Petroleum refining

Alkylation is a combining process that creates alkylate, a premium, high-octane blending stock. A large acid settler tank within the unit allows for separation of the acid/hydrocarbon emulsion created in the reactor. Sight glasses have traditionally measured settler level, but these are susceptible to plugging and require diligent flushing at regular intervals. Challenge Interface level control of the settler is required. Product/acid cross-contamination causes overall efficiency problems and can result in hazardous conditions. Inaccurate settler level indication is a frequent cause of physical acid carryover in the reactor effluent. Release of acid can cause extreme hazards to both process and personnel.

Storage and wash vessels in the alkylation unit include those for fresh and depleted acid and water, an acid analyzer settling pot, and a number of wash tanks. Caustic washes neutralize free acid carried over from the reaction zone and neutralize alkyl sulfates in the net effluent. Water washes decompose remaining esters and remove caustic and salt that might have carried over from the caustic wash. Challenge Interface level control in caustic and water washes prevents corrosion and fouling of the deisobutanizer and other downstream units. Levels rising above high set points can cause carryover of caustic or water, while too low levels can cause hydrocarbon carryunder.

Mixing finished gasoline requires level monitoring of blending stocks, in-line blending, additive storage, dilution systems and injection systems. Blend stocks are straight-run gasoline, alkylate, reformate, benzene, toluene and xylene. Additives include octane boosters, metal deactivators, anti-oxidant and knock agents, gum and rust inhibitors, and detergents. Challenge Level is normally measured only for inventory control and, in some cases, custody transfer. The precision of level indication that is required varies. If level is used only for internal inventory control, a less accurate measurement can be used. If level is used for custody transfer, highly accurate measurements are required.

The catalytic method of converting compounds into other compounds uses a chemical catalyst to increase the rate of a reaction. The catalyst is chemically unchanged after the reaction has occurred and can be recovered from the reaction mixture. Thirty of the top fifty commodity chemicals are created directly by catalysis. Challenge In the fluidized bed reactor-regenerator (FBRR) a fluid is passed through the catalyst at high enough velocities to suspend the solid and cause it to behave as though it were a fluid. Measurement in the reactor vessel must contend with severe process conditions. Finished product and, in some configurations, spent catalyst, is on level control.

The Fluid Catalytic Cracking Unit (FCCU) cracks heavy, low-value feedstocks into high-value, lighter molecular weight hydrocarbons which are blended to finished products. A cracker can produce a wide variety of yield patterns by operating in either Gasoline, Distillate or LPG modes. Challenge Catalytic crackers utilize a reactor and a catalyst regenerator with connecting risers where the reactions take place. Level controllers are often positioned on the first stage regenerator and at the top of the reactor. Level measurements involve fluidized solids levels at high temperatures. Conventional measurement techniques can be subject to plugging.

Catalytic reforming upgrades low-octane naphthas into high-octane gasoline blending components called reformates. Using heat and pressure with platinum catalysts to rearrange hydrocarbon molecules, this process converts low-octane gasoline fractions into petrochemical feedstocks and higher octane stocks suitable for finished gasoline blending. Challenge A hydrogen-rich gas stream is removed from the separator for recycling. Liquid product monitored for level at the bottom of the separator is sent to a stabilizer. Where stabilizer fouling has occurred due to the formation of ammonium chloride and iron salts, a monitored water wash system should also be included.

Variations in cat cracking include Selective Component Cracking (SCC) for polypropylene production, a two-vessel and external-reactor design for processing heavy residue feeds, and a UOP process for converting gas oils and resid feedstocks. All crackers employ a steam stripper to remove hydrocarbons entrained in the spent catalyst. Challenge Stripper level control allows sufficient residence time for stripping steam to displace hydrocarbons for recovery. It also maintains sufficient pressure to keep air in the regenerator from reverse flow into the reaction system, thereby causing a hazard. A waste heat recovery steam drum would also require monitoring.

Coking is the final means of converting the heaviest products of atmospheric and vacuum distillation. Feed is heated and cracked into light gases, gasoline blendstocks, distillates, and gas oil. Level applications for delayed and continuous (contact or fluid) coking include the fractionator, light gas oil stripper, steam, condensate, blowdown and settling drums, and vent gas knockout drums. Challenge Process conditions that level instrumentation must contend with include high temperatures, high pressures, foaming, and steam. An interface level gauge in drums that are susceptible to foaming will help avoid foamover and increase coke drum output.

A heat exchanger positioned near the bottom of the distillation column re-heats and vaporizes liquid and reintroduces the vapor several trays higher. This improves separation by introducing more heat into the column. For effective functioning of the reboiling process, level monitoring of the reboiler is required. Challenge In some steam reboilers, the level must be controlled so that only a percentage of tubes are covered. This allows a control scheme to regulate the heat transfer in the reboiler by controlling the percentage of the reboiler tubes covered by liquid. This is a critical control loop as heat transfer into the liquid is a strong function of the percentage of tubes covered.

Salt in the crude stream presents serious corrosion and scaling problems and must be removed. Salt is dissolved within the remnant brine of the crude oil. Desalting removes both salt and the residual free water. Though the refinery is the most economical place for desalting, pipeline requirements often necessitate field desalting. Challenge Level instrumentation is integral to single and two-stage desalting systems, multiple orifice plate mixers, and the settler tank of a chemical desalter. Interface level control keeps free water from hitting the desalter electrodes and prevents expensive damage. The Interface level should be kept constant otherwise electrical field changes will disturb electrical coalescence.

Inorganic chlorides, suspended solids, and trace metals found in untreated crude must be removed by chemical or electrostatic desalting. This reduces the risk of acid corrosion, plugging, fouling and catalyst poisoning in downstream units. Measurement of the oil/water interface in the desalter is crucial in separating the cleansed crude from contaminants. Challenge Coating and build-up on probes may create interface measurement errors. Instruments susceptible to electrostatic grid interference may require special filters. Interface dedicated level transmitters fitted with quick-disconnect probes provide optimum performance while reducing cleaning and maintenance time.

All unrefined crude oil stored in tanks has a percentage of water entrained within it, and while stored in tanks, separation naturally occurs with water collecting at the bottom of the tank beneath the oil. The two fluids are very distinct except for a “black water” or “rag” interface layer which is an emulsion of mixed oil and water. To dewater the tank, water is drawn off of the bottom of the tank and is then sent off to water treatment. Challenge Level controls designed for interface detection will sense the beginning of the oil/water interface during dewatering procedures and provide feedback to a control system which will terminate water draw-off when appropriate.

Upon arrival at the refinery terminal, crude oil is pumped into above-ground storage tanks with capacities of thousands to millions of gallons. Raw crude is stored in floating-or fixed-roof tanks field-built to API standards. Tank level measurement by non-contact radar has gained share over mechanical float type and servo gauges due to its accuracy, low maintenance, no moving parts and fast set-up. Challenge Tank level is maintained by valve actuation. By triggering an emergency cutoff, level controls prevent overflows and shut down pumps when level falls below low level. Safety certified controls may be necessary due to crude’s low flash point.

Diesel-powered engine-generator sets provide emergency power to operate critical nuclear plant systems in the event of a loss of station service power. The main diesel fuel storage tank provides a fuel capacity for one to seven days of full-load generator operation. The main storage tank is connected to an indoor day tank holding less than 1,000 gallons. Challenge Main storage tanks typically require a fuel level indicator with a remote indication transmitter. Sensors actuating electrical pumps connected to the main tank continuously monitor day tank fuel level. Day tank high-level alarms can lock out supply pumps until a system reset. Low levels and critical low-levels actuate alarms and the system will display the low-level conditions.

Following desalination, crude oil enters the distillation column where fractional distillation separates hydrocarbons into separate streams, cuts or fractions. For optimum operation of the distillation column, level controls must contend with occurances of foaming, bubbling and moderate-to-high temperatures. Challenge Sight glasses and displacer systems mounted in external chambers have traditionally provided distillation level measurement. Today, radar retrofitted in these existing chambers is gaining popularity due to radar’s less demanding maintenance schedule and ease of retrofit. High product temperatures necessitate temperature-tolerant level sensors.

Selecting a separation technology from among 20 leading varieties depends upon a chemical’s nature, the number of phases, and the capacity, speed and efficiency required. Distillation – separating substances based on differences in volatilities – is the most widely used separation and purification method. Today, approximately 40,000 distillation towers are operating in U.S. chemical plants. Challenge Level measurement at the bottom of a distillation tower controls the “bottoms” product rate. Poor level control could allow liquid to back up over the stripping trays causing damage and reduced yields. Too low level may cause pump cavitation. In related extraction towers, interface level control provides optimal separation from associated substances.

Heavier feedstock difficult to process by cat cracking or reforming can be converted by hydrocracking. By combining catalytic cracking and hydrogenation to crack feedstock in the presence of hydrogen, hydrocracking produces gasoline and distillate blending streams. About a dozen different hydrocracking process schemes are in current use. Challenge Level controls for the catalyst stripper, separator liquid and flash drum liquid must contend with severe process conditions to maintain optimum operation of the hydrocracker. These conditions include elevated temperatures and pressures, the presence of steam and high pressure hydrogen, and aggressive corrosives.

Catalytic Hydrotreating treats hydrocarbon liquids in the presence of hydrogen. This process removes 90% of the sulfur, nitrogen, oxygen, and metals from feedstocks. Hydrotreating can also provide hydrodearomatization (HDAr), heavy diesel hydrocracking (HDHDC), dewaxing, and performance enhancements of pyrolysis gasoline and diesel. Challenge Hydrotreatment will necessitate level indication of liquid/gas separators and water wash tanks. Level devices suited for high temperatures and pressures are essential in removing contaminants that can have detrimental effects on equipment, catalysts, and the quality of the finished product.

Isomerization in a variety of process configurations alters the arrangement of atoms to convert normal butane into isobutane, and normal pentane and hexane into highoctane gasoline components. Isomerization is similar to catalytic reforming in that the hydrocarbon molecules are rearranged, though isomerization only converts normal paraffins to isoparaffins. Challenge Near the end of the process, reactor effluent is cooled and separated into the liquid product isomerate and a recycle hydrogen-gas stream. Isomerate is caustic-washed and waterwashed, acid stripped, and stabilized before going to storage. Stabilizer bottoms and wash tanks require level monitoring.

Reboilers, or vaporizers, are heat exchangers that provide heat to the bottom of a distillation tower. They boil the bottom liquid to generate vapors which are returned to the tower to drive the distillation separation process. The reboiler may partially or completely vaporize the stream it receives from the bottom of the tower. Challenge Excess reboiler liquids (bottoms or blow-down) overflow a baffle where level is controlled by means of a level controller. If the reboiler level becomes too low, it will affect the maximum flow rate of bottoms product that can be drawn off. Inaccurate reboiler level can also degrade composition control for material balance control configurations.

The vast piping systems within today’s refineries transport hydrocarbon gas/vapor streams, light liquid gas/liquid streams, heavier liquid streams, water, and steam throughout the refinery. Certified, reliable and accurate flow measurement is essential for a refinery’s safe and efficient operation. Challenge Flow alarms and continuous flow controls monitor product streams, signal no-flow conditions caused by plugging or valve closure, and provide a defense against pump cavitation. Pumps operating in a reduced or no-flow condition can overheat and rupture the pump’s seal and cause a dangerous deviation in process pressure and temperature.

A heat exchanger removes vapor from the upper parts of the fractionator, cools it to a liquid, and pumps it into an accumulator (reflux drum). Reflux pumps then draw liquid from the bottom of the accumulator and pump part of it back (reflux) where it is reintroduced at a lower point in the column. This refluxing process improves separation in the column by assuring sufficient downward liquid flow meeting the rising vapor. Challenge Accurate and reliable level monitoring and control is necessary for the reflux accumulator to serve as a distribution point for reflux and distillate, and prevent excessive reflux from returning back to the tower.

The heavy fraction remaining following the distillation of crudes is called petroleum resids. A variety of solvent-extraction processes yield deasphalted oil (DAO) from these resids. These oils serve as downstream feedstocks for catalytic crackers and hydrocrackers. Depending upon the system configuration, level monitoring of the separator, preflash, stripper and hot oil phases may include surge and flash drums, separators and strippers. Challenge Level control is critical because interface level control of the separator feeds the flash drum, whose level feeds the stripper, etc. Application extremes include high temperatures, high pressures, and the presence of steam.