Process improvement of the tar-ammonia water separation system

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Process Improvement of the Tar-Ammonia Water Separation System Jiao Guohui (Anyang Iron and Steel Group Co., Ltd., Anyang 455004) 1 Existing Problems As the production scale has expanded, the gas purification system No. 1 put into operation in August 1997 in our plant’s recovery workshop can no longer meet the process requirements for separating tar from ammonia water; as a result, the quality of the recycled ammonia water deteriorates (with an oil content as high as 400 mg/L), the tar contains a high amount of residues, and the quality of the mixed liquid used for washing naphthalene in the gas pre-cooling system is poor, leading to low efficiency in naphthalene removal and frequent blockages in the pre-coolers. To ensure the quality of the recycled ammonia water, the technical department took measures to lower the interface between tar and ammonia water in the mechanical tar-ammonia water clarification tank, keeping the tar level in the tank below 900 mm and discharging the tar intermittently at regular intervals. This measure improved the quality of the circulating ammonia water. However, the lowering of the interface between tar ammonia water in the mechanized tar ammonia water clarification tank resulted in a decrease in the efficiency of the slag skimming machine, an extension of the settling time for tar sludge in the clarification tank, and a high content of sludge in the tar in the mixture of transferred tar and pre-cooling cycle naphthalene washing liquid, thereby increasing the operational difficulties. To address the above issues, while ensuring the proper operation of the tar-ammonia water separation system, and by drawing on the production experience of related processes, we designed our own mechanical tar-ammonia water clarification tank system for the continuous discharge of tar. Combined with super-centrifuge technologies for dehydration and dewatering, this approach improved the quality of both tar and ammonia water, enhanced the efficiency of naphthalene removal in the initial cooling stage, reduced the workload on workers, and effectively solved the problem of frequent clogging in the initial coolers, achieving satisfactory results. 2 Brief description of the process before improvement: The tar-ammonia mixture flows into a mechanical tar-ammonia clarifier, where it is clarified to form 3 layers: the upper layer consists of ammonia water, the middle layer is tar, and the lower layer is tar residue. The settled tar residue is continuously scraped by a scrapper to the nozzle and discharged outside the tank. The tar flows through the liquid level regulator to the tar intermediate tank (with some of it being sent for cyclic naphthalene washing), and then is pumped into the tar dehydration tank. After dehydration by standing, it is sent to the tar processing plant for further treatment. The clarified ammonia solution flows from the top into the intermediate ammonia tank, and then is pumped by a circulating ammonia pump to the coke oven gas collection system for reuse (with some of it being sent to naphthalene washing). Excess ammonia water flows into the excess ammonia tank, where it is pumped by an excess ammonia pump to the ammonia vaporization treatment process. 3 Improvements to the tar-ammonia water separation system (1) Tank No. 3 for tar dehydration was replaced with a tank for tar-ammonia water separation. Before the renovation of the tar-ammonia water separation system, the total volume of the equipment was 1440 m3, and the residence time of the tar-ammonia water mixture was short. The liquid level of the tar-ammonia separation is 4m. Under normal operating conditions, the thickness of the tar layer in the mechanical clarifier is 1000 mm, and that of the ammonia water layer is 3000 mm; after the modification, the thickness of the tar layer is 800 mm, while that of the ammonia water layer remains at 3200 mm. In the newly added tar-ammonia water separation tank, the tar layer is 6000 mm thick, while the tar emulsion layer is 4000 mm thick. The tar is sent to a super-centrifuge for further dehydration and deslagging; the tar emulsion layer flows from the top of the tar-ammonia water separation tank to the primary cooling and naphthalene washing system, where it is used to prepare a naphthalene washing mixture. This measure increased the equipment volume of the tar-ammonia water separation system by about 28%. The thickness of the tar layer increased by 500 mm, while the thickness of the ammonia water layer increased by 200 mm. The mechanical tar-ammonia water clarifier discharged tar continuously, and in combination with the super centrifuge technology for continuous dewatering and desludging, this approach effectively ensured the stability of the interface between tar and ammonia water within the mechanical tar-ammonia water clarifier. (2) A tar buffer tank was added between the mechanical clarifier and the oil makeup pump of the primary cooling system; this measure effectively ensured the stability of the feed to the tar-ammonia separation tank. (3) The emulsion from the tar-ammonia water separation tank flows directly at full flow into the inlet pipeline of the primary cooling naphthalene pump. The lower the average molecular weight of the tar in the naphthalene washing mixture, the more favorable it is for improving the efficiency of naphthalene washing. The naphthalene washing mixture has good fluidity at higher temperatures (usually, its temperature is controlled at 35°C), resulting in a large contact area with the gas in the primary cooler, which helps to improve the efficiency of naphthalene washing. Due to the cooling effect of the primary cooler, the temperature of the naphthalene washing mixture discharged from the bottom of the primary cooler is only 23°C, requiring additional direct steam heating. The emulsion outlet pipe is connected to the inlet pipe of the primary cooling naphthalene pump, allowing the tar emulsion at around 65°C to flow directly into the naphthalene washing pump, thereby raising the temperature of the naphthalene washing mixture to around 40°C. (4) Tar is dehydrated and desludged using a super centrifuge process. The tar containing residues at the bottom of the tar-ammonia separation tank is continuously pumped to a super centrifuge for dewatering and residue removal. After this process, the tar’s water content meets the quality standards for intermediate tar, and its residue content is extremely low; thus, no further heating for dewatering is required in a separate tar dewatering tank. (5) After the improvement of the tar-ammonia water separation system. The tar-ammonia mixture from the gas-liquid separator, primary cooler, electrostatic tar collector, and blower is initially clarified and separated in a mechanical clarifier to remove slag; thereafter, the ammonia water flows from the upper part into an intermediate ammonia water tank, and is then pumped to the coke oven gas collection system using a circulating ammonia water pump. Excess ammonia water flows into the excess ammonia tank, where it is pumped by an excess ammonia pump to the ammonia vaporization system for further treatment. The tar at the bottom of the mechanical clarifier, along with some tar residues and ammonia, flows automatically into the tar buffer tank (the total volume of tar and ammonia is about 50 m3/h). It is then pumped by the make-up oil pump from the primary cooling and naphthalene washing system to the tar-ammonia separation tank. The tar stays in this tank for approximately 8 hours; due to gravity, it separates into three layers: the upper layer is the ammonia layer, the middle layer is the tar-ammonia emulsion layer, and the lower layer is the tar layer. The tar-ammonia water emulsion is used for the initial cooling and washing of naphthalene; the slag-containing tar at the bottom of the tar-ammonia water separation tank is continuously sent to a supercentrifuge via a raw tar pump for dewatering and slag removal. After dehydration and deslagging in a super centrifuge, the tar flows automatically into the intermediate tar tank; from there, it is pumped by a transfer tar pump to tanks 1 and 2 for further static dehydration until it meets the required standards, after which it is sent to the tar processing workshop ； Ammonia water flows in automatically at the inlet of machines No. 6 and 7 in the mechanical clarification tanks ； Tar residue is discharged into tar residue trucks for coal blending. 4 Improvement effects: (1) The new process exhibits good complementarity with the super-centrifuge-based dehydration and dreg removal process; actual sampling data from the super-centrifuge for tar analysis are shown in Table 1. The test results show that the dewatering and deslagging process using super centrifuges meets the requirements of the tar-ammonia water separation system, with an average dewatering efficiency of 89.5%. The changes in the toluene-insoluble content indicate that the new process increases the residence time of tar and ammonia water in the tar-ammonia water separation system, thereby reducing the moisture and slag content in the intermediate tar. Table 1 Tar analysis data (%). Date, Supercentrifuge inlet tar, Supercentrifuge outlet tar, Dehydration efficiency, Toluene-insoluble matter, Moisture, Toluene-insoluble matter, Moisture: 2006-07-19: 6.63, 13.36, 0.02, 2.28, 3.5; 2006-07-20: 6.83, 22.55, 5.37, 1.49, 3.8; 2006-07-21: 6.44, 20.45, 5.76, 1.89, 1.2. (2) Continuous discharge of tar from the mechanical tar-ammonia clarification tank was achieved, the interface between tar and ammonia in this tank was stabilized, the separation efficiency between tar and ammonia improved, and the workload on workers was reduced. (3) It solved the problem of repeated heating of the mixed naphthalene washing solution in the primary cooler cycle, saving 1.2 t/h of steam and achieving energy-saving and environmental protection effects. (4) Improved the naphthalene washing efficiency. By stabilizing the ratio of the circulating oil in the primary cooling system and improving the quality of this oil, the efficiency of naphthalene removal was enhanced. As a result, the number of heat treatment cycles required for the primary cooler was reduced from 6–8 times per month to 0.5 times per month, and the amount of naphthalene present after the cooler dropped from 0.55 g/m3 to 0.18 g/m3. (5) It improved the quality of recycled ammonia water and residual ammonia water, reducing the biochemical treatment load. The oil content in the residual ammonia water before the modification is shown in Table 2 (the tar in the residual ammonia water before the modification was removed using a air flotation oil removal unit). After the modification, the air flotation oil removal unit was shut down, and the oil content in the remaining ammonia water is shown in Table 3. As can be seen from Table 3, the oil content in the residual ammonia water after the modification has decreased significantly. Table 2: Oil content in residual ammonia water at the inlet and outlet of the air flotation oil removal unit before renovation (mg/L). Time: 2005-05-07, 2005-05-10, 2005-05-17, 2005-05-19, 2005-05-24, 2005-05-30, 2005-06-02, 2005-06-07. Inlet: 824, 1071, 2056, 1064, 981, 1584, 836, 1841. Outlet: 465, 564, 595, 408, 514, 497, 512, 464. Table 3: Oil content in residual ammonia water of the air flotation oil removal unit after renovation (mg/L). Time: 2006-12-25, 2006-12-26, 2006-12-27, 2006-12-28, 2006-12-29, 2006-12-30. Oil content: 851, 241, 219, 513, 212, 1. 5 Conclusion: (1) Compared with System 2, System 1 has a smaller total volume, and the residence time of tar-ammonia water within this system is shorter, which limits the separation capacity of the equipment. (2) The naphthalene washing pump has a head of 35 m and a flow rate of 153 m3/h; the low pressure at the nozzle affects the dispersion effect, so it needs to be replaced with a tar pump that has a head of 65 m and a flow rate of 189 m3/h. (3) Due to the limitations of the technical parameters of super centrifuges, the moisture content of tar at the bottom of the tar-ammonia water separation tank should be <25%; otherwise, the separation performance of the super centrifuge will not meet the technical requirements.