LIQUID-LIQUID CONTACTING PROCESS

Abstract: A liquid-liquid contacting process for separating a component from a mixture in which the mixture flows through or over a packed bed of strands (4) of low surface-energy material, with gas (5) introduced so as to flow upwardly through the bed. This gives an increased efficiency of contacting due to the hydrophobic property of the bed material and its effect on the aqueous phase and on the bubble size as the gas becomes dispersed in the organic phase.

Description: DESCRIPTION LIQUIDLIQUID CONTACTING APPARATUS A wide variety of liquid-liquid contacting processes are known for the separation of a component from a mixture. The processes are applicable when the components to be separated distribute themselves differently in two liquid phases. Apparatus which has been developed for the efficient contacting of two liquid phases includes columns fitted with perforated plates or packings which may be dumped ( i.e. in random position or regular. Also well-known are mixer-settlers in which the two liquid phases are mixed and then settled before removing the phases separately. It has been observed that when a gas is introduced into a contactor, the efficiency of contacting is improved by the increased turbulence generated by gas agitation. However, this improvement is accompanied by a significant reduction in capacity, and in some cases to increase the capacity. The object of the invention is to increase the efficiency of contacting without significant reduction in capacity. According to the invention, a contacting process for aqueous and organic liquids involves flowing the same over a surface of low surface energy material provided by or on strands, and introducing gas to said bed. The manner in which the invention works is not clearly understood, but such explanations as are possible are included later in this description. The invention is based on the discovery that a very marked increase in the efficiency of contacting results from the mentioned factors (i.e. the bed of said material and the gas introduction). The mechanism is different from that of gas agitation, because the gas becomes dispersed in the organic phase, and the reduction in capacity with the known gas agitation is avoided. Examples of contacting apparatus where this invention increases the efficiency are: (1) In the mixer section of a mechanically agitated mixer-settler contactor (Figure 1). A solvent stream (1) and an aqueous solution (2) are fed to the mixer section which is stirred by a stirrer (3). Below the stirrer is a disc of knitted polypropylene fibre (4). Gas is introduced at (5) so that it rises into the knitted disc. The mixture of liquid phases flows to the settling section (6) from which the extract is removed (7) separately from the water (8). (2) In a gas agitated mixer with a lift tube as illustrated in Figure 2. The tubular draft tube (9) is fitted with a large diameter section at the base (10). Knitted polypropylene (11) is fitted inside this expanded section. Gas is introduced into the mixer through a sparge pipe (12) arranged so that some of the gas rises through the knitted polypropylene section. (3) In a countercurrent liquid-liquid extraction column as in Figure 3. The aqueous solution is added to the column via a distributor (13) and the organic solvent at the base (14). Gas is introduced at (15). The column is packed with discs of knitted polypropylene (16) which cover the full cross section of the column. After being brought into contact countercurrently the extract (organic) phase is removed from the column at (17) and the aqueous raffinate at (18). Gas is removed from the top of the column (19) and may be recycled to the base (15). In these examples quoted above, the gas when brought into contact with the organic phase in the presence of the low surface energy polypropylene surface is dispersed in the organic phases. With the kerosine-water-air system for example the air is dispersed in the form of very small (lmm) diameter bubbles probably due to the low surface-energy material) to the extent that the density of the disperse phase may easily be halved. In consequence, the interfacial area for mass transfer is obviously increased per unit mass ofsolvent. In addition, the viscosity of the disperse phase is reduced so that the interfacial area is also increased because of the smaller droplet size. The mass transfer rates were compared in a nominal 15 cm diameter column using the water-kerosinebenzoic acid system. The column was packed with stainless steel-polypropylene discs as described in BP 1,409,045. The conditions were: water phase velocity 27cm/min kerosine phase velocity 40cm/min Without air dispersion, the height of a transfer unit was 3.7 metres. With air added equivalent to a velocity of 50cm/min the height of a transfer unit was reduced to 1.3 metres. The capacity of the column ( as limited by the rate of coalescence) was at least 10% greater with an air-dispersed phase over a wide range of solvent/ aqueous phase ratios It is believed that the knitted condition of the strands is not essential, but the presence of small loops in the strands or filamentary material is important in the generation of gas dispersed droplets. The significance of the low surface-energy material depends upon this material being effectively exposed to the liquids/gas and consequently the material could be coated as a surface layer on a relatively inert carrier material. Low surface-energy materials such as polypropyline are effectively hydrophobic and it is this property canbined with its effect on bubble size which may produce the good results.

Claims: CLAIMS 1. A contacting process for aqueous and organic liquids involving flowing the same over a surface of low surface-energy material provided by or on strands, and introducing gas to said bed. 2. A process as claimed in Claim 1 wherein the strands are in the form of loops. 3. A process as claimed in Claim 1 or Claim 2 wherein the strands comprise low surface-energy material on an inert carrier. 4. A process as claimed in any preceding claim wherein the liquids and gas are mechanically stirred. 5. A process as claimed in any preceding claim wherein the liquids and gas are fed to a lift tube to flow through a passage lined with said low surface-energy material. 6. A process as claimed in any of Claims 1-3 wherein the liquids flow in counter-current directions through a column and the low surface energy material is provided in the column. 7. A contacting process for aqueous and organic liquids substantially as described with reference to the accompanying drawings.

Inventors: PRIESTLEY, RONALD
Applicants: PRIESTLEY, RONALD
Application year: 1982
Main classification: B01D11
IPC codes: B01D11