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Figure 3.4 shows a typical Mechanical Refrigeration System which uses Propane as the refrigerant.

Figure 3.4 : Mechincal Refrigeration NGL Recovery System

The boiling point of Propane is -42°C (-43.7°F). It is often used as a refrigerant in gas processing.

It is not used in domestic refrigerators because it is flammable. If leaked, it could cause a fire or explosion.

Within this system, there are actually two interconnected sub-systems. They are:

• the refrigeration system which reduces the temperature of the refrigerant

• the natural gas system where the reduction in temperature of the refrigerant is used to recover the NGL There are two points of contact between the two systems.

Activity

Take a close look at Figure 3.4. See if you can determine where these two points of contact are.

The first point of contact is the chiller. It is in this vessel that the low temperature created by the re-frigeration system is used to refrigerate the natural gas.

The second one is the refrigerant condenser where the heat generated during refrigerant com-pression is dissipated.

Let’s now work our way through the two systems starting with the refrigeration system. We can go through this rather quickly as it is very similar to the domestic refrigerator we looked at earlier.

Take a look at Figure 3.5 on the next page. It is the refrigeration system from Figure 3.4 which I have isolated from the rest of the drawing.

We will start at the same point as we did with the domestic refrigerator, that is, at the discharge of the compressor.

The refrigerant leaving the compressor is a high pressure, high temperature gas.

As it passes over the refrigerant condenser the refrigerant exchanges heat with cold gas coming from the gas/gas heat exchanger.

The refrigerant condenses to high pressure, relatively cool liquid as it enters the refrigerant accumulater,

Some of the liquid refrigerant leaves the

acumulator and flows across the expansion valve, where partial vapourisation takes place. This gas/

liquid mixture now flows into the chiller.

Warm natural gas is passing through the coils in the chiller, as we will see shortly. This is the gas which needs to be refrigerated in order to form NGL. The heat content of this gas is transferred to the refrigerant in the chiller. The net effect of this is to vapourise some of the liquid refrigerant, whilst reducing the temperature of the natural gas. The amount of natural gas flowing through the chiller will vary. Therefore, the amount of liquid refrigerant vapourised in the chiller will vary, affecting the liquid level. This level is maintained by a level controller operating the expansion valve.

The refrigerant leaves the chiller as a cold, low pressure gas and enters the compressor suction knockout drum.

This drum removes any entrained liquids which may be carried over by the refrigerant gas from the chiller.

The refrigerant gas leaves the compressor suction knockout drum and enters the compressor as a cold, low pressure gas.

After compression the refrigerant leaves the compressor as a hot, high pressure gas and the process starts again.

Figure 3.5 : Refrigeration System

Now let’s go through the actual gas liquids recovery part of the system. Figure 3.6 is once again the relevant part of Figure 3.4 which has been isolated from the rest of the drawing.

The natural gas enters the NGL recovery system after it has been dehydrated. This is to remove any chance of hydrate formation as the gas passes through the refrigeration process. (A separate unit in the Petroleum Processing Technology Series covers dehydration).

The incoming natural gas stream enters the gas / gas heat exchanger. It is called a gas / gas exchanger because both the medium to be cooled and the cooling medium are gas.

Figure 3.6 : NGL Recovery System

As the warm incoming natural gas enters the system it passes through the exchanger tubes and is cooled by the cold natural gas leaving the system through the exchanger shell. This pre-cools the incoming gas and pre-warms the outgoing gas.

The pre-cooled gas leaves the gas/gas exchanger and enters the refrigerant chiller. As it flows through the tubes in the chiller, it is refrigerated to a temperature of say -15°C (-59°F).

As the natural gas is refrigerated, the heavier hydrocarbons turn into natural gas liquids. This mixture of residual cold gas and liquids then enters the NGL/gas separator where the NGL is separated from the gas.

The cold gas leaves the separator from the top of the vessel and flows through the gas/gas heat exchanger where, as we have seen, ~ cools down the incoming warm gas.

Next the gas flows through the refrigerant condenser. Here it condenses the refrigerant and is itself warmed up further to a level where it can be put into a pipeline as a sales gas.

Note : Depending on operating pressures, this gas may require to be further compressed prior to being exported as sales gas.

The NGLs accumulate at the bottom of the NGLs gas separator, where a liquid level is maintained by a level controller.

From there they flow to an NGL flash separator.

In this vessel any small amounts of light

hydrocarbon gases which may still be entrained in the NGLs are separated. These gases are usually disposed of via a flare system.

From there, the NGLs are pumped via a level control valve to a transport system for sale.

The system we have just been looking at is a very common system which you could find on an offshore oil and gas production platform. However there are other systems. I mentioned one earlier where refrigeration is achieved using what is called an auto refrigeration system. We will look at one of these systems shortly. Before you move on to this however have a go at the following Test Yourself question on mechanical refrigeration.