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41-407: The Merox process requires an alkaline environment which, in some process versions, is provided by an aqueous solution of sodium hydroxide (NaOH), a strong base , commonly referred to as caustic . In other versions of the process, the alkalinity is provided by ammonia , which is a weak base. The catalyst in some versions of the process is a water-soluble liquid. In other versions, the catalyst

82-484: A Raschig ring section to prevent entrainment of any disulfides in the vented air. The disulfides are withdrawn from the separator and routed to fuel storage or to a hydrotreater unit. The regenerated lean Merox caustic is then pumped back to the top of the extractor for reuse. The conventional Merox process for the removal of mercaptans (i.e., sweetening) of jet fuel or kerosene is a one-step process. The mercaptan oxidation reaction takes place in an alkaline environment as

123-562: A pH greater than 7.0. The adjective alkaline , and less often, alkalescent , is commonly used in English as a synonym for basic, especially for bases soluble in water. This broad use of the term is likely to have come about because alkalis were the first bases known to obey the Arrhenius definition of a base, and they are still among the most common bases. The word alkali is derived from Arabic al qalīy (or alkali ), meaning '

164-441: A flapper closer to the nozzle. The nozzle pressure increases, which increases the output pressure to the actuator through a pneumatic amplifier relay. The increased output pressure to the actuator causes the valve stem to move. Stem movement is fed back to the beam by means of a cam. As the cam rotates, the beam pivots about the feedback axis to move the flapper slightly away from the nozzle. The nozzle pressure decreases and reduces

205-452: A kerosene cut with a higher content of sulphur than this limit. UOP has developed many versions of the Merox process for various applications: In all of the above Merox versions, the overall oxidation reaction that takes place in converting mercaptans to disulfides is: The most common mercaptans removed are: In some of the above Merox process versions, the catalyst is a liquid. In others,

246-412: A smart positioner on a control valve: Control valves are classified by attributes and features. A wide variety of valve types and control operation exist. However, there are two main forms of action, the sliding stem and the rotary. The most common and versatile types of control valves are sliding-stem globe, V-notch ball, butterfly and angle types. Their popularity derives from rugged construction and

287-467: A typical digital valve controller, the control signal is read by the microprocessor, processed by a digital algorithm, and converted into a drive current signal to the I/P converter. The microprocessor performs the position control algorithm rather than a mechanical beam, cam, and flapper assembly. As the control signal increases, the drive signal to the I/P converter increases, increasing the output pressure from

328-423: Is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. This enables the direct control of flow rate and the consequential control of process quantities such as pressure , temperature , and liquid level. In automatic control terminology, a control valve is termed a "final control element". The opening or closing of automatic control valves

369-452: Is a two-step process. In the first step, the feedstock LPG or light naphtha is contacted in the trayed extractor vessel with an aqueous caustic solution containing UOP's proprietary liquid catalyst. The caustic solution reacts with mercaptans and extracts them. The reaction that takes place in the extractor is: In the above reaction, RSH is a mercaptan and R signifies an organic group such as a methyl, ethyl, propyl or other groups. For example,

410-512: Is fed back to the microprocessor. The stem continues to move until the correct position is attained. At this point, the microprocessor stabilizes the drive signal to the I/P converter until equilibrium is obtained. In addition to the function of controlling the position of the valve, a digital valve controller has two additional capabilities: diagnostics and two-way digital communication. Widely used communication protocols include HART , FOUNDATION fieldbus , and PROFIBUS. Advantages of placing

451-426: Is impregnated into charcoal granules. Processes within oil refineries or natural gas processing plants that remove mercaptans and/or hydrogen sulfide (H 2 S) are commonly referred to as sweetening processes because they result in products which no longer have the sour, foul odors of mercaptans and hydrogen sulfide. The liquid hydrocarbon disulfides may remain in the sweetened products. These may be used as part of

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492-542: Is usually done by electrical , hydraulic or pneumatic actuators . Normally with a modulating valve, which can be set to any position between fully open and fully closed, valve positioners are used to ensure the valve attains the desired degree of opening. Air-actuated valves are commonly used because of their simplicity, as they only require a compressed air supply, whereas electrically operated valves require additional cabling and switch gear, and hydraulically actuated valves required high pressure supply and return lines for

533-417: The I/P converter. This pressure is routed to a pneumatic amplifier relay and provides two output pressures to the actuator. With increasing control signal, one output pressure always increases and the other output pressure decreases Double-acting actuators use both outputs, whereas single-acting actuators use only one output. The changing output pressure causes the actuator stem or shaft to move. Valve position

574-469: The Merox unit. The caustic solution leaving the bottom of the mercaptan extractor ("rich" Merox caustic) flows through a control valve which maintains the extractor pressure needed to keep the LPG liquified. It is then injected with UOP's proprietary liquid catalyst (on an as needed basis), flows through a steam-heated heat exchanger and is injected with compressed air before entering the oxidizer vessel where

615-415: The actuator stem to move upward. Through the cam, stem movement is fed back to the beam to reposition the flapper closer to the nozzle. When equilibrium conditions are obtained, stem movement stops and the flapper is positioned to prevent any further decrease in actuator pressure. The second type of positioner is an analog I/P positioner. Most modern processing units use a 4 to 20 mA DC signal to modulate

656-405: The calcined ashes ' (see calcination ), referring to the original source of alkaline substances. A water-extract of burned plant ashes, called potash and composed mostly of potassium carbonate , was mildly basic. After heating this substance with calcium hydroxide ( slaked lime ), a far more strongly basic substance known as caustic potash ( potassium hydroxide ) was produced. Caustic potash

697-448: The catalyst is in the form of impregnated charcoal granules. Process flow diagrams and descriptions of the two conventional versions of the Merox process are presented in the following sections. The conventional Merox process for extraction and removal of mercaptans from liquefied petroleum gases (LPG), such as propane, butanes and, mixtures of propane and butanes, can also be used to extract and remove mercaptans from light naphthas. It

738-402: The catalyst. An alkaline environment is provided by caustic being pumped into reactor on an intermittent, as needed basis. The jet fuel or kerosene feedstock from the top of the caustic prewash vessel is injected with compressed air and enters the top of the Merox reactor vessel along with any injected caustic. The mercaptan oxidation reaction takes place as the feedstock percolates downward over

779-422: The catalyst. The reactor effluent flows through a caustic settler vessel where it forms a bottom layer of aqueous caustic solution and an upper layer of water-insoluble sweetened product. The caustic solution remains in the caustic settler so that the vessel contains a reservoir for the supply of caustic that is intermittently pumped into the reactor to maintain the alkaline environment. The sweetened product from

820-475: The caustic settler vessel flows through a water wash vessel to remove any entrained caustic as well as any other unwanted water-soluble substances, followed by flowing through a salt bed vessel to remove any entrained water and finally through a clay filter vessel. The clay filter removes any oil-soluble substances, non-metallic compounds (especially copper) and particulate matter, which might prevent meeting jet fuel product specifications. The pressure maintained in

861-418: The circulating caustic solution and interfere with the Merox reactions. Therefore, the feedstock is first "prewashed" by flowing through a batch of aqueous caustic to remove any H 2 S. The reaction that takes place in the prewash vessel is: The batch of caustic solution in the prewash vessel is periodically discarded as " spent caustic " and replaced by fresh caustic as needed. The flow diagram below depicts

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902-406: The contact trays where the LPG intimately contacts the downflowing Merox caustic that extracts the mercaptans from the LPG. The sweetened LPG exits the tower and flows through: a caustic settler vessel to remove any entrained caustic, a water wash vessel to further remove any residual entrained caustic and a vessel containing a bed of rock salt to remove any entrained water. The dry sweetened LPG exits

943-533: The context of chemistry and chemical engineering . There are various, more specific definitions for the concept of an alkali. Alkalis are usually defined as a subset of the bases. One of two subsets is commonly chosen. The second subset of bases is also called an " Arrhenius base ". Alkali salts are soluble hydroxides of alkali metals and alkaline earth metals , of which common examples are: Soils with pH values that are higher than 7.3 are usually defined as being alkaline. These soils can occur naturally due to

984-440: The control set point to the control valves. Pressure is typically modulated between 20.7 and 103 kPa (3 to 15 psig) to move the valve from 0 to 100% position. In a common pneumatic positioner, the position of the valve stem or shaft is compared with the position of a bellows that receives the pneumatic control signal. When the input signal increases, the bellows expands and moves a beam. The beam pivots about an input axis, which moves

1025-413: The control valves. This introduces electronics into the positioner design and requires that the positioner convert the electronic current signal into a pneumatic pressure signal (current-to-pneumatic or I/P). In a typical analog I/P positioner, the converter receives a DC input signal and provides a proportional pneumatic output signal through a nozzle/flapper arrangement. The pneumatic output signal provides

1066-453: The controller. The HART , Fieldbus Foundation, and Profibus are the most common protocols. An automatic control valve consists of three main parts in which each part exist in several types and designs: Taking the example of an air-operated valve, there are two control actions possible: There can also be failure to safety modes: The modes of failure operation are requirements of the failure to safety process control specification of

1107-425: The equipment and the flow paths involved in the process. The LPG (or light naphtha) feedstock enters the prewash vessel and flows upward through a batch of caustic which removes any H 2 S that may be present in the feedstock. The coalescer at the top of the prewash vessel prevents caustic from being entrained and carried out of the vessel. The feedstock then enters the mercaptan extractor and flows upward through

1148-439: The ethyl mercaptan ( ethanethiol ) has the formula C 2 H 5 SH. The second step is referred to as regeneration and it involves heating and oxidizing of the caustic solution leaving the extractor. The oxidations results in converting the extracted mercaptans to organic disulfides (RSSR) which are liquids that are water-insoluble and are then separated and decanted from the aqueous caustic solution. The reaction that takes place in

1189-454: The extracted mercaptans are converted to disulfides. The oxidizer vessel has a packed bed to keep the aqueous caustic and the water-insoluble disulfide well contacted and well mixed. The caustic-disulfide mixture then flows into the separator vessel where it is allowed to form a lower layer of "lean" Merox caustic and an upper layer of disulfides. The vertical section of the separator is for the disengagement and venting of excess air and includes

1230-465: The feedstock be prewashed to remove any H 2 S that would interfere with the sweetening. The reaction that takes place in the batch caustic prewash vessel is: The Merox reactor is a vertical vessel containing a bed of charcoal granules that have been impregnated with the UOP catalyst. The charcoal granules may be impregnated with the catalyst in situ or they may be purchased from UOP as pre-impregnated with

1271-402: The feedstock jet fuel or kerosene, mixed with compressed air, flows through a fixed bed of catalyst in a reactor vessel. The catalyst consists of charcoal granules that have been impregnated with UOP's proprietary catalyst. The oxidation reaction that takes place is: As is the case with the conventional Merox process for treating LPG, the jet fuel or kerosene sweetening process also requires that

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1312-421: The hydraulic fluid. The pneumatic control signals are traditionally based on a pressure range of 3–15 psi (0.2–1.0 bar), or more commonly now, an electrical signal of 4-20mA for industry, or 0–10 V for HVAC systems. Electrical control now often includes a "Smart" communication signal superimposed on the 4–20 mA control current, such that the health and verification of the valve position can be signalled back to

1353-450: The input signal to the pneumatic positioner. Otherwise, the design is the same as the pneumatic positioner While pneumatic positioners and analog I/P positioners provide basic valve position control, digital valve controllers add another dimension to positioner capabilities. This type of positioner is a microprocessor-based instrument. The microprocessor enables diagnostics and two-way communication to simplify setup and troubleshooting. In

1394-418: The output pressure to the actuator. Stem movement continues, backing the flapper away from the nozzle until equilibrium is reached. When the input signal decreases, the bellows contracts (aided by an internal range spring) and the beam pivots about the input axis to move the flapper away from the nozzle. Nozzle decreases and the relay permits the release of diaphragm casing pressure to the atmosphere, which allows

1435-444: The plant. In the case of cooling water it may be to fail open, and the case of delivering a chemical it may be to fail closed. The fundamental function of a positioner is to deliver pressurized air to the valve actuator, such that the position of the valve stem or shaft corresponds to the set point from the control system. Positioners are typically used when a valve requires throttling action. A positioner requires position feedback from

1476-496: The presence of alkali salts. Although many plants do prefer slightly basic soil (including vegetables like cabbage and fodder like buffalo grass ), most plants prefer mildly acidic soil (with pHs between 6.0 and 6.8), and alkaline soils can cause problems. In alkali lakes (also called soda lakes ), evaporation concentrates the naturally occurring carbonate salts, giving rise to an alkalic and often saline lake. Examples of alkali lakes: Control valve A control valve

1517-466: The reactor is injected air will completely dissolve in the feedstock at the operating temperature . Alkaline In chemistry , an alkali ( / ˈ æ l k ə l aɪ / ; from the Arabic word al-qāly , القلوي ) is a basic , ionic salt of an alkali metal or an alkaline earth metal . An alkali can also be defined as a base that dissolves in water . A solution of a soluble base has

1558-422: The refinery or natural gas processing plant fuel, or they may be processed further. When dealing with kerosene, the Merox process is usually more economical than using a catalytic hydrodesulfurization process for much the same purpose. It is rarely (if ever) required to reduce the sulphur content of a straight-run kerosene to meet the 3000 ppm sulphur specification of jet fuel , because very few crude oils have

1599-423: The regeneration step is: After decantation of the disulfides, the regenerated "lean" caustic solution is recirculated back to the top of the extractor to continue extracting mercaptans. The net overall Merox reaction covering the extraction and the regeneration step may be expressed as: The feedstock entering the extractor must be free of any H 2 S. Otherwise, any H 2 S entering the extractor would react with

1640-410: The valve stem or shaft and delivers pneumatic pressure to the actuator to open and close the valve. The positioner must be mounted on or near the control valve assembly. There are three main categories of positioners, depending on the type of control signal, the diagnostic capability, and the communication protocol: pneumatic, analog, and digital. Processing units may use pneumatic pressure signaling as

1681-723: Was traditionally used in conjunction with animal fats to produce soft soaps , one of the caustic processes that rendered soaps from fats in the process of saponification , one known since antiquity. Plant potash lent the name to the element potassium , which was first derived from caustic potash, and also gave potassium its chemical symbol K (from the German name Kalium ), which ultimately derived from al k ali. Alkalis are all Arrhenius bases , ones which form hydroxide ions (OH ) when dissolved in water. Common properties of alkaline aqueous solutions include: The terms "base" and "alkali" are often used interchangeably, particularly outside

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