reduce alkylate costs with solid-acid catalysts _ hydrocarbon processing _ october 2007

09/05/12 Reduce alkylate costs with solid-acid catalysts | Hydrocarbon Processing | October 2007

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Reduce alkylate costs with solid-acid catalysts

1 0.05.2007 | Mukherjee, M., Exelus, Inc., Liv ingston, New Jersey ; Nehlsen, J., Exelus Inc., Liv ingston, New Jersey

New developm ents in alky lation processes m itigate risk while reducing operating costs

Key words:

T?he phase-out of methy l tertiary buty l ether (MTBE) leaves refiners searching for methods to keep octane and Reid vapor pressure (Rvp) of blended

gasoline within specifications. The required inclusion of ethanol for reformulated gasoline blends has raised Rvp of summer gasoline. More alky late is

needed to offset the octane losses while meeting vapor pressure requirements. Y et, the alky lation process has safety issues due to the liquid-acid cataly sts

used to process alky late. New developments in solid-acid cataly sts (SACs) remove most safety concerns for this refining operation. Several examples

illustrate possible options for alky lation technologies to meet blending requirements of "cleaner" gasoline.

Changes in gasoline pool. Ethanol is replacing MTBE as a blending stock in gasoline. Unfortunately , ethanol increases vapor pressure of blended

gasoline. Alky late can be used to reduce the vapor pressure of ethanol-containing blends; it also replaces octane loss and volume due to the removal of

MTBE from the gasoline pool. Result: In the US, alky late usage is expected to double over the next few y ears.

Alky late is currently produced using either sulfuric acid or hy drofluoric acid (HF) as the cataly st. Both acids are dangerous and can lead to corrosion-

induced accidents and impede firefighting efforts. Figs. 1 and 2 illustrate several dangers associated with liquid-acid alky lation units.1 ,2 HF is particularly

dangerous due to its ability to form stable aerosols in an accidental release. HF is prone to corrode metallic equipment. Sulfuric acid is less dangerous, but

its use still requires brick-lined vessels and it has high consumption rates. Storage of fresh and spent acid poses additional safety concerns. Sulfuric acid

units also require costly refrigeration to maintain the proper reaction temperature.

Fig. 1 Accidents result from using liquid acids in alky lation processes.

A ruptured sulfuric-acid tank killed an operator and spilled

99,000 gallons of acid.1



Fig. 2 In another accident, a corroded valve on an iso-

stripper led to a fire at an HF alky lation unit.2

SAC alky lation eliminates the hazards and costs associated with using and regenerating corrosive liquid acids. Key to the economic v iability of a SAC

paraffin alky lation process is adequate cataly st stability . Unfortunately , most solid-acid technologies are significantly more expensive than liquid-acid

sy stems and require complex reactor sy stems to compensate for short cataly st life. A new engineered SAC has demonstrated significantly higher stability ;

thus enabling solid-acid alky lation as a competitive alternative to conventional liquid-acid technologies.3

Engineered SAC design. Liquid acids are well-defined chemical compounds with fixed properties. Conversely , SACs have many properties that can be

tuned. For many y ears, solid acids have promised safer and cleaner alky lation. However, the short lifetimes of most solid acids involve expensive processes

with complex reactors and large cataly st inventories; thus making SACs uncompetitive with liquid-acid technology .

To compete with liquid acids, new SACs must be engineered for optimum performance, rather than selected from existing materials. Furthermore, these

cataly sts should be designed to work with practical reactor designs to control final process costs.4 SACs need a long serv ice life while still producing

alky late with an octane rating higher than that achieved in liquid-acid sy stems.

To achieve these results, a new SAC has been engineered on multiple levels as illustrated in Fig. 3.5 The cataly st particle shape and size are controlled to

prov ide the proper reaction environment by manipulating the rate of inter-phase and intra-pellet mass transport. Accordingly , the cataly st reduces

constraints on reactor design, simplifies the process and lowers total costs. The cataly st pore structure has been optimized on both the macro- and micro-

scale to enhance diffusion of large coke molecules out of the cataly st pores, thus reducing cataly st deactivation from pore blockage.

Fig. 3 The new engineered SAC sy stem promotes

alky lation production and mitigates coke

formation on the cataly st.

The strength and distribution of the active cataly st sites are tuned to promote alky lation over coke formation. The active sites are adjusted to facilitate the

formation of 2,3,3- and 2,3,4-trimethy lpentane—both compounds have octane ratings above 100. Cracking and isomerization to dimethy lhexanes (DMH)

are minimized to maintain a high total alky late octane. By carefully tuning the cataly st properties at multiple levels, all desired processing attributes can be

achieved: long serv ice life, high product octane and simple process design.

SAC perform ance. As discussed earlier, long cataly st serv ice under commercially relevant process conditions are essential for the economic v iability of

SAC alky lation processes.4 We have quantified the cataly st serv ice life using a stability parameter (SP), which is an intrinsic property of a cataly st that

depends on the number and density of available active sites and their resistance to deactivation. The effects of SP on operating costs and capital expenses is

shown in Fig. 4. The higher the SP value, the lower are the operating costs and capital expenses. Essentially , a higher SP value indicates a more robust

cataly st that relaxes processing conditions, which allows using a less expensive process design. In relation to the sulfuric-acid alky lation process, a solid-

acid alky lation cataly st with an SP value of 0.002 or lower is not economical.



Fig. 4 Operating and capital costs for various SP

values.

To compete favorably with liquid-acid cataly zed alky lation processes, solid-acid processes must have an SP > 0.002. However, most SACs have SP values

that are significantly lower (about 0.0005?–?0.001). This explains why solid-acid alky lation has been uneconomical as compared to liquid-acid sy stems.

New engineered SACs must outperform conventional solid acids to compete with liquid-acid sy stems. Fig. 5 compares the engineered cataly st to many

different conventional solid acids. All isoparaffin alky lation tests were conducted in the same reactor under identical feed composition, space velocity ,

reactor pressures and particle sizes but over a range of temperatures since the modified zirconia and chlorinated alumina cataly sts work best at 20°C?–?

30°C, while the zeolite cataly sts perform best around 7 0°C?–?90°C.

Fig. 5 Long cataly st serv ice life can be obtained when

using an engineered SAC.

The new engineered cataly st sy stem exhibits an SP > 0.004. It can achieve a better operating performance over conventional SACs and even reduce costs

compared to sulfuric acid sy stems.

The engineered SAC produces alky late with a high octane rating over a wide range of olefin feeds, operating temperatures (60°C?–?90°C), olefin space

velocities (0.1 to 0.5 1/hr) and feed composition (I/O ratios from 10 to 15). As summarized in Table 1 , the process robustness minimizes feed pretreatment

requirements. The octane values were computed using gas chromatograph product analy sis, and verified with independent engine testing.6 ?The cataly st's

unique design allows processing a wide variety of feedstocks while maintaining high total product quality .

Alky lation with isobuty lene. Alky lation using isobuty lene has posed technical challenges for many y ears. Although HF units are capable of processing

isobuty lene into high-quality alky late, sulfuric acid sy stems do not. Isobuty lene-containing feeds contribute to high acid consumption and as much as an

eight-point drop in RON in conventional sulfuric-acid alky lation sy stems.

Excess isobuty lene supplies from the MTBE phase-out have caused some producers to switch to olefin dimerization or indirect alky lation (dimerization

followed by hy drogenation). An alky lation process capable of handling isobuty lene-containing feeds offers considerable advantages over these process

options. Using isobuty lene can offer a 10- to 12-point MON advantage over the dimerization product, and can double production capacity for a given

amount of olefin. This leads to a tremendous gain in octane-barrels. Such an advantage is particularly important to MTBE producers now facing declining

demand and considering switching to another technology to utilize isobuty lene.

The ability to directly alky late isobuty lene into a high-quality product has great potential. The engineered SACs have the capability to process feeds

containing from 0% to 100% isobuty lene at temperatures of 7 0°C?–?90°C. As shown in Fig. 6, the resulting alky late maintains a high octane rating.



Fig. 6 Alky late octane y ielded when reacting

isobuty lene-containing feeds.

T he process. The new alky lation process using the engineered SAC is shown in Fig. 7 . Two multi-staged fixed-bed reactors are used in the reaction section.

One reactor is used for reaction while the other is being regenerated. The olefin stream is mixed with isobutane returning from the distillation section and

with the reactor effluent recirculation stream before being fed to the reactor. The alky lation reaction is mildly exothermic. The robust cataly st is insensitive

to small changes in temperature; the heat of reaction is removed by a heat-exchanger located on the recy cle loop outside the reactor. Table 2 summarizes

ty pical process conditions for the SAC process.

Fig. 7 The SAC alky lation process uses two fixed-bed reactors—one

online and the second on regeneration.

Cataly st regeneration is done using a circulating loop of hy drogen/hy drocarbon mixture at an elevated temperature of 250°C. The SAC does not require

any neutralization or washing equipment to post-treat the alky late product. Corrosion-resistant materials are not required. This simple flow scheme results

in low capital costs and the flexible ability to retrofit existing plants.

Alky lation cy cle lengths are designed for 12?–24 hours to simplify reactor operation. After the alky lation cy cle, the reactor is taken off stream and the SAC

is regenerated. During this time, the second reactor maintains the constant alky late production. Due to the small coke buildup on the cataly st surface,

hy drogen consumption is kept to a minimum.

Econom ic and environm ental benefits. The engineered SAC can also offer significant capital cost sav ings relative to liquid-acid units—especially

sulfuric acid. The sav ings are from multiple sources. First is eliminating corrosive acid from the process. Removing the liquid acid eliminates the acid

neutralization equipment, product washing vessels and storage tanks for fresh and spent acids. Equipment design is also simplified, thus eliminating the

need for brick-lined vessels and proprietary contactors. Second is the change in process conditions. The SAC operates at 50°C?–?100°C, and the heat is

supplied by the chemical reaction. Sulfuric-acid units require refrigeration and operate around 5°C, which requires expensive compressors and

refrigeration loops. Eliminating refrigeration also considerably lowers power costs. Table 3 summarizes capital expenses, raw material consumptions and

utility requirements.



Differences in cataly st regeneration procedures y ield considerable sav ings, which may be considered either capital or operating costs. The SAC is

regenerated in the reactor with hy drogen; thus, no waste is generated. Sulfuric acid requires a large regeneration plant that is either operated onsite (large

capital cost) or by another party (large operating cost).

Compared to other solid-acid technologies, engineered SACs can offer sav ings due to simplify ing operation and less processing equipment required. For

example, the engineered SAC requires only two reactors due to the long onstream cy cle time. Longer cy cle times enable complete regeneration of the off

stream reactor, thus lowering equipment cost. The ability to perform a complete regeneration (at 250°C) after each cy cle also reduces the precious-metal

content of the cataly st to a fraction of that required by other SACs, which are ty pically reactivated at reaction temperatures of 25°C?–?80°C. The reduction

in precious-metal content contributes capital sav ings of $50 million for a 10,000-bpsd plant and reduces the financial risk associated with large precious

metal inventories.

Low cataly st stability hinders the total process design. Having additional reactors or cy cling the solid cataly st in and out of the reactor are methods to

artificially reduce the space velocity .8 ,9 However, compensating for poor cataly st performance using these methods cannot ultimately result in better

economics. Rather, additional capital costs v ia using more or complex vessels occurs, and higher operating costs result due to very high cataly st

inventories.

Another important, but more abstract, sav ings is the reduced risk. SACs are inherently safer than liquid acids since they are noncorrosive. The transport,

storage and use of sulfuric acid for alky lation have led to numerous accidents and fatalities. The presence of sulfuric acid is responsible for the direct risk of

exposure and complicates firefighting efforts. Spill control protocols and equipment must be in place when using sulfuric acid. HF is even more dangerous.

A SAC eliminates these risks and the associated equipment, planning and liability .

Process dem onstration. The engineered SAC performance was validated in a pilot unit designed to simulate a commercial reactor design.1 0 The key to

successful piloting is not simply using a large reactor or processing vast amounts of feed. Rather, a well-designed pilot plant must achieve the same

behavior as the commercial unit as represented by matching a number of key reaction and transport parameters. The pilot plant for this process was

developed to match the expected commercial conditions. Table 4 compares the critical scale-up parameters for the two reactors. From Table 4, the scale-

up parameters are perfectly matched enabling a seamless scale-up from the pilot unit to the first commercial-scale plant.1 1

The product octane and cataly st activ ity obtained over the pilot test is shown in Fig. 8. Variations in octane resulted from altering process conditions

including feed composition and olefin space velocity . The octane values obtained are slightly lower than those reported in Table 1 because the test was done

using an untreated MTBE raffinate feed that contained over 3,500 ppm of di-olefins, 7 00 ppm of oxy genates and 0.6 ppm of acetonitrile. Cataly st stability

was confirmed by measuring the cataly st performance through multiple cy cles of alky lation and regeneration using hy drogen. Alky lation cy cles were

ty pically 12 hours, followed by a two-hour regeneration cy cle. The onstream time reported here refers to the cumulative time for the alky lation cy cles

only .

Fig. 8 The results of the pilot study indicate high

octane product at various process conditions.

As ev ident from Fig. 8, there is no activ ity loss over repeated cy cles of alky lation/regeneration as measured by olefin conversion, product octane or

alky late y ield. The product octane is significantly higher than that obtained with liquid-acid processes under similar operating conditions and feedstocks.

Revam p opportunities. The small-capital requirements of this process lead to numerous opportunities to replace older units:

Liquid-acid alky lation units can be converted to SAC operations by replacing the reaction section with two fixed beds. The separation train is retained.

An added attraction of the SAC process is the ability to use some feedstocks without pretreatment.

Cat-poly and olefin dimerization units are particularly well-suited for retrofit. The existing reactors can be used with only minor modifications. A de-

isobutanizer is added to the separation train.

MTBE unit replacement. Alky lation is a valuable addition to any MTBE facility by converting the unused n-butenes into additional high-value product.

However, MTBE is being phased out in certain countries, notably the US. Current isobuty lene producers can replace MTBE production with alky lation

without sacrificing capacity . The SAC process is capable of directly alky lating isobutane with isobuty lene to produce high-octane alky late.



Given the current push for eco-friendly processes to produce ultra-clean fuels, the engineered SAC technology can cover the void in the gasoline

market. HP

Nom enclature

= Cataly st effectiveness factor

kIn t = Intrinsic kinetic rate constant, 1/s

= Residence time, s

dp = Particle diameter, m

v = Superficial velocity , m/s

= Fluid v iscosity , kg/m/s

= Fluid density , kg/m3

Cp = Fluid specific heat, J/kg/K

k = Fluid thermal conductiv ity , W/m/K

ks = Inter-phase mass transfer coefficient, m/s

D = Fluid diffusiv ity , m2 /s

h = Fluid heat transfer coefficient, W/m2 /K

Dz = Axial dispersion, m2 /s

L = Reactor length, m

LIT ERAT URE CIT ED

1 US Chemical Safety and Hazard Investigation Board, "Investigation Report: Refinery Incident, Motiva Enterprises, LLC," Report No. 2001-05-I-DE,

2002.2 US Chemical Safety and Hazard Investigation Board, "Case Study : Oil Refinery Fire and Explosion," Report No. 2004-08-I-NM. 2005.3 D'Aquino, R. and L. Mavridis, "Solid-acid cataly sts shape up for alky lation," Chemical Engineering Progress, January 2007 , pp. 8–9.4 Mukherjee, M. and J. Nehlsen, "Consider cataly st developments for alky lation production," Hydrocarbon Processing, September 2006, pp. 85–96.5 Nehlsen, J., M. Mukherjee and R. Porcelli, "Apply an integrated approach to cataly tic process design," Chemical Engineering Progress, February

2007 , pp. 31–38.6 Hutson, T. and R. Logan, "Estimate alky late y ield and quality ," Hydrocarbon Processing, September 197 5, pp. 107 –110.7 "Refining Processes 2002," Hydrocarbon Processing, November 2002, p. 86.8 D'Amico, V., et al., "Consider new methods to debottleneck clean alky late production," Hydrocarbon Processing, February 2006, pp. 65–7 0.9 Meister, J. M., et al., "Optimize alky late production for clean fuels," Hydrocarbon Processing, May 2000, pp. 63–7 5.1 0 Mukherjee, M., et al., "Scale-up strategy applied to solid-acid alky lation process," Oil & Gas Journal, No. 104, Vol. 26, 2006, pp. 48–54.1 1 Jackson, K., "HPIn Construction," Hydrocarbon Processing, March 2007 , p. 28.

T he authors

Mitrajit Mukherjee is the founder and president of Exelus. He has spent the better part of his 17 y ear career developing v iable SAC solutions for

isoparaffin alky lation processes. Prior to starting Exelus, he held positions at Cataly tica and ABB Lummus Global. Mr. Mukherjee holds a BS degree in

chemical engineering from the Indian Institute of Technology and an MS degree from Southern Illinois University . He is a member of the American

Chemical Society and the American Institute of Chemical Engineers. He can be reached by e-mail at [email protected].

Jam es Nehlsen is a senior research engineer at Exelus specializing in the development of new cataly tic processes. He holds a PhD in chemical

engineering from Princeton University , where he developed new desulfurization technologies. Dr. Nehlsen is also a New Jersey Science and Technology

Fellow. He has authored or co-authored seven technical publications on desulfurization and fuel cell dy namics, and is a member of the American Chemical

Society and the American Institute of Chemical Engineers. He can be reached by e-mail at [email protected].

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reduce alkylate costs with solid-acid catalysts _ hydrocarbon processing _ october 2007

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