At present, our government attaches great importance to the work of energy saving and emission reduction, and many binding targets such as Chinese energy saving and emission reduction during the "13th Five-Year Plan" period will be stricter. For large air separation plant, how to save energy consumption and reduce operation cost on the premise of ensuring safe, stable and reliable operation is an important issue in the design process of air separation plant.
In recent years due to metallurgy, chemical fertilizer and petrochemical the rapid development of industry, air separation device level expanding, for example Hangyang manufacturing has put into use the class 60 thousand large scale air separation unit there 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 operating energy consumption of this system mainly consists of two parts: one is the energy consumption needed to overcome the air flowing through the adsorber, Valve and pipeline during adsorption. The other part is the energy consumption needed for regeneration, including the energy consumption of heat energy during regeneration heating, the consumption of sewage nitrogen during heating and cooling, and the consumption of overcoming the resistance of sewage nitrogen flowing through heating equipment, adsorber, muffler, pipeline, Valve, etc.
The following is a concrete analysis on how to effectively save energy and reduce consumption of molecular sieve purification system.
1. Control of absorption and regeneration resistance drop in purification system
1.1 Control of resistance drop of adsorbers with different structure forms
One form is horizontal or vertical horizontal bed adsorber. Air enters from the bottom of the bed plane of the adsorber and flows out from the top plane. The bed layer is usually double bed, the lower layer is activated alumina, and the upper layer is made up of molecular sieve. Built-in powder filter. The structure form adsorber domestic after nearly 30 years of accumulated experience, design increasingly mature, solve internal support grid frame anti-molecular sieve leak, bed are cloth wait problem, and reliable operation. After mass production, it has the advantages of relatively low manufacturing cost, convenient loading and unloading, etc. Users' inspection and maintenance are relatively simple. Therefore in the domestic air separation on major share. Due to the length-diameter ratio of the bed, the resistance of the adsorber is generally controlled at <6kPa.
The disadvantage of the horizontal bed structure adsorber is that the gas inlet and outlet need a certain uniform space, and the volume of bed accounts for about 35% of the total equipment space. Therefore, the space utilization rate of the equipment is relatively low, the volume of the large air separation is relatively large, and the equipment is placed horizontally, covering a large area. Therefore, measures such as controlling the height of the bed and the flow velocity are adopted to reduce the resistance drop in terms of energy saving. And adopt built-in thermal insulation layer and so on to reduce regeneration energy consumption.
The other is vertical radial flow adsorber, which is composed of upper and lower heads, cylinders, three-layer concentric porous cylinders, built-in powder filter, upper and lower tubes, skirts, etc, activated alumina and molecular sieve placed in two-layer concentric rings space. As shown in the figure on the right, the air passes through the absorbents layer in the radial direction from the peripheral passageway. Due to circumferential ventilation, it has the characteristics of large windward surface and relatively small bed resistance; bed layer space volume of equipment volume of about 50%, space effective utilization is high. So large air separation of the volume, weight can be reduced. And the floor space 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. However, since the proportion of non-ventilated section inside vertical radial flow adsorber is relatively large, the advantage of air separation of 20 thousand and below is not obvious.
The disadvantage of vertical radial flow structure adsorber is that due to the change of diameter, the flow velocity of every circle surface through which the air flows is different, so the adsorption capacity of inner and outer circles is different, and the bed layer should not be too thick, and the uniformity of the bed along the height direction is difficult to reach uniformity, and the adsorption performance of the molecular sieve is affected to some extent, so the designed adsorption time is 4 hours. Interior by multiple concentric bed layer difficult to manufacture, high 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 the resistance of pipelines and valves
In recent years, in the large air separation molecular sieve purification system, the three-bar valve is mostly used in switching valves. 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 most important thing is that the butterfly valve of this structure can well limit the maximum opening differential pressure and effectively prevent the molecular sieve punch. But the resistance of this type of valve is 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.
In the setting of the pipeline, the three-way resistance is the largest, and the elbow is the second, which requires the calculation of the total resistance of the pipeline. Try to reduce resistance.
1.3 measures to reduce regeneration resistance
Recycled nitrogen is emptied after passing the adsorber. In order not to increase the energy consumption of air compressor, reduce the flow pressure of terminal recycled nitrogen as much as possible. 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, so as to increase the pressure of the air compressor. Generally, energy-saving steam heater with low fin and no baffle plate is adopted.
As for the serial flow of steam heater and electric heater, all channels including standby electric heater can be opened when connected, and the electric heater can be used in low power group, the passage of an electric heater is added to reduce the resistance.
2, adsorption time control
The air separation plant is usually equipped with two adsorbers for switching. The adsorption time of one is equal to the regeneration time of the other. The design value of adsorptive time of the adsorber: usually 4 hours, and control the outlet CO2 of the adsorber to be less than 1ppm, in this way, it can also control the content of hazardous gas impurities in the air after adsorption, such as acetylene, moisture, hydrocarbon and so on, and ensure the safe operation of the air separation.
Because adsorption phase air pressure High, regeneration stage requirements low pressure, therefore, regeneration stage with pressure transform, regeneration time by step-down, heating, blowing cold, boost, four-stage. 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, and the effective efficiency δ = Q1/Q. Only when the ratio of Q2 decreases can the energy consumption be effectively reduced.
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; in addition, heating and lengthening cooling time during regeneration 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.
Design value of adsorber adsorption Time:
2.1 The adsorption time of the applied adsorber can be adjusted appropriately
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. Users can judge according to the CO2 value of the adsorber outlet. If the CO2 value of the adsorber outlet changes little or not, it means that the molecular sieve in the adsorber has a certain margin during this period of time and can be extended one by one. If the seasons change and the wind direction changes, the adsorption time can also be shortened. 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/h, consuming average electric power ~ 1588-degree/h. If the adsorption time is extended to 4.5 h, then sewage nitrogen will be consumed ~ 63500Nm ³/h, consumption average power consumption :~ 1502-degree/h. Averagely, 86 kWh of electricity can be saved per hour. Calculated by 0.58 kWh/yuan, the cost can be saved in one year: 86 x 0.58 x 24 x 360 = 430,000 yuan. Other recycled waste nitrogen reduce 7000Nm ³/h, reduce freezer run load.
Therefore, large air separation users, if possible, can regularly monitor the impurity content in the atmosphere, compare with the design value, maintain the normal and safe operation of the equipment, and also provide basis for energy-saving operation. Import setting online monitoring is easier to adjust.
2.2 Molecular sieve update can be used adsorption capacity stronger molecular sieve
With the development of molecular sieve, domestic manufacturers also constantly introduce molecular sieve with higher performance of absorbing CO2. Hangyang rely on dynamic adsorption laboratory, pair molecular sieve the actual use of performance dynamic experimental simulation, the conditions adsorption capacity and recycled energy dissipation of the experiment test. The same amount, commonly used 13X-APG molecular sieve adsorption capacity penetration time more than 120 minutes, strong adsorption of molecular sieve penetration time will increase to 240 minutes, also to analyze molecular sieve desorption (performance) whether easy.
In molecular sieve adsorbent performance need replacement when, based on local current atmospheric conditions replacement into better value of molecular sieve, can get more energy saving benefits. Especially in the atmosphere CO2> 500ppm more than adsorber, recommendations is changing molecular sieve when, the higher level of molecular sieve activated alumina bed layer also change adjustment, adjusting rear can extend adsorption time. 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. Also can use less waste nitrogen 23500Nm ³/h, this part waste nitrogen for shui leng ta cooling energy saving.
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. So long-period operation, reduced cycle times, no matter on equipment, valves, adsorbent also bring benefits to both.
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. This part of the user run fee we are calculated as well. Example comparison table 2.
Table 2 illustration: after using 0.5 MPa steam heater and electric heater in series, 40,000 air points is expected to reduce the cost of 1.91 million yuan/year; 20,000 air points can save 810,000 yuan/year, the investment cost of steam heater process is 1 ~ 2 years can take it back.
4. Operation on 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 0.6MPa under pressure, 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. So, where possible, due to air separation design condition considered summer etc. special high temperature design, in non-high temperature season, with certain design margin, will air inlet temperature try to down, more conducive to energy saving. Air separation greater, recycled energy consumption save more. 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 adsorber waste nitrogen emissions exit minimum temperature observation
Heating heat whether enough, can observe in heating phase adsorber waste nitrogen outlet temperature to determine.
High temperature heating gas into adsorber after heat heating internal adsorbent, and Parse internal CO2, H2O, hydrocarbons, etc. Gas after bed layer after high temperature gas heat to molecular sieve upper, arrived bed layer below, no excess heat to bed layer below the adsorbent, 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 summarize, in design of strength under heating conditions, exit temperature-2 ℃ the following, after regeneration CO2 normal; if lowest temperature reached 0 ℃ above, shows that regeneration effect is bad. 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 heating phase, can pass observation adsorber waste nitrogen emissions exit minimum temperature observation, to judge regeneration is enough.
By Formula recycled heat Q = V heating gas x Cv x Δ t × T time, recycled heat how much, by recycled gas, temperature, heating time the decision.
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 blowing cold peak
Into blowing cold stage, regeneration gas will bed layer the heat inside the purge out, adsorber waste nitrogen outlet temperature slowly rising, reach a peak, also called blowing cold peak temperature. 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.
General blowing cold peak greater than 80 ℃ above, not completely recycled, can reach recycled requirements, but cumulative time, need for activation.
5. Energy saving control in terms of heat insulating materials and heat insulating effect
5.1 of insulation material waterproof
Because in electric heater or steam heater interior, usually waste nitrogen go shell, the temperature reached 150 degrees above. 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 the daily maintenance process heater and hot pipe, the Orb of the thermal insulation layer of waterproof is also important.
Suppose internal water-absorbent 10 kg, heated to 100 ℃, and then evaporates, a need to take evaporative heat: follow Cp = 1 kcal/kg. ℃, latent heat of water evaporation: r = 600 kcal/kg. water weight: m = 10kg:
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.
So insulation materials waterproof important, sometimes some heater and heating tube know dripping phenomenon, but also the outer waterproof dressing.
5.2 High temperature valve, pipe heat preservation
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. A year down, 10.7 x 6 x 365 x 0.58 = $13647, (note electricity 0.58 yuan/degree, 4 hours heating once, six times a day heating), adsorber 4 back valves, 4X13647 = $54589.06.
These valves also include manual valves behind the electric heater or the steam heater. These high-temperature valves all need insulation and waterproof dressing. While small insulation materials insulation effect, guarantee insulation layer outer temperature is relatively low.
6, conclusions
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.
