 19th stage, which is the minimum load on the reboiler 66 of the second distillation column 64. The distillate flow rate of the second distillation column 64 was set to 1164 kmol/h, and the flow rate of propylene contained in this distillate was set to 1082 kmol/h.

In the third distillation column 67, mainly propylene and propane are separated. The number of stages in the third distillation column 67 was 150. The pressure at the top of the distillation column was 1618 kPa. The feed stage for the mixture was the 89th stage, which was the minimum load on the reboiler 69 of the third distillation column 67. The distillate flow rate of the third distillation column 67 was 1082 kmol/h, and the concentration of propylene contained in the bottoms was separated to be 99.5 mol%.

The results of comparing the input energy of the hybrid process of membrane separation and distillation shown in Figure 1 (Example) and the hybrid processes of membrane separation and distillation shown in Figures 6, 7, and 8 (Comparative Examples 2, 3, and 4) are shown in Figure 9. The input energy values of the reboiler, heat exchanger, and compressor were compared. Here, the input energy of the compressor is a value calculated from the required power and the conversion coefficient to primary energy (9.76 x ^10-3 GJ/h/kW).

The input energy of the hybrid process of membrane separation and distillation in Example 1 was approximately 36, 60, and 65% less than the input energy of the hybrid process of membrane separation and distillation in Comparative Examples 2, 3, and 4, respectively. By changing the propylene recovery method and the propane separation method as in Example 1, the reduction in the load of the reboiler 3 of the first distillation column 1 and the reboiler 17 of the third distillation column 15 was greater than the increase in the load of the reboiler 7 of the second distillation column 5, demonstrating that the input energy can be reduced throughout the entire process, even when the loads of the compressors 11 and 13 and the heat exchanger 8 are taken into account.

(Comparative Example 5)
The energy consumption of the entire process was calculated assuming a process similar to that of Example 1, with the first distillation column 1 recovering all ethane and components with boiling points lower than that of ethane from the distillate C. In this comparative example, only propylene and propane are introduced into the membrane separation device 9. Therefore, product-grade propylene is recovered from the permeate flow O of the membrane separation device 9, making the third distillation column 15 unnecessary.

The energy consumption of the entire process in Comparative Example 5 was 168.8 GJ/h, which was greater than the energy consumption of 148.9 GJ/h in Example 1. In other words, it was shown that the energy consumption of the entire process can be reduced by not recovering all of the ethane in the first distillation column 1, but by separating ethane from the top of the column using the distillation column 15 downstream of the membrane separation device 9 and recovering propylene from the bottom of the column.

Example 2
In the hybrid process of membrane separation and distillation in Example 1, the separation performance of the membrane capable of achieving energy saving compared to Comparative Example 1 was analyzed. Specifically, the propylene permeability was fixed and the paraffin permeability was changed to change the separation factor between propylene and paraffin, and the other conditions were the same as in Example 1, and a process simulation was performed.

10 shows the energy consumption of the entire process when the separation factor of the membrane is changed. In this example, if the separation factor of propylene and paraffin is greater than 8, the energy consumption is smaller than that of Comparative Example 1, and it was shown that efficient separation is possible.
This applies to the feed conditions in this example, and if the feed conditions are changed, the effect can be obtained even with a separation factor of less than 8 by selecting the optimal operating conditions for the entire process. Specifically, by changing the feed composition or the pressure on the non-permeate and permeate sides of the separation membrane, it is possible to reduce energy consumption even when the separation factor is small.

INDUSTRIAL APPLICABILITY The present invention can be advantageously used as a means for separating and concentrating at least one of an alkene and an alkane having the same carbon number, such as propylene and propane, from a mixture containing the alkene and the alkane, and has extremely high industrial applicability.
The present invention can be used as a means for introducing a membrane separation device into an existing distillation column, and can also be used as a means for designing a new device that combines membrane separation and distillation, and has extremely high industrial applicability.

1, 5, 15 Distillation column 9, 10 Membrane separation device 2, 6, 16 Condenser 3, 7, 17 Reboiler 4 Valve 11, 13 Compressor 8, 12, 14 Heat exchanger 18, 22, 25 Distillation column 19, 20, 26 Condenser 20, 24, 27 Reboiler 21 Valve 28, 32, 39 Distillation column 36 Membrane separation device 29, 33, 40 Condenser 30, 34, 41 Reboiler 31 Valve 37 Compressor 35, 38 Heat exchanger 42, 50, 53 Distillation column 47 Membrane separation device 43, 51, 54 Condenser 44, 52, 55 Reboiler 45 Valve 48 Compressor 46, 49 Heat exchanger 60, 64, 67 Distillation column 57 Membrane separation device 61, 65, 68 Condenser 62, 66, 69 Reboiler 63 Valve 58 Compressor 56, 59 Heat exchanger

Claims (6)

A method for concentrating an alkene and/or an alkane from a raw material mixture having a carbon number of 3 and containing an alkene and an alkane having the same carbon number, comprising the steps of:
an alkene concentration step in which the raw material mixture is supplied to a membrane separation device, the alkene is permeated through a membrane, and an alkane having the same carbon number as the alkene is separated and concentrated; and an alkene recovery step in which the membrane-permeated mixture from the concentration step is supplied to a distillation column, a light boiling fraction is distilled from the top of the column, and an alkene is recovered from the bottom of the column.
The method for concentrating comprising the steps of: The membrane separation device is a two-stage membrane separation device,
2. The method according to claim 1, wherein the alkene concentration step comprises supplying the raw material mixture to a first-stage membrane separation apparatus, supplying the membrane-permeated mixture to a distillation column, and supplying the membrane-retardant mixture to a second-stage membrane separation apparatus, and circulating the membrane-permeated mixture to the supply path of the first-stage membrane separation apparatus. The concentration method according to claim 1 or 2, wherein the ideal separation factor of the membrane separation device is 5 or more. a membrane separation apparatus for separating and concentrating an alkane having the same carbon number as the alkene by passing the alkene through a membrane; and a distillation column for distilling out a light boiling fraction by distillation;
a raw material supply line connected to the membrane separation device and supplying a raw material mixture having a carbon number of 3 and containing an alkene and an alkane having the same carbon number;
an alkane recovery line connected to the membrane separation device for recovering alkane that has not permeated through the membrane;
A membrane permeation mixture supply line connected to the membrane separation device and the distillation column and supplying the membrane permeation mixture to the distillation column;
A light fraction recovery line connected to the distillation column for recovering a distillate from the top of the distillation column;
an alkene recovery line connected to the distillation column for recovering alkene from the bottom of the distillation column;
A concentrating apparatus comprising: a first-stage membrane separation device and a second-stage membrane separation device which allow an alkene to permeate through a membrane and separate and concentrate an alkane having the same carbon number as the alkene;
a distillation column for distilling out light boiling fractions by distillation;
a raw material supply line connected to the first-stage membrane separation device for supplying a raw material mixture having a carbon number of 3 and including an alkene and an alkane having the same carbon number;
a membrane retentate mixture supply passage connected to the first-stage membrane separation device and the second-stage membrane separation device, for supplying the membrane retentate mixture of the first-stage membrane separation device to the second-stage membrane separation device;
A membrane permeation mixture circulation supply line connected to the first-stage membrane separation device and the second-stage membrane separation device, which circulates and supplies the membrane permeation mixture of the second-stage membrane separation device to the first-stage membrane separation device;
an alkane recovery line connected to the second-stage membrane separation device for recovering alkane that has not permeated through the membrane;
A membrane permeation mixture supply line connected to the first stage membrane separation device and the distillation column and supplying the membrane permeation mixture to the distillation column;
A light fraction recovery line connected to the distillation column for recovering a distillate from the top of the distillation column;
an alkene recovery line connected to the distillation column for recovering alkene from the bottom of the distillation column;
A concentrating apparatus comprising: The concentrator according to claim 4 or 5, wherein the membrane separation device has an ideal separation factor of 5 or more.

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