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Title:
CRYOGENIC UNIT AND METHOD FOR OPERATING A CRYOGENIC UNIT
Document Type and Number:
WIPO Patent Application WO/2017/046236
Kind Code:
A2
Abstract:
A cryogenic unit for treating gases. The unit may be part of an air separation unit (ASU) used in a power plant for electric power generation and/or steam generation and/or gasification plant such as IGCC.

Inventors:
RAUCHFUSS HARDY OLAF GERHARD (CH)
Application Number:
PCT/EP2016/071808
Publication Date:
March 23, 2017
Filing Date:
September 15, 2016
Export Citation:
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Assignee:
GENERAL ELECTRIC TECHNOLOGY GMBH (CH)
International Classes:
F25J3/04
Foreign References:
US20020174678A12002-11-28
US3086371A1963-04-23
US5254294A1993-10-19
Other References:
None
Attorney, Agent or Firm:
FOSTER, Christopher, Michael (CH)
Download PDF:
Claims:
CLAIMS

1 . A cryogenic unit for gas treatment comprising

a heat exchanger having a first side and a second side ; an expansion valve connected downstream of the first side of the heat exchanger,

a gas separation device connected downstream of the expansion valve and upstream of the second side of the heat exchanger,

at least one additional heat exchanger,

a mixing device connected to the at least one additional heat exchanger,

a supply of a liquid connected to the mixing device,

a supply of dry ice connected to the mixing device.

2. The cryogenic unit according to claim 1 , wherein the gas separation device is a distillation column.

3. The cryogenic unit according to claim 1 , further comprising a compressor upstream of the first side of the heat exchanger. 4. The cryogenic unit according to claim 1 , wherein the liquid is methanol.

5. The cryogenic unit according to claim 5, further comprising a first reservoir for storing the methanol. 6. The cryogenic unit according to claim 1 , further comprising a second reservoir for storing the dry ice.

7. The cryogenic unit according to claim 1 , further comprising a bypass line in parallel to the at least one additional heat exchanger.

8. The cryogenic unit according to Claim 1 , further comprising a first conversion unit for converting carbon dioxide into methanol to provide the supply of liquid.

9. The cryogenic unit according to Claim 1 , further comprising a second conversion unit for converting carbon dioxide into dry ice to provide the supply of dry ice.

10. The cryogenic unit according to Claim 1 , wherein the at least one additional heat exchanger is positioned upstream of the first side of the heat exchanger.

1 1 . The cryogenic unit according to Claim 1 , wherein the at least one additional heat exchanger is positioned between the first side of the heat exchanger and the gas treatment device.

12. The cryogenic unit according to Claim 1 , wherein the at least one additional heat exchanger is positioned between the second side of the heat exchanger and the gas separation device.

13. A method for operating a cryogenic unit for gas treatment, wherein the cryogenic unit comprises:

a heat exchanger having a first side and a second side, an expansion valve connected downstream of the first side of the heat exchanger,

a gas treatment device connected downstream of the expansion valve and upstream of the second side of the heat exchanger; the method comprising

cooling the gas being treated against treated gas by passing the gas being treated through the first side of the heat exchanger and the treated gas through the second side of the heat exchanger,

supplying the cooled gas being treated to the gas treatment device, mixing a liquid and dry ice generating a cooling mixture, and additionally cooling the gas being treated against the cooling mixture by passing the gas being treated and the cooling mixture through at least one additional heat exchanger.

14. The method according to Claim 13, further comprising:

additionally cooling the gas being treated upstream of the first side of the heat exchanger and/or

additionally cooling the gas being treated between the first side of the heat exchanger and the gas treatment device, and/or

additionally cooling the treated gas between the second side of the heat exchanger and the gas treatment device.

15. The method according to Claim 1 1 , further comprising separating the gas being treated into its components at the gas treatment device.

16. The method according to Claim 1 1 , wherein the liquid is methanol.

Description:
CRYOGENIC UNIT AND METHOD FOR OPERATING A CRYOGENIC UNIT

TECHNICAL FIELD

Embodiments of the invention relate to a cryogenic unit and method for operating a cryogenic unit. The cryogenic unit can be used in an air separation unit and/or a gas processing unit of a power plant for electric power generation and/or a steam generation and/ or a gasification plant.

DISCUSSION OF ART

Industrial power plants are known sources of C0 2 emissions and there is a need to reduce C02 emissions from power plants, for example, coal fired power plants.

One method to reduce the C0 2 emissions is to capture the C0 2 from the flue gas. One such method is known as oxy-fuel combustion technology.

In the oxy-fuel process, coal is combusted with (nearly pure) oxygen, for example, >95%. A flue gas is produced that consists mainly of highly concentrated C02 and water vapor. These two components are easily separated in a cooling process. The water is condensed and a C02-rich gas stream is formed, which can then be prepared for transport or storage.

The main problem with an oxy-fuel process is that separating oxygen from air requires a great deal of energy and thus creates a penalty to the power plant. An energy penalty is created when either the additional energy needed is supplied by the plant, i.e., it reduces the output available, or if the additional energy needed is supplied by an outside provider, i.e., demanding energy to generate the same amount of energy.

The oxygen provided to the system is generated in an Air Separation Unit (ASU), which employs a cryogenic unit to cool the gas to the required temperature for separation. Cryogenic units often have a compressor followed by a heat exchanger, where the heat exchanger has a first warm side and a second cold side. The gas being treated is passed through the warm side of the heat exchanger and is cooled. The cooled gas being treated is thus expanded in an expansion valve to be further cooled and is then supplied into a gas separation device.

The treated gas discharged from the gas separation device is passed through the cold side of the heat exchanger, in order to cool the gas being treated passing through the warm side of the heat exchanger, and is then forwarded to further treatments.

Since cooling is achieved by expansion of the gas being treated in the expansion valve, at start-up it takes usually a long time to reach the operating temperature at the heat exchanger, i.e. for the gas being treated to be cooled at the heat exchanger to the required design temperature.

As an example, if the cryogenic unit described above is part of an air separation unit (ASU) of e.g. an oxygen fired power plant, it could take up to two days from start up to reach the operating temperature (between -160/-190°C) at the outlet of the heat exchanger (i.e. for the air cooled at the heat exchanger to have a temperature in the range -160/-190°C at the outlet of the heat exchanger).

In view of the above, what is needed is an improved system and method to shorten and/or reduced the start-up time.

BRIEF DESCRIPTION

In an embodiment, the cryogenic unit can be part of an air separation unit (ASU) to produce oxygen from air.

In another embodiment, the cryogenic unit can also be part of a gas processing (GPU) unit in which flue gas is compressed and cooled in order to separate carbon dioxide from other gas. DRAWINGS

Further characteristics and advantages will be more apparent from the description of an embodiment of the cryogenic unit and method, illustrated by way of non-limiting example in the accompanying drawings, in which:

FIG. 1 is a diagram of an exemplary cryogenic unit as incorporated as part of an air separation unit of a power plant for electric power generation and/or steam generation.

FIG. 2 is a diagram of a cryogenic unit as in accordance with an embodiment of the present invention,

FIG. 3 is another diagram of a cryogenic unit in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Reference will be made below in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference characters used throughout the drawings refer to the same or like parts, without duplicative description.

As used herein, the terms "substantially," "generally," and "about" indicate conditions within reasonably achievable manufacturing and assembly tolerances, relative to ideal desired conditions suitable for achieving the functional purpose of a component or assembly.

As also used herein, the term "fluidly connected" means that the referenced elements are connected such that a fluid (to include a liquid, gas, and/or plasma) may flow from one to the other.

Accordingly, the terms "upstream" and "downstream," as used herein, describe the position of the referenced elements with respect to a flow path of a fluid and/or gas flowing between and/or near the referenced elements.

Additionally, as used herein, the term "fill" includes both fully and partially filling a containing object with a filling material or object. As also used herein, phrases such as "heat exchange" or "heating contact" means that the referenced objects are in proximity of one another such that heat/thermal energy can transfer between them.

Accordingly, referring to FIG. 1 , the cryogenic unit 1 is e.g. part of a power plant for electric power generation and/or steam generation and/or gasification plant such as IGCC.

The power plant is preferably an oxyfuel power plant, i.e. a power plant having a boiler in which a fuel such as coal or oil is burned in the presence of oxygen or oxygen enriched gas or substantially pure oxygen and recirculated flue gas.

Figures 1 and 2 show an oxyfuel power plant having, in addition to the cryogenic unit 1 , a boiler 20 supplied with oxygen 21 from the cryogenic unit 1 and fuel 22.

The cryogenic unit 1 can comprise a compressor 2 for compressing the gas being treated 3 and a heat exchanger 4, to remove the compression heat from the gas being treated.

The gas 3 (e.g. air in case the cryogenic unit is part of an air separation unit ASU or flue gas generated during combustion of a fuel such as coal or oil in case the cryogenic unit is part of a gas processing unit) is compressed at the compressor 2 and is then cooled at the heat exchanger 4, to remove the compression heat; at the compressor a cooling means such as water from an external source or air can be used.

The cryogenic unit 1 further has a cleaning system 5, such as a filter for dust removal and/or molar sieves for carbon dioxide and/or moisture removal by absorption.

The gas is forwarded to the cleaning system 5 where dust, humidity and carbon dioxide are removed (the treatments occurring in the cleaning system 5 depend on the particular application of the cryogenic unit 1 , e.g. in case the cryogenic unit 1 is part of a gas processing unit carbon dioxide is not removed).

The cryogenic unit 1 further includes a heat exchanger 7 having a first side

7a and a second side 7b. The gas is forwarded to the heat exchanger 7 (namely through the first side 7a of the heat exchanger 7). Here the gas is cooled against the treated gas passing through the second side 7b of the heat exchanger 7.

As an example, before entering the heat exchanger 7 (i.e. between the cleaning system 5 and heat exchanger 7) the gas has the ambient temperature; after having passed through the heat exchanger 7 (i.e. between the heat exchanger 7 and the expansion valve 9) the gas has a temperature between about -160/-190°C, after having passed through the expansion valve (i.e. between the expansion valve 9 and the gas separation device 10) the gas has a temperature between about -170/-190°C.

The streams are thus passed through the second side 7b of the heat exchanger 7 cooling the gas being treated passing through the first side 7a.

The cooled gas is thus made to pass through the expansion valve 9 where it is further cooled following expansion.

The expansion valve 9 is connected downstream of the first side 7a of the heat exchanger 7.

The gas separation device 10 is connected downstream of the expansion valve 9 and upstream of the second side 7b of the heat exchanger 7.

In one embodiment, the gas separation device 10 is a separation device for separating the gas into its components, for example, (nitrogen, oxygen, argon, etc.) The separated components leave the gas separation device 10 in separate streams, e.g., a N 2 stream and an 0 2 stream.

In one embodiment, the separation device may be a distillation column.

For example, two or more than two streams can be separated at the gas separation device 10. In the attached figures all streams separated at the gas separation device are collectively indicated by reference 7b.

For example, reference 23 in Figures 1 and 2 indicate nitrogen and other gases, from the cryogenic unit 1 that have been separated at the gas separation device 10. These streams pass through the heat exchanger 7 via a different path than from the oxygen stream. The oxygen stream leaving the gas separation device is then supplied to boiler 20, while the other gas (nitrogen, argon, etc.) is vented via 23 or used in another way.

The cryogenic unit 1 further has at least one additional heat exchanger 12. As mentioned above, to cool down the ASU to the required temperatures to start producing 0 2 requires a great deal of time. For example, if the cryogenic unit described above is part of an air separation unit (ASU) of e.g. an oxygen fired power plant, it could take up to two days from start up to reach the operating temperature (between -160/-190°C) at the outlet of the heat exchanger

With an oxygen fired power plant, it is typical that the system be disrupted.

For example, the load on the system is dependent upon the current electricity demand, thus there are constant changes to the system load. Likewise, there are planned and un-planned shut-downs for maintenance.

Thus, given that the start-up time for the ASU is so time-consuming, it is of great need to be able to shorten this time period in order to increase flexibility.

The additional heat exchangers 12 can be provided in different positions of the cryogenic unit 1 and are used to provide additional cooling to the gas being treated, in addition to the cooling provided by the treated gas passing through the second side 7b of the heat exchanger 7.

In an embodiment, for example, as shown in Figure 3, different positions are possible for the at least one heat exchangers 12 in the cryogenic unit 1 , for example, at least one heat exchanger 12 can be positioned:

- upstream of the first side 7a of the heat exchanger 7, and/or

- between the first side 7a of the heat exchanger 7 and the gas separation device 10 (i.e. upstream and/or downstream of the expansion valve 9), and/or

- between the second side 7b of the heat exchanger 7 and the gas separation device 10. In another embodiment, the additional heat exchangers are used as an intermediate step in the heat exchanger 7. For example, a portion of the gas stream can be diverted to the additional heat exchanger 12 and then returned to the heat exchanger 7.

The at least one heat exchanger 12 is particularly useful at start up in order to reduce the start-up time. The at least one heat exchanger 12 can also be used during operation in case additional cooling is needed.

Bypass lines 17 can be provided in parallel to the at least one additional heat exchanger 12.

The streams of gas are thus passed through the at least one heat exchanger 12 where the streams are further cooled.

The streams of gas can also pass through the at least one heat exchanger 12 without undergoing further cooling.

The streams of gas can also be bypassed according to the operating conditions and needs.

In different embodiments, the main flow can pass through the at least one heat exchanger 12 or bypass 17.

Each of the at least one additional heat exchanger 12 is connected to a mixing device 13. The mixing device 13 is in turn connected to a supply of a liquid such as methanol and/or a supply of dry ice (solid carbon dioxide).

A first reservoir 15 for storing the methanol and a second reservoir 16 for storing the dry ice are provided. Each reservoir 15 and 16 is fluidly connected to the mixing device 13 and the additional heat exchangers 12.

The mixing device 13 can be directly supplied by the first conversion unit 29 and/or second conversion unit 30 or the mixing device 13 can be directly supplied by the first reservoir 15 for the methanol and/or by the second reservoir 16 for the dry ice.

In addition, it is possible that the first conversion unit 29 is connected to and supplies methanol into the first reservoir 15 and/or the second conversion unit 30 is connected to and supplies dry ice into the second reservoir 16. At start up methanol from the first reservoir 15 and dry ice from the second reservoir 16 are supplied to the mixing device 13; for example the mixing device 13 can comprise a tank in which the liquid methanol is contained and one or more feeders to feed the solid dry ice into the liquid methanol. Agitators could also be provided.

When the solid dry ice is supplied into the methanol (also identified in industry by the abbreviation MeOH), the solid dry ice sublimates, passing from the solid state to the gas state (at least partially); the gaseous carbon dioxide in thus at least partly dissolved in the liquid methanol. This sublimation requires a large amount of heat to occur (because of the high heat of changing of state); the heat for making the sublimation of carbon dioxide to occur is taken from the methanol, which thus becomes colder (e.g. between -60 to -72). Therefore the consequence of mixing dry ice with methanol is the generation of a cold mixture of methanol with carbon dioxide.

The final temperature depends mainly on the amount of dry ice supplied into the methanol, because of the large heat required for making the sublimation to occur; the exact starting temperature of methanol is less relevant.

As an example, methanol and dry ice can be mixed in a ration 1 : 1 by weight.

It is clear that any liquid can be used instead of methanol, provided that it maintains its liquid state at the operating temperatures reached by the sublimation of dry ice.

Use of methanol is advantageous because it can be produced from the carbon dioxide generated in the power plant and because (even if it contains dissolved carbon dioxide) it can be used as a fuel or supplemental fuel in the power plant itself or in other applications; this way the heat absorbed by the methanol is not lost, but is used in the boiler or other applications.

The mixture of methanol with carbon dioxide is used in the at least one heat exchanger 12.

Different possibilities for making the methanol and the dry ice available exist. Methanol and dry ice can be supplied into the first and second reservoirs, 15, 16 respectively, by external sources. For example, methanol and/or dry ice can be bought on the market and supplied into the reservoirs 15 and 16 as needed.

Alternatively or in addition, methanol and dry ice can be produced on site if carbon dioxide is available. This is possible in case the cryogenic unit 1 is used in a power plant, such as an oxyfuel power plant.

In another embodiment, any carbon capture & storage plant (CCS) and/or carbon capture & utilization plant (CCU) where C02 is separated and prepared for storage can implement the present invention.

At the boiler 20 combustion of fuel (e.g. coal or oil or in general any carbon containing fuel, either solid, liquid or gaseous) occurs with generation of flue gas that is sent through an air quality control system 25 including e.g. a dust removal unit such as a fabric filter or electrostatic precipitator, a deSOx unit for sulphur removal, a deNOx unit for nitrogen removal (if required according to the specific application), a dryer, etc..

The cleaned flue gas is supplied to a gas processing unit 26 for separating the carbon dioxide from other gas constituting the flue gas; the carbon dioxide is thus supplied to a pump/compressor 27 for storage (the other gas comprising mainly nitrogen, argon, etc. can be vented from the GPU).

A part of the carbon dioxide separated from the flue gas can be used to convert carbon dioxide into methanol at a first conversion unit 29 and/or to convert carbon dioxide into dry ice at a second conversion unit 30. Processes to convert carbon dioxide into methanol are known in the art; processes to convert gaseous or liquid carbon dioxide into dry ice are known as well.

With specific reference to Figure 2, when the streams of gas are cooled in the at least one additional heat exchanger 12, their temperature is further reduced compared to the temperature at the outlet of the gas separation device 10. Therefore these streams are able to cool the gas being treated at the heat exchanger 7 to a lower temperature than without the at least one additional heat exchanger 12. Additional heat exchangers 12 can be located at different positions in the system. The cooling occurring there has also the effect of reducing the temperature of the gas being treated directed towards the gas separation device 10.

After having passed through the at least one additional heat exchanger 12, the mixture can be used in different ways. For example the mixture can be used as a fuel in the boiler 20 or as a supplemental fuel in the boiler 20; in this respect the mixture is supplied from the additional heat exchangers to the boiler via lines 31 . This is advantageous, because the carbon dioxide contained in the methanol is not vented into the atmosphere, but is treated and collected in the air quality control system 25 and gas processing unit 26.

The gas process unit has a compressor 2 and a gas cleaning system 5 for dust, moisture etc. removal. The gas processing unit further has first and second heat exchangers 7, with a first side 7a for the gas being treated, which in this example is flue gas, and a second side for the treated gas (e.g. nitrogen to be vented, separated carbon dioxide). Downstream of the second heat exchanger 7 a gas separation device in the form of e.g. a distillation column is provided. Also in this example, additional heat exchangers 12 can be provided in different positions.

Naturally the additional heat exchangers 12 are supplied with a cooling mixture as explained in the previous examples and can be connected to a first reservoir 15 and/or second reservoir 16 and/or first and/or second conversion units 29, 30.

The present invention also refers to a method for operating a cryogenic unit 1 for gas treatment.

The method comprises

- cooling the gas being treated against treated gas by passing the gas being treated through the first side 7a of the heat exchanger 7 and the treated gas through the second side 7b of the heat exchanger 7,

- supplying the cooled gas being treated to the gas treatment device, e.g. for separating the gas being treated into its components, - mixing a liquid such as methanol and dry ice generating a cooling mixture, this mixture in preferably liquid such that it can be pumped and distributed,

- additionally cooling the gas being treated against the cooling mixture by passing the gas being treated and the cooling mixture through one or more additional heat exchangers 12.

Preferably, additionally cooling the gas being treated occurs by:

- additionally cooling the gas being treated upstream of the first side 7a of the heat exchanger 7, and/or

- additionally cooling the gas being treated between the first side 7a of the heat exchanger 7 and the gas separation device 10, and/or

- additionally cooling the treated gas between the second side 7b of the heat exchanger 7 and the gas separation device 10.

Naturally the features described may be independently provided from one another.

In practice the materials used and the dimensions can be chosen at will according to requirements and to the state of the art.

Additionally, while the dimensions and types of materials described herein are intended to define the parameters of the invention, they are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

This written description uses examples to disclose several embodiments of the invention, including the best mode, and also to enable one of ordinary skill in the art to practice the embodiments of invention, including making and using any devices or systems and performing any incorporated methods.

The patentable scope of the invention is defined by the claims, and may include other examples that occur to one of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

As used herein, an element or step recited in the singular and proceeded with the word "a" or "an" should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated.

In the appended claims, the terms "including" and "in which" are used as the plain-English equivalents of the respective terms "comprising" and "wherein." Moreover, in the following claims, terms such as "first," "second," "third," "upper," "lower," "bottom," "top," etc. are used merely as labels, and are not intended to impose numerical or positional requirements on their objects.

Furthermore, references to "one embodiment" of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.

Moreover, unless explicitly stated to the contrary, embodiments "comprising," "including," or "having" an element or a plurality of elements having a particular property may include additional such elements not having that property.

Since certain changes may be made in the above-described invention, without departing from the spirit and scope of the invention herein involved, it is intended that all of the subject matter of the above description shown in the accompanying drawings shall be interpreted merely as examples illustrating the inventive concept herein and shall not be construed as limiting the invention.