An Innovative Method for Amine Treating of Hydrocarbon Liquids

Herbert W. Wizig Merichem Chemicals & Refinery Services LLC
Process Technology Division

Abstract

This article describes an innovative system for removing H₂S from a hydrocarbon liquid stream. A FIBER-FILM™ Contactor is used to bring the LPG stream into non-dispersive contact with a diethanolamine solution. This system achieves efficient mass transfer without excess amine carryover, even at high throughput rates.

Introduction

The main problem area in conventional liquid-liquid treating systems is inefficient contact between phases, poor turndown capability, and the creation of carryover or carryunder of one phase into the other. Conventional amine contactors for removing H₂S from gases and liquids in the refining, gas, and gas liquids industries use packing, trays, or mix valves to achieve intimate contact between two immiscible phases. These systems frequently require several treating stages due to low contacting efficiency. A downstream amine settler containing a coalescer is often needed to capture entrained amine. Turndown capability is also frequently less than desirable.

The FIBER-FILM™ Contactor Treating Principle

Merichem Company licenses FIBER-FILM™ Contactor, a well-known caustic treating technology in the oil refining, gas, and gas liquids industries. This technology uses a patented contacting device that produces non-dispersive mass transfer between immiscible phases, such as hydrocarbon liquids or gases and caustic. In 1974, Merichem introduced the FIBER-FILM™ Contactor, a retaining cylinder packed with very fine, proprietary metal fibers. This revolutionary system virtually eliminates the problems associated with dispersive treating.

In a FIBER-FILM™ Contactor the caustic phase flows along the fibers, continually being renewed as it preferentially wets the metal. Hydrocarbon also flows through the cylinder parallel to the caustic-wet fibers. The large surface area and tight packing of the fibers bring ultra thin films of caustic and hydrocarbon into close contact. The interfacial area produced is extremely large, and contaminants easily diffuse over the microscopic distance between phases.

However, since surface tension causes the caustic to cling to the metal fibers, while the hydrocarbon phase flows more freely, emulsions simply do not form. The result is a highly efficient mass transfer of impurities from one phase to the other without emulsification or high pressure drop.

FIBER-FILM™ Contactor systems produce excellent treating results without creating the typical problems associated with dispersive systems that employ mixing devices. The FIBER-FILM™ Contactor process improves mass transfer efficiency between phases yet virtually eliminates aqueous phase carryover. With shorter separation times and less waste generation, FIBER-FILM™ Contactor Technology requires smaller processing vessels, thus saving valuable plant space and reducing capital expenditures.

Application to Amine Treating

Recently this technology has been applied to systems using other aqueous treating reagents, including monoethanolamine (MEA), monodiethanolamine (MDEA), and diethanolamine (DEA) in water. This latest adaptation of the proven FIBER-FILM™ Contactor Technology is called AMINEX^SM.

A major refiner located in Southeast Asia has chosen Merichem's AMINEX^SM Technology to remove H₂S from a LPG stream from a resid FCCU. This article is based on the actual design of this amine treating system.

As shown in Figure 1, the untreated LPG stream enters the AMINEX^SM system after passing through one side of two parallel basket strainers. The strainers remove solid particles (such as iron sulfide scale) larger than 150 microns.

The LPG stream enters the top of the FIBER-FILM™ Contactor where it contacts metallic fibers wetted with 25 wt % DEA solution from the amine regeneration unit. While the LPG and DEA streams flow concurrently down through the Contactor shroud, most of the H₂S is extracted into the DEA, according to the following chemical reaction:

H₂S + 2(R₂NH) → (R₂NH₂)₂S
(DEA)

The amine solution is recycled on manual flow control by one of two centrifugal pumps. The recycling rate is approximately 30 volume % of the LPG stream. Recycling the amine solution provides more intimate contact between the LPG and amine phases than could be achieved with a once-through system.

Lean amine enters the system via one of two charge pumps after passing through one side of two parallel basket strainers. The amine transfer rate between the H₂S extraction system and the amine regeneration system is approximately 2.6 volume % of the LPG. This low amine transfer rate indicates how efficient the AMINEX^SM treatment process is. A proprietary coalescer pad is included in the separator vessel to prevent trace amine losses. Such losses could occur due to amine carryover during unexpected but potential flow surges.

The LPG stream exits the separator at the opposite end from the FIBER-FILM™ Contactor and flows to Merichem FIBER-FILM™ Contactor H₂S THIOLEX^SM caustic pre-wash and mercaptan extraction systems for removal of the remaining H₂S and mercaptans. A Merichem caustic regeneration system, REGEN^SM, is included to minimize caustic usage for mercaptan extraction. The REGEN^SM system oxidizes the mercaptans to disulfides and removes the disulfides from the caustic with a FIBER-FILM™ Contactor solvent wash system.

Annual Operating Cost Factors

Based on the factors noted in Table I and a 350-day operating year, annual requirements for utilities, chemicals, and manpower for the LPG amine treating system were estimated as follows:

AMINEX^SM Process Advantages

This refiner chose the AMINEX^SM system over the conventional countercurrent design for the following reasons:

• The AMINEX^SM system is simple. It does not rely on tray or packing internals that are subject to corrosion and plugging. Nor does it require additional settlers and downstream equipment to capture amine carryover.
• The AMINEX^SM process is reliable. It is designed for year-around operation, virtually shutdown free.
• The AMINEX^SM process operates efficiently over the client's anticipated range of throughputs and operating conditions. In spite of the high throughput rate, a higher amine regeneration capacity is not needed. Amine is recycled within the AMINEXSM unit for even greater treating efficiency.
• The AMINEX^SM system takes a much lower pressure drop than conventional countercurrent systems.
• The AMINEX^SM system is flexible. System capacity can easily be expanded in the future.