The safety of hydrofluoric acid (HF) catalyzed alkylation and the banning of HF units have focused the attention of refiners on an important process.
Refiners held a lengthy discussion of process and safety issues related to HF alkylation at the most recent National Petroleum Refiners Association question and answer session on refining and petrochemical technology, held in Philadelphia, Oct. 3-51 1990.
Details on this important meeting and its format can be found in OGJ, Mar. 25, p. 51.
OXYGENATES IN FEED
What effect do oxygenates in the olefin feed have on the operation of the HF alkylation unit?
UTLEY: Generally oxygenates in the alkylation feed will form a light acid soluble oil (ASO) that is difficult to reject from the unit, so the acid purity will fall quite rapidly. The low acid purity will likely cause an increase in the organic fluoride content of the alkylate normal butane and propane streams. The light ASO can be rejected in the rerun column, but the temperature must be reduced substantially to do it. The oxygenate-produced ASO, the increased fluoride content of the products, and the cool temperatures in the rerun column all contribute to very large increases in acid con-sumption at constant acid purity. Our specification for total oxygen content is 35 ppm.
STEGELMAN: Sulfur or oxygenates in the olefin feed will form light acid soluble oil which is difficult to remove in the rerun tower without reducing temperature severely. ASO is often green in color as it is removed from the rerun column. The main problem is that its presence will not be readily detected in a routine acid analysis. It will most likely be weathered off in the test that we use in the laboratory and not appear in the weighted portion of the sample which is being analyzed for ASO content.
MORGAN: Data from two of our units show that we see no significant effects from oxygenate levels less than 50 ppm. Oxygenate levels above this make ASO, as previously discussed, and we have the same problems purging it out of the unit. As long as we can maintain acid purity, unit operation and alkylate quality have not been affected due to high oxygenate levels in the feed.
GESICK: As already stated, the major effect of oxygenates in the olefin feed is reduced acid purities resulting in high acid consumption. Also, since the operating acid purity is lower, there may be some small increase in octane.
The light compounds formed by the oxygenates can be very difficult to remove from the acid.
The Meraux refinery recently experienced a decline in operating acid purity which we attributed to oxygenates. At the time this problem occurred, we were injecting methanol into our gas con chiller unit to remove hydrates. Our normal practice of internal acid regeneration in the Isostripper did little to improve the acid purity. We could only increase the purity by the use of our external regeneration, and we also found we had to run the external regeneration with a lower top temperature to allow the light compounds to drop out with the constant boiling mixture (CBM) and polymer.
What is the general experience with oxygenates in the alkylation feed without an MTBE unit? What variables in the FCC unit affect this?
TIEMAN: We have seen few instances of oxygen contamination of HF alkylation units without MTBE processes in the same complex. We have had several occurrences when oxygen contamination has occurred, and it was usually traced to alcohol in the saturate butane make-up. On UOP FCC effluents we have seen no evidence Of C3 or C4 oxygenates coming out with the olefins.
STEGELMAN: We did not appear to have a problem before the MTBE unit came on-stream. However, we have found some acetone, which does not appear to come from the MTBE unit since it is not found in the purchased methanol. This was not checked for before the MTBE unit, so we suspect it may have always been coming from the catalytic cracker, although this sounds unusual.
MACKEY: We feel the major source of the oxygenates in our alkylation unit is from the fuel butane stream which we purchase in significant volume from outside sources. We feel that this stream contamination is probably the result of being treated with alcohols at the production facilities to control water. We utilize external acid regeneration to control the water content in the acid to below 1 %. Historically our acid losses have been about 0.16 pounds per barrel of alkylate overall. We do not feel that we get oxygenates through from the FCC unit.
MORGAN: We routinely measure oxygenate levels from our FCC units in the propylene/butylene stream between 10 and 50 ppm. These oxygenates are mainly acetone, isopropanol, and sometimes small quantities of ether compounds. We have not been able to correlate FCC operating variables to the oxygenate levels measured. Additional oxygenates introduced into the alkylation feed from the MTBE unit are dimethyl ether and methanol. Although both of these affect the alkylation unit, we watch the methanol level closely since upsets in the MTBE methanol recovery section can slug significant quantities into the alkylation feed stream.
DAVID L. SMITH (Alcoa): A number of refiners have reported problems with trace oxygenates in their feed to the alkylation unit which have then resulted in ASO problems. An effective method to remove these trace levels of oxygenates, whether there is an upstream MTBE unit or not, is via selective adsorption. Alcoa has developed a product, Selectsorb CD, that can attain effluent levels on the oxygenates well below 1 PPM.
KENNETH PECCATIELLO (W. R. Grace & Co.): We have seen some FCCUs that have acetone showing up in the effluent. It appears from first glance that it can be correlated with reactor temperature, the amount of riser steam going into the unit at the time, and high catalyst circulation rates. These high rates will tend to entrain 02 from the regenerator.
SEALLESS PUMPS
What has been the industry experience with sealless pumps for hydrofluoric acid? How do they com. pare to double mechanical and tandem seals?
RATERMAN: We do not currently use sealless pumps in HF acid service, but we are investigating their potential use. We currently use either a single mechanical carbon on silica carbide seal or tandem seals made from the same material. We use an acid-free iso-butane flush stream to these seals and usually get about 1 year of service from them. In addition to investigating the use of sealless pumps, we are switching some of our carbon on silica carbide tandem seals to a silicon carbide on silicon carbide with an ATF barrier fluid between seals to see if we can improve their performance.
STEGELMAN: Several of our our licensees have had very good service with sealless pumps, such as HMD pumps from a United Kingdom manufacturer. Phillips corporate engineering has reviewed several U.S. manufacturer pump options and likes the design and construction of the Sundyne can motor pumps, type HS with Monel construction. However, we do Not have any operating experience with these pumps in HF service.
TIEMAN: We do not have any experience with the sealless pump in operation, but there is one location where a sealless pump has been ordered and will be put in service in the future.
UTLEY: I believe that industry has gained experience with sealless pumps, but there is still a long way to go. There are still problems with material compatibility. The clearances are tight, so fouling and plugging are problems. Finally, the head and volume are limited, so applicable services are restricted. It is hard to compare the two types of pumps. We still use a single seal with an alfa centered silicon carbide face on both the rotating and sta-tionary face. The stationary face is hydro-padded. We get 1-3 years of service out of these seals depending on the number of upsets and power failures that we have.
We tried double mechanical seals flushed in between the two, and had a lot of trouble with the flush circulating system. Every time we had trouble with this system we backed acid into the flush. This system was on our depropanizer charge pumps with the ability to expand to all of the alkylation pumps. With the trouble we had on the depropanizer pump, we were hesitant to expand it to any other pumps. We eventually changed the depropanizer charge pump back to a single seal.
CARON: At the Sun Sarnia refinery we do not use sealless pumps in HF acid service. We are beginning to install tandem seal pumps for this service.
EMERGENCY BLOCK VALVES
What type of emergency block valves are being used in acid service? How are they being tested? Are they being installed such that they can by fully stroked?
BRANHAM: Ashland's present emergency block valves for acid service are gate valves that are constructed to UOP HF-1 rating. These are equipped with remote electric motor operated actuators. The valves are usually tested weekly. The valves are installed so that they can be fully stroked, but usually a valve's travel is restricted to only about 50%, as this is viewed as an adequate test.
MORGAN: We are using Tufline plug valves for our emergency acid shutoff systems. The valves are installed on both the suction and discharge of all pumps containing significant amounts of HF to protect against seal failures.
These valves are remotely operated and tested weekly. Since the pumps are spared, the valves can be fully stroked. We also install emergency shutoff valves on HF-containing lines such as acid transfer lines and reflux pump discharge piping. Since these lines are spared or in non-continuous service, the valves can also be fully stroked when tested.
STEGELMAN: Durco and Pacific valves have been used satisfactorily by many of our licensees. Some are being installed such that they can be fully stroked on a specific frequency. At Borger, we have also used Tufline valves with good success. At the present time, all of our isolation valves are Durco valves. We are working on a dump system and several other modifications, and we have not yet specified the valves, but they will probably be Durco or Tufline.
LEAK MONITORING
How are people doing ambient monitoring for hydrofluoric leaks in and around their hydrofluoric alkylation unit?
MACKEY: We have installed an atmospheric monitoring system utilizing Gastech HF detectors, model 2321. We have 22 of these monitors placed throughout the alkylation unit in the acid-containing areas. They feed into an alarm board located in the alkylation change room, with a common trouble alarm that goes from there to the main control room. The alarm board consists of the unit layout indicating the sensor locations. The detectors alarm at a low level of 5 ppm, and a high level of 10 ppm. The system requires a quarterly preventive maintenance schedule to calibrate the sensors.
It seems to work acceptably well. We do get occasionally a spurious alarm from significant humidity or temperature swings. The operators have seen satisfactory results with very minor packing leaks, etc.; these alarms picked it right up. Overall we are satisfied with the installation.
BRANHAM: I might add that Ashland has field tested several portable monitors for HF. These include a GMD Systems Autostop portable monitor for HF. This basically is a paper tape type of system and it relatively cheap and easy to maintain. There is also an MDA TLD-1 toxic gas detector. The third one is a Gastech model 2321, which uses a diffusion electrochemical technique. This one is rather expensive.
STEGELMAN: Phillips has felt that elevated color tv surveillance cameras and hydrocarbon detectors have been the best means of providing reliable ambient monitoring for HF leaks in an alkylation unit. This is because an HF leak would normally be associated with a hydrocarbon release. However, with the advances being made in HF detectors recently, we think that it will not be long before very reliable detectors will be available that can be used for this service.
WILLIAM SCHLESING (BP Oil Co.): Mr. Mackey, on the detectors that you are using, at this point, I assume that they are not tied into any sort of a remote shutdown. Do you have any plans on tying them in, in the future, where they automatically shutdown on an alarm?
MACKEY: We went through a very extensive risk management assessment of the unit.
The conclusion we reached is, as far as the automatic shutoffs and so forth tied to these detectors, that we did not feel that it was necessary at this stage. We have the unit manned with two people 24 hr a day, and we wanted them to make an initial assessment of the situation and take appropriate action. So we have not coupled these to shutoffs at this time.
WILLIAM SCHLESING (BP Oil Co.): I assume that the main concern is to avoid potential false alarms and an unplanned shutdown.
MACKEY: That is right.
RELEASE CONTROL
What plans do you have to control possible hydrofluoric acid releases?
BRANHAM: Prevention is the key issue in all cases. Release potential is mitigated by maintaining minimum acid inventory in the unit at all times. Planned procedures include quick isolation of the source and the moving of an acid to a more secure storage location within the unit. Water sprays would be used to contain the HF to the immediate area.
MACKEY: We are also involved with two different levels of water mitigation for any potential HF leak. We have installed a deluge system over all of the rotating equipment that plumbs up a specific shower head that is triggered in the event that we encounter any kind of HF leak. We also have installed remotely actuated and operated elevated fire water monitors that are capable of putting out 1,000 gpm of water from each of five locations around the acid-containing areas of the plant.
Obviously if you have that much water, the containment also becomes an issue. So both our fire water systems and the drainage systems have been beefed up to accommodate the high water flow for a reasonable period of time. We feel that the use of water in this kind of quantity, with this sort of redundancy, is a safe and brute force way to deal with the issues that are very prominent these days. We have never had to use the system in practice other than to demonstrate it for many agencies and other refinery interested parties that have come through our facility to view it. Basically we feel that it is a functional and viable way to mitigate HF releases.
RATERMAN: Mobil was one of the participants in the Industry Cooperative HF Mitigation Assessment Program which conducted JF release tests at the Department of Energy test site in Nevada, near Las Vegas, in 1988. Based upon our experience with these tests, we have opted for a two part protection system in the event of a major acid leak. The major line of defense will be an acid evacuation system which will not rely on a pump to transfer the acid to the safe location. This will be backed up by an effective water spray system which will blanket the entire area. The spray system's design was based upon the desert results which studied the effect of using water sprays to knock down the acid. The results of these tests can be obtained from the U.S. Department of Commerce National Technical Information Service located in Springfield, Va. The report is auth-ored by the above-named group.
SCHAUB: Our safety services division has done Hazop studies and fault-tree analysis for clients with HF alkylation. They worked to-gether with our environmental services division which has done HF cloud modeling on computer, using some of the "what if?" scenarios. We have a lot of experience with these kinds of studies by virtue of our long standing work in the nuclear power industry. We have designed systems for unit surveillance and water deluge, using the test data generated from the study Mr. Raterman just mentioned.
STEGELMAN: The following mitigation measures are currently being considered as a means of reducing the risk attributed to releases of HF acid: (a) the installation of water spray systems, (b) the installation of ambient or environmental monitoring systems, (c) installation of emergency isolation or shutoff valves, if they do not already exist, (d) reducing HF inventory by acid settler compart-mentalization, (e) removal of acid circulation pumps (f) installation of a rapid acid dump or deinventory system, and (g) performing site specific risk analysis to locate any places where you might have the chance of a leak.
I might point out in reducing HF inventory, we have, at Borger, Tex., two identical settlers; one with conventional acid cooler bundles and the other with rod baffle cooler bundles.
We can operate about 1.5 to 2 tricocks, lower level in the settler with rod baffle bundles, and obtain the same acid circulation. So this is a means of reducing acid inventory in the settler.
ELTON C. EUBANKS (Chevron U.S.A. Inc.): Has anyone had a problem with sampling HF with aluminum bombs?
MACKEY: We do not use aluminum. We use solid Monel bombs.
STEGELMAN: We also use Monel bombs.
UTLEY: We do not use aluminum either. We use Monel.
MICHAEL SCHINDLER (Mobil Oil Corp.): Mr. Mackey, when you said that you beefed up your water handling system in case you have to use the elevated water monitors, would you expand on that.
MACKEY: Our fire water system is capable of delivering about 9,000 gpm of water in that quadrant of the refinery. What we did was to dike off the acid-containing areas where we feel that most of the water would wind up flowing in the event of an emergency.
Then we beefed up the sewer system and the neutralization pit where this water would go in order to contain that flow for about 20 to 30 min of time.
DANIEL KNEPPER (Cenex): Mr. Mackey, when you say elevated monitors, how high in the air are they?
MACKEY: They are 30 ft in the air, and are remotely operated by a joy stick, not dissimilar to a computer game. Our operators test all five of these monitors out once a week.
DANIEL KNEPPER (Cenex): What kind of a spray pattern are you using? You are not using a fog, so what is the spray pattern?
MACKEY: The monitors are capable of variable patterns. They can go from a full fog to a direct stream. We have educated and trained the operators to use the proper pattern as they set up. We drill them periodically by simulating a release, etc.
We have a video tape available if you would like to get a copy of it.
DUMP SYSTEM PHILOSOPHIES
What philosophies are refineries using to design hydrofluoric acid dump systems for their HF alkylation unit? Is the dump system for HF or HF and hydrocarbon? Does the system dump to a vented or to a closed receiving vessel? What is the vent pressure if the acid receiving vessel is vented? Should the HF be pressure-transferred or pumped?
RATERMAN: Our design philosophy is to transfer the total acid inventory to a remote vented drum within 5 min. The drum is vented through a caustic scrubber. The drum pressure will depend upon a number of operating conditions, but basically it floats on the scrubber, which is not pressure controlled, but vented to the flare system. Pressure transfer is used so that the dumping will not be dependent upon the operation of a pump.
STEGELMAN: Our current design philosophy for an HF acid emergency dump system for an alkylation unit is to provide for the transfer of acid from the unit to remote storage in 10 min. The dump system is for both HF acid and hydrocarbon. The system dumps to a vented receiving vessel that is initially maintained just above atmo-spheric pressure. A backpressure controller is provided to keep the HF acid above its bubble point, thus preventing overloading of the acid-relief neutralizer during emergency transfer of the acid. Gravity transfer is preferred over the use of sealless pumps. Pressure transfer is the preferred method of moving the liquid from the receiving tank back to the alkylation unit.
TIEMAN: After looking at different systems, we are currently designing, and prefer to transfer by pressure. Although it is rather expensive, we are going to an unvented drum. The reasons are that it is the most simple and reliable system that we could find.
MACKEY: We are designing an emergency dump system utilizing about a 10 min evacuation period for any one of the three major acid-containing vessels. Our design is focused on using a pump because of space limitations, but we do like the idea of having a non pump system by virtue of its simplicity. Utilizing a pump that is seldom, if ever, used is the major critical aspect of a dependable system.
MONEL 400 REGENERATORS
What is the present thinking on the maximum allowable temperature for Monel 400 in an acid regenerator? What is a typical regenerator life span?
GESICK: I do not have any comment on the maximum allowable temperature for Monel 400, but at Superior, our first acid regenerator lasted about 16 years. The second one lasted 11 years. The regenerator at Meraux also lasted 11 years. The Superior system runs about a 500 F. bottoms temperature and a 155 F. top temperature. The Meraux system runs about a 450 F. bottoms temperature and a 140 F. top temperature. The purchase of a sub-stantial corrosion allowance in the nozzles, particularly the small diameter nozzles, can serve to extend the life of the regenerator. Long weld necks, or double extra strong, should be purchased on anything that is 4 in. in diameter or under.
Our experience has been that repair welding on Monel vessels that have been in this service is unpredictable at best. We elected to purchase a new vessel following our last turnaround rather than gamble on a successful nozzle replacement in the vessel. Most of our corrosion in these vessels is in the bottom sections of the towers. When they have failed to the point of replacement, it has been in the bottom section of the tower.
MACKEY: In the recent 4 or 5 years, we have had some fairly aggressive corrosion in our acid regenerator. Our corrosion has been primarily in the trayed area of the column, as opposed to the bottom. We feel that our general operation of the vessel seems to be within normal operating parameters. We have not been able at this point to identify the exact cause and mechanism of the corrosion. We are going to use different tray designs in that portion of the vessel and anticipate extended vessel life.
BRANHAM: Our regenerators are made of solid plate Monel 400, and the expected life is 15 years. We will be shortly replacing one that does have 20 year service. The nozzles and trays are expected to last 5-10 years. The typical operating temperature for our regenerators is usually somewhere around 300-350 F. at the bottom.
CARON: We have experience with two units. In one we limit the acid regenerator to 300 F. with a typical regenerator life span of about 10 years. Our other regenerator operates at about 295 F. This vessel was replaced in 1965, and has been in service since with only minimal repairs.
RATERMAN: Most of the corrosion observed by Mobil has been in the acid feed heater of the regenerator which operates at about 300 F. At temperatures above 350 F., the corrosion rates for Monel in this service will become very high. To avoid high tube temperatures, saturated steam at pressures less than 100 psi should be used in the heater. In addition, pitting of the Monel heater tubes is caused by oxygen at concentrations as low as 1 to 2 ppm, regardless of tem-perature. Oxygen contamination usually results from using acid moved by air rather than acid which has been transferred by nitrogen. On the average, the Monel heaters in this service survive about 10 years or more.
STEGELMAN: Metal temperatures above 250 F. cause accelerated corrosion in Monel 400. Experience indicates the practical limit for alloy 400 in HF service is about 300 F. We design for a maximum rerun tower acid feed temperature of 290 F. Our rerun at Borger has operated for 8.5 years and has had no general corrosion on the shell. There is some roughness where condensables off the trays have run down the wall. There is also some corrosion on the trays.
TIEMAN: Our experience, and that of our licensees, has been similar, with a typical life span in the range of 5-10 years. Our nominal design life is about 10 years. This has been reached by quite a few licensees with good initial fabrication quality and oper-ating conditions.
UTLEY: I agree with what has been said. We feel that the temperature range for Monel 400 is between 160 and 300 F. metal temperature. The process temperatures can be slightly higher. I do not know what a typical regenerator is, but Monel columns should last somewhere around 20 years.
HEROS DERGREGORIAN (Giant Refining Co.): We do not have extensive corrosion in our regenerators, especially in the area of stripping isobutane in the hottest section of the column. I notice that everybody is referring to the problem of the corrosion at high temperature. Can you overcome that by lowering the operating temperature and increasing your stripping isobutane to achieve similar stripping?
STEGELMAN: We have found that we can adjust the stripping isobutane slightly and get some drop in acid rerun feed temperature. We generally do not move our feed temperature over about 10 F. during a long period of operation.
HEROS DERGREGORIAN (Giant Refining Co.): I would like to ask the panel or somebody in the audience to bring me up to date as to the future situation for HF alkylation as far as the problem of elimi-nating it in our future design. Secondly, has there been any development work on an alternative to sulfuric acid alkylation replacing HF?
MACKEY: I presume that you are alluding to some of the legislative efforts with regard to HF processes. In California there is a very aggressive effort now by one of the regulatory agencies to eliminate hydrofluoric acid units in southern California, in partic-ular. They are promulgating some rules to potentially legislate out HF units before 1995. Obviously, those of us that have units out there are very concerned about this. We are working with the agencies to attempt to establish a more rational basis for making conclusions regarding the hazards or the lack of hazards around HF relative to anything else that is used in the refining industry. So, the process is under discussion as far as that goes.
I believe that you are probably also aware that there is also a piece of federal legislation involving HF units that is on a more extended time frame. I believe that it is about 12 or 13 years to phase them out. I think that as an industry, it is critical for us to represent the seriousness of mixing politics and technology about this unit, or any other, in terms of its viability. One aspect, though, that will certainly affect this sort of legislation is any kind of serious accident or significant release of HF, or any other toxic, from our businesses. I think it is particularly critical for everyone in our business to take a very aggressive stand about mitigating potential releases and being sure that the type of systems that are being installed to prevent releases are effective and functional. Additionally, a comprehensive site-specific hazards assessment is very important.
TIEMAN: As to the second part of your question, UOP and Cepsa, from Spain, have jointly introduced the commercialization of a fixed bed, heterogeneous catalyst alkylation process for the alkylation of benzene to form linear-alkyl benzene for detergent production. In that process, though, that reaction is much easier and does not require the activity that the motor fuel application of alkylation does. So, at this time a solid bed system is not being offered for the motor fuel application.
MARTIN W. PERGA (Cenex): My comment is relevant to Mr. Mackey's point on HF safety. We are doing some piping rework in our alkylation unit where we were putting in drain valves and we are going to stress relieve that piping. We had these valves broken apart prior to stress relieving and on the top bonnet of those valves, the bolts were stretched. We measured torques holding these valves together from finger-tight, that is, you could loosen them with your fingers up to 55 lb-ft torque. The bolting was visibly stretched. This was not a micrometer measurement.
I might just caution you that these were new valves tested at the factory. It is a reputable manufacturer. They were not satisfactory, as we were going to put them in the plant.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
