CN118532889A - Cryogenic air separation plant and method for cryogenically separating air - Google Patents

Abstract

The invention provides a cryogenic air separation plant and a method of cryogenically separating air having a main air compressor, a single expansion turbine, and a turbocharger, main heat exchanger and rectifying column system driven thereby. The method for separating air at low temperature is suitable for the situation that the pressure of the first product after internal compression is not higher than 6bara, and the range of the liquid production ratio is 20% -30%. The feed air is compressed in a main air compressor to a first pressure air stream at least 6bara above the operating pressure of the higher pressure column, a first portion of the first pressure air stream is compressed in a turbocharger to form a second pressure air stream, the second pressure air stream is partially cooled in the main heat exchanger and then directed to an expansion turbine, and after expansion depressurization, is directed to a rectification column system; the second part of the first pressure air stream is fed into the rectification column system after being completely cooled in the main heat exchanger and throttled and depressurized. The air separation plant of the invention does not comprise an air booster driven by externally supplied energy.

Description

Low temperature air separation equipment and method for low temperature air separation

Technical Field

The present invention belongs to the field of air separation, and relates to a method and a device for low-temperature air separation, and in particular to a method and a device for low-temperature air separation which adopt an internal compression process to obtain a pressurized gas product.

Background Art

It is a known technology to produce air products such as oxygen, nitrogen, argon, etc. by low-temperature distillation of air. The air separation equipment of low-temperature distillation generally includes a main air compressor, an air precooling and purification system, a main heat exchanger and a distillation tower system, and uses the introduction of cryogenic liquid (such as liquid nitrogen) from the outside or the use of air expansion to generate the cold required for low-temperature separation. The distillation tower system may include a single tower, or a double tower and a triple tower connected by heat exchange. As an example, a double-tower distillation tower system consists of a low-pressure tower and a high-pressure tower with relatively low and high operating pressures, respectively, and can simultaneously produce gaseous and liquid nitrogen and oxygen products.

When the customer needs a gas product with a pressure higher than the operating pressure of the high-pressure tower or the low-pressure tower, the prior art uses an external compression or internal compression process. The external compression process refers to the process of vaporizing (for liquid products) and reheating the liquid or gaseous product taken out of the distillation tower in the main heat exchanger without changing its pressure, and compressing the corresponding reheated gaseous product to the pressure required by the customer using a gas compressor.

The internal compression process refers to the process of increasing the pressure of the liquid product taken out of the distillation tower to the pressure required by the customer by using a liquid pump, and then evaporating, pseudo-evaporating and reheating it in the main heat exchanger to obtain a gas product with appropriate pressure.

When the gas product required by the customer is a pressure oxygen product, for the sake of safety and oxygen utilization, the internal compression process of raising the liquid oxygen stream to the pressure required by the customer with a liquid oxygen pump, and then using the positive flow of high-pressure air in the main heat exchanger to vaporize and reheat it is preferred. In the prior art, an air booster is often used after the main air compressor to obtain a positive flow of high-pressure air. For example, US5,515,687 and CN108253732A both disclose a method of compressing at least a portion of the air compressed, pre-cooled and purified by the main air compressor in a re-compressor (i.e., an air booster) provided with energy from the outside to a positive flow of high-pressure air with a pressure higher than the pressure of the liquid oxygen to be vaporized at room temperature.

In view of the fact that the investment and energy consumption of the cryogenic air separation process are generally considerable, in order to improve production efficiency, reduce energy consumption, and save investment and operating costs, technical personnel in this field are committed to optimizing the process flow for specific product types.

Summary of the invention

The present invention is directed to a method for cryogenically separating air and a cryogenic air separation plant, thereby more efficiently and cost-effectively producing liquid and gas products of a given pressure and composition.

In order to achieve the above-mentioned invention object, on one hand, the present invention discloses a method for low-temperature separation of air, which uses an air separation device, and the air separation device has a main air compressor, an air precooling and purification system, a single expansion turbine, a single turbocharger, a main heat exchanger and a distillation tower system having a high-pressure tower and a low-pressure tower. The turbocharger is driven by the expansion turbine. First, all the feed air is compressed to a first air pressure in the main air compressor to form a first pressure air flow, and the first air pressure is at least 6 bara higher than the operating pressure of the high-pressure tower; then, the first part of the first pressure air flow is compressed to a second air pressure in the turbocharger to form a second pressure air flow, and the second pressure air flow is partially cooled in the main heat exchanger, introduced into the expansion turbine, expanded and depressurized, and then input into the distillation tower system; at the same time, the second part of the first pressure air flow is completely cooled in the main heat exchanger, throttled and depressurized, and then input into the distillation tower system; a liquid product is obtained in the air separation device, and the liquid production ratio of the air separation device ranges from 20% to 30%. A liquid first product stream is obtained in the distillation tower system, which is raised to a higher first product pressure in a liquid state, heat-exchanged with a second part of the first pressure air stream in the main heat exchanger, evaporated or pseudo-evaporated and reheated, and then removed from the air separation device as a first pressure gas product, wherein the air separation device does not include an air booster driven by externally supplied energy.

In the above method, the first air pressure ranges from 12 bara to 17 bara, and the second air pressure ranges from 17 bara to 25 bara.

Further, the first product stream comprises liquid oxygen, the first product pressure is not higher than 6 bara, and the first pressure gas product is oxygen; preferably, the first product pressure is not higher than 4.5 bara.

The liquid product produced by the above method comprises one or more of liquid oxygen, liquid nitrogen or liquid argon.

On the other hand, the present invention discloses a low-temperature air separation device, comprising: a main heat exchanger; a distillation tower system having a high-pressure tower and a low-pressure tower; a main air compressor, characterized in that it compresses all feed air to a first air pressure and forms a first pressure air flow, and the first air pressure is at least 6 bara higher than the operating pressure of the high-pressure tower; a unique turbocharger, which is driven by the expansion turbine and is used to compress a first part of the first pressure air flow to a second air pressure and form a second pressure air flow; a unique expansion turbine, which is used to receive the second pressure air flow after partial cooling in the main heat exchanger; a device for introducing the second part of the first pressure air flow into the main heat exchanger, and after complete cooling, throttling and reducing the pressure to introduce it into the distillation tower system; a device for introducing the second pressure air flow after expansion and reduction of pressure into the distillation tower system. A device for raising the liquid first product stream obtained in the distillation tower system to a higher first product pressure; a device for exchanging heat between the liquid first product stream at the first product pressure and the second part of the first pressure air stream in the main heat exchanger, and removing the first pressure gas product formed after evaporation or pseudo-evaporation and heating from the air separation device; a device for removing the liquid product obtained in the distillation tower system from the air separation device, the liquid ratio of the air separation device ranging from 20% to 30%; in particular, the air separation device does not include an air booster driven by externally supplied energy.

The present invention designs a simple and efficient air separation process, which obtains gas and liquid products through low-temperature air separation, wherein the pressure of the gas product (especially oxygen) is not higher than 6 bara, and the liquid ratio of the liquid product is in the range of 20% to 30%. Compared with the traditional air separation process, in addition to the main air compressor, the present invention does not use an air compressor, but only uses a turbocharger driven by an expansion turbine, thereby saving investment and energy consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and spirit of the present invention can be further understood through the following detailed description of the invention and the accompanying drawings.

FIG1 is a schematic diagram of a flow chart of Example 1 provided by the present invention.

In the figure: 20-main air compressor; 21-air precooling and purification system; 22-turbocharger; 23-expansion turbine; 24-aftercooler; 25-main heat exchanger; 26-throttle valve; 27-liquid pump; 28-condenser evaporator; 29-high-pressure tower; 30-low-pressure tower; 50-distillation tower system; 1-feed air; 2-first pressure air flow; 3-first part of the first pressure air flow; 4-second pressure air flow; 5-second part of the first pressure air flow; 6-first product flow; 7-nitrogen; 8-dirty nitrogen; 9-liquid oxygen; 10-liquid nitrogen; 11-first pressure gas product; 100-air separation equipment.

DETAILED DESCRIPTION

The specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. However, the present invention should be understood to be not limited to the embodiments described below, and the technical concept of the present invention can be implemented in combination with other known technologies or other technologies with the same functions as those known technologies.

The terms "first" and "second" are used only for descriptive purposes and do not refer to the limitation of time sequence, quantity, or importance. They cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features, but are only for distinguishing one technical feature from another technical feature in the technical solution. Therefore, the features defined as "first" and "second" can explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "multiple" is two or more, unless otherwise clearly and specifically specified. Similarly, the qualifiers similar to "one" appearing in this article do not refer to the limitation of quantity, but describe technical features that have not appeared in the previous text. Similarly, unless it is a noun modified by a specific quantitative quantifier, it should be regarded as including both singular and plural forms in this article, and the technical solution can include both singular and plural technical features. Words such as "only" and "only" should be understood as specific quantitative quantifiers, indicating that there is only one noun defined by them.

In this article, modifiers such as "about" and "approximately" that appear before a numeral usually include the number itself, and their specific meanings should be understood in conjunction with the context. When a numeral appears as an endpoint of a numerical range, the numerical range includes the values of both endpoints; similarly, when a numerical value is an endpoint of an open-ended range, such as when it is connected with "at least", "at most", "not more than", "not less than", "not higher than", or "not lower than", the open-ended numerical range also includes the values of the endpoints.

It should be understood that in the present invention, "at least one (time)" means one (time) or more (times). "And/or" is used to describe the association relationship of associated objects, indicating that three relationships may exist. For example, "A and/or B" can mean: only A exists, only B exists, and both A and B exist, where A and B can be singular or plural.

As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. Unless otherwise specified, all terms (including technical and scientific terms) used herein have the same meaning as any commonly understood meaning in the ordinary art of the field to which the invention belongs. It should also be understood that terms, such as those defined in common dictionaries, should be understood to have a meaning consistent with their meaning in the context of this specification and the related art, and should not be understood in an idealized or overly formal sense unless otherwise specified herein. For the sake of brevity and/or clarity, well-known functions or configurations may not be described in detail.

Here, natural pressure losses are usually not included in the pressure data. If the pressure difference between the corresponding parts is not greater than the natural line loss caused by the pressure loss in the pipeline, heat exchanger, cooler, adsorber, ordinary regulating valve (non-throttle valve), etc., the pressure is rated as "equal" here. The "pressure" and "pressure range" in the present invention can accommodate a certain error, and the unit used is absolute pressure. For a specific set of cryogenic air separation equipment or cryogenic air separation process, the pressure ranges do not overlap.

The air separation device 100 in the present invention refers to a combination of a set of devices that receive feed air and can produce gas products and liquid products. The feed air is unpurified air at room temperature and pressure, the gas product is generally a gaseous product that is reheated to room temperature through a heat exchanger and whose pressure and purity meet the requirements, and the liquid product is generally a liquid product that is not reheated and whose pressure and purity meet the requirements. The liquid product is often directly input into a pipeline network or a storage container. The higher the output of the liquid product directly taken out of the air separation device as a product, the more cooling capacity the air separation device requires. In practice, the liquid production ratio is used to characterize the molar ratio of all liquid products relative to oxygen products in the air separation device. The liquid product can include liquid oxygen (LOX), liquid nitrogen (LIN) or, in the case of argon products, liquid argon (LAR). The oxygen product includes liquid oxygen (LOX) and gaseous oxygen (GOX), and the calculation method is as follows:

The distillation tower system in the present invention refers to a device that receives low-temperature feed air and separates the air into gaseous or liquid oxygen, nitrogen and other components by gas-liquid mass transfer. The distillation tower system 50 is shown in a highly simplified form, including a tower body installed in a cold box and various equipment installed in the tower, such as tower plates, packings, condenser evaporators, etc.; pipes and valves connecting the towers and outputting the distillation products, etc. The distillation tower system 50 includes at least one low-pressure tower 30 operating at a pressure level of 1.0 bara to 3.0 bara and a high-pressure tower 29 operating at a pressure level of 4.0 bara to 7.0 bara, wherein the low-pressure tower and the high-pressure tower are thermally connected via a main condenser evaporator 28.

"Compressor" in the present invention refers to a device used to compress at least one gas flow from the initial pressure when entering the compressor to the final pressure at which the flow is withdrawn from the compressor. A compressor installed in a housing may contain a single or multiple compression stages. The main air compressor compresses all or a major part of the amount of air fed into the air separation device, that is, the entire feed air flow. A compressor that compresses part or all of the feed air compressed in the main air compressor to a higher pressure is called a recompressor or air booster. In the present invention, an air booster specifically refers to a compressor that is driven entirely by externally supplied energy, that is, a booster that is not driven by the expansion of a fluid previously compressed in the air separation device. The air booster defined above is not included in the air separation device of the present invention.

The air precooling system is set at the downstream of the main air compressor, and uses air cooling tower, water cooling tower, refrigerator or a combination of them to cool the feed air to the range of 10℃～25℃, and then send it to the air purification system. The purpose of the air purification system is to remove water, carbon dioxide, hydrocarbons and other substances in the feed air that may freeze under cryogenic conditions. Generally, parallel adsorbers filled with adsorbents such as molecular sieves and alumina are often used.

The "main heat exchanger" is used to cool the feed air in indirect heat exchange with the return stream from the distillation column system, such as heat exchange with dirty nitrogen or cryogenic air separation products. The main heat exchanger can be formed by a single heat exchange section or a plurality of heat exchange sections connected in parallel and/or in series, which have "channels" designed as fluid channels separated from each other and having heat exchange surfaces. Full cooling means that the cooled stream enters the main heat exchanger at the hot end and then is led out of the main heat exchanger from the cold end, that is, the channel through which the stream flows runs through the entire main heat exchanger; while partial cooling means that the cooled stream is led out of the main heat exchanger from a position between the hot end and the cold end, that is, the channel through which the stream flows does not run through the entire main heat exchanger.

An "expansion turbine" or "expander" is used to expand a gaseous or at least partially liquid stream under pressure and to perform work in the process. In the present invention, the expansion turbine 23 is coupled to the turbocharger 22 via a common shaft, thereby driving the turbocharger 22. In an ideal state, the work performed by the expansion turbine is entirely transferred to the corresponding mechanically connected turbocharger without using energy supplied externally, for example, by an electric motor. Mechanical connection in this context is understood to mean a fixed or mechanically adjustable speed relationship between these rotating parts by mechanical components, such as gears, belts, transmissions and the like. In the present invention, only one expansion turbine and a turbocharger driven entirely by it are used.

In the embodiment 1 represented in FIG. 1 , the feed air 1 is first compressed to a first air pressure in the main air compressor 20 and generates a first pressure air stream 2. The first air pressure is at least 6 bara higher than the operating pressure of the high pressure column, that is, the first air pressure ranges from 12 bara to 17 bara. After impurities are removed in the air precooling and purification system 21, the first pressure air stream 2 is divided into two parts, and the first part 3 is further compressed to a second air pressure by the turbocharger 22 to form a second pressure air stream 4. An aftercooler 24 cooled by cooling water can be provided after the turbocharger 22 to remove the heat of compression from the air separation device 100. After the second pressure air stream 4 is partially cooled in the main heat exchanger 25, it enters the expansion turbine 23, expands and releases to the pressure of the high pressure column 29, and is fed into the high pressure column. After the second part 5 of the first pressure air stream 2 is completely cooled in the main heat exchanger 25, it is released to the operating pressure of the distillation column system 50 through the throttle 26, and is fed into the corresponding high pressure column and/or low pressure column.

The distillation tower of Example 1 generates liquid products including liquid oxygen 9 and liquid nitrogen 10. After being led out of the distillation tower system 50, the two no longer participate in the heat exchange in the main heat exchanger 25 as reflux. Dirty nitrogen gas 8 and pure nitrogen gas 7 are also generated. After being reheated in the main heat exchanger in gaseous form, they can be taken out as products or realize the functions of precooling and regenerating the purifier. Stream 6 is a liquid first product stream, which is liquid oxygen in this example, and can also be liquid nitrogen or liquid argon as required. The liquid pump 27 is configured to increase the pressure of the first product stream to a higher pressure. The pressurized stream 6 is evaporated or pseudo-evaporated and reheated in the main heat exchanger 25, and then removed from the air separation device as a first pressure gas product 11.

In order to vaporize the pressurized first product stream in the main heat exchanger, a normal temperature air stream with matching pressure and flow rate needs to be provided to the main heat exchanger. In the present invention, this stream is the second part 5 of the first pressure air stream 2. When the first product stream is liquid nitrogen or liquid argon, the first air pressure is roughly equal to the pressure of the first product stream after pressurization, that is, the first product pressure; when the first product stream is liquid oxygen, the first air pressure is a multiple of the first product pressure obtained after pressurization, for example, 2 times. The present invention is particularly suitable for the first product pressure not higher than 6 bara, preferably not higher than 4.5 bara, and the first air pressure ranges from 12 bara to 17 bara.

In Example 1, taking the first pressure gas product as oxygen as an example, the liquid products are liquid oxygen stream 9 and liquid nitrogen stream 10, and the oxygen product is liquid oxygen stream 9 and the first pressure gas product 11. The calculation method of the production-liquid ratio is to use the molar number of each stream:

When the liquid production ratio of the air separation device is in the range of 20% to 30%, especially between 20% and 25%, the method and device of the present invention are particularly beneficial. By selecting the appropriate first air pressure and the second air pressure, the turbocharger is completely driven by the work done by the expansion turbine, thereby reducing energy consumption and the investment in purchasing an additional air compressor.

The preferred specific embodiments of the present invention are described in this specification. The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Any technical solutions that can be obtained by those skilled in the art through logical analysis, reasoning or limited experiments according to the concept of the present invention should be within the scope of the present invention.

Claims (6)

1. A method for cryogenically separating air, using an air separation device, wherein the air separation device has a main air compressor, an air precooling and purification system, a single expansion turbine, a single turbocharger, a main heat exchanger and a distillation column system, wherein the distillation column system has a high-pressure column and a low-pressure column, and the turbocharger is driven by the expansion turbine, characterized in that: - all feed air is compressed in the main air compressor to a first air pressure to form a first pressure air stream, wherein the first air pressure is at least 6 bara higher than the operating pressure of the high pressure column, - a first part of the first pressure air flow is recompressed to a second air pressure in a turbocharger to form a second pressure air flow, and the second pressure air flow is partially cooled in a main heat exchanger and then introduced into an expansion turbine, expanded and depressurized, and then introduced into the distillation column system, - the second part of the first pressure air flow is completely cooled in the main heat exchanger and then further reduced in pressure before being fed into the distillation column system, - obtaining a liquid product in the air separation device, wherein the liquid product ratio of the air separation device is in the range of 20% to 30%, - a first liquid product stream is obtained in the rectification column system, the first product stream is raised in liquid state to a higher first product pressure, heat-exchanged with the second part of the first pressure air stream in the main heat exchanger, evaporated or pseudo-evaporated and reheated, and then removed from the air separation device as a first pressure gas product, in, The air separation plant does not contain an air compressor driven by externally supplied energy. 2. The method for low-temperature separation of air according to claim 1, characterized in that the first air pressure ranges from 12 bara to 17 bara, and the second air pressure ranges from 17 bara to 25 bara. 3. The method for cryogenically separating air according to claim 2, characterized in that the first product stream comprises liquid oxygen and the first product pressure is not higher than 6 bara. 4. The method for cryogenically separating air according to claim 3, characterized in that the first product pressure is not higher than 4.5 bara. 5. The method for cryogenically separating air according to claim 1, wherein the liquid product comprises one or more of liquid oxygen, liquid nitrogen or liquid argon. 6. A cryogenic air separation device, characterized in that it comprises: - Main heat exchanger, a distillation column system having a high-pressure column and a low-pressure column, a main air compressor for compressing all feed air to a first air pressure and forming a first pressure air stream, wherein the first air pressure is at least 6 bara higher than the operating pressure of the high pressure column, a single turbocharger, driven by the expansion turbine, for compressing a first portion of the first pressure air flow to a second air pressure and forming a second pressure air flow, - a single expansion turbine adapted to receive the second pressure air stream after it has been partially cooled in the main heat exchanger, - a device for introducing the second part of the first pressure air flow into the main heat exchanger, after complete cooling, throttling and reducing the pressure to introduce it into the distillation column system, - a device for introducing the second pressure air stream after expansion and decompression into the distillation column system, - means for raising the liquid first product stream obtained in the rectification column system to a higher first product pressure, - means for exchanging heat in the main heat exchanger the first product stream in a liquid state at the first product pressure with the second part of the first pressure air stream, and for removing the first pressure gaseous product formed after evaporation or pseudo-evaporation and reheating from the air separation plant, - a device for removing the liquid product obtained in the distillation column system from the air separation device, the liquid production ratio of the air separation device having a value interval of 20%-30%, in, The air separation plant does not contain an air compressor driven by externally supplied energy.