Method for operating distillation column

Abstract

Provided is a technique for reducing the lower-limit value for the supply flow rate of a distillation column. This method for operating a distillation column includes: a step for adjusting a fluid being treated, which is supplied to the distillation column, to a turndown-operation-time supply flow rate that is lower than a design-operation supply flow rate; a step for increasing a turndown flux flow rate and a turndown reboiler flow rate, and raising a column-internal air-liquid load, in order to make the properties of a column-bottom product flowing out from the column bottom heavier than those in the design operation while satisfying specifications required in design, or making the properties of a column-top product flowing out from the column top heavier than those in the design operation while satisfying specifications required in design, for the distillation column to which the fluid being treated is supplied at the turndown-operation-time supply flow rate; and a step for verifying that the properties of the column-top product or the column-bottom product satisfy required specifications.

Description

How to operate a distillation column

[0001] The present invention relates to a method for operating a distillation column.

[0002] Liquefied natural gas (LNG) is produced from natural gas (NG) using an LNG plant. This LNG plant serves as a gas treatment device after removing impurities from the natural gas. The LNG plant includes a distillation column (scrub column) that separates and recovers heavy hydrocarbons from the NG to obtain light hydrocarbons primarily composed of methane, and a distillation column (stabilizer) that further precisely separates the heavy hydrocarbons obtained after the light hydrocarbons have been distilled and separated in the scrub column (see, for example, FIG. 11 of Patent Document 1).

[0003] Each distillation column installed in an LNG plant has a lower limit of the feed flow rate determined for stable operation based on the mechanical structural factors of the distillation column (for example, column diameter, column height, internal structure such as trays). Therefore, if the feed flow rate of NG at start-up is further reduced below the lower limit based on the mechanical structural factors, both the light hydrocarbon component (LNG) and the heavy hydrocarbon component (condensate) will no longer meet the specifications. As a result, it was necessary to send these to the flare stack and continue to burn them until the feed flow rate reached the required minimum feed flow rate and satisfied the design requirements. For this reason, an increase in the amount of hydrocarbons burned increases the amount of CO, which is a GHG (Greenhouse Gas). ２ Furthermore, as reductions in greenhouse gas emissions are required not only for LNG plants but also for distillation columns installed in other types of gas processing equipment, equally strict regulations are being required for operation at a feed flow rate lower than the design lower limit of the distillation column.

[0004] Special Publication No. 2022-534589

[0005] The present invention has been made under such circumstances, and provides a technique for lowering the lower limit of the feed flow rate of a distillation column by manipulating the operation without changing the design of the equipment.

[0006] The present invention is a method for operating a distillation column, comprising the steps of: adjusting a feed flow rate of a fluid to be treated that is supplied to the distillation column to a feed flow rate for turndown operation that is lower than the feed flow rate for design operation; increasing the turndown reflux flow rate and turndown reboiler flow rate beyond the assumed reflux flow rate and assumed reboiler flow rate that would be assumed if the heavier product were not to be produced during the turndown operation, thereby increasing the column vapor / liquid load, for the distillation column to which the fluid to be treated is supplied at the feed flow rate for turndown operation, by making the properties of a bottom product, which is a product or intermediate product, flowing out of the bottom of the column heavier than those of the design operation while satisfying the required design specifications for the bottom product; and verifying that the properties of a top product, which is a product or intermediate product, flowing out of the top of the distillation column satisfy the required design specifications for the top product, even during the step of increasing the column vapor / liquid load. Alternatively, the present invention is a method for operating a distillation column, comprising the steps of: adjusting a feed flow rate of a fluid to be treated that is supplied to the distillation column to a feed flow rate for turndown operation that is lower than the feed flow rate for design operation; and increasing the turndown reflux flow rate and turndown reboiler flow rate beyond the assumed reflux flow rate and assumed reboiler flow rate that would be assumed if the heavier product were not to be produced during the turndown operation, thereby increasing the vapor-liquid load in the column, for the distillation column to which the fluid to be treated is supplied at the feed flow rate for turndown operation, by making the properties of an overhead product, which is a product or intermediate product, flowing out of the top of the column heavier than those in design operation while satisfying the required design specifications for the overhead product.

[0007] The method for operating a distillation column may have the following features: (a) the distillation column is provided in a gas treatment device; (b) the turndown operation is performed at the start of operation of the distillation column; or, the turndown operation is performed in response to a change in the operating conditions of equipment located upstream of the distillation column; (c) the supply flow rate during the turndown operation is a value within a range of 10 to 40% of the supply flow rate during the design operation; and (d) the supply flow rate of the treated fluid during the turndown operation is equal to or less than the lower limit of the supply flow rate determined by a structural factor corresponding to the structure of the distillation column.

[0008] In the present invention, during turndown operation, the bottom product is made heavier than in design operation, and at the same time, the overhead product is made heavier, thereby increasing the turndown reflux flow rate and the turndown reboiler flow rate compared to when the properties of the bottom product or overhead product are not adjusted. As a result, the vapor / liquid load in the column is increased, and the lower limit of the feed flow rate during turndown operation can be lowered.

[0009] 1 is a block flow diagram showing a schematic configuration of an LNG plant. FIG. 2 is a configuration diagram of heavy component recovery equipment and distillation equipment installed in the LNG plant. FIG. 3 is a flow scheme highlighting the parts where major operating conditions have been changed with a focus on the distillation equipment during turndown operation. FIG. 4 is an action diagram (tray capacity graph) showing the change in the gas-liquid load in the column before and after application of the stabilizer operation method of the present embodiment. FIG. 5 is a flow scheme highlighting the parts where major operating operations have been changed with a focus on the heavy component recovery equipment during turndown operation. FIG. 6 is a configuration diagram showing another example of heavy component recovery equipment (an example using a gas expander).

[0010] As an example of a gas processing apparatus equipped with a distillation column to which the operating method of the present invention can be applied, an LNG plant for liquefying natural gas (NG) to produce LNG will be described. Figure 1 shows a block diagram of the LNG plant. Each block is equipped with a large number of devices and equipment, including static equipment such as towers, tanks, and heat exchangers, dynamic equipment such as pumps, compressors, and turbines, and piping connecting these static and dynamic equipment.

[0011] In the LNG plant of this embodiment, NG, which is a feed fluid supplied from a wellhead, is subjected to pretreatment to remove various impurities after the liquid (condensate) contained in the NG is separated in a gas-liquid separation facility 11. The condensate separated in the gas-liquid separation facility 11 is stored in a condensate tank 18 for shipping. The NG after gas-liquid separation is subjected to acid gas removal facility 12 to remove carbon dioxide and hydrogen sulfide, which are acid gases that may solidify in the LNG during liquefaction. The acid gas removal facility 12 may be configured, for example, as an acid gas absorption facility equipped with an absorption tower that countercurrently contacts an absorbing solution that absorbs acid gases with natural gas and an absorption solution regeneration tower. Alternatively, the acid gas removal facility 12 may be configured, for example, as a DDR-type zeolite membrane made of zeolite, or as a membrane separation facility equipped with a gas separation membrane made of multiple hollow fiber membranes within its main body.

[0012] The NG from which the acid gases have been removed is further subjected to removal of moisture and mercury in the moisture and mercury removal equipment 13. For example, the moisture and mercury removal equipment 13 includes a moisture adsorption tower filled with an adsorbent that adsorbs moisture, and a mercury adsorption tower filled with a mercury adsorbent that adsorbs mercury. The order in which impurities are removed is not limited to the above example. For example, the mercury adsorption tower may be placed before the acid gas removal equipment 12.

[0013] The natural gas from which the impurities have been removed is then cooled and separated by distillation in a heavy component recovery facility 14 into light hydrocarbons mainly composed of methane (hereinafter simply referred to as "light components") and heavy hydrocarbons (hereinafter simply referred to as "heavy components") which are liquid hydrocarbon components having 5 or more carbon atoms. The detailed configuration of the heavy component recovery facility 14 will be described later.

[0014] The heavy components separated in the heavy component recovery system 14 also contain small amounts of ethane, propane, and butane. In the distillation system 16, ethane, propane, and butane are precisely separated by distillation into condensate, which is a heavy hydrocarbon of C5+ or more. Most of the light component liquids of ethane, propane, and butane separated by distillation in the distillation system 16 are returned as reflux liquid to the top of a scrub column 23 (described later) of the heavy component recovery system 14. At the top of the scrub column 23 of the heavy component recovery system 14 to which the reflux liquid of ethane, propane, and butane is returned, excess C5+ components contained in the gas composition are brought into direct contact with the supercooled reflux liquid (ethane, propane, and butane), and the heavy components of the C5+ components contained in the gas components at the top of the scrub column 23, which is mainly composed of light components, are absorbed, thereby satisfying the design requirements. The remaining mixture of ethane, propane, and butane that has been separated by distillation in the distillation equipment 16 and not returned to the heavy component recovery equipment 14 is sent to the liquefaction equipment 15 together with the light components, mainly composed of methane, that have been separated by distillation in the heavy component recovery equipment 14. On the other hand, the condensate is sent to a condensate tank.

[0015] Here, a portion of the light components separated by distillation in the distillation equipment 16, for example, propane, is stored in a refrigerant component tank 19. The propane stored in the refrigerant component tank 19 is replenished as a refrigerant component of the external refrigerant used in the external cooler 26 of the liquefaction equipment 15. The liquid transfer line from the distillation equipment 16 to the refrigerant component tank 16 and the refrigerant replenishment line from the refrigerant component tank 16 to the external refrigerant cooler 26 of the liquefaction equipment 15, which are indicated by dashed arrows in Figure 1, are not used all the time but are used as needed.

[0016] The light components from which the heavy components have been removed in the heavy component recovery facility 14, and the ethane, propane, and butane combined from the distillation facility 16 are combined and then cooled and liquefied in the liquefaction facility 15 to produce liquefied natural gas (LNG). The liquefaction facility 15 is provided with a main cryogenic heat exchanger (MCHE) for liquefying the light components using a main refrigerant (a mixed refrigerant consisting of methane, ethane, propane, butane, isopentane, nitrogen, etc.) or a nitrogen refrigerant alone. The main cryogenic heat exchanger (MCHE) may be, for example, a spiral wound or cold box type heat exchanger.

[0017] There are also designs in which a portion of the LNG is then vaporized (end flashed) to obtain cryogenic cold at a temperature of -160°C, thereby increasing LNG production. Whether or not an end flash is provided, the liquid after leaving the main cryogenic heat exchanger is sent to an LNG tank 17 for shipping.

[0018] In addition, a portion of the liquid (condensate) separated from the natural gas in the gas-liquid separation equipment 11 is processed to remove light components in a condensate stabilizer (not shown) for recovering heavy components, and then stored in a condensate tank 18 for shipping together with the condensate obtained in the distillation equipment 16.

[0019] In an LNG plant having the above-described configuration, as already described, the heavy component recovery system 14 separates natural gas by distillation into light components mainly composed of methane and heavy components having 5 or more carbon atoms. The distillation system 16 precisely separates the natural gas by distillation into light components such as ethane, propane, and butane contained in the heavy components, and a condensate, which is a heavy hydrocarbon of C5+ or more. To perform these distillation separations, the heavy component recovery system 14 and the distillation system 16 each include a distillation column (scrub column 23, stabilizer 32).

[0020] As explained in the Background Art, these distillation columns have a lower limit of the feed flow rate, which is determined by the mechanical structural factors of the distillation column. Examples of structural factors include the internal structure of the distillation column, such as the column diameter, column height, and tray structure. This embodiment provides a technology for lowering the lower limit of the feed flow rate by improving the operation method of each distillation column provided in the heavy component recovery system 14 and the distillation system 16. Before explaining the details of the improvement in the operation method, an example of the configuration of the heavy component recovery system 14 and the distillation system 16 will be explained with reference to FIG. 2.

[0021] Methods for separating methane and heavy components include an autorefrigerant separation method and a distillation separation method. The autorefrigerant separation method separates heavy components using an autorefrigerant obtained by depressurizing the NG. The distillation separation method separates heavy components using an external refrigerant such as propane refrigerant or a mixed refrigerant without depressurizing the NG. Figure 2 shows a heavy component recovery facility 14 for an autorefrigerant type heavy component recovery method.

[0022] The heavy component recovery equipment 14 includes a NG supply line 201 to which NG is supplied after being treated to remove impurities upstream, a cold box 21 for cooling the NG supplied from the NG supply line 201, a feed separator 22 for performing gas-liquid separation of the NG cooled and partially liquefied in the cold box 21, and a scrub column 23, which is a distillation column in this embodiment, for distilling the NG and separating it into light components mainly composed of methane and heavy components having 5 or more carbon atoms.

[0023] The NG supplied from the NG supply line 201 is cooled in the cold box 21 and separated into gas and liquid by the feed separator 22. The separated gas is depressurized and expanded, and then supplied as a gas-liquid mixed fluid to the scrub column 23. The liquid separated into gas and liquid by the feed separator 22 is also depressurized and expanded, and then supplied as a gas-liquid mixed fluid to the scrub column 23. The gas-liquid mixed fluid supplied to the scrub column 23 via the feed separator 22 corresponds to the fluid to be treated in the scrub column 23.

[0024] Furthermore, a portion of the NG supplied via the NG supply line 201 is extracted as stripping gas from a stripping gas line 203 before the cold box 21 and supplied to the bottom of the scrub column 23. The stripping gas plays a role in regasifying the light component gas and liquefying the heavy component gas in the self-gas by bringing the cold liquid of light components that has descended to the bottom of the scrub column 23 into direct gas-liquid contact with the self-gas (stripping gas) at a higher temperature. For this reason, the scrub column 23 may be equipped with a reboiler instead of a stripping gas supply mechanism.

[0025] The scrub column 23 distills the supplied gas-liquid mixture of NG to recover light components, mainly methane, as gas, and sends the gas to the downstream cold box 21 via a gas supply line 202 installed at the top of the column. Liquid heavy components (C5+) are separated from the bottom of the column. These heavy components are sent to the downstream distillation equipment 16 as a bottom product, which is an intermediate product extracted from the bottom of the scrub column 23.

[0026] The light components, mainly composed of methane, flowing out from the scrub column 23 are used as a refrigerant for the cold box 21, then pressurized by the LNG feed gas compressor 24, and further cooled by the LNG feed gas compressor discharge air cooler 25 and the external refrigerant cooler 26. The cooled light components are sent to the liquefaction facility 15 as an intermediate product (top product) extracted from the top of the scrub column 23.

[0027] The distillation equipment 16 includes a stabilizer 32, which is a distillation column according to the present embodiment that precisely separates the heavy components separated in the scrub column 23 by distillation into light gases containing ethane, propane, butane, and nitrogen, and a C5+ or higher condensate. The heavy components flowing out from the bottom of the scrub column 23 are heated in a stabilizer preheating heat exchanger 31 and then supplied to the stabilizer 32. The stabilizer 32 distills the heavy components supplied from the scrub column 23, separating them into light components including ethane, propane, butane, and nitrogen, and a C5+ or higher condensate. The stabilizer 32 is equipped with a stabilizer reboiler 34 that heats the column bottom liquid and returns it to the stabilizer 32.

[0028] The gas extracted from the top of the stabilizer 32 is cooled in an air-cooled (or water-cooled) stabilizer condenser 331 to become a gas-liquid mixed fluid, and the gas-liquid mixture is separated in a stabilizer reflux separator 332. The liquid extracted from the stabilizer reflux separator 332 is re-supplied as reflux to the top of the stabilizer 32 by a stabilizer reflux pump 333.

[0029] The gas extracted from the stabilizer reflux separator 332 is cooled by an external refrigerant (e.g., propane refrigerant) in an LPG production cooler 38 to become a gas-liquid mixed fluid, which is then separated into gas and liquid in an LPG separator 36. The liquid mixture of ethane, propane, and butane extracted from the LPG separator 36 is pumped by a scrubber reflux pump 37, and a portion of it is supplied to the top of the scrub column 23 via an LPG recycle line 204. The remaining ethane, propane, and butane mixed liquid that is not used as LPG recycle is extracted as an intermediate product (top product) from the top of the stabilizer 32, and is combined with the main fluid of light components, mainly composed of methane, on the scrub column 23 side at the outlet position of the LNG feed gas compressor discharge air cooler 25, and then sent to the liquefaction facility 15.

[0030] The light gas mainly composed of methane and nitrogen extracted from the LPG separator 36 is sent to a boil-off gas (BOG) compressor (not shown). The BOG compressor pressurizes the end flash gas, the boil-off gas (BOG) evaporated from LNG in the LNG tank 17, and the light gas separated into gas and liquid by the LPG separator 36, and supplies the gas as fuel gas to a fuel gas consumer such as a gas turbine or a boiler. The light gas mainly composed of methane and nitrogen extracted from the LPG separator 36 can be pressurized by a newly provided light gas booster compressor (not shown) and joined at the outlet of the LNG feed gas compressor discharge air cooler 25, instead of being sent to the BOG compressor and used as fuel gas, and can be recovered as LNG feed gas for a product.

[0031] On the other hand, the C5+ or higher condensate extracted from the bottom of the stabilizer 32 is used as a heat medium to heat the heavy components supplied to the stabilizer 32 in the heat exchanger 31, and then cooled in the stabilizer product air cooler 35. The cooled condensate is sent to the condensate tank 18 as the product (bottom product) extracted from the bottom of the stabilizer 32.

[0032] In an LNG plant equipped with the heavy component recovery equipment 14 and distillation equipment 16 configured as described above, during normal operation, NG corresponding to the amount of LNG that can be produced is supplied. Therefore, an appropriate amount of fluid to be treated corresponding to the supply flow rate of the NG is supplied to the scrub column 23 of the heavy component recovery equipment 14 and the stabilizer 32 of the distillation equipment 16 (NG after impurity removal to the scrub column 23, and heavy components after distillation separation in the scrub column 23 to the stabilizer 32).

[0033] In this application, operation under conditions specified at the time of design is called "design operation," and the NG supply flow rate in design operation is set to 100%. In this case, operation at an NG supply flow rate within the range from the lower limit determined by the mechanical structural factors of the scrub column 23 and the stabilizer 32 of the distillation equipment 16 to 100% is called "normal operation."

[0034] Generally, a distillation column for distilling a treated fluid has a defined operating range within which stable operation is possible. This operating range is determined by the mechanical structural factors of the distillation column, which are based on the diameter and height of the distillation column, as well as the internal structure of the distillation column, such as trays and packings. Limits are set within this operating range based on various factors, such as jet flood (entrainment), blowing (liquid blow-by), and the cap flow rate limit when a cap is installed on a tray. The distillation column is operated so that the liquid flow rate down each tray and the gas flow rate up each tray fall within the operating range determined at the time of design (e.g., the range shown in gray in Figures 4(a) and 4(b)).

[0035] In the heavy component recovery system 14 and the distillation system 16 of this embodiment, the scrub column 23 and the stabilizer 32 described in FIG. 2 also have defined operating ranges. These scrub column 23 and stabilizer 32 are designed so that the liquid flow rate and gas flow rate in the column are within the applicable operating range during normal operation. Therefore, even if the NG supply flow rate is reduced by, for example, 10% to accommodate temporary operational fluctuations that occur in the LNG plant, the scrub column 23 and the stabilizer 32 are designed to be able to continue stable distillation processing operation within their respective operating ranges.

[0036] On the other hand, as explained in the Background Art section, during startup, it takes time for the properties of each intermediate product and product to meet the design requirements of the scrub column 23 and stabilizer 32. During startup, the NG feed flow rate may be lower than the lower limit (turndown constraint) based on the mechanical structural factors of the scrub column 23 and stabilizer 32. Therefore, the turndown constraint becomes a bottleneck, making it difficult to meet the product specifications, and off-specification products must be continuously sent to the flare stack. In response to this issue, the distillation column operation method according to this embodiment enables further reduction of the NG feed flow rate when starting operation of the LNG plant while satisfying the turndown constraint. The specific details of the distillation column operation method according to this embodiment will be described below with reference to Figures 3 and 4 in addition to Figure 2.

[0037] In Figure 2, it is assumed that NG is supplied to the scrub column 23 via the NG supply line 201 at a supply flow rate consistent with the design operation. In the following explanation, the NG supply flow rate for this design operation is assumed to be 100%. In the design operation in which NG of a predetermined composition is supplied at a supply flow rate of 100%, the scrub column 23 and the stabilizer 32 are operated so as to satisfy the required design specifications. Here, the required design specifications include the allowable ranges for the flow rates, compositions, and distillation properties of each bottom product and each top product obtained by distillation separation in the scrub column 23 and the stabilizer 32.

[0038] Examples of operational variables for producing overhead and bottom products that satisfy the design requirements for the scrub column 23 include an operating pressure range of 20 to 50 barg and an overhead operating temperature range of −25 to −70°C. In particular, the flow rate of the LPG recycle supplied via the LPG recycle line 204 is significantly affected by the concentration of C5+ heavy components, particularly BTEX (Benzene, Toluene, Ethylbenzene, and Xylene), contained in the gas-liquid mixture (hereinafter also referred to as the “feed gas”) supplied via the feed separator 22. The flow rate of the stripping gas supplied via the stripping gas line 203 can be in the range of 3 to 15 mol% of the feed gas flow rate. The flow rates of the LPG recycle and the stripping gas significantly affect the liquid and gas flow rates in the scrub column 23.

[0039] Furthermore, examples of operational variables for producing a top product and a bottom product in the stabilizer 32 that satisfy the required design specifications include an operating pressure range of 5 to 20 barg and a top operating temperature range of 10 to 80° C. In particular, the reflux flow rate supplied from the stabilizer reflux separator 332 to the top of the stabilizer 32 and the reboiler flow rate at which the bottom liquid heated in the stabilizer reboiler 34 is returned to the stabilizer 32 have a significant effect on the liquid flow rate and gas flow rate in the stabilizer 32.

[0040] In particular, in the example shown in FIG. 2, the stabilizer 32 is normally operated so that the Reid Vapor Pressure (RVP) of the condensate in the condensate tank (a mixture of the condensate and gas separated in the gas-liquid separation equipment 11 and treated to remove light components in the stabilizer) satisfies the required design specification. Here, 9 psia (pound square inch absolute) is shown as an example of the required design specification. Generally, the RVP can be in the range of 8 to 12 psia.

[0041] Next, an operating method for turndown operation in which the NG supply flow rate is reduced below the design operation when starting up the LNG plant will be described. The NG supply flow rate is not particularly limited as long as it is in a range higher than the lower limit determined by the constraints on mechanical structural factors (turndown constraints). However, an example will be given in which turndown operation is performed at a supply flow rate lower than that range, without any equipment modifications. The supply flow rate for turndown operation can be set to a value within a range of 10 to 40% of the design flow rate. FIG. 3 shows a case in which the supply flow rate of NG during turndown operation, supplied via the NG supply line 201, is set to 20%, within a range of 10 to 40% of the design operation.

[0042] First, we will show a method for starting operation of an LNG plant and operating the NG supplied to the scrub column 23 and stabilizer 32, which are distillation columns, at a supply flow rate (in this example, 20% of the design operation supply flow rate) below the lower limit due to turndown constraints. At the start of operation of the LNG plant, the supply of NG is started, and then the supply flow rate is gradually increased. Once the supply flow rate reaches 20% of the design operation supply flow rate (hereinafter also referred to as the "design flow rate"), this supply flow rate is maintained. This operational adjustment also ensures that the supply flow rate of the heavy components supplied to the stabilizer 32 of the distillation equipment 16 is 20% of the design flow rate (a process for adjusting the supply flow rate to the turndown operation). If we assume that the reflux flow rate and reboiler flow rate for the design operation in the stabilizer 32 are 20% of the design flow rate, it can be seen that the liquid flow rate and gas flow rate in the stabilizer 32 fall outside the appropriate operating range, as shown at the operating point in the tray capacity graph in Figure 4(a).

[0043] Therefore, in this embodiment, the stabilizer 32 to which the heavy components (fluid to be treated) are supplied at the supply flow rate during the turndown operation is set so that the properties of the condensate flowing out from the column bottom are made heavier than those in the design operation while satisfying the design requirements. Specifically, the condensate is made heavier so that the RVP in the condensate tank is 8 psia or less, which is within the range of the design requirements.

[0044] The settings for heavier condensate include adjusting the NG feed rate to 20% of the design rate, followed by increasing the reflux and reboiler flow rates of the stabilizer 32. The reflux and reboiler flow rates (hereinafter referred to as the "turndown reflux flow rate and turndown reboiler flow rate") are increased from the reflux and reboiler flow rates (hereinafter referred to as the "assumed reflux flow rate and assumed reboiler flow rate") assumed when the NG feed rate is adjusted to 20% of the design rate. By making these operational adjustments, both the liquid and gas flow rates (hereinafter referred to as the "intracooler gas-liquid load") in the stabilizer 32 increase (a process for increasing the intracooler gas-liquid load). In FIG. 3, the bold lines indicate the equipment / lines whose operating conditions changed significantly as a result of increasing the turndown reflux and turndown reboiler flow rates from the assumed reflux and assumed reboiler flow rates.

[0045] FIG. 4(b) shows a tray capacity graph when the condensate RVP is lowered to 8 psia. This graph shows the operating range of the stabilizer 32 after increasing the column gas / liquid load, showing the operating points corresponding to the liquid and gas flow rates during 20% ​​turndown operation. As shown in the graph, by increasing the turndown reflux flow rate and turndown reboiler flow rate, the stabilizer 32 can be operated appropriately within the applicable operating range even during turndown operation. Thus, the operating range of the stabilizer 32 changes significantly between when the condensate RVP is 8 psia and when it is 9 psia. Therefore, FIGS. 4(a) and 4(b) illustrate a case where, without changing the mechanical design, the design requirements are met while the stabilizer 32 is operating within the applicable operating range, even though it is outside the applicable operating range at 9 psia (FIG. 4(a)). By increasing the RVP to 8 psia, the stabilizer 32 can be operated within the applicable operating range (FIG. 4(b)).

[0046] Next, even when the process of increasing the gas-liquid load in the column by increasing each flow rate up to the turndown reflux flow rate and the turndown reboiler flow rate is performed, it is necessary to verify that the properties of the overhead product flowing out from the top of the stabilizer 32 satisfy the required design specifications (a process of verifying that the overhead product satisfies the required design specifications). In order to verify that the required design specifications are satisfied, it is necessary to actually measure and analyze the flow rates and properties of the overhead product and bottom product.

[0047] In the distillation equipment 16, the mixture of ethane, propane, and butane after gas-liquid separation in the LPG separator 36 is used as reflux for the scrub column 23.

[0048] Although the operation method of the stabilizer 32 during turndown operation has been described above as an example, the same concept can also be applied to the scrub column 23 of the heavy component recovery facility 14. That is, as shown in Fig. 5 , the NG supplied to the stabilizer 32 via the NG supply line 201 is accepted in its entirety into the scrub column 23 (a step of adjusting the supply flow rate), even if the supply flow rate is less than the design supply flow rate and is below the lower limit of turndown operation determined by constraints on mechanical structural factors (20% of the design operation in this example).

[0049] Next, the properties of the heavy components (C5+) in the gas flowing out from the top of the scrub column 23 are set to be heavier than in design operation while still satisfying the design requirements. In the scrub column 23, the liquid flow rate of the LPG recycle supplied via the LPG recycle line 204 must also be increased. Therefore, if liquid is present in stream 205, which handles the excess LPG liquid flow rate, the required flow rate is supplied from the liquid in stream 205 to the scrub column 23 via the LPG recycle line 204. In addition, the supply flow rate of the stripping gas supplied via the stripping gas line 203 is increased above the expected flow rate (a step of increasing the gas-liquid load in the column). The LPG recycle flow rate and the supply flow rate of the stripping gas correspond to the "turndown reflux flow rate and turndown reboiler flow rate" when performing the step of increasing the gas-liquid load in the column in the scrub column 23. If the amount of LPG liquid is still insufficient, it is possible to ensure the reflux flow rate by externally replenishing LPG liquid mainly composed of butane (see the dashed arrow line also shown in Figure 5). Furthermore, even when the step of increasing the gas-liquid load in the column is being carried out, it is necessary to actually measure and analyze the flow rate and properties of the overhead product of the scrub column 23 in order to verify that the properties of the overhead product flowing out from the top of the scrub column 23 satisfy the required design specifications (step of verifying that the overhead product satisfies the required design specifications).

[0050] The method of operating a distillation column according to this embodiment has the following advantages. During turndown operation, the properties of the bottom products of the stabilizer 32 and scrub column 23 are made heavier than in design operation, and the turndown reflux flow rate and turndown reboiler flow rate are increased compared to when the properties of the bottom products are not adjusted. As a result, it becomes possible to increase the vapor-liquid load in the stabilizer 32 and scrub column 23. These operational adjustments enable turndown operation at a feed flow rate below the NG turndown lower limit (e.g., 20% of design operation), which previously fell outside the appropriate operating range of the stabilizer 32 and scrub column 23 and was therefore impossible to implement to meet the required design specifications.

[0051] For example, when the feed gas supply flow rate is limited to a value below the lower limit during turndown operation, which is determined by the constraints imposed by the mechanical structural factors of the scrub column 23 and the stabilizer 32, the overhead product and the bottom product are generated as off-specification products that do not meet the design requirements, and these must be combusted by flaring. This is because the CO ２ In contrast, this embodiment increases the turndown limit by increasing the amount of CO within the range that satisfies the design requirements. ２ It is possible to reduce emissions and product loss.

[0052] 3 and 5, a method has been described in which the turndown reflux flow rate and the turndown reboiler flow rate are adjusted to increase the gas-liquid load in the column so that the bottom product is heavier while satisfying the design requirements, with attention being paid to the properties of the bottom product of the stabilizer 32 and the scrub column 23. However, the target of attention to satisfy the design requirements while adjusting the turndown reflux flow rate and the turndown reboiler flow rate is not limited to the bottom product.

[0053] For example, the feed flow rates of the treated fluid to the stabilizer 32 and the scrub column 23 may be adjusted to the feed flow rates during turndown operation (a step of adjusting the feed flow rates), and then, focusing on the overhead product flowing out from the top of the column, the turndown reflux flow rate and the turndown reboiler flow rate may be increased so as to make the overhead product heavier than in design operation while satisfying the design requirements (a step of increasing the gas-liquid load in the column). Thereafter, it is verified that the properties of the bottom product flowing out from the bottom of the stabilizer 32 satisfy the design requirements (a step of verifying that the design requirements of the bottom product are satisfied).

[0054] Furthermore, the reason for performing turndown operation is not limited to reducing a large amount of hydrocarbons by burning off-specification gas in a flare stack during the period when the LNG plant starts operating and until the design requirements of the liquefaction equipment 15 are met. The distillation column operating method according to the above-described embodiment may be applied when turndown operation is unavoidable from normal operation due to a change in the operating conditions of equipment located upstream due to trouble with other equipment other than the distillation equipment 16 or the heavy component recovery equipment 14, or a decrease in the feed flow rate of raw NG from the wellhead.

[0055] The above-described operating method for increasing the turndown reflux flow rate and the turndown reboiler flow rate during turndown operation is not limited to application to the heavy components recovery system 14 configured as described with reference to FIG. 2 and the like. For example, this operating method may be applied to a heavy components recovery system 14a configured to obtain a lower temperature autogenous refrigerant using an expander 271 as shown in FIG. 6. The heavy components recovery system 14a shown in FIG. 6 is configured such that a portion of the gas after gas-liquid separation in the feed separator 22 is depressurized and expanded in the expander 271 and supplied as a low-temperature gas-liquid mixture to the scrub column 23. The remaining gas not supplied to the expander 271 is subcooled in the scrubber reflux cold box 211, then depressurized and expanded by a pressure reducing valve (not shown), and supplied as reflux liquid to the top of the scrub column 23. Also, in FIG. 6, reference numeral 28 denotes a scrubber reboiler that heats the bottom liquid of the scrub column 23, and reference numeral 272 denotes a compounder that recovers the power that rotates the expander 271 and compresses the light components.

[0056] Furthermore, this operating method is not limited to application to the stabilizer 32 and the scrub column 23. For example, instead of the stabilizer 32, a plurality of fractionation columns (deethanizer, depropanizer, debutanizer) may be provided to sequentially separate by distillation ethane, propane, and butane contained in the C5+ or heavier components. The operating method of the distillation column according to this embodiment may also be applied to these fractionation columns.

[0057] Furthermore, the plant in which the distillation column operating method of the present embodiment is installed is not limited to the example of an LNG plant (gas processing facility). The distillation column operating method of the present embodiment can also be applied to distillation columns installed in various plants, such as NGL (ethane, propane, butane) recovery units, oil refineries that perform distillation and desulfurization of various intermediate products based on crude oil, and chemical plants that produce petrochemical products, intermediate chemicals, polymers, etc. based on ethylene, etc.

[0058] 11 Gas-liquid separation equipment 12 Acid gas removal equipment 13 Water and mercury removal equipment 14, 14a Heavy component recovery equipment 15 Liquefaction equipment 16 Distillation equipment 17 LNG tank 18 Condensate tank 19 Refrigerant component tank 201 NG supply line 202 Gas supply line 203 Stripping gas line 204 LPG recycle line 21 Cold box 211 Scrubber reflux cold box 22 Feed separator 23 Scrub column 24 LNG feed gas compressor 25 LNG feed gas compressor discharge air cooler 26 External refrigerant cooler 271 Expander 272 Compounder 28 Scrubber reboiler 31 Stabilizer preheating heat exchanger 32 Stabilizer 331 Stabilizer condenser 332 Stabilizer reflux separator 333 Stabilizer reflux pump 34 Stabilizer reboiler 35 Stabilizer product air cooler 36 LPG separator 37 Scrubber reflux pump 38 LPG production cooler

Claims

1. A method for operating a distillation column comprising the steps of: adjusting a feed flow rate of a fluid to be treated that is supplied to the distillation column to a feed flow rate for turndown operation that is lower than the feed flow rate for design operation; for the distillation column to which the fluid to be treated is supplied at the feed flow rate for turndown operation, increasing the turndown reflux flow rate and the turndown reboiler flow rate above the assumed reflux flow rate and the assumed reboiler flow rate that would be assumed if the heavier product was not produced during the turndown operation by making the properties of a bottom product, which is a product or intermediate product, flowing out of the bottom of the column heavier than those of the design operation while satisfying the design requirements for the bottom product, thereby increasing the turndown reflux flow rate and the turndown reboiler flow rate above the assumed reflux flow rate and the assumed reboiler flow rate that would be assumed if the heavier product production was not performed during the turndown operation, thereby increasing the gas-liquid load in the column; and verifying that the properties of an overhead product, which is a product or intermediate product flowing out of the top of the distillation column, satisfy the design requirements for the overhead product even during the step of increasing the gas-liquid load in the column.

2. A method for operating a distillation column comprising the steps of: adjusting a feed flow rate of a fluid to be treated that is supplied to the distillation column to a feed flow rate during turndown operation that is lower than the feed flow rate for design operation; for the distillation column to which the fluid to be treated is supplied at the feed flow rate during turndown operation, increasing the turndown reflux flow rate and turndown reboiler flow rate above the assumed reflux flow rate and assumed reboiler flow rate that would be assumed if the heavier product was not produced during turndown operation by making the properties of an overhead product, which is a product or intermediate product, flowing out from the top of the distillation column heavier than those of the design operation while satisfying the design specifications for the overhead product, thereby increasing the turndown reflux flow rate and turndown reboiler flow rate beyond the assumed reflux flow rate and assumed reboiler flow rate that would be assumed if the heavier product production was not performed during the turndown operation, thereby increasing the gas-liquid load in the column; and verifying that the properties of a bottom product, which is a product or intermediate product, flowing out from the bottom of the distillation column satisfy the design specifications for the bottom product even during the step of increasing the gas-liquid load in the column.

3. The method for operating a distillation tower according to claim 1 or 2, characterized in that the distillation tower is installed in a gas treatment device.

4. The method for operating a distillation column according to claim 1 or 2, characterized in that the turndown operation is carried out at the start of operation of the distillation column.

5. A method for operating a distillation tower as described in claim 1 or 2, characterized in that the turndown operation is carried out in conjunction with a change in the operating conditions of equipment located upstream of the distillation tower.

6. A method for operating a distillation column as described in claim 1 or 2, characterized in that the supply flow rate during turndown operation is within the range of 10 to 40% of the supply flow rate during design operation.

7. A method for operating a distillation tower as described in claim 1 or 2, characterized in that the supply flow rate of the treated fluid during the turndown operation is equal to or lower than the lower limit of the supply flow rate determined by a structural factor corresponding to the structure of the distillation tower.

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