Patent Application
20130055757
The present disclosure relates to methods and systems for operating a natural gas liquids recovery column within a liquefied natural gas plant.
In the production of liquefied natural gas (LNG), chilled natural gas is introduced to a natural gas liquids (NGL) recovery column which serves to separate methane to be liquefied from valuable natural gas liquids, i.e., C2+ hydrocarbon components to be recovered. The natural gas liquids are removed at the NGL column bottom and are further processed in a series of fractionation columns to recover ethane products, propane products, butane products and C5+ products. The NGL column is operated at a lower temperature and higher pressure than the other, aforementioned fractionation columns. When the NGL recovery column is operating relatively close to critical conditions for the fluid at the top of the column, minor fluctuations in the system pressure can result in the formation of NGL mist or fine droplets which can become entrained with the methane leaving the top of the NGL column, a condition referred to as “carry-over.” Demisters in the form of mesh pads are commonly placed at the top of the column to capture such mist, yet such pads alone are sometimes ineffective and the carry-over problem persists. Carry-over results in difficulties in methane liquefaction operations downstream of the NGL column. For instance, the heavier components of the NGL mist may freeze out in the liquefaction operations, resulting in clogging of flow paths.
It is known to provide the NGL recovery column with reflux, in which case the light components leaving the top of the column are condensed in a heat exchanger in which latent heat is removed. The condensed liquid is then separated in a subsequent reflux drum, and returned to the top of the column where the condensed liquid is sprayed into droplets upon which mist is collected and removed from the upward flow of gas. Such reflux operations may reduce carry-over of hydrocarbon mist from the NGL recovery column, at the high energy cost involved in condensing vapor components to liquid components. Furthermore, such operations have been found not to be adequate to control carry-over of hydrocarbon mist under all conditions.
It would be desirable to have a reliable and energy-efficient method for preventing carry-over of hydrocarbon mist from an NGL recovery column in an LNG plant.
In one aspect, the invention relates to a method for preventing hydrocarbon mist from escaping the top of an NGL recovery column in an LNG plant. The method includes the steps of flowing a hydrocarbon gas stream including a hydrocarbon mist upwardly in an NGL column, spraying droplets of a hydrocarbon liquid into the gas stream to strip at least some of the hydrocarbon mist from the upwardly flowing gas stream, collecting the droplets of the hydrocarbon liquid in a tray within the column, removing a liquid sidestream of the collected hydrocarbon liquid from the column, and returning at least a portion of the liquid sidestream to form the droplets which are sprayed.
In another aspect, the invention relates to a system for preventing hydrocarbon mist from escaping the top of an NGL recovery column in an LNG plant. The system includes a column for recovering NGL. The column includes a natural gas inlet for feeding natural gas to the column, an NGL outlet at a lower portion of the column for dispensing natural gas liquids from the column, a gas outlet at an upper portion of the column for removing gaseous components from the column, at least one tray for collecting liquids within the column, a spray mechanism for generating a spray of droplets which are collected as liquids in the at least one tray, a liquid inlet for receiving and feeding liquid to the spray mechanism and a liquid outlet for receiving liquid from the at least one tray. The system further includes a loop for fluidly connecting the liquid outlet with the liquid inlet to transport a sidestream of liquid, the loop including a pump in fluid communication with the liquid outlet for receiving and pumping the at least a portion of the sidestream to the liquid inlet.
These and other objects, features and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings where:
A method is disclosed to prevent the aforementioned problem of carry-over of hydrocarbon mist from the NGL recovery column (also referred to as the NGL column) in an LNG plant, when the NGL column is operated relatively close to critical conditions. By critical conditions is meant the combination of temperature and pressure which creates a thermodynamic condition where the vapor and liquid states cannot be distinguished from each other. The method involves the addition of a wash loop at the column top, in which a suitable hydrocarbon composition is recirculated as a liquid working medium stream in a loop and sprayed at the top of the column in the form of a distributed spray to capture NGL droplets.
Referring to
A sidestream 85 of the liquid collected in the chimney tray 84 is removed from the column through an outlet in the column and heated to a temperature suitable for flashing methane from the stream, e.g. between about −40° C. and −50° C., prior to separation in a working medium holding drum 60. In the embodiment shown, the heating may occur by placing streams 85 (to be heated) and 71 (to be cooled, to be described hereinafter) in a cross exchange relation in cross exchanger 76. Flash valve 88 is used to reduce the stream pressure to facilitate separation of liquid and vapor phases. Methane preferentially accumulates in the vapor phase.
The working medium holding drum 60 functions to receive and hold working medium 55, as well as to separate gaseous components 62 from liquid working medium stream 64. The drum 60 has a liquid inlet for receiving fresh working medium 55, also referred to as makeup hydrocarbon liquid, a gaseous components outlet for releasing gaseous components 62, also referred to as a gas stream including methane, and a liquid working medium stream outlet from which liquid working medium stream 64, also referred to as a methane depleted sidestream, is removed. The working medium 55 can be added as needed (makeup) to the working medium holding drum 60 in fluid communication with the top of the column 110. In alternative embodiments, not shown, makeup hydrocarbon liquid can also be added directly to the liquid sidestream 85 between the column 110 and the working medium holding drum 60 or to the methane depleted sidestream 64. Working medium liquid 55 is added in an amount to maintain sufficient liquid in the circulating loop for spraying droplets in the column. Makeup working medium can be added to the drum 60 as needed to maintain sufficient liquid for a circulating loop, to account for losses over time. After being flashed in the drum 60, the released gaseous components 62 will be directed to other process sections of the plant or recycled to gas streams for further processing. It should be noted that the collected liquid 83 in the chimney tray 84 of the column is not necessarily the same composition as the liquid working medium 55 which is added to working medium holding drum 60, since some light components are dissolved into the working medium due to the high pressure in the column. The majority of the dissolved light components will be flashed out in the drum 60. It is also possible to change the composition of the liquid working medium 55 which is loaded to the drum 60 over time as appropriate, if the feed natural gas composition changes or the operating conditions change. The working medium 55 can be supplied from an external source independent of the LNG system, or it can be diverted from a convenient source within the system. For instance, the working medium 55 can be supplied from a point within NGL column 110 or from one of the other NGL component recovery columns.
The wash loop uses suitable hydrocarbon components or their mixtures as the liquid working medium. The working medium can be selected to be sufficiently heavy not to vaporize and be lost from the top of the column. The working medium can also be selected to have a suitable freezing point to avoid freezing within the column. Freezing temperatures at atmospheric pressure (1.013 bar) for candidate hydrocarbon components are listed in Table 1. In the table, iCn refers to iso-Cn, and nCnrefers to normal-Cn.
One of the advantages of the wash loop according to the present disclosure is that it becomes possible to independently control the chemical composition of the working medium in the loop. By having a liquid spray of the working medium in the top of the tower, it is ensured that there will always be a liquid phase present in this section of the tower. The wash loop protects the LNG refrigeration system from excessive loads since there is no phase change within the wash loop as there are in conventional reflux systems. The wash loop can save refrigeration duty when compared with a conventional design in which a reflux stream is created by increasing the column internal vapor and liquid flows. This is because the wash loop only chills a single phase flow (in which gas may be entrained), whereas the conventional design must recondense the additional vapor flow from the column top to generate the reflux stream. Thus the present system is more energy-efficient than a conventional reflux system.
Liquid stream 64 is pumped by pump 66 to a desired flow rate and a sufficient height to be supplied to the top of column 110.
Before the stream is returned to the column 110, it must be cooled. The stream is preferably cooled to a temperature substantially equivalent to the temperature within the upper portion of the NGL column. By “substantially equivalent” is meant as close as practically possible to the temperature in the upper portion of the NGL column, even within a few degrees C., and even within about 3 degree C. of the temperature in the upper portion of the NGL column. Stream 71 enters cross exchanger 76 where cooling of stream 71 occurs by exchanging heat with stream 85. Since the NGL column operates at a low temperature, generally between about −50° C. and −110° C., especially about −90° C., cooled stream 73 may require further cooling in heat exchanger 78 utilizing refrigeration from the LNG plant. Preferably, stream 75 has been cooled to a temperature between about −50° C. and −110° C. before being fed to column 110. Cooled stream 75 can then be fed to column 110 via spray mechanism 80. Stream 75 may contain up to about 3% methane. The spray mechanism can be any suitable liquid distribution mechanism as would be apparent to one skilled in the art.
A flow control system can be implemented for controlling the flow rate at which the working medium stream is returned to the column 110. The flow control system includes a controller 70 for controlling the flow rate. The controller 70 is in communication with a liquid level detector 72 for measuring the liquid level in the chimney tray 84. If the level becomes low, the control system is reset, and controller 70 operates valve 74 to resupply fluid to the desired level. Flow meter 68, also in communication with controller 70, measures the flow rate of stream 67. If the rate becomes lower or higher than control limits, the controller 70 operates valve 74 to increase or decrease the rate, respectively.
An alternative embodiment is shown in
Where permitted, all publications, patents and patent applications cited in this application are herein incorporated by reference in their entirety, to the extent such disclosure is not inconsistent with the present invention.
Unless otherwise specified, the recitation of a genus of elements, materials or other components, from which an individual component or mixture of components can be selected, is intended to include all possible sub-generic combinations of the listed components and mixtures thereof. Also, “comprise,” “include” and its variants, are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that may also be useful in the materials, compositions, methods and systems of this invention.
From the above description, those skilled in the art will perceive improvements, changes and modifications, which are intended to be covered by the appended claims.
