Eliminating Chloride Corrosion and Fouling
of Reformer Stabilizer Columns

Reprinted from Hydrocarbon Technology International

F. J.  Suarez, Merichem Company

In the oil refining industry, cycloparaffins contained in virgin straight run naphtha streams are dehydrogenated to aromatics using catalytic processes known as reformers. The product from these units is then used as a high octane motor fuel blending agent or its components are separated for their petrochemical value. In order to maintain the required catalytic activity, reformer units employ a continuous addition of organic chloride compounds to the reactor system. These compounds are cracked into hydrocarbons and hydrogen chloride (HCL), causing various operating and maintenance problems. This article looks at a new method for preventing the ravages of these compounds.

The HCL produced in the reformer reactor is distributed between the gas and liquid phases in the reactor products separator. As expected, the bulk amount stays in the gas phase where it creates a separate set of problems. Minute amounts of HCL mainly in the 1ppm to 15ppm weight range, remain in the liquid product which passes on to the stabilizer column for the fractionation of LPG and light gases as the overhead product from the liquid gasoline bottoms fraction.

The stabilizer feed also contains traces of water and ammonia which, along with the trace levels of HCL, produce undesired side reactions and the bulk of the operating and maintenance problems associated with stabilizer operation.

The HCL is deionized by the water to become a very acidic species which attacks the carbon steel components, particularly distillation trays in the fractionation column. Ammonia reacts with the chloride ion to form deposits which become part of the iron chloride scale that lays on the distillation trays. Any HCL not deionized breaks through the column and can make the offgas and LPG streams corrosive as well, often making them unfit for fuel use without additional treatment.

Depending on the amount of impurities present in the feed and other factors of which the writer is not aware, the net result is costly corrosion, and plugging and downtime of the fractionator and perhaps the entire reformer unit. In addition, an investment for overhead products treatment is usually necessary.

Despite the fact that the reforming process has been a mainstay in the oil refining industry for many decades, the source of the problems created by these chlorides has not been very well addressed. Instead, the approach has been to deal with the effects instead of the cause by such techniques as using special metallurgy, making changes in operating conditions, adding chemical inhibitors, or undergoing frequent maintenance shutdowns.

Reactive Measures

Many refiners have changed fractionator tray metallurgy to very expensive alloys, changed operating conditions to affect water dew points, added filming or neutralizing amines to the column in hope of protecting the metal surfaces, or have simply given up and resorted to frequent maintenance shutdowns. Most of these reactive measures have been very expensive, often producing less than satisfactory results.

Merichem Company possesses and licenses a well known caustic treating technology in the oil refining industry named FIBER-FILM™ Contactor. This technology uses a patented contacting device that produces nondispersive mass transfer between immiscible phases such as hydrocarbon and caustic without creating the typical problems associated with dispersive systems that employ mixing devices. The main problem area is inefficient contact between phases and the creation of carryover or carryunder of one phase into the other.

In early 1989, JGC Corporation of Japan, one of the largest worldwide and more widely known engineering and construction firms in the refining and petrochemical industries, approached Merichem to solve this problem on behalf of its client, the Showa Shell Sekiyu KK.

Showa Shell was in the design stage of a new UOP licensed continuous regeneration reformer (CCR) unit for its Kawasaki oil refinery in Japan. The original treating system conceived by Showa Shell was to be a single treating stage combination of static mixer and coalescer. A two-stage treating system was later proposed by JGC in order to prevent plugging of the stabilizer reboiler due to the potentially high NaCL content of the stabilizer bottoms with a one-stage system.

Finally, JGC, who had worked with Merichem in designing caustic treating systems for other customers, conceived the idea of substituting a FIBER-FILM™ Contactor system to reduce investment costs and increase the proposed system's reliability with respect to HCL removal while minimizing sodium entrainment. After numerous technical consultations and extensive economic comparisons, JGC and Showa Shell finally decided to switch the earlier proposed treating schemes for an alkaline water wash single stage treating process which Merichem licenses under the servicemark of AQUAFINING^SM In effect, this combined the three treating steps originally envisioned in the conventional system.

The advantages foreseen by JGC and Showa Shell in using this AQUAFINING^SM process more than made up for the fact that FIBER-FILM™ Contactor technology had never been commercialized in this application.

The AQUAFINING^SM unit was designed to treat a reformer stabilizer feed stream containing 15ppm weight of HCL to undetectable levels and product a product containing a maximum of 0.1ppm weight sodium including both NaCL and NaOH (see Table 1).

Figure 1 depicts Showa Shell's system. The reformer stabilizer feed stream enters the HCL removal system by first passing through one side of two parallel basket strainers (BS-1 Or 2) to remove any solid particles larger than 150µ. It then enters the top of the FIBER-FILM™ Contactor (FFC 1) where it contacts alkaline water. Stabilizer feed and alkaline line water flow concurrently downward through the contractor shroud where the HCL is removed. The treated stabilizer feed leaves the FIBER-FILM™ Contactor, passes through the exits the separator vessel (V-1) at the opposite end from contractor to the stabilizer column. A proprietary coalescer pad (SP-2) is installed in V-1 to remove final traces of alkaline water contained in the stabilizer feed. The chemical reaction for HCL neutralization is:

Process water is continuously injected through metering pumps, P-5 or 6, and along with the fresh caustic proceeds to the static mixer (SP-1) where it is blended with the recycled alkaline water. The spent water is removed from the system on level control.

A pH controller (PHC), sensing the circulating water pH leaving the vessel, resets another pH controller sensing the circulating water pH after the make-up caustic and water have been thoroughly mixed through the static mixer, SP-1. The net output of this pH-pH cascade loop resets the stroke in the fresh caustic metering pumps, P-3 or 4, to ensure a steady pH of the final alkaline water.

The basic design of the AQUAFINING^SM unit was completed by JGC in May 1989 with the delivery of Merichem's proprietary equipment to Japan in July 1989. The CCR unit started up in May 1990 and though sufficient data has not bee gathered yet on the AQUAFINING^SM System, the preliminary results indicate that the unit is meeting all the required specifications.

One of the most important specifications that the unit has met is the pH of the water in the stabilizer overhead receiver. When this project was initially conceived it was thought that the operating performance of the unit, with respect to HCL removal, would be gauged by the amount of HCL detected in the treated stabilizer feed. However, a calculation revealed that it would take 0.09ppb weight HCL in the treated product to create a potential corrosion problem in the stabilizer column because the final pH of the overhead receiver water phase would be less than 5.5. Since the required product HCL level would be impractical and perhaps impossible to detect, the pH of the overhead receiver water became the primary specification. Since startup the unit has maintained an overhead receiver water pH of 6.0.

The other critical specification for determining process performance is the amount of sodium salts left in the treated stabilizer feed. It is expected that any carryover of sodium salts will end up in the column reboiler and increase the fouling and maintenance requirements of the equipment. The 0.1ppm weight Na+ specification was chosen based on Merichem's other commercial experience with treating units where the product passes immediately to a downstream fractionator. This level of entrainment will allow Showa Shell to sustain the desired reboiler service factor.

Showa Shell has implemented the AQUAFINING^SM system with full expectations that it will not experience any stabilizer column or reboiler shutdowns - between normal turnarounds planned for the CCR unit. The estimated annual operating cost of the unit is shown in Table 2. The projected operating costs in US dollars are based on US Gulf Coast costs for utility, chemicals, and manpower. The sum total of these costs indicate a very low operating cost of US$ 0.007 per barrel of throughput or US$0.056 per ton.

The author: 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.