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Groundwater Remediation
Published in Kathleen Sellers, Fundamentals of Hazardous Waste Site Remediation, 2018
Vapor emissions from groundwater treatment units are often laden with water vapor. The first step in treatment is often to remove liquids or reduce the relative humidity of vapor streams. (Relative humidity is the ratio of the partial pressure of the compound in the vapor phase to its vapor pressure.)190 Commonly used techniques include the following: A demister is a knitted mesh, typically made of stainless steel or plastic fibers. A demister entrains small droplets of liquid as the vapor flows upward through the mesh.191A knock-out tank works very simply: by increasing the cross-sectional area in flow (relative to the influent pipe), the velocity of the vapor stream slows. As a result, water droplets and particulates fall out of the vapor stream by gravity.191 A knockout tank can be as simple as a 55-gal drum with a vapor inlet near the bottom of the drum, an outlet for the vapor at the top of the drum, and a tap for removing collected liquids from the bottom of the drum. Another common design for a knockout tank brings the influent vapors in through a pipe inserted through the top of the tank and down through a demister. As the vapors expand, droplets form; the demister helps to remove the droplets as the vapors flow upward though the demister to exit the tank through a port located above the demister. Knock-out tanks are commonly used in dual-phase extraction systems and soil vapor extraction systems.192Either cooling or heating can be used to control the water content of a vapor stream. Cooling a vapor stream can cause water or organic liquids to condense. Condensation can be an effective form of treatment for off-gases containing >5,000 to 10,000 ppmv organics.193 However, the organics which have a high enough vapor pressure to be removed in a remediation system which does not heat the soil or water will not condense readily in a heat exchanger unless the temperature is lowered significantly. Heating a vapor stream lowers the relative humidity, as discussed below in Section 3.7.2.
A critical review of separation technologies in lignocellulosic biomass conversion to liquid transportation fuels production processes
Published in Chemical Engineering Communications, 2022
Paola Ibarra-Gonzalez, Lars Porskjaer Christensen, Ben-Guang Rong
As described in the previous section, on the condensing system, bio-oil is separated from non-condensable gases. For instance, in a system including spray towers placed in series, the non-condensable gases are gradually recovered at the top of the columns. In a study performed by Jones et al. (2013) two spray towers in series were considered in the pyrolysis plant design. In the first spray tower, the non-condensable gases were recovered together with some remaining bio-oil components and therefore a second column was required to separate completely the non-condensable gases from the bio-oil. In addition, in some process configurations, the spray towers are coupled with a demister, as presented in Figure 4. Aerosols as part of the non-condensable fraction still contain liquid droplets (Rengel 2007), which should be removed. The demister assists in the separation of the liquid droplets from the permanent gases. Commonly, an electrostatic precipitator is used as a demister. The electrostatic precipitator allows the removal of liquid droplets from permanent gases by means of an electrostatic field of force produced by a discharge electrode and a collecting electrode (Mussatto 2016).