Improved design of two-stage filter cartridges for high sulfur natural gas purification
Introduction
High sulfur natural gas produced from natural gas wells is transmitted to a gas purification plant through a gas gathering system. Entrained contaminants including solid particles and oil droplets are moved along with the gas to the purification plant [1]. The most common contaminants include deposits of elemental sulfur, iron sulfides/oxides, waxes, water, chemical additives, silica and sand, etc. [2]. If the contaminants are not removed from the gas before reaching the desulfurization unit, the desulfurizer can become contaminated, which will result in amine solvent degradation and foaming that will affect the quality of purified gas [3]. Researchers have suggested a number of control measures to reduce amine solvent degradation and foaming, including improving the desulfurizers, adding defoamers, solution filtration and prevention of equipment corrosion, but removal of the contaminants from the feed gas is considered to be the primary measure to ameliorate [4], [5], [6], [7], [8]. In addition, the liquid contaminants can generate corrosive substances, and chloride in the liquid can cause stress corrosion of austenitic stainless steel [9], [10]. Therefore, a feed gas filtration unit has been established to remove the contaminants at the first step in the gas purification process, which plays a key role in the safe operation of the natural gas purification plant.
Natural gas is flammable and combustible, its pressure is generally more than 4 MPa, and can reach 12 MPa in practice. For this reason, natural gas filtration should be initially studied using experimental work and numerical simulations, which can be followed by safe field tests. Li et al. [11] compared the repeatability and reliability of light-scattering spectrometry and membrane filter sampling system to measure the mass concentration of outlet liquid droplets, and evaluated the gas-liquid separation performance of three types of cartridge filters under laboratory conditions. Innocentini et al. [12] tested the penetration characteristics of filter materials such as cellulose, polyester, polypropylene and stainless steel fiber, at the absolute pressures of 93 ∼ 693 kPa, and found that the pressure drop across these materials increased with the pressure of the gas. Azadi et al. [13], [14] measured particle concentration and particle diameter distribution using isokinetic sampling methods at various nodes in a natural gas gathering system and concluded that the performance of the filtering system required further improvement.
High sulfur natural gas contains significant quantities of hydrogen sulfide, fluctuating working conditions and a high liquid content. These conditions pose significant challenges to the conventional filtration system. Engel et al. [2] comprehensively analyzed the existing problems of filtration and separation systems in gas processing operations. These studies considered unsuitable technologies, inept compatibility, deficient vessel design, inappropriate sealing surfaces, poor media, lack of or inappropriate maintenance procedures and instrumentation deficiencies. The authors found that these factors directly produced high operation costs and/or an inability to attain sweetening specifications. Richard et al. [15] upgraded the amine filtration system in a gas plant, with amine-solution filters and an inlet gas liquid coalescing filter to achieve the projected performance of the formulated amine.
In the study reported herein, the problem of filtering contaminants from natural gas was analyzed in a gas purification plant. The structure of the two–stage filter cartridges was redesigned to improve the filtration performance. In addition, a matching filtration performance was conducted using a laboratory-scale evaluation system. The resulting filter cartridges were used in a field evaluation to determine the performance of the improved two-stage filtration unit.
Section snippets
Analysis of the problems in a feed gas filtration unit
The feed gas filtration unit is positioned at the front of a natural gas purification system and generally contains two-stage filters, which includes two pre-filters and a coalescing filter. The feed gas filtration unit process is shown in Fig. 1. The pre-filter is used to remove particles from the gas stream that have a diameter greater than 1 µm, and the coalescing filter removes liquid droplets with diameter of 0.3 µm and more. The designed gas flow rate of the filtration unit is 300 × 104 Nm3/d,
Improved filter cartridges design
Considering the ISO 12500 standard, it is more meaningful to compare the different coalescence filters at the steady coalescence state of gas-liquid filtration process, when the filter cartridge is saturated with liquid. The laboratory conditions used for this analysis included simple droplets where the upper limit of the droplet size is 5 ∼ 8 μm, and an inlet concentration of about 200 mg · m−3 to achieve a steady coalescence stage over a short period of time. This inlet concentration was much
Application of improved filter cartridges
A combination unit in a natural gas purification plant in southwest China was chosen as the site to conduct the field tests on the improved filter cartridges. This plant had two series units as a combination unit. The conventional filter cartridges were separately installed as the control group and the improved filter cartridges were installed as the trial group. Process data were recorded during the test, including the inlet gas pressure, the gas flow from the filtration unit, the pressure
Conclusions
The feed gas in a natural gas purification plant contains high concentrations of hydrogen sulfide, high liquid content and experiences fluctuating working conditions. Factors including operating conditions, filter performance and operation costs, must be comprehensively considered when choosing the type and materials of the filter cartridges used to clean up the gas stream. This reported work focused on the problems existing in natural gas filtration plants where the pressure drop of a
Acknowledgement
This work was supported by China National Natural Science Foundation (No. 51376196) and China National Science and Technology Major Project (No. 2016ZX05017).
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