Application of ammonia absorption refrigeration process in NHD decarburization

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In 1999, our plant changed from using two sets of decarburization systems with the original thermal potassium-alkali dual-activator method to a single set of NHD-based decarburization system, with a designed annual production capacity of 150 kt of synthetic ammonia. Since NHD decarburization is a physical absorption process, it requires operation at low temperatures (–5 to –8°C), and the design utilizes the cooling capacity of 3 ice makers with a capacity of 1.84×106 kJ/h each. When the plan was finalized, our factory proposed that the cooling capacity for the decarbonization system be provided not by ammonia compression refrigeration, as has been used until now, but by ammonia absorption refrigeration utilizing low-temperature waste heat. Because after NHD decarburization, there is a significant amount of low-grade thermal energy remaining in the syngas; if this heat is not utilized, it not only increases the consumption of cooling water in the syngas water cooler but also results in the waste of approximately 2.8×107 kJ/h of heat that could have been recovered. Given that our plant has only two 35 t/h thermal boilers for generating steam, and that there is a shortage of steam needed for production as well as for heating both inside and outside the plant during winter, we discussed the matter with the design institute and decided to use this heat for heating the staff residences in winter, and as a heat source for ammonia absorption refrigeration in summer. To utilize the waste heat generated in production for heating the residential area, our factory has taken necessary measures, such as conducting gas detection of the heating water after it passes through the generator ； An air-water separator is installed at the highest point of the heating water system; the water vapor discharged from the top of the separator is then passed through a sampling cooler, and analyzed once per shift using a combustible gas analyzer. Should any leakage inside the generator be detected, the system is stopped immediately for repair. In addition, two blast plates were installed to prevent overpressure in the heating water pipeline. In summer, when used for ammonia absorption refrigeration, it can generate a cooling capacity of over 8.36×106 kJ/h, thereby eliminating the need for 5 ice makers with a capacity of 1.83×104 kJ/h each. This portion of the cooling capacity is used for NHD decarburization, while the remainder is sent to synthesis. 1 Process flow The ammonia absorption refrigeration process flow is shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload/0605161613431688.jpg The ammonia absorption refrigeration process mainly consists of absorption, distillation, condensation, throttling expansion, and evaporation. The low-pressure gaseous ammonia at –8°C and 0.2 MPa, cooled by the ammonia evaporator, merges with some gaseous ammonia coming from the synthesis workshop. It first enters a subcooler where it exchanges heat with liquid ammonia at 1.5 MPa, and then proceeds to the absorber unit, where it is absorbed by a dilute ammonia solution (with a concentration of 30% and a temperature of 46°C). The heat generated during this absorption reaction is removed by cooling water at a temperature below 28°C. The absorbed concentrated ammonia solution (42%) flows into the concentrated ammonia storage tank, and then an ammonia pump pumps it into the solution heat exchanger, where it exchanges heat with the dilute ammonia solution coming from the bottom of the distillation tower. After being heated to a temperature close to its bubble point (92°C), the concentrated ammonia enters the middle section of the distillation tower, is sprayed onto high-efficiency corrugated packing, and exchanges heat and mass with the rising ammonia solution; as a result, its concentration decreases, after which it flows into the reaction zone. A small portion of the ammonia gas with a concentration of over 99.8% at the top of the distillation tower is condensed in the reflux condenser, while most of it goes to the condenser, where it is condensed by the circulating cooling water into high-pressure liquid ammonia. The 166°C shift gas from the low-pressure system supplies heat to the generator, heating the solution at the bottom of the distillation column. The dilute solution at the bottom of the tower is cooled to 46°C by heat exchange with a concentrated ammonia solution in a solution heat exchanger, and then sent to the absorber to absorb gaseous ammonia, thereby increasing its concentration to 42%, with a flow rate of 64 t/h. High-pressure ammonia gas is also released from the top of the distillation tower and, after being condensed, supplied to users. Concentrated and dilute ammonia solutions are continuously cycled in this manner. 2 System improvements: (1) The original design height of the distillation tower was 28 m (by utilizing the existing secondary decarboxylation tower). Due to the difficulties associated with installing the reflux condenser at the top of the tower, 9 m was removed from the distillation tower; as a result, the total height of the tower together with the reflux condenser is 23 m. The top reflux condenser was replaced with a bellows-type condenser, and its area was reduced from 709 m2 to 250 m2. (2) The floating valve trays were replaced with high-efficiency perforated plate corrugated packing. (3) The vacuum pump was replaced with a steam ejector. (4) A new steam pipeline is installed on the low-temperature reformer pipeline; the purpose is to supply high-pressure steam rapidly when the low-temperature reformer is shut down, so as to prevent a drop in the temperature of the distillation column generator and avoid large fluctuations in the ammonia absorption system. (5) An primary gauge shall be added to the liquid ammonia storage tank, and a self-regulating valve shall be installed on the liquid ammonia delivery pipeline. 3 Operation status of ammonia absorption refrigeration system: Installation was completed at the end of April 2000, and it began operations officially on May 1. Normal production was resumed in a very short time. By May 3, 10–13 m3/h of refrigerant (liquid ammonia) had been produced (equivalent to about 7 tons of ammonia per hour). The cooling capacity is approximately 9.1×106 kJ/h. Before the plant is put into operation, 8 ice machines will be used for NHD decarburization in the gas production section and for ammonia coolers in the synthesis workshop (by the end of April). After ammonia absorption refrigeration became normal in May, the number of ice makers was reduced from 8 to 4–5. By taking measures such as cooling liquid ammonia through a subcooler (reducing its temperature from 40°C to below 20°C), the heat load on the synthesis condenser was reduced. As a result, the outlet pressure of the ammonia cooler dropped from 0.26 MPa to 0.2 MPa, and the temperature of the exhaust gas at the outlet fell from 0°C to around –3°C. This increased the amount of ammonia that could be condensed, while simultaneously reducing the system resistance, thereby improving the operating conditions of the synthesis system. 4 Proposals for improving ammonia absorption refrigeration systems. At present, the cooling capacity of ammonia absorption systems does not reach the designed level of 10.8 t/h, and the main reasons are as follows. (1) The temperature of the low-variation gas entering the distillation column generator did not reach the design value (166°C). Mainly, the temperature of the wash water entering the wash water heater before the generator is too low. It is designed for 120–130°C, but the actual temperature is 104°C. Therefore, a relatively larger amount of heat is removed. The solution to this problem is to change the configuration of the 642 carbon black water heat exchanger from parallel to series, in order to recover as much of the residual heat from the carbon black water as possible and reduce its discharge temperature below 80°C (currently it is 90°C). Too much water should not be added to the quench chamber of the degasifier and to the separators at various stages. (2) The area of the current ammonia water heat exchanger is 168 m2. Production operations have shown that this heat exchanger has a relatively small heat exchange area, which affects the efficiency of heat exchange; the temperature of the ammonia solution entering the absorber reaches 56°C at 70% load (the design value being 46°C), and the temperature of the concentrated ammonia solution entering the distillation tower is only 72°C (the design value being 92°C). This affects the production capacity of the distillation tower. (3) The oil feeding rate of this system should be adjusted to 7.5–8 t/h as much as possible (currently 7.2 t/h), in order to supply more waste heat to the ammonia absorption unit. Furthermore, taking into account the characteristics of the medium-low pressure shift process in our plant, when the CO content in the shift gas is high due to catalyst aging or other reasons, it is possible to consider adding high-pressure steam to the shift system. This not only helps to reduce the CO content at the exit of the shift gas but also provides more heat for ammonia absorption refrigeration. For each 0.1% decrease in low-temperature variability, ammonia production can increase by 1%. 5 Comparison between the designed water consumption and actual water consumption of this device: The designed cooling water consumption for 10.8 t/h of liquid ammonia is shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload/0605161614274048.jpg As can be seen from Table 1, the total design cooling water consumption is 1323 m3/h; in other words, the cooling water required per ton of liquid ammonia is 122.5 m3. Currently, the actual amount of liquid ammonia used for ammonia absorption refrigeration is only around 7 t/h, and the total water consumption should be 857.5 m3/h. The actual cooling water consumption for 7t/h liquid ammonia is shown in Table 2. That is, the actual cooling water consumption for 7 t/h of liquid ammonia is 475.4 m3/h. http://www.nmtech.com.cn/jishuwang/upload/0605161615079031.jpg Based on the above comparison, it can be seen that the actual water consumption is much lower than the designed value. The main reason for this lower water consumption is that the heat exchange area of the cooling water in the ammonia absorption refrigeration system is designed to be quite generous; in practice, the temperature difference Δt at the inlet and outlet of each heat exchanger’s cooling water is greater than 10°C, while the temperature difference Δt in the return condenser can even reach 20°C℃ ； Secondly, the water temperature entering the cooler is 22°C, which is relatively low (the design value is 28°C). If ammonia absorption refrigeration is not used in the retrofit of the decarbonization system, the heat required by the distillation tower generator (1.8×107 kJ/h) must also be removed by the low-temperature gas-water cooler, requiring at least 880 m3/h of water ； After the ammonia absorption refrigeration system is started up, only 260 m3/h of cooling water is required. Additionally, after 3 to 4 synthetic ice machines are shut down, the cooling water flow to the corresponding vertical condensers decreases by 500 m3/h. The above data show that, by utilizing the existing plant for renovation, the comprehensive water consumption for ammonia absorption refrigeration purposes is actually lower. 6 Characteristics of the ammonia absorption refrigeration process: (1) When using ammonia for absorption refrigeration, the power consumption of the ammonia water pump is 65 kW·h, which results in lower energy consumption compared to compression refrigeration. (2) The equipment is simple and easy to manufacture; aside from the ammonia water pump, all are stationary devices that can be installed outdoors. (3) It has high operational flexibility and is generally suitable for scales of (400～1200)×104 kJ/h, capable of operating normally within a design range of 30% to 110%. (4) Plant investment and operating costs increase as the evaporation temperature decreases. Our factory utilizes waste heat for ammonia absorption refrigeration; the total investment is approximately 2.8 million yuan. By adopting this approach for NHD decarburization, it is possible to avoid the need for at least 4 units of the 8AS17 type refrigerators along with related equipment and facilities, resulting in savings of around 1.5 million yuan in investment costs. Calculations show that the additional capital investment required for the ammonia absorption refrigeration process can be recovered in less than a year. Furthermore, under the same conditions, the outlet gas temperature of the ammonia synthesis evaporator in summer has dropped from 0°C in the past to around –2°C, which increases the net value of ammonia. At the same time, reducing the volume of gas circulated into the synthesis tower brings about more significant economic benefits. Based on the current operational results, the ammonia absorption refrigeration process for NHD decarburization features simple equipment, cost-effectiveness, and ease of operation, thus opening up new avenues for the recovery of low-grade thermal energy.