CHEVRON BUCKEYE CO2 PLANTTREATING OF NATURAL GAS USING THE RYAN/HOLMES SEPARATION PROCESSSENIOR PROJECT REPORTBAAS 4393 – Senior ProjectBachelor of Science in Industrial TechnologyThe University of Texas at the Permian BasinApril 28, 2008Mark Garner
Presentation Outline Section I - INTRODUCTION Objective Background and History of the Buckeye CO2 Plant Process Purpose Section II – BUCKEYE CO2 PLANT PROCESS OVERVIEW Plant Inlet Dehydration Compression Refrigeration NGL Treating Section III – RYAN HOLMES PROCESS Process History Chevron Buckeye CO2 Plant Ryan Holmes Process Current Operating Conditions Other Ryan Holmes Process Types Chevron Buckeye Plant Single Tower Process Section IV – CONCLUSION Section V - APPENDIX Flow Schematics NGL Analysis PRC Inlet Analysis PRC Outlet Analysis Compressor Photo PRC Photo
Objectives The objective of my Senior Project is to: • Explain how the Ryan /Holmes Process is used at the Chevron Buckeye CO2 Gas Plant to treat high carbon dioxide (CO2) content natural gas streams. Why? • The use of this process in gas treating facilities is becoming more wide spread as oil producers increase the use of carbon dioxide injection to improve the oil recovery rates in mature producing fields. • The Ryan/Holmes process is one of the more complex methods used to separate light hydrocarbons from carbon dioxide. Because of all the forces at work in the process, it is hard for a new plant operator to really understand how the Ryan/Holmes process works. • Once this project is complete, I plan to use this report as a training tool for new process operators at the Chevron Buckeye CO2 Plant.
Background and History • The Buckeye Gas Plant is located near the center of the Chevron Vacuum Field Area. This field is approximately 25 miles northwest of Hobbs, New Mexico and has been in operation since the early 1950’s.The Buckeye CO2 Plant was built in 1998, with the plant coming online in late October of 1998. • The CO2 Plant was built in response to the injection of carbon dioxide into the Texaco Central Vacuum Field. Texaco selected CO2 injection as a way to revive the fields lagging production rate.
Why Inject Carbon Dioxide? • There are several reasons CO2 is used to help recover reservoir oil that was not recoverable before with other production methods. • CO2 acts as a solvent for the oil in the formation. When the carbon dioxide mixes with the oil and water in the formation it reduces the viscosity of the produced fluid. This allows the fluid to more freely flow to the producing wells well bore.
More Reasons To Inject CO2 • CO2 will wash the oil off the formation rock due to it acting as a solvent. • When CO2 and water are mixed it will form a mild carbonic acid. This acid created by the mixing of CO2 and water then helps to open up channels in the limestone formation rock. • By injecting CO2 into a formation the CO2 will increase the pressure in the formation. This increase in pressure will help to drive fluid to the lower pressure areas created by the producing wells.
Drawbacks TO CO2 Injection • CO2 is expensive to purchase • Once CO2 has been injected for a period of time, the gas associated with the produced fluid rapidly becomes contaminated with high levels of CO2 which makes it unfit to be processed in most of the gas plants located throughout the Permian Basin. • That is the reason for the Buckeye CO2 Plant
Process Purpose • The Ryan/Holmes process is used at the Chevron Buckeye Plant to recover hydrocarbons from the carbon dioxide rich gas stream that is produced with the oil production from the Vacuum Field. • An approximate mixture of 86% CO2, 12% light hydrocarbons and 2% nitrogen enters the plant, is compressed to boost the pressure and sent to the Ryan/Holmes process. • Through the use of the process, a natural gas liquid (NGL) mixture of 50% propane, 28% butane, 11% pentane, 9% hexanes plus and trace amounts of methane, ethane and carbon dioxide is stripped from the inlet gas and sold. • The vapor stream that exits the process is now composed of 91% carbon dioxide, 4% methane, 3% ethane with trace amounts of propane, butane and hydrogen sulfide. • This gas stream is then compressed to 1800 PSI and sent back to the field to be re-injected into the oil producing formation to start the cycle of injection, production, processing and compression over again.
Plant Process Overview • The location of the Ryan/Holmes process in the Buckeye CO2 Plant is in the Propane Recovery Column. • The 139 ft tall Propane Recovery Column (PRC) uses the Ryan/Holmes process to strip propane, butane, pentane and heavier hydrocarbon liquids from the high content CO2 gas coming to the plant. • Before any plant inlet gas can be sent to the PRC it must first be dehydrated and compressed. • An overview of these systems follows.
Major Plant Systems • Plant Inlet • Dehydration • Compression • Refrigeration • NGL Treating
Plant Inlet • The plant inlet gas comes from various Vacuum Field gas gathering systems and enters the Buckeye CO2 Plant at approximately 80 psig and 75 deg F. • The gas from each source is measured in separate orifice plate style meter runs. • After measurement, the gas is commingled in a single steel pipe that goes to the two-phase Inlet Separator. • Liquids caught in the Inlet Separator are pumped back to the field facilities via pipeline. • Inlet gas then flows from the top of the Inlet Separator vessel to the Inlet Filter Separator. Any liquids entrained in the gas are coalesced and removed from the stream by the coalescing elements in the separator.
Dehydration • After leaving the Inlet Filter Separator, the gas then flows to the Inlet Gas TEG (Triethylene) Contactor which utilizes TEG to remove free water from the gas and dehydrate the gas to a water dew point of -25 deg F. • Note that dehydration takes place upstream rather than downstream of the CO2/Recycle Compressors. • This is done to avoid the need for exotic materials of construction due to the fact wet gas containing CO2 and small amounts of hydrogen sulfide is very corrosive to unprotected carbon steel.
CO2/Recycle Compression • From the TEG Contactor the now dehydrated inlet gas goes to the 1st Stage Suction Scrubber of the plant CO2/Recycle Compressors. • The plant CO2 Process/Recycle compressors currently consist of four, 3000 horse power electric motor driven, compressors. • All four units are three stage compressors with the 1st stage compression discharge feeding the Propane Recovery Column (PRC) and the 2nd and 3rd stages moving the processed gas that leaves the PRC back to the field for re-injection. • All three stages are in series with the discharge of the previous stage feeding the suction of the next stage. • These compressors are all connected to a common pipe manifold system that allows the compressors to equally split the load between them. • The operating pressures for each stage are: 1st Stage - 90 psi suction to 310 psi discharge, 2nd Stage - 310 psi suction to 850 psi discharge 3rd Stage - 850 psi suction to 1850 psi discharge pressure to field re-injection.
Refrigeration • To produce the needed gas liquids used by the Ryan/Holmes process in the Propane Recovery Column, it is necessary to chill the gas stream after it leaves the top of the PRC as overhead gas. • To condense the liquids present in the vapor phase of the overhead gas stream, the gas is chilled to approximately -5 deg F in the PRC Reflux Condenser. • The condenser is a large gas exchanger vessel that utilizes refrigerant grade propane on the shell side to cool the gas, with the overhead gas on the tube side, to a point where the propane (C3+) and heavier hydrocarbons present in the overhead stream will condense and be recovered in the PRC Reflux Accumulator. • As the liquid builds up in the accumulator vessel, it is pumped back to the top of the PRC to be used as a reflux liquid.
Refrigeration Continued • The plant refrigeration system can be thought of as a giant refrigerator with propane being used like Freon and the compressors acting like the small compressor located behind your refrigerator. • Propane vapor is compressed and cooled to condense to a liquid. • This liquid propane is then allowed to flash back to a vapor in a specially designed heat exchanger (PRC Reflux Condenser). • When the refrigerant propane is flashed to a vapor it gets very cold and this is the effect used to chill the gas coming out of the PRC.
NGL Treating • The Natural Gas Liquid produced by the PRC is mostly free of any hydrogen sulfide and only needs some polishing to meet sales pipeline specifications. • This combined propane, butane, pentane and hexane stream is treated in the NGL Treater using Sulfa Clean material to remove any small amounts of remaining H2S. • The NGL’s are then passed through a filter and sent to the common carrier pipeline after passing through a custody transfer meter. • The produced NGL’s will eventually make their way to a fractionator plant located at Mont Bellevue, TX.
Ryan Holmes Process Description History • In 1979, Arthur S. Holmes and James M. Ryan jointly developed a way to separate a feed gas mixture containing methane and carbon dioxide. • This distillation column used the fact that CO2 under certain cold temperatures and elevated pressures, would go to a liquid state, while the methane and ethane in the natural gas mixture would remain in a vapor phase. • By applying this principle it was possible to design a distillation column that would produce a methane overhead product mostly free from CO2 and a CO2 bottoms product that had very little methane in it. • This process was different from existing processes due to the fact the distillation column was operated at pressures and temperatures that under normal conditions CO2 solids (dry ice) would form. • To prevent the formation of CO2 solids in the column a warm external additive liquid was pulled from the bottom of the column and introduced to the upper portions of the column.
More History – A Drawback to the Process • Using the original Ryan/Holmes process required overhead condenser temperatures of -125 deg F and pressures in excess of 515 psig. Because of the low operating temps and high pressures this required the column and associated equipment to be constructed out of stainless steel and other cold resistant metal alloys. • A couple of years later in 1981 a patent for an improved Ryan/Holmes process was filed by John V. O’Brien, with Koch Process Systems. • This patent took advantage of the unexpected discovery the distillation column could be operated at a temperature above the triple point (the point on the CO2 phase chart where CO2 exists as a solid, liquid and vapor at the same time) of CO2. • As long as the column is operated above the triple point formation of carbon dioxide solids does not occur. This effect was achieved by increasing the amount of additive fluid in the upper portion of the column. • Temperatures of -70 to -40 deg could now be used in the distillation column and the same recovery levels maintained as with the colder operating temperatures. • The overall effect of this allowed distillation columns to be made with regular carbon steel and greatly reduced the need for large refrigeration systems and extremely low dew point dehydration processes. This made the Ryan/Holmes process much more economically viable.
Buckeye CO2 Plant Ryan/Holmes Process Propane Recovery Column (PRC) • The Chevron Buckeye CO2 Plant uses a single tower, 67 tray distillation column Ryan/ Holmes process to strip propane and heavier hydrocarbons from the gas sent to the plant. • The dehydrated, hydrocarbon rich CO2 stream is compressed to approximately 310 psi by the 1st Stage of the plant CO2/Recycle process compressors. • The compressed gas is then cooled to the desired PRC inlet temperature of 125 deg F. The cooled gas then enters the PRC at an intermediate tray.
PRC Continued • The PRC column operates at approximately 0 degree F at the top and at approximately 340 deg F at the bottom. • The column works to separate carbon dioxide, methane, ethane, hydrogen sulfide, and nitrogen from the propane and heavier NGL’s. • The PRC is unique is because it uses two reflux streams, the flow of which must be ratio matched to the PRC inlet flow volume to minimize energy usage by the hot oil system used to heat the liquids that accumulate in the bottom of the column and propane refrigeration system used to chill the gas as it leaves the column. • The lower reflux stream, known as additive, is actually a slipstream of the PRC bottoms cooled in a side reboiler, and air cooler, and a column overhead interchanger. Its function is to ease the separation of propane and CO2. • However, it cannot accomplish 95% recovery of the propane on its own.
Propane Recovery Column • To achieve higher recovery propane recovery rates, the overhead vapor from the column is cooled in the PRC Reflux condenser to approximately 0° F. using a propane refrigerant system to produce a two-phase stream. • The two-phase stream is separated into a liquid and a vapor (PRC Residue) stream in the Reflux Accumulator. • The portion of the PRC overhead stream that does not condense in the PRC Reflux Accumulator then flows through the Refrigerator Sub-Cooler, Additive Sub-Cooler, then to the CO2/Recycle Compressor 2nd Stage Suction Scrubber. • The liquid from the Reflux Accumulator is returned to the top tray of the column by the reflux pumps to counter flow against the rising gas stream. • This reflux fluid is used to help strip out propane and heavier hydrocarbons that may still be present in the overhead vapor stream.
PRC Continued • The PRC Reboiler on the bottom of the column maintains the liquid in the bottom of the PRC at approximately 340° F. • The heat for this is provided by exchanging hot oil with the PRC Bottoms liquid. The hot oil is heated to 475 deg F. by the Plant Heaters and then circulated to the PRC Bottoms Reboiler. • The bottom product from the PRC is pumped by the PRC Additive Pump to the PRC Side Reboiler to provide heat to vaporize the liquid that is collected on the PRC intermediate tray. • After moving through the PRC Side Reboiler the additive stream passes through the PRC Bottoms Air Cooler and then on to the Additive Sub Cooler. • The cooled additive is sent to the top of the PRC to counter flow against the vapor moving to the top of the column. • The main purpose of the additive is to help warm the top of the tower to prevent the formation of CO2 solids in the cold area of the tower.
PRC Continued • The natural gas liquid (NGL) product recovered by the Ryan/Holmes Process comes from two separate draws from the PRC. • The main source stream is recovered from the middle of the PRC column, between PRC tray 54 and 55. This stream is withdrawn as a vapor at approximately 225 deg. F. at 310 psi. • This vapor stream consists mainly of propane and butane. It is cooled and condensed in an air cooler before it is sent to the NGL treating system. • The second source comes from the remainder of the PRC Bottoms not used as additive. This stream is composed of mostly pentanes and hexanes plus. • The amount of heavier hydrocarbon components added to the product stream is controlled by the PRC Bottoms level controller. As the level in the bottoms rises, more liquid from the PRC Bottom is added and as the level falls in the bottom, less liquid is added.
PRC Continued • Another important feature of the PRC that needs to be mentioned are the water draw points located near the mid point of the tower. • These water draws are employed to help remove any residual water in the tower. • Because of the temperature profile of the PRC, residual moisture in the column feed is trapped and can eventually build to a quantity that would interfere with the proper operation of the tower. • The residual moisture cannot leave with the overhead stream because it is too cold (-5 F) or the bottom stream because it is too hot (340 F). • To prevent this moisture from forming a hydrate (a mixture of ice and hydrocarbon) in the upper part of the tower, two vapor draws, are employed to bleed off any trapped moisture. • These vapor streams are recycled to the Plant Inlet Separator to drop out any liquid and then mixed back in to the inlet gas.
PRC Lab Analysis At present the PRC column is processing 35 million standard cubic feet per day of inlet gas. Above is a table of the Gas Analysis for the gas sent to the column, the vapor that leaves the column and the NGL’s recovered in the column.
PRC Continued • From the table note how the CO2 content in the PRC Inlet is different from the PRC Residue. • The CO2 content by mole percent has increased due to the removal of the C3+ from the gas stream. • The amount of C3+ in the residue stream should be lower. The process as it is currently configured should be recovering 95% of the propane in the gas stream with no more than 1000 ppm of CO2 in the recovered gas liquids. The tower was not running efficient at this time. • Any propane plus liquid not recovered, is product that could be recovered and sold at favorable market conditions. • The only consolation regarding this situation is this hydrocarbon is not really lost. It will be injected back to the formation with the rest of the re-injection stream and will be available to be recovered sometime in the future.
PRC Continued • The operation of the PRC is controlled and monitored by the plant Distributed Control System (DCS). • The PRC operating parameters have been input to the DCS computers and through the use of remote temperature, pressure and flow sensors the plant operators can monitor and make adjustments as needed to the process. • The tower is very sensitive to pressure and temperature changes. • Because of this it is critical to have all monitoring systems working properly and for plant operations personnel to be aware of changing conditions. • The following slides are the Operators Station screen shots for the PRC control system. • The operators screen gives the real time operating conditions of the PRC Tower. Temperatures, pressures, flow and H2S content is monitored on the screens. • The system is set up to alarm if certain variables fall outside the set normal operating parameters.
PRC Control Panel Screen Views • Screen 1 is the DCS operator’s station control screen for the upper portion of the PRC. • Note the flow to the tower (FIC-352) is at 35.9 million cubic feet per day rate, the temperature at tray 9 is 3.7 degrees F. and the reflux flow rate (FIC-419) is at 70.2 gallons per minute. • On the left side of the screen, point 3-10 represents the overhead vapor out of the tower, point 1-11 the reflux flow back to the tower, point 3-11 is additive back to the tower and point 3-15 is the NGL draw point.
PRC Control Panel Screen Views – Screen 2 • Screen 2 is the DCS operator station display screen for the bottom half of the PRC column. • On this screen the PRS Bottoms Reboiler (3-17) and Side Reboiler (3-16) temperatures are monitored. • Note that on the lower left of the screen the PRC Bottoms temperature is 330.7 deg. F. and the temperature at tray 60 is 270.4 deg. F. • On the lower right is the NGL production rate that is being sent to the pipeline. At the time of this screen shot the PRC was sending liquid to the pipeline at a rate of 21.3 barrels per hours.
Other Ryan/Holmes Processes • As stated before, the Ryan/Holmes Process at the Chevron Buckeye CO2 Plant is a single tower process. • The single tower design is unique in regards to Ryan/Holmes Processes. • The only other single tower system being used at this time is at the Williams Energy, Dry Trails Gas Plant located in Oklahoma. • Most other systems utilize a four, three or two tower process.
Three Column Ryan/Holmes Processes • With the multiple tower process the number of towers used is dictated by the composition of the feed gas and the desired product. • The three column system is used when inlet gas containing less than 50% CO2 is to be processed. • The first column separates methane from the gas stream. • The second column in the system produces a CO2 product overhead that also contains a small amount of H2S. • The third tower produces natural gas liquids and a NGL additive stream that is recycled back to the previous two columns. • It is this additive that makes the process work. • In the first tower the additive prevents CO2 Solids formation and in the second tower the additive works to break the carbon dioxide/ethane azeotrope that is created with high CO2/natural gas mixtures. • By breaking this azeotrope high ethane recovery rates are possible.
Four Column Ryan/Holmes Processes • For high CO2 concentrations above 50% a four column process is recommended. • This configuration uses the ethane recovery column as the initial separation step. • A mixture of CO2 and methane is then sent to a bulk CO2 removal column and demethanizer to produce a methane product. • The CO2 is produced as a liquid and is pumped to injection or sales.
Two Column Ryan/Holmes Process • Two column Ryan/Holmes processes have been designed for special situations. • These systems can be used when a methane product is not required and produced with the CO2 stream. • Another variation of the two column system relaxes the ethane recovery column operation allowing ethane and methane to be present in the CO2 product stream. • Do these conditions this sound familiar?
Chevron Buckeye Plant Single Tower Process • The Ryan/Holmes process at the Buckeye CO2 Plant uses a single tower with 67 internal trays. • For all intents and purposes the single tower at the Buckeye is nothing more than a two column Ryan/Holmes process system where the towers have been stacked one on top of the other. • With the single tower configuration the Additive Recovery Column (ARC) is placed at the bottom of the tower stack, below the Propane Recovery Column (PRC) section. • The main advantage to this set up is the entire ARC and its associated reflux condenser, drum and pumps, along with an additional reboiler, can be eliminated. • Also, since the reflux for the ARC is no longer required, the total heat requirements for the system are reduced.
Conclusion • In conclusion, I hope I have helped to promote some understanding on the Ryan/Holmes process used at the Chevron Buckeye CO2 Plant. • The process uses the principle that gases can be separated from a mixture by the application of temperature and pressure variations. • All gases have different conditions in which they exist as a solid, liquid or vapor. • By careful application of this scientific fact, a natural gas mixture of carbon dioxide, methane, ethane, propane, butane, pentane and hexane plus can be separated in to their individual pure states as needed, by manipulating pressure and temperature and this is what the Ryan/Holmes process does.