RU2776906C1 - Ammonia-water absorption cooling system - Google Patents

Abstract

FIELD: refrigeration.

SUBSTANCE: invention relates to refrigeration equipment. Proposed is an ammonia-water absorption cooling system. Liquid ammonia (10) is evaporated to produce ammonia vapours and the refrigerating effect. The ammonia vapours are absorbed in depleted aqueous ammonia solution, resulting in an enriched solution. Gaseous ammonia (17) is desorbed under high pressure from the enriched solution in order to produce gaseous ammonia and regenerate the depleted solution (18). The gaseous ammonia (17) is condensed and expanded, reducing the pressure, in order to be further used in the evaporator (1), wherein the absorption stage includes cooling and condensing a two-phase mixture (36) of gaseous ammonia and the depleted solution in at least one evaporation condenser (30). The first part (13a) of the depleted aqueous solution from the desorber is mixed with the gaseous ammonia in order to form a two-phase mixture directed to the evaporation condenser, and the second part (13b) of said solution is mixed with at least partially condensed flow (39, 43) from the evaporation condenser (30) in order to form the inlet flow of the further-located absorber.

EFFECT: increase in the efficiency of the ammonia-water cooling system.

14 cl, 4 dwg, 2 tbl

Description

Technical field

The invention relates to an ammonia-water absorption cooling system and a method for upgrading it. A preferred application for an absorption cooling system is the cooling of process gas in an ammonia synthesis plant.

State of the art

Ammonia absorption cooling system generally includes:

an ammonia evaporator in which substantially pure liquid ammonia evaporates;

an absorber in which ammonia vapor from the evaporator is absorbed by a depleted solution of ammonia and water (aqueous ammonia solution) to obtain an enriched solution with the transfer of some heat to the cooling medium;

a desorber in which gaseous ammonia is extracted from the enriched solution, and the lean solution mentioned above is separated for further use in the absorber;

a condenser in which the gaseous ammonia withdrawn from the stripper is condensed to produce liquid ammonia, which is sent back to the evaporator.

Evaporation of ammonia in the evaporator creates the desired cooling effect, for example by extracting heat from another stream.

The driving force behind this process is the heat supplied to the desorber to separate the refrigerant vapor from the rich solution. Compared with the conventional vapor compression refrigeration system (VCRS), the absorption refrigeration system uses low-grade heat instead of electrical energy as an energy source to produce a cooling effect.

The system described is also known as an aqua ammonia refrigeration plant (AARP).

The evaporator and absorber operate at the first pressure; the desorber and the air cooler are operated at a second pressure, the second pressure being higher than the first. Usually the first pressure is in the range from vacuum to 5 bar.

The terms lean solution and rich solution refer to the ammonia content of the aqueous solution, i.e., the lean solution contains less ammonia than the rich solution.

Absorption cooling systems are used, among other things, for cooling process gas in ammonia plants. Accordingly, the process gas transfers heat to the ammonia evaporator. For example, an ammonia evaporator is an indirect heat exchanger in which the process gas is cooled on one side and the evaporating ammonia is on the other side.

The ammonia plant mainly comprises a head section for generating make-up synthesis gas containing H ₂ and N ₂ from hydrocarbon feedstock, a compression section for increasing the pressure of the make-up gas to the synthesis pressure, and a synthesis loop for catalytically converting the make-up gas into a gaseous product, mainly made up of ammonia.

Said make-up gas and product gas are produced at elevated temperatures and the heat content (enthalpy) is typically recovered in a series of heat exchangers, including, for example, waste heat boilers and/or boiler feed water (BFW) heaters. ). Typically, absorption refrigeration systems are used to cool a product gas downstream of a water cooler or gas/gas heat exchanger to condense and separate ammonia from said product gas.

There is a need to modernize existing ammonia plants to increase their current or potential productivity and/or reduce the consumption of energy or other valuable resources.

In an ammonia-water refrigeration system, the absorber gives off heat and therefore requires a cooling medium. This cooling medium in most cases (particularly in ammonia production) is fresh cooling water. However, the amount of cooling water may be limited and its use as a cooling medium may be costly. Therefore, when planning the modernization of the ammonia synthesis plant, it is also advisable to improve the water-ammonia cooling system.

Water absorption cooling is disclosed, for example, in WO 2012/042496.

Disclosure of invention

The objective of the present invention is to eliminate the disadvantages of the existing technology described above. The invention is aimed at improving the efficiency of the ammonia-water cooling system. In particular, the invention is directed to reducing the consumption of cooling water for cooling the absorber. The invention is also directed to a method for retrofitting an ammonia-water cooling system, in particular when this cooling system is part of an ammonia plant and is used to cool at least one ammonia synthesis process gas.

These tasks are solved by the method in accordance with paragraph 1 of the claims. The invention modifies the absorption step by introducing a preliminary step carried out in an evaporative condenser. The absorption step includes: mixing gaseous ammonia from the evaporation step and a depleted ammonia solution to form a two-phase mixture; at least partial condensation of this two-phase mixture in at least one evaporative condenser; feeding the thus obtained and at least partially condensed mixture to an absorber, where any remaining gaseous ammonia can be absorbed in a lean solution.

Another feature of the invention is the water-ammonia cooling system according to the claims.

Another feature of the invention is a method of modernization (re-equipment) of the water-ammonia cooling system in accordance with the claims.

Some preferred embodiments are represented by the appended dependent claims.

Gaseous ammonia and lean solution, according to various embodiments, may be mixed before entering the flash condenser, or may be mixed directly in the flash condenser.

The evaporative condenser transfers heat from the mixture of ammonia gas and lean solution to the evaporating medium, usually water, indirectly (i.e. without direct contact) in a closed circuit. This water, in turn, transfers heat to the surrounding air through direct contact.

Preferably, said two-phase mixture of gaseous ammonia and lean solution enters the inside of at least one heat exchange section of the evaporative condenser; cooling water is sprayed on the outer surface of this heat exchange section; inside the evaporative condenser, ambient air circulates in direct contact with the atomized cooling water. Due to heat and mass transfer with the surrounding air, water is cooled, which can be repeatedly passed through a closed circuit. Accordingly, it can be said that the actual heat sink of the evaporative condenser is the ambient air and the achievable cooling effect depends on the temperature and humidity of this ambient air.

Said mixture passes inside the heat exchange section, and evaporating cooling water and air act on its outer surface. This heat exchange section may be, for example, a condenser tube, more preferably a coil. To improve heat transfer, this heat exchange section may be provided with fins. Water is sprayed onto the surface of the heat exchange section, causing at least a portion of the sprayed water to evaporate and heat to be removed. Preferably, the water to be sprayed and the air stream flow in countercurrent, with the water moving down and the air stream going up. At the bottom of the evaporative condenser, water can be collected.

The evaporative condenser provides at least partial condensation of the two-phase mixture of ammonia gas and lean liquor, reducing the amount of heat to be removed in the absorber, for example by transferring to the absorber cooling water. Accordingly, thereby reducing the amount of cooling water required for the operation of the absorber.

In some embodiments, at least one evaporative condenser provides partial condensation of the mixture. The first part of the gaseous ammonia contained in this two-phase mixture is condensed in said at least one evaporative condenser, and the second part of the gaseous ammonia is condensed in the absorber. Preferably, the first part and the second part constitute or substantially constitute the total amount (eg, at least 99%) of the gaseous ammonia initially contained in the mixture.

In some embodiments, at least one evaporative condenser ensures complete condensation of the mixture. The term complete condensation means that the ammonia gas has undergone complete condensation, with the exception of the residual amount of uncondensed ammonia due to the presence of liquid-vapor equilibrium in the condenser and the presence of a non-condensable fraction. In the fully condensing embodiment, at least 99% of the ammonia gas is preferably condensed.

Thus, at least one evaporative condenser can be provided to ensure complete condensation during normal operation; a downstream absorber may provide subcooling of the condensate effluent and may prevent the possibility of some ammonia gas remaining in the evaporative condenser effluent due to, for example, operating conditions not well suited for ammonia condensation.

By mixing the ammonia gas from the ammonia evaporator and the lean ammonia solution, a two-phase mixture is obtained, preferably having a vapor fraction of 5% to 20%. The vapor fraction consists mainly of gaseous ammonia.

The applicant has found that this mixture has a relatively high temperature, about 50°C, for example, in the range from 45 to 65°C. The present invention is based on the claim that this mixture can be cooled and partially condensed in an evaporative condenser due to its relatively high temperature compared to ambient temperature.

For example, in a preferred embodiment, an evaporative condenser may be adapted to cool the mixture of ammonia vapor and lean solution to a temperature of 30 to 40°C.

In a preferred embodiment, in the evaporative condenser, the vapor fraction of this two-phase mixture is reduced by at least 30%, preferably by at least 50%. Accordingly, the mixture at the inlet of the evaporative condenser has a first vapor fraction, and the mixture leaving after the evaporative condenser has a second vapor fraction ranging from 0.5 to 0.7 of the first vapor fraction.

The evaporative condenser feed stream may contain all or only a portion of the ammonia gas from the evaporative condenser and the lean solution from the stripper. Preferably, the full amount of ammonia gas is used.

In some embodiments, the lean solution is split between an evaporative condenser and an absorber. Specifically, a first part of the lean solution is mixed with ammonia gas to form an evaporative condenser feed stream, and a second part of the lean solution is mixed with an evaporative condenser effluent. The first part of the lean solution, which is preferably from 20% to 100%, the second part of the lean solution forms the remaining difference up to 100%.

Lean liquor separation can be advantageous in some embodiments by reducing the capital cost of the evaporative condenser.

In accordance with the above, part of the absorption process also takes place in the evaporative condenser, where the gaseous ammonia condenses into a liquid phase and passes into an aqueous solution, thereby enriching it.

In existing technology, the heat removed from the ammonia gas and lean solution is completely transferred to the absorber cooling water. In the present invention, on the contrary, part of the heat is released to the environment through an evaporative condenser. Thus, the need for cooling water is reduced.

Another advantage is that the ability of the lean solution to absorb gaseous ammonia depends not only on the concentration of ammonia in the solution, but also on the temperature of the solution. In particular, a lower temperature corresponds to a higher ammonia absorption capacity. Therefore, cooling the solution in the evaporative condenser improves the performance of the downstream absorber, namely by increasing the amount of ammonia that can be transferred to the solution.

Another advantage of the invention is that the saved cooling water, which is no longer required by the absorber, can be used elsewhere. For example, in an ammonia synthesis plant, the saved cooling water can be used to provide additional cooling capacity in one or more ammonia water refrigeration units (AARP), or in a Li-Br absorption unit.

The advantages of the invention will be better understood on reading the following detailed description relating to the preferred embodiment.

Brief description of the drawings

Below the invention is discussed in more detail with reference to the accompanying drawings, in which:

in fig. 1 is a simplified diagram of a water-ammonia absorption refrigeration plant in accordance with the prior art;

in fig. 2 is the diagram shown in Fig. 1 modified in accordance with an embodiment of the invention;

in fig. 3 shows another embodiment of the invention;

in fig. 4 shows another embodiment of the invention.

Detailed description of the invention

In FIG. 1 shows a water-ammonia absorption refrigeration system, including: an ammonia evaporator 1, an absorber 2, a desorber 3 with a heat exchanger 4, a condenser 5, an ammonia solution pump 6, pressure relief valves 7 and 8.

Liquid ammonia 10 evaporates in the ammonia evaporator 1. When the liquid ammonia 10 evaporates, gaseous ammonia (ammonia vapor) 11 is formed and the stream 12 is cooled, creating a cooling effect. Stream 12 is, for example, a process stream from an ammonia synthesis reaction.

Gaseous ammonia 11 is absorbed in the absorber 2 in a depleted aqueous ammonia solution 13. The mixture of gaseous ammonia and the depleted solution gives off heat to the cooling water 14 of the absorber 2. This absorber 2 is, for example, a shell and tube apparatus in which cooling water circulates in its tube space.

Enriched ammonia solution 15 is removed from absorber 2. This enriched solution 15 is pumped by pump 6 of ammonia solution into heat exchanger 4 and further into desorber 3.

The enriched high-pressure solution 16 supplied by the pump 6 is heated in the heat exchanger 4 and enters the desorber 3.

This desorber 3 removes gaseous ammonia 17 from the rich solution and recovers the depleted ammonia solution 18. The desorption process requires heat input, which is provided by the heat transfer medium 19, for example, a low pressure flow.

Stripper 3 in the preferred embodiment can be a distillation column, in which gaseous ammonia exits through the top of the column, and the regenerated lean solution is removed from its bottom.

The depleted aqueous ammonia solution 18 heats the incoming enriched solution in the heat exchanger 4 and, after depressurizing the pressure relief valve 8, forms a depleted solution stream 13 directed to the absorber 2.

The gaseous ammonia 17 recovered from the stripper 3 is condensed in a condenser 5, which is, for example, an air-cooled condenser. The high pressure liquid ammonia 20 thus obtained is depressurized by means of a pressure relief valve 7 to form a liquid ammonia stream 10 directed to the evaporator 1.

Before condensation in the condenser 5, a step of cooling the gaseous ammonia 17 may be performed. In addition, heat recovery may take place between the liquid ammonia 10 and the gaseous ammonia 11.

It should be understood that the main energy from the process comes from the coolant 19, which provides heat for the regeneration of the depleted solution. On the other hand, the process transfers a significant amount of heat to the cooling water 14 and thus requires a large amount of this water.

Evaporator 1 and absorber 2 operate at a first pressure, and desorber 3 and condenser 5 operate at a second pressure greater than the first. Pump 6 and valves 7, 8 determine the level of high pressure and low pressure in the process. In typical embodiments, low pressure is in the range of 100 to 450 kPa and high pressure is in the range of 900 to 1400 kPa.

In FIG. 2 is a diagram modified in accordance with an embodiment of the invention. In FIG. 2 uses the same numerals as in FIG. 1 for the corresponding features, which therefore do not need to be described again.

An evaporator condenser 30 is installed between evaporator 1 and absorber 2. This evaporative condenser belongs to the low-pressure part of the plant, i.e., it operates at approximately the same pressure as the absorber.

The evaporative condenser 30 includes a coil 31 and an atomizer 32. Water 33 is collected from the bottom of the condenser 30 and sprayed onto the coil 31 by a pump 34. The evaporative condenser 30 also has an ambient air inlet 35.

Gaseous ammonia 11 and lean solution 13 are mixed to form a two-phase mixture 36, which is fed into the coil 31. Flowing through the tube 31, the mixture 36 is cooled and partially condensed due to the transfer of heat to the water 33 sprayed onto the coil 31, which partially evaporates.

The atomized water falls down in countercurrent and in direct contact with the air 35. As a result, some water vapor is transferred to the air, increasing its humidity, and the temperature of the water decreases. The air leaves the condenser as air stream 37 having a greater moisture content than inlet stream 35. Preferably, outlet stream 37 is saturated or close to saturated.

The water 33 is enclosed in a closed circuit, except for the addition 38, which compensates for the amount transferred to the air.

The cooled and partially condensed two-phase mixture exits the evaporative condenser 30 as stream 39 to the downstream absorber 2.

It should be understood that the heat that can be removed by the evaporative condenser 30 ultimately depends on the temperature of the ambient air 35 as measured by the wet bulb temperature. The temperature of a wet bulb bulb depends on temperature and humidity according to a known relationship.

For example, if the ambient temperature 35 is 28°C and the relative humidity is 65%, the temperature on the wet bulb is 22.5°C. Assuming a temperature drop across the ends of the evaporative condenser equal to 14°C, the incoming mixture 36 can be cooled to about 36.5°C. The mixture 36 in most cases has a temperature in the range from 45 to 65°C. This means that the evaporative condenser 30 can significantly reduce the temperature of the mixture 36 and thus can significantly reduce the capacity of the absorber and the amount of cooling water 14 required to operate it.

For example, the incoming mixture has a vapor fraction of about 12%, while the outlet mixture 39 has a vapor fraction of about 6% to 7%.

Under certain conditions of reduced air temperature, the evaporative condenser can allow a significant amount of heat to be removed from the mixture without using water 33. In this case, the spraying of water 33 can be temporarily stopped, causing the evaporative condenser to operate as an air cooler.

The invention can be applied to retrofit an ammonia synthesis plant including an ammonia water refrigeration plant (AARP) as shown in FIG. 1. Stream 12 in this case is an ammonia synthesis process stream. For example, stream 12 may be a hot make-up gas (containing N ₂ and H ₂ ) for ammonia synthesis, or ammonia gas produced in an ammonia converter.

The retrofit method may include installing an evaporative converter 30 between ammonia vaporizer 1 and absorber 2, and appropriate pipe connections, such as in FIG. 2.

The lean solution 13 and gaseous ammonia 11 can be mixed in a suitable device.

In FIG. 3 illustrates an embodiment where an ejector 40 is used to mix the lean solution and ammonia gas sent to the condenser 30.

In particular, in the embodiment shown, only a portion 13a of the lean solution is supplied to the ejector 40. The remaining part 13b of the depleted solution is sent to the absorber 2, where it is sprayed into the annular space by the atomizer 41.

The effluent 39 of the evaporative condenser 30 is fed to the gas-liquid separator 42. The liquid fraction 43 is sent to the nebulizer 41 together with the lean solution 13b, while the gaseous fraction 44 is fed to the annulus for condensation.

Fig. 4 illustrates another embodiment that uses a contact chamber 50 installed before an evaporative condenser 30. In the contact chamber 50, ammonia gas 11 is brought into contact with a lean solution 13a. The mixture 36 sent to the evaporative condenser 30 is withdrawn from the chamber 50. The ammonia gas stream 51 exiting from the top of the chamber 50 is directed to the annulus of the absorber 2. The effluent 39 of the evaporative condenser 30 mixes with the lean solution 13b to form a liquid feed stream which is sprayed into absorber 2 sprayer 41.

Example

The following is an example for comparison. Table 1 shows the data for the basic circuit according to the prior art (FIG. 1), while Table 2 shows the data for the embodiment of the invention according to FIG. 2. The comparison shows that the required amount of cooling water (stream 14) is reduced by 50%.

Claims (26)

1. The method of water-ammonia absorption cooling, including: an evaporating step including evaporating liquid ammonia (10) in an ammonia evaporator (1) to obtain gaseous ammonia (11) and a cooling effect; an absorption step comprising absorbing gaseous ammonia into the lean aqueous ammonia solution (13) and removing heat during the absorption process to obtain a rich aqueous ammonia solution (16); a desorption step comprising recovering gaseous ammonia (17) from the rich solution in the stripper (3) to obtain gaseous ammonia and a lean solution (18) for further use in the absorption step; condensing the gaseous ammonia (17) obtained in the desorption stage to obtain liquid ammonia for further use in the evaporator, moreover, the evaporation stage and the absorption stage are carried out at a first pressure, and the desorption and condensation stage are carried out at a second pressure higher than the first pressure, characterized in that during the absorption step: mixing at least part of the gaseous ammonia (11) from the evaporation step and at least part of the depleted ammonia solution (13) to obtain a two-phase mixture (36); feeding the two-phase mixture to at least one evaporative condenser (30) in which at least a portion of the gaseous ammonia contained in the mixture is condensed; the thus obtained at least partially condensed exhaust mixture (39) is fed into the absorber (2), wherein the first part (13a) of the depleted aqueous solution from the desorber is mixed with gaseous ammonia to form a two-phase mixture sent to the evaporative condenser, and the second part (13b) ) of said solution is mixed with the at least partially condensed stream (39, 43) from the evaporative condenser (30) to form an inlet stream of the downstream absorber. 2. The method according to claim 1, wherein the first part of the gaseous ammonia contained in the two-phase mixture is condensed in at least one evaporative condenser and the second part of the gaseous ammonia is condensed in the absorber. 3. The method according to claim 1, wherein all of the gaseous ammonia contained in the two-phase mixture is condensed in at least one evaporative condenser. 4. The method according to any one of paragraphs. 1-3, in which the gaseous ammonia withdrawn from the ammonia evaporator has a temperature in the range from -33 to 20°C. 5. The method according to any one of paragraphs. 1-4, in which the two-phase mixture obtained by mixing gaseous ammonia and lean solution has a mole fraction of steam in the range from 5 to 20%. 6. The method according to any one of the preceding claims, wherein the biphasic mixture resulting from mixing the ammonia gas and the lean solution has a temperature in the range of 45 to 65°C. 7. The method according to claim 6, wherein the two-phase mixture is cooled in at least one evaporative condenser to a temperature of 30 to 40°C. 8. A process according to any one of the preceding claims, wherein the vapor fraction of the two-phase mixture in the evaporative condenser is reduced by at least 30% by condensing ammonia gas. 9. The method according to any one of the preceding paragraphs, in which in the evaporative condenser the two-phase mixture passes inside at least one section (31) heat exchange; cooling water (33) is sprayed onto the outer surface of the heat exchange section and ambient air (35) circulates in the evaporative condenser in direct contact with the sprayed cooling water. 10. The method of claim. 1, in which the first part of the depleted aqueous solution is from 20 to 100% of the total depleted solution withdrawn from the stripper. 11. A method according to any one of the preceding claims, wherein the absorber uses water as a cooling medium. 12. Ammonia absorption cooling system, comprising: ammonia evaporator (1) for evaporating liquid ammonia (10); an absorber (2) configured to absorb gaseous ammonia in the depleted aqueous ammonia solution to obtain a rich aqueous ammonia solution; a desorber (3) for extracting gaseous ammonia from said rich solution and separating the lean solution for subsequent use in the absorber; a condenser (5) for condensing the gaseous ammonia withdrawn from the desorber to obtain liquid ammonia for further use in the evaporator, characterized in that it comprises at least one evaporative condenser (30) installed between the ammonia evaporator and the absorber, so that the two-phase mixture (36) containing gaseous ammonia from the ammonia evaporator and the depleted aqueous solution from the stripper is cooled and at least partially is condensed in at least one evaporative condenser, and the thus obtained partially condensed effluent stream of at least one evaporative condenser is sent to the absorber, and the first part (13a) of the lean aqueous solution from the stripper is mixed with gaseous ammonia to form a two-phase mixture sent to the evaporator condenser, and the second part (13b) of the solution is mixed with the at least partially condensed stream (39, 43) from the evaporative condenser (30) to form an inlet stream for the downstream absorber. 13. A method for upgrading a water-ammonia cooling system containing an ammonia evaporator (1), an absorber (2), a desorber (3) and a condenser (5), moreover, in the specified water-ammonia cooling system, liquid ammonia evaporates in the ammonia evaporator; the ammonia gas stream thus obtained is absorbed in the depleted aqueous ammonia solution in the absorber to form a rich aqueous ammonia solution; gaseous ammonia is recovered from the rich solution in the desorber, thereby regenerating the lean solution, and the gaseous ammonia is further condensed to liquid ammonia in the condenser, and during the modernization, at least one evaporative condenser (30) is installed between the evaporator (5) and the absorber (2) and a line is installed for supplying gaseous ammonia (11) withdrawn from the evaporator (1) and a depleted aqueous solution (13) of ammonia from desorber (3) into at least one evaporative condenser (30), in which the two-phase mixture (36) of gaseous ammonia and depleted solution is partially condensed, and the stream of partially condensed mixture leaving the evaporative condenser is sent to the absorber (2), and the first part (13a) of the depleted aqueous solution from the desorber is mixed with gaseous ammonia to form a two-phase mixture sent to the evaporative condenser, and the second part (13b) of this solution is mixed with at least partially condensed stream (39, 43) from the evaporative condenser (30) to form an inlet stream for the downstream absorber. 14. The method of claim 13 wherein the ammonia water cooling system is part of the ammonia synthesis unit and is used to cool at least one process stream of the ammonia synthesis process.

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