Catalysts for light-ends refining processes were the focus of much discussion at the most recent National Petroleum Refiners Association Question and Answer Session on Refining and Petrochemical Technology.
The topics discussed included:
- Optimization of hy drotreating catalyst performance
- New developments in alkylation catalysts
- Recycling of reformer catalyst
- Effects of sulfur on etherification catalysts.
This is the second in a series of abstracts of the transcript of this meeting. For details on the format of this meeting, held October 4-6, 1995, in San Antonio, see Part 1 of this series (OGJ, June 3, p. 49).
Hydrotreater pretreating
What factors are involved in the determination of the optimal design for a trash collection and liquid/vapor distribution system at the top of a hydrotreating reactor bed? What on-line techniques are available to measure or determine the flow distribution in a reactor?
Van Iderstine:
Some factors to consider to minimize fouling on the top of a hydrotreating reactor are as follows: feed storage in gas-blanketed tanks should prevent polymers and gums that could cause equipment and catalyst bed plugging; feed filtering to 25 will remove particles that could cause bed plugging; and grading of the top of the catalyst bed by size and activity to ensure adequate filtering and flow distribution and to prevent catalyst overactivity in the top of the bed.
In our atmospheric resid desulfurization (ARDS) unit, we have moved away from trash baskets in our second in-line reactor, but we have left them in the guard reactor to date. However, we expect to move away from trash baskets in the guard reactor also as the developments in graded beds advance. Many of the catalyst companies offer graded catalyst bed advancements.
As for on-line monitoring for flow distribution, we utilize bed thermocouples and monitor the temperature radially across the reactor. We look for any differences in the radial bed temperatures which could indicate flow maldistribution. In light oil hydrotreaters a radial distribution of less than 5 F. would be expected. In heavy residual units we believe that the differential could be as high as 20 F. without having significant maldistribution. Anything beyond that could indicate a problem.
Shen:
There are two methods generally practiced for trash collection or to reduce pressure drop buildup from particulates in the feed. One is to use trash baskets. These are cylindrical baskets made of screen wire or Johnson screen. Their purpose is to provide additional surface area or paths for flow when flow is restricted by particulate buildup.
Usually the particulates are small particles of iron sulfide scale with coke. Because these particles are small, in the 1 to 5 range, the baskets do not act as filters. The catalyst itself does the filtering.
While trash baskets can help reduce pressure drop buildup, they do cause maldistribution in the first bed of the reactor. For this reason, some refiners prefer not to use them. If the feed is low-sulfur, and the heater and feed/effluent exchange metallurgy is resistant to sulfur corrosion, and there are no particulates in the feed, such as coke particles from coker stocks, trash baskets may not be needed.
The second method is to use a graded bed of different catalyst sizes. Large particles spread out the particulates further into the bed, thus avoiding pressure drop buildup. Switching the entire bed, say, from 1/16-in. to 1/8-in. catalyst helps, but at the expense of catalyst activity. For this reason, only a portion at the reactor top is larger catalyst as part of a graded bed approach.
To further reduce pressure drop buildup, the use of hollow cylinders in a graded bed provides additional void space between the catalyst particles for the accumulation of particulates. For those feeds that are reactive and prone to polymerization and coke buildup, refiners have used low activity catalyst material at the top of the first catalyst bed. By doing this, any coke and polymer buildup is spread throughout the low catalytically active material instead of deposited in a small layer at the top of active catalyst and plugging off the bed.
Radioactive tracers can be used to determine the reactor residence time distribution and therefore how close the reactor is to the plug flow (OGJ, May 23, 1977, p. 61).
Good vapor-liquid distribution across the reactor cross sectional area is essential for best catalyst performance. Flow distributors of various designs have been used to achieve uniform flow. Some are clearly better than others. One study found some distributor designs were very sensitive to out-of-level trays, whereas others were not.
The larger the reactor, the more difficult it is to ensure the tray will be truly level, so this can be an important factor. As a guideline, if the radial temperature difference exceeds 10 F., the distributor should be looked at.
Abrahams:
I wanted to add one experience we had with a unit that processes coker naphtha along with other intermediate naphtha from our crude unit. There is a small, radial guard bed reactor before the downflow reactor, and the radial reactor would foul heavily. We replaced all the catalyst in it with active support balls to give us more void space, and we saw our run lengths double before the delta P got too high.
Hansen:
On our resid hydrodesulfurization (HDS) unit we do not use trash baskets; rather, we depend upon gradient bed loading.
Johns:
A lot of units are taking trash baskets out and going to graded beds. On some units, especially lighter ones, this is good. There are a few rules of thumb for design of the trash baskets.
There should be as many trash collection baskets as possible, and they should be as big as is practical, for dirty service. Typically, we like to space baskets 1 ft apart. Each basket is generally made of woven wire mesh or Johnson screen. Standard mesh is 9 x 9/sq in. (1/8 in. between wires) with 1/16 in. wire. Baskets are 4 in. diameter, 12 in. to 18 in. long.
Liquid-vapor distribution needs to provide as equal a distribution of liquid as possible at all expected operating conditions. Drip points, distributor caps, or holes should be no more than 10 in. to 12 in. apart. Vapor will distribute itself across the catalyst bed by pressure drop; good vapor distribution usually requires an inlet cone to absorb its inlet downward momentum.
Multiple thermocouples within each bed (three or four vertical thermowells with at least three thermocouples per thermowell per bed is usually reasonable) can provide an indication of radial temperature gradients, which is the usual indication of a flow maldistribution.
Keller:
To obtain uniform gas and liquid flow in a catalyst bed, the gas and liquid must first be evenly distributed across the reactor. This is achieved by using a well designed and properly installed gas and liquid distributor at the top of the reactor bed. To maintain uniform gas and liquid flow throughout the catalyst bed, the catalyst must be properly loaded to achieve uniform void fraction and pellet orientation.
Ultramar has obtained the best results for trash collection at the top of the catalyst bed by using graded bed materials. Graded bed materials are available in a wide range of sizes, shapes, and activities. They have more voids than typical hydrotreating catalyst to hold trash. The optimum graded bed design is unique for each unit depending on feed contaminant type and level, unit constraints, and unit run history. We have had good success using Haldor Topsoe Raschig rings.
Some companies, such as Tru-Tec Inc., offer on-line isotopic tracer studies to analyze the liquid flow pattern in a reactor. Catalyst bed and reactor skin temperatures also provide some indication of flow distribution (or maldistribution) in a catalyst bed.
Charles S. McCoy (McCoy Consultants):
I agree with the graded bed comments. The trouble with trash baskets is that they extend the shutdown longer than they extend the run length.
Brian M. Moyse (Haldor Topsoe Inc.):
I basically agree with the panel's comments, and wanted to add a couple of items. One of them is obvious. Clearly you do not want to use your reactor to collect trash, so you should do everything possible to remove any problem materials upstream. If you do have problems anyway, Mr. Van Iderstine gave a very good description of what you should look for. I would add that you would also be well served by looking at catalyst samples taken from earlier runs and examining the deposits in your trash baskets.
Finally, on the point about distribution, we have worked with Tru-Tec looking at distribution, and we had one real success and one mediocre result from that. But it was an interesting technique and distribution these days is very important as we push the units harder.
Rick Bertram (UOP):
It has been our experience that graded beds do help minimize the impact of fines buildup in the top bed. Unfortunately, this is only a partial answer. To achieve the 3 to 4-year cycle lengths required by many of our licensees, cartridge or backwash filters for the feed are necessary.
Regarding reactor flow distribution, several Unicracking units have been retrofitted with Gaysco thermowells. This has increased the thermocouple density at the bottom elevation of the catalyst beds to 16-20, as opposed to 3-6. With the additional thermocouples we can get a much more accurate measurement of radial temperature profiles and flow
distribution.
Johns:
I will follow up and say that on most all of our reactors in heavy-duty service we still use the trash baskets and graded top fill as well.
Alkylation "co-catalysts"
Alkylation unit co-catalysts are being used more frequently. What yield, octane, and acid consumption benefits have been seen?
Abrahams:
One of our plants is currently evaluating Betz's Alkat-XL co-catalyst. Preliminary results of actual use on the unit indicate the following: yield has improved 3% to 4%, acid consumption has been reduced by 19%, the octane number has increased 0.7 numbers, and the T90/95 and end point have decreased some.
Other than the acid consumption, the results require the use of modeling techniques to quantify the benefits. For this reason, we have been actively pursuing trials at our two sites that incur outside regeneration costs and have been less energetic where we have internal regeneration facilities. The improvements also increase isobutane use, so
the benefits are lower if you are isobutane limited.
Emanuel:
We have also introduced the co-catalyst at our Port Arthur refinery within the last 6 months. While using the co-catalyst, we have seen that acid consumption has decreased by approximately 10%, and there appears to be a very slight increase in our alkylate yields. But we have not been able to really quantify the yields or octane benefits at this time.
Frondorf:
We have also tested the Betz product over a period of time. I can give you some vendor numbers and then where we fit in.
Yield improvements are reported anywhere from 3% to 7%, acid consumption improvements anywhere from 10% to 27%, and octane improvements anywhere from 0.3 to 0.7 numbers. For the trials that we have run over a period of time, our results have fallen within those ranges.
The system is set up to model your unit using the neural network type of modeling effort, so there is a lot of data that need to be collected. We have worked with Betz over the past 2 years and have run two 60-day trials in that period of time.
Even with all that data and the modeling effort, it is still difficult to measure improvements due to the scatter and the changes on the unit. It would appear to us that the benefits will be most important to those units that are run what I would call severely, meaning outside of the new design recommendations from Stratco, and where you will have more opportunities for improvements.
HF additives
What has been the recent operating experience with HF alkylation additives to improve the vapor pressure characteristics of the circulating acid stream?
Johns:
Alkad operations are continuing at Texaco's El Dorado, Kansas HF alkylation unit. Additive operations since September 1994 have confirmed that the alkylate octane and distillation have improved compared to operations without the additive with similar rates and C3/C4/C5 feedstock compositions. Inspection of the unit during the April 1995 turnaround confirmed that there were no significant changes in corrosion or fouling compared to prior inspections.
Parker:
Phillips and Mobil have been jointly developing a reduced vapor pressure liquid catalyst alkylation process, Revap. Phillips is currently completing detailed design to convert the HF unit at its Woods Cross, Utah, refinery to the Revap technology. My understanding is that Mobil is also in the detailed design phase to convert the HF unit at its Torrance, California refinery.
A detailed paper on this Revap process was presented at the API Operating Practices Symposium on May 9, 1995. This paper includes cost and yield information along with the general description of the design changes needed.
Solid acid catalysts
Do any of the solid acid catalyst systems under development show significant economic advantage versus H2SO4 or HF alkylation? If so, when will they be commercialized?
Parker:
During the research process, we looked at solid catalyst and concluded that a liquid catalyst system was superior. Major problems that we observed with solid catalyst systems were short run lengths between regenerations, the inability to regain activity on the catalyst after regeneration, and high capital investment and operating costs.
Costs are high because you need large volume solid beds, and heat removal equipment for the highly exothermic reaction is expensive. You also need regeneration and solids handling equipment.
As for comparing the operating cost, the catalyst cost at our Phillips unit at Sweeny is running about 5 cents/bbl of alkylate. Our acid losses are about 0.08 lb/bbl of alkylate. We are currently running about 3 years between turnaround intervals and are considering moving to 4 years based on the condition of the unit. I would add that we have pretreatment in front of that unit and it seems to make a big difference.
Solis:
We are following very closely the work that several groups are carrying out for the development of processes using a solid catalyst for motor gasoline alkylation. I would like to say that this question already has an answer for other types of petrochemical alkylation.
Cepsa, through its affiliate Petresa Canada, has started up the first linear alkylbenzene Plant (LAB), where benzene is alkylated with C10 to C12 olefins in a solid bed catalyst reactor system. The solid bed alkylation unit has a capacity of 75,000 tons/year of LAB and has been installed at Becancour, Canada. The plant, started up in May 1995, is operating today at design capacity.
Philip Geren (Haldor Topsoe Inc.):
The fixed-bed alkylation process which is being commercialized by Haldor Topsoe appears to have about a $1.10 to $3/bbl of alkylate economic advantage when compared to sulfuric acid alkylation. In answer to the second part of the question, the status of commercialization is that there presently are about 20 companies, under secrecy, evaluating the technology for specific applications. Three of these could be considered to be hot prospects. And we have reason to believe that it may be possible to sign the first license agreement in this calendar year or early in 1996.
The three hottest prospects are for retrofit of equipment to existing catalytic polymerization, HF alkylation, and sulfuric alkylation units to practice the Topsoe process. The process employs a liquid super acid that is dispersed upon the surface of a particulated solid substrate in a fixed-bed reactor.
W. Robert Epperly (Catalytica Advanced Technologies):
As reported earlier, Catalytica, Conoco, and Neste have operated a 7 b/d pilot plant for over 5,000 hours. Progress has been made toward our target of a 3 cents to 4 cents/gal advantage, based on bench-scale results.
Further work is needed over the next 2 years to reach our targets and to have the technology ready for licensing. We are having discussions with several companies, under confidentiality, to join our partnership.
Frank Himes (UOP):
UOP is continuing to develop the solid catalyst system previously announced to the industry in this and similar forums. The material is a solid phase regenerable catalyst and UOP is in the process of confirming product quality, yield, catalyst stability, and similar parameters in our pilot plants.
Earlier this year, an initial detailed design was completed and costs were developed by a major contractor. Based on these estimates, the technology is competitive with current alkylation technology. While certain technical achievements remain to be accomplished, UOP expects to be in a position to support a commercial design by early 1996.
David Parnell (TPA Inc.):
What happens to the quality of the alkylate from these solid catalyst processes? Do the octane values stay basically the same as they are with the liquid acids, or do the octane values get better or worse? Also, what happens to other properties of the alkylate? What is the expected life of the catalyst or cycle time for the regenerable type
catalysts?
Philip Geren (Haldor Topsoe Inc.):
As the reactions are the same, the octane quality is approximately the same as with sulfuric and HF acid. Yield is a function of how much or how little acid soluble oil byproduct is made. And in a fixed-bed process with short residence time during which the olefins are within a reactive zone of a fixed bed, the acid-soluble oil (ASO) production is about one-fifth of that which is expected in a sulfuric acid plant.
Frank Himes (UOP):
We are targeting similar octane and yield to the liquid catalyst systems, and we think we will be able to make that.
Reforming catalyst
Have continuous reforming operators found it economical to separate the "whole pills" from the dust collector fines and reuse as makeup to the unit? If so, what separation method was used? Were the whole pills regenerated ex situ before being added back to the unit and if so, how?
Keller:
No, it is not economical for us to recover whole pills from the fines. We operate at the UOP design elutriation gas rate. Our CCR fines loss is about 8 lb/day. Chips and whole pills makeup 25 wt % of the fines. Our potential incentive to recover whole pills is less than $15/day before handling and regeneration costs.
Emanuel:
We do not separate the whole pills and return as makeup.
Frondorf:
Our Corpus Christi refinery has been recovering whole catalyst pills from fines for many years. We do find this recovery to be economical on the whole pill percentage, typically 10% to 15% of the amount of fines retained before shipment back for recovery. A lot depends on the ability to seal and store the fines in a dry condition and the ability of the screen operator to effectively make the cut between whole catalyst and chips.
We purchased a small portable screener a number of years ago to do this, so we have already spent that small amount of capital to do that. In our case, we do not regenerate the recovered catalyst prior to reuse, although this is desirable. We load this catalyst directly into the surge drum.
Ric Zima (Ashland Petroleum Co.):
We hold our CCR catalyst sweepings and fines until we have between 20 and 30 drums, then we send them down to CRI to undergo a density grading. We have found it economical, and we do reuse the whole pill catalyst. The dust is sent out for platinum recovery. The whole pill catalyst is regenerated ex situ and returned to makeup inventory for reuse.
Angelo Furfaro (UOP):
UOP commercial feedback from licensees is mixed. Most return the dust collector material for metals recovery, and several have tried separating the whole pills from the fines. UOP recommends that all material (fines, chips, and whole pills) collected from the dust collectors be returned for metals recovery.
UOP is aware that a number of refiners choose to screen the material collected from the dust collectors and use the whole pill fraction as makeup catalyst. There are two risks associated with screening this material and adding the whole pill fraction of the dust to the unit as makeup catalyst.
First, the screened whole pill fraction usually still contains chips and dust, which lead to regenerator screen plugging, reactor centerpipe plugging, and/or catalyst transfer problems. On more than a few occasions, this practice has been identified as a leading cause for such problems.
Second, the whole pill fraction recovered will typically contain 4-5 wt % coke. If this material is used as makeup catalyst, it will return to the regenerator with approximately 8-10 wt % coke. Without an adjustment in regenerator conditions, this catalyst will be damaged when it passes into the high oxygen areas of the regenerator. This can result in phase-damaged catalyst, which is very soft and inactive. The result is usually a significant increase in fines (which means more catalyst makeup will be required) with faster screen plugging and more catalyst transfer problems.
However, in response to Mr. Frondorf's suggestion, if the refiner has invested in the hardware to make the proper separations, UOP recommends that the whole pills be screened several times to ensure no smaller diameter particle material (chips or dust) remains. UOP then recommends that these whole pills be sealed in air-tight warehouse storage until the next regenerator screen cleaning.
During screen cleaning the recovered whole pills can be added back to the disengaging hopper manway so that the catalyst coke is burned before the catalyst returns to the reactors. UOP believes this is preferable to ex situ carbon burning the recovered whole pills since the conditions are sure to be properly controlled in the regenerator.
Etherification catalysts What is the impact of various sulfur species on deactivation of resin catalysts in methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME) units?
Solis:
The impact of sulfur compounds on the resin is a function of the chemical nature of the compound. For instance, Lewis bases are attracted to the acid sites and tend to neutralize the site by forming salts on the surface of the resin.
Metal sulfides would ion exchange and deactivate the resin with the metal cation components. If, however, the sulfur species was not basic and offered no metal for ion exchange, it is likely that it would flow through the resin without impact.
Abrahams:
Analysis of spent catalyst from both our MTBE and TAME units indicates that sulfur is accumulating through the run.
We do not have information on the effects of different sulfur species, but in total the analysis shows that sulfur accounts for between 8% and 18% of the active site deactivation. This indicates to us that nitrogen compound poisoning is not the only contributor in catalyst deactivation.
Hansen:
It is our understanding that the sulfur species and levels normally found in MTBE and TAME unit feeds that originate from FCC units are not normally reported to be harmful. H2S, while not usually present, has been reported to be a catalyst poison when it reacts with acid sites, rendering them unavailable. This poison can only be removed by acid regeneration or catalyst replacement.
Steven L. Stieg (Rohm & Haas Co.):
As the gentleman mentioned, the more basic sulfur species, the dialkyl sulfides, have been found to cause deactivation of polymeric catalysts. This work is covered in patent literature and, to reiterate, a thorough spent catalyst analysis can identify the routes of deactivation of that polymer catalyst.
David L. Smith (Alcoa Industrial Chemicals):
After conferring with the various etherification catalyst suppliers and the licensors of MTBE and TAME processes, I have concluded that there is no consensus on whether the various sulfur compounds deactivate the catalyst. But in the event that there is concern, we have a product called Selexsorb CDX that has been used to remove dimethyl sulfide, dimethyl disulfide, and mercaptans, as well as propionitrile from the isoamylene stream.
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