Overcoming tight emulsion problems
A refiner’s trials of a membrane coalescence mechanism were scaled up to plant level and delivered significant product recovery from tight aqueous emulsions
HERNANDO SALGADO, Cartagena Refinery, Ecopetrol
LUIS MARIÃ‘O Ramgus S. A., Pall Corp.
ROSÃNGELA PACHECO, Barrancabermeja Refinery, Ecopetrol
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One of the most challenging problems in refining and petrochemical processes is the separation of liquid emulsions and dispersions, which are often formed during the purification of products, especially when intimate contact between hydrocarbon and aqueous phases is involved. This is the case in the treatment of hydrocarbon fuels such as fuel gas, LPG, naphtha or jet fuel, when amines and caustic solutions and water are used to remove or wash contaminants such as H2S, CO2, mercaptans or naphthenic acids.
In many cases, these liquid solutions or solvents can be entrained by the main hydrocarbon product, forming tight emulsions and dispersions which are difficult to separate and are often discarded as effluents to the wastewater treatment unit. On the other hand, entrainment of these liquid solvents could potentially affect product specifications or cause operational upsets in downstream processes.
Nevertheless, emulsions and dispersions can be separated by coalescence, taking advantage of the interfacial tension between hydrocarbon and aqueous phases. In the coalescence process, two drops of a single phase and identical composition make contact with each other, forming a single bigger drop and in this way minimising their specific surface (surface per volume unit).
There are different types of coalescing media, from fibre glass of moderate performance through to special polymer membranes which can provide high performance in separating tight emulsions such as those with some content of surfactant compounds.
In many refining and petrochemical processes, formed emulsions are very stable due to the presence of small quantities of surfactant compounds which can be additives in regular use such as corrosion inhibitors or anti-foaming and anti-fouling additives, as well as contaminants such as naphthenic acids and sulphur compounds.
The tightness of an emulsion can be measured in terms of its interfacial tension, which is the free energy in the contact zone of two immiscible liquid phases. Interfacial tension is a consequence of the superficial tension of both liquid phases, and can be expressed in units of force per distance, generally in d/cm.
The presence of surfactants leads to the formation of micelles (of hydrocarbon in an aqueous phase) or inverse micelles (of an aqueous phase in a hydrocarbon), depending on the concentration of each phase, either the continuous or the disperse phase. The internal energy of an emulsion increases proportionally with surface and interfacial tension; therefore, the lower the interfacial tension is, the lower the free energy. Consequently, the emulsion is tighter, leading to the formation of smaller drops which are more difficult to separate.
The high coalescence and separation performance of polymer membranes makes them suitable for separate tight emulsions, which cannot be treated by using conventional coalescing materials and equipment.
Membrane coalescence mechanism
Membranes for coalescence are polymer fibres which vary in diameter sizes and surface treatments, depending on the application. The linked structure of the fibres forms very fine pores which allow droplets in the range 0.2-50 µm to be collected and transformed into a dispersion of bigger drops of 500-5000 µm diameter. However, due to the small pore size of the fibres’ structure, particulate matter must be removed from the fluid before it is processed through the membrane.
The membrane coalescence mechanism is defined by the following steps (see Figures 1 and 2):
1. Removal of particulate matter in the pre-filter
2. Adsorption of droplets on the membrane fibres
3. Movement of droplets to membrane fibre intersections due to entraining by process flow
4. Coalescence of two tiny droplets to form a bigger one when a second droplet reaches the same fibre intersection
5. Release of bigger drops from the fibre intersections due to accumulation of drops and entrainment by process flow
6. Repetition of steps 2-5 with progressively bigger drops and larger pores in the fibre structure.
Membrane coalescence on the industrial scale
According to the mechanism discussed above, membrane coalescence is generally designed on an industrial scale with the following processing arrangement: pre-filtration, coalescence and separation.
Generally, pre-filtration occurs in a vessel with filtration cartridges, with a mesh size depending on the amount and size of particles previously measured or estimated. The purpose of this step is to remove the solid particles which could increase emulsion stability and at the same time to protect the membrane’s functionality by preventing clogging of pores.
While pre-filtration is arranged in a separate vessel, coalescence and separation occur in the same piece of equipment, although they can be carried out in separated compartments. A typical process arrangement is shown in Figure 4.
Even though separation is not a formal step in the coalescence mechanism, it is required to achieve the main goal of coalescence, which is the separation of the two liquid phases involved in the emulsion. The selection of the proper type of arrangement of the coalescer-separator depends on the particular application, requiring (or not) an additional membrane cartridge to assure the separation of phases, as in the vertical arrangement.
The vertical arrangement is generally used to separate an aqueous contaminant dispersed in a hydrocarbon continuous phase with interfacial tension as low as 3 d/cm and a small difference in density between the phases. In such applications, the separation membrane is made of a hydrophobic material to retain the aqueous phase in a different compartment and to allow the hydrocarbon to flow through downstream in order to facilitate and assure the separation process.
The horizontal arrangement (see Figure 3) is generally used to separate hydrocarbons from a continuous aqueous phase with a grater difference in density. In this case, after coalescence is carried out, the separation of phases is achieved by settling the aqueous phase.
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