Implementation of an Improved Method for Treating a Sulfuric Acid Alkylation Reactor Effluent Stream at Tosco's Bayway Refinery

Felipe Suarez, Merichem Chemicals & Refinery Services LLC
Michael Berlyant, Tosco Refining Company, Bayway Refinery

Project Background

The alkylation unit is one of the mainstays of the oil refining industry for the production of high-octane gasoline components. The sulfuric acid based process has become the leading alkylation process being applied primarily because of its environmental and safety advantages over the HF acid alternative.

In 1999, Tosco Refining Company decided that it had become necessary to improve the operation of the caustic and water wash treating steps that treat the sulfuric acid alkylation reactor effluent stream. The treaters are operated just upstream of the deisobutanizer (DIB) column where the C4 hydrocarbons are fractionated from the final alkylate gasoline product.

Over the 40 years since the stirred auto-refrigerated alkylation unit was placed in operation, the throughput had been increased 80% above its original design capacity. As a consequence, the caustic and water washing systems could not adequately perform its dual function of removing acidic impurities in the reactor effluent product to assure product quality and to protect the downstream processing equipment such as heat exchangers and the DIB fractionation tower.

In reviewing the options available for improving the caustic and water wash steps, Tosco's objectives were as follows:

• Increase the reliability of the caustic wash system to eliminate acid breakthrough to the water wash system. This would reduce corrosion and plugging problems and thus maintenance costs and downtime in the DIB section.
• Substantially reduce the caustic carryover from the caustic wash to the water wash and water carryover from the water wash to the DIB section. This would reduce the amount of dissolved solids entering the DIB section and thus plugging and downtime. When the alky unit is shutdown, the olefins must be downgraded to fuel or burned as a substitute for refinery fuel. Plugging can also result in poor DIB tower fractionation and reduce the alkylation reaction efficiency.
• Increase caustic utilization in the caustic wash and minimize water use in the water wash. This would reduce operating costs and aqueous waste handling.
• Minimize capital investment needed to accomplish the desired improvements.

Tosco considered the option to replace the existing equipment with larger equipment that employed the mixing/settling process historically practiced. The alternative to change the process using new electrostatic precipitation equipment was also considered. Finally, Tosco selected Merichem's FIBER-FILM^TM Contactor technology to retrofit the existing caustic wash vessel to convert it to the THIOLEX^SM process and retrofit the existing water wash vessel to convert it to the AQUAFINING^SM process. In order to understand the selection process, it is important to first review the technical aspects of sulfuric acid alkylation reactor effluent treating.

Process Considerations

Sulfuric acid alkylation reactors are designed to produce intimate contact between the sulfuric acid (the reaction catalyst), the C3, C4 and C5 olefins and the isobutane (primary reactants) to convert the lighter unsaturated hydrocarbons to iso-paraffins that become high-octane motor gasoline components. The higher the mixing energy, the more complete the alkylation reaction and the higher the octane values. In fact, alkylation reactors are designed to form a tight emulsion between the H₂SO₄ (sulfuric acid) and the hydrocarbon feedstocks, which then creates a difficult separation problem between the final product and the aqueous acid phase.

In the Tosco case, the alkylation reactor effluent contains approximately 300 ppm vol. H₂SO₄ in the form of entrained (free) acid and acid esters, an intermediate impurity formed in the reactor by the following reaction mechanism:

These acidic compounds must be removed ahead of the DIB tower in order to protect this section of the plant from free acid corrosion or acid formation via ester breakdown from the heat applied to the DIB tower feed. The industry has historically used caustic (NaOH) to neutralize these acid impurities and remove them from the product in accordance to the following reactions:

Other alkylation reaction by-products named neutral esters are also formed by the following mechanism:

These compounds can be partially removed in the water wash step if the operating temperature is elevated to 120°F by the following hydrolysis reaction:

The process historically employed by Tosco employed a mix valve and a 14.3 wt % NaOH solution to remove the acid impurities. The caustic wash was followed by a water wash to remove the carryover caustic left in the treated hydrocarbon. In Tosco's case, the caustic wash vessel was only providing seven (7) minutes of hydrocarbon residence time for phase separation; a time period that was insufficient to reduce the hydrocarbon caustic content to its desired level of less than 10 ppm wt as Na⁺. Under Stokes' law, a residence time of at least thirty (30) minutes would normally by needed to separate the small particles of caustic to achieve the desired carryover target. This problem is inherent to any dispersive mixing/settling caustic wash system where small particles of caustic are created to provide the mass transfer surface area for impurity removal. Tosco would have needed to replace the vessel in order to have increased settling time to achieve proper phase separation.

On the other hand, FIBER-FILM^TM Contactor technology employs a non-dispersive mass transfer mechanism in which the aqueous phase is constrained onto a metallic fiber and contacted co-currently with the hydrocarbon phase passing between the aqueous phase wetted fibers. This mechanism does not disperse small particles of the aqueous phase into the hydrocarbon allowing the use of the existing short residence time vessel.

In order to take advantage of this non-dispersive process, the retrofit vessels were provided caustic and water recycle pumps that increase the effectiveness of impurity removal while minimizing the consumption of fresh caustic and water.

Scope of Project

Under the contract issued by Tosco, Merichem was required to supply the technology-related minimum scope of supply which included the Process License, Basic Engineering Design and Proprietary Equipment consisting of FIBER-FILM^TM Contactors, basket strainers, and a coalescer pad. Merichem also provided a Review of Contractor's Detailed Design for Process Performance, Operating and Maintenance Manuals and Startup Assistance.

Tosco sub-contracted the detailed design and construction and procured all new equipment including pumps, instruments, analyzers and relief valves for installation by their sub-contractors.

Process Flow Description

In the system (Figure 1), untreated alkylation reactor (alky) effluent flows through one side of a parallel set of 300-micron basket strainers FL-4 A or B to remove entrained solids larger than 300 microns. The untreated alky effluent then enters the top of the THIOLEX^SM FIBER-FILM^TM Contactor, FFC-1, where it contacts the caustic treating solution to remove H₂SO₄ and acid esters. The hydrocarbon flows co-currently with caustic in the spaces between the caustic wetted fibers. When the caustic and alky effluent exit the Contactor, the caustic continues to adhere to the metallic fibers, which extend down into the caustic phase in the bottom of the separator vessel. The caustic droplets then disperse into the aqueous phase. The alky effluent disengages from the fiber bundle and flows out at the opposite end of the phase separator D-10 to the water wash (AQUAFINING^SM) system.

Fresh 19.61 wt % caustic is added to the system using P-22 A or B on flow control. The fresh 19.61 wt % caustic is diluted to 14.6 wt % with a slip spent water stream cascaded from the water wash using automatic flow ratio control.

The caustic treating solution entering the Contactor flows downward adhering to the fibers until it enters the aqueous layer in the bottom of the phase separator, D-10, where it disengages from the fibers. Most of the caustic which leaves the vessel is pumped back to FFC-1 for recycling using P-10 A or B. Spent caustic is withdrawn from the system via level control to maintain a constant interface level between the caustic and hydrocarbon layers in the vessel. Recycle caustic, fresh 19.61 wt % caustic, and dilution water all enter the suction of P-10 A or B which will mix the three streams. The outlet from P-10 A or B flows through one of the two parallel 150 micron basket strainers (FL-3 A or B) used to remove scale or particulates that could deposit unto the top of the FIBER-FILM^TM Contactor.

The alky effluent leaving D-10 then flows to the inlet of the Contactor FFC-2 where it contacts a mixed fresh and circulated water stream. The water and alky effluent flow downward through the fiber-packed Contactor where any caustic carryover from D-10 is extracted from the hydrocarbon into the water phase. The water in the separator vessel (D-11) is circulated via one of two circulating pumps (P-23 A or B) to the water inlet of the Contactor. Before entering the recycle pumps, the recycled water is first heated in exchanger E-4 using DIB bottoms as the heating medium. The recycled water temperature is sufficiently increased on temperature control such that the combined temperature of the hydrocarbon and water phases is kept at 120°F in the reactor zone of the FIBER-FILM^TM Contactor. This temperature is necessary to promote the neutral ester hydrolysis reaction earlier mentioned. The outlet from P-23 A or B first flows through one of the two parallel 150 micron basket strainers (FL-5 A or B). A slip stream of the recycled water is transferred on flow ratio control for use as dilution water in the caustic wash system.

Fresh make-up water is added to the system using a flow control station. The fresh make-up water is added into the recycle stream prior to the circulation pumps (P-23 A or B). The effluent water is sent to tank D-700, via a level controller on vessel (D-11).

When the water and alky effluent exit the Contactor, the water continues to adhere to the metallic fibers, which extend down into the aqueous phase in the bottom of the separator vessel and is then recycled. The alky effluent disengages from the fiber bundle and passes through a proprietary coalescer pad (SP-1) and leaves the separator at the end opposite from the Contactor. The alky effluent then passes on to the DIB section of the alkylation unit.

Project Completion and Processing Performance

Tosco successfully started up the revamped caustic and water wash units on March 4, 2000 or seven months after the execution of the contract with Merichem. The vessel modifications and final interconnections of the system were completed during a one-month alkylation unit maintenance turnaround. The unit took only 24 hours for full commissioning achieving on spec product through the entire startup period.

During the first year of operation the system performed as anticipated meeting or exceeding the process performance guarantees. The key parameters that indicate actual improvements in the current operation versus the prior operation can be taken from table 1.

Conclusions

The alkylation process will continue to be of great importance to the production of not only high octane gasoline components but also environmentally friendly low sulfur highly paraffinic motor gasoline. Refiners continue to push their FCC units to even higher throughputs and conversion levels generating increasing volumes of unsaturated (olefins) light hydrocarbons used as the primary alkylation feedstocks. As a consequence, alkylation units are being pushed beyond original design capacities at a cost to performance and plant efficiencies. Revamps where existing equipment can be debottlenecked without resorting to the installation of new grass roots process units will usually provide substantial economic value to the capital strapped and profit squeezed refining industry. In Tosco's case, the refinery was able to increase alkylate production by 2,400 BPSD resulting in substantial annual savings for the refinery.

Savings in operating and maintenance costs via retrofit of existing equipment typically provide an excellent payout particularly when technology is being upgraded. Waste reduction programs continue to be stressed because they further reduce operating costs and make the refinery friendly to the communities and its citizens therefore the efficiency of alkylation effluent treating equipment must continue to improve.

The authors: Felipe Suarez is vice president and general manager of the process technology division of Merichem Co., Houston, Texas. He has served as special projects manager, technical services manager, raw materials technical services manager, proprietary technology group manager, assistant sales manager and assistant general manager. From 1969 to 1980, he worked for Murphy Oil, Meraux, Louisiana, with experience in refinery process design and operations. He holds a BS degree in chemical engineering from Louisiana State University in Baton Rouge, Louisiana.

Michael Berlyant is a senior staff engineer at Tosco Refining Company's Bayway Refinery in Linden, New Jersey, formerly the Exxon Bayway Refinery. He provides the day-to-day monitoring and process engineering technical support for the alkylation unit. He received his Masters degree in 1970 from the Yaroslav Institute of Technology in Russia where he was lead engineer. He has also worked for Raytheon Engineering, Chevron in St. James, Louisiana and Exxon Bayway. He has been with Tosco since 1993.