ALKYLATION PROCESS HAZARDS MANAGEMENT DOES IT MATTER WHICH ACID YOU USE? PRESENTED BY BRUCE SCOTT

Transcription

1 ALKYLATION PROCESS HAZARDS MANAGEMENT DOES IT MATTER WHICH ACID YOU USE? PRESENTED BY BRUCE SCOTT MAY 1991

2 The American Petroleum Institute published Recommended Practice 750 in January, 1990, and the U.S. OSHA will promulgate similar rules before the end of Both of these documents attempt to guide the management of process hazards. Several states in the U.S. and many European countries have adopted process hazards regulations. The goal of all these efforts is to reduce accidental releases of flammable or toxic substances which have catastrophic consequences, or more specifically, which pose a serious danger to people both within and beyond the workplace. In general, these regulations require a number of management systems to be put into place without specifying the process details. This paper will focus on the alkylation process, and particularly on the choice of acid catalysts, from the standpoint of hazard management of details beyond the requirements of the above regulations. The alkylation process, which chemically reacts isobutane and light olefins in the presence of a strong acid catalyst into a premium gasoline component, has been well described elsewhere. From the hazard viewpoint, the process presents two problems: the acid catalyst which is corrosive and toxic, and the light hydrocarbons which are highly flammable and potentially explosive if released. Of the two, the hydrocarbon represents the greater hazard, while the acid catalyst will have the greatest impact on the cost of mitigation. A recent survey of refinery disasters included several alkylation episodes; all were fires, in none was a major acid release the principal problem. However, three recent mishaps in the U.S. have focused regulatory attention on the hazards of hydrofluoric (HF) acid. One major local regulatory body has voted to ban HF within its jurisdiction. Alkylation plants are somewhat unique in that they contain large volumes of liquefied petroleum gasses (LPG), on a par with LPG storage facilities, but are also refinery process plants. Therefore, they take special effort to insure that the normal process procedures and training standards include not only process information, but also information and procedures relevant to the large volumes stored. In most cases the LPG hazard management is independent of the flavor of acid used: both have similar volumes of hydrocarbon, and neither acid impacts the flammability of the LPG. A key element of the management of LPG hazard is directed at preventing the release in the first place. When the plants are new they are constructed to standards and with materials that are specifically designed to make the plant as safe as it can be made. As the plant ages it deteriorates from that high standard, and a comprehensive program of equipment inspection, including rigorous record keeping and review, is needed to detect the deterioration before it can result in a release of LPG. Both Sulfuric Acid (H 2 ) and HF catalyzed processes produce acid esters from the reaction of acid with olefins. These esters are unstable at higher temperatures found in the bottoms of the distillation columns in the plants. The decomposition products released as these esters break down are almost always acidic in nature, and if in contact with water can cause rapid corrosion. The concern is somewhat different in HF catalyzed plants than in H 2 plants. Because of the volatile nature of HF acid, the columns, which normally run dry, are used to recover the acid carried over from the settler. Acidic components are neutralized as the final products are recovered from the plant. Because acid and esters are expected to be present throughout the distillation section the potential for corrosion is great if water enters this area. In most HF plants fired reboilers are specified to avoid the use of high pressure steam. In H 2 plants any acid carried out of the reaction section is neutralized immediately. The columns operate wet and no further treatment is usually done. In both HF and H 2 plants a rigorous equipment inspection program should focus on the parts of the plant most subject to corrosion. In HF plants, care must be paid to the water content of the circulating acid in the plant as excessive water may concentrate in the overhead of the isostripper and cause corrosion in that area. If the isostripper or the depropanizer are operated at low temperatures, HF acid may descend into the reboiler furnaces, causing corrosion in the furnace tubes. A key area of corrosion is in the product defluorination section, -1-

3 where the reaction of HF with alumina releases water and some free acid. This can cause corrosion in the lines carrying LPG to the caustic treater. In the H 2 process the effluent from the reaction section is normally neutralized with an alkaline water solution. The area of the piping system where the acidic effluent and the aqueous alkali are first contacted is susceptible to high corrosion rates. Alloy piping is normally specified in this area. The equipment inspection program must also pay particular attention to the low spots in the LPG train where acidic water can collect. Overhead accumulator boots, coalescer boots, and reflux lines are examples of areas that require careful inspection. Low point bleeders in LPG piping and on pump cases require particular attention because they are not normally used and can trap acidic water. These should be radiographed on a regular frequency. In both plants pressure relief systems which may be exposed to acidic material should be inspected regularly. Additional inspection should be requested whenever process upsets occur that might result in a excursion of acid esters into the LPG-containing portions of the plant. Piping specifications should be updated and reviewed for suitability for infrequent exposure to dilute acid. Deviations from established piping and material specifications should not be allowed in the alkylation plant because of the large volumes of LPG involved and the potential for acid exposure. The alkylation plant should be included in the LPG equipment auditing procedures setup for the refinery. In addition to the steps taken to reduce the probability of an LPG release, other steps must be taken to mitigate the effect of such a release should it occur. All LPG facilities need to be adequately protected against the impact of fire, but some things specific to alkylation plants should be considered as well. An alkylation plant contains several large reservoirs of light hydrocarbon. In case of a release in the plant it is essential that these containers be isolated from the leak to minimize the volume of flammable hydrocarbon involved. Alkylation plants are equipped with remotely operated valves for process control purposes, but in the event of a fire these are frequently rendered unusable as electrical leads or air tubing are destroyed. Gate valves (or plug valves) installed for equipment isolation during normal operations may be engulfed in the fire, rendering them unusable also. Separate power operated isolation valves should be considered for the major LPG containing vessels in the alkylation plant. These should be capable of being operated from multiple remote locations so that an operator always has access to the operating station. The motive force (air, electricity or nitrogen) should have sufficient backup capacity to be always operable in case the primary source of power is disabled by fire. Also, valves should be designed to fail into a safe mode. Electrical leads and tubing runs should be fireproofed. The valves and the control systems must be regularly tested to insure they are in operable condition when they are needed. Some of the larger reservoirs that should be considered for isolation include the reactor/settler system, the treating sections, and the individual distillation columns. If a hydrocarbon release results in a fire it may be necessary to provide water deluge to cool nearby LPG containing vessels and columns to insure their structural integrity. Relatively small fires have become major fires when vessels have ruptured or columns toppled because of overheated metal. Consider installing spray or deluge headers around major hydrocarbon vessels. Local fire protection codes may provide guidance on the volume of water needed. Due to the extreme potential consequences of an alkylation vessel overpressure, older relief systems should be reviewed to insure that additions to the system since the alkylation plant was built have not reduced the available capacity for the alkylation plant. New plants will not have this potential problem, and care should be taken to maintain the capacity of the alkylation plant relief system. Pay particular attention to acid neutralizers in the relief system; if undersized, a large release my blow the alkali out, allowing acidic material to go into the flare system. -2-

4 If release mitigation systems are to have a positive impact on plant hazards some system of leak detection must be in place. Consider a network of hydrocarbon detectors at strategic locations through a plant. Leak detection technology is undergoing rapid development at this time and wide area or line of sight monitors for hydrocarbons and HF are expected to become available at some time soon. The possibility of an LPG (or HF) vapor cloud release from an alkylation plant suggests the desirability having on hand a real-time method of assessing the direction and extent of the cloud movement. Computer systems are available, with built-in weather stations, to predict cloud dispersion as the release occurs. Some refiners do a series of computer studies covering a range of weather conditions and release sizes and keep this data available in a notebook in the control room. The second major challenge in limiting the hazards related to operating an alkylation plant is the acid catalyst used to make the chemistry work. Both the HF and the H 2 plants contain a large inventory of concentrated acid, and both can cause pernicious injuries to people directly exposed to the acid. The difference in acid properties, however, has a distinct impact on the hazard mitigation effort. In the U.S. HF has been identified as a hazardous air pollutant in current Federal and State legislation. H 2 has not. In case of liquid exposure both will cause serious, painful bums. HF also has the property of penetrating tissue and reacting with the calcium and magnesium in the blood, causing hypocalcemia. HF is volatile, and when spilled will form a vapor cloud. It is also known to form a stable aerosol cloud when released under pressure. Both vapor and aerosol cause a serious inhalation hazard to plant personnel and to people living and working around the refinery. H 2 is not volatile. It has been suggested that H 2 mixed with isobutane may form an aerosol if released under pressure. Experience and preliminary testing indicate no aerosol is formed. A program of definitive testing will be done by a consortium of oil companies in the summer of 1991 to determine if any aerosol can be formed. As with the LPG parts of the plant, the principle focus of an acid hazard management program must be on prevention. Here also, new plants are built to exacting standards and materials specifications, and deteriorate with use. The materials of construction requirements for the two acids are not the same. Both concentrated acids are contained in carbon steel, and both become very corrosive when diluted with water. Frequent analysis of acid is essential in both to control acid strength. Alloy 20 Cb3 is used in H 2 plants where traces of acid may be exposed to water. STRATCO recommends that acid velocities be kept below 2 FPS in H 2 plants. Monel must be used in HF plants to avoid excessive erosion-corrosion. Both acids become corrosive to carbon steel at elevated temperatures. In H 2 plants the acid is kept away from the hot areas of the plant and this is not normally a problem. Regeneration of the acid in HF plants is done at elevated temperatures and Monel is used to resist the resulting corrosion. HF acid contaminated with even small amounts of oxygen can cause severe pitting and cracking of steel and Monel. In addition to corrosion, HF acid can cause stress cracking in steel above a threshold hardness, particularly in the heat affected zones of welds. Special steels which do not harden during welding, or extensive use of post weld heat treatment are used to protect against cracking. Special bolting is used where flanges may be exposed to wisps of HF. HF can also cause hydrogen blistering of steel, and killed or clean steels may be specified to limit this damage. The special material needs of HF plants place special emphasis on online equipment inspection, and may require a separate materials warehouse. Repair materials and spare parts are certified suitable for use in HF service may then be stored in a separate identifiable area. -3-

5 Minimizing the inventory of the acid in the plant requires close control of levels in the various vessels. Levels are monitored in H 2 plants with conventional glass level gauges, but, because HF etches glass, nuclear level devices are normally specified in the HF plants. In both processes the quality of the acid directly impacts the quality of the alkylate product, and the water content of HF acid directly affects corrosion in the plant. Frequent sampling and analysis of the acid catalyst is required. H 2 is a common laboratory chemical and its properties and hazards are well known to refinery chemists. Protective clothing required during sampling includes gloves, face shield, goggles and an acid resistant apron or jacket. Continuous on-line H 2 analyzers are available which will eliminate most of the handling of acid samples. Sampling and testing of HF exposes plant operators and lab technicians to acid vapors, and as the acid is uncommon, special training is required. Both must wear special acid resistant clothing and be provided with safe breathing air. Another major difference between the two processes is the handling of the contaminated acid. Acid in both plants is contaminated by esters and by water and impurities in the plant feed and must be purged from the reaction section. In H 2 plants the purge goes to tankage from which it is transported to a regeneration plant. This may be adjacent in the refinery, or may be remote. Care must be taken to insure that dissolved hydrocarbons are removed from the spent acid before it enters tankage, and the tank must be blanketed to prevent explosive conditions in the vapor space of the tank. Sometimes tank vent vapors are scrubbed to remove traces of sulfur dioxide. In the HF process contaminated acid is purged directly to a regeneration column where the acid is stripped out of the contaminants and returned to the process. The resulting oily stream can contain significant quantities of an azeotrope of HF and water and thus must be carefully neutralized by the refinery before disposal. Due to the high temperatures and the possibility of weak acid in the neutralization system, the acid regenerator merits special attention in the on line inspection program. In the event of an acid release, as in an LPG release, mitigation of the impact before the acid can spread is a critical part of the hazard management plan. Here, again, the properties of the acid impact on the strategy and equipment used in the plan. The boiling point of HF acid is 67 F (19.4 C) at atmospheric pressure. In the HF alkylation plant the reactions take place at about 100 F (37.8 C) which means the acid will vaporize out of any containment penetration. For this reason isolation of the acid-containing elements of the plant is a critical feature in reducing the volume of acid released through a leak. This isolation should be done with remotely operated valves using all the design features discussed in the LPG section. Rigorous testing of these isolation valves is crucial because of the tendency of HF to form a hard scale on the inner surface of steel can result in valve blockage. In addition to isolating an acid release in an HF plant, it may be desirable to be able to move the acid contents of a leaking vessel to a secure containment location. To do so rapidly may involve large acid transfer pumps, several remote operated valves and if remote storage vessels are envisaged, potentially complex pressure relief and acid vapor handling facilities. H 2 is a non-volatile liquid. Stopping the mixer drivers will allow the acid to drop from the settler into the Contactor Reactor where it will remain. Even if a Contactor Reactor shell is breached acid leakage will be limited to that one vessel. HF plants are routinely designed with acid pump-out pumps and remote acid storage tanks. -4-

6 Extensive testing has shown that a release of superheated HF will form a cold, dense vapor cloud that may persist for some distance. The situation will be worsened if an aerosol cloud is formed. In any case, it is highly desirable to spray copious quantities of water on the cloud. If enough water can be applied on the cloud up to 90% of the HF can be knocked to the ground and downwind impact of the release greatly reduced. Under the conditions of the recent desert tests this required at least 40 volumes of water for each volume of HF released. Remotely operated water cannons, or an extensive network of water spray curtains are commonly installed for this purpose. Provisions must be made to handle the large volumes of acidic water runoff from a water mitigation system. The possibility of a toxic vapor cloud also suggests a safe haven approach to control room design. In addition to being blast resistant for protection against an LPG cloud, new control rooms should be designed to be sealed off from the inside. The air intake system should have an HF alarm and should be easy to close off against toxic vapors. The control room should have direct access to a personnel protective equipment (PPE) storage room. Access from the PPE room to the plant must be designed to avoid accidentally flooding the control room with toxic HF vapors when people go out to handle the release. The safe haven control room gives operators and others working in the plant a place to go when the evacuation horn sounds. There they can seal themselves in and develop a safe response plan. As mentioned above, it has been suggested that H 2, intimately mixed with superheated isobutane, may form an aerosol cloud. Testing will be done during the summer of 1991 to establish the conditions, if any, under which such a cloud may form. Until formation of an aerosol can be confirmed no mitigation system is recommended for this possibility in an H 2 alkylation plant. When an acid release occurs, particularly a small release, access to the site of the release can be critical in preventing it from becoming larger. Care should be given in the layout of an alkylation plant to provide adequate access to field instruments and other sensitive locations. Accessibility must take into account the protective clothing the operator will be wearing when he approaches an acid leak. In an H 2 plant this will be boots, gloves, rain suit, face shield, and goggles while in an HF plant it may be a full air-fed acid suit with self contained breathing tanks or an air hose to an in-plant breathing air system. Written standards for both types of plants should include procedures for cleaning, neutralizing, and disassembling for maintenance any equipment that has been in acid service. This standard should take account of the different volatility of the acids, and the possibility of trapping HF under iron fluoride scale inside steel equipment. In conclusion, alkylation plants, regardless of the choice of acid catalyst, can be operated safely, and with minimum process risk to employees or neighbors. Both types of plants require a comprehensive plan for process hazards management, with that plan taking full account of the differing physical properties of the acids involved. The cost to the refiner of the control and actual cost will vary considerably from plant to plant and from location to location, the order of magnitude costs will be about $1 2 million for the H 2 plant, and about $10 15 million for the HF plant. Given the current regulatory attention to HF safety in the U.S., it is doubtful that any new HF alkylation plant could obtain a permit without the above mitigation facilities, if permits to build are granted at all. -5-