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.
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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 petroleum courses 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. 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 petroleum courses 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. 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. 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. In lease production of natural gas, the marketing specifications as prescribed by the gas sales contract must be considered when selecting the system for processing wellhead gas for liquid recovery. Natural gas at the wellhead can contain liquefied hydrocarbons, free water. Technical Data used in the field explained in petroleum courses in Islamabad .Water vapor, acid gases, and other undesirable components. To make wellhead gas merchantable, these components must be reduced to a composition that will satisfy the marketing specifications. The first part of this chapter is devoted to the removal of liquefiable hydrocarbons from natural gas. The latter part of the chapter describes techniques for removal of some of the other components. The removal of liquefiable hydrocarbons, which are called “condensate,” is necessary for efficient transmission of natural gas in pipelines. If hydrocarbons condense in the pipeline, additional horsepower is required to overcome the increased pressure drop. Where the heating value of the natural gas is specified by the gas sales contract, there must be control of the condensate removal to satisfy this limitation. A final consideration for removal of condensate is that additional revenue is derived over that from sale of natural gas. In many instances this additional revenue will readily pay out the cost of process equipment required to produce gas of merchantable quality. Some more details of petroleum courses in Islamabad are as under. Before various methods for removing condensate from natural gas are discussed, some of the physical phenomena involved in the formation of condensate are examined. Retrograde condensation is a phenomenon that occurs at the high temperatures and pressures frequently encountered in condensate hydrocarbon reservoirs. In the retrograde condensation region, as shown in Fig. 14.1, condensate forms at constant temperature with a reduction in pressure or at constant pressure with an increase in temperature, both of which are contrary to the normal expectations for condensation. If fluid in the reservoir existed at Point A, condensation would occur as the pressure in the reservoir declined from A to B. Less frequently encountered is the case shown by Line C-D. Here the fluid is in the single-phase region at Point C. As the temperature is increased at constant pressure from C to D, condensation occurs. TSK Training for Skills and Knowledge is the best institute in Rawalpindi Islamabad for Pakistani Students who wants to join petroleum courses in Rawalpindi. Underground storage* is most advantageous when large volumes are to be stored. underground storage is especially advantageous for high-vapor-pressure products. Technical Data used in the field explained in petroleum courses in Islamabad. Solution-mined and conventionally mined caverns are not typically used for underground storage of refrigerated products. Underground storage allows most of the surface area (except for the entry wells) to be used for other purposes. This is especially beneficial in high-value, congested areas. Types of underground storage are caverns constructed in salt by solution Ming or conventional mining, caverns constructed in nonporous rock by conventional mining, caverns developed by conversion of depleted coal. limestone, or salt mines to storage, and depleted reservoirs. The solution-mined cavern is constructed by drilling a well or wells into the salt and circulating low-salinity water over the salt interval to dissolve the salt. a typical solution-mined cavern. Some more details of petroleum courses in islamabad are as under. Conventionally mined caverns can be constructed any place a nonporous rock is available at adequate depth to withstand product pressures. An engineer or geologist experienced in underground storage should evaluate any specific site for the feasibility of constructing underground storage. Most product caverns are constructed in shale, limestone, dolomite, or granite. This type cavern is operated “dry” (the product is recovered by pumping). The cavern may be operated by brine displacement of product or pump-out methods Most solution-mined caverns are operated by the brine displacement technique (Fig. 1 I .9). A suspended displacement string of casing is installed near the bottom of the cavern. and product is injected into the annulus between the product casing (casing cemented at cavern roof) and the displacement casing, forcing brine up the displace ment casing. The procedure is reversed for product recovery. In this type operation, a brine storage reservoir usually is provided. Some solution-mined caverns are operated “dry” by installing a pump at cavern depth either within the cavern or in a well connected to the cavern by fracturing. Both submersible electrically driven pumps and deep-well vertical multistage pumps are used for this purpose. 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. Vapor-recovery systems are of two basic types. One type connects a vacuum line to the tank and transports the tank vapors to a processing or gasoline plant. Technical Data used in the field explained in petroleum courses in Rawalpindi. The other type consists of a small compressor located by the tank battery. which compresses the tank vapors to a pressure suitable for lease use or sales. Vacuum-Line System. The vacuum-line system usually is found only in large oil fields, where many tank batteries can be connected together into a relatively short gathering system. This system must employ well-maintained and properly functioning pressure/vacuum relieving devices and dependable control valves to prevent the tanks from collapsing or air from entering the gathering system. Some more details of petroleum courses in rawalpindi are as under. Compression System. The compressor system is usually electrically driven and all components are skid mounted Some of these systems use a vane-type compressor and inject a refined oil by way of a lubrication system to seal the vanes against the compressor walls. In these systems an actual liquid recovery is accomplished by the sealing oil absorbing the condensed hydrocarbons from the compressed vapors and transporting them to storage with the returning sealing oil. Applications for this type of system are twofold: (I) compression of the rich stocktank vapors for sale to a gasoline plant and (2) the recovery of liquids from the rich stock-tank vapors. Liquid hydrocarbons also can be recovered from the compressed vapor-recovery unit (VRU) vapor by either one or both of the following means. A vapor cooling system (air-cooled or water-cooled) heat exchanger can be installed complete with a separator downstream of the hot compressed VRU vapor stream. Multiple vapor cooling systems may he used in between the stages of a multiple-staged compressron system. 2. A mechanical refrigeration unit may be installed downstream of the VRU for a higher-yield liquid hydrocarbon recovery. 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 Islamabad. Much can be done by the producer to prevent undue losses of crude oil by evaporation. Products should be introduced into storage as cool as possible and kept that way. Technical Data used in the field explained in petroleum courses in Rawalpindi. Some types of heat-exchange equipment should be employed between an emulsion treated, or other heating equipment, and the tanks to cool the oil before it enters storage. If fluid heat exchangers are used, a preventive maintenance program should be employed to guard against buildup of scale. paraffin. salt. etc.. which are common to many pro diced fluids. Most modern tank batteries are equipped with lease automatic custody-transfer (LACT) units. The run tank should be of sufficient size to allow approximately I2 hours’ settling time. Where batteries are equipped with storage for bad oil. this storage should be kept to a minimum and the battery treating capacity should be capable of treating a certain amount of bad oil. Some more details of petroleum courses in rawalpindi are as under. Steel tanks should be painted with a reflective or white paint. Tests show the vapor-space temperature of a tank painted with aluminum paint to average 4%“F above atmospheric temperature. while a red-painted tank averaged 14°F above atmospheric temperature. Tanks should be selected with smaller diameters. Greater heights. and smaller capacities, all other considerations being equal. These factors will allow the stored product to have relatively smaller surface areas and vapor spaces as well as a shorter length of storage time before being sold. Down comer pipes prevent undue agitation in the tank. They are usually made by installing a line inside the tank from the inlet connection in the tank deck to 1 ft above the tank bottom. The down comer must have a vacuum breaker hole at the top to allow gas to escape and thereby prevent agitation, splashing, and accumulation of static electricity. All tank openings should be maintained closed and pressure on the tank should be as high as practical (at least r/z in. of water column). Tanks in a battery all should be connected together into a common vent line to keep breathing and filling losses to a minimum. Bypass thief hatches are manufactured that will do much to prevent evaporation losses when a tank is gauged through the thief hatch. These special hatches have the tank-battery vent lines connected to them and will close off or isolate all other tanks except the one being gauged. This allows all other tanks in the battery to maintain their pressure while the tank being gauged is depressed The producer may install one of several types of ground level gauging and sampling devices available that will permit gauging and sampling without opening the tank. The tank remaining closed goes far toward eliminating evaporation losses. 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