Teknologi Pemrosesan Gas (TKK 564) Instructor: Dr. Istadi ( staf/istadi )

Transcription

1 Teknologi Pemrosesan Gas (TKK 564) Instructor: Dr. Istadi ( staf/istadi ) istadi@undip.ac.id

2 Course Syllabus: (Part 1) 1. Definitions of Natural Gas, Gas Reservoir, Gas Drilling and Gas production (Pengertian gas alam, gas reservoir, gas drilling, dan produksi gas) 2. Overview of Gas Plant Processing (Overview Sistem Pemrosesan Gas) and Gas Field Operations and Inlet Receiving (Operasi Lapangan Gas dan Penerimaan Inlet) 3. Gas Treating: Chemical Treatments (Pengolahan Gas: secara kimia) and Sour Gas Treating (Pengolahan Gas Asam) 4. Gas Treating: Physical Treatments (Pengolahan Gas: secara fisika) 5. Gas Dehydration (Dehidrasi Gas) 6. Gas Dehydration (Dehidrasi Gas) 7. Hydrocarbons Recovery (Pengambilan Hidrokarbon)

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4 Retrogade Condensa7on! Phenomenon associated with the behavior of a hydrocarbon mixture in the critical region wherein, at constant temperature, the vapor phase in contact with the liquid may be condensed by a decrease in pressure; or at constant pressure, the vapor is condensed by an increase in temperature! Dew point control is also necessary if a potential for condensation is present in a process because of temperature or pressure drops! The latter happens when the gas is in the retrograde condensation region

5 Retrogade Condensa7on in P- T Diagram Pressure temperature diagram for a hypothetical raw natural gas that contains predominately methane (85 mol% methane with 4.8 mol% of C3+), with trace components up to heptane

6 ! At any temperature and pressure combination outside the envelope, the mixture is single phase.! At temperatures and pressures inside the envelope, two phases exist.! Three points on the envelope are important:! The cricondentherm (A), the maximum temperature at which two phases can exist! The cricondenbar (C), the maximum pressure at which two phases can exist! The critical point (B), the temperature and pressure where the liquid and vapor phases have the same concentration! The retrograde condensation effect can be seen by following the vertical dashed line

7 ! Dropping the pressure causes a liquid phase to form (retrograde condensation), which will be present until the pressure is below the envelope! The dashed curve inside the envelope denotes the pressure and temperature of the mixture when the vapor quality is 95 mol%! the cricondentherm of a mixture strongly depends on the molecular weight of the heavy components! The cricondenbar increases with increased molecular weight

8 HYDROCARBON RECOVERY PROCESSES! Inlet gas pressures make a major difference in plant configuration.! High pressures permit use of expansion, J-T or turboexpander, to provide all of the cooling if low ethane recovery is desired.! For low inlet pressures, either external refrigeration or inlet compression followed by expansion is needed to cool the gas, regardless of extent of ethane recovery.! Required outlet pressure helps decide which approach should be taken.

9 PROCESS FOR HYDROCARBONS RECOVERY! 1. Dew point control and fuel conditioning! 2. Low ethane recovery! 3. High ethane recovery

10 DEW POINT CONTROL AND FUEL CONDITIONING! Dew point control and fuel conditioning exist to knock out heavy hydrocarbons from the gas stream! There are three methods:! Low Temperature Separators (LTS)! Twister! Vortex Tube

11 LOW TEMPERATURE SEPARATOR(LTS)

12 LTS.! The LTS process consists of cooling and partial condensation of the gas stream, followed by a low temperature separator.! When inlet pressures are high enough to meet dischargepressure requirements to make pressure drop acceptable, cooling is obtained by expansion through a J-T valve or turboexpander. Otherwise, external cooling is required.! Water usually is present, and to prevent hydrate formation the separator downstream of the expander is warmed above the hydrate-formation temperature to prevent plugging.! An alternative to heating is injection of either ethylene glycol or methanol, which is then recovered and dried for reuse

13 ! If inlet pressures are too low for expansion, the stream is cooled by propane refrigeration.! The advantage of direct refrigeration is that the pressure drop is kept at a minimum! Hydrate formation must be considered with either feed dehydration upstream of the unit or inhibitor injection! Glycol injection is usually the more cost effective, but if used, it increases the required refrigeration duty

14 TWISTER! It is used in one offshore facility for dew point control and dehydration.

15 ! Gas enters and expands through a nozzle at sonic velocity, which drops both the temperature and pressure and causes droplet nucleation.! The two-phase mixture then contacts a wing that creates a swirl and forces separation of the phases by centrifugal force.! The gas and liquid are separated in the diffuser; the liquid is collected at the walls and dry gas exits in the center

16 The Twister SWIRL valve improves the separa7on of two- phase flow across a pressure reduc7on valve, such as a choke valve, Joule Thomson (JT) valve or control valve.

17 Advantages of the Twister system! Simplicity. No moving parts and no utilities required.! Small size and low weight. A 1-inch (24-mm) throat diameter, 6 feet (2 m) long tube can process 35 MMscfd (1 MMSm3/d) at 1,450 psia (100 bar).! Driven by pressure ratio, not absolute pressure.! Relatively low overall pressure drop. System recovers 65 to 80% of original pressure.! High isentropic efficiency. Efficiency is around 90% compared with 75 to 85% for turboexpanders.

18 Drawbacks of The Twister System! Requires a clean feed. Solids erode the tubing and wing, necessitating an inlet filter separator.! Limited turndown capacity. Flow variability is limited to ±10% of designed flow. This limitation is mitigated by use of multiple tubes in parallel.

19 VORTEX TUBE! The vortex tube, also known as the Ranque-Hilsch vortex tube, is a mechanical device that separates a compressed gas into hot and cold streams. It has no moving parts! Vortex tubes use pressure drop to cool the gas phase but generate both a cold and warm gas stream.! If streams are recombined, the overall effect is comparable to a J-T expansion.! The principle of operation is the Ranque-Hilsch tube, developed in the 1940s and commonly marketed as a means to provide cold air from a compressed air stream.

20 VORTEX TUBE PRINCIPLES! Pressurized gas is injected tangentially into a swirl chamber and accelerates to a high rate of rotation.! Due to the conical nozzle at the end of the tube, only the outer shell of the compressed gas is allowed to escape at that end.! The remainder of the gas is forced to return in an inner vortex of reduced diameter within the outer vortex.

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22 ! For dew point control, and dehydration, the device has the vortex tube and a liquid receiver connected to the tube.! Gas enters the tube tangentially through several nozzles at one end of the tube, expands, and travels spirally at near sonic velocities to the other end. As it travels down the tube, warm and cool gas separate.! The cool gas goes into the center of the tube. Warm gas vents in a radial direction at the end, but the cool gas is reflected back up the tube and exits just beyond the inlet nozzles.! Condensation occurs in the cool gas, and the liquid is moved to the walls by centrifugal force, where it collects and drains into the receiver below.! The working pressure of the tube is 500 to 3,050 psig (36 to 210 barg), and flow rates are 20 to 140 MNm3/h.! The turndown ratio is 15% for a single tube but can be increased by use of multiple tubes in parallel; the optimum pressure drop is 25 to 35%! Turndown ratio: the ratio of the maximum flow to the minimum flow of a meter

23 Advantages of Vortex Tubes! Simplicity and light weight.! It could be useful where limited turndown is acceptable.! It will be of most value when no compression is required.

24 MEMBRANE METHOD FOR FUEL CONDITIONER! Gas enters the membrane on the discharge side of the compressor, and the residual gas provides fuel to the compressor engine or turbine.! The low pressure permeate is recycled to the suction for recompression to recover the permeate

25 ADVANTAGES OF MEMBRANE TECHNOLOGY! the process is simple and requires no moving parts.! relatively small and light weight.! this technology is used on several offshore installations.! Unlike the Twister and vortex tube, membranes have the advantage of a turndown ratio down to 50%, with no performance penalty.! This property may not be an advantage for fuel gas conditioning, where flow rates should be stable.

26 ! In fuel conditioning, the selectivity is not a major issue because of the relatively small fraction of gas that needs to be recompressed, and the enriched stream is recycled without requiring additional compression.! Membrane permeability is the product of the solubility and diffusion coefficient.! For separation of light gases, the primary mode of selectivity is the diffusion coefficient.! For dew pointing, solubility drives the selectivity.! These membranes are silicone rubber compounds that preferentially absorb the heavy components

27 2. LOW ETHANE RECOVERY! The focus of the previous section was removal of heavy components (C3+) to avoid condensation or to lower the heating value! the objective is to produce a lean gas and recover up to approximately 60% of the ethane in the feed gas! Two process schemes are used to obtain this level of ethane recovery:! 1. Cooling by expansion or external refrigeration! 2. Lean-oil absorption

28 Cooling by Expansion or External Refrigera7on! At constant pressure and temperature, the ethane concentration in the liquid decreases with increasing C3+ fraction, which lowers the ethane concentration in the vapor and, thus, increases the percent ethane recovered.

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30 ! Inlet gas is initially cooled with cold residue gas and cold liquid from the cold separator before going to the propane chiller and to the cold separator.! Vapor from the separator is the sales gas, and the liquid goes to a fractionator to strip out light ends and recover liquid product.! The column operates at a lower pressure than does the cold separator.! Because of system pressure drop and because the fractionator runs at the lower pressure, the recycle stream must be recompressed.! Alternatives to the process include:! Reduction or elimination of the recycle by adding reflux to the fractionator! Running the fractionator at a higher pressure and use of a pump to feed the column from the cold separator

31 ! With high inlet gas pressures, replacing the propane system with an expander is an attractive option.! However, inlet compression may be necessary to obtain the temperatures required to obtain the desired recoveries.! Both J-T and turboexpanders are used. Crum (1981) points out situations where a J-T system may be preferable to turboexpanders, although recent advances in turboexpander technology may temper some of them:! Low gas rates. J-T is more economically viable at low gas rates.! Low ethane recovery. For ethane recoveries of 10 to 30%, J-T expansion may be sufficient.! Variable flow rates. J-T is insensitive to widely varying flow rates, whereas turboexpanders lose efficiency when operating off of design rates.

32 Lean Oil Absorp7ons! Early gas processing plants used lean oil absorbers to strip NGL from natural gas (Cannon, 1993), and the process is still used in about 70 gas plants today.! The process involves three steps:! Absorption. An absorber contacts a lean oil to absorb C2+ plus from raw natural gas.! Stabilization. The rich oil demethanizer (ROD) strips methane and lighter components from the rich oil.! Separation. The still separates the recovered NGL components as product from the rich oil, and the lean oil then returns to the absorber.

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34 3. HIGH ETHANE RECOVERY! The above processes provided limited recovery of ethane.! To obtain 80 to 90% or more ethane recovery requires separation temperatures well below what is obtainable by use of propane refrigeration alone.! In principle, direct-refrigeration processes could be used by cascading propane cooling with ethane or ethylene refrigeration or by use of a mixed refrigerant that contains methane, ethane, and propane.! The primary motivation for use of only direct refrigeration would be low inlet gas pressures.! If significant inlet compression is required to produce refrigeration by expansion, then cascade or mixed-refrigeration cooling, with or without expansion, may be attractive.! No matter which option is used, obtaining high ethane recoveries from low inlet-pressure feed streams requires substantial compression, of either the feed stream, the refrigerants, or both.

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37 Tugas Kuliah (kumpulkan paling lambat 18 November 2009)! JOULE- THOMSON (J- T) Expansion:! Jelaskan konfigurasi J- T Expansion! Jelaskan Prinsip Dasar J- T Expansion! Jelaskan fenomena J- T Expansion di dalam P- H atau P- T Diagram! Jelaskan Aplikasi- aplikasi J- T Expansion terutama untuk pengambilan hidrokarbon di dalam gas! TURBO Expansion:! Jelaskan konfigurasi Turbo Expansion! Jelaskan Prinsip Dasar Turbo Expansion! Jelaskan fenomena Turbo Expansion di dalam P- H atau P- T Diagram! Jelaskan Aplikasi- aplikasi Turbo Expansion terutama untuk pengambilan hidrokarbon di dalam gas! JAWABAN SETIAP MAHASISWA TIDAK BOLEH SAMA. PEKERJAAN HARUS DITULIS TANGAN. CARILAH LITERATUR DARI BERBAGAI SUMBER (harus ada DAFTAR PUSTAKA dan dirujuk)

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