Corresponding author: Andrei Krasnov (ufa-znanie@mail.ru)

Academic editor: Aleksandr I. Malov

Many Russian gas fields in the Arctic are now in the final development stage, so there is a need for additional gas compression along the gas collection system between the wells and the gas processing plant. After the compression stage, the gas is cooled in air cooling units (

Almost every major gas field in Russia is located in the Arctic, so the harsh climate makes development of such deposits a real challenge from the very start. A large portion of explored reserves and almost all the gas produced in the region belong to Cenomanian deposits characterised by low formation pressure and temperature values (

Different types of air cooling unit (

Consequently, ensuring hydrate-free operation is the key factor for proper performance of ACUs handling crude natural gas, so elaboration of measures to ensure such hydrate-free operation is a relevant applied research task.

Hydrate-free operation of pipelines can be assured in a number of ways, such as temperature increase or gas pressure reduction, injection of hydrate inhibitors or gas dehydration (

Another way to ensure hydrate-free operation of

The existing air cooling units for natural gas primarily employ automatic systems for variable frequency control of the

The temperature of the inner wall of the cooled tubes is the key criterion restricting hydrate-free operation of ACUs for crude natural gas and one of the major parameters that drives hydrate formation is the specific humidity of the gas, characterised by the dew point temperature (

The risk of hydrate formation cannot be detected instrumentally owing to absence of standard measuring instruments and a corresponding methodology. Moreover, this would be extremely hard to do from a technical perspective. One

Mathematical modelling of

Equilibrium conditions of hydrate formation for almost all known natural gases have already been empirically identified and explored. Formation of natural gas hydrates in tubes depends primarily on the gas composition, pressure, temperature and dew point. The hydrate-forming components of natural gas include methane, ethane, propane, i-butane and n-butane, carbon dioxide, hydrogen sulphide, nitrogen and oxygen. If the gas contains as much as several per cent of ethane, let alone propane and i- or n-butane, the conditions for hydrate formation change drastically. For example, adding 1% of propane to the gas at a temperature of 283.15 K results in a significant reduction in the hydrate formation pressure – from 6.99 MPa for pure methane to 4.36 MPa for the mixture, and adding 1% of isobutane – to 3.04 MPa for the mixture (

Current best practices offer numerous techniques for calculating equilibrium conditions for hydrate formation, since experimental methods are extremely laborious and require expensive equipment. In the absence of proper harmonisation, laboratory experiment results and findings of theoretical computations are poorly compatible. As a result, even given robust computational methods, engineering practices employ simple empirical equations to identify the conditions for hydrate formation in natural gases.

The most frequently used methods are approximate calculation ones, including the Curzon–Katz, Skhalyakho–Makogon and Ponomarev methods (

The conditions for hydrate formation in gases with different specific gravity are determined from the hydrate equilibrium curves. These curves divide the area of possible thermobaric states into two segments: the area where hydrates exist and that where they do not. The higher the specific density of the gas, the lower the hydrate formation pressure.

To identify the area of possible hydrate formation, the specific humidity and density of the transported gas, as well as its temperature and pressure, must be known (

The temperature at which gas hydrates remain in thermodynamic equilibrium (equilibrium hydrate formation temperature) is calculated as follows (

_{hydr}_{0}_{term}

_{hydr}_{1}_{term}

where _{term}_{0}_{1}

The terminal pressure value is calculated as follows:

_{term}^{−0.616}. (3)

The functions of reduced density of gas can be calculated as follows:

_{0}^{−0.225}, (4)

_{1}^{−0.246} (5)

Reduced density of gas

where _{i}_{i}

Knowing that _{term}

_{hydr}^{−0.246} − 25.397 ∙ ln(

The gas temperature corresponding to the dew point temperature (

Т_{DP}^{0.05032} ∙ ^{0.0564}, (8)

where ^{3}.

The mathematical model of the hydrate formation process was integrated into the general mathematical model of

Fig.

Fig. ^{3}.

A manual

Screenshots of modelling in

Dependence of the dew point (°С) on gas pressure and specific humidity

Credible information about the current gas temperature values at the

^{th}International Conference on Compressors and their Systems IOP. Publishing IOP Conf. Series: Materials Science and Engineering 2017.