Research Article |
Corresponding author: Andrey N. Krasnov ( ufa-znanie@mail.ru ) Academic editor: Aleksandr I. Malov
© 2018 Andrey N. Krasnov.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Krasnov AN (2018) About creation of the simulation model of the thermal mode in air cooling units for crude natural. Arctic Environmental Research 18(2): 71-75. https://doi.org/10.3897/issn2541-8416.2018.18.2.71
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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 (ACU). Cooling crude (wet) gas in low-temperature environments using ACUs involves a risk of hydrate plugs forming in the ACU’s heat transfer tubes. Variable frequency control of speed fans is typically used to control performance of the ACUs and the control criterion is the gas temperature at the ACU outlet. Even so, the chances of hydrate forming in the bottom of the tube bundle remain large owing to inhomogeneous distribution of the gas temperature in the tube bundles and the temperature jump between the inner surface of the tube wall and the gas flowing through that tube, despite the high gas temperature in the outlet header. To enable forecasting of possible hydrate formation, the mathematical model of the ACU’s thermal behaviour that forms the basis of control system’s operating procedure must ensure proper calculation not only of the gas temperature at ACU outlet but also the dew point at which condensate formation begins and the hydrate formation temperature. This article suggests a simulation model for crude gas ACU thermal behaviour that enables modelling of both the temperature pattern of the gas inside the tube and the areas of condensate and hydrate formation. The described thermal behaviour model may be used in ACU management systems.
hydrate formation, air cooling unit, ACU, equilibrium conditions, variable frequency control, specific humidity, dew point, simulation model
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 (ACU) are used to cool the gas. ACUs are environmentally friendly systems that help significantly reduce water intake by industrial facilities and do not require any pre-conditioning of the cooling agent, thereby dramatically cutting the gas cooling operating costs (
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 ACU handling wet gas is by altering the equipment design, i.e., building ACUs with longitudinally washed arrays of finned tubes, with cooling air circulated with the help of an air louver system (
The existing air cooling units for natural gas primarily employ automatic systems for variable frequency control of the ACU fan speed (
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 (DPT) (
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 ACU would need at least 45 temperature sensors distributed throughout the system in a sophisticated pattern (
Mathematical modelling of ACU thermal behaviour is a challenging task, since the thermal behaviour is closely linked to the ACU’s gas dynamic behaviour, which means this will be a nonlinear model. The issues of ACU modelling are described in multiple publications, such as (
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 (
Thydr = 2.322 – F0 + 8.028 ∙ ln(Р), at Р ≥ Рterm (1)
Thydr = 2.322 – F1 + 25.397 ∙ ln(Р), at Р ≥ Рterm (2)
where P – pressure of gas in the ACU tube, MPa; Рterm – terminal pressure value corresponding to a critical hydrate existence temperature of 273.15 K; F0 and F1 – functions of reduced density of gas.
The terminal pressure value is calculated as follows:
Рterm = 19.317 + 12.171 ∙ (Δ − 0.548)−0.616. (3)
The functions of reduced density of gas can be calculated as follows:
F0 = 9.207 ∙ (ρ- − 0.546)−0.225, (4)
F1 = 0.258 + 27.795 ∙ (ρ- − 0.544)−0.246 (5)
Reduced density of gas ρ- is calculated as follows:
where k – the number of hydrate-forming components of the gas mixture; ai – volume fraction of the i hydrate-forming component in the source gas; Δi – relative density of the i hydrate-forming component.
Knowing that P < Pterm in the ACU, we can combine formulae (2) and (5) to obtain the following equation:
Thydr = 2.58 + 27.795 ∙ (ρ- − 0.544)−0.246 − 25.397 ∙ ln(Р). (7)
The gas temperature corresponding to the dew point temperature (DPT) can be determined using the formula below (
ТDP = 282.84 ∙ Р0.05032 ∙ W0.0564, (8)
where W – specific humidity of the saturated gas, g/m3.
The mathematical model of the hydrate formation process was integrated into the general mathematical model of ACU thermal behaviour. A simulation was developed for a specific type of ACU – the 2AVG-75, which is most frequently used in gas field operations. The existing model was enhanced with additional blocks for real-time calculation of the DPT at which condensate formation begins and the hydrate formation temperature.
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Fig.
A manual DPT analyser Chandler was installed at the ACU outlet to verify the model’s suitability and the DPT value calculated using the model was compared with the analyser readings. The root-mean-square deviation of the calculated values from the real-life values did not exceed 1.5% (the representativeness of the sample consisted of 237 measurements).
Credible information about the current gas temperature values at the ACU outlet, the equilibrium temperature of hydrate formation and dew point temperature is crucial for ensuring effective service of ACUs and preventing local hydrate formation in such equipment. Furnishing an ACU with physical sensors takes a lot of effort and is uneconomical. For this reason, the ACU management system operating procedure involves a mathematical model that enables real-time calculation of said temperature values. An experimental test demonstrated suitability of the model.