WO2022165450A1

WO2022165450A1 - Subsea dehydration of natural gas using solid desiccant

Info

Publication number: WO2022165450A1
Application number: PCT/US2022/070086
Authority: WO
Inventors: John Timothy CULLINANE; P. Scott Northrop
Original assignee: Exxonmobil Upstream Research Company
Priority date: 2021-01-28
Filing date: 2022-01-07
Publication date: 2022-08-04

Abstract

Techniques described herein relate to a subsea dehydration system including two or more solid desiccant dehydration units. Each solid desiccant dehydration unit includes solid desiccant beds arranged as parallel pipes oriented substantially horizontally and packed with solid desiccant material. The solid desiccant dehydration units are configured to perform a cyclic dehydration process in which at least one solid desiccant dehydration unit performs an adsorption function for selectively adsorbing water from a wet natural gas stream, while at least one other solid desiccant dehydration unit simultaneously undergoes a regeneration function for desorbing adsorbed water from corresponding solid desiccant beds and cooling the corresponding solid desiccant beds to a suitable temperature prior to performing the adsorption function. Moreover, the subsea dehydration system is configured to periodically switch the direction of flow such that the solid desiccant dehydration units alternate between performing the adsorption function and undergoing the regeneration function.

Description

SUBSEA DEHYDRATION OF NATURAL GAS USING SOLID DESICCANT

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority benefit of United States Provisional Patent Application No. 63/142741, filed January 28, 2021, entitled SUBSEA DEHYDRATION OF NATURAL GAS USING SOLID DESICCANT, the entirety of which is incorporated by reference herein.

FIELD

[0002] The techniques described herein relate to the oil and gas field and, more specifically, to natural gas processing within a subsea environment. More particularly, the techniques described herein relate to the subsea dehydration of natural gas using solid desiccant.

BACKGROUND

[0003] This section is intended to introduce various aspects of the art, which may be associated with embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.

[0004] During the production of hydrocarbon fluids from underground reservoirs, the produced fluids, which include primarily natural gas and oil, may also include water, both as a free liquid phase and as water vapor. When production wells are located offshore in deep water, it may be advantageous to complete the wells subsea and produce the well stream into a flowline. The well stream may be transported via flowline to shore, tied back to a host facility on the topsides, or processed subsea. However, the presence of water can result in hydrate formation, corrosion, and scaling in the flowlines, resulting in blockages, reduced production, and integrity issues. Moreover, the water vapor may condense along the flowline because of the lower ambient temperature in the subsea environment. In natural gas production, the condensation of liquid (e.g., hydrocarbon and/or water) may also increase the pressure drop because of the multiphase nature of the flow. [0005] In recent years, significant efforts have gone into developing subsea separation systems to physically separate the natural gas, oil, water, and sand that may be found in hydrocarbon production streams. Such subsea separation systems include, for example, multiline pipe separators such as harp separators. These subsea separation systems may be designed to produce single phase natural gas and oil streams that may be compressed or pumped, respectively. The separated water stream may then be injected into a disposal well, discharged, or sent to a topsides facility for further processing.

[0006] However, physical separation alone only removes free liquid water from the hydrocarbon streams. Water in the vapor phase exits the subsea separation system with the natural gas, and such water is likely to condense if the ambient temperature of the sea is lower than the dewpoint of the natural gas. Moreover, the water may form hydrates if the temperature is sufficiently low in the flowline, such as along the walls of the flowline.

[0007] Chemicals, such as methanol or glycol, are often injected into the flow to prevent or slow the formation of hydrates. Similarly, chemical corrosion inhibitors are also often injected into the flow. These chemicals add to operating costs for the overall hydrocarbon production system. To address corrosion concerns, the flowline is often designed to be cleaned and inspected by periodic “pigging”. In this case, the flowline design becomes more complex and costly due to facilities for launching the pig, catching the pig, and the like.

[0008] The removal of water from natural gas is a critical part of producing a saleable gas stream. In general, natural gas must be dehydrated down to a specified maximum water content to avoid operational issues. The maximum water content is determined such that the water vapor is removed down to a specified dewpoint so that condensation will not occur at the expected minimum temperature within the flowline/pipeline. The conventional approaches for dehydrating natural gas in onshore or topsides facilities are to contact the natural gas stream with a liquid or solid desiccant with an affinity for water. This contacting usually takes place in a pressure vessel, such as towers for absorption via liquid desiccant or vessels that hold solid desiccant. The water is removed by the liquid or solid desiccant, and the desiccant is then typically regenerated and reused. However, much of the equipment used to implement these conventional approaches is not well suited for subsea environments, where external pressures are high and the equipment must be designed to be easily retrievable. Moreover, even when such equipment is capable of being implemented within subsea environments, it is often difficult, time-consuming, and/or cost- prohibitive to do so. Accordingly, there exists a need for improved systems and methods for the subsea dehydration of natural gas.

SUMMARY

[0009] An embodiment described herein provides a subsea dehydration system. The subsea dehydration system includes at least two solid desiccant dehydration units, wherein each solid desiccant dehydration unit includes solid desiccant beds arranged as parallel pipes that are oriented substantially horizontally and packed with solid desiccant material. The at least two solid desiccant dehydration units are configured to perform a cyclic dehydration process in which at least one solid desiccant dehydration unit performs an adsorption function for selectively adsorbing water from a wet natural gas stream within corresponding solid desiccant beds, while at least one other solid desiccant dehydration unit simultaneously undergoes a regeneration function including a desorption step for desorbing adsorbed water from corresponding solid desiccant beds and a cooling step for cooling the corresponding solid desiccant beds to a suitable temperature prior to performing the adsorption function. In addition, the subsea dehydration system is configured to periodically switch a direction of flow corresponding to the at least two solid desiccant dehydration units such that the at least two solid desiccant dehydration units alternate between performing the adsorption function and undergoing the regeneration function.

[0010] In various embodiments, the subsea dehydration system is configured to operate the desorption step of the regeneration function at a relatively low temperature of 250-500 °F. In addition, in some embodiments, the regeneration function is performed using a regeneration gas stream including a slipstream of the dehydrated natural gas stream. In such embodiments, the subsea dehydration system may include a condenser to condense at least a portion of vaporized water within a spent regeneration gas stream exiting the at least one other solid desiccant dehydration unit and a separator to separate the condensed water from the cooled spent regeneration gas stream. In addition, in such embodiments, the subsea dehydration system may be configured to recombine the cooled spent regeneration gas stream with the dehydrated natural gas stream. Additionally or alternatively, in some embodiments, the subsea dehydration system may be configured to utilize the lower ambient temperatures experienced within the subsea environment of the subsea dehydration system to operate the condenser at a low temperature that is limited by the hydrate formation temperature of the spent regeneration gas stream.

[0011] In some embodiments, each of the parallel pipes comprising the solid desiccant beds is packed with multiple sections of solid desiccant material separated by support grids or screens. Furthermore, in some embodiments, the subsea dehydration system includes at least three solid desiccant dehydration units, where the at least three solid desiccant dehydration units are configured to perform a cyclic dehydration process in which at least one solid desiccant dehydration unit performs the adsorption function, at least one other solid desiccant dehydration unit undergoes the desorption step of the regeneration function, and at least one additional solid desiccant dehydration unit undergoes the cooling step of the regeneration function.

[0012] Another embodiment described herein provides a method for subsea natural gas dehydration. The method is executed by a subsea dehydration system including at least two solid desiccant dehydration units, with each solid desiccant dehydration unit including solid desiccant beds arranged as parallel pipes that are oriented substantially horizontally and packed with solid desiccant material. The method includes flowing a wet natural gas stream through at least one solid desiccant dehydration unit to perform an adsorption function in which water is selectively adsorbed from the wet natural gas stream, producing a dehydrated natural gas stream. The method also includes simultaneously flowing a regeneration gas stream through at least one other solid desiccant dehydration unit to perform a regeneration function including a desorption step in which adsorbed water is desorbed from the corresponding solid desiccant beds and a cooling step in which the corresponding solid desiccant beds are cooled to a suitable temperature prior to performing the adsorption function. The method further includes periodically switching a direction of flow such that the at least two solid desiccant dehydration units alternate between performing the adsorption function and undergoing the regeneration function.

[0013] In various embodiments, the regeneration gas stream includes a slipstream of the dehydrated natural gas stream. In such embodiments, performing the regeneration function includes flowing the regeneration gas stream through the at least one other solid desiccant dehydration unit at a relatively high temperature to desorb the adsorbed water from the corresponding solid desiccant beds, producing a spent regeneration gas stream, as well as cooling the spent regeneration gas stream to condense at least a portion of the water within the spent regeneration gas stream. In addition, in such embodiments, performing the regeneration function also includes separating the condensed water from the spent regeneration gas stream, recombining the spent regeneration gas stream with the dehydrated natural gas stream, and flowing the regeneration gas stream through the at least one other solid desiccant dehydration unit at a substantially lower temperature to cool the corresponding solid desiccant beds to the suitable temperature prior to performing the adsorption function. Furthermore, in some embodiments, the separated water is pumped back to an upstream production separator, and the separated water is then disposed of along with the water stream exiting the production separator.

[0014] In some embodiments, the method includes operating the desorption step of the regeneration function at a relatively low temperature of 250-500 °F. Furthermore, in some embodiments, the method includes performing the adsorption function within the at least one solid desiccant dehydration unit, performing the desorption step of the regeneration function within the at least one other solid desiccant dehydration unit, and performing the cooling step of the regeneration function within at least one additional solid desiccant dehydration unit. In other embodiments, the method includes performing the adsorption function within at least two solid desiccant dehydration units and performing the regeneration function within at least one additional solid desiccant dehydration unit.

[0015] In some embodiments, the method includes orienting the parallel pipes horizontally, at an incline of less than 5 degrees from horizontal, at an incline of less than 10 degrees from horizontal, or at an incline of less than 15 degrees from horizontal. In addition, in some embodiments, the method includes constructing the solid desiccant beds as parallel pipes that are oriented substantially horizontally and packed with multiple, separate sections of solid desiccant material separated by support grids or screens. Furthermore, in some embodiments, the method includes measuring an amount of water remaining within the dehydrated natural gas stream using a moisture analyzer, utilizing such measurements to anticipate moisture breakthrough conditions for the at least one desiccant dehydration unit, and effectively controlling bed cycle timing based on the anticipated moisture breakthrough conditions.

[0016] Another embodiment described herein provides a solid desiccant dehydration unit. The solid desiccant dehydration unit includes solid desiccant beds that are arranged as parallel pipes oriented substantially horizontally and packed with solid desiccant material that is capable of selectively adsorbing water from a wet natural gas stream. In various embodiments, the solid desiccant dehydration unit is installed within a subsea dehydration system including at least one other solid desiccant dehydration unit. Moreover, in some embodiments, the parallel pipes are oriented horizontally, at an incline of less than 5 degrees from horizontal, at an incline of less than 10 degrees from horizontal, or at an incline of less than 15 degrees from horizontal.

[0017] In various embodiments, each solid desiccant bed includes support grids or screens on both an upstream and a downstream side of the solid desiccant material within the corresponding pipe. Moreover, each solid desiccant bed may include multiple, separate sections of solid desiccant material separated by the support grids or screens.

DESCRIPTION OF THE DRAWINGS

[0018] Advantages of the present techniques may become apparent upon reviewing the following detailed description and drawings of non-limiting examples in which:

[0019] FIG. 1 is a schematic view of a conventional dehydration system that utilizes a liquid desiccant to dehydrate a wet natural gas stream;

[0020] FIG. 2 is a schematic view of a conventional dehydration system that utilizes a solid desiccant to dehydrate a wet natural gas stream;

[0021] FIG. 3 is a schematic view of an exemplary implementation of an improved subsea dehydration system according to embodiments described herein;

[0022] FIG. 4 is a schematic view of another exemplary implementation of an improved subsea dehydration system according to embodiments described herein; and

[0023] FIG. 5 is a process flow diagram of a method for removing water from a wet natural gas stream using a subsea dehydration system.

[0024] It should be noted that the figures are merely examples of the present techniques and are not intended to impose limitations on the scope of the present techniques. Further, the figures are generally not drawn to scale, but are drafted for purposes of convenience and clarity in illustrating various aspects of the techniques. DETAILED DESCRIPTION

[0025] In the following detailed description section, the specific examples of the present techniques are described in connection with preferred embodiments. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present techniques, this is intended to be for example purposes only and simply provides a description of the embodiments. Accordingly, the techniques are not limited to the specific embodiments described below but, rather, include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.

[0026] At the outset, and for ease of reference, certain terms used in this application and their meanings as used in this context are set forth. To the extent a term used herein is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Further, the present techniques are not limited by the usage of the terms shown below, as all equivalents, synonyms, new developments, and terms or techniques that serve the same or a similar purpose are considered to be within the scope of the present claims.

[0027] As used herein, the terms “a” and “an” mean one or more when applied to any embodiment described herein. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated.

[0028] The terms “about” and “around” mean a relative amount of a material or characteristic that is sufficient to provide the intended effect. The exact degree of deviation allowable in some cases may depend on the specific context, e.g., ±1%, ±5%, ±10%, ±15%, etc. It should be understood by those of skill in the art that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described are considered to be within the scope of the disclosure.

[0029] As used herein, the phrase “at least one,” wherein used in reference to an entity, should be understood to mean that one or more instances of the specified entity are present. More specifically, the phrase “at least one solid desiccant dehydration unit” refers to one or more solid desiccant dehydration units, while the phrase “at least one other solid desiccant dehydration unit” refers to one or more other solid desiccant dehydration units. Similarly, the phrase “at least two solid desiccant dehydration units” refers to two or more solid desiccant dehydration units.

[0030] As used herein, the term “configured” means that the element, component, or other subject matter is designed and/or intended to perform a given function. Thus, the use of the term “configured” should not be construed to mean that a given element, component, or other subject matter is simply “capable of’ performing a given function but that the element, component, and/or other subject matter is specifically selected, created, implemented, utilized, and/or designed for the purpose of performing the function.

[0031] As used herein, the term “dehydration” refers to the treatment of a natural gas stream to partially or completely remove water, while the term “dehydrated natural gas stream” (or “dry natural gas stream”) refers to a natural gas stream that has undergone a dehydration process. Typically the dehydrated natural gas stream has a water content of less than 7 Ib/MMscf for pipeline quality, and less than 1 part per million (ppm) for cryogenic processing, or a water content of less than 0.01 volume percent (vol%), preferably less than 0.001 vol%, or more preferably less than 0.0001 vol%.

[0032] As used herein, the terms “example,” exemplary,” and “embodiment,” when used with reference to one or more components, features, structures, or methods according to the present techniques, are intended to convey that the described component, feature, structure, or method is an illustrative, non-exclusive example of components, features, structures, or methods according to the present techniques. Thus, the described component, feature, structure or method is not intended to be limiting, required, or exclusive/exhaustive; and other components, features, structures, or methods, including structurally and/or functionally similar and/or equivalent components, features, structures, or methods, are also within the scope of the present techniques. [0033] The term “fluid” refers to gases, liquids, and combinations of gases and liquids, as well as to combinations of gases and solids, and combinations of liquids and solids.

[0034] The term “gas” is used interchangeably with “vapor,” and is defined as a substance or mixture of substances in the gaseous state as distinguished from the liquid or solid state. Likewise, the term “liquid” means a substance or mixture of substances in the liquid state as distinguished from the gas or solid state. [0035] A “hydrocarbon” is an organic compound that primarily includes the elements hydrogen and carbon, although nitrogen, sulfur, oxygen, metals, or any number of other elements may be present in small amounts. As used herein, hydrocarbons generally refer to components found in natural gas and oil.

[0036] The term “liquid solvent” refers to a fluid in substantially liquid phase that preferentially absorbs one component over another. Moreover, a liquid solvent that preferentially absorbs water may be specifically referred to as a “liquid desiccant.” Liquid desiccants, such as glycols, preferentially absorb water, thereby removing at least a portion of the water from a gas stream. Examples of commonly-used liquid desiccants include monoethylene glycol (MEG), diethylene glycol (DEG), and triethylene glycol (TEG).

[0037] The term “stream” indicates a material that is flowing from a first point, such as a source, to a second point, such as a device processing the stream. The stream may include any phase or material, but generally includes a gas or liquid. The stream will be conveyed in a line or pipe, and according to embodiments described herein, reference to the line or pipe may also refer to the stream the line is carrying, and vice versa.

[0038] “Natural gas” refers to a multi-component gas obtained from a crude oil well or from a subterranean gas-bearing formation. The composition and pressure of natural gas can vary significantly. A typical natural gas stream contains methane (CEL) as a major component, i.e., greater than 50 mol % of the natural gas stream is methane. The natural gas stream can also contain ethane (C2H6) and/or higher molecular weight hydrocarbons (e.g., C3-C20 hydrocarbons). The natural gas can also contain contaminants, such as acid gases (e.g., CO2 and/or EES), water, nitrogen, iron sulfide, wax, crude oil, and the like.

[0039] As used herein, the term “pipe” refers to a cylinder or tube through which a fluid can flow. It will be appreciated by one of skill in the art that, while the term “pipe” may be used herein in reference to standard pipes typically used within the oil and gas industry, in other embodiments, the term “pipe” may also refer to any other suitable types of elongated, cylindrical pressurecontaining vessels.

[0040] Substantial” when used in reference to a quantity or amount of a material, or a specific characteristic thereof, refers to an amount that is sufficient to provide an effect that the material or characteristic was intended to provide. The exact degree of deviation allowable may depend, in some cases, on the specific context.

[0041] The term “topsides facility” refers to a facility that is above a sea surface, such as a platform, a barge, an FPSO (floating production, storage, and offloading vessel), and the like. The topsides facility may also be a shore installation, for example, placed near an offshore gas or gas and oil field.

[0042] The terms “well” and “wellbore” refer to holes drilled vertically, at least in part, and may also refer to holes drilled with deviated, highly deviated, and/or lateral sections. The term also includes the wellhead equipment, surface casing string, intermediate casing string(s), production casing string, and the like, typically associated with hydrocarbon wells.

[0043] Certain aspects and features are described herein using a set of numerical upper limits and a set of numerical lower limits. It should be appreciated that ranges from any lower limit to any upper limit are contemplated unless otherwise indicated. All numerical values are “about” or “approximately” the indicated value, and account for experimental errors and variations that would be expected by a person having ordinary skill in the art.

[0044] Overview

[0045] Embodiments described herein provide an improved system and method for the subsea dehydration of natural gas using solid desiccant. More specifically, embodiments described herein provide a subsea dehydration system that includes two or more solid desiccant dehydration units that are configured to implement a cyclic dehydration process for removing water from a natural gas stream. To implement the cyclic dehydration process, at least one solid desiccant dehydration unit performs an adsorption function for the adsorption portion of the cycle, while at least one other solid desiccant dehydration unit undergoes a regeneration function for the regeneration portion of the cycle. Moreover, the solid desiccant dehydration units are configured to periodically switch functions such that the at least two solid desiccant dehydration units alternate between performing the adsorption function and undergoing the regeneration function during operation of the subsea dehydration system.

[0046] According to embodiments described herein, each solid desiccant dehydration unit includes multiple solid desiccant beds, where the solid desiccant beds are arranged as parallel pipes oriented substantially horizontally (or at a mild slope) within the solid desiccant dehydration unit. In other words, as opposed to conventional dehydration systems that include large towers with conventional adsorption beds, the subsea dehydration system described herein includes more compact units in which each solid desiccant bed is composed of a horizontally-oriented (or slightly-inclined) pipe that is packed with solid desiccant. Configuring the solid desiccant beds in this manner allows for the use of relatively small-diameter pipe rather than the thick-walled pressure vessels that are typically required to withstand the high external pressures exhibited within subsea environments. In addition, using such small-diameter pipe readily accommodates the substantially horizontal orientation of the solid desiccant beds and allows the pipes to be easily insulated for subsea service, thus preventing unwanted condensation of free water within the beds during the adsorption portion of the cycle and allowing for heat retention during the regeneration portion of the cycle. Furthermore, using such small-diameter pipe allows for the easy replacement of individual pipe segments, as will be required at the end of each bed’s useful life.

[0047] In various embodiments, the subsea dehydration system described herein utilizes a slipstream of the dehydrated natural gas stream generated during the adsorption portion of the cycle to regenerate the solid desiccant beds during the regeneration portion of the cycle. Moreover, in various embodiments, the subsea dehydration system is configured to recycle and reuse the resulting spent regeneration gas stream via gas blending with the dehydrated natural gas stream, as described further herein. Such gas blending simplifies the overall subsea dehydration system by avoiding the need for a separate compressor for the spent regeneration gas stream.

[0048] Conventional Natural Gas Dehydration Systems and Their Limitations

[0049] As described herein, the removal of water from natural gas is a critical part of producing a saleable natural gas stream. In general, a maximum water content is specified for natural gas to avoid operational issues. For the transport of natural gas in flowlines/pipelines, hazards of free water include plugging by hydrate formation or corrosion, particularly in the presence of acidic contaminants such as carbon dioxide (CO2). A common standard for the maximum water content of natural gas within continental U.S. pipelines is 7 pounds of water per million standard cubic feet (Ib-water/MMscf) of natural gas, which is equivalent to a dewpoint of approximately 35 °F (where temperature references are to 1,000 pounds per square inch absolute (psia) unless otherwise noted). For cryogenic processing, lower dewpoints are required to prevent freezing within the process equipment. Moreover, natural gas liquids (NGL) recovery processes may require a maximum water content of around 0.2 Ib/MMscf (which corresponds to a dewpoint of around -60 °F), and liquefied natural gas (LNG) production may require very low dewpoints of below -260 °F.

[0050] Various processes have been developed for dehydrating natural gas to pipeline specifications. One common method includes drying the natural gas with a liquid desiccant, such as tri ethylene glycol (TEG), as described further with respect to FIG. 1.

[0051] FIG. 1 is a schematic view of a conventional dehydration system 100 that utilizes a liquid desiccant to dehydrate a wet natural gas stream 102, producing a substantially dry natural gas stream 104. This is accomplished by flowing the wet natural gas stream 102 into a contactor 106, which removes the water from the wet natural gas stream 102. The resulting dry natural gas stream 104 is then flowed out of the contactor 106 as an overhead stream.

[0052] The wet natural gas stream 102 is generally obtained from a subsurface reservoir 108 via any suitable type of hydrocarbon recovery operation. The wet natural gas stream 102 includes one or more non-absorbing gases (i.e., primarily methane), as well as contaminants such as water, nitrogen, and acid gases (e.g., EES and CO2). The concentration of water within the wet natural gas stream 102 is generally dependent on the temperature and pressure in the subsurface reservoir 108 and will be at saturation levels for natural gas produced in the presence of water. For example, at higher temperatures, the equilibrium water content of the natural gas will be higher than at lower temperatures. Moreover, natural gas with H2S and CO2 may hold higher concentrations of water. [0053] As shown in Fig. 1, the wet natural gas stream 102 is flowed into an inlet separator 110 upon entry into the conventional dehydration system 100. When entering the inlet separator 110, the wet natural gas stream 102 may be under a large amount of pressure. However, the pressure of the wet natural gas stream 102 may vary considerably, depending on the characteristics of the subsurface reservoir 108 from which the natural gas is produced. For example, the pressure of the wet natural gas stream 102 may range between atmospheric pressure and several thousand psig. For natural gas treating applications, the pressure of the wet natural gas stream 102 may be boosted to about 100-500 psig (or greater), if desired.

[0054] The inlet separator 110 cleans the wet natural gas stream 102, for example, to prevent foaming of liquid solvent during a later acid gas treatment process. This is accomplished by separating the raw natural gas stream into liquid-phase components and gas-phase components. The liquid-phase components include heavy hydrocarbons, free liquid water, sand, and other impurities such as brine, fracturing fluids, and drilling fluids. Such components are flowed out of the inlet separator 110 via a bottoms line 114 and are typically sent to an oil recovery system 116 or other type of treater. The gas-phase components include natural gas and some amount of impurities, such as water vapor and acid gases. Such components are flowed out of the inlet separator 110 as the overhead natural gas stream 112.

[0055] From the inlet separator 110, the natural gas stream 112 is flowed into the contactor 106. The contactor 106 uses a liquid desiccant stream 118, such as a liquid glycol stream including tri ethylene glycol (TEG) or another type of glycol, to absorb water in the natural gas stream 112. The liquid desiccant stream 118 is typically stored in a lean desiccant tank 120. A high-pressure pump 122 forces the liquid desiccant stream 118 from the lean desiccant tank 120 into the contactor 106 under suitable pressure. For example, the high-pressure pump 122 may boost the pressure of the liquid desiccant stream 118 to within the range of about 500-1,200 psia, depending on the pressure of the wet natural gas stream 102.

[0056] Once inside the contactor 106, gas within the natural gas stream 112 moves upward through the contactor 106. Typically, one or more trays 124 or other internals are provided within the contactor 106 to create indirect flow paths for the natural gas stream 112 and to create interfacial area between the gas and liquid phases. At the same time, the liquid from the liquid desiccant stream 118 moves downward and across the succession of trays 124 in the contactor 106. The trays 124 aid in the interaction of the natural gas stream 112 with the liquid desiccant stream 118

[0057] The contactor 106 operates on the basis of a counter-current flow scheme. In other words, the natural gas stream 112 is directed through the contactor 106 in one direction, while the liquid desiccant stream 118 is directed through the contactor 106 in the opposite direction. As the two fluid materials interact, the down-flowing liquid desiccant stream 118 absorbs water from the up-flowing natural gas stream 112 to produce the dry natural gas stream 104.

[0058] Upon exiting the contactor 106, the dry natural gas stream 104 is flowed through an outlet separator 126. The outlet separator 126, also referred to as a scrubber, allows any liquid desiccant carried over from the contactor 106 to fall out of the dry natural gas stream 104. A final dehydrated natural gas stream 128 is flowed out of the outlet separator 126 via an overhead line 130, while any residual liquid desiccant 132 drops out through a bottoms line 134.

[0059] A spent or rich desiccant stream 136 flows out of the bottom of the contactor 106. The rich desiccant stream 136 is a glycol solution that is rich in the absorbed water. The rich desiccant stream 136 may be at a relatively low temperature, such as about 90-105 °F. In addition, the rich desiccant stream 136 may include a relatively high proportion of water, such as, for example, about 8-10 weight percent (wt%) water. Accordingly, the conventional dehydration system 100 typically includes a desiccant regeneration system for regenerating the liquid desiccant stream 118 from the rich desiccant stream 136.

[0060] Specifically, within the desiccant regeneration system, the rich desiccant stream 136 flowing out of the contactor 106 is heated within a heat exchanger 138 and then flowed into a regenerator 140 at relatively low pressures of about 10-45 psia. The regenerator 140 is a large pressure vessel, or interconnected series of pressure vessels, that is configured to regenerate the liquid desiccant stream 118 from the rich desiccant stream 136. In particular, the regenerator includes a reboiler 142 that is coupled to a distillation column 144, among other components.

[0061] First, the rich desiccant stream 136 is flowed through a tube bundle 146 in the top of the distillation column 144. High-temperature water vapor and off-gases 148 being released from the distillation column 144 preheat the rich desiccant stream 136 as it flows through the tube bundle 146, before the water vapor and off-gases 148 are released via an overhead line 150.

[0062] After being preheated within the distillation column 144, the rich desiccant stream 136 is released from the tube bundle 146 as a warmed desiccant stream 152. The warmed desiccant stream 152 is flowed into a flash drum 154, which operates at a pressure of about 50-100 psig, for example. The flash drum 154 may have internal parts that create a mixing effect or a tortuous flow path for the warmed desiccant stream 152.

[0063] Residual gases 156, such as methane, H2S, and CO2, are flashed out of the flash drum 154 via an overhead line 158. In addition, any entrained heavier hydrocarbons, such as hexane or benzene, within the warmed desiccant stream 152 are separated within the flash drum 154 as a liquid of lesser density than the desiccant stream. The resulting hydrocarbon stream 160 is then flowed out of the flash drum 154 via a bottoms line 162. [0064] Furthermore, as the temperature and pressure of the warmed desiccant stream 152 drops within the flash drum 154, the hydrocarbons within the warmed desiccant stream 152 are separated out, producing a partially-purified desiccant stream 164. The partially-purified desiccant stream 164 is then released from the flash drum 154. The partially-purified desiccant stream 164 is typically flowed through a filter 166, such as a mechanical filter or carbon filter, for particle filtration.

[0065] The resulting filtered desiccant stream 168 is flowed through a heat exchanger 170. Within the heat exchanger 170, the filtered desiccant stream 168 is heated via heat exchange with the liquid desiccant stream 118. The resulting high-temperature desiccant stream 172 is then flowed into the distillation column 144 of the regenerator 140. As the high-temperature desiccant stream 172 travels through the distillation column 144, water vapor and off-gases 148, such as H2S and CO2, are removed from the high-temperature desiccant stream 172.

[0066] The high-temperature desiccant stream 172 is then flowed out of the bottom of the distillation column 144 and into the reboiler 142. The reboiler 142 boils off residual water vapor and off-gases from the high-temperature desiccant stream 172. The components that are boiled off travels upward through the distillation column 144 and are removed as the water vapor and offgases 148 in the overhead line 150.

[0067] The regenerator 140 also typically includes a separate stripping section 174 fed from the liquid pool in the reboiler 142. The stripping section 174 includes packing that promotes further distillation. Any remaining impurities, such as water, H2S, and/or CO2, boil off and join the water vapor and off-gases 148 in the overhead line 150. The high-temperature desiccant stream 172 is then flowed into a surge tank 176, from which it is released as the regenerated liquid desiccant stream 118.

[0068] The regenerated liquid desiccant stream 118 is then pumped out of the surge tank 176 via a booster pump 178. The booster pump 178 may increase the pressure of the liquid desiccant stream 118 to about 50 psig, for example. The liquid desiccant stream 118 is flowed through the heat exchanger 170, in which the liquid desiccant stream 118 is partially cooled via heat exchange with the filtered desiccant stream 168. The liquid desiccant stream 118 may then be stored in the lean desiccant tank 120. The high-pressure pump 122 may then force the liquid desiccant stream 118 from the lean desiccant tank 120 through a cooler 180 prior to being returned to the contactor 106. The cooler 180 may cool the liquid desiccant stream 118 to ensure that the glycol will absorb the water when it is returned to the contactor 106. For example, the cooler 182 may chill the liquid desiccant stream 118 to about 100-125 °F.

[0069] While the conventional dehydration system 100 described with respect to FIG. 1 is commonly used to dehydrate natural gas, it has several serious limitations, particularly with respect to subsea applications. Specifically, flowlines installed within subsea environments experience colder ambient temperature of, for example, around 40 °F or below. As a result, natural gas streams flowing through such subsea flowlines may require a lower dewpoint than natural gas streams flowing through typical flowlines in order to avoid subsea methanol injection facilities. For example, a water content of less than 2 Ib/MMscf may be required for subsea applications. However, the conventional dehydration system 100 described with respect to FIG. 1 is only capable of drying natural gas to a water content of around 3-4 Ib/MMscf (which corresponds to a dewpoint of around 20-30 °F). In some cases, enhanced regeneration schemes are applied to lower the achievable water content to around 2 Ib/MMscf (which corresponds to a dewpoint of around 2 °F). Such enhanced regeneration schemes may include, for example, the use of vacuum stripping, stripping gas, DRIZO™ dehydration technology, Coldfinger dehydration technology, or the like. However, the use of such enhanced regeneration schemes typically requires long tiebacks to a surface regeneration facility, which can be cost-prohibitive.

[0070] Another limitation of the conventional dehydration system 100 described with respect to FIG. 1 is that the desiccant regeneration system operates at low pressure, and the liquid desiccant must be pumped back to a high pressure prior to entering the contactor 106. However, due to the relatively high pressures within subsea environments, the contactor 106 must be very thick and expensive to be able to effectively manage such pressure changes. Moreover, the rejected water vapor 148 typically contains co-absorbed hydrocarbons or inerts (i.e., the off-gases), which must be vented. Such venting typically requires mechanical pumping and/or compression, which can also be costly for subsea applications. In addition, for sour or BTEX-containing systems, such venting may require routing to a thermal oxidizer for destruction prior to venting. Furthermore, the contactor 106 is typically large and heavy, e.g., greater than 15 feet in diameter and more than 100 feet tall, and for high-pressure applications, the vessel must have thick, metal walls. Due to these limitations, it may be cost-prohibitive to implement the conventional dehydration system 100 within subsea environments.

[0071] Due to the dewpoint limitations of the conventional liquid desiccant dehydration system 100 described with respect to FIG. 1, solid desiccant dehydration systems have also been developed to achieve very low dewpoints, e.g., dewpoints of below around -260 °F. One such solid desiccant dehydration system is described with respect to FIG. 2.

[0072] FIG. 2 is a schematic view of a conventional dehydration system 200 that utilizes a solid desiccant to dehydrate a wet natural gas stream 202. Specifically, the conventional dehydration system 200 includes a two-tower configuration that performs a cyclic dehydration process for removing water from the wet natural gas stream 202. To implement the cyclic dehydration process, a first solid desiccant tower 204 performs an adsorption function for the adsorption portion of the cycle, while a second solid desiccant tower 206 performs a regeneration function for the regeneration portion of the cycle. Moreover, the two towers 204 and 206 periodically switch functions such that both the adsorption function and the regeneration function may be continuously performed within the conventional dehydration system 200.

[0073] As will be appreciated by one of skill in the art, each solid desiccant tower 204 and 206 includes solid desiccant beds 208 for adsorbing water from the wet natural gas stream 202, producing a dehydrated natural gas stream 210. Moreover, when the solid desiccant beds within the solid desiccant tower 204 or 206 that is currently performing the adsorption function become saturated with water, the solid desiccant tower 204 or 206 switches to the regeneration function, which includes both a desorption step and a cooling step. During the desorption step of the regeneration function, a regeneration gas stream 212 is flowed counter-currently through the solid desiccant tower 204 or 206 to desorb the water from the solid desiccant beds 208. Moreover, during the cooling step of the regeneration function, a cool regeneration gas is passed through the dried solid desiccant beds 208 to return the solid desiccant material to the ideal temperature for adsorption. In this manner, the solid desiccant tower 204 or 206 is prepared to perform the adsorption function again in the next cycle of the dehydration process.

[0074] In operation, the regeneration gas stream 212 is typically passed through a compressor 214 and a heater 216 prior to being flowed into the solid desiccant tower 204 or 206 that is currently performing the desorption step of the regeneration function. The compressor 214 and the heater 216 increase the pressure and temperature, respectively, of the regeneration gas stream 212 such that the solid desiccant material within the solid desiccant beds 208 can be completely regenerated. In particular, the regeneration gas stream 212 is typically heated to a temperature of around 500 °F, thus allowing very low dewpoints of around -260 °F to be achieved.

[0075] In addition, the hot, wet regeneration gas stream exiting the solid desiccant tower 204 or 206 is typically flowed through a condenser 218 that is configured to decrease the temperature of the hot, wet regeneration gas stream such that the bulk of the water within the regeneration gas stream is condensed. The resulting multiphase stream is then flowed through a separator 220 that is configured to separate the multiphase stream into a water stream 222 and a spent regeneration gas stream 224. The spent regeneration gas stream 224 is then often recycled within the conventional dehydration system 200 or flared or vented to atmosphere.

[0076] Furthermore, the conventional dehydration system 200 typically includes a controller (not shown) that periodically switches the functions of the solid desiccant towers 204 and 206 by activating and deactivating a series of three-way valves 226 that are interconnected via a series of conduits 228. The three-way valves 226 and the conduits 228 are arranged such that the solid desiccant towers 204 and 206 can alternate functions during implementation of the cyclic dehydration process.

[0077] While the conventional dehydration system 200 described with respect to FIG. 2 is commonly used to dehydrate natural gas, it has several serious limitations, particularly with respect to subsea applications. Specifically, the solid desiccant towers 204 and 206 are large and heavy, and for high-pressure applications, the towers must have thick, metal walls. In addition, the solid desiccant towers 204 and 206 are difficult to replace when the solid desiccant beds 208 within the towers reach the end of their useful life, since the individual beds cannot be easily replaced without transporting the entire towers to the topsides facility. As a result of these limitations (and others), it is difficult, costly, and time-consuming to implement such conventional dehydration systems within subsea environments. Accordingly, there exists a need for improved systems and methods for the subsea dehydration of natural gas, as provided by embodiments described herein.

[0078] Improved Subsea Natural Gas Dehydration System Described Herein

[0079] FIG. 3 is a schematic view of an exemplary implementation of an improved subsea dehydration system 300 according to embodiments described herein. Similarly to the conventional dehydration system 200 of FIG. 2, the improved subsea dehydration system 300 described herein is configured to remove water from a wet natural gas stream 302 by performing a cyclic dehydration process. However, the configuration of the improved subsea dehydration system 300 described herein varies considerably from the configuration of the conventional dehydration system 200 of FIG. 2. In particular, the improved subsea dehydration system 300 described herein includes at least two solid desiccant dehydration units, where each solid desiccant dehydration unit includes solid desiccant beds that are arranged as parallel pipes oriented substantially horizontally (or at a mild slope) within the solid desiccant dehydration unit. For example, referring to the exemplary implementation shown in FIG. 3, the improved subsea dehydration system 300 includes a first solid desiccant dehydration unit 304 and a second solid desiccant dehydration unit 306 that periodically switch between performing the adsorption function for the adsorption portion of the cycle and the regeneration function for the regeneration portion of the cycle. This is illustrated in FIG. 3, which depicts the portion of the dehydration process in which the first solid desiccant dehydration unit 304 performs the adsorption function to produce a dehydrated natural gas stream 308, while the second solid desiccant dehydration unit 306 undergoes the regeneration function in preparation for the next cycle of the dehydration process.

[0080] According to embodiments described herein, each solid desiccant dehydration unit 304 and 306 includes multiple solid desiccant beds 310A-F, where the solid desiccant beds 310A-F are configured as parallel pipes arranged substantially horizontally (or at a mild slope) within the solid desiccant dehydration units 304 and 306. For example, in various embodiments, the solid desiccant beds 310A-F are arranged horizontally, at an incline of less than 5 degrees from horizontal, at an incline of less than 10 degrees from horizontal, or at an incline of less than 15 degrees from horizontal. Moreover, each solid desiccant bed 310A-F is packed with solid desiccant material that is capable of selectively adsorbing the water from the wet natural gas stream 302. For example, the solid desiccant material may include silica gel, 3A zeolite, 4A zeolite, 5A zeolite, 13X zeolite, alumina, or any other suitable solid desiccant material or combination of solid desiccant materials. In addition, acid resistant versions of the aforementioned solid desiccant materials may also be used. Furthermore, the solid desiccant material may be packed into the pipes in any suitable manner to form the solid desiccant beds 310A-F. For example, in some embodiments, the solid desiccant material is arranged as solid pellets or other extruded shapes within the pipes. In addition, due to the substantially horizontal orientation of the solid desiccant beds 310A-F, support grids or screens may be included on both the upstream and downstream side of the solid desiccant material within the pipes comprising the solid desiccant beds 310A-F. Such supports grids/screens prevent the solid desiccant material from moving around or settling into the top or bottom of the pipes.

[0081] In various embodiments, constructing the solid desiccant beds 310A-F out of such horizontally-oriented (or slightly-inclined) pipes (as opposed to large, heavy towers or columns) allows for the use of relatively small-diameter pipe rather than the thick-walled pressure vessels that are typically required to withstand the high external pressures exhibited within subsea environments. Specifically, in some embodiments, the solid desiccant beds 310A-F are constructed from pipe that has an inner diameter within the range of, for example, around 12 to 60 inches, or around 24 to 48 inches, as well as a wall thickness of around 0.25 to 1.0 inch, although the inner diameter and thickness of the pipe may vary depending on the details of the specific implementation. Moreover, in some embodiments, the pipe corresponding to each solid desiccant bed 310A-F has a length of, for example, around 2 to 20 feet, although the length of each pipe may also vary depending on the details of the specific implementation. In general, the construction of the solid desiccant beds 310A-F from such relatively-small diameter pipe according to embodiments described herein provides considerably flexibility in terms of the overall size and configuration of the solid desiccant dehydration units 304 and 306.

[0082] Furthermore, in various embodiments, using such small-diameter pipe readily accommodates the substantially horizontal orientation of the solid desiccant beds 310A-F and allows the pipes to be easily insulated for subsea service, thus preventing unwanted condensation of free water within the solid desiccant beds 310A-F during the adsorption portion of the cycle and allowing for heat retention during the regeneration portion of the cycle. Furthermore, using such small-diameter pipe allows for the easy replacement of individual pipe segments, as will be required at the end of each bed’s useful life.

[0083] According to the exemplary implementation shown in FIG. 3, the first solid desiccant dehydration unit 304 performs the adsorption function, in which the wet natural gas stream 302 is concurrently flowed through the solid desiccant beds 310A-C within the first solid desiccant dehydration unit 304. As the wet natural gas stream 302 flows through the solid desiccant beds 310A-C, the water is selectively adsorbed by the solid desiccant material within the solid desiccant beds 310A-C, producing the dehydrated natural gas stream 308. Meanwhile, the second solid desiccant dehydration unit 306 undergoes the regeneration function, which includes both a desorption step and a cooling step. During the desorption step of the regeneration function, a high- temperature regeneration gas stream is flowed counter-currently through the second solid desiccant dehydration unit 306 to desorb the water from the solid desiccant beds 310D-F. More specifically, the regeneration gas stream is concurrently flowed through the solid desiccant beds 310D-F within the second solid desiccant dehydration unit 306 in a counter-current direction at a comparatively low flow rate (e.g., around 7-10% of the adsorption flow rate) such that the water is desorbed from the solid desiccant material, producing a hot, wet regeneration gas stream that is flowed out of the top of the second solid desiccant dehydration unit 306. In this manner, the solid desiccant material within the second solid desiccant dehydration unit 306 is prepared to perform the adsorption function again in the next cycle of the dehydration process.

[0084] In some embodiments, the solid desiccant beds 310A-C and 310D-F within the respective solid desiccant dehydration units 304 and 306 are manifolded or otherwise configured such that the flow rate of the gas stream flowing through each solid desiccant dehydration unit 304 and 306 remains substantially constant throughout the corresponding solid desiccants beds 310A- C and 310D-F. This is due to the fact that large flow rate differences between the solid desiccant beds 310A-C or 310D-F within a particular solid desiccant dehydration unit 304 or 306 may result in some of the beds becoming saturated more quickly than others. This, in turn, may result in water breakthrough of the product gas or other similar negative operating conditions.

[0085] In various embodiments, the regeneration gas stream is a slipstream of the dehydrated natural gas stream 308, as shown in FIG. 3. However, it will be appreciated by one of skill in the art that the regeneration gas stream can be obtained from a variety of sources. For example, in some embodiments, the regeneration gas stream is an independent stripping gas, such as nitrogen. [0086] In addition, during the desorption step of the regeneration function, the regeneration gas stream is heated to a suitable temperature to provide for the complete regeneration of the solid desiccant material within the solid desiccant beds 310D-F. In various embodiments, this is accomplished using a heater 312, as shown in FIG. 3. According to embodiments described herein, because a low dewpoint is not required, a lower temperature regeneration cycle can be implemented within the subsea dehydration system 300. For example, in various embodiments, the regeneration gas stream is only heated to a temperature of around 250 to 500 °F, depending on the details of the specific implementation. This provides several advantages over conventional dehydration systems, such as the conventional dehydration system 200 of FIG. 2, which typically require a regeneration temperature of around 500 °F or above. Specifically, a lower temperature regeneration cycle reduces the degradation of the solid desiccant material through hydrothermal aging, thus prolonging the useful life of the solid desiccant beds 310A-F. This is particularly important for subsea applications, for which equipment maintenance and replacement operations are relatively expensive. In addition, a lower temperature regeneration cycle reduces the amount and quality of energy required for the regeneration cycle, thus reducing capital expenditures for the overall hydrocarbon recovery process.

[0087] While only the heater 312 is shown in FIG. 3, it will be appreciated by one of skill in the art that the equipment requirements will vary depending on details of the specific implementation and the type of regeneration gas that is used. For example, in some embodiments, a gas blower/compressor (not shown) is also included within the subsea dehydration system 300 to increase the pressure of the regeneration gas stream to a suitable level. Furthermore, it will be appreciated by one of skill in the art that, while FIG. 3 depicts a counter-current flow scheme for the regeneration gas stream, the subsea dehydration system 300 may alternatively be arranged with a co-current flow scheme.

[0088] In various embodiments, the hot, wet regeneration gas stream exiting the second solid desiccant dehydration unit 306 is flowed through a condenser 314 that is configured to decrease the temperature of the hot, wet regeneration gas stream such that the bulk of the water within the regeneration gas stream is condensed. Here, the cold ambient conditions experienced in subsea environments can be used to advantage by using the cold water surrounding the subsea dehydration system 300 to chill the wet regeneration gas stream within the condenser 314. The temperature in this case is limited by the approach temperature of the condenser 314 (e.g., a minimum temperature of around 55 °F for a 40 °F subsea temperature), as well as by the hydrate formation temperature of the regeneration gas stream.

[0089] The resulting multiphase stream is then flowed through a separator 316 that is configured to separate the multiphase stream into a water stream 318 and a spent regeneration gas stream 320. In various embodiments, the separator 316 employs a simple phase separation process to separate the liquid water from the cooled regeneration gas stream. Moreover, according to embodiments described herein, the resulting water stream 318 may be pumped back to an upstream production separator (not shown), where the term “production separator” refers to a separation device that separates a three-phase production fluid into oil, water, and gas streams. In general, the oil stream is flowed back to the surface, while the gas stream (e.g., the wet natural gas stream 302) is dehydrated according to embodiments described herein. The water stream, however, is often pumped back downhole or otherwise disposed of within the subsea environment. Accordingly, in various embodiments, the water stream 318 from the separator 316 may be pumped back to the production separator and disposed of along with the water stream exiting the production separator. This may further simplify the overall gas processing system 300 by avoiding the need to separately dispose of the water stream 318 from the separator 316.

[0090] In various embodiments, gas blending is used to dispose of the spent regeneration gas stream 320. More specifically, the spent regeneration gas stream 320 is blended with the dehydrated natural gas stream 308, as shown in FIG. 3. Because the cyclic dehydration process provided by the subsea dehydration system 300 produces a dehydrated natural gas stream that is drier than necessary for dewpoint specifications, some moisture can be reintroduced into the natural gas stream without negatively impacting the results of the dehydration process. Table 1 shows the impact of blending the spent regeneration gas stream 320 with the dehydrated natural gas stream 308 at the projected process conditions.

Table 1 : Approximate Blended Gas Water Content Scenarios

[0091] As evidenced by Table 1, gas blending can be effectively implemented while still providing a natural gas stream that meets dewpoint specifications. Moreover, if high pressure is used for the regeneration portion of the cycle, the spent regeneration gas stream 320 will at a pressure of only around 20 psi less than the dehydrated natural gas stream 308. Therefore, the pressure of the dehydrated natural gas stream 308 can be easily lowered by this small amount to allow effective blending with the spent regeneration gas stream 320. This avoids mechanical compression of the spent regeneration gas stream 320 and allows for reclamation of the regeneration gas volume for sales.

[0092] During the cooling step of the regeneration function, a cool regeneration gas stream (e.g., a slipstream of the dehydrated natural gas stream 308) is then passed through the dried solid desiccant beds 310D-F to return the solid desiccant material to the ideal temperature for adsorption. In various embodiments, this is accomplished by turning off the heater 312. The cool regeneration gas stream is then flowed through the hot, dried solid desiccant beds 310D-F to cool them in preparation for the adsorption portion of the next cycle. Moreover, in various embodiments, the spent regeneration gas stream from the cooling step of the regeneration function is then blended with the dehydrated natural gas stream 308, as described above with respect to the spent regeneration gas stream from the desorption step of the regeneration function. [0093] Furthermore, once the solid desiccant beds 310A-C within the first solid desiccant dehydration unit 304 become saturated with water, the flow though the subsea dehydration system 300 is reversed, as described herein, such that the first solid desiccant dehydration unit 304 is regenerated while the second solid desiccant dehydration unit 306 adsorbs water from the wet natural gas stream 302. Moreover, it should be understood that, while the adsorption and regeneration functions are both generally performed continuously during the operation of the subsea dehydration system 300, the first solid desiccant dehydration unit 304 and/or the second solid desiccant dehydration unit 306 may sometimes enter a standby mode. For example, when the solid desiccant dehydration unit undergoing the regeneration function is completely cooled, it may then enter a standby mode until the other solid desiccant dehydration unit completes the adsorption function.

[0094] As will be appreciated by one of skill in the art, the subsea dehydration system 300 also includes a controller (not shown) that periodically switches the functions of the two solid desiccant dehydration units 304 and 306 by activating and deactivating a series of three-way valves 322 (and/or other types of valves) that are interconnected via a series of conduits 324 (e.g. pipes or flowlines). The three-way valves 322 and the conduits 324 are arranged such that the two solid desiccant dehydration units 304 and 306 can seamlessly alternate functions during implementation of the cyclic dehydration process.

[0095] In various embodiments, the controller is configured to change the operating parameters of the equipment within the subsea dehydration system 300, such as in response to a command received from a computing system located at the topsides facility. For example, in some embodiments, the controller adjusts the operating temperature for the regeneration portion of the dehydration process by turning on or off the heater 312 as appropriate. As another example, in some embodiments, the controller adjusts the regeneration gas flow rate. This may be particularly useful for instances in which the subsea dehydration system 300 is servicing multiple wells that are producing natural gas streams with varying water concentrations and, thus, a steady regeneration gas flow rate is not highly effective.

[0096] Furthermore, in some embodiments, the subsea dehydration system 300 includes a moisture analyzer 326 that is configured to measure the amount of water remaining within the dehydrated natural gas stream 308. The controller (optionally with input from the computing system located at the topsides facility) may then utilize such measurements to anticipate moisture breakthrough conditions and, thus, effectively control bed cycle timing. In such embodiments, the overall number of cycles may be reduced, resulting in significant energy savings and extending the useful life of the solid desiccant beds 310A-F.

[0097] The schematic view of FIG. 3 is not intended to indicate that the subsea dehydration system 300 is to include all of the components shown in FIG. 3, or that the subsea dehydration system 300 is limited to only the components shown in FIG. 3. Rather, any number of components may be omitted from the subsea dehydration system 300 or added to the subsea dehydration system 300, depending on the details of the specific implementation. Moreover, the subsea dehydration system 300 may be arranged into any number of alternative configurations without changing the overall technical effect of the subsea dehydration system 300. For example, while only two solid desiccant dehydration units 304 and 306 are depicted in FIG. 3, a person of skill in the art will appreciate that one or more additional solid desiccant dehydration units may be included within the subsea dehydration system 300 described herein. For example, in some embodiments, a three- unit cycle or four-unit cycle is used to perform the subsea dehydration process. An exemplary implementation of such a three-unit cycle is described with respect to FIG. 4. Furthermore, while FIG. 3 depicts three solid desiccant beds 310A-C and 310D-F within each solid desiccant dehydration unit 304 and 306, respectively, it will be appreciated by one of skill in the art that any number of solid desiccant beds may be included within each solid desiccant dehydration unit, depending on the details of the specific implementation.

[0098] FIG. 4 is a schematic view of another exemplary implementation of an improved subsea dehydration system 400 according to embodiments described herein. Like numbered items are as described with respect to FIG. 3. The subsea dehydration system 400 of FIG. 4 is similar to the subsea dehydration system 300 of FIG. 3. However, the subsea dehydration system 400 of FIG. 4 includes a three-unit configuration in which the first solid desiccant dehydration unit 304, the second solid desiccant dehydration unit 306, and a third solid desiccant dehydration unit 402 alternate between performing the adsorption function for the adsorption portion of the cycle, the desorption step of the regeneration function for the regeneration portion of the cycle, and the cooling step of the regeneration function for the regeneration portion of the cycle. In some embodiments, separating the desorption and cooling steps of the regeneration function in this manner may reduce the length of time between cycles and, thus, increase the efficiency of the overall dehydration process.

[0099] The schematic view of FIG. 4 is not intended to indicate that the subsea dehydration system 400 is to include all of the components shown in FIG. 4, or that the subsea dehydration system 400 is limited to only the components shown in FIG. 4. Rather, any number of components may be omitted from the subsea dehydration system 400 or added to the subsea dehydration system 400, depending on the details of the specific implementation. Moreover, the subsea dehydration system 400 may be arranged into any number of alternative configurations without changing the overall technical effect of the subsea dehydration system 400. For example, while FIG. 4 depicts three solid desiccant beds 310A-C, 310D-F, and 310G-I within each solid desiccant dehydration unit 304, 306, and 402, respectively, it will be appreciated by one of skill in the art that any number of solid desiccant beds may be included within each solid desiccant dehydration unit, depending on the details of the specific implementation.

[0100] Furthermore, in some embodiments, the three-unit configuration of FIG. 4 includes two solid desiccant dehydration units that perform the adsorption function, while one solid desiccant dehydration unit undergoes both the heating step and cooling step of the regeneration function. The three solid desiccant dehydration units then alternate functions as appropriate. This embodiment provides the significant advantage of being able to process a larger amount of natural gas at one time.

[0101] In various embodiments, the subsea dehydration system described herein also includes an inlet separator that is configured for bulk liquid removal upstream of the solid desiccant dehydration units. In such embodiments, the cold ambient conditions experienced in subsea environments can be used to advantage by using the cold water surrounding the subsea dehydration system to chill the wet natural gas stream and, thus, condense a large amount of the liquid within the wet natural gas s stream. Again, the temperature in this case is limited by the approach temperature of the inlet separator, as well as by the hydrate formation temperature of the wet natural gas stream. In general, reducing the temperature of the wet natural gas stream by around 25 °F results in the condensation and removal of approximately half of the stream’s water content. In such embodiments, performing this additional step minimizes the amount of solid desiccant material that is needed for the dehydration process, allows the solid desiccant beds to be constructed out of smaller-length sections of pipe, and reduces the chance of free water condensing out in the solid desiccant dehydration units in the case of an unplanned shutdown.

[0102] Moreover, because the solid desiccant beds described herein are constructed from relatively-thin-diameter pipe, the solid desiccant beds are longer than solid desiccant beds within conventional dehydration systems. However, while conventional applications have bed height limitations due to mechanical considerations, the substantially horizontal orientation of the solid desiccant beds described herein allows for longer beds to be easily utilized. The only downside is that higher-than-normal pressure drops may occur for longer beds, and such high pressure drops can crush or attrit the sieve material. Accordingly, in some embodiments, each solid desiccant dehydration unit includes split sold desiccant beds to prevent the pressure drop from damaging the solid desiccant material. In other words, each solid desiccant bed may include multiple, separate sections of solid desiccant material separated by support grids or screens, rather than only one section as shown in FIGS. 3 and 4. For example, in some embodiments, each solid desiccant bed (or pipe) includes around 2 to 4 separate sections of solid desiccant material, depending on the details of the specific implementation.

[0103] Furthermore, in some embodiments, the subsea dehydration system described herein is configured as a modular unit that can be easily installed/uninstalled within subsea environments and transported between different gas production facilities. Options for configuring such a modular unit are well-known to those of skill in the art.

[0104] Improved Method for Subsea Dehydration of Natural Gas by Solid Desiccant

[0105] FIG. 5 is a process flow diagram of a method 500 for removing water from a wet natural gas stream using a subsea dehydration system. The method 500 is executed by the subsea dehydration system described herein, such as the subsea dehydration system 300 described with respect to FIG. 3, the subsea dehydration system 400 described with respect to FIG. 4, or any suitable variations thereof. In particular, the method is executed by a subsea dehydration system including at least two solid desiccant dehydration units, wherein each solid desiccant dehydration unit includes solid desiccant beds arranged as parallel pipes that are oriented substantially horizontally and packed with solid desiccant material.

[0106] The method 500 begins at block 502, at which a wet natural gas stream is flowed through at least one solid desiccant dehydration unit to perform an adsorption function in which water is selectively adsorbed from the wet natural gas stream, producing a dehydrated natural gas stream. At block 504, a regeneration gas stream is simultaneously flowed through at least one other solid desiccant dehydration unit to perform a regeneration function including a desorption step in which adsorbed water is desorbed from the corresponding solid desiccant beds and a cooling step in which the corresponding solid desiccant beds are cooled to a suitable temperature prior to performing the adsorption function.

[0107] In some embodiments, the regeneration gas stream includes a slipstream of the dehydrated natural gas stream. In such embodiments, performing the desorption step of the regeneration function may include: (1) flowing the regeneration gas stream through the at least one other solid desiccant dehydration unit at a relatively high temperature to desorb the adsorbed water from the corresponding solid desiccant beds, producing a spent regeneration gas stream; (2) cooling the spent regeneration gas stream to condense at least a portion of the vaporized water within the spent regeneration gas stream (e.g., by utilizing the lower ambient temperatures experienced within the subsea environment of the subsea dehydration system); (3) separating the condensed water from the spent regeneration gas stream; and (4) optionally recombining the spent regeneration gas stream with the dehydrated natural gas stream. In addition, performing the cooling step of the regeneration function may include flowing the regeneration gas stream through the at least one other solid desiccant dehydration unit at a substantially lower temperature to cool the corresponding solid desiccant beds to the suitable temperature prior to performing the adsorption function. Moreover, the regeneration gas stream may then be dried and recombined with the dehydrated natural gas stream, as described above. In this manner, the regeneration gas stream is continuously (or, optionally, intermittently) recycled for reuse within the subsea dehydration system and/or recovered for sales. Furthermore, in various embodiments, the water separated from the spent regeneration gas stream is pumped back to the upstream production separator and disposed of along with the water exiting the production separator.

[0108] In various embodiments, the desorption step of the regeneration function is operated at a relatively low temperature of 250-500 °F, which is substantially lower than the regeneration temperature for conventional dehydration systems. In addition, in various embodiments, the parallel pipes corresponding to the solid desiccant beds are oriented horizontally, at an incline of less than 5 degrees from horizontal, at an incline of less than 10 degrees from horizontal, or at an incline of less than 15 degrees from horizontal. In addition, in various embodiments, each parallel pipe includes an inner diameter of 12-60 inches, a wall thickness of 0.25-1.0 inch, and a length of 2-20 feet, and is packed with multiple, separate sections of solid desiccant material separated by separate grids or screens. However, it will be appreciated by one of skill in the art that the orientation, dimensions, and configuration of the parallel pipes are susceptible to any number of modifications or variations without changing the overall technical effect of the corresponding solid desiccant beds.

[0109] At block 506, the direction of flow is periodically switched such that the solid desiccant dehydration units alternate between performing the adsorption function and undergoing the regeneration function. In various embodiments, the timing for switching the functions of the solid desiccant dehydration units is determined by the controller corresponding to the subsea dehydration system. In some embodiments, the method also includes measuring an amount of water remaining within the dehydrated natural gas stream using a moisture analyzer, as well as allowing the controller to utilize such measurements to anticipate moisture breakthrough conditions for the solid desiccant dehydration unit that is currently performing the adsorption function and, thus, effectively control the bed cycle timing for the solid desiccant dehydration units.

[0110] The process flow diagram of FIG. 5 is not intended to indicate that the steps of the method 500 are to be executed in any particular order, or that all of the steps of the method 500 are to be included in every case. Moreover, any number of additional steps not shown in FIG. 5 may be included within the method 500, depending on the details of the specific implementation. For example, in some embodiments, the method 500 also includes flowing the wet natural gas stream through an inlet separator to provide for bulk liquid removal prior to flowing the wet natural gas stream through the at least one solid desiccant dehydration unit. In addition, in some embodiments, the method 500 includes performing the adsorption function within the at least one solid desiccant dehydration unit, performing the desorption step of the regeneration function within the at least one other solid desiccant dehydration unit, and performing the cooling step of the regeneration function within at least one additional solid desiccant dehydration unit. In other embodiments, the method 500 includes performing the adsorption function within at least two solid desiccant dehydration units and performing the regeneration function within at least one additional solid desiccant dehydration unit. Moreover, one of skill in the art will appreciate that the method 500 is susceptible to any number of other modifications or variations without changing the overall technical effect of the method 500.

[oni] While the embodiments described herein are well-calculated to achieve the advantages set forth, it will be appreciated that such embodiments are susceptible to modification, variation, and change without departing from the spirit thereof. Indeed, the present techniques include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.

Claims

CLAIMS What is claimed is:

1. A subsea dehydration system, comprising at least two solid desiccant dehydration units, wherein each solid desiccant dehydration unit comprises solid desiccant beds arranged as parallel pipes that are oriented substantially horizontally and packed with solid desiccant material, and wherein the at least two solid desiccant dehydration units are configured to perform a cyclic dehydration process in which at least one solid desiccant dehydration unit performs an adsorption function for selectively adsorbing water from a wet natural gas stream within corresponding solid desiccant beds to produce a dehydrated natural gas stream, while at least one other solid desiccant dehydration unit simultaneously undergoes a regeneration function comprising a desorption step for desorbing adsorbed water from corresponding solid desiccant beds and a cooling step for cooling the corresponding solid desiccant beds to a suitable temperature prior to performing the adsorption function; and wherein the subsea dehydration system is configured to periodically switch a direction of flow corresponding to the at least two solid desiccant dehydration units such that the at least two solid desiccant dehydration units alternate between performing the adsorption function and undergoing the regeneration function.

2. The subsea dehydration system of claim 1, wherein the subsea dehydration system is configured to operate the desorption step of the regeneration function at a relatively low temperature of 250-500 °F.

3. The subsea dehydration system of claim 1 or 2, wherein the regeneration function is performed using a regeneration gas stream comprising a slipstream of the dehydrated natural gas stream, and wherein the subsea dehydration system comprises: a condenser to condense at least a portion of vaporized water within a spent regeneration gas stream exiting the at least one other solid desiccant dehydration unit; and a separator to separate the condensed water from a resulting cooled spent regeneration gas stream; and

- 32 - wherein the subsea dehydration system is configured to recombine the cooled spent regeneration gas stream with the dehydrated natural gas stream.

4. The subsea dehydration system of any of claims 1 to 3, wherein the subsea dehydration system comprises a condenser to condense at least a portion of vaporized water within a spent regeneration gas stream exiting the at least one other solid desiccant dehydration unit, and wherein the subsea dehydration system is configured to utilize lower ambient temperatures experienced within a subsea environment of the subsea dehydration system to operate the condenser at a low temperature that is limited by a hydrate formation temperature of the spent regeneration gas stream.

5. The subsea dehydration system of any of claims 1 to 4, wherein each of the parallel pipes is packed with multiple sections of solid desiccant material separated by support grids or screens.

6. The subsea dehydration system of any of claims 1 to 5, wherein the subsea dehydration system comprises at least three solid desiccant dehydration units, and wherein the at least three solid desiccant dehydration units are configured to perform a cyclic dehydration process in which at least one solid desiccant dehydration unit performs the adsorption function, at least one other solid desiccant dehydration unit undergoes the desorption step of the regeneration function, and at least one additional solid desiccant dehydration unit undergoes the cooling step of the regeneration function.

7. A method for subsea natural gas dehydration, wherein the method is executed by a subsea dehydration system comprising at least two solid desiccant dehydration units, with each solid desiccant dehydration unit comprising solid desiccant beds arranged as parallel pipes that are oriented substantially horizontally and packed with solid desiccant material, and wherein the method comprises:

- 33 - flowing a wet natural gas stream through at least one solid desiccant dehydration unit to perform an adsorption function in which water is selectively adsorbed from the wet natural gas stream, producing a dehydrated natural gas stream; simultaneously flowing a regeneration gas stream through at least one other solid desiccant dehydration unit to perform a regeneration function comprising a desorption step in which adsorbed water is desorbed from the corresponding solid desiccant beds and a cooling step in which the corresponding solid desiccant beds are cooled to a suitable temperature prior to performing the adsorption function; and periodically switching a direction of flow such that the at least two solid desiccant dehydration units alternate between performing the adsorption function and undergoing the regeneration function.

8. The method of claim 7, wherein the regeneration gas stream comprises a slipstream of the dehydrated natural gas stream, and wherein performing the regeneration function comprises: flowing the regeneration gas stream through the at least one other solid desiccant dehydration unit at a relatively high temperature to desorb the adsorbed water from the corresponding solid desiccant beds, producing a spent regeneration gas stream; cooling the spent regeneration gas stream to condense at least a portion of the water within the spent regeneration gas stream; separating the condensed water from a resulting cooled spent regeneration gas stream; recombining the cooled spent regeneration gas stream with the dehydrated natural gas stream; and flowing the regeneration gas stream through the at least one other solid desiccant dehydration unit at a substantially lower temperature to cool the corresponding solid desiccant beds to the suitable temperature prior to performing the adsorption function.

9. The method of claim 8, comprising: pumping the separated water back to an upstream production separator; and disposing of the separated water along with a water stream exiting the upstream production separator.

10. The method of any of claims 7 to 9, comprising operating the desorption step of the regeneration function at a relatively low temperature of 250-500 °F.

11. The method of any of claims 7 to 10, comprising utilizing lower ambient temperatures experienced within a subsea environment of the subsea dehydration system to cool the spent regeneration gas stream.

12. The method of any of claims 7 to 11, comprising: performing the adsorption function within the at least one solid desiccant dehydration unit; performing the desorption step of the regeneration function within the at least one other solid desiccant dehydration unit; and performing the cooling step of the regeneration function within at least one additional solid desiccant dehydration unit.

13. The method of any of claims 7 to 11, comprising: performing the adsorption function within at least two solid desiccant dehydration units; and performing the regeneration function within at least one additional solid desiccant dehydration unit.

14. The method of any of claims 7 to 13, comprising orienting the parallel pipes horizontally, at an incline of less than 5 degrees from horizontal, at an incline of less than 10 degrees from horizontal, or at an incline of less than 15 degrees from horizontal.

15. The method of any of claims 7 to 14, comprising constructing the solid desiccant beds as parallel pipes that are oriented substantially horizontally and packed with multiple, separate sections of solid desiccant material separated by support grids or screens.

16. The method of any of claims 7 to 15, comprising: measuring an amount of water remaining within the dehydrated natural gas stream using a moisture analyzer; utilizing such measurements to anticipate moisture breakthrough conditions for the at least one solid desiccant dehydration unit; and effectively controlling bed cycle timing based on the anticipated moisture breakthrough conditions.

17. A solid desiccant dehydration unit, comprising solid desiccant beds that are arranged as parallel pipes oriented substantially horizontally and packed with solid desiccant material that is capable of selectively adsorbing water from a wet natural gas stream.

18. The solid desiccant dehydration unit of claim 17, wherein the solid desiccant dehydration unit is installed within a subsea dehydration system comprising at least one other solid desiccant dehydration unit.

19. The solid desiccant dehydration unit of claim 17 or 18, wherein the parallel pipes are oriented horizontally, at an incline of less than 5 degrees from horizontal, at an incline of less than 10 degrees from horizontal, or at an incline of less than 15 degrees from horizontal.

20. The solid desiccant dehydration unit of any of claims 17 to 19, wherein each solid desiccant bed comprises support grids or screens on both an upstream and a downstream side of the solid desiccant material within the corresponding pipe, and wherein each solid desiccant bed comprises multiple, separate sections of solid desiccant material separated by the support grids or the screens.

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