To date, many diverse impurities are known. For example, it can be mechanical pollution. They are contained both in compressed air and in natural gas itself. After these impurities get into the installation, for example, in the gas pumping line, then all devices operating in this place are exposed to these particles. This leads to the fact that their wear is greatly increased.
Moisture as an impurity
Naturally, one must understand that mechanical impurities are not the only problem. A rather great danger is a substance such as ordinary liquid, water. It may sound strange, but it is during the extraction of natural gas that the problem with water is quite acute. This is expressed in the fact that the formation of water vapor is possible. Their occurrence, as well as density, depends on the location of the place of extraction. Although it is important to note here that pairs of this type are present in any case, their concentration may simply be different.
Why is moisture dangerous?
The reason is quite obvious - corrosion of metal pipes through which the gas moves. Basically, all devices, like pipelines, consist precisely of iron alloys. The presence of moisture inside such a pipe will cause hydrates to form. In addition, the formation of ordinary ice is not excluded. The presence of any of these impurities will lead to the following effect: a plug is formed inside, which will impede the transportation of natural gas. In addition, the emergence of hydrates disrupts the work of most regulatory bodies responsible for maintaining proper pressure.
From all this it follows that gas dehydration is a procedure for removing impurities such as moisture. This problem is most acute when transporting blue fuel. It is also worth adding here that most of the facilities for the extraction of volatile matter are located far from infrastructure in the Far North. For these reasons, all installations must comply with certain rules. The first and most important requirement is the ability to function at a fairly wide range of external temperatures, including the lowest. The second important requirement is the reliability of the plants, and the third is the availability of operation and maintenance.
Drying Methods
Gas dehydration is a method that eliminates the formation of hydrates or ice plugs. Currently, there are four main methods for conducting this operation.
- Cooling. In this case, cold is used, both natural and artificially created.
- The absorption method involves the use of liquid scavengers.
- The adsorption method is carried out using an already solid absorber.
- Another way is combined. For example, absorption and cooling may be used.
Gas cooling method
The first way to dry the gas is through cooling. The method is based on the fact that the humidity of the gas varies depending on the ambient temperature. For example, if you cool a gas that was previously warm, then part of the wet steam will condense. The condensate that will precipitate in this case is simple enough to remove, and the gas itself will have a lower dew point. In order to achieve the necessary drying of the gas in this way, it is necessary that it is cooled to a temperature that will be slightly lower than the minimum value in the pipeline.
As for natural cooling, it is possible during the winter period and if the pipeline was laid above the ground. Also, the air temperature should be lower than the indicators of the soil. There is another way that you can use. For this, the field must have reserves of natural and oil gas. In this case, it becomes possible to cool the oil gas with cold in the heat exchanger. Cold is taken in it after the throttle procedure of natural fuels. It is possible to carry out cooling by throttling oil gas.
There is another cooling method that is often used in various gas processing plants. The artificial method is carried out using ammonia or propane. For this, evaporation of these substances is carried out. So, it is worth noting that dehydration of natural gas in any case will partially remove substances such as heavy hydrocarbons.
Liquid scavengers
Absorption gas dehydration, or rather its essence, lies in the fact that some liquid substances are able to absorb moisture from the air. In order for a substance to be used as such an absorber, it must be suitable according to several requirements:
- the moisture capacity of the substance must be very high;
- the absorber must not be toxic;
- stability;
- the item must not have corrosive characteristics;
- the solubility of the absorber in a substance such as gas or liquid hydrocarbon should be minimal;
- absorber regeneration is simple.
The best substances that fit this description are DEG and TEG. They are deciphered as diethylene glycol and triethylene glycol. If we consider mainly only Russian places and methods of production, then it is most often used DEG. A chemical reagent such as calcium chloride is used as a substance.
Solid type absorbers
Adsorption gas dehydration is built on approximately the same principle as the previous method, however, instead of liquid matter, solid materials are used here. In this case, these objects are called adsorbents. They differ in that their area is quite large, and besides, all of it is covered with many capillary pores. Such materials can be successfully used raw materials such as activated alumina, bauxite, florite and several other substances.
It is very important to note here that if the gas does not preliminarily undergo purification from some other impurities, such as heavy hydrocarbons or hydrogen sulfide, the ability of adsorbents to absorb moisture will be significantly reduced. This suggests that the gas dehydration scheme cannot be used as an independent system. In order to regenerate solid scavengers, either dry hot gas or air must be used.
Design of Drying Units
Drying equipment consists of several prefabricated elements, they are equipped with all means of control, as well as control for the operation. All this equipment has the following features:
- the regeneration process is carried out in a completely closed circuit, moreover, without the consumption of natural fuel;
- The structure includes automatic logic circuits responsible for managing and controlling the entire system;
- the base frame for this device is separate;
- maintenance and use of such installations is as simple as possible;
- the flexibility of the stations and their reliability are quite high;
- during assembly, they are manufactured in such a way that the equipment can function 100% under very low temperatures.
Devices with liquid absorbers
If the absorption method was chosen as the method of gas drying, then in this case the absorber will be the main apparatus. This is understandable. The gas dehydration unit has other elements, however this one is the most important. The absorber is a bubble column used as mass transfer. There are also special plates that have round or conical caps. They will be responsible for ensuring that the necessary level of liquid is always maintained on the plates.
Using drying methods
As you can see, there are a number of different methods for drying gas, but it is worth adding that they are not always possible to use. When choosing a method, it is very important to take into account the area where mining is carried out and, of course, economic costs and benefits.
Thus, in laboratories, most often only one is used - the chemical method of drying. It is important to understand here that in fact there are so many substances that can act as absorbents or adsorbents.
Although the problem is that the use of chemicals is a one-time process, a similar fact means that its use is not completely cost-effective on an industrial scale. Given the rather large volumes of the gas industry in Russia, we can conclude that this will be very costly. All this prompted the industry to develop completely physical drying methods. In the end, it is worth adding only that the absorption method from the point of view of economic costs, as well as the complexity of the technical design, is much simpler and cheaper than adsorption, which led to the spread of liquid absorbers.