Natural Gas Liquefaction Stage to Become LNG

Liquefaction is the method by which natural gas at normal temperature and pressure is cooled to very low temperatures in order to change its physical state from gas to liquid, thus facilitating transport over long distances safely. The liquefaction process has two main phases, the first is cooling and condensation and the second is what is called 'flashing', in the latter process nearly atmospheric pressure is low. The process 'flash' is an adiabatic process by which heat is not added or removed to the process but the boiling point of the liquid is low. This can be done by a Expander or simply by means of a valve which takes the name of the process J-T valve (Joule Thomson effect). In making this last stage in the preparation of LNG vapors called 'flash' occur. The amount of vapors created depends on the pressure of your LNG tank and which are sending the LNG.

Basic Principles of Refrigeration

The cooling is currently used not only in LNG liquefaction plants but also in other applications such as condensation of reflux in fractioning of light hydrocarbons, control hydrocarbon dew point, etc.

Several factors come into play regarding the selection of the refrigerant, and these factors are given by the temperature requirements necessary refrigerant availability, cost, and proven experience. we will mainly focus on the mechanical refrigeration (vapor compression and expansion), although other methods such as Refrigerations, less used in the hydrocarbon industry.

Natural Gas Refrigeration Cycle

A refrigeration cycle can be displayed in four distinct stages (evaporation, compression, condensation and expansion). Below are the four stages of cooling natural gas to obtain liquefied natural gas (LNG):

1. Evaporation

The liquid refrigerant absorbs heat as it evaporates, and this is where a cooling in the cooling cycle occurs. A refrigerant after passing gaseous state has no cooling capacity. The vapors leaving the evaporator saturated or slightly reheated, to go to the next stage.

2. Compression

In any compressor refrigeration cycle is the work element that adds to the process. The compressor pulls the vapors that are no longer cooling capacity and compresses. Control of this stage is critical because if the compressor removes the vapors more rapidly than the down pressure system formed. The vapors leaving the compressor at high pressure and superheated.

3. Condensation

In the condenser the refrigerant heat is removed, it is sometimes simply a heat exchanger (HE) by means of a fan. The air in the atmosphere is at a lower temperature than the superheated vapor leaving the compressor, thus helping to reduce its temperature. The vapors are saturated and pass liquid state.

4. Expansion

The liquid expands n here condenser coolant heat is removed, it is sometimes simply a heat exchanger by means of a fan. The air in the atmosphere is at a lower temperature than the superheated vapor leaving the compressor, thus helping to reduce its temperature. The vapors are saturated and pass liquid state.

Diagram Pressure and Enthalpy

The pressure and enthalpy diagram (right of the page) shows the relationship between pressure, temperature and enthalpy. To the right of the 'bell' we gases, to liquids and left point found within the 'bell' are points liquid-gas phase. There are currently several cycles liquefaction commercially available. Depending on the capacity of liquefaction you want to install you can choose from a variety of processes.

There are currently several cycles liquefaction commercially available. Depending on the capacity of liquefaction you want to install you can choose from a variety of processes.