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Application of Mass Flow Meters in Measurement of Crude Alcohols - Focus on Microfluidic Control Technology

1 Introduction
The mass flowmeter has a range of 0~50t/h, accuracy of ±0.2%, four-wire, 24VDC power supply, and 4~20mA output. It is based on the Coriolis force principle to measure the mass flow of a fluid. The main body of the measuring structure is two parallel titanium measuring tubes. The fluid flows through two parallel measuring tubes. The measuring tube is subjected to the reverse vibration caused by the Coriolis force. The vibration causes the deformation of the tube, and the phase difference between the inlet and the outlet follows. The change (Figure 1).
As the flow of fluid through the measuring tube increases, the phase difference increases. The vibration phase of the tube can be measured by the inlet and outlet electromagnetic phase sensors. The system balance is guaranteed by the anti-phase vibration of the two measuring tubes.
The mass flow meter complies with the National Metering Verification Rules for Mass Flowmeters (JJG897-95) promulgated by the State Bureau of Technical Supervision in 1995. Compared with other flowmeters, it has the following advantages:
(1) The measured value is affected little by the changes in fluid temperature, pressure, viscosity, conductivity, and flow characteristics;
(2) The measuring tube has a high vibration frequency (1000 Hz) and is affected little by the low-frequency vibration of the field pipeline;
(3)Easy to install, no need for a straight pipe section, horizontal or vertical;
(4) Flow, density and temperature can be measured simultaneously.
2 Problems and Solutions After Installation
2.1 Problems
After the mass flow meter was installed, some abnormal phenomena occurred in the measurement system during the debugging process:
(1) Flowmeters sometimes appear to be missing, resulting in inaccurate crude alcohol accumulation;
(2) When the flow rate is more than 5t, the flowmeter shows 0.
2.2 Reasons
(1) From the information provided by the instrumentation, the main alarms are 586#, 587#, 701#, which respectively reflect the two conditions that the fluid velocity is fast and the fluid contains bubbles.
(2) From the perspective of usage, the crude alcohol includes two routes: all the way from the methanol separator outlet pipe into the crude alcohol flash tank, the pressure is reduced from 13.5 MPa to 4 MPa, and then further reduced to 0.4 MPa by the regulating valve. A portion of the gas was flashed off; the other was from the neat alcohol scrubber because the liquid level of the neat alcohol scrubber was not well controlled and some of the gas was trapped. After two-way convergence, the meter is sent to the distillation section for purification. This situation has the potential to entrain airborne alcohol flowing through the flow meter.
(3) There is a 90° downward elbow at 5m from the outlet pipe of the flowmeter, so that the bubbles in the fluid flow upward and accumulate in the horizontal pipeline containing the flowmeter, which has a serious impact on the measurement results. Sharp high-frequency whistling in the tube also illustrates the existence of this phenomenon.
2.3 Solution
According to the analysis, the main cause of the failure is the gas-liquid two-phase flow in the medium flowing through the flowmeter. To this end, two solutions are proposed: one is to add a buffer tank in front of the main pipe to eliminate the flashing bubbles after the pressure drop, and the second is to install an air eliminator to release the flashed bubbles. After comparison, the buffer tank is large in size, and more capital is needed for retrofitting. The workload of reconstruction is large, and the difficulty is also great. The small size of the gas eliminator requires less funds for retrofitting, and the workload for modification is small. In the end, a solution for installing an air eliminator was adopted.
The air eliminator is flanged and has orifice plates, bulkheads and float valves. When the liquid to be measured enters the container, the gas contained in the liquid to be measured breaks through the liquid film and collects on the top of the gas eliminator along the inclined partition after the perturbation and throttling of the orifice plate. As the gas increases and falls, when the liquid level drops to a certain height, the float valve opens under the action of the float gravity, discharges the gas, and then the liquid level rises, the float floats, once the float rises to a certain height, The exhaust valve closes.
When using an air eliminator, a back pressure of 0.022MPa is required after the request. The instrument controls the original pipe to have exactly an upward rise, which can act as a back pressure valve. With the addition of the air eliminator, the flow meter is set to cut small signals (<0.1 t/h) to prevent interference from vibration signals.
3 Usage after change
After installing the air eliminator, the flow meter display is still sometimes absent, and sometimes the air pipe alarm message is also issued. After careful observation, it was found that the air release valve of the air eliminator was exhausted for a long period of time. Due to the excessive gas content, the air eliminator did not exhaust the air bubbles.
To solve the above problems, the following two methods can be used: one is to connect a gas release valve in series to further eliminate air bubbles; the other is to expand the gas exhaust hole from 6mm to 8mm, and at the same time, reasonably adjust the vent sealing gasket and the spring so that both Do not leak liquid and intermittent exhaust. The instrumentation uses a second method and incorporates a net alcohol wash column into the flowmeter line into the crude alcohol flash tank to reduce the gaseous components entering the fluid.

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