Thread Content
Changhua Group Company in Qingdao, Shandong is a urea manufacturer with an ammonium hydroxide production capacity of 200 kt/year. In 1998, the urea plant was put into operation; the associated semi-water gas desulfurization system consisted of a desulfurization tower with dimensions of φ4,000mm×29,000mm, equipped with two stages of structured packing. This tower operated in parallel with the desulfurization tower of the old system. Since 1999, the desulfurization system has frequently encountered problems: the tower had to be cleaned 3 to 4 times a year, and its desulfurization efficiency was low, at only 80% to 90%. After 2000, as the supply of raw coal became strained and its quality deteriorated, the mass concentration of H2S in the raw gas increased from around 1.0 g/m3 (under standard conditions) to over 2.5 g/m3 (under standard conditions). Production cannot proceed normally; it is often necessary to reduce output, and the tower has to be shut down for cleaning almost every two months, which severely impacts the company’s economic performance. The renovation of the desulfurization system is urgent. Starting in 2003, a scientific evaluation of the system was conducted to formulate a phased renovation plan; modifications and adjustments were made to aspects such as the packing, internal components of the tower, regeneration tanks, injectors, the purification of feed gas before desulfurization, circulation rates, and gas distribution. In 2006, an additional pre-desulfurization unit was added, enabling the system to handle feed gas with an H2S concentration of 4.0 g/m3 (under standard conditions) or higher, with excellent operational results. 1 Analysis of desulfurization efficiency and reasons for tower blockage (1) In new projects, 250Y structured packing was used for desulfurization. Although it provides a large mass transfer area, as the sulfur load in the tower increases and the distribution within the tower changes, it becomes unsuitable for the requirements of desulfurization processes. Moreover, repeated removal and reinstallation of this packing leads to severe damage to it; after reinstallation, it tends to collapse or shift, resulting in a reduced mass transfer area and increased initial resistance. (2) The solution circulation volume is too low, resulting in insufficient spray density and liquid-to-gas ratio in the desulfurization tower; especially as the production volume of gas increases and the H2S content rises, the solution circulation volume cannot be increased. (3) One regeneration tank with a diameter of φ4,000 mm and another with a diameter of φ5,000 mm cannot meet the requirements of the upgraded production process. (4) Changes in the desulfurization catalyst: Due to frequent shutdowns for tower cleaning, even during production operations, the desulfurization efficiency remains low, severely affecting normal production. Within 1 year, seven or eight types of desulfurization catalysts were tried; each time a catalyst was replaced, it caused fluctuations in the process, and due to the sensitive operating conditions, problems arose easily. It was not until May 2001, when the \"888\" desulfurization catalyst was used, that the correct role of this catalyst could be determined, with the guidance of technical personnel from Toa Company. (5) The treatment of semi-water gas prior to desulfurization, such as tar removal, dust removal, and temperature reduction, is not thorough enough; in summer, the desulfurization temperature exceeds 55°C, which severely affects the absorption and regeneration processes in desulfurization. 2 Replace the structured packing, modify the internal components of the tower, and renovate the regeneration tank. After thorough consideration, during the major maintenance in May 2003, the structured packing that had been in use for 5 years was replaced with rectangular saddle ring packing of φ76mm diameter. Although the mass transfer area decreased significantly, it is believed that improvements to the distribution within the tower should not have a substantial impact on the mass transfer efficiency. At the same time, the original two sections of packing space in the regular packing (6m per section) were changed to three sections, each 5m long; inter-section distributors were added, and the initial distributor was modified. It improved the initial distribution efficiency of the liquid, enhanced the inter-section distribution efficiency, and optimized the internal structure of the desulfurization tower. After the system was put into operation, it was found that the desulfurization efficiency not only did not decrease but also increased to around 99%. By the end of 2003, as the fertilizer market improved, production was increased by 25% step by step, and desulfurization continued to meet the required standards, further confirming the effectiveness of this renovation. In 2004, the fertilizer market continued to improve, with production volumes rising further. The price of raw coal soared, and its quality declined; the H2S concentration in semi-water gas increased from 1.0 g/m3 (under standard conditions) to over 2.5 g/m3 (under standard conditions), making desulfurization once again a bottleneck. The company has decided to modify the regeneration tank in order to increase the solution circulation rate and meet production requirements. During the major overhaul in November 2004, the original regeneration tank with a diameter of φ4,000 mm was replaced by a new one with a diameter of φ7,000 mm. The regeneration pump, desulfurization pump, and injectors were all replaced as well. The solution circulation rate increased by 50%. Practice has shown that after these modifications, the desulfurization process completely changed from its previous passive state, meeting the new production requirements; the desulfurization efficiency remained above 98%, and it became possible to control the resistance of the desulfurization system more steadily. It changed the situation of cleaning the tower 5 to 6 times a year, laying a solid foundation for stable production. 3 Improve the quality of the feed gas and optimize production management. There was no coking removal device before desulfurization, and the scrubber tower was relatively small; as a result, the temperature of the gas could not be reduced. Impurities such as tar and ash entered the desulfurization system along with the semi-water gas, directly affecting the desulfurization absorption process and foam regeneration. Issues such as the absence of foam in the regeneration tank and turbidity in the desulfurization liquid often occurred, which in turn affected the efficiency of desulfurization and the control of tower resistance. To this end, the \"two removals and one reduction\" process for semi-water gas (removal of tar, removal of coal dust, and temperature reduction) was modified. First, the desulfurization circulating water is made to circulate separately, completely separated from the gas generation wastewater, which effectively ensures the water quality and temperature. Secondly, two dust removal and cooling towers with a diameter of φ6600mm were added; together with the existing gas washing and cooling tower, this results in three stages of water washing. Even during production in summer, the temperature of the desulfurization liquid can be kept below 40°C, **improving the effectiveness of the \"two removals and one cooling\" process. Desulfurization requires favorable operating conditions as well as appropriate hardware; management of desulfurization, then, is the essential software. The production management of desulfurization will be discussed from several aspects below. (1) Control of the desulfurization circulation volume and total system solution volume. The total volume of the solution must be such that it allows for an appropriate residence time for the desulfurization liquid to circulate. When the composition of the solution is suitable, and the gas flow rate is high along with a high H2S content, it is necessary to increase the circulation rate of the solution in order to ensure effective desulfurization – this means increasing the liquid-to-gas ratio and the spraying density. It is also important to take into account the time required for sulfur precipitation in the rich liquid tank and the oxidation time in the regeneration tank; therefore, the circulation rate needs to be adjusted accordingly. After selecting the appropriate liquid-to-gas ratio and spray density, it is necessary to strictly and steadily control the solution circulation rate, in order to avoid reducing it arbitrarily to save energy or depending on the load level; otherwise, long-term operation can easily lead to tower blockage. (2) Control of solution temperature. When the solution temperature is too low, both the absorption and desulfurization reaction rates decrease, which affects the desulfurization efficiency and hinders water balance. Crystal formation of alkalis may occur, posing a serious threat to the resistance control in the desulfurization tower ； If the temperature is too high, the solubility of H2S gas in the desulfurization solution decreases, which reduces the driving force for absorption and lowers the purity of the gas. Meanwhile, the solubility of oxygen during the regeneration process also decreases, which hinders oxidative regeneration. It also leads to an intensification of side reactions involving Na2S2O3, resulting in a faster sulfur precipitation process and finer sulfur particles that make separation more difficult. Additionally, the corrosiveness of the solution increases as the temperature rises. Therefore, the appropriate solution temperature must be strictly controlled (around 40°C); under normal circumstances, production proceeds smoothly at temperatures between 30 and 50°C. (3) Strengthen the management of the regeneration tank and ejector. The liquid level in the regeneration tank must be controlled smoothly, to prevent large fluctuations and splashing; it is necessary to ensure that the flotation foam overflows in a steady and even manner. The injectors should be distributed evenly throughout the regeneration tank, and they must function properly. Check the nozzle and throat for blockage or scaling, as well as the verticality of the ejector, based on whether air is being drawn in by suction or backflow is occurring; otherwise, it should be addressed or replaced promptly. The self-priming air volume should be determined based on the level of hydrogen sulfide and the production load, and it is adjusted by controlling the level of regeneration pressure as well as the opening degree of the air intake valve of the injector. When the air valve is fully open, the amount of regenerating air increases, resulting in a higher blowing intensity, which facilitates the oxidative regeneration of the solution and ensures thorough CO2 stripping. However, if the air volume is too high, the foam layer becomes unstable, resulting in high suspended sulfur levels. If the air volume is excessively high for an extended period, the solution potential will be high, leading to intensified side reactions. (4) Strengthen the control of solution components. Controlling the components of the desulfurization solution is crucial; strict control is required over the levels of Na2CO3, total alkalinity, pH, suspended sulfur, and by-products. Otherwise, issues such as corrosion, tower blockage, and reduced efficiency may occur, leading to a vicious cycle in production. (5) Strengthen continuous sulfur melting management. Ensure proper low-level separation, increase the concentration of foam entering the sulfur melting tank, and reduce unnecessary entry of desulfurization liquid into the sulfur melting tank. For the venting of molten sulfur, low-temperature separation venting should be employed as much as possible, allowing the residual molten sulfur to settle and then returning it to the system after cooling. 4 A new pre-desulfurization unit has been added to improve the ability to remove sulfur at high levels. Regarding raw coal, the company does not have any geographical advantages, and due to limited transportation capacity, it is forced to use coal with high sulfur content. With the renovation and expansion of the ammonia synthesis system in 2006, the two existing semi-water gas desulfurization units with a diameter of φ4,000 mm were far from sufficient to meet the new production requirements. Based on the need to burn high-sulfur coal, a hierarchical desulfurization approach was adopted. It was decided to install an additional desulfurization system with a diameter of φ6600mm before the existing semi-water gas desulfurization process, that is, before the Roots blower; this system is equipped with a regeneration tank with a diameter of φ10000mm. **This approach increased the capacity of the desulfurization system, yielding significant improvements in performance as well as economic benefits.