NATURAL GAS PURIFICATION FROM HIGH HYDROCARBONS IN A VORTEX TUBE

ABSTRACT

The pressure of gas extracted from gas fields in your country is decreasing sharply. It is known that during the initial gas preparation process, the gases separated from separators and degasifiers are burned in a flare due to their low pressure. The cumulative effect has been studied in detail for air, methane, hydrogen, argon, helium, ammonia, carbon dioxide, water vapor, and other gases and vapors. This gas also contains gas condensate, which can be used to obtain liquefied gas. In order to extract this valuable raw material, analytical results are presented on the development of a cumulative refrigeration machine that allows cooling gases even at low pressure.

АННОТАЦИЯ

Давление газа, добываемого на газовых месторождениях в вашей стране, резко падает. Известно, что в начальном процессе газоподготовки газы, отделенные от сепараторов и дегазаторов, из-за низкого давления сжигаются в факеле. Кумулятивный эффект был подробно изучен для воздуха, метана, водорода, аргона, гелия, аммиака, углекислого газа, водяного пара и других газов и паров. Этот газ также содержит газовый конденсат, из которого можно получить сжиженный газ. Для извлечения этого ценного сырья показаны аналитические результаты разработки дымовой охлаждающей машины, позволяющей охлаждать газы даже при низком давлении.

Keywords: Cumulative efficiency, deceleration temperature, gas velocity, internal friction forces, pressure difference, throttling, single-nozzle, double-nozzle.

Ключевые слова: Кумулятивный эффект, температура торможения, скорость газа, силы внутреннего трения, перепад давления, дросселирование, односопло, двухсопло.

Introduction. It was invented by a scientist named Samara (effect) or rank. The process of separating a gas mixture into cold and hot streams during its expansion in a vortex tube is called the vortex effect. The design of the gutter is shown in Figure 1.

The gas flow is transmitted at the speed of sound through a nozzle located tangent to the pipe. Due to the circulation of gas inside the trough, part of the flow changes direction towards the diaphragm. In this case, the gases around the bullet are cooled, and the layer outside it is heated. The efficiency of flow cooling can significantly exceed the efficiency of a conventional throttled Joule-Thomson. For example, when expanding gas from 0.3-0.6 to 1 MPa, the gas temperature of 20...80% decreases by 20...70 ⁰C compared to the initial temperature, while the rest of the gas is heated.

Literature analysis and its results. The rank effect is approximately directly proportional to the ratio of the gas pressure before entering the pipe P₁ to the pressure at its outlet P₂, if (P₁/P₂)≤4…5. If we increase the value of the pressure ratio even more, the separation effect will increase not proportionally, but less, and finally, (P₁/P₂)>11…13, the growth will stop altogether. The spontaneous effect is directly proportional to the absolute decrease in temperature. In a vortex tube, the total cooling effect during the period of gas expansion is equal to the sum of the Joule-Thomson and rank effects.

The maximum cooling effect can be achieved with the cold flow fraction X = 0.2…0.3, and the maximum cooling can be achieved with x = 0.5…0.6. A fan is installed at the hot end of the pipe to regulate the flow ratio. The cooling of the vortex chute compared to the throttle valve when expanding the gas pressure from 0.6 to 0.1 MPa is 14 times more efficient, 3.2 times less than in the expander.

Figure 1. Vortex tube:

1-diffuser with four nozzles; 2-cold flow tube; 3-hot flow tube.

The vortex effect has been studied in detail for air, methane, hydrogen, argon, helium, ammonia, carbon dioxide, water vapor and other gases, as well as vapors. The results of the studies show that the cooling of a gas in a vortex tube depends on its composition in very small quantities. The conversion factors of the heat transfer coefficient α for various gases are given in Table 1.

Table 1.

The conversion factors of the heat transfer coefficient α for various gases

Experimental data on the efficiency of cooling and heating of various gases are given in Table 1.2.

Table 1.1.

Experimental data on the efficiency of cooling and heating of various gases

In simplified form, the essence of the vortex effect is as follows: as a result of the adiabatic outflow of gas from the nozzle, the thermodynamic temperature of the gas T decreases to this value:

                                                        (1)

Where t₀=deceleration temperature, K, U-gas velocity, m/s; g-gravity acceleration, m/s²; CP-gas heat capacity, kcal/kg⁰S; A = 427 kg m/kcalТемпература замедления t₀ остается неизменной.

To reduce it, it is necessary to somehow obtain some of the kinetic energy of the gas flow. This process can be effectively carried out in a sprayer, mechanical work is performed. In a vortex tube, the kinetic energy of the gas flow is transferred to another flow, as a result of which the first flow is cooled and the second is heated.

The exchange of thermal and kinetic energy of two flows rotating in the same direction due to internal friction forces occurs in the turbulent regime of oppositely moving flows. In these cases, rotation of approximately the same thermodynamic temperature and constant angular velocity is established in front of the diaphragm. If the internal flow velocity were zero, the cooling effect would be at its maximum. But this velocity will not be zero, and in practice it will be very large. The approximate value of the cooling effect in a drainpipe is found using this formula:

                                     (1.1)

Where u is the average gas leakage velocity from the nozzle, m/s; u is the average tangential velocity of the cold flow, m/s; ua is the average axial velocity of the cold flow, m/s; ∆t_cold is the temperature reduction due to throttling, ⁰С

Equation (1.1) expresses the dependence of the cooling effect on the properties of gases (pressure, temperature, proportion of cold flow) and the ratio of the sizes of the diaphragm and nozzle.

To calculate the vortex tube, the nozzle cross-sectional area FC is determined at a given gas flow rate and pressure. But it should be borne in mind that the gas velocity in the nozzle inlet section must be equal to the speed of sound. In this case, the most effective inlet is one with two nozzles at a right angle and a nozzle height to width ratio of 0.5:1 (Fig. 1.1).

Figure 1.1. Diffuser with four nozzles

Differentiating formula (1.1), we obtain an expression for calculating the optimal value of the aperture corresponding to the maximum value of ∆t_hol for given values ​​of P_кр, P₂ and χ:

Where Pcr is the pressure in the critical section of the nozzle; t1 is the flow temperature in the critical section of the nozzle;  is the deceleration temperature of the cold flow. The length of the hot flow pipe is determined relative to l_hot=20d, and the cold flow pipe relative to l_cold=5d.

Conclusion. The degree of purification of natural gas depends on the operating modes of the device. In the vortex tube, not only the condensation process occurs, but also the absorption of hydrocarbons with the help of a compressed air. For this reason, the degree of gas purification in the toilet is significantly higher than with conventional condensation. The concentration of hydrocarbon C5 in the purified gas is 2.5 ... 3, the deviation from its average content is also 2 ... 3 times. The concentration of hydrocarbons C₆-C₈ is determined based on chromatographic analysis and fluctuates from 0.2 ... 0.6 to 0.02 ... 0.03% at a temperature of -50 ... -60 ℃, up to 0.04% at a temperature of -42 ... -48 ℃ and up to 0.06% at a temperature of -35 ... 0 ℃.

References:

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