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Solving the High Boiling Problem

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Solving the High Boiling Problem WATER AND WASTEWATER Treating industrial wastewater (Dis)Solving the high boiling problem Sulzer equipment can cost effectively remove pollutants from large wastewater flows. Depending on the boiling points of the pollutants to be removed in relation to that of water, Sulzer recommends either a wastewater stripper or a liquid-liquid extraction unit. large number of pollutants in before the wastewater is discharged, as, would be very hig h— leading to high industrial wastewater are organic for example, in the case of pollution with costs and additional environmental Achemicals that are dissolved in organic solvents. pollution. water. These are only biodegradable in Depending on the source of the waste - the rarest cases. Therefore, this waste - water, the typical concentrations of the Wastewater stripper water cannot be cleaned in a municipal solvents lie in the range of 1 –20 % by Stripping represents a well-proven and sewage treatment plant together with weight. Such concentrations of pollutants effective method of treating wastewater domestic wastewater. The substances are too low and the calorific value of the contaminated with solvents. In this often interfere with the microorganism wastewater flow is too low for thermal process, wastewater is fed into the top metabolism or are even toxic. Such pol - treatment, which is basically incineration. of a rectification column and steam is lutants must therefore be removed The requirement for a secondary fuel passed through the column from the Part of an extraction column on a truck. During the contact of the two liquids in the extraction column, the phenol is transferred from the water to the extract. The latter has a higher affinity for phenol than water. 20 | Sulzer Technical Review 3 /2011 4358 WATER AND WASTEWATER (Concentration figures quoted in % by weight) Vent Cooling water Condenser Decanter Feed (100 kg/h) 92 % Water 8% Ethyl acetate Distillate (8 kg/h) > 97 % Ethyl acetate Stripping < 3% Water bottom in the opposite direction. This solvent-rich flow. Typical solvents that column steam can be directly fed from an can be removed through direct stripping Heat exchanger Live steam external steam network, but can also be without a decanter are alcohols and 1 generated in an evaporator. Figure ketones. A decanter is required for esters, Bottom product (92 kg/h) shows a diagram of this process. ethers, short-chain hydrocarbons, and > 99.99 % Water Simple wastewater stripping is suitable other substances that do not mix with for use for all solvents with a lower water. This process is also well suited 1 Wastewater stripper with optional decanter, boiling point than water and for those for wastewater that is simultaneously including a wastewater example. that form a low boiling azeotrope with polluted with several components. water. The solvent, which will be con - densed as a concentrate at the head of Problem: high boiling substances content in the azeotrope, direct stripping the column, accumulates in the vapor If wastewater is polluted with a chemical is therefore not practical. phase. The maximum achievable solvent that has a boiling point above 100 °C at Separation is also difficult in the case concentration is determined by the ther - atmospheric pressure, the wastewater of water polluted with acetic acid. Acetic modynamic equilibrium and by economic stripping process above is of limited use. acid has no azeotrope with water and considerations. Wastewater that is prac - If the substance forms an azeotrope with has a higher boiling point than water, so tically free from solvents can be removed water that has a lower boiling point than that all the water would be removed as from the bottom of the column. After it pure water and that has a sufficiently a low boiling substance out of the dis - has been used for the preheating of the high enrichment, then wastewater strip - tillate in direct “stripping.” This process polluted wastewater flow, it can either ping can be performed with a decanter would not be stripping in the true sense, be fed into the sewer system or be as described earlier. The solvent can but would be wastewater evaporation recycled in the process. The concentrate thereby be effectively enriched in the dis - and would thereby be extremely energy obtained at the head of the column can tillate of a stripping column and can be intensive. Treatment would also be either be treated further, with the solvent removed well, despite the high boiling possible using extractive or entrainer being recycled, or can now be incinerated point. distillation, but both are complicated more cost efficiently. If the thermodynamic equilibrium is and also require a lot of energy. If the solvent forms a heterogeneous less favorable, however, direct stripping azeotrope with water, and shows a mis - cannot be used. Phenol, for example, has cibility gap, the decanter presented as a boiling point of 182 °C. Although it 2 Flow diagram of a liquid-liquid extraction an option in Figure 1 will be required. forms an azeotrope with water, the unit, including solvent treatment and a waste - water stripper. The water phase from the decanter is boiling point of this azeotrope is only fed back into the stripping column, as 99 °C with a phenol content of 9 % by it has a composition similar to the feed weight. This almost corresponds with and is saturated with the solvent. The the solubility of phenol in water at 3 organic phase can be correspondingly 25 °C1. No concentration will take place treated further or be disposed of as a in wastewater saturated with phenol. concentrate. Because the boiling point of the azeotrope Feed Acetic acid/ The costs of this process are mainly is very close to that of water, the Phenol 1 t 2 n a t determined by the thermodynamics of stripping column will also need to have c a r t x the water-solvent mix. Depending on the a very high separation performance. E vapor-liquid equilibrium, a different This would require a high number of 4 water concentration will result at the stages and a high reflux ratio. The cor - 1 Extraction column head of the column, and will therefore responding column would therefore be 2 Decanter also lead to a different energy require - high and would consume a great amount 3 Extractant recovery Wastewater stripper ment for a sufficiently concentrated, of energy. Considering the high water 4 Wastewater Sulzer Technical Review 3 /2011 | 21 WATER AND WASTEWATER In many cases, ketones, esters, or ponent from wastewater using extraction ethers are used as extraction agents for is much more complicated than single- phenol and acetic acid. During the stage stripping. Key factors in an contact between the two liquids in the efficient process are the right choice of extraction column, a material transport the extraction agent and the operating of the phenol takes place from the water conditions of all the connected columns to the extraction agent, as the latter has that have been adapted to the separation a higher affinity for the phenol than task. water. The water will thereby become The extraction agent must have depleted of phenol and the extraction certain properties: it must not be agent enriched. At the same time, how - miscible with wate r— or at least be only ever, the water will become saturated slightly miscibl e— it must have a high with the extraction agent that is being affinity for the transfer component, and used, so that the water itself cannot be it should be as environmentally friendly directly discharged after the extraction. 3 Internals of the Kühni-type ECR column. The extraction stage alone cannot purify the water, but the component with a high boiling point can be removed without an evaporation step. The water 4 Pilot liquid-liquid extraction column thereby takes up another materia l—the with 60 mm diameter. Solution: Extraction extraction agent. This agent has a lower In such cases, liquid-liquid extraction boiling point than water and/or forms offers an elegant solution to the separa - a low boiling azeotrope, and therefore tion problem. This process is based on it can be simply removed with the strip - the different solubility of a substanc e— ping process described earlier. In some the so-called transfer componen t— in cases, a solvent that is already present two liquids that cannot be mixed with in trace quantities in the feed is used for each other or that can only be partially the extraction. In this way, no additional mixed. If these two liquids are brought materials are brought into the process. into contact with each other, a two-phase The transfer componen t— in this case, liquid-liquid dispersion results, and the phenol or acetic aci d— must be removed transfer component divides itself accord - from the loaded extraction agent in order ing to the thermodynamic equilibrium to make the agent reusable for extraction. between the two liquids. This is normally carried out in a rectifi - The liquid that has a higher affinity cation unit. Figure 2 shows the basic for the transfer componen t— and that flow diagram of the complete process. takes these u p— is generally described as the extraction agent, while the liquid Comparison phase enriched with the transfer compo - Biodegradable solvents can also be nent is described as the extract. The removed by means of biological waste - donor phase with depleted transfer com - water treatment in a sewage treatment ponent is called the raffinate. Liquid- plant. They are, however, mostly recycled liquid extraction is mostly carried out at for cost reasons. A comparison of the ambient temperature and pressure, so two processes described above, on the that no additional energy is required for basis of the two flow diagrams, indicates heating or cooling.
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