1. Necessity
During the evaporation process, the electrolyte preheating temperature is one of the main process operating conditions, which is particularly important. Through calculation, every time the electrolyte temperature increases by 10℃, the consumption of heating steam can save 170kg/t, accounting for about 5% of the total consumption. In the general process, when the electrolyte is sent to evaporation, the temperature is only about 75 ℃. The three-effect four-body two-stage co-current evaporation operation is used. The boiling point of the Ⅰ effect solution is 145 ℃, that is, the electrolyte must be heated to the boiling point. The temperature rise reaches 70°C. If it is completely heated with raw steam, the steam consumption only used for electrolyte preheating is 1190kg/t·alkali. If the preheating temperature is close to the boiling point of the feed I effect, it will be of great benefit to the stable operation of the evaporation device and reduce the amount of heating steam. Generally, the electrolyte is preheated using sensible heat of evaporated condensed water. Due to the imperfect preheating process and equipment, the temperature of the electrolyte after preheating is often much lower than the boiling point of the feed liquid in the evaporator, which inevitably continues to preheat in the evaporator and consumes a portion of the heating steam. According to data reports, the temperature of most chlor-alkali plant electrolytes after preheating is 45 to 50°C lower than the boiling point of the feed efficiency, causing steam consumption to increase by 0.7 to 0.9t/t·100% NaOH, accounting for the total vaporized steam Consumption of 25% to 30%, therefore, in order to save steam, measures should be taken to increase the electrolyte preheating temperature.
２ Introduction of common electrolyte preheater
2.1 Tube heat exchanger
Tubular electrolyte preheaters are commonly used, usually placed horizontally, in the electrolyte pipe, between the condensate pipes, in a counter-current mode, in order to increase the flow rate of the two fluids, there are passage spacers in the tube side and the shell side The number of tube passes is usually 4 to 6 passes, and a pass spacer is set at intervals of 30 to 50 cm in the shell pass. The heat transfer coefficient of the electrolyte preheater using condensed water is not high, about 600～1000kJ/m2·h·℃. The advantages of the tube heat exchanger: simple structure, easy maintenance, and low price; the disadvantages are also prominent: low heat transfer coefficient, large volume, and more metal materials.
2.2 Spiral plate heat exchanger
The spiral-plate preheater is made of two parallel thin steel plates. It has two spiral channels separated from each other. In the center of the preheater, there is a central partition. The side is provided with nozzles, and the fluid entering these two nozzles can be introduced into the nozzles on the left and right sides of the outermost layer of the cooler through two different channels along the spiral line. When preheating the electrolyte with a spiral-plate preheater, the electrolyte and condensate transfer heat through the common wall surfaces on both sides of the respective channels. Because in the spiral plate preheater, the flow rates of electrolyte and condensate water are much higher than in the tube preheater, and the heat transfer coefficient can reach 2400~3500kJ/m2·h·℃. The advantages of the spiral plate preheater: high heat transfer coefficient, small footprint, and excellent performance; the disadvantage is that the corrosion of the electrolytic alkaline solution makes the spiral plate preheater prone to alkali embrittlement, and leakage maintenance is difficult.
3 Use of spiral electrolyte preheater
3.1 Introduction to the evaporation process
The production scale of caustic soda in a certain plant is 100,000 t/a per year, the product design specification is 30% liquid caustic soda, and the three-effect four-body two-stage downstream flow process is adopted. The dilute alkali from the electrolysis is preheated in two stages to the effect evaporator. After the electrolyte evaporates part of the water in the effect evaporator, it enters the effect evaporator, continues to evaporate and precipitates some salt, and then enters the effect evaporator. When the alkali concentration is increased to 19%, most of the salt is crystallized and precipitated. The lye and the precipitated NaCl crystals are mixed together and pumped into the hydrocyclone for salt and alkali separation. The clear liquid of the overflow pipe enters the intermediate alkali tank and the bottom stream of salt slurry After flowing into the high-level tank, the salt and alkali are further separated by a centrifuge. The separated alkali liquid is pumped into the intermediate alkali tank. The alkali liquid in the intermediate alkali tank enters the concentrated force forced circulation evaporator to continue evaporation. When the alkali concentration NaOH reaches 30%, the pump is used. After being clarified, the clear liquid is pumped through the cooler and continuously cooled with cold water. After the temperature drops to (40±5)°C, it enters the clarifying tank. The clear liquid is sent to the concentrated alkali storage tank and prepared as qualified alkali for sale.
The Ⅰ effect evaporator is heated by steam of about 14MPa, the secondary steam from the Ⅰ effect evaporator is used as the heat source for the Ⅱ effect evaporator and the concentrated effect evaporator, and the secondary steam from the Ⅱ effect evaporator is used as the heat source , Ⅲ effect, concentrated effect are vacuum evaporation. Production over the years has shown that the boiling point of the effect I solution is 145°C, the effect II is 125°C, the effect III is 75°C, and the concentration effect is 85°C.
3.2 Electrolyte preheater process control and equipment
(1) Industrial control situation
Electrolyte preheating adopts two-stage preheating, the first stage uses II effect condensate, and the second stage uses I effect condensate. After preheating, the condensate flows into the hot water tank, and then sent to the brine process to wash the salt sludge.
(2) Equipment operation
There are currently 4 sets of carbon steel spiral plate preheaters with F=45m2, 2 sets in Group A and B respectively. Due to the “alkali embrittlement” effect of alkali on carbon steel equipment, corrosion and cracking of the weld zone is prone to occur during the operation of this equipment. The service life of the carbon steel spiral plate electrolyte preheater is basically about one year, and the shortest time is only In 8 months, the equipment needs to be updated at least once a year. Without special maintenance equipment, the old spiral plate preheater can not be repaired, so it has to be scrapped and the loss is large.
3.3 Analysis of low temperature of electrolyte preheating
The industrial control index of the electrolyte preheating temperature is 115℃. After two-stage preheating, the actual temperature is only 100℃, which is 45℃ away from the effective boiling point of I. The reason for the low preheating temperature is ① the area of the preheater is not enough. The design scale is 100,000 t/a (100% NaOH), the actual working time is only about 300 days per year after deducting the tank washing and maintenance time, the evaporation process should produce caustic soda 14.3t/h, use electrolyte 118m3/h, according to the material 1. Heat balance calculation, using Ⅰ effect, Ⅱ effect and concentrated effect condensate to preheat the electrolyte from 75 ℃ to 115 ℃ through two stages of preheating, the area of the spiral preheater needs 360m2, of which the first level is 240m2, the second level 120m2 (The heat transfer coefficient of the spiral plate preheater is 3344kJ/m2·h·℃). ② The amount of condensate is not enough. In the evaporation process of our plant, the secondary steam from effect Ⅰ is used for heating effect Ⅱ and concentrated effect. The temperature of the condensation water of effect Ⅱ and concentrated effect is about 140℃. Both of them can be used to preheat the electrolyte. The Ⅱ effect condensate is used in the first-level preheater, and the concentrated effect condensate is directly discharged to the hot water tank, resulting in insufficient hot water in the first-level preheater. In summary, the preheater must be modified and the process control must be strengthened to increase the electrolyte temperature to 115°C after preheating.
4 Improvement measures
4.1 Selection and calculation of preheater
4.1.1 Selection
If the spiral plate preheater continues to be used, its total area should reach 300m2 . For the narrow evaporation process of the site, it should not be used, and another type should be selected.
According to relevant information, the new plate heat exchanger has a high utilization rate of heat energy, and the heat transfer coefficient is 3 to 5 times that of the spiral plate heat exchanger. The selection of plate heat exchanger has the following advantages: ① saves heat transfer area, small equipment, small installation area, and mass is smaller than the heat exchanger based on the same heat load, which reduces basic investment; ② easy to disassemble and convenient maintenance . The plates of the plate heat exchanger can be assembled on site, the number of plates can be increased or decreased at will, and any damaged piece in the heat exchanger can be removed at any time, and the maintenance time is short. ③High thermal efficiency. Adopting countercurrent heat transfer, completely turbulent, the heat recovery rate can be as high as 94-98%. Only the edges are exposed to the atmosphere, and the heat loss is negligible.
4.1.2 Area calculation
① Output 100,000 t/a (100% NaOH)
② The annual production time is 300 days (7200h)
③ Raw material electrolyte ρ = 1.193g/L, containing NaOH 10.47%
④Alkali loss The evaporation process itself loses 2%. Based on the concentrated evaporation loss, the production of 1t100% NaOH loses 20kg, the recovered brine takes away 14kg of alkali, and the total alkali loses 34kg;
⑤ Production 1t100% NaOH requires electrolyte (1000 + 34) / 10147% = 9877kg;
⑥ Alkali production 1034 × 106/6200 = 14.3t/h;
⑦ Plate heat exchanger K takes 1000kJ/m2·h·℃ (material: all titanium) electrolyte C takes 3185kJ/kg·℃
⑧ Calculation uses preheating process
It is the same as the original process. The first stage uses Ⅱ effect and concentrated effect condensate, and the second stage uses Ⅰ effect condensate. According to the heat balance calculation, the amount of Ⅰ effect condensate is 4,000 kg/t, and the total amount of Ⅱ effect and concentrated effect condensate is 2,800 kg/ t;
⑨ Calculation of two-stage preheater
Endothermic electrolyte 14.3× 9877× 3.85×(115-t1)kJ/h
Condensate heat release 4000×14.3×(65541-46016)=11107079kJ/h
t1=94.5℃
Δ T = 2312K
F2 = Q/K · Δ T = 48m2
⑩ Calculation of first-stage preheater
Condensate 140℃t2
Q put = 2800×14.3×4.18×(140-t2)
The electrolyte absorbs heat 14.3× 9877× 3.85×(94.5-75) t2=76.6°C
Find Δ T = 13.1K
F1 = Q/K · Δ T = 80m 2
4.2.2 Preheater material selection
The preheater separately flows through alkali liquid and condensed water, which has certain corrosivity. The selection of materials requires anti-corrosion. According to the specific conditions of the plant, a plate heat exchanger made of titanium is selected.
4.3 Improvement measures and effects
(1) Improvement measures
The original one idle F=240m2 titanium plate heat exchanger of a factory was converted into four F=40m2 plate heat exchangers by processing and purchasing some accessories, replacing the original two-stage and one-stage spiral preheaters respectively Heater, the process is unchanged, 2 units per stage (the evaporation is divided into two groups A and B). The process is improved, and the original straight-line concentrated steam condensate is connected to the first-level preheater to increase the amount of first-level preheating water.
(2) Effect
The operation after the improvement shows that the electrolyte preheating temperature rises to 112°C, which is 12°C higher than that before the improvement, and the steam-saving effect is obvious.