Modifying the Benfield decarburization process using WD-3 activator

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The Synthetic Ammonia Plant of Sichuan Lianxing Chemical Co., Ltd. (hereinafter referred to as Lianxing Ammonia Plant) is a small fertilizer plant that uses natural gas as raw material, designed by China Chengda Chemical Engineering Company. Its original design called for an annual ammonia production capacity of 25,000 tons; after upgrades and expansions, this capacity has been increased to 40,000 tons per year. Its decarbonization system utilizes a modified Benfield process. Due to the insufficient capacity of the hot potassium alkali solution using diethanolamine (DEA) as an activator to absorb carbon dioxide, the system load is close to its design limit; equipment such as circulation pumps is operating at near full capacity, yet the purity of the feed gas still does not meet the production requirements. At the same time, due to the insufficient activation performance of DEA, a higher concentration of DEA is required after expansion to improve absorption capacity; furthermore, because DEA is prone to degradation, the loss of activator per ton of ammonia amounts to 0.15 kg. To achieve the goal of energy conservation and consumption reduction, the WD-3 activator developed by Huisheng (Shanghai) Chemical Engineering Co., Ltd. (hereinafter referred to as Huisheng Company) was used to upgrade this decarburization system, yielding satisfactory results. 1 Problems and Analysis 1.1 Low absorption capacity of the solution: In a typical Benfield carbonation process, the capacity of 1 m3 of solution to absorb CO2 is 21–25 m3 (under standard conditions)«1», and this value depends on the concentration of potassium carbonate in the solution, the concentration of the activator DEA, and the gas-liquid mass transfer conditions within the tower. Before the renovation, the potassium carbonate concentration in the decarboxylation system of Lianxing Ammonia Plant was 29%–31%, while the DEA concentration was 3.3%–3.9%. The low-temperature gas entering the absorption tower contained 17% CO2; the flow rate of the purified gas was 17,000 m3/h, and the capacity of 1 m3 of solution to absorb CO2 was (22.8±0.5) m3. In the same low-temperature Benfield decarburization process, when a sterically hindered amine is used as an activator, the capacity of 1 m3 of solution to absorb CO2 can reach 28–30 m3«2», which indicates that in addition to modifying the components inside the tower to improve decarburization performance, using an efficient activator is a more effective measure. 1.2 High solution circulation volume and high tower load: Before this renovation, the circulation volumes of the lean solution and the semi-lean solution were 19.8–21.5 m3 and 130–135 m3 respectively; both the lean solution pump and the semi-lean solution pump were already operating near full capacity, leaving no room to increase the solution circulation volume any further. On the other hand, since the processing capacity of the tower has increased by 60% compared to its designed capacity, the operation of the tower is approaching its design limit. Taking the lower section of the absorption tower as an example, the liquid-phase spray density is nearly 100 m3/m2; both the liquid holdup and pressure drop in the tower are high, which increases the gas-liquid absorption intensity as well. 1.3 High DEA concentration in the solution leads to significant degradation losses. When the ammonia synthesis capacity of the associated ammonia plant was increased from 25,000 t/year to 40,000 t/year, the concentrations of potassium carbonate and DEA were increased appropriately based on the conventional Benfield solution; the DEA concentration was raised by 10% to 30% compared to the usual 3%. Since DEA is prone to degradation reactions in the thermal potassium-alkali decarboxylation tower, which causes the concentration of the activator to decrease continuously, the higher the concentration of DEA, the faster its degradation rate. For the dual-star ammonia plant, the amount of DEA that needs to be added to the decarbonization system each month is over 400 kg, with a DEA consumption of up to 0.15 kg per ton of ammonia produced. 1.4 The poor purity of the feed gas leads to an increase in the methanation temperature. In the decarboxylation system of Lianxing Ammonia Plant, the CO2 content in the purified gas exiting the absorption tower is approximately 1,200×10—6, and the methanation temperature rises to 39–41°C; this indicates that the capacity of the decarboxylation system is insufficient after the expansion of the plant. 2. Reasons for the renovation 2.1 Necessity of renovating the solution system Based on the above analysis, the original Phil decarboxylation system in Lianxing Ammonia Plant suffered from issues such as low absorption capacity, high solution circulation rates, unstable absorption solutions, high reagent consumption, and low purification efficiency. From the perspective of improving the mass transfer efficiency in the gas-liquid absorption process, using highly effective activators with better activation properties than DEA (such as sterically hindered amines) or employing efficient mass transfer equipment (such as structured packing) can both address the issue of low absorption capacity. The performance of the activator plays a decisive role in the absorption capacity of the solution; therefore, this modification focuses primarily on the solution system. On the other hand, the high level of loss of activators in the Phelde carbonation solution, coupled with the fact that the degradation products of these activators are detrimental to the absorption process, makes it urgent and necessary to modify this Phelde solution. 2.2 Approaches to modifying potassium-alkali solutions for activation The activation heat potassium-alkali method has undergone numerous technical improvements, and Table 1 lists several typical potassium-alkali decarburization methods using activation heat. Among them, the method using steric amine activators has the highest absorption capacity, but its drawback is the high cost of steric amines and high volatility losses. The Benfield method and the modified arsenic-alkali method are decarburization techniques from before the 1980s, which are still used in many plants in China. However, their absorption capacity is generally low, and there is a problem of degradation and loss of the activator. In the 1990s, the U.S.-based company UOP introduced the ACT-1 activator, which enables an increase in production capacity of 5% to 25% or a boost in purification efficiency of 25% to 85% compared to the Benfield activator. The consumption of this activator per ton of ammonia produced is 0.02 kg; however, it has not yet been used in China. http://www.nmtech.com.cn/jishuwang/upload/0605091644298850.jpg Based on an analysis of the advantages and disadvantages of domestic and international decarboxylation activators, Huisheng Company has developed a highly efficient and low-loss decarboxylation activator named WD-3. Its absorption capacity is 30% to 100% higher than that of Benfield activators, while its desorption capacity is comparable to that of Benfield activators. Figure 1 shows the absorption properties of several activators relative to a 2.1% DEA solution at different potassium carbonate conversion rates. As can be seen from Figure 1, the carbon dioxide absorption rate of the hot potassium alkali solution containing 1.3% WD-3 activator is significantly higher than that of the 3% DEA solution ； Adding 0.5% WD-3 activator to a 2.1% DEA solution increased the absorption rate by over 20%, which is comparable to that of a 3% DEA solution ； By adding 1% WD-3 activator to a 2.1% DEA solution, the absorption rate increased by more than 25% compared to that in a 3% DEA solution. It can also be seen from Figure 1 that when the conversion rate is greater than 40%, the carbon dioxide absorption rate of the 3% DEA solution decreases significantly compared to that of the 2.1% DEA solution, indicating that increasing the DEA concentration further is not an effective way to enhance the solution’s absorption capacity. On the contrary, the absorption performance of the solution at high conversion rates **improved** after adding the WD-3 activator or using the WD-3 activator alone. http://www.nmtech.com.cn/jishuwang/upload/0605091645458883.jpg Another advantage of the WD-3 activator is its stable performance and low tendency to degrade. Apart from reacting with strong acids, it has low chemical reactivity and exhibits excellent resistance to degradation and oxidation: no degradation was observed even after being cooked in a hot potassium carbonate solution and a carbon dioxide atmosphere for 1 month ； When a hot potassium-alkali solution containing the WD-3 activator was boiled for 10 days under continuous exposure to air and carbon dioxide, only a slight change in color occurred; less than 0.1% of the WD-3 activator oxidized, whereas under the same conditions 3% of the DEA oxidized. Given the high efficiency and good stability of the WD-3 activator, this renovation plan aims to add WD-3 activator to the existing Phil system without shutting down the operation, in order to modify the current thermal potassium-alkali decarboxylation process. This will enable an increase in the absorption capacity of the decarboxylation unit by more than 10%, a reduction in the circulation volume of the absorbent solution by 10%–15%, and a reduction of CO2 levels in the purified gas to below 1,000×10^-6, thereby achieving energy savings and reduced consumption. 3 Modification Methods and Steps 3.1 Modification Method Based on the research results regarding the absorption properties of WD-3 activator and DEA solution at different concentrations, and taking into account the high DEA concentration in the original Phil solution (greater than 3%), it was decided that during the initial modification the concentration of WD-3 activator in the solution should be kept around 0.6%; no further DEA would be added, thereby reducing the DEA concentration to 1.5%–2%. Then, the more economical concentrations of WD-3 activator and DEA are determined based on the production conditions. 3.2 Modification steps: (1) Calibrate parameters such as the composition and properties of the solution in the original decarburization system, the temperature and pressure in the absorption tower and regeneration tower, the temperature and pressure in the medium-low shift and methanation reactors, as well as the composition, flow rate, and pressure of the low-shift gas and purified gas. (2) 520 kg of WD-3 activator was dissolved in a underground tank using decarburization liquid and condensate water from the regeneration tower; after stirring for 1 hour, it was left to age for 4 hours. (3) With all other operating parameters of the decarburization system remaining unchanged, the solution containing WD-3 activator from the underground storage tank was gradually pumped into the regeneration tower using a feed pump over a period of 4 hours. (4) Monitor the changes in pressure drop of the absorption tower; take samples of the lean liquid every 4 hours to analyze its foaming properties, and observe and record the temperature rise in the methanation reactor. (5) Reduce the circulation rate of the semi-poor liquid by 10%–15%, and observe and record the temperature rise changes in the methanation reactor. (6) Determine an appropriate circulation rate for the semi-poor liquid, and operate it over a long period to observe the system stability as well as the degradation of the WD-3 activator and DEA. 4 Reform Results and Analysis 4.1 System Stability The results of 2 months of operation show that the WD-3 activator can operate stably in the Benfield decarburization unit. Based on the data regarding the foaming properties of the solution (Table 2), the bubble height of the solution decreased from 5 cm before the modification to 3.5 cm, while the defoaming time dropped from 2 s to 1 s. It should be particularly noted that the improvement in the foaming performance of the solution was not achieved through the use of defoamers; no defoamers were added to the system during the 2 months following the addition of the WD-3 activator, indicating that the WD-3 activator itself contributes to the stable operation of the decarburization system. 4.2 Efficiency of the WD-3 activator: Upon adding the WD-3 activator to the decarburization system, the circulation volume of the semi-poor liquid was significantly reduced, while the purity of the feed gas also improved compared to before the modification. As shown in Table 2, during this renovation, the circulation rate of the semi-poor liquid can be reduced to 111 m3/h, a 17% decrease compared to before the renovation. In fact, the circulation rate of the semi-poor liquid once dropped to 107 m3/h, yet the decarburization system was able to operate stably; at that time, 1 m3 of solution had a CO2 absorption capacity of 27 m3. http://www.nmtech.com.cn/jishuwang/upload/0605091647128140.jpg As can be seen from the conversion rate data of the rich liquid at different semi-poor liquid circulation rates in Table 2, the conversion rate of the rich liquid after the modification can be increased to the range typical of designs for Benfield carbonation towers; this indicates that the WD-3 activator is suitable for situations where the processing volume increases and thus the gas-liquid absorption intensity rises. On the other hand, after modification using the WD-3 activator, the circulation rate of the semi-poor liquid can be adjusted within a certain range according to operational requirements. For example, in order to lower the regeneration temperature and reduce heat consumption, a moderate circulation rate of the semi-poor liquid (120 m3/h) can be used; this allows the conversion rate of the poor liquid to increase from 0.2 before the modification to around 0.25. This not only reduces energy consumption but also decreases the loss of the activator. 4.3 Stability of the WD-3 activator: When it was put into operation in early June 2000, the DEA concentration in the decarburization solution was 3.5%, while the concentration of the WD-3 activator was 0.65%. After 2 months of operation, the DEA concentration had dropped to 2.1%, representing a loss of 40%, which is equivalent to a loss of 0.15 kg per ton of ammonia. In contrast, the concentration of the WD-3 activator remained above 0.6%, with a loss of no more than 0.015 kg per ton of ammonia – one-tenth of the DEA loss rate. This indicates that the WD-3 activator possesses excellent activation performance as well as low consumption, making it suitable for upgrading catalytic thermal potassium-alkali decarboxylation systems. 5 Conclusion By adding 0.65% WD-3 activator to the Benfield decarburization system without stopping the operation, more than two months of stable operation have shown that this modification achieved good results: (1) The circulation rate of the semi-poor liquid was reduced by 11%–17% ； (2) The temperature rise in the methanation reactor is reduced by 1–3 compared to before the modification℃ ； (3) The ammonia loss per ton of WD-3 activator is no more than 0.015 kg, **lower than that of DEA. These results indicate that the WD-3 activator boasts good activation performance and low consumption, making it comparable to UOP’s ACT-1 activator and suitable for upgrading activated hot potassium-alkali decarboxylation systems. By combining adjustments to the operating parameters of the decarburization section as well as the upstream and downstream sections, along with modifications to the internals of the tower, the WD-3 activator decarburization technology will contribute to energy savings, reduced consumption, improved efficiency, and enhanced product competitiveness in ammonia synthesis plants in China.