At present, our government attaches great importance to the work of energy saving and emission reduction, and many binding targets such as China's energy saving and emission reduction during the "13th Five-Year Plan" period will be stricter. ForLarge air separation plant, How to save energy consumption and reduce operation cost on the premise of ensuring safe, stable and reliable operation isAir separation equipmentImportant issues in the design process.
Due to the rapid development of metallurgy, fertilizer and petrochemical industry in recent years,Air separation equipmentThe level continues to expand, for example, the 60,000 level which has been put into use made by Hangzhou Hangyang Co., Ltd.Large air separation plantThere are already more than 20 sets, and it has expanded to 80 thousand, 100 thousand and 120 thousand levels.
Molecular sieve purification system is the key device to purify the air before the air in the air separation plant enters into the low temperature cold box. It is to use the absorption method to remove the moisture, carbon dioxide, acetylene, hydrocarbon and other substances in the air of raw materials, it is necessary to prevent the follow-up low temperature equipment from blocking or even exploding, and ensure the long-term safe and reliable operation of the air separation plant.
The purification system of molecular sieve usually consists of two adsorbers, heating equipment, Valve, pipeline and instrument electronic control. One of the two adsorbers is adsorbed and the other is regenerated. After one is saturated, put another regenerated adsorber into use and switch to use. Therefore the system energy consumption is mainly composed of two parts: Part for adsorption overcome air flows through adsorber, valves, pipe the required energy consumption. The other part is recycled the required energy consumption, including recycled when heated heat energy consumption, heating, blowing cold when waste nitrogen amount of consumption and overcome waste nitrogen flows through heating equipment, adsorber, muffler, pipes, valves, etc. resistance consumption.
The following is a concrete analysis on how to effectively save energy and reduce consumption of molecular sieve purification system.
1. Purification system in adsorption and regeneration resistance drop control
1.1 different structural forms the orb resistance drop control
A form for horizontal or vertical horizontal bed adsorber, air from adsorber bed layer flat lower part, from flat outflow. Bed layer is generally bunk bed, lower activated alumina, upper for molecular sieve. Built-in powder filter. After nearly 30 years of experience accumulation in China, the design of this structural adsorber is becoming more and more mature, which solves the problems such as anti-molecular sieve leakage of the internal support grid frame, uniform distribution of bed and so on, and the operation is reliable. Scale Production rear, with manufacturing cost relatively low, mounted, discharge convenience. User inspection, repair are relatively simple. Therefore in the domestic air separation on major share. Because bed layer aspect ratio of the relationship, General adsorber resistance control in <6kPa.
Horizontal bed structure form adsorber downside is the gas import needs certain equispaced space, bed layer volume total equipment space is about 35%. So the equipment space utilization is low, in large scale air separation on larger volume, and is horizontal place, large area. Therefore in energy saving adopted on bed height, control flow rate measures reduce resistance drop. And adopt built-in thermal insulation layer and so on to reduce regeneration energy consumption.
The other is the vertical radial flow adsorber, by upper, lower head, cylinder, 3-layer concentric porous cylindrical, built-in powder filter, upper, lower connecting tube, skirt crimping block composition, activated alumina and molecular sieve placed in two-layer concentric rings space. The figure, air from peripheral runner radial through adsorbent layer, because weeks to ventilation, has Windward large, bed layer relatively little resistance characteristics; the space volume of the bed accounts for about 50% of the equipment volume, and the space utilization rate is relatively high. Therefore, the volume and weight of large air separation can be reduced. And covers an area is also small. In energy-saving on, in larger air separation on application vertical radial flow adsorber, equipment resistance can control <4kPa, and volume small, recycled heating energy consumption also minimize, so it has some advantage. And 20 thousand and the following air separation due to vertical radial flow adsorber interior don't ventilation segment proportion relatively large, advantage is not obvious.
Vertical radial flow structure adsorber disadvantage is due to the diameter changes, air flows through each circumference the velocity different, therefore inner, outer ring adsorption abilities are different, bed layer cannot be too thick, and the bed layer along the height uniformity difficult to achieve consensus, molecular sieve adsorption performance of some impact on, so the design adsorption Time 4 h. The interior is composed of multiple concentric beds, which is difficult to manufacture and high in cost. Inner space is narrow and small, hard to stand, maintenance difficulty large internal grid rigidity degree and life requirements are also higher. Such as for dan ceng chuang adsorber, outer moisture high molecular sieve after water absorption hardness decline, runtime powder multi-, easily plug valves and equipment, the bunk bed adsorber effect is good.
1.2 reduce pipe, valve resistance
In recent years large air separation plant molecular sieve purification system in switch valve applied in the IS three valve. It has good sealing performance, and because there is no friction between the valve plate and the sealing surface of the valve seat, it has the characteristics of higher reliability and longer service life. The main thing is the structure butterfly valve can well limit maximum opening differential pressure, effectively prevent molecular sieve punch. But the type valve resistance slightly higher. Therefore the domestic such as Hangyang pump valve of three-bar valve, in reducing resistance on take amplification valve diameter measures to reduce resistance. And the same level of domestic valve cost relative imported valve to low, choose when you can pipe straight etc. measures to reduce resistance, and therefore get wider application.
Pipe settings on, tee most resistance, elbow followed by, need pipeline total resistance calculation. Try to reduce resistance.
1.3 reduce regeneration resistance measures
Recycled waste nitrogen after adsorber after emptying, in order not to increase air compressor energy consumption, minimize end recycled waste nitrogen flow pressure. If the series structure of steam heater and electric heater is adopted, the resistance on the series process should be reduced as much as possible. The resistance of the selected steam heater itself must be small, otherwise it will increase the pressure of polluted nitrogen, thus increased air compressor pressure. Generally use low fin no of baffle energy-saving steam heater.
For steam heater and electricity heater series flow path, can consider when connected in series, open including spare electric heater including all channels, electric heater open small power group power use, increase been given a heater channel, less resistance.
2, adsorption time control
Air separation equipmentUsually equipped with 2 adsorbers, switch to use. A adsorption time is equal to another's regeneration time. Adsorber adsorption time design value: usually 4 h, and control adsorber exit CO2 <1ppm, this can also control after adsorption air in acetylene, moisture, hydrocarbons, etc. under low temperature harmful gas impurities content, on ensuring the safe operation.
Because the air pressure in the adsorption stage is high and the pressure in the regeneration stage is low, there is pressure change in the regeneration stage, and the regeneration time consists of four stages, namely, depressurization, heating, blowing cooling and boosting pressure. The regeneration heating process is to detach the impurities such as water, CO2, hydrocarbon and so on in the absorbents, so as to achieve the purpose of using the absorbents again. But the total heat Q needed in the process of degassing, part of which is the heat Q1 needed for degassing the impurities, the other part is the waste heat Q2 which is inevitably used to heat the absorbents, equipment, pipelines and so on when heating. Q = Q1 + Q2, effective efficiency & eta; = Q1/Q, only Q2 per cent ratio decline, can effectively reduce the energy consumption.
When the adsorption time is prolonged, the cycle times in one day will be reduced, which is equivalent to reducing the energy consumption times of the adsorber, pipeline, Valve, absorbents and so on, and increasing the active efficiency; other recycled when heating, blowing cold time extension, can also reduce regeneration gas. To achieve drop energy consumption purposes.
Of course, the adsorption time can not be extended infinitely, because the resistance of the positive and negative air flow through the bed of the equipment should also be considered. At design time need optimization comparison.
Adsorber adsorption time design value:
2.1 have insured for the Orb adsorption time can be appropriate adjustment
In the design of air separation, the adsorption time of the adsorber is mainly determined by the size of the equipment, the type and dosage of the adsorbents filled inside. The adsorption capacity is by design was environmental conditions maximum proposed. And in the atmosphere CO2 value changes, at run time, as the season different, wind and other factors, CO2 value with likely allowance. The user can adsorber exit CO2 value determine, such as adsorber export CO2 value little change or not changes, description adsorber in molecular sieve during this time have some allowance, can successive extension. If season changes, wind direction change, also can shorten adsorption time. Regeneration time temporary by heating, blowing cold time average extension distribution OK.
A set of ultra-large air separation with 60,000 air separation and 360,000 air treatment takes 4 hours to adsorb, which will consume polluted nitrogen ~ 70500Nm & sup3;/h, average power consumption ~ 1588-degree/h. If the adsorption time is extended to 4.5 h, then sewage nitrogen will be consumed ~ 63500Nm & sup3;/h, average power consumption :~ 1502-degree/h. Average per hour can save 86 degrees electric, by 0.58-degree/$ calculation, a year cost savings: 86 x 0.58 x 24 x 360 = $430 thousand. Other recycled waste nitrogen reduce 7000Nm & sup3;/h, reduce freezer run load.
So large air separation user, with conditions can atmospheric impurity content in regular monitoring, and design value compared maintenance equipment normal safety run, for energy saving operation provides the basis. Imported set online monitoring more easy to adjust.
2.2 Molecular sieve update can be used adsorption capacity stronger molecular sieve
With the molecular sieve development, domestic manufacturers are constantly introduced adsorption CO2 higher performance of molecular sieve. Relying on the dynamic adsorption laboratory, Hangzhou Hangyang conducts dynamic experiment simulation on the actual performance of molecular sieve, and conducts experiment tests on the adsorption capacity and regeneration energy consumption under working conditions. With the same amount, the penetration time of the common 13X-APG molecular sieve's adsorption capacity is more than 120 minutes, and the penetration time of the molecular sieve with strong adsorption capacity can be increased to more than 240 minutes. At the same time, it is also necessary to analyze the adsorption (performance) of the molecular sieve) whether easy.
When the performance of the molecular sieve adsorbents declines and needs to be replaced, the molecular sieve with better cost performance can be replaced according to the current local atmospheric conditions, which can obtain larger energy saving benefits. Especially for the adsorber whose CO2 is more than 500ppm in the atmosphere, it is suggested to use molecular sieve of higher level when changing molecular sieve, and the activated alumina bed should also be changed and adjusted, which can prolong the adsorption time after adjustment. Table 1. The list illustrates the energy consumption value saved by a certain 60,000 air separation after the replacement of absorbents.
From Table 1 above can be drawn: adsorption time extension 2 hours for steam energy consumption cost reduction 1.63 million yuan/year, such as electricity recycled, can save 2.31 million yuan/year. In addition, sewage nitrogen 23500Nm & sup3;/h can be used less, and this part of sewage nitrogen can be used for cooling and energy saving of water cooling tower.
The increase of adsorption time also prolongs the life of molecular sieve. The circulating changes of high temperature, high humidity and gas pressure in each cycle will generate irreversible gradual change to the lattice structure in the molecular sieve. Therefore, long-cycle operation reduces the number of cycles, which also brings benefits to equipments, valves and adsorbents.
3. Selection and comparison of heating source
The regeneration heat source of the molecular sieve purification system includes electric heating and steam heater heating sewage nitrogen, and the heated sewage nitrogen is used to heat the absorbents. Generally, those with stable steam pressure sources will choose steam as the regeneration heat source.
3.1 Comparison of energy saving cost before and after the utilization of relatively low pressure steam
For many air separation users who use electric energy for regeneration, the design of air separation may be due to the unstable steam source or insufficient steam pressure, so it is suggested that the steam heater and electric heater should be connected in series to control the temperature, achieve the purpose of warming the absorbents. We also calculated the operating costs of this part of users. Example: compare table 2.
Table 2 illustration: with the connection of 0.5MPa steam heater and electric heater, 40,000 air points is expected to reduce the cost of 1.19 million yuan/year; 20,000 air points can save 810,000 yuan/year, the investment cost of steam heater process is 1 ~ It can be taken back in 2 years.
4. Operational energy saving measures
4.1 control of air inlet temperature
4.1.1 if the inlet temperature rises, the outlet temperature will be higher
The control of the inlet temperature of the adsorber can directly reduce the absorption heat and regeneration energy consumption during the absorption process. Air inlet temperature lifting 1 degree, water content about enlargement 6%, such as air inlet temperature 12 ℃, in 6MPa MPA, after adsorption air temperature about 16.7 ℃, up 4 ℃; air inlet temperature 17 ℃, after adsorption air temperature 23.5 ℃, rising 6.5 ℃. This is because of the increase of water content, which leads to the increase of absorption heat and the higher temperature rise. The higher the outlet temperature is, it means that the air entering the tower needs to consume more cooling.
Of course, according to the conservation of energy, the cooling of the air-cooling tower to 12 ℃ needs more energy than cooling to 17 ℃, but the cooling of the air-cooling tower mainly uses natural water and chilled water to cool down the energy loss. However, if the latter two parts -- the molecular sieve purification system warming, regeneration and expansion machine system are used to complete the cooling, the loss will be large and the energy consumption will be higher. Therefore, if possible, since the working condition of air separation design takes the special high temperature working condition such as summer into consideration, there is a certain design margin in non-high temperature season, and the inlet air temperature should be lowered as far as possible, more conducive to energy saving. The larger the air separation is, the more energy consumption will be saved. But if the inlet temperature of the designed molecular sieve is too low, it will also lead to problems such as poor control and easy freezing.
4.2 adjustment and control of total heat of regeneration
4.2.1 The observation of the minimum temperature of the waste nitrogen emission outlet of the adsorber
Whether the heating heat is enough can be determined by observing the temperature of the nitrogen outlet of the adsorber at the heating stage.
After the high-temperature heating gas enters the adsorber, heat will heat the internal absorbents and analyze the internal CO2, PPM, hydrocarbon and so on. After the gas passes through the bed layer, the high temperature gas transmits the heat to the upper layer of the molecular sieve. When it reaches the lower layer of the bed layer, there is no extra heat transmits to the adsorbents below the bed layer, therefore, first of all, the heat of the lower layer of alumina absorbed will be brought out, which will cause the temperature to begin to drop to a lowest point, and then rise. The temperature of the lowest point generally needs to reach below 0℃. Whether the lowest temperature is low enough indicates the degree of analytic drying inside the bed during the heating stage.
According to the conclusion, under the heating condition of designed gas volume, when the outlet temperature reaches below-2℃, the carbon dioxide after regeneration is generally normal; if the lowest temperature reaches above 0℃, it indicates that the regeneration effect is not good. There is even an extreme situation that the heating low reaches 5℃, which indicates that the internal absorbents are not dry. Later it was found out that it was the recycled gas after the steam heater leaked to warm, which caused the internal dryness of the bed to fail to meet the requirements.
Therefore, in the heating stage, we can judge whether the regeneration is enough by observing the minimum temperature of the exhaust nitrogen outlet of the adsorber.
According to the formula regeneration heat Q = V heating gas volume ×cv × & Delta; T×T time, the amount of regeneration heat is determined by regeneration heat volume, temperature difference and heating time.
If the lowest temperature in the heating stage is not low enough, we can increase the input of heat by Appropriately increasing the gas volume in the heating stage, or prolonging the heating time, increasing the heating temperature and other measures.
4.2.2 blow-cold peak
Entering the stage of blowing-cooling, the renewable gas will blow out the heat inside the bed, and the temperature of the nitrogen outlet of the adsorber will gradually rise to reach a peak value, which is also called the peak temperature of blowing-cooling. Through the peak temperature of blowing cooling, we can judge the highest temperature inside the bed, so as to judge the overall degree of attachment of the adsorbents in this regeneration cycle.
Generally, the peak value of cooling is above 80℃, and incomplete regeneration can also meet the regeneration requirements, but it needs to be activated after accumulating for a certain period of time.
5. Energy saving control in terms of heat insulating materials and heat insulating effect
5.1 waterproof of thermal insulation materials
Because in the interior of electric heater or steam heater, the sewage nitrogen usually goes through the shell, and the temperature reaches more than 150 degrees. There is a big difference between the temperature and the external environment, and heat insulation materials are adopted. This point has been calculated and confirmed during the design. But in the daily maintenance process, the waterproof of the external insulation layer of heater and heating pipe and adsorber is also very important.
Assume that the internal water absorption is 10 kg, heat it to 100℃, and then evaporate. The evaporating heat energy needs to be taken away: According to Cp = 1 kcal/kg. ℃, latent heat of water evaporation: r = 600 kcal/kg. weight of Water: m = 10 kg calculation:
Q=Cp×m×Δt+r.m=(10×1×(100-20)+10×600)=6800Kcal
Due to the heating over 1 h in one cycle, the electricity is equivalent to 7.9 kWh. This part of energy consumption is also quite large in a year.
Therefore, the waterproof of thermal insulation materials is very important. Sometimes, some heaters and heating pipes have dripping phenomenon, so it is more necessary to conduct external waterproof binding.
5.2 heat preservation of high temperature valve and pipeline
Large air separation valve such as DN1400, supposing the average weight: 300 kg, during the heating process, it also needs to absorb heat continuously. It is equivalent to heating up from 20℃ to 160℃, and the specific heat of carbon steel is 0.11 kcal/kg. ℃, which is calculated to consume 10.7 degrees of electricity. After one year, 10.7 × 6 × 365 × 0.58 = 13647 yuan, (charge of electricity supply is 0.58 yuan/kWh, heating once every 4 hours, heating 6 times a day), 4 valves behind the adsorber, 4 × 13647 = 54589.06 yuan.
These valves also include manual valves behind the electric heater or the steam heater. These high-temperature valves all need insulation and waterproof dressing. Meanwhile, pay attention to the heat preservation effect of the heat preservation material to ensure that the temperature outside the heat preservation layer is relatively low.
6. Conclusion
Through the energy-saving analysis of each link of the molecular sieve purification system, it can be concluded that the adsorption time can be prolonged appropriately to bring about energy-saving effect on the premise that environmental conditions permit and there is a safe margin. For users who need to update the molecular sieve after operating for many years, it is suggested to compare with the working condition in the design to upgrade and transform; for users who have steam available, steam heating is preferred to reduce the operating cost. And put forward energy saving suggestions when operating molecular sieve purification system. Through the above analysis and taking reasonable measures, the comprehensive effect of energy conservation and consumption reduction of molecular sieve system can be achieved and the operation cost can be reduced.
