Tackle tough separations

There are a plethora of devices in the marketplace that are designed to improve separation and thus improve product efficiency.  Learn how to determine which one is ideal for each particular process application.

By Thomas H. Wines, Pall Corp.

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The use of polymer and fluoropolymer materials of construction allows for expanded use of coalescers since they can withstand an array of aggressive chemical applications over a range of temperatures from -40° F (-40° C) up to 300° F (149° C).

Whereas liquid–liquid coalescers have many benefits in breaking tough emulsions, there are some limitations to consider. Higher concentrations of solids can be problematic and lead to excessive changeout of disposable prefilters. Generally, the solids range at which liquid-liquid coalescers can economically operate with disposable filters is less than 10 ppm. Above this concentration, further pretreatment will be required, such as backwash cartridge filters, mixed-media packed beds, or hydrocyclones.

The operational limits for removing free liquids must also be understood. The coalescer will not be able to remove contaminants that are in solution. If the clarified outlet stream is cooled, condensation of previously dissolved contaminants can occur, resulting in a hazy product. You must carefully consider the location of the coalescer, as well as changes in downstream process conditions.

Coalescers typically will have a service life of one to two years when adequately protected by prefiltration. Despite the long life, the equipment will eventually require disposal and replacement.

The problem of surfactant disarming must be considered for liquid-liquid coalescers constructed from glass-fiber medium, as well as for low-IFT-emulsion systems (less than 20 dyne/cm). An efficient separation cannot be achieved under such circumstances. For these conditions, non-disarming fluoropolymer or polymer coalescers should be considered. Such materials also have wider compatibility for chemical streams, in addition to a wider operating-temperature range.

Coalescers at work
Liquid-liquid coalescers can be applied to a number of processes to protect equipment, recover valuable streams and to meet environmental discharge limits. Some examples are given below.

• Improve extraction efficiency: High-efficiency liquid-liquid coalescers can allow a higher degree of mixing in extraction processes since they can break stable emulsions, thereby enhancing the mass transfer and reducing the size of equipment for extraction.

• Guard solid-bed catalysts/desiccants: Catalysts and desiccants are often sensitive to free water since it may contain dissolved salt, metals or caustic.

• Purify final product: Water condensation originating from steam stripping in refinery operations can lead to off-specification, hazy products.

• Recover liquid catalysts: Liquid or homogeneous catalysts will form emulsions in product streams. Such contamination can foul downstream equipment and poison solid-bed catalyst so that it has to be replaced.

An example is the recovery of caustic used to remove mercaptans (organic sulfur compounds) from gasoline. Caustic and air are injected into the gasoline feed upstream of the fixed-bed catalyst reactor. In the reactor, mercaptans are extracted into the caustic and are then converted to disulfide oils by oxidation and catalytic action. The reactor effluent typically contains some caustic that results in off-spec, hazy gasoline, high costs of caustic makeup, and corrosion of downstream piping.

Installing a high-efficiency liquid-liquid coalescer system was found to be an effective solution to recover the caustic carryover.

• Recycle Solvents: A number of unit operations require solvents, including extraction, crystallization and leaching. These solvents can be purified and reused. High-efficiency liquid-liquid coalescers have been used for solvent recovery in the manufacture of active pharmaceutical ingredients (API).

• Protect distillation/stripper columns: Distillation columns can be adversely affected by aqueous contaminants. The distillation energy requirement will be increased by having to vaporize the free aqueous contaminant, and any dissolved solids will plug the trays or packing. For sour-water stripper columns, oil and other hydrocarbons, if present, can lead to fouling of the internals.

In an ethylene plant, a liquid hydrocarbon feed stock is thermally cracked in a furnace along with steam. The effluent passes through oil and water quench towers where solids and heavy organic compounds are separated. The gas is further compressed, treated and distilled to obtain the final products. 

The quench water removes primarily pyrolysis gasoline (py-gas) from the cracked gas stream along with solid contaminants. Significant amounts of py-gas in the quench water can cause fouling and reduced efficiency of process equipment, including heat exchangers, strippers and boilers. A high-efficiency liquid-liquid coalescer system was found to be an effective way to protect the equipment and recover the py-gas.

Emulsions are present in many processes and require efficient separation to meet industry&rsquos increasing demands. High-efficiency coalescers can tackle these separation challenges.

Dr. Thomas H. Wines is senior marketing manager of the fuels and chemicals group for Pall Corp., East Hills, N.Y. He has more than 16 years of experience troubleshooting filtration systems in the refining, gas processing and chemical industries. E-mail him at Tom_Wines@Pall.com.

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