Vacuum Stripping and Gas Stripping
Transcription
Vacuum Stripping and Gas Stripping
PROCESS & COMPRESSION SYSTEMS Vacuum Stripping and Gas Stripping WATER Vacuum stripping and gas stripping towers for dependable water deaeration Total capability As part of the original Serck Baker waterflood technology portfolio, Cameron’s HiperVac and HiperStrip deaerators are the product for more than 30 years operating experience on over 100 installations worldwide. Supplied as either stand-alone units or as part of a full waterflood system, the vacuum stripping and gas stripping deaerators provide reliable and consistent water deaeration. Gas vacuum deaeration Vacuum deaeration is the most common deaeration technology. Packed towers are continuously evacuated by specially selected vacuum systems, reducing the oxygen partial pressure to create a driving force for mass transfer from the liquid to the gas phase. The design of the Gas Vacuum Deaerator is an economic balance between tower diameter, number of stages, packed bed heights and vacuum pump size. Computeraided design procedures help select the optimum configuration by taking into account salinity and temperature, air leakage under vacuum conditions, liquid distribution and 500,000 bpd vacuum stripping module mounted for offshore India the foaming tendency of the water. Materials of construction are selected to give excellent corrosion resistance (in a service where oxygen levels are high enough for corrosion to occur but too low to replace the passive film on 316 stainless steel). World Wide Experience Cameron Dearator systems have been applied to a variety of water sources with a wide range of feed water temperatures: Location Temperature Water Source Brunei 20º F River and produced water mixed Arabian Gulf 18º F Seawater Indonesia 18º F River delta USA 15º F Produced water Alaska 4º F Ice melt/seawater UK 4º F Estuary Data Sheet | TC9814-061 Tower internals In general, packed beds are preferred to tray towers because material selection is more reliable, tower fabrication and lining is easier, installation of internals is simpler and susceptibility to misalignment is lower. How it works Water enters at the top of the deaeration tower. It is distributed evenly across the vessel cross-section. The water trickles down over a bed of polypropylene mass transfer packing, where it is broken up into thin films, thereby forming a large interfacial area between the water and the surrounding vapor phase. A vacuum system extracts all gases from the vapor phase, thereby lowering the partial pressure oxygen in the vapor phase to near zero. This creates a driving force for oxygen molecules dissolved in the water to diffuse to the liquid surface and on into the vapor phase, thereby reducing the concentration of oxygen in the water. Two-stage efficiency The first stage of the tower performs bulk removal of oxygen at a relatively high pressure, whilst in the second stage, final removal is achieved at a lower absolute pressure (higher vacuum). For successful removal of oxygen, it is necessary to achieve the optimum balance between a number of variables including tower diameter, packing shape, size and quantity, vacuum capacity, internals efficiency, etc. Deaerator internals Top distribution system The top distributor may be selected as a header and lateral system, or as a tray distributor depending on the application. A header and drilled lateral system is typically used to evenly distribute water over the surface of the packing. If even distribution is not achieved, the mass transfer packing will not perform properly. The top distributor must work at all flow rates. Mass transfer packing The mass transfer packing must create a very large liquid/vapor contact area and must have a high number of drip points per unit volume, while allowing large flows of water to pass through. The optimum packing size for most applications is 2”, though this may vary. Second stage packing Two stages of mass transfer packing are typically used. At any given temperature, it is necessary to achieve a certain pressure to achieve the desired oxygen residual. This is typically 25 mm Hg absolute for warm water applications. Theoretically, it is possible to use a single stage tower and extract the entire vapor at this low pressure, but the necessity of using an ejector to achieve the low pressure required would mean having to pump large volumes of motive air. It is more efficient to extract the 40,000 bpd top distribution tray PROCESS & COMPRESSION SYSTEMS bulk of oxygen at higher pressure in the first stage without an ejector, leaving a relatively light duty for the lower pressure stage. Interstage seal This is the most important component in any vacuum deaeration system. The interstage seal is required to maintain a pressure differential between the stages of the tower. This seal is typically of the order of 25 mm Hg or 14 inches of water. Some manufacturers use crude orifice plate devices or downcomers, which while performing well at design conditions, may not hold up sufficient liquid level at low flows to maintain the necessary seal. Slugging and bypassing of vapor can result. Cameron’s system uses an upcomer/seal pot arrangement, which guarantees a minimum water hold up, and therefore pressure seal at all flow rates, even when there is no flow. Following the seal section, the water falls onto a corrugated redistribution plate designed to optimize distribution and turndown. 2,300,000 bpd 32 Horizontal Hiperfilters SFX for Prudhoe Bay, Alaska Vacuum system Sump section The vacuum system must extract two vapor After passing through the second stage packing, the water passes to a streams continuously. The first stage stream sump section to allow for level control, residence time, oxygen scavenger will be at about 40 to 50mm Hg abs while reaction time, extraction pump NPSH and injection pump rundown. the second stage will be 20 to 25 mm Hg abs (lower in cold water). There are two types of vacuum pumps available: Single-stage – 100 mm Hg abs minimum suction pressure Two-stage – 40 mm Hg abs minimum suction pressure If a single stage pump is used, it is necessary to raise both extract streams to at least 100 mm Hg abs. If a two-stage pump is used, it can be connected directly to the first stage and only the second stage extract need be raised in pressure. In both cases, atmospheric air ejectors are used to achieve the necessary vapor compression. Skid mounted vacuum pump package PROCESS & COMPRESSION SYSTEMS Gas stripping deaerators Where natural gas of suitable quantity and quality is available, gas stripping via the gas stripping deaerators may be an attractive alternative to vacuum deaeration. A stream of natural gas is introduced in counter-current flow to the water. The oxygen-free gas creates a driving force for mass transfer. Gas stripping systems may employ either packed bed or tray towers. The design of packed towers takes into account backmixing of the gas phase as a result of the high liquid to gas ratios employed for deaeration. 52,500 bpd HiperVac tower for West Africa FPS Cutaway of vacuum stripping deaeration tower 5 of 8 towers supplied for Arco’s 2 million bpd project in Prudhoe Bay, Alaska L O C AT I O N S United States of America 11210 Equity Dr., Suite 100 Houston, TX 77041 USA TEL +713.849.7500 United Kingdom Cameron House 61-73 Staines Road West Sunbury-on-Thames Middlesex, TW16 7AH England, UK TEL +44.1932.732000 O T H E R L O C AT I O N S Singapore 2 Gul Circle (Gate 2) Jurong, Singapore 629560 TEL +65.6861.3355 Abu Dhabi Japan Australia Malaysia Brazil Mexico Calgary Russia Colombia France w w w. c - a - m . c o m © 2011 Cameron | 12/11 TC9814-061 Saudi Arabia