Writer： admin Time：2022-11-10 00:00:00 Browse：80℃

Zero air loss heat of compression dryer is a new type of desiccant dryer. It does not consume the product air processed through it. In fact, HOC dryer is a thermal swing adsorption (TSA) equipment. Firstly, the desiccant beads utilize the exhaust temperature of front air compressor. That is, it uses the “compression heat”. Then desorb the condensed water under high temperature and high pressure. Afterwards, the product air of the dryer is recycled for cooling. Finally the adsorbent gets “regeneration”.

In fact, the “waste heat” of any factory can act as renewable energy. The most convenient thing in the compressed air system is the final exhaust temperature of the air compressor. Even if the temperature is not high enough, we can also increase it by using an external heater. The figure below is a simulated flow chart of a “Heat of Compression Zero Air Consumption” dryer.

(1) Workflow Analysis of Heat of Compression Dryer:

1) The air compressor (M) inhales the ambient moist air. The parameters are as follows: temperature t0 (°C), pressure P0 (MPa), and water content M0 (kg). After compression, the temperature rises to t1. At the same time, the pressure rises to P1. In addition, the amount of water M0 does not change.

2) The outlet pressure (P1) of the air compressor is 0.7MPa (gauge pressure). The secondary exhaust temperature (t1) is around 120°C. This temperature is lower than the TSA desorption temperature. Therefore, it is necessary to use the electric heater E to improve.

3) The regeneration air volume of tower B can be either full flow or partial exhaust flow. After being heated, it enters the B tower of HOC dryer. Therefore, the regeneration air temperature rises to t2. At the same time, the pressure rises to P2. The water content M0 still remains unchanged. Therefore, the relative humidity is greatly reduced. This becomes a carrier for desorbed water.

4) Regeneration tower (B) state before desorption:

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

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 dew point temperature of the inlet pressure of the previous A tower.

5) Regeneration tower (B) state during desorption:

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

② The temperature t3 of the regeneration tail air leaving the tower is higher than the vaporization point (100°C) —— otherwise, there will be liquid water in the tower of heat of compression dryer;

③ The total water volume of the regeneration tail air 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) —— resulting in a higher desorption temperature —— this “positive feedback” cycle is unique to the “zero air consumption” dryer.

6) Adsorption tower (A) inlet temperature t4≤40℃. A water cooler L1 must be installed between the two towers. It is a two-column connection channel —— this is clearly different from other adsorption dryers down the piping structure.

7) The high temperature and high pressure regeneration exhaust air is cooled in L1. And it discharge a large amount of liquid water. After the temperature drops to t4, it enters the A tower. Then carry out adsorption drying. A tower outlet gas (t5≥t4). One of them enters Tower B for “cold purge”. t6>t5 after the air flow leaves the B tower of HOC dryer. It is cooled to t7 (≈40°C) by the “second cooler” L2. Enter the air pipeline network as “product air”. Since P7<P6<P5, the mixed flow of the two airflows leads to a decrease in P5. The magnitude of the decrease is determined by the proportion of cold purge blowing.

(2) Energy consumption analysis of HOC air dryer

The energy consumption of the zero air loss dryer includes two parts. That is, “heat loss” and “power loss”. In addition to “compression heat”, the former is mainly supplemented by electric heaters. The latter is all added to the “shaft power” of the air compressor drive motor.

1) Heat loss of HOC air dryer

The “compression heat zero air loss ” dryer adopts the whole process of high pressure desorption. The vaporization temperature of the adsorption condensation water increases with the increase of the pressure in the tower. Under the working pressure of 7bar, the regenerating air inlet temperature needs to be above 250℃～280℃. The temperature apart from the tower is not lower than 100℃～120℃. This is the same reason that the vaporization temperature decreases at low pressure. For example, the well-known Everest air pressure is 0.3 bar. The water vaporization temperature (boiling) is 75°C.

We can use Antoine’s formula to calculate the phase transition temperature of saturated steam at different pressures. Its general formula is: ln(P)=A-B/(T-C). The difference between the “pressure-phase transition temperature” of different substances is that the coefficients A, B, and C are different. For water vapor, the Antoine formula is specifically expressed as: ln(P)=9.3876-3826.36/(T-45.47). The unit of pressure P in the formula is MPa. The applicable range of temperature T is 290K～500K.

Specific calculation and conculutions:

Calculate the vaporization temperature of water at 0.8MPa according to the above formula, T≈175℃——that is, to desorb condensed water at 0.8MPa, the temperature of the desiccant beads should not be lower than 175℃. And in order to prevent liquid water from accumulating at the bottom of the tower, the temperature of the desorption air apart from tower should be higher than 100 ℃. At the same time, the inlet temperature is higher than 250°C. Only in this way can the average temperature of the adsorption column reach 175 °C. The exhaust temperature of the last stage of the two-stage compressor is not so high.

Therefore, we must add an external heating device —— the heater power must meet the requirements. That is to meet the heat of vaporization and various “rigidity” consumptions required for condensing water. The so-called “just consumption” refers to ineffective consumption that cannot be avoided. This includes the heat of the regeneration exhaust air for heating the adsorbent and the tower body, etc. Such losses do not vary with the size of the load.

According to the air state equation: the pressure and temperature change synchronously in the isovolumic process. The air compressor exhaust temperature increases from t1 to t2. This will increase the pressure of the air entering column B (P2>P1). According to the Antoine formula, the vaporization temperature of the condensed water will also increase. ——This positive feedback process cannot stop until 223°C (500K). This theory is extremely unfavorable for “zero air consumption” desorption.

2) Power loss of heat of compression dryer

The air compressor drive motor converts the kinetic energy of the main shaft into the potential energy of the compressed air. That is compression energy. So make it have the ability to do external work.

Take the atmospheric temperature and pressure as the external reference. The ability of compressed air to do external work is called effective energy E. Its expression formula is: E=pV ln (P/Pa).

In the formula: V—volume, p—air absolute pressure, Pa—atmospheric absolute pressure (0.1MPa).

Effective energy E depends on the pressure and volume of the compressed air. When the pressure is equal to the external pressure, the effective energy is zero. The higher the pressure, the higher the effective energy value. When the air flows, the effective energy takes the form of power. We call it “pneumatic power” P. P=Pa**Qa**InP/Pa (W). And Qa—— the volume flow m³/s converted in atmospheric state.

The meaning of pneumatic power:

① The pneumatic power unit is the same as the electric power unit. Both of them are all W. So it’s good for the unified calculation of the energy consumption of the whole dryer;

② Pneumatic power is the pure energy supplied to the end-use air equipment. It has excluded the energy loss during air production and air transmission;

③ Under a certain flow rate, the power of various types of equipment in the pneumatic pipeline is closely related to the inlet and outlet air pressure. The air quality requirements of the power system are not limited to cleanliness. That is, there are also requirements for water, dust and oil content. In addition, pressure drop is also an important technical indicator. Many pneumatic actuators require even higher air pressure than “cleanliness”. If there is not enough pressure, the working capacity of the pneumatic terminal decreases. In severe cases, it will even lead to “paralysis” of the device!

④ The regeneration path of “zero air consumption dryer” is very long. The pressure difference between inlet and outlet is the largest. The aerodynamic power loss against the background of atmospheric pressure is also the largest.

The pneumatic power loss must be included in the constant calculation of the energy of the zero air lossheat of compression dryer.

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