JPS6348581B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for transporting and compressing impure process gases, such as in dual purpose pressure plants for the production of nitric acid and caprolactam. More particularly, the invention removes and prevents the precipitation of crystalline ammonium nitrate in nitrous gas compressors and other process gas streams in which such precipitation occurs. Regarding the method.

Ammonium nitrate precipitates formed, for example, by unreacted ammonium from the catalytic oxidation of _NH3 to NO, reduce the flow capacity of nitrogenous gas compressors and
Increases power consumption and makes rotating parts of the compressor unstable. Furthermore, such salt precipitation can become a safety hazard if salt accumulation is not prevented or limited.

Since the day the first nitrogenous gas compressor was built in a nitric acid plant 30 years ago, water injection or atomization has been routinely used to remove salt deposits. Compressors are constructed with multiple rows of spray nozzles in their flow channels to inject water periodically during plant operation. The time interval between each cleaning operation is 4~
Varying over 36 hours, each cleaning operation time is 10-30
It's a minute. Typical water additions during cleaning operations are 0.5 to 2.0 grams per kmole of process gas. Some nitrogenous gas compressors even provide continuous water injection in addition to discontinuous cleaning.

Water is also commonly added continuously to the compressor seal system to prevent labyrinth blockage caused by salt deposits. This water injection, which can be in an amount of up to 500 kg/hour, remains in the product by evaporating into the process gas or being removed as condensate. Significant amounts of water are thus used to suppress salt precipitation. However, the addition of such wash water to the process gas or to the condensate is undesirable. This is because this water must be balanced by a corresponding reduction in the amount of water added to the absorption system, thus reducing the absorption efficiency or the highest concentration of product that can be achieved. be.

Despite prolonged water injection into the compressor, cleaning does not remove enough deposits to restore full performance. This is because the inertia of the ejected water droplets prevents them from wetting or moistening the entire surface on which salt precipitation occurs. In some compressors, the salt buildup on these surfaces is so extensive that it is necessary to shut down the compressor from time to time to allow more thorough cleaning to restore original compressor capacity.

The jet water droplets also have a significant erosive effect on the compressor, particularly on the rotor blades of axial compressors and on the riveted joints of centrifugal compressors.

Furthermore, more effective removal of salt deposits is particularly desirable in order to maintain a greater average flow capacity in the compressor to increase plant productivity.

Thus, the main object of the present invention is to provide a new and improved method for the removal of salt deposits, thereby avoiding the various serious deficiencies mentioned above.

The physical conditions for removing crystalline salt deposits are that the salt must be converted to a liquid state or the temperature must be raised to a temperature at which the salt sublimes or decomposes. By comparing the vapor pressure of water on a mixture of ammonium nitrate and water and the crystallization temperature (melting point) for the above mixture in equilibrium, we find that there is a temperature-dependent upper limit on the vapor pressure at which crystalline ammonium nitrate can exist. I discovered that. This relationship is illustrated graphically in FIG. In FIG. 1, curve 1 shows the vapor pressure of water above a saturated solution of the salt, and curve 2 shows the saturation point or dew point of the process gas. These curves show that crystalline ammonium nitrate cannot exist under any circumstances at temperatures above 170° C. or at water vapor pressures above 0.25 bar.

Adiabatic or polytropic compression of the process gas provides a relationship between pressure and temperature. Thus, one skilled in the art can calculate the change in water vapor pressure as a function of the temperature change in the flow channels in the compressor. FIG. 2 shows the change in vapor pressure of water in a given compressor as a function of temperature. Curves 1 and 2 correspond to curve 2 for water vapor content of 35°C to 163°C.
This shows that salt precipitation is possible between ℃ and ℃.

These basic data are used by the invention to prevent the presence of salt deposits by appropriately increasing the vapor pressure of the water by special injection of external steam. If the salt deposits have to be removed by increasing the vapor pressure of the water, i.e. by the addition of external steam, the vapor pressure of the water is equal to that of the salt at the same temperature present in the compressor. Must be greater than the saturation pressure above the precipitate. An example of minimal steam addition is illustrated in curve 3 of FIG. For conventional compressors, this curve for minimum steam consumption is tangent to the saturation curve for salt deposits at approximately 110° C. and a water vapor pressure of 0.25 bar.

If the polytropic index of the compressor and the water content of the process gas before steam injection are known, the steam consumption can be calculated. This steam consumption is usually approximately 0.067 to 0.075 kg per m ³ of injected gas (actual volume). The % reduction in nitrogen oxide loading during steam cleaning is therefore strongly dependent on the suction pressure, thus ranging from about 14% when the suction pressure is 0.9 bar to about 2 when the suction pressure is 5 bar. %
It changes over time.

Deposits can also be prevented in compressor sealing systems by replacing water with pure steam or a mixture of steam and air, which steam can form at all temperatures, in the sealing system or in additional drainage pipes. Even the highest temperature should give a saturated salt solution.

Thus, in accordance with the present invention, a novel method for eliminating or preventing the formation of salt precipitates as described above is provided. The features of this method and the structural arrangements for carrying out the method are described below and defined in the claims.

The technical effect obtained by the method of the invention is that the physical conditions for salt removal are immediately established and that the precipitate can be deposited virtually independently of where it is deposited or how difficult it is to access the precipitate. This produces the effect of instantly cleaning all surfaces covered by objects.

Practical tests have shown that the normal required amount of steam is not significantly higher than the above-mentioned minimum required amount and that the precipitates present are already dissolved after 15-20 seconds of processing.

The method and apparatus for supplying steam are illustrated in FIGS. 3 and 4 with three different applications: low pressure compressors, high pressure compressors, and high pressure compressor sealing systems.

Example 1 A thermal nitrogen oxide-containing gas 1 from an atmospheric combustion plant was cooled to 30° C. in a gas condenser 2. The cooled gas 3 (pressure was 0.9 bar absolute) was compressed to 4-3.0 bar absolute in an axial compressor. The temperature at outlet 5 of the compressor was 200°C. The hot gas was then cooled and passed to an absorption tower where nitric acid was produced (not shown). The compression capacity when the compressor is clean is 1800 kmol/h, which corresponds to a volume of 50000 m ³ /h at inlet 3. During nitric acid production, the charging amount gradually decreased due to ammonium nitrate precipitation. This rate of reduction corresponds to a reduction of approximately 5% in the daily charge. The compressor was not subjected to continuous water injection, but was treated with steam from the steam reservoir (steam source) 6 every 8 hours. The charging amount during this process is approximately 30 kmol/
Only time decreased. Steam saturated at 5 bar absolute pressure was injected into the suction side of the compressor through a perforated tube (nozzle) 7 placed at right angles to the gas flow and approximately 11/2 m in front of the compressor. The perforated tube has a diameter of 100 mm and contains approximately 80 holes of 15 mm in diameter.
The injection of steam is regulated by a valve 8 and the amount of injection is recorded by a measuring device 9. The generated condensate was discharged to a condensate receiver 10. During the steam cleaning process, 3500 kg/hour of steam was introduced for about 15 seconds. The total amount of steam during the cleaning period was between 15 and 25 kg, depending on how quickly the nitrogen oxide charge could be changed without disturbing the combustion equipment. The temperature at the inlet 3 of the compressor during steam addition is 45℃
The temperature at outlet 5 of the compressor decreased to 195°C. Temperature and pressure changes in other parts of the nitric acid plant were negligible during steam injection.
During the steam addition, the flow rate of nitrogen oxide-containing gas 1 from the combustion plant was reduced by about 14%. After steam injection, the compressor was again cleaned and operated at full capacity.

Example 2 Thermal nitrogen oxide-containing gas 1 (Figure 4) was cooled to 30°C in a condenser 2 and then compressed in a centrifugal compressor. The pressure of the compressor suction example 4 was 4.5 bar absolute, the pressure at the compressor outlet 5 was 10 bar absolute and the temperature was 100°C. Compressor capacity is 1800
in kilomoles/hour, which is at the compressor inlet 4
Corresponding to the amount of 10000m ³ /hour.

The compressor was cleaned every 8 hours using 700 Kg/hour of steam for approximately 1/2 minute each (total steam amount was 6-10 Kg). Steam saturated at 10 bar absolute pressure is supplied to the steam source (steam storage tank) 6.
was introduced into the process gas through two nozzles located approximately 11/2 meters in front of the compressor and arranged to uniformly mix with the process gas stream. Changes in temperature and pressure before and after the addition of steam were negligible and did not disrupt production.

During the addition of steam, the flow rate of nitrogen oxide-containing gas 1 was reduced by about 2%. After steam addition, the compressor was again operated at full capacity.

Example 3 Steam from the steam source 6 was injected continuously into the labyrinth gland seals 17 and 18 of the compressor, but by means of special pipes and nozzles. Airlocks 11, 12 are attached to both packing boxes of the compressor, which also include ventilation devices 13, 14 and devices 15 and 16 for discharging steam and condensate. Approximately 1 kg/hour of steam is passed through the labyrinth seal 17, and approximately 2.5 kg/hour is
Time steam was introduced through the labyrinth seal 18. The conventional water injection of about 200 kg/hour was thus replaced and effective prevention of salt formation was achieved.

Based on the results from the above example and on the basis of additional tests carried out, per ^m3 of process gas.
It was concluded that an amount of steam should be added between 0.02 and 3 Kg, but preferably between 0.067 and 0.075 Kg ^per cubic meter of process gas.

Furthermore, there is no need to inject steam into the compressor and other parts of the plant for periods longer than 15 minutes. However, it is desirable to add steam at intervals of one minute or less during plant operation. Since the flexibility of the steam injection is so great in this method, there is no reason to implement an optimum interval for the interval of the steam injection.

By replacing the conventional water washing method with the salt removal method of the present invention, the cleaning operation has become much more efficient. Unwanted water addition to the nitric acid production process was reduced by up to 90% and the required cleaning time was significantly reduced. Furthermore, the period between each wash operation was shortened and the average material charge to the system was increased. The injected steam eliminated the erosion problems caused by injecting water into the compressor.

Compressor construction can also be simplified by using steam for cleaning instead of water cleaning. The provision of a perforated tube or similar form of steam nozzle upstream of the compressor can replace the conventional multiple rows of small spray nozzles, which currently must be installed in the compressor flow slot. Must be. This requires drilling through the compressor casing, resulting in a complex and more expensive construction geometry.

We have thus obtained significant benefits from the cleaning method and apparatus according to the present invention. Even though it is not surprising that salt precipitation is eliminated when the vapor pressure of water is increased above the equilibrium pressure for a saturated solution of the salt, it is surprising that the salt dissolves in such a short time as in the method of this invention. That's true. Although the addition of water to the compressor must normally continue for up to 30 minutes without returning to its original flow capacity, complete salt cleaning can be achieved in just 15 to 30 seconds with steam injection. achieved. Even though the current steam consumption of the low pressure compressor is relatively high, this short cleaning time significantly reduces the total water addition to the process. The short cleaning time means that production losses during cleaning are small. In high-pressure compressors, the immediate requirements for steam are much lower, so that the instantaneous addition of water is less than during water washing.

The prejudice against removing salt deposits by steam injection is probably due to the fear that the higher temperature of the steam at the inlet will lead to an unacceptable temperature rise at the compressor outlet. However, water vapor has a greater specific heat capacity than the process gas, and our tests have shown that the addition of some amount of steam actually results in a decrease in outlet temperature during cleaning.

Although this invention has been described primarily for the prevention and removal of salt deposits in nitrogen-containing gas compressors and auxiliary seal systems where precipitation problems are greatest, the method of this invention is useful for other methods where such problems occur. Obviously, it can also be used in areas.
In fact, this technique is generally suitable for other systems in which salt precipitates form and in which the necessary conditions for immediate dissolution of salt precipitates can be established by adjustment of temperature and vapor pressure. It is something that

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between vapor pressure and temperature in a system where crystalline ammonium nitrate exists, Figure 2 is a graph showing the relationship between water vapor pressure and temperature, and Figures 3 and 4 are graphs showing the relationship between the vapor pressure and temperature of a system in which crystalline ammonium nitrate exists. 1 shows a schematic cross-sectional view of a steam supply device according to FIG. In the figure: 1...Nitrogen oxide-containing gas, 2...Gas condenser,
3...Cooled gas, 4...Compressor inlet (suction side),
5... Compressor outlet, 6... Steam source (steam storage tank), 7... Nozzle (perforated pipe), 8... Valve, 9... Measuring device, 10... Condensate receiver, 11, 12... Air lock, 13, 14... Ventilation Device, 15, 16... Steam, condensate discharge device, 17, 18... Labyrinth packing seal.

Claims (1)

Claims: 1. In a method for removing and preventing the formation of salt deposits in a predetermined area of a plant for transporting and compressing process gases containing ammonium nitrate and similar impurities, the said area is injected with steam and water. A method characterized in that it is supplied in such an amount that the vapor pressure is greater than the saturated vapor pressure of water above the salt deposit at the temperature of the zone. 2 Steam is placed upstream of the nitrogen-containing gas compressor,
Alternatively, the method according to claim 1, wherein the method comprises occasionally injecting into the inlet of the compressor. 3. The method according to claim 2, wherein steam is added to the nitrogen-containing gas compressor in an amount of 0.01 kg to 3 kg per m ³ of process gas. 4. A method according to claim 3, in which steam is supplied into the nitrogen-containing gas compressor in an amount of 0.067 Kg to 0.075 Kg per m ³ of process gas. 5. The method according to any one of claims 1 to 4, in which steam is injected for a short period of 15 minutes or less. 6. The method according to claim 5, wherein steam is injected for a short period of one minute or less. 7. The method according to claim 1, wherein steam is injected into a sealing system of a compressor. 8. A patent claim in which steam is injected together with air and the amounts of steam and air are adjusted so as to obtain a water vapor content that always corresponds to an amount that saturates the salt deposits at the highest temperature present in the sealing system. The method described in scope item 7. 9. The plant is equipped with a steam injection device connected to a steam source 6, said device comprising a steam pipe connected to a nozzle 7 installed upstream of the zone in which salt deposits are removed or the formation of salt deposits is prevented. Consisting of
Steam is applied to a given area of the plant in which process gases containing ammonium nitrate and similar impurities are to be transported and compressed in an amount such that the vapor pressure of the water is greater than the saturated vapor pressure of the water above the salt deposit at the temperature of that area. Apparatus for carrying out the method of removing and preventing the formation of salt deposits in said areas by feeding. 10. The device according to claim 9, wherein the compressor of the device includes a nozzle for injecting steam. 11 The compressor used is an axial compressor or a centrifugal compressor, both of which have a compressor casing without a device for mounting water washing nozzles arranged in longitudinal rows, 10. The apparatus according to claim 9, comprising one or more steam nozzles 7 projecting into the gas inlet 3 of the compressor on the inlet or suction side of the compressor.