Uncover the "zero gas consumption" adsorption compressed air pipe dryer

"Zero gas consumption dryer" is a heated adsorption dryer that does not consume the product gas of the machine.

It is a typical temperature swing adsorption (TSA) device - the adsorbent desorbs condensed water under high temperature and high pressure by using the exhaust temperature of the last stage of the air compressor ("compression heat"), and then circulates the product gas of the machine for cooling to obtain active "regeneration".

In fact, the "waste heat" of any factory can be used as renewable energy. The convenient thing in the air compressor system is the exhaust temperature of the last stage of the air compressor - even if the temperature is not high enough, it can be easily improved by using external heating devices.

The following figure is a practical process of "zero gas consumption of compression heat" dryer (other processes are not excluded)

1、 Process analysis:

1. The air compressor (m) sucks ambient humid air with temperature t0 (℃), pressure P0 (MPA) and water content M0 (kg). After compression, the temperature rises to T1, the pressure rises to P1, and the water volume M0 remains unchanged.

2. The outlet pressure (P1) of the air compressor is 0.7MPa (gauge pressure), and the secondary exhaust temperature (T1) is about 120 ℃. This temperature is lower than the desorption temperature of TSA, which should be raised by the electric heater E.

3. The regeneration gas flow of Tower B can be either full flow or partial exhaust gas flow (two mixed flows before entering the inter tower cooler L1). After heating, it enters tower B. the regeneration temperature rises to T2 and the pressure rises to P2. The water content M0 remains unchanged - the relative humidity is greatly reduced - and it becomes the carrier of desorbed water.

4. Regeneration tower (b) status before desorption:

① The initial temperature (TB) of the adsorption column is slightly equal to the final temperature (≈ 40 ℃) of the front blowing cooling;

② The pressure in the tower (PB) is slightly lower than the working pressure (0.7MPa);

③ The water content (MB) of the adsorption column is equal to the water content (kg) corresponding to the inlet pressure and dew point temperature of the previous tower a.

5. Regeneration tower (b) status during desorption:

① The average temperature of the adsorption column rises to the average temperature (TP) of the inlet and outlet of the regeneration gas;

② The temperature T3 of the regenerated tail gas leaving the tower is higher than the vaporization point (100 ℃) - otherwise, liquid water will be trapped in the tower;

③ Total water volume of regeneration tail gas leaving the tower m3 = M0 + MB; (and M0 > MB);

④ The pressure in the tower is higher than the exhaust pressure of the air compressor (P2 > P1) - which causes the desorption temperature to rise more - this "positive feedback" cycle is unique to the "zero gas consumption" dryer.

6. The inlet air temperature T4 of the adsorption tower (a) is ≤ 40 ℃, and the water cooler L1 must be set between the two towers. It is the connecting channel between the two towers, which is obviously different from the lower pipe system structure of other adsorption dryers.

7. The high-temperature and high-pressure regeneration tail gas is cooled in L1 and a large amount of liquid water is discharged. After the temperature drops to T4, it enters tower a for adsorption and drying. One of the outlet gas from tower a (T5 ≥ T4) enters Tower B for "blowing cold air". After leaving Tower B, T6 ＞ T5, the gas stream is cooled to T7 (≈ 40 ℃) by the "second cooler" L2 and enters the gas pipe network as "product gas". Since P7 ＜ P6 ＜ P5, the mixed flow of the two air streams at the inlet end of the pipe network leads to the decrease of P5, and the decreasing amplitude is determined by the proportion of the blowing cold air.

2、 Energy consumption analysis

The energy consumption of the zero gas consumption dryer is divided into "heat loss" and "power loss". The former is mainly supplied by the electric heater in addition to the "compression heat", while the latter is all added to the "shaft power" of the drive motor of the air compressor.

According to GB / t2589-2008 general rules for calculation of comprehensive energy consumption, all kinds of energy (electricity, water, gas, etc.) consumed by power equipment shall be converted into "standard coal" according to the specified "conversion coefficient". The "power loss" caused by the pressure drop is calculated by the reduction of the "active energy" of the compressed air.

1. Heat loss

The "compression heat zero gas consumption" dryer adopts the whole process high-pressure desorption. The vaporization temperature of the adsorbed condensed water increases with the increase of the pressure in the tower. Under the working pressure of 7 bar, the temperature of regeneration gas entering the tower is required to be above 250 ℃ ~ 280 ℃, and the temperature from the tower is not less than 100 ℃ ~ 120 ℃ (Professor Yu Yongzhang advocates that the inlet temperature is 300 ℃). This is the same reason that the vaporization temperature decreases under low pressure - for example, the well-known Everest pressure is 0.3bar, and the water vaporization temperature (boiling) is 75 ℃.

The phase transition temperature of saturated vapor of various substances under different pressures can be obtained by Antoine formula. The general formula is: ln (P) = A-B / (T-C). The difference of "pressure phase transition temperature" of different substances lies in the difference of coefficients a, B and C. For water vapor, Antoine formula is specifically expressed as: ln (P) = 9.3876-3826.36 / (t-45.47), where the unit of pressure P is MPa (absolute pressure), and the applicable range of temperature T is 290k-500k. According to the above formula, the vaporization temperature T of 0.8MPa water is ≈ 175 ℃ -- that is, the temperature of the adsorption bed cannot be lower than 175 ℃ when the condensed water is desorbed under 0.8MPa. In order to prevent liquid water from accumulating at the bottom of the tower, the temperature of desorbing gas leaving the tower must be higher than 100 ℃, and the temperature of entering the tower must be higher than 250 ℃ to ensure that the average temperature of the adsorption column reaches 175 ℃ - the exhaust temperature of the last stage of the two-stage compressed air compressor is not so high.

Therefore, an external heating device must be added - the power of the heater must meet the vaporization heat required for condensed water and the accompanying "rigidity" consumption. The so-called "rigid consumption" refers to all other ineffectual consumption except vaporization heat, including the radiation and convection loss of heating adsorbent, heating tower body and accessories, external wall of tower body to the environment, and the heat of non recyclable regenerated tail gas, etc. this kind of loss does not change with the load, and should be paid according to the quantity under the "fixed time sequence control" -- only the proportion changes according to the gas state equation, The pressure and temperature of the constant volume process change synchronously. The increase of the exhaust temperature of the air compressor from T1 to T2 will increase the pressure of the gas entering Tower B (P2 > P1). According to Antoine's formula, the vaporization temperature of condensed water will also increase - this positive feedback process will not stop until 223 ℃ (500K). This theory is extremely disadvantageous to "zero gas consumption" desorption.

The "heat balance" calculation is used to determine the required small power of the external heater. The calculation basis is based on the allowable limit working conditions (temperature, pressure and flow) of the equipment, strictly abide by the laws of physics, use general engineering data and calculate with mathematical formulas. The "heat balance" engineering calculation does not actively seek the accuracy of the results, but the calculation process must clearly reflect the context of energy income and expenditure within the equipment boundary: total system energy income = total system energy expenditure - this is the universal principle of the "first law of thermodynamics".

In GB / t2589-2008 general principles for comprehensive energy consumption calculation, compressed air belongs to "energy consuming working medium" in terms of energy property.

The definition of energy consuming working medium is: the working substance consumed in the production process that is not used as raw material and does not enter the product, and needs to directly consume energy during production or preparation. There are mainly new water, softened water, compressed air, oxygen, nitrogen, etc. Compressed air is an energy consuming working medium with small "conversion coefficient" and low energy density.

3、 Evaluation of "zero gas consumption" dryer

1. The concept of "zero gas consumption" comes from the "zero emission" in the field of environmental protection ("carbon emission" approaches "zero"). Compressed air is non-toxic, harmless and low-cost "energy consuming working medium". Raw materials are taken from the environment free of charge. Besides the cost of electricity, the purification cost is extremely low. There are no regulatory restrictions and cost barriers on the "quality" and "quantity" of emissions.

2. The "zero gas consumption" dryer destroys the arrangement order of the air source system - the absence of the rear cooler and the oil-water filter causes the downstream equipment and materials to be directly scoured by high temperature, high pressure and high concentration "dirty gas" (moisture, oil mist and solid particles), completely overturning the significance of the purification equipment;

3. Untreated "dirty gas" first enters the electric heater, which is a pressure vessel. High temperature, high pressure and high concentration "dirty gas" flows in the electric heater, which is very easy to deposit carbon and scale. The manufacturers and regulatory authorities must not take this potential safety hazard lightly;

4. Supporting centrifugal air compressor is a passive "non intrinsically safe" protective measure for this type of dryer. The ambient air of heavy chemical industry concentrated areas contains aerosols, various nitrogen oxides and 2 μ M U.M or less. The centrifuge has no effect on these impurities, and large flow of "dirty gas" will cause serious pollution to downstream equipment and materials;

5. The rear cooler is a weak link in the safe operation of the air compressor system. The explosion accident caused by improper operation or management was discussed by Jiang Qiang, the former chief engineer of the second steam turbine. The outdoor aftercooler is moved between the two towers to become an indispensable component of the dryer, which will only increase the operation risk;

6. As a power source, the centrifugal air compressor is inferior to the oil-free screw machine in most cases. The zero gas consumption dryer takes the centrifuge as the supporting object, and reduces the general equipment to the special equipment that cannot be mass produced. This is not the direction of technology development;

7. The power of the electric heater can be reduced by using the exhaust "waste heat". However, a large amount of water brought by the regeneration gas will increase the load of the inter tower cooler (L1) and the consumption of cooling water will also increase. We all know the importance of water and will not mention it here;

8. Under the same working condition, the unit comprehensive energy consumption (converted into standard coal) of the "zero gas consumption" dryer at the cost of "high water consumption" and "high pressure drop" is larger than that of the air blast heat dryer, and it can not be compared with the non heat regeneration dryer with simple structure and variable control;

9. The "zero gas consumption" dryer has a large volume, which greatly increases the material consumption, manufacturing, logistics, finance and after-sales costs, while the product gas quality (pressure dew point and exhaust pressure) decreases;

10. In the sales field, some Shanzhai brands swagger in the name of "zero gas consumption". Such fake and shoddy products have greatly damaged the reputation of the compressed air purification industry and the interests of users.