JPH10121071A - Process and apparatus for adjusting calorific value of liquefied natural gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a process and an apparatus for satisfactorily adjusting the calorific value of LNG while preventing the solidification of impurities such as methanol by a simple inexpensive structure by mixing a liquefied natural gas with a liquefied petroleum gas having a temperature higher than that of the former. SOLUTION: As regards the side of an LNG delivery passage 12, the LNG is sent to a mixer 30 by opening a shut-off valve 18, operating an LNG delivery pump 14 and regulating the flow rate of the LNG by means of a flow control valve 16. On the other hand, as regards the side of an LNG delivery passage 22, when the concentration of impurities in an LPG held in an LPG tank 20 is above the allowance, namely when the concentration of impurities is so high that the impurities cooled by mixing with the LNG in the mixer 30 are solidified and stop the passage (e.g. when the concentration of a methanol impurity in a propane-based LPG is 50ppm or above), a shut-off valve 52 is shut, a shut-off valve 28 is opened, and an LNG delivery pump 24 is operated. The flow rate is regulated by means of a flow rate control valve 26. The impurity-containing LPG is sent to a membrane separator 40, the separated impurities are discharged out of the system, and the remaining LPG is fed to the mixer 30 and mixed with a low-temperature LNG, whereupon the stoppage of the passage can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to liquefied natural gas (hereinafter referred to as LNG) and liquefied petroleum gas (hereinafter referred to as LP).
The present invention relates to a method and apparatus for adjusting the calorific value of LNG by mixing G).

[0002]

2. Description of the Related Art Conventionally, as a means for adjusting the calorific value of LNG, a method of mixing LNG with LNG has been known.

However, in this method, when the LPG contains impurities (mainly methanol) at a concentration higher than a predetermined concentration, the LPG is mixed with low-temperature LNG, whereby the impurities solidify and become semi-conductive. It becomes a solid and may block the inside of the passage. Such coagulates cannot be removed by ordinary physical or chemical methods and, in the worst case, can cause operation interruption.

For example, when methanol of about 500 ppm is present in LPG, the freezing point of LPG itself (for example, in the case of LPG containing a large amount of propane,
At about -133 ° C, which is much higher than the freezing point of propane (around the temperature of -187.7 ° C), methanol solidifies into a snow-white state at about -133 ° C. There is a risk of causing a situation.

Therefore, Japanese Patent Application Laid-Open No. 4-55119 discloses L.
There is disclosed a method in which NG is heated by a so-called open rack type preheater or the like to a temperature at which the methanol does not solidify, and then the LNG is mixed with the LNG.

[0006]

In order to carry out the method disclosed in the above publication, a large open rack type LNG heating device, a pipe for sending LNG to the heating device,
Control valves for passage switching and flow rate adjustment must be specially installed as additional equipment. Therefore, there is a disadvantage that the equipment area is greatly increased and the cost is increased.

SUMMARY OF THE INVENTION In view of such circumstances, an object of the present invention is to provide a method and an apparatus that can perform good LNG fuel adjustment with a simple and inexpensive configuration while avoiding solidification of impurities such as methanol.

[0008]

As a means for solving the above-mentioned problems, the present invention is to reduce the calorific value of the liquefied natural gas by mixing the liquefied natural gas with a liquefied petroleum gas having a higher temperature than the liquefied natural gas. In the method for adjusting the calorific value of the liquefied natural gas to be adjusted, impurities which solidify at the temperature of the liquefied natural gas are removed from the liquefied petroleum gas by a membrane separation device, and then the liquefied petroleum gas is mixed with the liquefied natural gas. is there.

That is, according to the method of the present invention, the liquefied petroleum gas is mixed with the liquefied petroleum gas after removing impurities having a high freezing point contained in the liquefied petroleum gas in advance. Even so, blockage of the passage due to solidification of the impurities can be avoided. Further, it is not necessary to remove 100% of the impurities in the membrane separation apparatus.
Any material can be used as long as the impurity concentration can be reduced below the allowable value (that is, the concentration at which the impurities are likely to solidify and block the passage). Therefore, it is not necessary to use a specially expensive and high-performance separation membrane, so that it is inexpensive and simple. The structure can prevent the solidification of the impurities.

The present invention also provides a liquefied natural gas delivery passage,
A liquefied petroleum gas delivery passage for delivering liquefied petroleum gas having a higher temperature than liquefied natural gas, and a mixer connected to both passages for mixing the delivered liquefied natural gas and liquefied petroleum gas are provided. In the liquefied natural gas calorific value adjusting device for adjusting the calorific value of the liquefied natural gas, a membrane separation for removing impurities solidified at the temperature of the liquefied natural gas from the liquefied petroleum gas in the middle of the liquefied petroleum gas delivery passage A device is provided.

In this apparatus, the liquefied petroleum gas delivery passage is selectively provided with a bypass passage that bypasses the membrane separation device, and a state in which the liquefied petroleum gas passes through the membrane separation device and a state that passes through the bypass passage. It is more preferable to provide a passage switching means for switching. With this configuration, when the impurity concentration in the liquefied petroleum gas is already below the allowable value before the treatment, that is, when there is no need for membrane separation, the liquefied petroleum gas is passed through the bypass passage to perform membrane separation. Unnecessary deterioration of the separation membrane in the device can be prevented.

Further, a plurality of membrane separation devices are arranged in parallel in the liquefied petroleum gas delivery passage, and an apparatus selection means for selecting a membrane separation device to be used among these membrane separation devices is provided. For example, a long-term continuous operation can be realized by using another membrane separation device while performing maintenance of one membrane separation device.

[0013]

FIG. 1 shows a first embodiment of the present invention.
A description will be given based on FIG.

The apparatus shown in FIG.
A PG tank 20 and a mixer 30 are provided. LNG
The tank 10 is connected to the mixer 30 via the LNG delivery passage 12.
In the middle of the LNG delivery passage 12, an LNG delivery pump 14, a flow control valve 16, and a shutoff valve 18 are provided. The LPG tank 20 is connected to the L of the mixer 30 via the LPG delivery passage 22.
An LPG delivery pump 24 and a flow control valve 2 are connected to the PG inlet port.
6, shut-off valve 28, and membrane separation device 4 which is a feature of the present invention.
0 is provided.

The membrane separation device 40 separates a main component of LPG (for example, propane) and an impurity (for example, methanol) which solidifies under the temperature of LNG flowing into the mixer 30. Various structures such as a hollow fiber membrane module structure and a spiral type membrane module structure can be applied as long as they have such separation performance, regardless of the specific structure of the entire apparatus. As an example, an apparatus having a hollow fiber membrane module structure is shown in FIGS.

The membrane separation device 40 shown in FIG. 1A has a cylindrical main body 41, and a mixed fluid inlet 42 is provided on a side wall of the main body 41. Lids 43 and 44 are attached to both ends of the main body 41, and a cylindrical hollow fiber membrane 45 is provided between the lids 43 and 44 as shown in FIG. The internal space (i.e., the space on the outlet side of the impurities) is gasified by evacuation by a vacuum pump 48 as shown in FIG. 1 or introduction of an inert gas.

The separation membrane, which is the material of the hollow fiber membrane 45,
Any substance may be used as long as it selectively permeates or selectively removes the above impurities contained in LPG. For example, when it is desired to remove methanol as an impurity from LPG mainly containing propane, this methanol is selected. Suitable is a separation membrane that is permeable to the specific, specifically, a cellulose acetate membrane, a polyamide membrane, a polyimide membrane, a silicon membrane, or the like.

Returning to FIG. 1A, one end of the hollow fiber membrane 45 passes through the lid 43 in the axial direction, and the other end is closed by the lid 44. This lid 44 is locally attached to the LPG passage 4
An LPG outlet 47 is provided on the downstream side (right side in the figure) of the LPG passage 44a, and an impurity outlet 46 is provided on the downstream side (left side in the figure) of the lid 43. Then, of the mixed fluid (LPG containing methanol) introduced into the main body body 41 from the mixed fluid inlet 42, impurities (methanol) are gasified inside the hollow fiber membrane 45 and discharged out of the system from the impurity outlet 46. While
The main component of LPG (propane or the like) passes through the outer portion of each hollow fiber membrane 42 and the LPG passage 44a in the main body 41, and is returned to the LPG delivery passage 22 from the LPG outlet 47.

Further, in the apparatus shown in FIG.
In the delivery passage 22, the shutoff valve 28 and the membrane separation device 4
0 is provided.
A shielding valve 52 is also provided in the middle of the bypass passage 50.

Next, a method of adjusting the calorific value of LNG using this apparatus will be described.

First, on the LNG delivery passage 12 side,
With the shut-off valve 18 opened, the LNG delivery pump 14 is operated, and while the flow rate is
Is supplied with LNG.

On the other hand, on the LPG delivery passage 22 side, L
When the impurity concentration in the LPG stored in the PG tank 20 exceeds the allowable value, that is, when the impurity cooled by mixing with LNG in the mixer 30 is high enough to solidify and block the passage (for example, In propane-based LPG, the concentration of methanol, an impurity, is 50p
pm or more), the shutoff valve 52 is closed and the shutoff valve 28 is closed.
Is opened, the LNG delivery pump 24 is operated, and LPG containing impurities is supplied into the membrane separation device 40 while adjusting the flow rate with the flow rate control valve 26. The impurities separated from the LPG by the membrane separation device 40 are discharged out of the system, and the remaining LP is removed.
G is supplied to the mixer 30. Membrane separation device 4 as described above
Since the impurity concentration in the LPG is significantly reduced by the membrane separation at 0, even if this LPG is mixed with the low-temperature LNG in the mixer 30, the impurities in the LPG will not solidify and block the passage. And good driving can be continued.

Here, it is not always necessary to remove 100% of the impurities in the membrane separation apparatus 40, but it is sufficient that the impurities can be removed to such an extent that the impurity concentration is equal to or less than the allowable value. For example, when the impurity concentration in the LPG stored in the LPG tank 20 before the separation treatment is 500 ppm, if the impurity is removed by about 80 to 90% by the membrane separation device 40, the impurity concentration becomes an allowable value. 100100 ppm or less. Therefore, there is no need to use a high-performance separation membrane,
Solidification of impurities can be prevented with a cheaper and simpler structure than before.

On the other hand, when the impurity concentration in the LPG stored in the LPG tank 20 is already below the allowable value, there is no need to pass this LPG through the membrane separation device 40, so that the shielding valve 28 is closed to close the LPG. LPG may be supplied directly to the mixer 30 by opening the valve 52 and bypassing the membrane separation device 40. Thus, unnecessary deterioration of the separation membrane can be avoided, and the life of the separation membrane can be extended.

As shown in FIG. 3 as a second embodiment, if another membrane separation device 40 'and a shielding valve 28' are arranged in parallel with the membrane separation device 40 and the shielding valve 28, While the maintenance of the membrane separation device 40 is performed with the 28 closed, the shut-off valve 28 ′ is opened to open the membrane separation device 40.
By using ', continuous operation can be performed and the processing efficiency can be increased. It goes without saying that this advantage can be obtained even when there are three or more membrane separation devices.
It is also possible to arrange devices having different separation performances from each other as membrane separation devices and to select a membrane separation device to be used depending on the impurity concentration and the type of impurity.

[0026]

As described above, according to the present invention, after the impurities solidifying at the LNG temperature are removed from the LNG calorie adjusting LPG in advance by the membrane separation device, the LPG is removed from the LNG.
Since it is mixed with G, unlike the conventional method and apparatus using the LNG heating means, it is possible to prevent solidification of impurities with a simple and inexpensive structure, and it is possible to maintain good operation. is there.

The LPG transmission path includes an LNG transmission path, an LPG transmission path for transmitting LPG having a higher temperature than LNG, and a mixer connected to both paths for mixing LNG and LPG to be transmitted. In the one in which the membrane separation device is provided in the middle of the delivery passage,
A bypass passage for bypassing the membrane separation device;
According to the PG provided with the passage switching means for selectively switching between the state in which the PG passes through the membrane separation device and the state in which the PG passes through the bypass passage, the impurity concentration in the LPG is already below the allowable value before the treatment. In other words, when it is not necessary to perform membrane separation, by passing this LPG through a bypass passage, it is possible to prevent unnecessary deterioration of the separation membrane in the membrane separation apparatus and extend the life of the separation membrane. Is obtained.

Further, a plurality of membrane separation devices are arranged in parallel in the LPG delivery passage, and a device selection means for selecting a membrane separation device to be used among these membrane separation devices is provided. While performing maintenance of the membrane separator, other membrane separators can be used,
The effect of enabling continuous operation for a long period of time and increasing the processing efficiency of LNG is obtained.

[Brief description of the drawings]

FIG. 1 is a flow sheet of an LNG calorie control apparatus according to a first embodiment of the present invention.

FIG. 2A is a partial cross-sectional front view showing an example of a membrane separation device provided in the calorific value adjusting device, and FIG. 2B is a partial cross-sectional perspective view of the membrane separation device.

FIG. 3 is a flow sheet of the LNG calorie control device according to the second embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 LNG tank 12 LNG delivery path 14 LNG delivery pump 20 LPG tank 22 LPG delivery path 24 LPG delivery pump 28, 28 'Shut-off valve (path switching means and apparatus selection means) 30 Mixer 40, 40' Membrane separation device 50 Bypass passage 52 Shutoff valve (passage switching means)

Claims (4)

[Claims] 1. A method for adjusting the calorific value of a liquefied natural gas, wherein the liquefied natural gas is mixed with a liquefied petroleum gas having a higher temperature than the liquefied natural gas. A method for controlling the calorific value of a liquefied natural gas, comprising removing impurities solidified at the temperature of the liquefied natural gas with a membrane separator, and mixing the liquefied petroleum gas with the liquefied natural gas. 2. A liquefied natural gas delivery passage, a liquefied petroleum gas delivery passage for delivering liquefied petroleum gas having a higher temperature than liquefied natural gas, and a liquefied natural gas and a liquefied petroleum gas connected to and connected to both passages. A liquefied natural gas calorie control device that adjusts the calorific value of the liquefied natural gas by mixing the liquefied natural gas from the liquefied petroleum gas in the middle of the liquefied petroleum gas delivery passage. A calorie control device for liquefied natural gas, comprising a membrane separation device for removing impurities solidified at a gas temperature. 3. The liquefied natural gas calorie control apparatus according to claim 2, wherein the liquefied petroleum gas delivery passage includes a bypass passage that bypasses the membrane separation device, and a state in which the liquefied petroleum gas passes through the membrane separation device. And a passage switching means for selectively switching to a state of passing through the bypass passage. 4. The liquefied natural gas calorie control apparatus according to claim 2, wherein a plurality of membrane separation devices are arranged in parallel in the liquefied petroleum gas delivery passage, and one of these membrane separation devices is used. A liquefied natural gas calorie control device comprising a device selecting means for selecting a membrane separation device.