CHAPTER-11 MAJOR EQUIPMENT & THEIR OPERATION

Transcription

1 CHAPTER-11 MAJOR EQUIPMENT & THEIR OPERATION 11.1 CAUSTIC PREWASH VESSEL It is vertical vessel designed in remove naphthenic acids H2S by adsorbing with 3 be caustic. It has two feed distributors at the bottom lower one facing down and upper facing top. In general lower distributors are used for proper distribution of feed in stagnant caustic level. Only during caustic change over upper distributor is used so that caustic does not carry kerosene with it REACTOR SECTION There are two vessels in this section reactor and caustic settler REACTOR The fixed bed reactor is normally a vertical designed to contain the charcoal bed upon which merox catalyst is impregnated internal equipment is limited to an upper inlet distributor lower side outlet collector and bottom drain screen. Flow is down ward through the bed. The inlet distributor is designed to provide a uniform lateral flow distribution over the reactor cross section. The distributor holes should be oriented upward. The holes in the inlet distributor should be protected from plugging during charcoal loading by wrapping the distributor with canvas of plastic covering. The outlet collector and vessel drainpipes are specially constructed stainless steel slotted screens of a proprietary design manufactured by Johnson Screen. These slots should be closely inspected before commissioning to ensure that any damage serving to open these slots is repaired. Any time charcoal is unloaded these screens should be cleaned with a wire brush since charcoal fines collecting here can often lead to screen plugging problems. All flanges should be checked for proper resistant gaskets and proper bolt tension. The most recent merox reactor designs specify that the bottom head of the reactor be filled with concrete. The concrete should be carefully screed for levelness between the support rings and center pipe. After the concrete has cured it is coated with epoxy resin for protection CAUSTIC SETTLER The caustic settler is a horizontal vessel designed to permit gravity separation of aqueous caustic solution from the hydrocarbon product. An inlet distributor is provided to avoid excess inlet turbulence and promote an even flow distribution through the vessel. CHAPTER-11 1

2 11.3 POST TREATMENT Many merox units will have some equipment after the merox unit proper for the removal of other contaminates (amine caustic corrosion inhibitors copper etc) and/or impurities (COS surfactants etc). The ultimate product destination use and specification will determine the type of post treatment equipment WATER WASH When a product is to be used, as aviation turbine fuel component product water washing is typically needed to remove soluble surfactants that might otherwise cause failure of water separation related product specifications. This water-washing step will also remove any entrained sodium soaps or caustic solution. Occasionally product water washing is used strictly to remove entrained caustic. The simplest type of water wash is a vertical vessel operated batch wise. Hydrocarbon enters the bottom of the vessel and jets through a distributor with holes oriented down ward. The distributor hole area and orientation is designed to provide adequate mixing and should be checked for conformance to specification. The number and size of the distributor holes will vary with the application. The vessel is normally operated half full of water and is replaced batch wise. These are also usually equipped with skim line near the interface to drain off any interfacial scum, which might accumulate. This scum is usually an emulsion that forms at the water/hydrocarbon interface and will lead to water carry over down stream unless it is drained. A full diameter stainless steel wire mesh blanket is provided in the upper section of the vessel below the hydrocarbon outlet to coalesce and remove most of the entrained water drop lets of emulsions. The mesh blanket should be adequately secured to its support with tie down wires. Periodic inspection should be conducted to evaluate mesh blanket integrity. The water wash is normally supplied with two parallel inlet distributors one near the bottom and the 2 nd several feet up. Normal operation utilizes the bottom distributor with holes oriented upward is used only when it is necessary to change out water. This upward hole orientation allows a calm water phase essentially free of undissolved hydrocarbon to be with drawn from the bottom of the vessel. Steam condensate or other high purity water is required and is added via a break tank and pump. Water change out is normally made when the total alkalinity exceeds approximately 0.2 WT% NaOH or a ph of about 12.5 A continuous water wash is sometimes provided when minimum operator attention is desired or when the frequency of batch change out is deemed impractical. These are equipped with a water distributor near the normal hydrocarbon water interface and an automatic drain operated by level control. These are also usually equipped with skim lines near the interface like the batch water washes and they are used for the same purpose. CHAPTER-11 2

3 SALT FILTER The salt filter is a vertical vessel containing a bed of granular rock salt NaCL. Its function is to remove free water entrained in a hydrocarbon product plus a small portion of the dissolved water. It is installed upstream of a product clay filter in order to protect the clay from premature failure due to free water attack of the crystalline clay structure. As the salt remove water aqueous salt brine is formed which gravitates to the bottom of the vessel for disposal by periodic draining. As aqueous salt brine is formed salt catalyst dissolve the solid salt bed settles and eventually must be replenished. This is normally accomplished by bypassing the vessel from time to time opening the top man way and topping off the salt bed with fresh rock salt. Consumption of salt depends on many factors but typically averages approximately 20 pounds per 1000 barrels of product treated (60 Kg per 1000 cubic meters) when operated at 100 F (38 C). The bed should be checked and replenished before it is 50% consumed. It is recommended to check the level at threemonth intervals until a consumption history established. The hydrocarbon inlet distributor is designed to allow even flow distribution (up flow) and is wrapped with stainless steel screen to exclude salt. The brine collector is a stainless steel screen pipe of a proprietary design CLAY FILTER The clay filter is a vertical vessel containing bed attapulgus or fullers earth type clay. Its function is to remove oil soluble surfactants metal compounds particulates corrosion inhibitors and other compounds, which adversely affect thermal stability or water separation, which are related to product specification. The attapulgus clay specified is 30 x 60-mesh particle size and sleeted for efficient long life performance. The clay filter is preceded by a salt filter to remove free water and some dissolved water. Attapulgus clay is hygroscopic that is free water will cause the crystalline clay particle to soften forming an amorphous mud like mass which quickly compacts and cause a severe pressure drop to develop. The clay filter is provided with a small vent line piped from the top head of the vessel to a location near grade with the discharge directed to an oily water sewer. This vent should be checked frequently for a separate air phase if present the air phase should be vented. A separate air phase, which penetrates the bed with result in channeling the premature clay replacement. Clay is replaced when product specification testing dictates it has lost its adsorptive capacity. Life can not be predicated and varies widely for different feed stock a typical life ranges from 10,000 to 50,000 bbl/ton (1600 to 8000 m 3 /ton). An inlet distributor (top) is provided to allow even flow distribution with holes oriented upward to prevent impingement on the bed of clay. The outlet collector pipe assembly (bottom) is a Johnson Screen proprietary design to retain the clay yet permit hydrocarbon CHAPTER-11 3

4 to pass unrestricted. These slots should be checked initially to verify the correct size and to see that damage has not created larger openings. During each clay bed change out clean the collector pipe assembly to avoid problems of slot blinding and again verify that damage has not occurred. During loading the inlet distributor should be covered to prevent clay from entering it. The sock loading procedure is recommended to minimize the possibility of channeling as well as excessive dust SPECIAL EQUIPMENT There are two special equipment for this unit AIR MIXER This is a specific piece of equipment common to all fixed bed sweetening units. It consists of a central mixing chamber in which hydrocarbon and air meet from opposing directions. The common exit for hydrocarbon containing dissolved air is perpendicular to the inlets. A sintered stainless steel filter cylinder is provided to disperse the air if the airlines contain any dirt this filter will become plugged. Care should be taken to follow all safety precautions which handling these reagents SPECIAL CHECK VALVE The purpose of this piece of equipment is to prevent a back flow of either caustic of hydrocarbon to the source of air whether it is a dedicated compressor or the plant air system. It is installed directly on the airline just up stream. It is installed directly on the airline just up steam of the injection point into the sour hydrocarbon stream. The features of this check valve include its low cracking requirement (2-4 psig kg/cm 2 ) and positive spring loaded closing action. Although no check valve is 100% fail safe these valves have proven themselves quite reliable over the years. The valve must be installed is a vertical position and it is recommended to use stainless steel tubing on its inlet side in order to prevent extended pipe threads either hindering the operation of or damaging the valve internals REACTOR BED HOT WATER WASHING AND CATALYST REIMPREGNATION Hot water washing the reactor bed is to remove the absorbed materials such as naphthenic acids, high molecular hydrocarbons, surfactants, and particulate matters, which when allowed to accumulate will block off the catalyst active sites. Catalyst reimpregnation is necessary when other means such as hot water washing, higher reaction temperature, stronger caustic saturation, lower kerosene feed rate, will no longer provide sufficient mercaptan oxidation. CHAPTER-11 4

5 PROCEDURE FOR HOT WATER WASHING 1. After the unit is shut down, isolate the reactors from the rest of the unit Start pumping out the kerosene. Keep the reactor under a positive pressure by steam of N2/fuel gas blanketing. This is to avoid pulling a vacuum on the reactor. 2. When all kerosene is pumped out, begin the LP steam flow from the water heater (15-JA-002, to the reactor. Keep the steam rate at about kg/hr/m3 of charcoal bed for at least one hour. Drain out the condensate from reactor bottom. This is to remove the residual kerosene and heavy hydrocarbon in the bed. Note: At no time should be bed temperature be allowed to exceed 195 deg. C. The activated charcoal will self ignite at this temperature. 3. Begin introducing demineralized water through the water heater into the reactor. Keep the water temperature close to boiling (90-95 deg. C ) by inducting steam into the water heater. When the reactor is full, open reactor bottom to drain. Keep the draining rate close to the hot water fill rate, i.e M3/hr/M3 of charcoal bed. 4. Check the colour of drain water. When the colour is clear, water washing is completed if no catalyst reimpregnation is to follow. Keep the steam flow for minutes to displace the water and any residual materials. 5. After stopping the steam flow, begin a nitrogen purge down through the reactor for minutes at a moderate rate. This is to help cool the bed and displace the condensate. 6. After the reactor is cooled down, catalyst bed can be alkalized with fresh 10 0 deg. Be caustic solution and is ready for kerosene treating. 7. If catalyst reimpregnation is to follow the hot water wash, the hot water washing should be continued until the reactor effluent ph is down to Stop the water and steam flow, flush out the caustic circulation system by opening up the suction lines from the reactor bottom to the circulation pump, 15-P-03m and pump discharge to spent caustic tank. 9. Refill the reactor with water after the reactor has been pumped out. 10. Start circulation from bottom to top of reactor. Both the kerosene outlet and the bottom pump out of the reactor are open to the pump suction. 11. Inject steam occasionally to keep the circulating water temperature at deg. C. 12. Add 5 gallons of glacial acetic acid to the acetic acid pot. Route a slip steam from the circulation pump discharge to the educator 15-JA-001. Start educating acetic acid to the circulation system. CHAPTER-11 5

6 13. After completing one circulation around the reactor, check the reactor effluent ph. If ph is higher than 6.5, add another batch of 5 gallon acetic acid. Continue this until the water ph is this acetic acid addition is necessary to neutralize the trace alkalinity on the bed and provide a slight acidic medium. 14. Drain the water and stop circulation when the level is below the distributor pipe. 15. Keep a moderate steam flow through the reactor for minutes. 16. Nitrogen/steam purge reactor for minutes. 17. After reactor is cooled off, start impregnation procedure. ************************ CHAPTER-11 6