Patent Application
20200355429
The present invention relates to a low-temperature rectification air separation process and device.
The use of cryogenic distillation to separate air into nitrogen and oxygen products is a common and mature technology. At least two air separation towers operating at different pressures—a medium pressure tower and a low pressure tower—are brought into communication by means of heat exchange via a main condensing evaporator. A pressurized, purified and cooled air feed gas is input into the medium pressure tower and/or the low pressure tower, and by means of rectification, gaseous and/or liquid nitrogen and oxygen are obtained. All or part of the nitrogen and oxygen undergo heat exchange with the air feed gas in the main heat exchanger to obtain gaseous nitrogen and oxygen products at room temperature. The design of an air separation device and process is generally based on the requirements of customers for the status, pressure and output of nitrogen and oxygen products.
When it is necessary to produce oxygen and/or nitrogen products with a higher pressure—for example, greater than 40 bara, an external pressurization method by which room temperature oxygen or nitrogen, which has been reheated by means of the main heat exchanger, is pressurized by means of a corresponding booster, or an internal pressurization method by which low-temperature liquid oxygen or liquid nitrogen is boosted to a desired pressure by means of a pump and then reheated by means of the main heat exchanger may be selected. In particular, when producing high pressure oxygen, due to production safety and equipment cost considerations, the internal pressurization process is generally used.
During the internal pressurization process, a high pressure warm stream is required for evaporating and vaporizing the high pressure liquid oxygen in the main heat exchanger, wherein this warm stream is generally a high pressure air feed gas, or may also be circulating high pressure nitrogen. If a high pressure air feed gas is used, the air feed gas that has reached a pressure for the medium pressure tower by means of a main air compressor needs to be further pressurized to a higher pressure by means of a booster, and this is an energy-consuming process.
A technical problem to be solved by certain embodiments of the present invention is to improve the utilization of the rectification capacity of an air separation tower, especially a low pressure tower.
Another technical problem to be solved by certain embodiments of the present invention is to reduce the energy consumption required for the pressurization of an air feed gas.
Still another technical problem to be solved by certain embodiments of the present invention is how to flexibly provide nitrogen with different output and pressure to a customer.
In one aspect, the present invention provides a method for producing nitrogen and oxygen by means of the cryogenic distillation of air, comprising: providing a first tower working at a higher pressure, i.e. a medium pressure tower, and a second tower working at a lower pressure, i.e. a lower pressure tower, wherein the first tower and the second tower are brought into communication by means of heat exchange via a main condensing evaporator; providing at least one air pre-cooling system, one air purification system, one main air compressor, at least one air booster, at least one main heat exchanger, and one supercooler; further treating an air feed gas, which has been pressurized to a first pressure range by means of the main air compressor, by means of the pre-cooling system and the purification system, then sending part of the air feed gas to the main heat exchanger for heat exchange with gas products produced by means of rectification and then to the first tower, subjecting the other part of the air feed gas to pressurization by means of the air booster and several stages of expander boosters, to heat exchange in the main heat exchanger with gas and liquid products produced by means of rectification and then to expansion or throttling for decompression to the first pressure range, and then sending this other part to the first tower, or supercooling a part of separated liquid by means of the supercooler, followed by throttling and sending to the second tower; rectifying the air feed gas in the first tower, extracting oxygen-enriched liquid air at the bottom of the first tower, pure liquid nitrogen at the top, and optionally impure liquid nitrogen in the middle part, and sending same to the supercooler for supercooling and then to the second tower as reflux liquids; extracting pure liquid oxygen in the main condensing evaporator, and sending the pure liquid oxygen to a liquid oxygen pump for pressurization and then to the main heat exchanger for heat exchange with the air feed gas pressurized by means of the air booster and the several stages of expander boosters, followed by evaporation and vaporization for output as a product; extracting impure nitrogen from the second tower, and sending the impure nitrogen to the supercooler for warming and then to the main heat exchanger for further reheating; extracting pure nitrogen at a first nitrogen product pressure from the top of the first tower and sending the pure nitrogen to the main heat exchanger for reheating. In the method, a part of pure nitrogen at the first nitrogen product pressure is partially reheated in the main heat exchanger, decompressed to a second nitrogen product pressure by means of a nitrogen expander, then further reheated by means of the main heat exchanger, and output as a product; and the nitrogen expander is braked by means of a first expander booster, the first expander booster further pressurizes the part of air feed gas that has been pressurized by means of the air booster and reaches a second pressure range to a third pressure range, and the air feed gas within the third pressure range enters, directly or optionally after undergoing further pressurization, the main heat exchanger for heat exchange with the gas and liquid products produced by means of rectification, and is then decompressed to the first pressure range by means of a liquid expander and then sent to the first tower, or a part of liquid is supercooled by means of the supercooler, then throttled and sent to the second tower.
In another aspect, the present invention further provides an apparatus for producing nitrogen and oxygen by means of the cryogenic distillation of air, comprising: a first tower working at a higher pressure and a second tower working at a lower pressure, wherein the first tower and the second tower are brought into communication by means of heat exchange via a main condensing evaporator; at least one air pre-cooling system, one air purification system, one main air compressor, one air booster, a first expander booster, at least one main heat exchanger, one nitrogen expander, at least one liquid expander, one liquid oxygen pump and one supercooler; a pipeline for sending an air feed gas to the first tower via the main air compressor, the air pre-cooling system, the air purification system and the main heat exchanger; a pipeline for sending oxygen-enriched liquid air at the bottom of the first tower to the supercooler for supercooling and to the second tower; a pipeline for sending pure liquid nitrogen at the top of the first tower to the supercooler for supercooling and to the upper part of the second tower; optionally, a pipeline for sending impure liquid nitrogen in the middle part of the first tower to the supercooler for supercooling and to the second tower; a pipeline for extracting impure nitrogen from the second tower and sending the impure nitrogen to the supercooler for warming and to the main heat exchanger for reheating; a pipeline for extracting pure liquid oxygen from the main condensing evaporator and sending the pure liquid oxygen to the liquid oxygen pump for pressurization and then through the main heat exchanger; and a pipeline for extracting pure nitrogen from the top of the lower tower and sending the pure nitrogen to the main heat exchanger; and further comprising a pipeline for sending a part of pure nitrogen, which has been reheated, from the main heat exchanger to the nitrogen expander, and returning the expanded pure nitrogen to the main heat exchanger for reheating, wherein the nitrogen expander is braked by means of a first expander booster, and the main air compressor, the air booster and the first expander booster are sequentially connected in series, and are connected to the main heat exchanger, the liquid expander and then the lower tower via a pipeline.
In certain embodiments of the present invention, a nitrogen product is only extracted from the top of the first tower, and the pressure of this nitrogen product is generally a medium pressure of 5-6 bara. If a customer needs nitrogen with a higher pressure, the reheated nitrogen can be further pressurized using a booster. If a customer needs nitrogen with a lower pressure, instead of the common practice of extracting low pressure nitrogen from a pure nitrogen tower at the top of a low pressure tower, the present invention uses the expansion of medium pressure nitrogen to obtain desired low pressure nitrogen. Furthermore, the above-mentioned nitrogen expander may be braked by means of an expander booster for compressing air. It can be seen therefrom that the present invention can flexibly produce nitrogen with different pressures, while the energy consumption for producing pressurized air can be reduced by utilizing the expansion work of nitrogen.
After medium pressure nitrogen is extracted at the top of the first tower, the amount of pure liquid nitrogen that can be used as a reflux liquid to the second tower will be reduced accordingly. This will make greater use of the rectification capacity of the second tower, and will also reduce the required diameter of the second tower, making the transportation thereof more convenient.
The drawings of the present disclosure are only intended to illustrate the present invention for construing and explaining the spirit of the present invention, but do not limit the present invention in any respect.
In the present disclosure, the term “air feed gas” refers to a mixture mainly comprising oxygen and nitrogen.
The term “pure nitrogen” covers gaseous fluids with a nitrogen content of not less than 99 mole percent, and the term “impure nitrogen” covers gaseous fluids with a nitrogen content of not less than 95 mole percent, with the content of nitrogen in the “impure nitrogen” being less than that in the “pure nitrogen”.
The term “oxygen-enriched liquid air” refers to a liquid fluid with a molar percentage of oxygen greater than 30, and the term “pure liquid oxygen” covers liquid fluids with a molar percentage of oxygen greater than 99, with the content of oxygen in the “pure liquid oxygen” being higher than that in the “oxygen-enriched liquid air”.
The term “pure liquid nitrogen” refers to a liquid fluid with a molar percentage of nitrogen greater than 99, and the term “impure liquid nitrogen” refers to a liquid fluid with a molar percentage of nitrogen greater than 96, with the content of nitrogen in the “impure liquid nitrogen” being less than that in the “pure liquid nitrogen”.
The low temperature rectification of the present disclosure is a rectification method carried out at least in part at a temperature of 150 K or less. “Tower” herein means a distillation or fractionation tower or zone in which liquid and gas phases come into countercurrent contact for effectively separating a fluid mixture. The operating pressure of the “first tower” in the present disclosure is generally 5 to 6.5 bara, which is higher than the general operating pressure of the “second tower” by 1.1 to 1.5 bara. The second tower can be installed vertically at the top of the first tower or the two towers are installed side by side. The “first tower” is also generally referred to as a medium pressure tower or a lower tower, and the “second tower” is also generally referred to as a low pressure tower or an upper tower. The main condensing evaporator is generally located at the top of the “first tower”, and it can make pure nitrogen produced at the top of the first tower condense by means of heat exchange with pure liquid oxygen produced at the bottom of the second tower to obtain pure liquid nitrogen at the top of the first tower, while the liquid oxygen is partially evaporated. Types for the main condensing evaporator include a tube and shell type, a falling film type, an immersion bath type, etc., and in the present invention, an immersion bath type condensing evaporator may be used.
The air pre-cooling system in the present invention is used for pre-cooling high temperature air (70-120° C.) discharged from the main air compressor to a temperature suitable for entering the air purification system (generally 10-25° C.). High-temperature air generally comes into contact with ordinary circulating cooling water and low-temperature water (generally 5-20° C.) in an air cooling tower for heat exchange to achieve the purpose of cooling. Low-temperature water can be obtained by bringing ordinary circulating cooling water into contact with gas products or by-products produced by the air separation apparatus, such as contact with impure nitrogen for heat exchange, or by means of a refrigerator.
The air purification system refers to a purification device that removes dust, water vapor, CO2, hydrocarbons etc. from the air. In the present invention, a pressure swing adsorption method is generally used, wherein an adsorbent is involved which may optionally be a molecular sieve plus alumina, or a molecular sieve only.
In the main heat exchanger, the compressed, pre-cooled and purified air feed gas undergoes non-contact heat exchange with gas and/or liquid products produced by means of rectification, and is cooled close or equal to the rectification temperature of the first tower, generally less than 150 K. Common main heat exchangers include split or integrated types, etc. The main heat exchangers are divided into high pressure (>20 bara pressure) and low pressure (<20 bara pressure) heat exchangers according to suitable pressure ranges. In the present invention, both a high pressure plate heat exchanger and a low pressure plate heat exchanger, or an integral combined heat exchanger may be used.
In the present disclosure, the first pressure range is consistent with the range of the working pressure of the first tower or medium pressure tower, and is generally 5 to 6 bara, and the air feed gas at atmospheric pressure can be compressed by the main air compressor to reach this pressure range. The second pressure range is a pressure range achieved by pressurizing the air feed gas within the first pressure range by means of the air booster, and is generally 40 to 60 bara. The third pressure range is achieved by further pressurizing the air feed gas within the second pressure range by means of the first expander booster and/or the second expander booster, and is generally 60 to 75 bara. The air feed gases within the second and third pressure ranges are required to be capable of exchanging heat with pressurized liquid oxygen in the main heat exchanger and causing same to evaporate and vaporize, and therefore, the specific pressure thereof is determined by the pressure of the liquid oxygen that needs to be vaporized.
The first nitrogen product pressure refers to the pressure of the pure nitrogen extracted from the top of the first tower or medium pressure tower, and is generally 4 to 5 bara. According to customer requirements, the pure nitrogen with the first nitrogen product pressure can be expanded and decompressed to obtain the second nitrogen product pressure, which is generally about 1.1 bara; alternatively, the pure nitrogen with the first nitrogen product pressure can be pressurized by means of the nitrogen booster to obtain the third nitrogen product pressure, which is generally greater than 7 bara. The second and third nitrogen product pressures can both be flexibly determined according to customer requirements.
The Rahman's principle points out that when the upper tower or the low pressure tower is used to produce pure oxygen, the rectification capacity of the low pressure tower is not fully utilized. In the present invention, one or more of the following measures are used to improve this situation, in order to increase the efficiency of the entire air separation system, reduce energy consumption, and even reduce the volume of the tower. One of the measures is to introduce part of the air feed gas directly into the upper tower, i.e., the low pressure tower, so as to utilize the excess rectification capacity of this tower; a second one of the measures is to draw the pure nitrogen produced at the top of the medium pressure tower as a nitrogen product, and correspondingly, the amount of pure liquid nitrogen obtained after condensation by the main condenser will be reduced, that is, the amount of the reflux liquid sent to the low pressure tower will be reduced. On the one hand, the reduction of the reflux liquid will make further use of the rectification capacity of the low pressure tower; on the other hand, the reduction of the reflux liquid requires a reduction in the processing capacity of the low pressure tower, and the diameter of the low pressure tower can be reduced accordingly, causing easier transportation. Furthermore, compared with pure nitrogen with a pressure of about 1 to 2 bara extracted from the top of the low pressure tower as a nitrogen product, the pressure of pure nitrogen drawn from the top of the medium pressure tower is generally 4 to 5 bara. If the pressure of a nitrogen product required by a customer is greater than 4 to 5 bara, such as 10 bara, the energy consumption for pressurizing the pure nitrogen extracted from the medium pressure tower to 10 bara is greatly reduced than that for pressurizing the pure nitrogen extracted from the low pressure tower to 10 bara. If the pressure of a nitrogen product required by a customer is less than 4 to 5 bara, such as 1 bara, the pure nitrogen extracted from the medium pressure tower can be expanded to 1 bara, and the expansion work can be used for power generation or a shaft-linked expander booster, thereby reducing the energy consumption of the entire air separation system.
As shown in
The air feed gas introduced into the first tower 11 is rectified in the first tower to produce oxygen-enriched liquid air 110 at the bottom of the tower and pure nitrogen at the top of the tower. By means of indirect heat exchange in the main condensing evaporator 12 with the liquid oxygen produced at the bottom of the second tower, a part of pure nitrogen is condensed into pure liquid nitrogen. The part of pure liquid nitrogen mentioned above is used as a reflux liquid to the first tower. Optionally, a part is sent to a storage tank as a liquid nitrogen product, and the other part 112 is supercooled and then input into the upper part of the second tower 13 as a reflux liquid. Furthermore, gases that are supercooled and input into the second tower also include the oxygen-enriched liquid air 110, and optionally, the impure liquid nitrogen 111 extracted from the middle part of the first tower 11 and the part of liquid air feed gas 108. The above-mentioned streams are throttled and decompressed to about 1.3 to 1.4 bara and then transported to the second tower 13, and participate in rectification therein, and then, the impure nitrogen 121 with a pressure of about 1.3 bara can be extracted from the upper part of the second tower, while the pure liquid oxygen 122 with a pressure of about 1.4 bara is obtained at the bottom of the second tower. When a customer needs a high pressure oxygen product, the pure liquid oxygen can be pressurized to about 88 bara by means of a liquid oxygen pump 31, and then evaporated and vaporized by the high pressure air feed gas 106, 104 in the high pressure main heat exchanger 2 to obtain the high pressure oxygen product. When a customer also needs a medium pressure oxygen product, a part of high pressure liquid oxygen from the liquid oxygen pump can be throttled and decompressed to obtain medium pressure liquid oxygen with a pressure of about 30 bara, which is then similarly evaporated and vaporized by the high pressure air feed gas 106, 104 in the high pressure main heat exchanger 2 to obtain the medium pressure oxygen product.
In this embodiment, the only nitrogen product is the medium pressure pure nitrogen 123 with a pressure of about 5.5 bara drawn from the top of the first tower 11. When a customer needs both low pressure and high pressure nitrogen products, the following operations can be carried out. In the middle part of the low pressure main heat exchanger 1, part 124 of the medium pressure pure nitrogen 123, which has been partially reheated, is extracted, decompressed to a desired pressure, which is referred to as a second nitrogen product pressure, by means of the nitrogen expander 23, then returned to the main heat exchanger, and completely reheated to obtain a second nitrogen product. The nitrogen expander 23 is braked by means of the first expander booster 24, thereby converting the expansion work into energy required for the compressed air feed gas. After the remaining part of the medium pressure pure nitrogen 123 is completely reheated by means of the low pressure main heat exchanger 1, this remaining part can be output at the first nitrogen product pressure as a first nitrogen product, or pressurized to a third nitrogen product pressure, as required by a customer, by means of a nitrogen booster and output as a third nitrogen product.
The main difference between the embodiments shown in
The rest of the embodiment shown in
The following simulation calculations compare the operating costs of the air separation apparatus.
The simulation calculations listed in the following table are carried out using ASPEN software for an air separation system with an oxygen output of 100,000 Nm3/h. In the air separation system, a main heat exchanger is involved which is of an aluminum plate-fin type, and a main air compressor (MAC) and an air booster (BAC) are involved which are both steam turbines driven by high pressure steam. The calculation of the operating cost is based on a high pressure steam price of 100 RMB/ton and is evaluated based on 5 years of operation.
Due to the extraction of additional 27000 Nm3/h of medium pressure pure nitrogen, the recovery of O2 obtained according to the present invention is slightly lower than that of the comparative example; however, the loss here is much less than the overall energy saving achieved by the present invention. In the above table, the “medium pressure pure nitrogen extraction” refers to the flow rate of the medium pressure pure nitrogen extracted from the top of the first tower, the “low pressure pure nitrogen extraction” refers to the flow rate of the low pressure pure nitrogen extracted from the top of the pure nitrogen tower, and the “flow rate of conversion of medium pressure pure nitrogen to low pressure pure nitrogen” refers to the flow rate of the part of medium pressure pure nitrogen that is drawn from the middle part of the main heat exchanger and sent to the nitrogen expander 23. In the present invention, due to the utilization of the work done by the nitrogen expander, the work required for producing the air feed gas with substantially the same pressure and flow rate by the air booster (BAC) is reduced, and the corresponding high pressure steam consumed is also reduced. Based on a total of five years, an operating cost of about 10 million can be saved.
While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.
The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.
“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.
“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.
Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.
Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.
All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.
This application is a § 371 of International PCT Application PCT/CN2017/113525, filed Nov. 29, 2017, which is herein incorporated by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2017/113525 | 11/29/2017 | WO | 00 |
