Application of Simple Orifice Flowmeter in Production

1 Brief process of vinyl chloride synthesis

After drying, acetylene gas and hydrogen chloride gas are mixed in a certain ratio (1:1.05~1.1), preheated in a preheater, and enter the first set of converters in series. The mixed gas of acetylene and hydrogen chloride is converted into vinyl chloride by the mercuric chloride catalyst adsorbed on the activated carbon in the converter tubes. The unreacted mixed gas in the first set of converters enters the second set to continue the reaction. The second set of converters is filled with new catalysts with high activity. The heat of the synthesis reaction is removed by circulating hot water.

2 Analysis of the causes of mercuric chloride consumption

1) The strength of the activated carbon carrier is low, and mercuric chloride is lost with the broken activated carbon; 2) The impure raw gas reduces mercuric chloride to mercurous chloride and metallic mercury; 3) The reaction temperature in the converter is high, and mercuric chloride is lost by sublimation; 4) The mixed gas flow rate of a single converter is large. For a newly installed catalytic converter, the main explanation is from the control of the mixed gas flow rate of the converter.

An important factor affecting the service life of the converter catalyst is the size of the mixed gas flow rate of a single converter. The mixed gas flow rate of a single converter is not easy to be opened too large, especially in the early stage, which is the most important factor leading to the rapid deactivation of the catalyst. At present, most companies do not install flow meters on the converter to monitor the flow rate, and the size of the ventilation volume is manually adjusted by the valve opening. Since the newly installed catalyst has been used for a long time on the second set of converters and the diameter of the converter intake pipeline is mostly above DN150, there is a lot of arbitrariness and uncertainty in adjusting the valve opening based on experience. However, if a flow meter is purchased, it will cost nearly 10,000 yuan. According to a set of PVC production equipment with a capacity of 100,000 tons, at least 40 flow meters need to be configured, which not only creates a great economic burden, but also requires a lot of manpower and material resources for the daily maintenance of the flow meter. Since the measurement of the mixed gas flow rate of the converter does not need to be very accurate, this problem can be solved by a homemade orifice differential pressure flowmeter.

3 Working principle of orifice differential pressure flowmeter

Orifice differential pressure flowmeter is an instrument that measures flow based on the differential pressure generated by the flow detection orifice installed in the pipeline, the known fluid conditions and the geometric dimensions of the detection orifice and the pipeline. When the fluid that fills the pipe flows through the throttling device in the pipe, the flow velocity will form a local contraction at the throttling device, so the flow velocity increases and the static pressure decreases, so a pressure difference is generated before and after the throttling device. The larger the fluid flow rate, the greater the pressure difference generated, so the flow rate can be measured based on the pressure difference. This measurement method is based on the flow continuity equation and the Bernoulli equation.

3.1 Calculation basis

The calculation formula based on the orifice flowmeter is:

Where: Q is the orifice flow rate, unit: Nm3/h; α is the flow coefficient: 0.738 (provided by professional orifice design manufacturers);

ε is the flow expansion coefficient: 0.997870d is the orifice opening diameter, unit: mm;
△p is the pressure difference before and after the orifice, unit: Pa;

To calculate Q, you need to know the density of the mixed gas ρ, and calculate the density of the mixed gas according to the formula ρ=m/V with 1 mole of mixed gas as the basic unit.

According to the formula: PV=nRT, calculate the volume of 1 mole of mixed gas:

P is the pressure before the mixed gas orifice, unit: Pa;
N is the number of moles of the mixed gas, here it is 1 mole;
R is the gas constant: 8.314J/mol·k
T is the mixed gas inlet temperature, unit K;

the above data can be substituted into the formula to calculate the volume V, unit m3.

The average molecular weight of the mixed gas can be obtained by detecting the composition of the mixed gas; the average molecular weight of the mixed gas is equal to the molecular weight of each gas multiplied by the proportion of this gas in the mixture: M average = M1×A1%+M2×A2%+M3×A3%

3.2 Calculation example

Known conditions: mixed gas inlet temperature: 70℃; orifice plate diameter: 57.5mm; pressure before the orifice plate 19.7KPa, pressure after the orifice plate 16.3KPa; the volume composition of the mixed gas is: one group of outlet contains acetylene: 22.5%, hydrogen chloride: 22.8%; VC: 51%, inert gas 3.7%. [page]

Calculation:

△p is the pressure difference before and after the orifice plate: 19800-16400=3400pa

Substitute the above data into the formula to get:

According to the formula: PV=nRT, calculate the volume of 1 mole of mixed gas:

P is the pressure before the mixed gas orifice plate, which is 121Kpa here;
N is the number of moles of mixed gas, which is 1 mole here;
T is the inlet temperature of the mixed gas, which is about 70℃; (343K)

Substitute the data into the formula to get:

121×V=1×8.314×343

V=23.57m3

A group of outlets contain acetylene: 22.5%, hydrogen chloride: 22.8%; VC: 51%, and inert gas accounts for about 3.5%;

The average molecular weight of 1 mole of mixed gas is:

The following is the corresponding relationship between the pressure difference and flow rate of the standard orifice flowmeter made by the manufacturer based on the relevant parameter information of the production site provided by the author. The diameter of the homemade orifice is consistent with the diameter of the standard orifice opening:

It can be seen from Table 1 that when the orifice pressure difference is 3458Pa, the orifice flowmeter displays a flow rate of 455.50m3/h.

The data calculated by the formula is 421.5m3/h.

The deviation rate is: (455-421)/455=7.5%.

It can be used on converters that do not require high flow accuracy and can fully meet production needs.

3.3 Analysis of factors affecting the measurement accuracy of orifice plates

Plate manufacturing and installation accuracy: The material selection and manufacturing of orifice flowmeters must be strict. The manufacturing of orifice plates requires that there should be no obvious scratches on the front face, the front and rear faces are parallel, the outlet edge is free of burrs, and the opening bevel angle is between 30 and 45 degrees. The processing of the orifice opening diameter must be very precise. The pressure holes in front and behind the orifice plate are as equidistant as possible and close to the orifice plate. The pressure pipeline should not be too long. The correct manufacturing, installation and use of pressure holes and pressure pipelines are the key to ensuring the true differential pressure value. Only by strengthening the standardization of manufacturing and installation can the goal be achieved.

Mixed gas temperature and composition changes: Changes in mixed gas temperature and conversion rate will affect the measurement accuracy of the orifice plate, so in daily production, try to ensure stable production operation and stable conversion rate.

4 Conclusion

Through the field use verification in the past year, the self-made orifice flowmeter can fully meet the production needs, and the daily detection is very convenient and fast. It provides a basis and standard for the personnel to scientifically adjust the converter flow, and saves the cost of purchasing flowmeters for enterprises, eliminating the daily maintenance and repair work. It has a large application space in the PVC industry.

References
[1] Liu Xinrong, ed. Flowmeter. Second Edition. Beijing: Water Conservancy and Electric Power Press, 1990
[2] Zhai Xiuzhen et al., ed. Differential Pressure Flowmeter. Beijing: China Metrology Press, 1995
[3] Sun Huaiqing, Wang Jianzhong, ed. Flow Measurement Throttling Device Design Manual. Beijing: Chemical Industry Press, 2000
[4] Jiang Renjie, ed. Instrument Maintenance Worker. Chemical Industry Press, 1993 (end)