Which separation method is better for gas-liquid separators?
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The process flow is as follows: The feed material is at 8 MPa/270°C; after being cooled to 45 degrees Celsius in a cooler, it enters the gas-liquid separator. There is no pressure reduction inside the separator, but the gas purification unit at the top of the subsequent separator does not require such high pressure, with an operating pressure of only 4 MPa. To reduce costs, could the operating pressure of the separator be lowered to 4 MPa? Will this affect the separation efficiency? The separation medium is a hydrogen-rich polyol. The process flow diagram is as follows: The main difference lies in whether pressure control is performed before or after the gas-liquid separator
Basically, there should be no need to install a pressure control valve. To put it another way, after the fluid passes through the cooler, the decrease in temperature causes some of the substances inside to turn into a liquid state or form droplets. These are then separated through gas-liquid separation. Furthermore, when fluid (whether liquid or gas) moves from a smaller space to a larger one (such as from a pipeline into a storage tank), the pressure changes accordingly due to the change in flow rate, which makes the formation of liquid or droplets more apparent and facilitates separation. PS. There are some issues with this kind of cascade control: the pressure source is 8 MPa, and even with pressure losses due to the heat exchanger, it won’t drop below 4 MPa. If control is to be achieved, the pressure transmitter should be installed downstream of the control valve, so as to monitor the pressure downstream and adjust the valve’s operation; it shouldn’t be installed upstream, as in that case it’s impossible to measure a value of 4 MPa
They are all backpressure valves; they only control the pressure ahead, while the pressure behind is controlled separately. You probably didn’t understand what was meant by that
It depends on the size of your traffic. If the flow rate is high, separation should occur at 8 MPa and 45°C; by reducing the pressure to 4 MPa now, cooling may occur due to throttling ； At the same time, a decrease in pressure may result in an excessive vapor phase in the separator, causing too much polyol to be carried away with the vapor phase and thus reducing the polyol recovery rate. Why not reduce the pressure after separating the vapor phase, which would require one more throttling element?
Placing the pressure control at the front results in a mixture of gas and liquid at the pressure measurement point, making it difficult to obtain accurate readings; as a result, control issues arise. Colleagues in the instrumentation field, please correct it～
The answer on floor 5 can be referred to by the original poster! Speaking of ideas!
The second process is good: after cooling, the liquid phase precipitates and is separated. In the first process, pressure is reduced before separation, and the liquid phase vaporizes again.
It’s a comprehensive issue – what purpose do you want the separator to serve? The first type is gas-liquid mixed throttling; as mentioned on the 5th floor, changes in pressure lead to changes in the composition of the gas and liquid phases as well as in temperature. Is it feasible from a process perspective, or what type is required from a process standpoint? Additionally, the first pressure measurement point should be located after the valve. If the volume is so large, such a high throttling pressure drop is a bit of a waste! If the gas pressure is to be increased by another 4 MPa later on, it will be difficult – requiring both a compressor and a cooler. So it’s still necessary to consider everything comprehensively.