Hydrocarbon gas drying

Excessive moisture if gas causes a number of serious problems during gas transportation in pipelines. In the gas treatment and transportation process, water vapor is being condensed and water condensate is being formed in the system due to the temperature lowering. Water condensate reacts with natural gas components thus leading to formation of hydrates. Hydrates settle in gas pipelines decrease their cross section and sometimes even lead to emergency situations. Besides, water in the system gives rise to equipment corrosion particularly if such acid components as Н2S and CO2 are presented in the feed gas. Therefore, natural and petroleum gases are subjected to drying prior to being fed into trunk pipelines and in the treatment process.

Selection of gas drying method depends on the feed composition, and primarily on the content of heavy hydrocarbons. Gases are divided into lean and rich ones under this criterion. A lean gas is a gas in which a content of hydrocarbons does not interfere with its pipeline transportation up to consumer.

Absorption and adsorption processes are used for lean gas drying. In condensate is occurring in the gas, the processing of gas is carried out using low temperature processes. Steam condensation occurs at gas cooling stage due to reduction of gas equilibrium moisture capacity.

Countercurrent absorption is mainly used for drying of lean gas, acid gas, gas downstream gas cleaning units from acid components using water solutions of different chemical agents, in gas conditioning for low temperature processing, etc.

Direct-flow absorption is mainly used at oil fields. Gas drying using this method is generally performed in horizontal scrubbers. Drying units manufactured in the former German Democratic Republic are used in the field of Russia (in Tymenskaya oblast, Tomskneft, Bashneft, Dagneft, etc.). Such units capacity is low ranging from 0.5 to 2.5 mln m3/day. Some characteristics and application of direct-flow processes of gas drying are mentioned herein.  ^[9]

Adsorption processes are applied both for lean gas conditioning for transportation, and for deep gas drying, i.e. prior to gas supply for low temperature gas treatment, for instance in helium recovery units. Also, these processes are applied for drying of liquefied gas used as engine fuel or cooling agent.

Designing of gas drying units includes: determination of required water dew point of gas, selection of concentration for initial and spent dehumidifier drier, justification of equipment selection for drying and regeneration units, etc.

1. Gas drying by glycols

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General requirements for natural gas driers:

a) high absorbing ability within wide range of concentrations, pressures and temperatures;

b) low pressure of saturated vapors ensuring inconsiderable losses connected with their evaporation;

c) boiling point different from water baling point to the extent enabling simple separation methods of absorbed water from the drier;

d) density shall be different from the same of hydrocarbon condensate to ensure sharp separation by simple methods;

e) low viscosity in operating conditions thus enabling good contact with gas in absorber, heat exchangers and other heat exchanging equipment;

f) high selective ability of gas components i.e. low intersolubility with them;

g) neutral properties, i.e. not to react with inhibitors used in gas production process;

h) low corrosion activity;

i) low foaming ability in conditions of contact with gas mixture;

j) high oxidation resistance and thermal decomposition resistance ability.

Application of two-component drier when a mixture is prepared directly at gas processing plant requires additional reservoirs for its storage and pumping. If two-component drier is applied because of necessity to change quality parameters (pour point, viscosity, etc.) then the second component shall meet the same requirements as all driers. It is advisable the difference between boiling point of absorbent components and water to be as big as possible,

At complex gas treatment plants, a portion of drier penetrates water basins and soil therefore it shall be nonpoisonous and completely biodegradable. Besides, the driers shall be cheap and non-toxic.

Glycols - ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), propylene glycol (PEG), glycol mixtures with their ethers, etc. meet these requirements to one extent or another.

Actually, highly concentrated DEG and TEG solutions are applied in gas absorption drying units as driers.

Water solutions of other glycols, particularly ethylene glycol and propylene glycol are used as hydrate inhibitors.

Glycols are diatomic fatty alcohols and mix with water in all ratios. Their water solutions do not induce equipment corrosion. This factor by contrast with other absorbents provides additional advantage, since the equipment is possible to be manufactured from low-priced steel.

2. Gas drying using solid sorbents

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The basic sorbents used for hydrocarbon gas drying are silica gels and molecular sieves.

2.1. Silica gels

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The advantages of silicagels: low regeneration temperature, i.e. low power consumption as compared with other mineral sorbents (aluminium oxide, zeolites), and relatively low costs.

KSM finely porous silicagel is very efficiently used for gas drying in industrial units. It has the largest adsorption capacity by contrast to another silicagel grades, provides lower drying degree, has higher mechanical resistance against friction and crushing. However, it quickly granulates in the presence of condensed moisture in the gas. For this reason a layer of finely porous silicagel is usually protected by a layer of adsorbent which is inert to condensate moisture.

When selecting a sorbent and its regeneration mode, the presence in gas of hydrocarbons that are heavier than butane shall be taken into account. Heavy hydrocarbons С₅ and higher are retained by silicagel and removed incompletely during regeneration. It must be understood that heating of silicagel to the temperatures over 220°С results in destructive changes of silicagel surface thus lowering its adsorption capacity. Heating above 250°С leads to drop of silicagel activity.

In the initial loading period silicagel has high activity about 15-20% max., which lowers down to 7% max during operation,

2.2. Zeolites

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Synthetic zeolites are the most expensive adsorbents, but their application in the drying unit considerably lowers operating costs. Zeolites provide very low dew point at high adsorption ability and they are stable if contacting with condensed moisture.

Unique structure of synthetic zeolites together with gas drying enables to remove heavy hydrocarbons from gas. Zeolites are more stable at low temperatures compared to silicagel. Experience on adsorbents use in conditions of north fields as well as laboratory tests have shown that silicagel is being cracked at multiple low temperature exposure: watered silicagel is destroyed to 15-20%, and regenerated one – to 5-7%; as for zeolite, it is not visually changed and do not reduce its service properties.

Actually, schemes comprising 3 and 2 gas drying adsorbers are capable to be implemented depending on specific quantity of recovered components, gas drying depth, characteristics of the used equipment and adsorbent properties.