Method of determining humidity of natural gas after glycolic drying

FIELD: physics; measurement.

SUBSTANCE: invention relates to measurement techniques and more specifically to measurement of humidity of gases particularly natural gas, containing a high level of technical trace constituents - compressor oil, vapours of desiccant alcohols (glycols), and higher hydrocarbons. Gas is passed over the cooling surface of a metallic mirror and the dew point temperature (DPT) is recorded. At operating pressure the gas is fed into an enclosed volume. Using the mirror, the entire gas is cooled to a temperature invariably lower than the dew point. Thermohygrometric equilibrium is established between the falling condensate and the surrounding gas and mass of the water precipitated on the mirror is measured. The absolute moisture content, adjusted to normal conditions, is found, corresponding to saturated gas at temperature of the mirror, using known tables or graphs which link humidity of gas with dew point temperature at operating pressure. The complete normalised humidity of the initial natural gas is calculated using a given mathematical relationship, and then using the same tables or graphs, the dew point temperature is found.

EFFECT: reduced errors.

3 dwg

 

The invention relates to the field of measuring equipment, namely, moisture measurement gas, in particular natural, containing a high level of technological impurity components - compressor oil vapor drying alcohols (glycols), higher hydrocarbons.

There are several ways to determine the humidity of gases [1].

Closest to the claimed condensation is a way of measuring the humidity of gases. The analyzed gas is passed over the metal mirror, which is gradually cooled. At the same time are monitoring the state of the mirror surface, and measure its temperature So At a certain temperature T=Ttothe surface of the mirror begins to sweat it condensed droplets of water - falls the dew. Temperature Ttocalled the dew-point temperature (TpTTP), is associated with a moisture content of gas and is a measure of the moisture[1], [5], [10].

Devices based on the described principle, called condensation hygrometers. Known condensation hygrometers: "Kharkiv 4", "Bovar", "CG-Chandler", "Dew Point Tester" and devices "Cong - prima", containing a cooled mirror, the surface temperature or changes over time or varies along the mirror[1], [7].

Condensation hygrometers are widely used in enterprises gases is th complex as measured by TTR value defines the safety margin to meet the required conditions of operation of the pipeline: if the temperature Tpcompared with the temperature of the walls of the pipeline or falls below it, on the inner surface of the pipe there is water condensed from the gas. This violated the terms competent operation of the pipeline internal surface control equipment (valves, valves, diaphragms, etc.) begins to be covered by a water film and to acquire crystalline; with a significant amount of water, the gas stream becomes a gas-liquid two - phase. This reduces the capacity of the pipeline, causing the need to increase the pressure differential between compressor stations, leads to a number of negative aspects. Therefore, before applying gas to pipeline its drain to a certain temperature, dew point temperature, normalized by OST; it depends on the climatic zone, which passes through the pipeline, and the time of year [2]. Typically, the drying gas used absorbers with a liquid absorbent (mainly glycols; next to certainty - diethylene glycol - deg) [10].

When measuring TTR gas at the outlet of the absorber through the condensation hygrometers difficulties associated with the fact that the cold climate the ski zone TTR vapors of the desiccant - Deg (Tis much (30-40°C) higher than TTR drained gas moisture (T). Hence, by lowering the temperature of the mirror hygrometer, in order to determine the temperature of condensation of water vapor, a pair deg begin to condense at much higher temperatures mirrors than water vapor. This fact greatly complicates the measurement process Tbecause the film deg is formed much earlier than the film of moisture. An additional point, introducing significant error into the determination of Tis the fact that condensed on the mirror pair deg absorb moisture from the gas fed to the hygrometer, which blurs the very moment at which condensation of water vapor [3, 7].

For example, if the temperature of the absorber plates, setting the temperature of the vapor deg out with dried gas is 10-20°C (typical values), then the equilibrium vapor concentration deg will be 0.5-1.2 mg/m3. The amount of water in the gas drained to -20°C (Tr=-20°C) will be ~0.8 g/m3. Thus, the concentration of deg in the vapor phase will be from 0.06% to 0.15% of the concentration of moisture in the gas. Given that the TTP in Daegu lies in the area of 10-20°C, dynamic hygrometers ("Cong-prima-2", "Cong-prima-4", "Bovar" and others),in which the gradual cooling of the mirrors from high temperatures to TTR gas moisture, will the "distance" from the TTP in Daegu to TTR in moisture, reducing the temperature at a rate of 2 degrees per minute [5], for 15-20 minutes. However, due to the constant flow of gas at the mirror hygrometer will have time to accumulate a significant condensate film deg, and to distinguish the beginning of the formation of the new film - film water - the problem is quite severe. It is a disadvantage of this method when measuring TTR gas at the outlet of the absorber.

The disadvantage of the above method is also the fact that it is impossible to correctly measure TTR, because the definition of TTP on the temperature at which condensation Ttoor the temperature of the evaporation Tandor both of these temperatures leads to significant errors, especially in the case of low TTR (minus 10-20°C), because the fixed temperature at which condensation Ttowill always be lower, and recorded the temperature at which evaporation of Tand- always above the true TTR [8].

The technical result of the proposed solution is the creation of a method of determining the moisture content of the gas after installation glycol dehydration.

The technical result is achieved in that in the method of determining the moisture content of natural gas glycol dehydration, namely, that the gas is passed over a cooled surface of a metal mirror, and determine the value of the temperature of the dew point (TTP), gas at an operating pressure Ppserved in a closed volume V, with the help of a mirror all the gas is cooled to a temperature TCobviously below TTR (Tp)set thermohygrometric balance drawn between the condensate and the surrounding gas, measure the weight fell on the mirror of water Meters, find the absolute, reduced to normal conditions, moisture content WA1(TC) known to tables or graphs, which links the moisture content of a gas with a dew point at the operating pressure corresponding to the saturated gas at a temperature TCand calculate the moisture content of the original natural gas ratio:

Pnat normal pressure, then under the same tables or graphs found TTP.

Figure 1 presents a device that implements the claimed method. The device comprises a closed vessel 1, which is thermocooler 2 with a cooled mirror 3, a temperature sensor mirror 4, a gas temperature sensor 5 and the heat sink 6. The whole bottom end of the vessel 1 and its walls, except for the mirror 3, is closed by the projection 7 (e.g., foam). In the upper part of the vessel is very slowly rotating fan 8, a mixing gas with the aim to equalize its temperature, pressure gauge 9; input valve 10 soedenjaetsja 1 with tubing the valve 11 is opened while leveling a new portion of the gas in the working position the valves 10 and 11 are closed; open for input light or microwave radiation 12, a radiating horn 13, receiving the mouthpiece 14, the radiation source 15, the light receiver 16, the recording device 17; numbers 18 and 19 designated incident and reflected light or microwave beams.

Recording device 17 allows to unambiguously associate the observed effect is the weakening of the light or microwave beam with the film thickness of the water and knowing the condensation surface, to calculate the total mass of water M, condensed on the mirror.

The entire device is pre-calibrated gas with a known amount of moisture received from generators wet type gas "pole", "Spring-2", "Spring-3", etc. [12; 7], or using high-precision humidity sensors [13; 7].

Figure 2 shows the graphs of a time course of the temperature of the mirror Tg, gas temperature Tgand the mass of water M, condensed on the mirror 3.

Figure 3 shows a graph of the one-to-one correspondence absolute moisture content WAndand dew point for natural gas with density R=0,70 g/m3at a pressure of P=0.1...72 MPa, is given in [11]. The absolute moisture content of WA(g/m3) figure 3 shows normal conditions is m, i.e. to 760 mm RT. Art.(0,1013 MPa) and 20°C, the numerical table, the corresponding dependence of W(P, Tp), which graphs figure 3, is shown in[4], [10].

The method is implemented as follows. Consider a specific example of one measuring cycle. Let for definiteness working conditions are such that the gas in the pipeline is at an operating pressure Pp=5 MPa and the gas temperature Tg=7°C, let the dew point of the moisture will be Tp=3°C.

Initially the volume 1 is filled with gas from a gas pipeline. To do this, open the valve 11 and gradually release the former portion of the gas. The pressure drop of 0.1 MPa. Then open the valve 10 and organize the purge gas to the cleaning vessel 1. After that, gradually opening the valve 10, is installed in the vessel operating pressure P0=5 MPa. Since the temperature in the gas will increase due to adiabatic compression, continue blowing heated gas with an operating pressure up until the temperature in the vessel 1 becomes equal to the temperature of the gas in the pipeline. Close the valves 10 and 11.

At time t=t1(see figure 2) on thermocooler voltage and the maximum current, at time t2the temperature of the mirror TCand the gas temperature Tgbegin to fall. At low temperature TCbelow 3°C in the grain is Alo starts to fall for the condensate, the mass grows with time and with decreasing temperature (figb).

The process of achieving thermodynamic and hygrometric equilibrium between the gas temperature and the temperature of the mirror takes a long time, since the gas is cooled slowly and the temperature asymptotically approaches the temperature TC. In order to achieve one hundred percent relative humidity, at time t3the refrigerator temperature set at 15-20°C higher to begin the evaporation of condensed moisture. The evaporation process in the presence of mixing gas with fan goes faster than the condensation process, the gas is quickly saturated with water vapor, and the temperature is compared with the temperature of the mirror TC. Mass condensed on the mirror of water is the same: in the region of t4comes hygrometric equilibrium relative humidity of the gas reaches 100% and then does not change (Fig).

After reaching the hygrometric equilibrium, i.e. after a time t4determine the gas temperature Tgthermometer 5, which is his dew-point temperature (TP2=Tg), and the registering device 17 measures the mass of condensate Meters

Knowing the dew point temperature of the gas Tgon figure 3 are absolute, reduced to normal the conditions moisture content W A1(TCin the unit volume, and knowing (from measurements) the amount of mass deposited condensate Meters, expect more given moisture contentso that the moisture content of gas WAgo as a sum of vegetariani WA1(TC) WA2: WA=WA1(TC)+A2.

We will show by numerical example of how this is done.

May we have in the vessel 1 when Rp=5.0 MPa hundred percent relative humidity, corresponding to the dew point temperature Tp=Tg=-20°C.

By measuring the attenuation of the signal reflected from a mirror, if the optical hygrometer, or by measuring the attenuation of the signal, if the microwave hygrometer, find the number rolled on the moisture mirror. Because this moisture fell from a known volume, you can calculate how much water in the gas was originally, that is to find the moisture content W, and then, using the table of saturated water vapor, to identify and TTP.

Let the volume of the vessel 1 is equal to 3 litres.

Further identified that the mirror condensed M=20.7 mg of water. Since this is the amount of water condensed from a volume of 3 l, the moisture content of each liter was reduced by the value of the

(WRadditional moisture content at operating pressure) which ACC is tstuat additional shows the absolute moisture content of the gas under normal conditions W A2calculated value

Since we have (created) gas with Tp=TC=-20° C, then it (see figure 3) shows the absolute moisture content is WA(TC)=0.027 g/m3. Thus the moisture content of the sample gas will be equal to

WA=WA1(TC)+WA2(M)=0,027 g/m3+was 0.138 g/m3=0,165 g/m3, which corresponds to a dew point of the sample gas with Tp=3°C(see figure 3).

The main advantage of the proposed method of determining the moisture content or TTR gas moisture, compared to the condensation method is that it allows measurements without the duct, and be used to analyze a small amount of gas (in the example, 3 years). This volume contains a very small amount of vapor deg, which is not able to interfere with the measurement of humidity. This allows measurement of W and TTR in heavily contaminated gases, in particular at the output of the absorbers glycol dehydration unit.

The method was tested in laboratory conditions. As a hygrometer, cooling the gas and determining the mass of the condensed water Meters, applied microwave hygrometer GMM-01, at one time used to determine TTF natural gas in underground gas storage in the village. steppe (With the ratovsky region), where he worked together with the standard hygrometers "Kharkov-1 and modern fiber-optic devices "Cong-prima-2" and "Cong-prima-4" [6]. The hygrometer was improved: greatly increased surface area of the mirrors and power cooling thermogradient.

Measurement of TTR in Gaza in the presence of glycol vapour showed that the method can be applied to solve the problem of determining Tpthe moisture in the dried natural gas after absorber glycol dehydration unit.

The main error of the instrument at the present time is mainly determined by the accuracy of measuring the mass of the precipitated condensate and the determination of the moisture content is the amount ~of 0.1÷0.2 mg/litre, which gives an uncertainty of ±3°C when determining TTR in the area of the temperature minus 20°C.

Sources of information

1. Caliph A.L., Turevskiy E.N., Roly-poly CENTURIES, Sugars VE, the machine P.I. Instruments for determining the moisture content of natural gas. M: IDC Gazprom, 1995, p.45.

2. OST 51. 40-93 Physico-chemical characteristics of natural gas, supplied and transported through pipelines.

3. Moskalev, I.N., Guillemots S. p. Temperature dew point humidity in glycol dehydration. Gas industry, No. 5, 2002, S. 60-63.

4. Gritsenko, A.I., Istomin V.A., Kulkov A.N., Suleymanov R.S. Collection and field preparation of gas on the Northern oil fields of Russia. M: edra, 1999,S. 474.

5. GOST 20060-83 combustible Gases, natural, methods of determining the content of water vapor and dew point of the moisture. M: Of standards, 1984, p.16.

6. Moskalev, I.N., Korotkin I.P., Ushakov V.A., and other Experience of the use of microwave hygrometer station UGS. // Gas industry, 2004, No. 5, S. 63-66.

7. Vyshyvany I.G., Kostyukov V.E., I. Moskalev. Condensation hygrometers: a condition and prospects of improving. // Automation, program and communication in the oil industry, 2007, No. 7, S. 2-12.

8. Moskalev, IGOR. The correct definition TTR condensation hygrometers. // Gas industry, 2004, №4, s-70.

9. Indicator conditionnot gases "Kharkiv 1M". Passport ASA 2.844,000 PSTO. - Vnpo "Ausgezeichnete", 1978.

10. Lanchakov GA, Kulkov A.N., Siebert G.K. Technological processes of preparation of natural gas and methods of calculation equipment. M.: Nedra, 2000, S. 280.

11. Carpenters V.M., Podreshetnikov VA, Goshawks, LN. Instrumentation and metering of natural gas and condensate. Leningrad: Nedra, 1998, 240 S.

12. Seleznev S. C., Derevyagin A.M., Agaltsov A.G., and others, Verifying complex "KONG". Science and technology in the natural gas industry, No. 4, 2003, S. 37-44.

13. Ivchenko Y.A., Mammoths G.M. Fedorov, A.A. Michell Instruments, Condensation hygrometers. Devices, 2004, №3 (45), S. 20-22.

The method of determining the moisture content of natural gas after glycolic wasp is key, namely, that the gas is passed over a cooled surface of a metal mirror, and determine the value of the dew point temperature (TTR), characterized in that the gas at an operating pressure PPserved in a closed volume V, with the help of a mirror all the gas is cooled to a temperature of mirrors TCobviously below TTR (TP)set thermohygrometric balance drawn between the condensate and the surrounding gas, measure the weight fell on the mirror of water Meters, find the absolute, reduced to normal conditions, moisture content WA1(TC) known to tables or graphs, which links the moisture content of a gas with a dew point at the operating pressure corresponding to the saturated gas at a temperature TCand calculate the moisture content of the original natural gas ratio:
PHat normal pressure, then under the same tables or graphs found TTP.



 

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