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Petroleum and Chemical Energy Saving Issue 4, 2006·3· Special Forum Energy Saving Measures in the Soda Ash Distillation Process of the Ammonia-Alkali Process Hu Shuya (China Tianchen Chemical Engineering Company, Tianjin Beichen 300400) Abstract The distillation process is one of the important links in the production of the ammonia-alkali process. Reasonable design is crucial for energy conservation, consumption reduction and cost reduction. This article analyzes the energy-saving measures that should be considered in the design of the distillation process.: Slow down the scabbing speed of distillation equipment and extend its service life ； Reasonable selection of ammonia cooler ； Pay attention to recovering the heat of ammonia evaporation waste liquid and reducing energy consumption. Keywords: Soda ash, ammonia-alkali method, ammonia distillation tower, energy-saving distillation process is one of the important links in the production of soda ash in the ammonia-alkali process. The production quality of the distillation process greatly affects the consumption of energy, ammonia, lime milk, etc. and the soda ash production cost of the entire plant. Therefore, the technological progress of the distillation process has attracted the attention of my country's soda ash industry scientific and technological workers. A large number of improvements and technological developments have been made in its process, equipment, management and other aspects. They are all seeking reasonable process devices in order to achieve energy saving, high efficiency, low consumption and safety as much as possible. In the production of soda ash by the ammonia-alkali method, NH3 exists as an intermediate medium. It circulates over and over again during the production process, and this cycle is achieved with the help of the distillation process. To drive NH3 out of the solution is a complex process and requires huge equipment. It is often related to the success or failure of process equipment design. Factors that should be considered in distillation process design include: ①Slow down the scabbing speed of distillation equipment, extend its service life and reduce the frequency of tower sweeping ； ②Reasonable selection of ammonia cooler form ； ③Choose a reasonable process flow to recover waste liquid heat and reduce energy consumption. Considering the above influencing factors in the design work and making reasonable engineering design are the keys to the design of the distillation section. The relevant measures taken in the design are analyzed as follows:: 1 Reduce scarring and extend the operating cycle The scarring problem in the distillation tower is a technical problem in the alkali production industry. So far, there is no satisfactory solution. The continuous operating time of ammonia steaming towers in China, except for the dry ash-added ammonia steaming tower, which is relatively long, is generally about 30 days. The shortest operating time is only 20 days, and the longest is about 90 days. In foreign countries, it is generally 6 to 18 months, and the longest can be 24 months. For this reason, the design of the steaming and suction section focuses on reducing scarring and extending the operating cycle. Numerous alkali workers at home and abroad have done a lot of research on the mechanism of scarring in distillation towers, and the means and methods of scarring prevention and removal, but there are not many effective, practical, economical and reasonable methods. Regarding the scarring problem of the distillation tower, the basically same view is: It is believed that the scarring is caused by the calcium sulfate supersaturation in the pre-ash bucket being not eliminated. As the reaction time prolongs and the temperature rises, calcium sulfate precipitates gradually form from top to bottom in the distillation section of the distillation tower, and the crystal form changes from calcium sulfate dihydrate (CaSO4·2H2O) to calcium sulfate hemihydrate (CaSO4·1/2H2O) and anhydrous calcium sulfate (CaSO4). The thickness of the scar thickens from top to bottom, and the firmness of the scar layer increases from top to bottom (these phenomena are consistent with actual phenomena). The scarring component is mainly calcium sulfate and other solids such as calcium carbonate and sand brought in by ash milk. They agglomerate and wrap each other, and scarring and siltation promote each other, causing scarring and blockage in the distillation tower, pre-ash barrel and liquid channels, thus forcing the distillation tower to be shut down for cleaning. According to the manufacturer’s experience, methods that can reduce distillation tower scarring within a certain range include:: (1) Choose a sieve plate ammonia evaporation tower ； (2) Increase the mixing intensity of the pre-ash bucket and increase the volume of the pre-ash bucket, that is, increase the residence time in the blended liquid. Reducing the sulfate content of refined brine is the fundamental way to solve the problem of scarring and blocking the tower, but it is not easy to further reduce sulfate. However, there is currently no more effective method for gray milk sand removal. Although increasing the CO2 content of the preheated mother liquor or adding condensate to the pre-ash barrel has been proven to be more effective, it comes at the expense of increasing the consumption of limestone, steam, and waste residue emissions, and can only be a stop-gap measure. Reducing the operating temperature of the distillation tower and the pre-ash barrel is an indicator related to production operations. What can be done in the design stage is to work on the selection of the mixing intensity of the pre-ash barrel and the volume of the pre-ash barrel to change the supersaturation of calcium sulfate in the pre-ash barrel, thereby changing the scarring and transferring the scarring, and achieving the effect of ·4· 2006 Issue 4 Petroleum and Chemical Energy Saving of reducing scarring in the tower and extending the operating cycle. The following measures were taken in the design: 1.1 Selection of sieve plate ammonia evaporation tower The ammonia evaporation tower is the main equipment for mother liquor distillation. It consists of a preheating section and a distillation section. The design of the ammonia evaporation tower must follow the principles of high efficiency, high operating flexibility, good process indicators and easy cleaning of scabs. In the 1970s, foreign researchers developed a sieve plate ammonia evaporation tower. The starting point for the design of this device was based on the simplest sieve plate tower. Bubble towers are mainly widely used in domestic production. In recent years, sieve plate towers have also been used in China. This tower has a large production capacity. The production capacity of a sieve plate ammonia tower with a diameter of 3 m is as high as 850-900 t/d, and the service life can be extended to 3-4 months. Structurally, the sieve plate tower changes all the bubble cap trays into sieve plate trays without internal and external overflow pipes. The equipment is much simplified. It is much easier to make than the bubble cap tower. It is light in weight and easy to install. Although cleaning still relies on manual labor and pneumatic picks, the large effective space in the tower circle and the absence of dead corners and curves make the cleaning work more convenient and the cleaning time cost is low. 1.2 Increase the volume of the pre-ash barrel and change the path of the liquid in and out of the pre-ash barrel. In the past, in the design of the pre-ash barrel of large-scale ammonia and alkali plants in my country, ordinary pre-ash barrels with door frame stirring were usually used, such as the old 600,000 t/a device in Haihua. The diameter of the pre-ash barrel is 4 m, the barrel height is 11 m, and the volume is 90 m3. The supporting ammonia evaporation tower has a diameter of 3.2 m, a blended liquid residence time of 30 min, and a residence time of 24 at maximum production capacity. min, the operating cycle of the ammonia distillation tower is 30 days. Based on the operation conditions of the ammonia evaporation tower and summarizing the ammonia evaporation operation experience of each manufacturer, it was believed that increasing the volume of the pre-ash barrel and increasing the residence time of the blended liquid in the pre-ash barrel would be beneficial to extending the operation cycle. Therefore, during the expansion of Haihua Phase II, the design of the pre-ash barrel was changed. The new pre-ash barrel has a diameter of 4.5 m, a barrel height of 17 m, a volume of 270 m3, and a residence time of 43 minutes at maximum production capacity. In the past design, the liquid inlet and outlet paths of the pre-ash bucket were entered from the lower part of the pre-ash bucket, and the upper part of the blended liquid was discharged into the ammonia evaporation tower. In the design of the new plant, the liquid path was changed: That is, the ash milk and preheated mother liquor enter from the upper part of the pre-ash barrel, the blended liquid is discharged from the lower part, and then returns upward to the ammonia evaporation tower. With this design, the path of the blended liquid is lengthened and the residence time is changed accordingly. The reaction process of the entire pre-ash bucket is: Gray milk and preheated mother liquor flow from top to bottom from the upper part of the pre-ash barrel. The reaction gradually improves, the temperature gradually increases, and calcium sulfate precipitates gradually form in the pre-ash barrel from top to bottom, and the crystal form changes. ； Due to the increased reaction time, there is enough reaction time in the pre-ash bucket to convert the calcium sulfate dihydrate (CaSO4·2H2O) in the blended solution into calcium sulfate hemihydrate (CaSO4·1/2H2O) and anhydrous calcium sulfate (CaSO4) ； In addition, due to the presence of sand particles in the ash milk, crystal seeds are provided for the adhesion of calcium sulfate, which gradually grow from top to bottom and adhere to the wall of the pre-ash barrel. When the blended liquid enters the ammonia evaporation tower, the supersaturation of calcium sulfate has been eliminated, so dense calcium scars are not easily formed in the ammonia evaporation tower, making the cleaning of the ammonia evaporation tower easier. Therefore, changing the design structure of the pre-ash bucket also changes the scarring process. Judging from the production situation, the effect is remarkable, and the operation cycle of the ammonia evaporation tower can reach 90 days. When cleaning the tower, the scars inside the tower are loose and easy to clean. ； As for the scab layer formed in the pre-ash bucket, due to the simple structure and large space of the pre-ash bucket, the cleaning work becomes relatively simple. Based on the above analysis, it is believed that due to the change in residence time, the calcium sulfate in the pre-ash barrel has enough reaction time to undergo crystalline transformation. ； Due to the change in path, the presence of sand particles in the ash milk provides seed crystals for the adhesion of calcium sulfate. This design is consistent with the actual operation situation. This design is beneficial to the operation of the ammonia evaporation tower. 2 Reasonable selection of ammonia condenser The function of ammonia condenser is: First, the evaporated ammonia gas is cooled to a specified temperature range, and at the same time a large amount of H2O vapor in the gas is condensed, and NH3 and CO2 are concentrated. ； Second, by preheating or heating the mother liquor and the cooling water in each section, the heat of the outlet gas can be recovered and used to reduce the steam heat energy. 2.1 Structural form of ammonia condenser Currently, domestic ammonia condensers include: Cooling square box type (traditional type), flat plate type, spiral plate type, bellows type (new type). 2.1.1 Cooling box type ammonia condenser The traditional cooling box type ammonia condenser consists of multiple cast iron cooling boxes, and the forms of domestic cooling boxes are basically the same. The advantages of the cooling box condenser are that the net cross-sectional area of the gas path is large and the fluid resistance is small, which is beneficial to the operation of steaming and absorbing ammonia. As a result, the operation is smoother, the temperature adjustment sensitivity is moderate, and it is not allergic, so the operation is safe and reliable. However, the cooling box is very heavy and has poor heat transfer performance. In order to enhance heat transfer, the temperature difference between the partition walls is increased and the temperature difference between the inlet and outlet of the water is reduced. Therefore, the cooling water consumption is large and the investment is also large. 2.1.2 Flat-plate ammonia condenser The flat-plate ammonia condenser has high heat transfer efficiency, but the fluid pressure drop is large, the elasticity is small, the operation is prone to allergies, the service life is short, and cleaning is frequent. Especially on the brine side, if the brine is not good, it needs to be cleaned once every 10 days. The main reason is that the gap between the plates is small, only 4 mm, and is easily blocked by scars and debris. 2.1.3 Spiral plate ammonia condenser The spiral plate ammonia condenser has a high heat transfer rate, and the fluid pressure drop is lower than that of the flat plate type. The heat transfer plate can be divided into 2 to 3 steps to exchange heat with different fluids. However, there are still disadvantages of flat plate heat exchangers, which are difficult to repair once damaged. Petroleum and Chemical Energy Saving Issue 4, 2006·5· 2.1.4 Bellows heat exchanger Bellows heat exchanger is a new heat exchange product developed on the basis of the traditional shell and tube heat exchanger structure, using thin-walled stainless steel corrugated tubes to replace the thick-walled smooth heat exchange tubes of the shell and tube heat exchanger. It inherits the advantages of the shell and tube heat exchanger, such as durability, safety and reliability, and wide range of uses. At the same time, it overcomes its shortcomings such as poor heat exchange capacity and easy scaling and blockage. Compared with other forms of heat exchangers, its comprehensive economic performance indicators are at a leading level. The heat exchange tube adopts a corrugated shape, and the cross-sections of the internal and external flow channels of the tube continuously mutate, causing the fluid flow to remain in a highly turbulent state even at very low flow rates, making it difficult to form a laminar flow bottom layer. This allows the main thermal resistance of convective heat transfer to be effectively overcome, and the heat transfer inside and outside the tube is simultaneously enhanced, resulting in a high heat transfer coefficient. At the same time, it makes it difficult for particles in the fluid to deposit and scale, and has the ability to naturally prevent and remove scale. 2.2 Comparison between corrugated tube heat exchanger and water tank type ammonia cooler. In summary, it is believed that corrugated tube heat exchanger has more selective advantages. Now, the following analysis will be made using the configuration of a single ammonia evaporation tower as an example.: (1) The equipment cost adopts water tank type ammonia cooler, and each ammonia steaming system is equipped with 12 water tanks. Using titanium heat exchange tubes, the average price of each water tank is 450,000 yuan, totaling 5.4 million yuan. ； If a bellows-type ammonia cooler is used, the heat exchange area will be 800 m2, and the equipment cost will be 3.7 million yuan. The difference between the two is 1.7 million yuan. (2) Civil construction costs. The height of the factory building for a water tank-type ammonia cooler is 52 m. If a corrugated tube heat exchanger is used instead, the height of the factory building is 42 m. The difference between the two is 10 m. The civil construction cost of a single tower saved is about 100,000 yuan. (3) The total weight of the water tank for a water tank-type ammonia cooler is 240 t, and the total weight of the equipment for a bellows-type ammonia cooler is about 47 t. The difference in installation fee alone is 300,000 yuan. It can be seen from this that the investment difference between the two options for single tower configuration is 2.1 million yuan. If a soda ash plant with a large production capacity needs to be equipped with 3 or 5 sets of towers, the investment will differ by 6 million to 10 million yuan. In recent years, domestic manufacturers have adopted corrugated tube ammonia coolers. The results are quite satisfactory, the heat exchange efficiency is high, the natural anti-scaling ability is strong, and no pickling is required for 2 years of use. It is a cost-effective equipment selection. 3. Choose a reasonable process to recover waste liquid heat. Ammonia consumption, steam consumption and lime milk consumption are the main energy consumption of ammonia and alkali production, and these three items are reflected in the steaming and suction section. Adopting a reasonable process flow, effectively utilizing heat exchange, and driving NH3 and CO2 out of the solution as much as possible to minimize the ammonia and lime content in the waste liquid are the basic points for selecting the steam and suction process. 3.1 Determination of distillation route The distillation process in an ammonia-alkali plant is divided into pressure distillation and vacuum distillation. The pressure distillation process is still the most widely used typical process in ammonia and alkali plants around the world. The entire system is operated under pressure, which is beneficial to improving equipment capabilities. Due to the presence of pressure, the decomposition of carbonate is relatively strong. The higher the pressure, the lower the residual CO2 in the preheated mother liquor, which can reduce the amount of lime milk. The disadvantage is higher energy consumption. The characteristics of vacuum distillation are to reduce energy consumption and extend the equipment operation cycle, which is beneficial to labor protection and environmental sanitation. ； Because the system temperature is low, it is beneficial to reduce the ammonia content of the residual liquid and reduce the loss of ammonia. But the disadvantages of vacuum distillation are: ①The production intensity of the device is limited by the allowable gas speed. If the allowable gas speed is exceeded, the operating indicators will be seriously damaged, and distortion consequences such as gas cap and foaming will occur. ； ②A lot of moisture is brought out of the gas phase, which increases the operating load of the system. ； ③The carbon dioxide displacement rate is low and the loss of lime is large. Weighing the pros and cons of pressure distillation and vacuum distillation, the design combined with the current situation at home and abroad, proposed a process route of separate distillation of mother liquor and weak liquid, and a combination of pressure distillation and vacuum distillation, that is, pressure distillation is used for mother liquor distillation, and vacuum distillation is used for weak liquid distillation. The ammonia, carbon dioxide and water vapor gases from the mother liquor distillation tower enter the ammonia absorption tower to make ammonia salt water. The gas containing ammonia, carbon dioxide and water vapor from the condenser of the light liquid distillation tower enters the vacuum ammonia absorption tower. In the vacuum ammonia absorption tower, the light ammonia brine sent from the carbonization process absorbs ammonia and carbon dioxide under high vacuum conditions to become crude ammonia salt water. 3.2 The thermal energy distillation in the waste liquid recovered by Shanfa consumes huge energy, which is one-third of the energy consumption of the entire alkali production process. ； However, the sensible heat brought out by the distillation residue accounts for 70% of the process heat energy. The degree of heat energy loss is self-evident, so this heat should be recovered in some way. Theoretically, 1,200 kg/t of alkali must be heated during the ammonia distillation process. In actual production, the steam consumption of my country's ammonia-alkali plants varies according to different production methods. For wet distillation, the steam consumption is about 1,700 to 1,800 kg/t of alkali. It is a process with the largest energy consumption and difficult waste heat recovery in the soda ash production of an ammonia-alkali plant. Flashing waste liquid and recycling flash steam is an effective way to reduce the energy consumption of ammonia evaporation. Foreign alkali plants often use secondary flash to recover heat. Solvay's several ammonia and alkali plants have low steam consumption in ammonia evaporation towers, which can represent the advanced level in foreign countries. Among them, the steam consumption of the φ3.5m tower of the French Donbas Alkali Plant and the φ2.75m tower of the Belgian Guyer Alkali Plant is 1300 and 1450 kg/t alkali respectively. ； The φ2.75 m and φ3.5 m towers of the Rossillono plant in Italy are slightly lower, reaching 1250 kg/t alkali, see Table 1. Table 1 Comparison of steam consumption of ammonia towers in foreign ammonia and alkali plants France Donbass Plant Belgium Guyer Plant Italian Rossillono Plant Tower inner diameter, m 2.75 2.75 2.75 3.5 Production capacity, t/d 500 275 275 600~700 Steam consumption, kg/t Alkali 1300 1450 1250 1250 Working time, month 6~18 8~24 12 24 ·6· 2006 No. 4 Petroleum and Chemical Energy Saving Exergy Analysis of Wuhan Petrochemical Cogeneration Hu Kunhou (Wuhan Petrochemical Plant, Wuhan, Hubei 430082) Abstract This article briefly introduces the concepts of cogeneration and exergy, applies exergy analysis method to evaluate the thermal system, truly reflects the thermal energy utilization level of the system and thermal equipment, and points out the principles of thermal energy utilization and cogeneration and the economic operation mode of the system, so as to truly realize the economic operation of thermoelectricity. Keywords Petrochemical combined heat and power, thermal energy utilization, energy saving Thermal energy is widely used in industrial enterprises and occupies a dominant position in energy utilization and consumption. Whether its utilization is reasonable or not directly affects the economic benefits of the enterprise, and combined heat and power is an economic operation mode of thermal energy utilization. The use of general thermal energy can be summarized into three forms: power heat, process heat and domestic heat. Thermal energy not only has quantity, but also has taste, which is the concept of quality. Without this concept and awareness, the rational design and production organization of the thermal system will be affected, resulting in a huge waste of energy. Therefore, when we evaluate thermal systems or thermal equipment, we should adopt correct methods, analyze from the above two aspects, find out the main reasons for unreasonable energy utilization, and adopt corresponding measures to achieve the purpose of energy saving and efficiency improvement. In terms of energy utilization analysis, exergy analysis is a more scientific evaluation method. 1 Cogeneration technology and exergy analysis 1.1 Cogeneration technology and requirements After the steam generated by the boiler drives the turbine generator unit to generate electricity, the discharged steam still contains most of the heat and is taken away by the cooling water, so the thermal efficiency of the thermal power plant is not high. If the heat of the steam-driven steam turbine process or the subsequent extraction or exhaust steam can be utilized, both power generation and heat can be supplied. This production method is called combined heat and power, and its English name is Combined Heat and Power (CHP). This process involves the production of both electrical energy and thermal energy. It is a highly efficient form of energy utilization that produces both heat and electricity simultaneously. Its thermal efficiency can reach 80% to 90%, and its energy utilization efficiency is more than double that of pure power generation. It utilizes different grades of thermal energy in a graded manner (i.e., high-grade thermal energy is used for power generation and low-grade thermal energy is used for heating), which improves energy utilization efficiency and reduces environmental pollution. Therefore, combined heat and power has comprehensive benefits such as saving energy, improving the environment, improving heating quality, and increasing power supply. Combined heat and power is more energy-efficient than separate heat and power. The thermal efficiency of large thermal power plants is 30% to 40%, while the thermal efficiency of combined heat and power projects can reach about 50%. In fact, cogeneration is not a new technology, but a management method or production combination of combined heat and power production that improves energy efficiency. ﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌ ﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌﹌ The temperature and pressure of the flash waste liquid have a certain relationship with the amount of steam recovered by the flash. The lower the flash liquid pressure, the lower the temperature of the flash waste liquid, and the greater the amount of steam recovered. However, the consumption of working steam will increase significantly as the flash pressure decreases. The selection of flash waste liquid pressure should be based on the principle that the sum of working steam and recovered steam is not greater than the steam volume required by the distillation system. Otherwise, not only will the volume of discharged mixed steam be too large to be processed, but the waste liquid pressure will be too low, which will also cause certain difficulties in waste liquid discharge. According to the characteristics of the distillation process, two-stage flash technology is used to evaporate ammonia waste liquid. The first-stage flash steam is passed into the preheating section of the ammonia evaporation tower, and the second-stage flash steam is passed into the dilute liquid ammonia evaporation tower. The first-stage flasher flashes out steam of about 92 kg/t alkali, and the flash steam pressure (gauge pressure) is about 0.0414 MPa. It enters the middle heating section of the mother liquor ammonia evaporation tower as a supplementary heat source for the ammonia evaporation tower. The amount of steam flashed out from the secondary flash evaporator is about 183 kg/t alkali, and the steam pressure becomes negative pressure and enters the light liquid distillation tower as part of the heat source of the light liquid distillation tower. Using this process, the steam consumption can be saved by about 275 kg per ton of alkali. Based on 40 yuan per ton of steam, for a 600,000 t/a soda ash plant, 165,000 tons of steam can be saved, and the annual steam cost can be saved by 6.6 million yuan. Therefore, the process technology is more advanced and energy-saving. About the author: Hu Shuya (1961~) graduated from Zhengzhou Institute of Technology in 1982 and is a senior engineer.