Energy-saving synthetic ammonia process and heat pump method decarburization
Energy-saving synthetic ammonia process and heat pump decarbonization Chen Zonghua (Lanzhou Chemical Machinery and Automation Research & Design Institute, Lanzhou, Gansu 730060) The production of synthetic ammonia consumes a lot of energy. How to minimize the energy consumption of synthetic ammonia production has always been an eternal topic for synthetic ammonia producers to consider. With the further intensification of energy shortages in the world, various energy prices will become higher and higher. Therefore, energy conservation and consumption reduction will become powerful measures for companies to reduce product costs and increase market competitiveness. As a means of energy-saving heat pump, it is one of the most popular technologies to reduce energy consumption and increase energy efficiency. This article will study the process of energy-saving ammonia synthesis and its main energy-saving measures, and focus on the great energy-saving benefits of heat-pumped decarburization. 1 Energy-saving synthetic ammonia process The traditional synthetic ammonia process energy consumption is generally high, in order to reduce energy consumption, must take energy-saving measures in the three major blocks of gas, purification and synthesis. In general, the following measures can be used to save energy and increase conversion efficiency. Specifically, the operating pressure of a reformer can be increased (a better furnace tube is selected) and a better catalyst is selected. 2) Reduce the water-carbon ratio to save a lot of steam. 3) Leave a larger conversion load of the reformer to the secondary reformer. 6) Reduce compression costs. New synthetic column internals can be used to reduce pressure drop; good catalysts can also be used to operate at low pressures. 7) Recovery of H2 from the purge gas of the synthesis loop to save raw materials, etc. Due to the differences in specific energy-saving measures, a variety of energy-saving synthetic ammonia processes have been created, such as Brown Process, Avi Process, Low Energy Kellogg Process, Danish Topsoe Process, Exxon Process, and German Wood Process. , Germany Linde process, the United States PARC process, India PDIL process. The following will focus on the representative natural gas as raw material 1.1 Brown process American Brown process design tons of ammonia energy consumption of 28GJ, the core technology is the use of excess air, gas turbines, heat pump decarburization, cryogenic purification, friendly operation and molecular sieve drying, The main features are as follows: 1) A mild stage of conversion and excess air in the secondary reformer. The conversion ratio of water to carbon in the reformer section of the conversion furnace is approximately 30% at the outlet 0H4 and 3.00 MPa at the temperature of 694. The excess air in the secondary reformer is 50%, which will generate a large amount of heat that can be utilized. Since more of the conversion load is transferred to the secondary reformer (exothermic reaction), the working conditions in the primary reforming furnace (endothermic reaction) are moderated, thereby reducing the loss of fuel consumption and extending the equipment. life. °C gas turbine exhaust is used for combustion in a reformer. 3) Low-energy benzene decarburization process (hereinafter referred to as heat pump decarburization process) adopts four-stage jet flashing and fifth-stage vapor flash pump suction flash evaporation. 4) Set up a cold box on the main flow after methanation to finally purify the syngas and separate excess N2 (referred to as cryogenic purification). 5) Three axially adiabatic ammonia synthesis towers are connected in series, with three waste heat boilers in series, with a high net ammonia value and 125 MPa high pressure steam as a byproduct (referred to as the friendly operation of the synthesis tower for short). British ICI's AMV process adopts some of the advanced technologies of Brown Process and Kellogg Process to design a ton of ammonia energy consumption of 28.27GJ. Its main features are: 1) The raw gas is saturated with process condensate (using high-gas-varying heat) to reduce the process Steam consumption. 2) A slight amount of reformer and secondary reformer plus excess air. A conversion furnace water-to-carbon ratio of 2.5, export CH4 for the study of fluid machinery, graduate master degree, the current Lanzhou Chemical Machinery and Automation Research and Design Institute vice president of chemical machinery research and Compressor Research Institute, senior Brown T Arts, craftsmanship and low energy Kellogg process. Engineer, has been fighting more than 30 papers 731 leaven Chen Zonghua. Energy-saving synthetic ammonia process and heat pump method Decarburization 16.3% Temperature 804°Q Pressure 42MPa The secondary reformer excess air 25%. This measure greatly reduces fuel gas consumption and steam consumption. 3) A gas turbine driven air compressor. The gas turbine exhaust gas containing 2 15.8% and 550°C is used for combustion in a reformer, recovering waste heat and saving fuel. 3) Improve the design of a reformer. Large furnace tubes are used to reduce the number of furnace tubes; ceramic fiber insulation materials are used to reduce heat loss. Low-energy consumption Benfield decarburization (modified Benfield method) with jet flash semi-lean liquid. 4) A four-stage injection flash and a fifth-stage low-energy benzene decarbonization process for steam flash pump suction are used. Type 1 ammonia synthesis catalyst, so the synthesis loop operating pressure is low (only 10. 2MPa), reducing the energy consumption of the synthesis gas compressor. 6) Use cryogenic purification process (but different from Brown process). Fresh gas does not enter the cryogenic system (cold box) directly, but part of it is withdrawn from the recycle gas to the cryogenic system. The excess N2 is recovered from the recycle gas by cryogenic separation, and H2 is recovered, and the hydrogen and nitrogen in the recycle gas are regulated. ratio. 1.3 Low-energy Kellogg Process Low-energy Kellogg Process The designed tonnage ammonia energy consumption is 29GJ. Its main features are as follows: 1) Improved conversion conditions. The reformer outlet pressure is increased to 35MPa and the outlet temperature is reduced to 793C. The mixture temperature in the inlet pipe is increased to 621C; the process air temperature in the secondary reformer is increased to 850C. 5) The H2O and trace C2 in the fresh gas are removed by molecular sieve to make fresh Gas can enter the entrance of the synthesis tower directly. 6) A horizontal synthesis tower using a new interlayer heat exchanger. 7) The use of four ammonia cooling and combined ammonia cooler. 8) Recovery of bleed in the purge gas using a Prison device. 9) Using a gas turbine driven air compressor, a high-temperature, high-temperature gas turbine exhaust is used for combustion in a reformer. 2 heat pump decarburization system In most energy-saving synthetic ammonia process (such as Brown process, AMV process), the use of heat pump decarburization is one of the most important energy-saving measures. The traditional typical decarburization process (for two-stage absorption, two-stage regeneration), heat consumption per cubic meter of CO2 is 50241J; after the jet decarburization process using a steam jet pump, the heat loss per cubic meter of C2 is 33491J, which is more than typical method. Energy-saving 33% Heat pump decarburization process after the use of steam compressors The decarbonization process per cubic meter of CO2 heat consumption is 19641J, which is 61% energy saving than the typical method. Obviously, heat pump energy saving effect is higher than that of injection 2) Addition of combustion air preheater to recover flue gas Excess heat makes the method more pronounced. The process flow of exhaust gas temperature dropping to 149C heat pump decarburization (see) is to improve the shortcomings of the basic process of the typical method of two-stage absorption and secondary regeneration. From the typical method, it can be seen that the lean liquid directly comes out from the bottom of the regeneration tower, and the temperature is too high for the absorption tower. If it is not cooled, it will inevitably lead to heat loss; and the temperature of the low-temperature change outlet is also high. Therefore, it is possible to use a low-saturation gas as a heat source and a condensate as a medium to set up a low-pressure boiler to generate approximately 0.4 MPa (G) of steam, and to pass this steam through a flash tank and a steam compressor. The steam and the output from the bottom of the regeneration tower are large. The nitrogen-fed dilute solution flashes the secondary steam flashed out of the flash tank and is sent to the bottom of the regeneration tower after being pressurized by a steam compressor. In this way, by recovering the heat of the low-conversion gas and the lean liquid, even if the amount of steam obtained by the regenerator is increased (the regenerated energy is increased and the concentration of the solution is adjusted), the solution is completely regenerated, and the temperature of the lean liquid after passing through the flash tank is reduced. (In this case, the lean liquid temperature is applied to the middle of the absorption tower, and the top of the absorption tower still needs to be cooled again. By lowering the temperature, the purification rate of the raw material gas at the top of the absorption tower is ensured). The rich liquid from the bottom of the absorption tower (degree of conversion is 0.75-0.9, pressure is about 2.5 MPa), after the energy is recovered by the hydraulic turbine (driven by the lean liquid pump), the pressure is reduced (close to normal pressure) directly to the top of the regeneration tower, due to the rich The liquid pressure is reduced and the temperature is also reduced. C2 and water vapor in the rich liquid are partially flashed at the top of the regeneration tower (after cooling and separation, CO2 and condensate are obtained respectively, and the condensate must be pumped back to the top of the regeneration tower). Then, the rich liquid flows down the packing of the regeneration tower, contacts with the bottom-up steam countercurrently, and simultaneously carries out mass transfer and heat transfer. The C2 in the liquid phase is continuously stripped, the alkali liquid is regenerated into an lean liquid, and finally the lean liquid is The bottom of the regeneration tower flows out of the tower and cools down through the flash tank to divide it into the middle and top of the absorption tower. Obviously, the core of the heat pump decarburization process is the use of steam compressors. Through the suction and pressurization of steam compressors, a large amount of heat required for regeneration is obtained at the expense of less mechanical energy, and the recovery is low. The variable gas and lean liquid heat achieve the dual purposes of energy utilization and energy saving. Steam compressors in heat pump devices, also known as thermal compressors, are generally centrifugal compressors. Table 1 lists the main technical parameters of the centrifugal steam compressor used in the Brown process and the AMV process, respectively. Since the steam compressor in the heat pump decarburization process is a centrifugal compressor that compresses water vapor (containing a small amount of (1)2) as the main medium and has a high rotation speed, the media compression factor and medium corrosivity must be taken into account in design calculations. , high temperature, vibration, strength and other factors, in order to achieve the purpose of efficient, reliable, safe, long-term operation of the equipment. Table 1 The main technical parameters of the centrifugal steam compressor technology Brown process AMV process compression medium inlet pressure / MPa (A) inlet temperature / ° c outlet pressure / MPa (A) outlet temperature / ° c mass flow / kgDh1 motor speed / r. Min1 motor power / kW / 3 Conclusion 1) energy-saving synthetic ammonia technology generally in the gas, purification and synthesis of the three major blocks were adopted to recover waste heat, reduce compression costs, save fuel (raw) materials and other major energy-saving measures to significantly reduce energy consumption. 2) Most of energy-saving synthetic ammonia processes such as brown process, AMV process, and low-energy Kellogg process use heat pump decarburization devices to save energy, and the decarbonization energy saving ratio accounts for about 40% of total energy conservation. 3) The heat pump method decarburization is 61% more energy-saving than the typical method. The energy-saving effect is extremely significant. It is recommended that all major domestic ammonia plants adopt this energy-saving decarburization process technology.
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