Taking the 4 #ethyl acetate parking space for automobile loading and unloading as an example, the exhaust gas recovery method in which the gas phase balance pipe is directly connected to the storage tank is currently adopted. The design idea is that the liquid level of the storage tank decreases during loading, and the gas phase space in the tank increases, forming a closed balance cycle between the tank car and the gas phase tube. However, in actual operation, the nitrogen gas inside the tank is generally not replaced. After the air in the tank is mixed with the ethyl acetate vapor, if it directly enters the storage tank, it may bring new safety risks.
The following is an analysis of the system operation combined with on-site process parameters and pressure verification.
Whether the exhaust gas of the tanker can enter the storage tank smoothly is essentially to determine whether the pressure of the tanker has sufficient driving force, that is, it must meet:
Tank Pressure> Tank Pressure + Total Pipeline Pressure Drop
At the same time, the pressure must not exceed the take-off pressure of the tanker breathing valve/safety valve.
Calculated according to the Clausius-Clapeyron equation:
Saturation vapor pressure at 35 ° C (summer) 16.2 kPa (gauge)
The saturated vapor pressure at 5 ° C (winter) is about 4 kPa (gauge pressure)
The tank car is a state of coexistence of gas and liquid. When the pressure reaches the saturated vapor pressure of the medium, the evaporation and condensation reach a dynamic equilibrium, and the pressure does not continue to rise.
Combined with the local temperature in Wuhu over the years, the saturated vapor pressure of ethyl acetate is extremely sensitive to temperature:
Main parameters of gas phase balance tube:
| Parameter | value |
|---|---|
| Specifications | DN150, inner diameter 154.1 mm |
| Straight Pipe Length | 200 m |
| Wall roughness | 0.15 mm |
| 90 ° elbow | 15, a single equivalent length of about 4.5 m |
| Total Equivalent Length of System | Approx. 267.5 m |
According to the liquid phase loading flow rate of 60 m ³/h, the gas phase flow rate is about 50 m ³/h.
Calculated according to the Darcy-Weissbach formula:
Frictional resistance along the path: about 4.8 kPa
Local resistance (elbows, valves, etc.): approx. 2.3 kPa
Main pipe pressure drop Delta P Pipe resistance: 7.1 to 8.5 kPa.
Note: If there is fluid in the pipeline, the pressure drop may increase by more than 50%. The design pressure drop of the flame arrester on the gas phase pipeline is generally 2-3 kPa. If the blockage or selection is too small, the pressure drop can reach 5-10 kPa.
On-site tankers are mainly divided into two categories:
| Tank type | Design pressure |
|---|---|
| Atmospheric pressure aluminum alloy tank | 3.6 kPa |
| stainless steel tank | 7.2 kPa |
When the tank is discharged, the liquid level decreases, the gas phase space expands, and the pressure decreases. When the pressure is lower than 1 kPa, the self-operated nitrogen sealing valve automatically replenishes nitrogen to maintain the tank pressure.
The initial pressure of the tanker is close to normal pressure, and the gas phase is compressed after the liquid enters, and the pressure gradually rises.
Tank pressure increased to 7.2 kPa, reaching respiration valve take-off pressure.
At this time, pushing the exhaust gas into the storage tank needs to be overcome:
Tank pressure + pipe pressure drop = 1 kPa + (7.1 to 8.5) kPa = 8.1 to 9.5 kPa
It is much higher than the upper pressure limit of the tanker 7.2 kPa, the exhaust gas cannot enter the storage tank through the balance pipe, the breathing valve is opened, and the steam is directly discharged.
The pressure of the tanker is maintained at around 7.2 kPa, and the breathing valve is frequently opened and closed;
There is basically no flow in the gas phase balance pipe, and the balance recovery system actually fails;
The storage tank continues to replenish nitrogen due to the continuous discharge pressure drop.
Due to the combined influence of the design pressure of the tanker, the pressure loss of the pipeline and the nitrogen seal pressure of the storage tank, the system pressure difference is insufficient, and the exhaust gas cannot be effectively recovered to the storage tank. The existing gas phase equilibrium recovery system may be difficult to function.
It is recommended to build a loading exhaust gas recovery system with active power. By adding air induction or pressurization facilities, a stable and reliable transmission pressure difference can be established to realize the fully airtight recovery of loading exhaust gas, and the intrinsic safety level and environmental compliance of the device can be improved from the source.
