Mechanical Refrigeration Systems

Because mechanical refrigeration systems frequently chill to as low as O”F, they involve essentially the same problems encountered in the low-temperature systems where the temperature is obtained by pressure expansion. Technical Data used in the field explained in  petroleum courses in Islamabad. The systems really are quite similar. The only essential difference is that the choke in the low-temperature separator systems is replaced by a chiller in the mechanical refrigeration systems.

Although glycol-injection systems are used extensively, there are many installations where inlet dehydration is used to lower the water dewpoint of the gas below the operating temperature in the chiller. There is merit to this system since the glycol does not come in contact with the condensate and glycol losses are, therefore, much smaller. Some of the other operating problems such as separation of glycol and condensate also are eliminated. The dehydrator, if it exists in the flowsheet, would ordinarily be placed between the inlet liquid separator and the chiller. The dehydrator could either be a liquid or granular desiccant type. In a refrigeration process, low temperature is achieved by cooling the gas using a refrigerant at low pressure. Some more details of petroleum courses in Islamabad are as under.

The heat in the incoming stream vaporizes the refrigerant at low pressure. The refrigerant must then have its pressure increased so that it can be liquefied. The pressure may be raised in two ways-compression or by absorbing the refrigerant in a liquid, which is then pumped to high pressure with the refrigerant being stripped and subsequently condensed. Compression refrigeration systems are more common, but either should work satisfactorily. TSK Training for Skills and Knowledge is the best institute in Rawalpindi Islamabad for Pakistani Students who wants to join petroleum courses in Rawalpindi.

Turbine Expansion Systems

The turbine expansion low-temperature liquid recovery system differs from the choke or valve expansion in that the turbine turns a shaft from which work is extracted. Technical Data used in the field explained in petroleumcourses in Islamabad.  A typical turbo-expander process is The gas enters through an inlet separator with any liquid separated at this point being introduced to a low point in the stabilizer tower. The gas then goes through heat exchange with the cold gas leaving the stabilizer. Another separator is installed if sufficient liquid is formed in the gas-to-gas exchanger with the liquid being introduced at an intermediate point in the stabilizer. The cold gas then flows to the expander where the pressure is reduced and low temperature achieved. The gas and liquid mixture  leaves the expander and flows to the separator that normally is on top of the stabilizer column. Sales gas flows back through the exchanger and may be compressed in the direct-connected centrifugal compressor before being put into the sales gas line. Since extremely low temperatures are reached in a typical turbo-expander plant, dehydration normally is a first step though some plants do use alcohol injection. Some more details of petroleum courses in Islamabad are as under.

The gas frequently is expanded below sales gas pressure and then recompressed to make use of the work that must be extracted from the shaft of the turbine. The stabilizer is either a demethanizer or deethanizer with the mixed hydrocarbon product being sold.

A fairly recent development in gas processing, the turbo-expander process is one of great simplicity and ease of operation. The favorable operating characteristics allow the plant to run unattended through long periods and its simplicity and relatively low investment cost make it an attractive option. TSK Training for Skills and Knowledge is the best institute in Rawalpindi Islamabad for Pakistani Students who wants to join oil field after petroleum courses in Rawalpindi.

Low-Temperature Separation Without Hydrate Inhibitor.

The basic unit for low-temperature separation without hydrate inhibitor includes essentially a choke, separator. and heat-exchange coils. Assuming that the inlet wellstream contains a minimum amount of free water and is of sufficient temperature to prevent formation of hydrates upstream of the choke, the operation is as follows. Technical Data used in the field explained in petroleum courses in Islamabad.

The wellstream enters the unit through the heat-exchange coil. where it is cooled through heat exchange with the liquid external to the coil. The well stream then passes through an adjustable choke used to control the flow rate through the system and establish a means for introducing the necessary pressure drop. The turbulence and temperature drop created by the expansion across the choke cause the formation of hydrates and the condensation of the liquefiable hydrocarbons. The hydrates and condensate are separated from the gas by means of centrifugal force, normally generated by locating the choke tangential to the shell of the separator, and by gravity. Some more details of petroleum courses in Islamabad are as under

The hydrates and condensate collect in the bottom of the separator where they absorb heat from the inlet coil, causing the hydrates to be melted. The liquid level is maintained by a level controller such that the coils are always submerged in the liquid.

Two possible operating problems might occur in this simple system. Either the wellstream could be near the hydrate temperature on entering the coil and further cooling would create hydrates upstream of the choke, or there is an insufficient amount of the liquid bottoms causing hydrates to build up inside the separator. In either case the system will malfunction. To use low-temperature separation successfully, the pressure of the gas upstream of the choke must be approximately twice the pressure in the low-temperature separator. Certainly, the higher the pressure upstream of the choke the lower the temperature that can be achieved in the low-temperature separator.

The common solution to this problem is to install an indirect heater upstream of the low-temperature separator. The indirect heater temperature would be maintained at a level to ensure wellstream gas temperatures above the hydrate temperature. Heat transfer is accomplished by flowing the wellstream through the coils in the indirect heater. The gas temperature is controlled by a thermostat located in the outlet end of the coil. A second thermostat can be located in the liquid section of the low-temperature separator to override the heater controls in the event the liquid temperature is too low. TSK Training for Skills and Knowledge is the best institute in Rawalpindi Islamabad for Pakistani Students who wants to join  petroleum courses in Rawalpindi.

Low-Temperature Separation Systems (Hydrate Formation)

A third phenomenon that must be considered is the possible formation of hydrates when water is present in the natural gas stream. Technical Data used in the field explained in petroleum courses in Islamabad.  Hydrates are materials that have fixed chemical compositions but exist without chemical bonds and are called “clathrates.” They form a solid similar to snow at temperatures above 32°F (the freezing point of water) when the gas is under pressure.

They appear to be hydrates of a mixture of the component gases and not a mixture of the hydrates of the individual gases. The hydrates form at a temperature that is characteristic of a given gas mixture rather than at the hydrate temperature for the individual components in the mixture. The hydrates normally include several water molecules for each hydrocarbon molecule so that the presence of liquid water is generally considered necessary for the formation of hydrates in sufficient quantity to cause plugging of a line, valve, etc. Turbulence accelerates the formation of hydrates and for this reason they frequently occur downstream from valves, regulators, chokes, orifice plates, sharp bends, etc. Fig. 14.4 can be used to estimate hydrate-forming conditions for different natural  gases. Caution also must be used in Fig. 14.4 because, as shown by the different hydrate-forming lines for O.&gravity gases, there can be considerable difference in the hydrate temperature of gases of the same gravity. Some more details of petroleum courses in Islamabad are as under.

If the composition of the gas is known, a composition dependent calculation of the hydrate temperature, either by hand3 or by computer, I will give a much better estimate  of the hydrate temperature than will  A necessary condition for hydrate formation is the presence of liquid water. Prediction of the temperature where free water will occur will help identify the first point at which hydrates might form. The chart  gives the water vapor content of sweet [no hydrogen sulfide (HlS) or CO21 natural gas as a function of temperature and pressure. As the temperature decreases at a given pressure the water content required for saturation also decreases. This will result in condensation of liquid water for a saturated gas stream as it is cooled. As an example, suppose a well is flowing 1 MMscf/D of natural gas at 1,000 psia saturated with water vapor but containing no liquid water at 110°F. The gas is cooled to 60°F because of ground and atmospheric cooling. At 1,000 psia and 1 lO”F, the gas contains 80 lbm water vapor/MMscf and at 60°F it contains only 18 IbmiMMscf. One day of gas production will result in the formation of 62 Ibm of free water because of the cooling. Referring if the gas flowing has a specific gravity greater than 0.6, hydrates are likely to form in the flow line at some point of turbulence. TSK Training for Skills and Knowledge is the best institute in Rawalpindi Islamabad for Pakistani Students who wants to join petroleum courses in Rawalpindi.

Low-Temperature Separation (LTS) Systems

The calculated dewpoint curve for the off-gas from a separator operating at 120°F and 1,000 psia. Technical Data used in the field explained in petroleum courses in Islamabad. The maximum dewpoint temperature is 136°F and  occurs at about 500 psia. Also are  constant mol% liquid lines. Over nominal line pressure drops very little liquid will form but that formed will be primarily the heaviest components in the gas stream.

A second phenomenon to consider in condensate removal from natural gas is the cooling that can occur when the pressure on the gas is decreased.

This temperature decrease can have one of two causes. When natural gas expands from a high pressure to a lower pressure without heat transfer or work being done (a constant  enthalpy expansion), there is an accompanying temperature drop or refrigeration effect normally referred as to the Joule-Thomson effect. If, however, the expansion occurs through a turbine then work is removed from the gas during the expansion and cooling occurs also. Advantage can be taken of the available pressure drop to lower the separation temperature of the hydrocarbon mixture and cause more liquid to form from the natural gas. Some more details of petroleum courses in Islamabad are as under.

Cooling from turbine expansion must be modeled along the lines of compression calculations and is not easily correlated. Cooling available from constant-enthalpy  expansion can be estimated by charts such. One must be cautious in using charts like because they are composition-dependent and cooling depends on  gas composition and amount of liquid formed as well as  the initial and final pressures. TSK Training for Skills and Knowledge is the best institute in Rawalpindi Islamabad for Pakistani Students who wants to join petroleum courses in Rawalpindi.