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Contamination of groundwater and soils with volatile organic compounds (VOCs) is a problem at many industrial sites. Soil contamination by VOCs is a source for continuous air pollution and is also a source for groundwater pollution. The level of volatile organic compounds allowed in discharged wastewater or drinking water is lowered every few years. Examples of contaminants are petroleum hydrocarbons, methyl fert-butyl ether, chlorinated hydrocarbons such as trichloroethylene and carbon tetrachloride. Many of the VOCs are potential carcinogens.

MTBE, widely used as an octane enhancer, has relatively high water solubility and vapor pressure. MTBE is very toxic and is also suspected for its carcinogenic properties. Recently, MTBE has been detected in lakes, reservoirs, and groundwater used as potable water supplies in concentrations exceeding, in some cases, allowed levels for taste, odor, and human health.

There is a need to develop separation systems that can remove organics from already contaminated sites and systems to prevent future contamination. There are several alternative methods which can be applied to the VOCs removal, including air stripping, carbon adsorption, biological treatment, steam stripping, and incineration. Some of them can generate secondary wastes. This, coupled with a growing interest in recycling both for economical and environmental reasons, opens new opportunities for membrane technologies like pervaporation. Pervapo-ration can be used for effectively removing VOCs from water, concentrating them for economical disposal or recycle/reuse using specially designed hydrophobic membranes.

The separation achieved is determined by the individual rates of permeation through the membrane and the relative volatility of the components of the feed mixture. Usually the VOCs permeate through hydrophobic membrane orders of magnitude faster than water, as a result of which the VOCs are highly concentrated.

MTBEs are examples of VOCs which can be found in the effluents from many chemical, pharmaceutical, or petrochemical, factories (Kujawski, 2000; Keller et al., 2007).

Pervaporation, in its simplest form, is an energy efficient combination of membrane permeation and evaporation. It is considered as an attractive alternative to other separation methods for a variety of processes. For example, with the low temperatures and pressures involved in pervaporation, it often has cost and performance advantages for the separation of constant-boiling azeotropes. Per-vaporation is also used for the dehydration of organic solvents and the removal of organics from aqueous streams. Additionally, pervaporation has emerged as a good choice for separation of heat-sensitive products.

Pervaporation involves the separation of two or more components across a membrane by differing rates of diffusion through a thin polymer and an evaporative phase change comparable to a simple flash step. A concentrate and vapor pressure gradient is used to allow one component to preferentially permeate across the membrane. A vacuum applied to the permeate side is coupled with the immediate condensation of the permeated vapors. Pervaporation is typically suited to separating a minor component of a liquid mixture, thus high selectivity through the membrane is essential. Figure 37.1 shows an overview of the pervaporation process.

Pervaporation can be used for breaking azeotropes, dehydration of solvents and other volatile organics, organic/organic separations such as ethanol or methanol removal, and wastewater purification.

Characteristics of the pervaporation process include

1. Low energy consumption

2. No entrainer required, no contamination

3. Permeate must be volatile at operating conditions

4. Functions independent of vapor/liquid equilibrium

Liquid transport in pervaporation is described by various solution-diffusion models. The steps included are the sorption of the permeate at the interface of the solution feed and the membrane, diffusion across the membrane due to concentration gradients (rate determining steps), and finally desorption into a vapor phase at the permeate side of the membrane. The first two steps are primarily responsible for the permselectivity. As material passes through the membrane a 'swelling' effect makes the membrane more permeable, but less selective, until a point of unacceptable selectivity is reached and the membrane must be regenerated. The other driving force for separation is the difference in partial pressures across the membrane. By reducing the pressure on the permeate side of the membrane, a driving force is created. Another method of inducing a partial pressure gradient is to sweep an inert gas over the permeate side of the membrane. These methods are described as vacuum and sweep gas pervaporation, respectively (Kumar M.S., 2007).

Literature searches for MTBE/water mixtures were given in Table 37.1. Separation factors are not available in the literature. It is concluded that from the literature survey that PEBA membrane is suitable for the MTBE removal from water.

Table 37.1 Separation of MTBE/water mixtures by pervaporation.

Membrane material

Feed concentration (MTBE )

Separation factor

Reference

PVAC-modified silica

0.01 vol%

68

(Yoshida and Cohen, 2004)

PEBA, PDMS

1 wt %

Ostrowska-Gumkowsha,

2003a)

H polysiloxane co styrene isoprene

1 wt %

3200

(Kujawski et al., 2003b)

FAS grafted ceramic

0 - 3.5 wt%

Not given

(Kujawski, 2007)

Commercial or-ganophilic

0.1 vol %

Not given

(Urkiaga et al., 2002)

PEBA 4033

1 wt %

33

(Kujawski and Roszak, 2002)

In this study PEBA membrane was developed for the pervaporation separation of MTBE/water mixtures. The effects of feed concentration on the separation performance were investigated experimentally and the flux and the selectivity of the membrane were evaluated. Pervaporation that uses PEBA membrane could be an alternative separation technology for the original environmental problems like this study.

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