Continuous synthesis system and method of 2-chloro-5-methylpyridine

Abstract

The invention provides a continuous synthesis system and method of 2-chloro-5-methylpyridine, the system comprises a tower reactor, a clinker kettle, a condenser, a first flash tank, a spray absorption tower, a second flash tank and a circulating pump, the synthesis method comprises the following steps: phosgene and an intermediate solution enter the tower reactor for reaction from bottom to top, after discharging, introducing into a clinker kettle for reaction, discharging reaction tail gas out of a system after condensation, after a certain retention time, discharging reaction synthetic liquid, introducing into a first flash tank for gas-liquid separation, introducing a gas-phase material into a spray absorption tower, discharging after absorption treatment, introducing a liquid-phase material into a second flash tank for secondary gas-liquid separation, and discharging after secondary gas-liquid separation; a gas-phase material obtained after secondary gas-liquid separation enters a clinker kettle, and a liquid-phase material is subjected to a post-treatment step to obtain a 2-chloro-5-methylpyridine product. By adopting the synthesis system and the synthesis method, the yield of the 2-chloro-5-methylpyridine is high, and the discharge of a large amount of wastewater with complex components is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of organic chemistry, and in particular relates to a continuous synthesis system and method of 2-chloro-5-methylpyridine. Background Art

[0002] 2-Chloro-5-methylpyridine is an organic intermediate with application value, which is widely used in the fields of pesticides, medicines, fine chemicals, etc. At present, the synthetic routes of 2-chloro-5-methylpyridine include cyclization method, direct chlorination method, 3-methylpyridine oxidation method, diazotization method, etc. Among them, cyclization method is a major industrial synthetic method.

[0003] In the cyclization method, the process of producing 2-chloro-5-methylpyridine using morpholine and propionaldehyde as raw materials was first used in Jiangsu Yangnong Chemical Group Co., Ltd. in 2000 with an annual industrial capacity of 700 tons, and then the production capacity was expanded to 2,000 tons per year. The process of the morpholine-propionaldehyde method is mainly as follows: using morpholine and propionaldehyde as raw materials to prepare N-propylene morpholine through a microchannel reactor, N-propylene morpholine and excess methyl acrylate are continuously used to prepare the intermediate cyclobutane, the cyclobutane and ammonia water are subjected to aminolysis under the action of a catalyst to obtain 5-methyl-3,4-dihydropyridin-2(1H)-one (hereinafter referred to as pyridone), and the pyridone is chlorinated and phosgenated to finally obtain the product 2-chloro-5-methylpyridine. The process route of chlorination and phosgenation of pyridone to obtain 2-chloro-5-methylpyridine is as follows: pyridone reacts with chlorine to generate an intermediate 5,6-dichloro-5-methylpiperidin-2-one (hereinafter referred to as intermediate one), and intermediate one is thermally decomposed at high temperature to release hydrogen chloride to 5-methyl-2-pyridone (hereinafter referred to as intermediate two), and intermediate two has tautomerism 2-hydroxy-5-methylpyridine (hereinafter referred to as pyridinol), which reacts with phosgene to obtain the product 2-chloro-5-methylpyridine.

[0004]

[0005] However, the phosgenation process of pyridone in the above-mentioned process generally has the problem of low yield and high wastewater treatment cost. The main reason for the low yield is that the tautomer pyridinol of intermediate two is thermally unstable and easily deteriorates by heat, resulting in a low yield. And the reaction of pyridinol and phosgene is a slow reaction under a light-poor system. If pyridinol cannot be converted into 2-chloro-5-methylpyridine in time, it is more likely to deteriorate when enriched to a certain concentration. Regarding the problem of high wastewater treatment cost, although the wastewater produced by the morpholine-propionaldehyde process is less than other process routes, such as the cyclopentadiene method, the hydrogen chloride of the process byproduct is easy to react with the product 2-chloro-5-methylpyridine to generate hydrochloride, and it is necessary to use dilute alkali in post-treatment to neutralize, so there is wastewater with complex components, and pyridine organic matter is more in the wastewater.

[0006] Patent application CN110078660A mixes 5-methyl-3,4-dihydropyridine-2(1H)-one, a catalyst and a solvent, reacts with chlorine and phosgene at high temperature, and combines the two steps of chlorination and phosgenation into one step, which simplifies the operation and reduces energy consumption. However, the organic layer needs to be neutralized with alkali, washed with water, and the water layer needs to be extracted. The post-processing process of this process is complicated, the amount of wastewater is large, and the yield of 2-chloro-5-methylpyridine is still low.

[0007] Patent application CN109824580A chlorinates and dehydrochlorinates 5-methyl-3,4-dihydropyridine-2(1H)-one in a microchannel reactor to obtain an intermediate 2-hydroxy-5-methylpyridine, and then phosgenates the intermediate through a microchannel reactor to synthesize 2-chloro-5-methylpyridine. In this process, a large amount of acid-binding agent needs to be added to absorb the generated hydrogen chloride. The acid-binding agent is easily precipitated in the organic phase, thereby clogging the equipment, and the post-processing is more complicated, which is not suitable for industrial production.

[0008] Therefore, it is necessary to provide an improved synthesis process of 2-chloro-5-methylpyridine to solve the above-mentioned problems existing in the phosgenation of pyridone in the existing morpholine-propionaldehyde process, thereby improving the yield and reducing the cost of three wastes treatment, thereby achieving the effect of reducing costs and increasing efficiency. Summary of the invention

[0009] In order to overcome the deficiencies of the prior art, the present invention aims to provide a continuous synthesis system and method for 2-chloro-5-methylpyridine.

[0010] The technical solution adopted by the present invention is:

[0011] A continuous synthesis method of 2-chloro-5-methylpyridine comprises the following steps:

[0012] (1) After phosgene and an intermediate solution are mixed, they enter a tower reactor from the bottom, react and exchange heat from bottom to top, and the reaction synthesis liquid is discharged from the top of the tower reactor, wherein the intermediate is 5,6-dichloro-5-methylpiperidin-2-one;

[0013] (2) The reaction synthesis liquid discharged from the tower reactor is passed into the clinker kettle for reaction. After a certain residence time, the reaction tail gas is condensed by the condenser and discharged from the system, and part of the condensate is refluxed into the clinker kettle;

[0014] (3) The reaction synthetic liquid in the clinker kettle is discharged into the first flash tank for gas-liquid separation;

[0015] (4) The gaseous material in the first flash tank enters the spray absorption tower and contacts with the absorption solvent in countercurrent. The absorbed gas is discharged from the top of the tower, and the recovered solution is discharged from the bottom of the tower and circulated to the feed port of the tower reactor;

[0016] (5) The liquid phase material of the first flash tank enters the second flash tank for secondary gas-liquid separation, and the gas phase material is mixed with the discharge material of the tower reactor and then enters the clinker kettle; the liquid phase material is subjected to post-treatment steps to obtain 2-chloro-5-methylpyridine product.

[0017] Furthermore, the synthesis method feeds a stabilizer at the same time when starting up, and the addition of the stabilizer can be stopped after a certain amount of the stabilizer is fed, and the stabilizer is circulated in the system, and the specific circulation process is clinker kettle→first flash tank→second flash tank→clinker kettle. The stabilizer is selected from one or more of diisopropylamine, dipropylamine, and diisobutylamine. The stabilizer is fed from the bottom of the tower reactor together with phosgene and intermediate one solution when starting up, or fed from other feeding positions in the system. The absolute molar amount of the stabilizer in the system accounts for 1% to 20% of the molar amount of the intermediate one in the system, preferably 5% to 15%, and the molar amount of the intermediate one in the system is the feed molar flow rate of the intermediate one × residence time.

[0018] The stabilizer of the present invention is selected from a specific secondary amine substance, which is conducive to the reaction equilibrium between the intermediate 2 and the tautomer of the intermediate 2, that is, pyridinol, moving toward the direction of the intermediate 2, so that the content of unstable and perishable pyridinol in the reaction system is small and will not be enriched in large quantities. The isomerization reaction path of the intermediate 2 and pyridinol is as follows:

[0019]

[0020] When a stabilizer is added, the isomerization reaction pathway is as follows:

[0021]

[0022] When secondary amine substances participate in the reaction as stabilizers, the required activation energy is greatly reduced, and the isomerization reaction tends to exist in the keto form. The stabilizer affects the reaction equilibrium between the tautomers and the content distribution between the two tautomers, thereby avoiding the reduction of the reaction yield. Moreover, through the process control of the two-stage flash tank, most of the stabilizer is recycled in the small circulation system of the clinker kettle-the first flash tank-the second flash tank, and the utilization rate of the stabilizer is high.

[0023] Furthermore, the solvent of the intermediate one solution in step (1) is selected from one or more of chloroform, dichloroethane, trichloroethane, and chlorobenzene, and the mass concentration of the intermediate one in the intermediate one solution is 10% to 50%, preferably 20% to 35%.

[0024] Furthermore, the molar ratio of the intermediate 1 to phosgene in step (1) is 1:2.0-10, preferably 1:2.5-4.0. The amount of pyridinol of different ages in the system follows a normal distribution. This process adopts phosgene-rich reaction conditions, which can reduce the distribution mean of the content of pyridinol of different ages, making pyridinol less likely to deteriorate. And the light-rich environment is conducive to increasing the reaction rate, and the pyridinol in the system can be quickly phosgenated, so that the pyridinol will not be enriched in large quantities, thereby ensuring the product yield.

[0025] Furthermore, the reaction temperature of the tower reactor in step (1) is 40-100°C, preferably 50-70°C; the reaction temperature of the clinker kettle in step (2) is 70-120°C, preferably 80-95°C.

[0026] Furthermore, in step (2), the residence time of the reaction synthesis liquid in the clinker kettle is 2 to 6 hours, preferably 3 to 5 hours.

[0027] Furthermore, the hydrogen chloride in step (2) is discharged through an exhaust port provided below or above the condenser, preferably discharged through the exhaust port below the condenser.

[0028] Furthermore, in step (3), the temperature of the first flash tank is 10-80°C, and the pressure is atmospheric pressure; in step (5), the temperature of the second flash tank is 80-130°C, and the pressure is 0.8 bar-2.0 bar.

[0029] Furthermore, the absorption solvent in step (4) is consistent with the solvent in the intermediate solution, and the absorption solvent is a frozen solvent with a temperature of -10°C to 20°C; a distributor is provided at the upper part of the spray absorption tower for evenly distributing the feed solvent and then spraying it downward, so that the absorption solvent contacts the gas to be absorbed in reverse.

[0030] Furthermore, the phosgene solution recovered from the bottom of the spray absorption tower in step (4) is mixed with the phosgene and the intermediate solution in front of the feed circulation pump and then pumped into the bottom of the tower reactor for circulation.

[0031] Furthermore, the post-processing step in step (5) includes conventional post-processing means such as light removal, desolventization, and distillation.

[0032] The present invention also provides a continuous synthesis system of 2-chloro-5-methylpyridine, comprising a tower reactor, a clinker kettle, a condenser, a first flash tank, a spray absorption tower, a second flash tank and a circulation pump, wherein:

[0033] The discharge port of the circulation pump is connected to the bottom feed port of the tower reactor; the upper discharge port of the tower reactor is connected to the feed port of the clinker kettle, and a liquid phase probe is provided at the feed port of the clinker kettle; the condenser is provided above the clinker kettle, and the inlet and outlet of the condenser are connected to the upper gas phase space of the clinker kettle; the bottom discharge port of the clinker kettle is connected to the feed port of the first flash tank, and the gas phase outlet of the first flash tank is connected to the bottom feed port of the spray absorption tower; the liquid phase outlet of the first flash tank is connected to the feed port of the second flash tank, and the gas phase outlet of the second flash tank is connected to the feed port of the clinker kettle; the bottom outlet of the spray absorption tower is connected to the feed port of the circulation pump.

[0034] Furthermore, the clinker kettle is a stirred tank reactor. By adopting a process setting in which a tower reactor and a stirred tank reactor are connected in series, the decomposition reaction of the intermediate one and the uniform distribution of phosgene in the reaction materials are mainly carried out in the tower reactor, and the phosgenation reaction is mainly carried out in the stirred tank reactor. The decomposition reaction of the intermediate one and the phosgenation reaction are carried out in stages, which is beneficial to improving the yield of the product 2-chloro-5-methylpyridine.

[0035] Compared with the existing synthesis process of 2-chloro-5-methylpyridine, this process has the following beneficial effects:

[0036] (1) Adding a stabilizer to the system for recycling is beneficial to reducing the distribution of unstable and perishable pyridinol in the circulating material. By adding a small amount of stabilizer, the yield of 2-chloro-5-methylpyridine can be significantly improved; and the stabilizer is recycled in the small circulation system of the clinker kettle-the first flash tank-the second flash tank, and the utilization rate is high.

[0037] (2) By setting up a multi-stage gas discharge device, the hydrogen chloride gas produced by the reaction decomposition is discharged in the clinker kettle and the first flash tank in turn through flash evaporation, so that the hydrogen chloride gas can be separated in time, avoiding the problem of back-mixing of hydrogen chloride in the kettle. There will be no large amount of hydrochloride in the reaction system, and the product does not need to be post-treated with dilute alkali neutralization, thereby reducing the discharge of a large amount of wastewater with complex components. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 The figure is a process flow chart of a continuous synthesis system of 2-chloro-5-methylpyridine.

[0039] In the figure, 1-tower reactor, 2-clinker kettle, 3-condenser, 4-first flash tank, 5-spray absorption tower, 6-second flash tank and 7-circulation pump. DETAILED DESCRIPTION

[0040] The present invention provides a continuous synthesis system of 2-chloro-5-methylpyridine, such as Figure 1As shown, it includes a tower reactor 1, a clinker kettle 2, a condenser 3, a first flash tank 4, a spray absorption tower 5, a second flash tank 6 and a circulation pump 7, wherein:

[0041] The discharge port of the circulation pump 7 is connected to the bottom feed port of the tower reactor 1, and is used to mix the solution of the raw material intermediate 1 and phosgene and then feed them into the tower reactor 1; the upper discharge port of the tower reactor 1 is connected to the feed port of the clinker kettle 2, and a liquid phase probe is set at the feed port of the clinker kettle 2 to strengthen the material distribution in the clinker kettle 2 and improve the utilization rate of phosgene; the condenser 3 is arranged above the clinker kettle 2, and the inlet and outlet of the condenser 3 are connected to the upper gas phase space of the clinker kettle 2, which is used to discharge the hydrogen chloride in the reaction tail gas from the system in time, and the phosgene and solvent are refluxed into the kettle after condensation; the bottom discharge port of the clinker kettle 2 is connected to the feed port of the first flash tank 4, and the first flash tank The upper gas phase outlet of 4 is connected to the bottom feed port of the spray absorption tower 5, which is used to remove hydrogen chloride, phosgene and solvent from the reaction synthesis liquid through the first gas-liquid separation, remove the gas phase and then spray absorption to discharge hydrogen chloride from the system in time, and phosgene and solvent are collected and absorbed; the lower liquid phase outlet of the first flash tank 4 is connected to the feed port of the second flash tank 6, and the upper gas phase outlet of the second flash tank 6 is connected to the feed port of the clinker kettle 2, which is used to remove stabilizer, phosgene, solvent and hydrogen chloride from the reaction synthesis liquid through the second gas-liquid separation, and circulate the gas phase back into the system; the bottom outlet of the spray absorption tower 5 is connected to the feed port of the circulation pump 7, which is used to circulate the captured phosgene solution back into the system.

[0042] Furthermore, the tower reactor 1 is filled with liquid, and in order to reduce the degree of back mixing in the tower, a plurality of horizontal mesh trays are arranged in the tower.

[0043] The present invention also provides a continuous synthesis method of 2-chloro-5-methylpyridine, which comprises the following steps:

[0044] (1) After phosgene and an intermediate solution are mixed, they enter a tower reactor 1 from the bottom, undergo a reaction stroke from bottom to top, undergo a reaction and heat exchange, and the reaction synthesis liquid is discharged from the top of the tower reactor 1 and enters the clinker kettle 2;

[0045] (2) The reaction synthesis liquid enters the clinker kettle 2 through the liquid phase probe to react, and after a certain residence time, the reaction tail gas is condensed by the condenser 3, the phosgene and the solvent are refluxed into the kettle, and the hydrogen chloride is discharged from the reaction system;

[0046] (3) The reaction synthesis liquid is discharged from the bottom of the clinker kettle 2 and enters the first flash tank 4 for gas-liquid separation. The gas phase material is mainly phosgene, hydrogen chloride and a small amount of solvent, and the liquid phase material is mainly the synthesis liquid;

[0047] (4) The gaseous material in the first flash tank 4 enters the spray absorption tower 5 from the lower part of the tower, and contacts with the solvent fed from the upper part of the tower in countercurrent, thereby capturing phosgene and a small amount of solvent. The absorbed hydrogen chloride is discharged from the top of the tower, and the recovered phosgene solution is discharged from the bottom of the tower and circulated to the feed port of the tower reactor 1;

[0048] (5) The liquid phase material of the first flash tank 4 enters the second flash tank 6 for secondary gas-liquid separation. The gas phase material mainly includes stabilizer, part of solvent, a small amount of phosgene and hydrogen chloride, which is mixed with the discharge material of the tower reactor 1 and then enters the clinker kettle 2; the liquid phase material is the synthetic liquid after the reaction, and the 2-chloro-5-methylpyridine product is obtained through the post-treatment step.

[0049] The process route of the synthesis method is as follows. It should be noted that the raw material intermediate 1 in the synthesis method is 5,6-dichloro-5-methylpiperidin-2-one, which is an intermediate in the synthesis of 2-chloro-5-methylpyridine, and is obtained by chlorination of 5-methyl-3,4-dihydropyridine-2(1H)-one. Intermediate 1 can be prepared by existing technology, for example, by the preparation method in US4612377A or CN106117128A.

[0050]

[0051] In order to make those skilled in the art better understand this scheme, the present invention is described in detail below in conjunction with embodiments. In the following embodiments, the product yield is calculated by the following formula:

[0052] Product yield = the output molar amount of 2-chloro-5-methylpyridine per unit time / the input molar amount of intermediate 1 per unit time * 100%.

[0053] Example 1

[0054] Phosgene and 20wt% of the intermediate solution are mixed and introduced into a tower reactor, the molar ratio of the intermediate to phosgene is 1:2.5, the reaction temperature is 50°C, and the solvent of the intermediate solution is ethylene dichloride; after the reaction synthesis liquid is discharged from the tower reactor, it enters the clinker kettle for further reaction, the reaction temperature is 90°C, and after a residence time of 3h, it enters the first-stage flash tank for gas-liquid separation, the temperature in the tank is 50°C, and the pressure is normal pressure; the gas phase material enters the spray absorption tower and is reversely contacted and absorbed with the absorption solvent ethylene dichloride, the temperature of the absorption solvent is 0°C, the liquid phase material of the first-stage flash tank enters the second-stage flash tank for secondary gas-liquid separation, the temperature in the tank is 100°C, and the pressure is 1 bar, the gas phase material is circulated to the clinker kettle, and the liquid phase material is driven out, desolventized, and distilled to obtain the 2-chloro-5-methylpyridine product. When starting up, the tower reactor is fed with a stabilizer diisopropylamine at the same time, and the absolute molar amount of the stabilizer in the system accounts for 5% of the molar amount of the intermediate in the system. After testing, the product yield of 2-chloro-5-methylpyridine was 96.5%.

[0055] Embodiments 2 to 11

[0056] The same process flow as in Example 1 was adopted, except for the operating conditions shown in the following table. The product yields of 2-chloro-5-methylpyridine in each example are also shown in the following table.

[0057]

[0058]

[0059] Comparative Example 1

[0060] The same process as in Example 1 was used, except that no stabilizer was added to the system. The product yield of 2-chloro-5-methylpyridine was 83%. When no stabilizer was added, the proportion of pyridinol components in the system increased, resulting in a significant decrease in yield.

[0061] Comparative Example 2

[0062] The same process as in Example 1 was used, except that sodium hydroxide was added to the system instead of the stabilizer, and the amount added was 5% of the molar amount of the intermediate 1 in the system. The product yield of 2-chloro-5-methylpyridine was 55%. When an alkaline reagent such as sodium hydroxide was added to the system, it could act as an acid binding agent to react with the hydrogen chloride generated by the reaction, but it could not affect the reaction balance, and the generated sodium chloride could not be separated. The strong alkali at high temperature would also destroy the product structure, and the yield was significantly reduced compared with Example 1.

[0063] Comparative Example 3

[0064] The same process flow as in Example 1 was adopted, except that the temperature of the primary flash tank was 100°C. The product yield of 2-chloro-5-methylpyridine was 86%. Due to the process control of the primary flash tank, the stabilizer was vaporized and entered the spray absorber, so that the stabilizer was recycled in the large cycle of tower reactor → clinker kettle → first flash tank → spray absorber → tower reactor. Compared with the small cycle of clinker kettle-first flash tank-second flash tank → clinker kettle in Example 1, part of the stabilizer was separated from the liquid phase in the first flash tank, which ultimately resulted in a reduction in the amount of stabilizer circulating in the system, and the yield was significantly reduced compared with Example 1.

Claims

1. A continuous synthesis method of 2-chloro-5-methylpyridine, characterized in that: The following steps are involved: (1) phosgene and an intermediate solution are mixed and then introduced into a tower reactor to react and exchange heat from bottom to top, wherein the intermediate is 5,6-dichloro-5-methylpiperidin-2-one; (2) the reaction synthesis liquid discharged from the tower reactor is passed into the clinker kettle for reaction, and after a certain residence time, the reaction tail gas is condensed by the condenser and discharged from the system, and part of the condensate is refluxed to the clinker kettle; (3) the reaction synthetic liquid in the clinker kettle is discharged into a first flash tank for gas-liquid separation; (4) The gaseous material in the first flash tank enters the spray absorption tower, contacts with the absorption solvent in countercurrent, and the absorbed gas is discharged from the top of the tower; (5) The liquid phase material in the first flash tank enters the second flash tank for secondary gas-liquid separation, the gas phase material enters the clinker kettle, and the liquid phase material is subjected to post-treatment steps to obtain 2-chloro-5-methylpyridine product.

2. The synthesis method according to claim 1, characterized in that A stabilizer is fed during start-up. The stabilizer is selected from one or more of diisopropylamine, dipropylamine, and diisobutylamine. The absolute molar amount of the stabilizer in the system accounts for 1% to 20% of the molar amount of the intermediate 1 in the system, preferably 5% to 15%. The stabilizer is recycled in the system.

3. The synthesis method according to claim 1, characterized in that The solvent of the intermediate one solution in step (1) is selected from one or more of chloroform, dichloroethane, trichloroethane, and chlorobenzene, and the mass concentration of the intermediate one in the intermediate one solution is 10% to 50%, preferably 20% to 35%.

4. The synthesis method according to claim 1, characterized in that In the step (1), the feed molar ratio of the intermediate 1 to the phosgene is 1:2.0 to 10, preferably 1:2.5 to 4.

0.

5. The synthesis method according to claim 1, characterized in that The reaction temperature of the tower reactor in step (1) is 40-100°C, preferably 50-70°C; the reaction temperature of the clinker kettle in step (2) is 70-120°C, preferably 80-95°C.

6. The synthesis method according to claim 1, characterized in that In the step (2), the residence time of the reaction synthesis liquid in the clinker kettle is 2 to 6 hours, preferably 3 to 5 hours.

7. The synthesis method according to claim 1, characterized in that In the step (3), the temperature of the first flash tank is 10-80°C, and the pressure is normal pressure; in the step (5), the temperature of the second flash tank is 80-130°C, and the pressure is 0.8 bar-2.0 bar.

8. The synthesis method according to claim 1, characterized in that The absorption solvent in the step (4) is consistent with the solvent in the intermediate solution, and the temperature of the absorption solvent is -10°C to 20°C.

9. The synthesis method according to claim 1, characterized in that The solution recovered from the bottom of the spray absorption tower in step (4) is circulated to the tower reactor.

10. A continuous synthesis system of 2-chloro-5-methylpyridine, characterized in that: It includes a tower reactor, a clinker kettle, a condenser, a first flash tank, a spray absorption tower, a second flash tank and a circulation pump, wherein: The discharge port of the circulation pump is connected to the bottom feed port of the tower reactor; the upper discharge port of the tower reactor is connected to the feed port of the clinker kettle; the condenser is arranged above the clinker kettle, and the inlet and outlet of the condenser are connected to the upper gas phase space of the clinker kettle; the bottom discharge port of the clinker kettle is connected to the feed port of the first flash tank, and the gas phase outlet of the first flash tank is connected to the bottom feed port of the spray absorption tower; the liquid phase outlet of the first flash tank is connected to the feed port of the second flash tank, and the gas phase outlet of the second flash tank is connected to the feed port of the clinker kettle; the bottom outlet of the spray absorption tower is connected to the feed port of the circulation pump.

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