Liquid ammonia is widely used in the production of chemical fertilizers, pesticides, medicines, etc., as well as ammoniation of chemical products. It can be used as a refrigerant in refrigeration systems and as a propellant for rockets and missiles in the defense industry. A hydrogen-nitrogen mixed gas prepared by ionizing pure liquid ammonia is commonly used as a shielding gas for industries such as semiconductors and metallurgy. The national standard stipulates that liquid ammonia products are classified into three grades according to the concentration of ammonia: 99.9% of superior products, “Fund Project: 99.8% of products for marine public welfare industry research projects (201305011), 99.6% of qualified products. Different quality liquid ammonia per The price difference is about 50 yuan. However, according to the place of production, the ex-factory price fluctuates greatly, about 2400~3300 yuan/t. At present, the synthetic ammonia production method commonly used in industry, although the process is very mature and can reach a large scale, its energy consumption is high and the pollution is serious. The production liquid of the method currently contains a large amount of ammonia-containing tail gas and by-product ammonia in the domestic chemical industry, which needs to be recovered and purified. The cost of purifying ammonia is much lower than that of direct ammonia synthesis, which will bring huge economic and environmental benefits. Since the ammonia water mixture vapor has a higher pressure, the higher the ammonia concentration of the gas component in the saturated state, is industrially purified by compressing the ammonia gas. However, the use of existing reciprocating piston compressors and general purpose refrigeration injection screw compressors for compressed ammonia is very difficult. The reciprocating piston compressor has poor running stability under the condition that the gas volume is large and the particles contain more impurities such as carbon ammonium. Due to the introduction of impurities such as lubricating oil, the oil-injected screw compressor can not meet the standard of the product quality; at the same time, impurities such as h2s in the medium gas are easily mixed into the lubricating oil, resulting in deterioration of the lubricating oil. The working chamber of the oil-free screw compressor is separated from the bearing housing by a sealing device, which can well avoid the above problems.

Nowadays, chemical companies have adopted an oil-free screw compressor with ammonia spray and preliminary calculations using chemical process software, but the specific physical process is not clear, and theoretical calculations are needed and supported by actual operational data.

Considering that the compression process is a heat and mass transfer process of a two-phase binary non-azeotropic mixture, it is very complicated. In this paper, the calculation of ammonia water property and the calculation of the balance tube flow rate of the screw compressor are firstly solved. Based on the research of the water jet screw compressor, the two-phase compression process is simulated by the idea of ​​wet compression and spray refrigerant compression. Calculate and compare the calculation results with the field operation data. Then the key parameters such as exhaust pressure, intermediate pressure and intermediate temperature were optimized and their effects on system performance were analyzed.

2 Application and technical parameters 2.1 Process flow As shown, the system is a two-stage ammonia gas compression system, and observe the nodes in the figure. The first stage compressor draws 0.19 MPa, 30 ft, 98% low pressure and low temperature superheated ammonia water vapor (1, while injecting 55, 0.62 MPa saturated ammonia solution (8. It is assumed that the inhalation and the spray can be instantaneously mixed into a two-phase flow ( 2. When the amount of liquid is small, the compression may be started after pure gas. The compressed exhaust Q is filled with a certain amount of 40 ft, 5% dilute ammonia 4, and after mixing 5 is cooled to a two-phase flow by the first-stage cooler ( 6, and enter the first-stage gas-liquid separator separation. The separated gas is the second-stage suction (7, the liquid is the first-stage spray, and the excess liquid is discharged to the ammonia crude distillation tower. The second-stage process and The first stage is similar, but no refilling is required after compression. The high-pressure ammonia gas separated by the second-stage gas-liquid separator (13 concentration should reach 99.6% or more, and then passed into the ammonia cooler to be cooled to liquid ammonia. 2.2 State change observation, in the figure 1-3s1 indicates compression of the superheated ammonia vapor without suction, and the dotted line 1-3s' indicates isentropic compression, and the solid line 1-3' indicates actual compression.

After the liquid is sprayed, when the amount of the liquid reaches a certain level, as shown in (a, the compressor inhales and mixes into a two-phase flow, and as the compression proceeds, the liquid evaporates, and finally a high-pressure superheated vapor is obtained; as shown, the liquid is sprayed When the amount is further increased to the critical value, the high pressure vapor at the compressor outlet reaches a saturated state; the amount of liquid spray continues to increase, as shown in (C, the high pressure two-phase flow at the compressor outlet).

After the compressor exhaust gas enters the compression, the cooler is cooled to the two-phase flow, and after the gas-liquid separation, most of the liquid is sprayed and returned to the compressor.

The gas after the first stage gas-liquid separation is introduced into the second stage compressor for further compression, and the second stage compression process is performed with the first stage.

2.3 System parameters, see Table 1 Indicators First stage Second stage Compressed medium Superheated Ammonia Water vapor Intake air temperature Intake pressure Exhaust pressure Theoretical volume Flow sprayed Saturated ammonia solution Note: Ignore a very small amount of oxygen and nitrogen, only consider ammonia and water.

Note: The spray state parameter is taken as the liquid state parameter in the gas-liquid separator after compression, and the pressure loss from the gas-liquid separator to the spray port is not considered.

The 1+ hand shouts 2.4 process gas compressor features to ensure the process compression reliability and make the compressor run smoothly. The two-stage oil-free screw ammonia compressor unit is designed with three sets of balance tubes. Unicom: Uni-stage exhaust balancing chamber and first-stage suction balancing chamber, secondary exhaust balancing chamber and secondary suction balancing chamber, and secondary exhaust balancing chamber and stage suction balancing chamber.

3 Calculation model 3.1 Performance calculation model (3) Calculate the compressor inlet flow rate m2 and the enthalpy entropy value of the gas-liquid component P. Suppose that m2 is liquid after spraying, containing liquid component ma, gas component. By mass conservation, energy conservation relationship, subscript 2 indicates the compressor inlet state, l indicates the liquid component, g indicates the gas component, and 3 and 11 indicate the first and second exhaust states, respectively. Simultaneous solution can obtain various state parameters at the entrance.

(2) Calculating the shaft power The gas composition is compressed in equal volume according to the isentropic compression liquid component.

It is assumed that the gas-liquid binary component is a dynamic equilibrium process that can enter a new equilibrium state after a very small pressure change. According to the material conservation and energy balance relationship, the heat generated by the compressor of the lower type will evaporate part of the liquid and continuously update the state of the compressor inlet gas. Until then, there is a subscript i indicating the compressor inlet state, and o indicating the compressor outlet state. .

3 Calculating the motor output power The transmission efficiency of the motor nd represents the ratio of the compressor shaft power ns to the motor output power Nd. It has the following relationship with the efficiency of the motor and the efficiency of the frequency converter. 3.2 Calculation of the ammonia water property The bubble point and dew point formula of the ammonia water mixture are taken from the parameters.

1 The bubble point temperature in the liquid state 2 The dew point temperature ç„“ entropy and specific volume in the gaseous state.

3.3 Balance tube calculation model The gas flow in the balance tube should be analyzed. The steady-state uncompressible fluid driven by the differential pressure on both sides should have a frictional adiabatic flow in the equal-section pipeline. It is known that the balance pipe length is 1, the pipe diameter D, the high pressure side pressure Pi, the low pressure side pressure, the high pressure side temperature Ti, and the mass flow rate in the pipe is calculated. M. Firstly, the formula of the critical length of the adiabatic friction is applied, wherein the subscript i=1, 2 , respectively, the f-wall friction coefficient of the high pressure side and the low pressure side of the pipeline, the stainless steel tube can be taken as 0.04k - the adiabatic index, the ammonia gas is taken as 1.29 R gas constant, and the ammonia gas is taken as 489, (kg-K then lists the adiabatic friction threshold) The pressure formula a/Butterfly 傩农 ru and the high pressure side can be obtained according to the equation of state supplemented by the following equations. The mass flow can be obtained. 4 Results analysis 4.1 Comparison of calculation results with actual operation data Selecting the operation data and calculation under a typical working condition The results are compared. The results are shown in Table 2. Table 2 Field operation data and calculation data comparison refers to the first level second level operation data estimation data operation data estimation data inspiratory pressure / MPa inhalation temperature household c inspiratory volume / Kghh spray volume / kg-h-1 balance gas volume / kg-h-1 volumetric efficiency /% exhaust pressure / MPa exhaust temperature household c cooling water flow / m3-h-1 cooling water temperature rise / C consumption Work AW pure ammonia production / kg-h-1 motor output power operating data: 1379kW, estimation According to: 1407kW analysis data comparison: 1 cooler heat load. The design values ​​of the two heat exchangers are 10 liters. The first stage cooler has a large heat load, but it is due to the first stage ammonia road effusion. More, the heat exchange efficiency is poor, the waterway valve is fully open to meet the heat transfer requirements during operation. The calculation shows that the thermal load of the first-stage cooler is greatly affected by the first-stage compression and rehydration. The selection parameter is: temperature 40 ft, the greater the pressure replenishment amount, the more it can absorb the dissolved ammonia gas, the more the heat release, the greater the heat load of the aftercooler.

For the second stage cooler, the heat exchange efficiency is higher, and due to the low feed water temperature in winter, the cooler can operate at a lower flow rate than the design temperature difference.

2 spray volume. When the unit is running, the amount of liquid should be controlled according to the exhaust temperature. Now check the calculation, fix the exhaust temperature to calculate the amount of liquid spray, and ignore the cooling effect of the cooling fan on the casing and part of the heat taken away by the bearing cavity lubricant. Considering the heat dissipation of the casing and the lubricating oil, the spray volume is about 70%~80% of the calculated value for the same exhaust gas temperature. Q balance gas volume. The first stage balance gas comprises two parts, which are respectively from the first stage exhaust end balance chamber and the second stage exhaust end balance chamber. Considering that the balance of the balance ring and the exhaust end of the carbon ring seal is poor, the pressure of the balance chamber at the exhaust end is equal to the discharge pressure.

4 motor output power. Additional motor power consumption includes motor energy loss, inverter energy loss, power consumption of the gearbox, and cooling fan power consumption. Considering the power consumption of the speed increasing box and the power consumption of the cooling fan, the motor transmission efficiency should be more than 85.5%.4.2 liquid ammonia quality control parameter. The saturated water vapor mixture at a specific concentration should be in accordance with the curve. Shown. The higher the pressure, the lower the temperature and the higher the concentration of the ammonia component. When changing the operating conditions of the liquid ammonia, care should be taken to control the temperature and pressure in the second-stage gas-liquid separator to separate a sufficient concentration of ammonia.

Increasing the pressure of ammonia to increase the quality of the product will increase the power consumption of the compressor. The lowering of the temperature of the gas-liquid separation increases the requirement for cooling water. For lower cooling water temperatures in winter, lower pressures can purify ammonia. Considering the summer environmental requirements, the gas-liquid separation temperature should not be lower than 40 ft, and the corresponding separation pressure should not be lower than 1.17 MPa. - 99.6% secondary exhaust pressure / MPa. When other parameters are constant, the compressor sprays The relationship between the amount of liquid and the compressor discharge temperature and power consumption is shown in . When the spray is below the critical value, the cooling is insufficient, and the exhaust gas temperature and power consumption are significantly increased; when the spray liquid is above the critical value, the exhaust gas temperature and power consumption hardly change. Therefore, in order to ensure the safe operation of the compressor, the amount of liquid spray must be higher than the critical value, the first stage is 0.19kg / s, the second stage 4.4 optimizes the two-stage discharge pressure fixed two-stage spray volume, select the first stage spray The liquid temperature is 45 ft, and the second-stage spray temperature is selected according to the solid line to control the concentration of the finally separated ammonia gas to reach 99.9%. The result of optimizing the calculation of the intermediate pressure, the solid line in the figure indicates the theoretical intermediate pressure dotted line indicates the calculated result. The intermediate pressure Pm at the lowest energy consumption, 4.5 optimizes the two-stage spray temperature to take the second-stage discharge pressure of 1.55 MPa, and the first-stage discharge pressure is 0.50 MPa, taking the two-stage compression as a whole to consider the fixed spray volume. Change the temperature of a certain spray liquid and observe the change of the energy consumption of the compressor of liquid ammonia. The calculation result is shown in 7. Since the spray temperature is the temperature required by the gas-liquid separator, there is a certain range. First, the spray temperature must be high enough. The first stage spray temperature should be determined according to the ambient temperature, and should not be lower than 40 ft. The second-stage spray temperature also needs to consider the nature of the ammonia: the dew point temperature of the aqueous ammonia solution decreases as the ammonia concentration increases. When the ammonia concentration rises to 100%, the dew point temperature reaches a minimum value, that is, the evaporation temperature of the pure ammonia solution. It is 39.7T at 1.55 MPa: the gas-liquid separator is lower than this temperature and the gas cannot be separated. Therefore, the temperature of the gas-liquid separator must be higher than the temperature to separate enough high-pressure gas. Secondly, there is an upper limit to the temperature of the spray. The higher the temperature of the spray, the less liquid is separated from the gas-liquid separator. After a certain temperature, sufficient liquid cannot be separated for spray cooling. The upper limit temperature is calculated as: the first stage is 62.5 U, the second stage is the relationship between the energy consumption per unit mass of liquid ammonia compressor and the second stage sputum temperature. In fact, for two separate compressors, the amount of liquid spray is fixed, and the spray is changed. The temperature, the compressor power consumption of liquid ammonia, decreases with the increase of the temperature of the spray, that is, it should show a downward trend. However, if the whole two-stage compression is considered, the increase of the first-stage gas-liquid separation temperature will inevitably lead to the second-stage cooling deficiency, so that the second-stage power consumption is increased to a higher degree than the first-level reduction, and the overall performance is an upward trend. .

5 Conclusion The cost of producing liquid ammonia by purification method is much lower than that of traditional ammonia synthesis method, which has good economic and environmental benefits. Due to its excellent stability and non-contamination of process gases, oil-free screw compressors enable the ammonia gas compression process using this machine to meet the industrial requirements for liquid ammonia purification.

By injecting an aqueous ammonia solution during the compression process, the compressor discharge temperature can be significantly reduced, the outlet liquid content can be changed, and the problem of the rotor being bitten due to excessive temperature can be prevented. The calculation can initially determine the amount of liquid spray and the power consumption of the compressor, and optimize the adjustment of the spray parameters accordingly.

According to the nature of the ammonia water, the concentration of the separated ammonia gas can be controlled by controlling the temperature and pressure of the gas-liquid separation. When gas-liquid separation, if the ammonia water can be cooled to a lower temperature, it is only necessary to compress the ammonia gas to a lower pressure, and the pressure corresponding to 40 psi is 1.18 MPa. Therefore, the temperature of the gas-liquid separation should be minimized, and This temperature depends on the ambient temperature and the cooling capacity of the cooling water.

For a system with two-stage compression, the fixed spray volume, the first-stage spray temperature is 45 ft, and the second-stage spray temperature is taken as the critical temperature corresponding to the 99.9% concentration, the intermediate pressure can be taken as the first-stage spray temperature. The lower the compressor, the lower the energy consumption, so the intermediate cooling temperature should be minimized. If it can be reduced from 45 to 35 feet, the compressor energy consumption can be reduced by about 5%. For a type of process screw compressor with a balance tube, the flow of the balance gas cannot be ignored. When the balance chamber and the exhaust chamber are not well sealed and the balance chamber pressure is high, the flow rate may be large, which in turn affects the performance specifications of the compressor and the entire system.

Small Accessories For Tin Can

Oil Can Lid,Tinplate Caps,Metal Press Caps,Plastic Spout Lid

Jiangsu Fast Pack Co., Ltd , https://www.fast-metalpack.com