Contribution to National Caustic Soda Technology Annual Meeting-Operation Summary of MVR Unit in All-Halogen Soda Production



The MVR device in full Stewed alkali production in the Operation Summary

  Abstract:Introduced the Chongqing City, Hui Ying-day chlor-alkali the run the the situation of the Chemical Co., Ltd.-MVR device in the in the the alkali production of the full-Stewed, summed up the successful experience, analysis of the problem of the existence of the the, To investigate the the affect the of the the factors that of the MVR device and and wholly-Stewed alkali and response measures.
   Key Words:MVR device, Full Stewed alkali is, Crafts, Equipment, Self-control, Problem

  1 Foreword
Chongqing Yingtianhui Chlor-Alkali Chemical Co., Ltd. (hereinafter referred to as Yingtianhui) applied MVR technology to the whole halogen alkali process for the first time in China. The MVR device was completed and put into operation at the end of 2011. The production scale is 135 m3/h for light brine treatment capacity and 60 t/h for evaporated water, corresponding to 150000 tons/year of ionic membrane caustic soda production capacity. Yingtianhui has a chlor-alkali production capacity of 100000 tons/year, and the actual production ratio of halogen in 2012 is 97%, basically realizing full halogen alkali production.
  2 Introduction to technology [1]
  Principle of 2.1 Technology
Mechanical vapor recompression technology, referred to as MVR(mechanical vapor recompression), commonly known as heat pump technology, is an advanced energy-saving technology that reuses the latent heat of condensation of the secondary steam generated in the evaporation concentration process, thereby reducing the demand for external energy supply in the evaporation concentration process [2]. Its working principle is to compress the low-temperature secondary steam by the steam recompressor to increase the pressure, temperature and enthalpy, and then enter the evaporator for condensation and heating to make full use of the latent heat of the secondary steam. In this way, the waste steam to be discharged is fully utilized, which not only recovers the latent heat, improves the thermal efficiency, but also recovers the steam condensate. In addition to the start-up of the MVR device, the whole evaporation process during normal operation basically does not need to supplement the raw steam, only consumes electricity, and its energy utilization efficiency is equivalent to 20-effect evaporation.
  2.2 flow diagram

Figure 1: MVR process flow diagram

  Brief description of 2.3 process
  (1) Brine system
After being mixed in the intermediate tank, the brackish water from different processes is pumped to the preheater for preheating, and a small amount of sodium sulfite is added to the pump outlet to ensure that it does not contain free chlorine. After the brackish water exchanges heat with steam condensate and concentrated brine in parallel, it enters the falling film evaporator for evaporation. In order to ensure the brine concentration reaches 310g/l, it is then sent to the forced circulation evaporator for evaporation. After reaching the concentration of strong brine and light brine, brine heat exchange, into the filtered refined salt water storage tank.
  (2) Steam generation system
When the MVR device system is started, raw steam should be added into the heating chamber to heat up the light salt water in the evaporator to generate secondary steam for recycling of the evaporation system. After the secondary steam produced by the system reaches equilibrium, close the raw steam valve and stop the use of raw steam. In addition, when the temperature of the feed brine is too low, a little raw steam needs to be added; when the feed load is adjusted from low load to high load, the raw steam needs to be temporarily added according to the temperature change.
  (3) Secondary steam system
In the process of concentration of light brine, the secondary steam generated simultaneously in the falling film evaporator and the forced circulation evaporator enters the secondary steam scrubber after the demister. Through steam washing, the salt content of the secondary steam entering the compressor is less than 10ppm and reaches the saturation state, so as to ensure the long-term safe and stable operation of the steam compressor. Finally, the secondary steam is pressurized and heated by the first and second stages of the steam compressor, and enters the heating chamber of the falling film evaporator and the forced circulation evaporator for condensation and heating, and the heat is transferred to the light brine of the evaporator through heat exchange. The light brine is evaporated to generate secondary steam, and the recovery and utilization of secondary steam is realized.

  3 Operation
  3.1 process aspects
  (1) Process index
The operating data of 10 hours under three working conditions of full load (Table 1), low load (Table 2) and normal load (Table 3) were recorded respectively.

Table 1 Operation data under full load

Table 2 Operation data under low load
Table 3 Operating data under normal load

(2) Data analysis
According to the operation process index of the unit, the production capacity, concentrated brine concentration, steam condensate quality, steam consumption, power consumption and other data are analyzed respectively, and the conclusion of the operation situation is drawn.

Table 4 Analysis of operating data under full load

Table 5 Analysis of Operation Data under Low Load
Table VI Analysis of Operation Data under Normal Load

  3.2 equipment
  (1) Steam fan
Stable operation. The actual line staggered peak operation, frequent start and stop, fan operation is good. There were no repairs in 2012. The fan uses air-cooled cooling, but when the temperature is high, the temperature has a certain rise, and then an air-cooled machine is added.
  (2) Falling film evaporation device
The falling film evaporation device runs well. The hole-type splitter plate is used for liquid distribution, and the distribution is very uniform. The unique circulation design and defoaming design are adopted, and there are no phenomena such as evaporation efficiency drop and evaporation tube main pipe.
  (3) Forced circulation evaporation device
Also known as crystallization evaporator, its role is mainly to prevent crystallization and ensure the concentration of brine. The operation was basically normal. In the later period, local leakage occurred and was quickly resolved.
  (4) Steam washing device
The main function is to improve the quality of secondary steam and eliminate overheating. The multi-nozzle hierarchical design, cycle washing quantitative discharge, the operation effect is better, the intuitive reaction is the steam condensate conductivity is low.
  (5) Plate heat exchange device
A total of 4 plate heat exchange devices, the heat source is steam condensate and brine. The two plate heat exchangers in which the heated medium is light brine run well. The two plate heat exchangers whose heated medium is brine are easy to scale and need to be cleaned once a month because of the high impurity content of brine.
  3.3 self-control aspects
The MVR device is controlled by DCS, and the operation of several important control points is briefly described:
  (1) Load regulation control
According to the design, the MVR device can realize fully intelligent automatic adjustment of the operating state when the system load, fan speed, raw steam flow, feed temperature, discharge concentration and other indicators change according to the production requirements. But this is an ideal control scheme, in the actual operation, in general, through the computer for manual intervention.
  (2) Fan interlocking
The fan is equipped with a complex and reasonable interlocking system, which is also the need for safe operation. During ultra-low load operation, the control requirements are very strict, especially when the secondary steam volume is extremely small, it is easy to cause the fan to chain stop and realize the protection function.
  (3) free chlorine control
In actual operation, the concentrated brine is directly into the secondary brine, which requires high control of free chlorine. During the operation in 2012, there was a case of exceeding the standard. It is recommended to add a free chlorine monitoring point and an automatic switching device for concentrated brine shunt at the concentrated brine, and return to the primary brine if it exceeds the standard.
  3.4 economic aspects
Based on the actual ionic membrane caustic soda output of 75000 tons and the halogen ratio of 97% in 2012, the base year of calculation is 25% of halogen doping in 2011.
  (1) Reduction of salt consumption
When using refined salt, due to the loss of transportation, storage and salt, the production of 1 ton of ionic membrane caustic soda needs to consume 1.54t of 100% salt. The use of all-halogen caustic soda, pipeline transportation, for a closed system, the production of 1 ton of ion-exchange membrane caustic soda only consumes 1.48t of 100-fold salt. In 2012, the cost savings due to the reduction of salt consumption was about 900000 yuan.
  (2) Brine salt price difference
In 2012, the raw material cost was saved by about 15.98 million yuan due to the price difference of halogen salt.
  (3) save excessive refined preparation
The concentrated brine from the MVR device is directly fed into the filtered refined brine storage tank at other times except for shutdown and regular return to primary brine. According to the index of excess refined preparation, about 50 tons of excess caustic soda, 140 tons of excess sodium carbonate and about 420000 yuan of excess refined preparation cost were saved in 2012.
  (4) Recovery of steam condensate
In 2012, a total of 190000 cubic meters of steam condensate were recovered, which was calculated by 2 yuan/square, saving about 380000 yuan.
  (5) Staggered peak power consumption
The main energy consumption of MVR device is electricity. As the design load is greater than the current production load, the area where the device is located implements step-by-step electricity price, and the peak-shifting operation can be implemented in ancient times. In 2012, the cost of peak-shifting electricity was saved by about 800000 yuan.
  (6) Increased cost of refined preparations due to brine quality
Calcium 1300mg/l, magnesium 600 mg/l and sulfate 6g/l in brine bring cost increase to primary refining and membrane denitration. The increased cost in 2012 is about 3.53 million yuan.
  (7) Operating cost of MVR
Mainly the consumption of electricity and steam. In 2012, it consumed 6.688 million degrees of electricity and 2660 tons of steam, equivalent to a cost of about 4.54 million yuan.
Based on the above benefits and costs, without calculating the depreciation of fixed assets, the total halogen alkali in 2012 will save about 10.41 million yuan compared with 25% of halogen in 2011.
  4 Questions to discuss
  4.1 concentrated brine returns to primary brine and directly to secondary brine
  (1) Current operation
The concentrated brine from the MVR device exchanges heat with the brine, and then is sent to the filtered refined brine storage tank for direct secondary refining treatment. When starting and stopping, the concentrated brine from the MVR device is returned to the salt pool for a refining treatment.
  (2) Directly into the secondary brine analysis
The concentrated brine from the MVR device directly enters the secondary brine. The advantage is that it can save the cost of one-time saline overfining and halve the amount of one-time saline treatment. The disadvantage is that the primary brine is changed from brine refining to brine refining. If the brine quality is not good, it is not convenient for brine treatment. Ion enrichment changes from the large circulation of primary brine to the small circulation of secondary brine, and the ion enrichment that cannot be treated in the secondary brine process is more obvious. In particular, the free chlorine in concentrated brine should be drained out, otherwise it will affect the secondary brine chelating resin and have greater risks. In addition, the brine and brine indirect heat exchange, easy to scale, affecting the brine temperature.
  (3) Return to primary brine analysis
The concentrated brine from MVR device is returned to the salt pool, which is the same as the original dilute brine returning to the salt pool. The first advantage is that the brine is directly heated and the temperature is controllable. The second is to dilute the brine impurity index, which is convenient for primary brine treatment. The third is to realize a large circulation of brine, which can remove some enriched impurities, especially to have a control device for free chlorine in brine. The disadvantage is that "clean" brine and "dirty" brine are mixed, which increases the amount of primary brine treatment and increases the amount of excess concentrate.
  (4) Discuss the results
In fact, this is mainly a trade-off between economy and risk. Directly entering the secondary brine can achieve better economic benefits, but the risk is greater; returning to the primary brine has a lower risk, but it will lose part of the economic benefits. According to the value theory, the choice of the scheme depends on the enterprise's comprehensive evaluation of the operation effect of the production device. In the actual operation, we adopted the method of combining the two schemes, that is, the period of high risk of starting and stopping, the primary brine is returned, and according to the ion enrichment cycle, the concentrated brine is regularly returned to the primary brine, and the concentrated brine is sent to the filtered refined brine storage tank in other periods.
  Recovery of 4.2 steam condensate
(1) Actual operation
During normal operation, steam condensate enters the mixed bed of desalted water process after heat exchange with light brine, brine and circulating water respectively, and is used as desalted water after treatment. When starting and stopping, steam condensate is reused as supplementary water of circulating water. In the second half of 2012, a technological transformation was carried out. A steam condensate buffer tank was added to the secondary brine process. The desalted water for the regeneration of the chelating resin tower was directly provided by this buffer tank, and the excess steam condensate was sent to the desalted water process.
  (2) Steam condensate recovery analysis
The steam condensate from the MVR device is of good quality and can be directly used as pure water. However, considering the high requirement of pure water for electrolysis and the high salt content in alkali, the steam condensate still enters the post-treatment process of desalted water. Later, through the decomposition of the use of pure water, an economical recycling scheme was adopted. The chelating resin tower regeneration process with low salt content of pure water directly used steam condensate, and the high salt content of pure water was sent back to the desalted water post-treatment process.
A small number of MVR devices are carbon steel equipment, which has an impact on the iron content of steam condensate during startup and shutdown. In actual operation, it was found that the condensate turned red once started, so this part of condensate cannot enter the desalted water system and can be sent to the circulating water system or directly discharged.
  (3) Discuss the results
Steam condensate shall be fully recycled to ensure its economy.
  Quality control of 4.3 raw brine and treatment of primary brine
  (1) Actual operation
As Chongqing Changshou bittern mining is a new well, the dissolution cavity is not stable enough and the quality of bittern is poor. The main indicators are:
NaCl   300g/l     Ca2     1300mg/l
Mg2 600mg/l SO42- 6g/l
Fe    6.8ppm     Al     5ppm
Mn 0.7ppm Ni 0.5ppm
SiO2 50cmos three 7cmos
I 0.06ppm Total Ammonium 8ppm
The biggest impact is Mg2. The primary brine treatment process uses the Kaifa process. The design capacity of pretreatment is not so large. In severe cases, the Mg2 content of pretreatment is still 200ppm. Only by means of large circulation can the Mg2 content be reduced, thus reducing the treatment capacity of Kaifa by half and making Kaifa pickling frequent.
followed by SO42-. The membrane denitration device has changed from the original 1/3 denitration to the current full denitration. If SO42-is too high, the processing capacity is slightly insufficient. In June 2012, the denitrification capacity was doubled through technological transformation.
  (2) Raw brine quality control and primary brine treatment analysis
The quality of raw halogen directly affects the economy of operation, the quality is poor, the amount of refined preparation is more, the cost is high. The second is the impact on the primary brine treatment device. For the existing chlor-alkali device, the primary brine treatment is changed from the original brine refining to brine refining. The capacity of the pretreatment system and salt sludge system will be insufficient and need to be reformed to meet the requirements. For the newly-built chlor-alkali device, the process and equipment suitable for brine refining should be considered in the design. The pretreatment system can use the new preconditioner of Kaifa membrane or ceramic membrane.
  (3) Discuss the results
Fluctuations in the quality of raw brine do not have a direct impact on the MVR unit, but affect the primary brine treatment system. If the quality of the original halogen is too poor, it is recommended to make a simple pretreatment in the original halogen production enterprise.
  4.4 brine heating method
  (1) Actual operation
The primary brine process requires a brine temperature of 60°C. The brine exchanges heat with the concentrated brine and steam condensate from the MVR unit. The brine is very easy to scale at 60 ℃, and it needs to be cleaned once a month to meet the requirements of a brine temperature.
  (2) Analysis of brine heating mode
The brine is heated by indirect heat exchange, and the problem of easy scaling has not been well solved. If concentrated brine is mixed with brine for direct heating, the economic loss of the aforementioned primary brine will occur.
  (3) Discuss the results
Now more expectations are the improvement of heat exchange equipment and the improvement of descaling process. The heat exchange equipment can be well disassembled and cleaned. There is a re-line descaling process for brine, which can solve this problem well.
  4.5 peak shifting operation
  (1) Actual operation
The MVR unit operates every day during low and flat valley electricity prices, and maintains salt water circulation and stops fans during other periods.
  (2) Staggering operation analysis
Staggered operation needs to meet several conditions: first, the area is the implementation of the ladder electricity price; second, the MVR device design load is large, there is enough capacity to stagger the peak; third, there is enough capacity of light brine storage tank, easy to adjust the operating load at different times. When the peak electricity price, the fan stops running, the system does not evaporate, but the brine circulation pump runs to maintain the system temperature and prevent crystallization. During normal production, the brine is heated by concentrated brine and steam condensate. After the MVR device is shut down, steam needs to be introduced into the heat exchanger for heating.
  (3) Discuss the results
According to the conditions and characteristics of peak-shifting operation, the mathematical models of light brine reserves, primary brine capacity, MVR operation capacity and step electricity price are established, and different operation modes are selected according to different indicators.
 Optimal Operation of 4.6 MVR Device
  (1) Actual operation
After denitration, the dechlorinated brine directly enters the MVR device for concentration. When the brine concentration is greater than 300g/l, the concentrated brine concentration is controlled at about 300g/l. When the brine concentration is lower than 300g/l, the concentrated brine concentration is controlled above 300g/l, which matches the brine concentration. The control method is to first adjust the fan to increase the speed without obvious change, and finally adjust the amount of raw steam to meet the requirements of concentrated brine concentration.
  (2) Optimized operation analysis of MVR device
If the feed concentration of the light brine is 170g/l, there is no steam consumption, and the concentration of the concentrated brine can meet the requirements directly by adjusting the fan speed. However, in actual operation, the feed concentration reaches 200g/l, resulting in insufficient secondary steam volume, and a small amount of raw steam needs to be added to meet the concentration requirements of concentrated brine. Due to the high concentration of sulfate and chlorate in the brine, especially sulfate, the boiling point of the solution will increase with the increase of temperature and concentration. To achieve the brine concentration requires more energy and there is a risk of crystallization.
  (3) Discuss the results
According to the actual operation data and some constraints, the mathematical model of brine concentration, light brine concentration, concentrated brine concentration, steam consumption and electricity consumption should be established, and the optimal operation index under different conditions should be obtained.
  5 Conclusions
The actual operation of Yingtianhui MVR device in 2012 shows that the technology is mature and reliable, the device operates normally, the economic benefits are obvious, the social energy consumption is reduced, the application of MVR technology and the construction of the device are successful, and it is recommended to promote it in the all-halogen alkali industry. At the same time, the application of MVR technology is a systematic project, in the quality of raw materials, pre-treatment, post-treatment, energy utilization and other aspects need to be considered comprehensively, and the scheme selection and device construction should be carried out according to local conditions, so as to make the application of all-halogen alkali more perfect, and further promote the application to the evaporation and concentration process of other salts.

[1] Yan Chenglin, Hu Hongming, Application of Mechanical Vapor Recompression (MVR) Technology in All-Halogen Alkali Process, China Chlor-Alkali, 20011(10):9-11.
[2] Li Shusheng, Wu Zongsheng, Research on Mechanical Vapor Recompression Evaporation Process of Saturated Brine in Tantian, Salt Industry and Chemical Industry, Volume 38, Phase 1, 2009(1):18-20.

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