Comparison between cryogenic nitrogen production and PSA nitrogen generators

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Comparison between Cryogenic Nitrogen Production and PSA Nitrogen Generators I. Introduction With the rapid development of industry, nitrogen is widely used in fields such as chemicals, electronics, metallurgy, food, and machinery. In China, the demand for nitrogen is increasing at a rate of over 8% per year. Nitrogen has inert chemical properties; under normal conditions it exhibits high inertness and does not readily react with other substances. Therefore, nitrogen is widely used as a shielding gas and sealing gas in the metallurgical, electronics, and chemical industries. The purity required for such shielding gases is generally 99.99%, with some applications demanding even higher purity levels of 99.998% or more. Liquid nitrogen is a convenient cold source that is being used increasingly in various fields such as the food industry, healthcare, and the storage of semen in the livestock sector. In the fertilizer industry, during the production of synthetic ammonia, if the feed gas for synthesizing ammonia – a mixture of hydrogen and nitrogen – is purified by washing with pure liquid nitrogen, the content of inert gases can be reduced to extremely low levels, with the content of sulfur monoxide and oxygen not exceeding 20 ppm. Pure nitrogen cannot be obtained directly from nature; the air separation method is primarily used. Air separation methods include: cryogenic method, pressure swing adsorption (PSA), and membrane separation. II. Process Flow and Equipment Overview of PSA Nitrogen Generators 1. Overview of the process flow: Air enters the air compressor after passing through an air filter to remove dust and mechanical impurities; it is then compressed to the desired pressure. Following rigorous oil removal, water removal, and dust removal purification processes, clean compressed air is produced, with the aim of ensuring the longevity of the molecular sieves used in the adsorption tower. There are two adsorption towers equipped with carbon molecular sieves; while one tower is in operation, the other undergoes pressure-reduced desorption. Clean air enters the working adsorption tower; as it passes through the molecular sieve, oxygen, carbon dioxide, and water are adsorbed by it. The gas that reaches the outlet consists of nitrogen along with trace amounts of argon and oxygen. The other tower (desorption tower) releases the adsorbed oxygen, carbon dioxide, and water from the micropores of the molecular sieve and discharges them into the atmosphere. In this way, the two towers take turns to carry out nitrogen-oxygen separation, thereby continuously producing nitrogen, as shown in Figure 2. The purity of nitrogen produced by pressure swing adsorption is 95%-99.9%; if higher purity nitrogen is required, nitrogen purification equipment must be added. 95%-99.9% of the nitrogen produced by the pressure swing adsorption nitrogen generator enters the nitrogen purification equipment; at the same time, an appropriate amount of hydrogen is added via a flow meter. In the deoxygenation tower of the purification equipment, catalytic reactions occur between hydrogen and the trace amounts of oxygen present in the nitrogen, thereby removing oxygen. The gas is then cooled in a water condenser, the water is removed using a vapor-water separator, and finally it is dried thoroughly in a dryer (two adsorption drying towers are used alternately: one is used for adsorption drying to remove water, while the other is used for heating to desorb and remove water). As a result, high-purity nitrogen is obtained, with a purity level of up to 99.9995%, as shown in Figure 3. At present, the largest production capacity for pressure swing adsorption nitrogen production in China is 3,000 m3/nh. III. Introduction to the Process Flow and Equipment for Cryogenic Nitrogen Production 1. Typical process flow for cryogenic nitrogen production: The entire process consists of air compression and purification, air separation, and liquid nitrogen vaporization. ⑴ The compressed and purified air enters the air compressor after dust and mechanical impurities have been removed by an air filter; it is then compressed to the desired pressure and sent to an air cooler to reduce its temperature. It then enters an air drying and purification unit to remove moisture, carbon dioxide, acetylene, and other hydrocarbons from the air. ⑵ Air separation: The purified air enters the main heat exchanger in the air separation tower, where it is cooled to saturation temperature by the backflow gas (product nitrogen and waste gas). It is then sent to the bottom of the distillation tower, from where nitrogen is obtained. The liquid air is sent to the condensation evaporator after throttling; this process causes part of the nitrogen coming from the distillation tower to condense. The condensed liquid nitrogen is used partly as reflux fluid in the distillation tower and partly as liquid nitrogen product that exits the air separation tower. The exhaust gas exiting the condensation evaporator is rewarmed to about 130 K in the main heat exchanger before being fed into an expander to perform expansion refrigeration and supply cooling capacity to the air separation tower. A portion of the gas after expansion is used for the regeneration and cooling of molecular sieves, and then it is discharged into the atmosphere through a silencer. ⑶ The vaporization of liquid nitrogen occurs when the liquid nitrogen coming out of the air separation tower is stored in a liquid nitrogen storage tank. When the air separation equipment needs maintenance, the liquid nitrogen in the tank enters the vaporizer, where it is heated before being sent to the product nitrogen pipeline. Deep cryogenic nitrogen production can yield nitrogen with a purity of ≥99.999%. IV. Technical and Economic Comparison between Cryogenic Nitrogen Production and Pressure Swing Adsorption Nitrogen Production 1. Process Comparison From the above discussion, it can be seen that the pressure swing adsorption nitrogen production process is simple, with fewer pieces of equipment; the main devices include only air compressors, air dryers, nitrogen production units via adsorption, and gas storage tanks. The cryogenic nitrogen production process is complex and involves a large number of equipment, with the main ones including air compressors, air coolers, air purification and drying units, heat exchangers, expanders, and fine flow towers. Nitrogen generator 2: Comparison of product types and purities. Cryogenic nitrogen production not only enables the generation of nitrogen gas but also liquid nitrogen, meeting the process requirements for liquid nitrogen. The liquid nitrogen can be stored in storage tanks; when there is a intermittent demand for nitrogen gas or minor maintenance is required of the air separation equipment, the liquid nitrogen in these tanks is fed into a vaporizer where it is heated before being sent to the nitrogen gas pipeline to satisfy the needs of the process equipment. The operating cycle of cryogenic nitrogen production (referring to the interval between two major heating cycles) is generally over 1 year; therefore, backup systems are usually not considered for cryogenic nitrogen production. On the other hand, pressure swing adsorption nitrogen production can only generate nitrogen; there is no alternative method, and a single unit of equipment cannot ensure continuous operation over long periods of time. Deep cryogenic nitrogen production can yield nitrogen with a purity of ≥99.999%. The purity of nitrogen is constrained by factors such as the nitrogen load, the number of tray levels, the efficiency of those tray levels, and the oxygen purity in the liquid phase; as a result, the range within which it can be adjusted is very limited. Therefore, for a set of cryogenic nitrogen production equipment, the purity of the product remains relatively constant and cannot be adjusted. The purity of nitrogen produced by pressure swing adsorption nitrogen generation is generally in the range of 95%-99.9%; if higher purity nitrogen is required, nitrogen purification equipment must be added. The purity of nitrogen is affected only by the nitrogen load of the product; under constant other conditions, the greater the amount of nitrogen discharged, the lower its purity ； Conversely, the higher it is. Therefore, for a set of pressure swing adsorption nitrogen production equipment, as long as the load permits, its product purity can be adjusted arbitrarily between 90% and 99.9%. 3. Operation control: Since the cryogenic method is carried out at extremely low temperatures, the equipment must undergo a pre-cooling startup process before it can operate normally. The startup time, from the moment the expander starts to when the nitrogen purity meets the required standards, is generally not less than 12 hours ； Before entering major maintenance, the equipment must undergo a warming and thawing period, usually lasting 24 hours. Therefore, nitrogen production equipment using cryogenic methods should not be started and stopped frequently, but rather should operate continuously for extended periods of time. When starting the pressure swing adsorption process, simply pressing a button is sufficient; qualified nitrogen gas can be obtained within 30 minutes of startup. If high-purity nitrogen is required, then by passing the gas through a nitrogen purification unit, high-purity nitrogen with a purity of 99.99%-99.9999% can be obtained in another 30 minutes approximately. To shut it down, just press a button. Therefore, pressure swing adsorption nitrogen production is particularly suitable for intermittent operation. Currently, nitrogen production using cryogenic methods generally employs advanced DCS (or PLC) computer control technology to achieve integrated control at the central level, at the machine location, and on-site, enabling effective monitoring of the entire production process of the equipment. Pressure swing adsorption nitrogen production is equipped with intelligent fully automatic control; nitrogen can be generated simply by pressing buttons, without the need for a dedicated operator. V. Conclusion For petrochemical plants, the required purity of nitrogen is usually 99.9%. Based on the above introduction and comparison of cryogenic nitrogen production and pressure swing adsorption nitrogen production, we can draw the following conclusions: a) When the continuous nitrogen demand exceeds 600 m3/nh and the intermittent demand is not very high, and the requirements can be met by the vaporization of liquid nitrogen, cryogenic nitrogen production should be employed. b) When the continuous nitrogen demand exceeds 600 m3n/h and the intermittent demand is high, such that liquid nitrogen vaporization can no longer meet this demand, a method combining cryogenic nitrogen production with pressure swing adsorption for intermittent gas supply can be employed. c) When the continuous nitrogen demand is less than 600 m3n/h, pressure swing adsorption can be used for nitrogen production. d) Pressure swing adsorption for nitrogen production is particularly suitable for applications with a nitrogen demand of less than 3000 m3n/h, a nitrogen purity of 95%, and intermittent operation. e) When the process unit requires liquid nitrogen, cryogenic nitrogen production should be used unless there is a possibility of obtaining liquid nitrogen from an external source.