Question 5.22: Discuss the following: 1. The similarities and dissimilariti...

1. The following presents a discussion on the similarities and dissimilarities between the wet scrubbing process and the gas stripping process. Wet scrubbing and gas stripping are both mass transfer unit operations. Only the direction of the movement of a given pollutant species is different. In a wet scrubber, such as in the previous example of odor control (Example 19), the offending species is H_{2}S (or some other malodorous gas) and it is controlled by being absorbed into a passing liquid phase. Mass transfer from gas to liquid defines a scrubbing situation.

On the other hand, in certain situations the desire is to remove a given species from a liquid into a passing gas phase. This is the definition of air stripping. Using the example of H_{2}S gas, when H_{2}S is dissolved in groundwater in small amounts, the offending odor of H_{2}S may prevent the use of an otherwise potable water source. Therefore, a stripping tower is one plausible technique to use to remove the H_{2}S from the water. Other possible solutions are aeration or tray tower technologies (47,50–55). The actual choice of removal technology will depend on the space available for the equipment used to treat the water. An aeration basin will require a large available area and will lose significant amounts of water to evaporation, as well as have high power (and hence operating) costs. A tray tower will be less costly to fabricate than a stripper system. However, if the water flow being treated is large, the large pressure loss in a tray tower and subsequent cost of operation will make a stripper tower the logical choice to treat the water.

Briefly, in mass transfer, a species must leave one phase and enter another phase. This movement of a molecule from one phase to another is treated extensively in standard academic texts by McCabe et al. (38). The two-film theory presented by McCabe et al. (38) is widely accepted as the model to explain how mass transfer occurs in both a scrubber and a stripper tower.

A simple, graphical explanation of the two-film theory of mass transfer is presented by Heumann (56). The concentrations of the species being scrubbed/stripped at the film interface will be less than the bulk concentrations of the species in the bulk phases as the specie transfers from one phase to the other. The difference in concentration between the bulk phases, and actually between the two-film interface, is the driving force to mass transfer. If the concentrations of the species at the film interface are equal to the bulk concentrations of the same species in the bulk phases, no mass transfer will occur.

In actual practice, the specie being treated in the system will have limited solubility in one of the two phases. In a scrubbing situation, the specie being scrubbed must cross the barrier of the gas film in order to pass into the liquid film. This resistance of passage of the molecule out of the gas film is the limiting factor to mass transfer in a scrubber system. So, with exceptions noted below, scrubbing is said to be gas film controlled.

The exceptions referred to are CO_{2}, NO_{x}, phosgene, or similar scrubbing situations. Although these gases have high solubility in water and one would think that as such gas film resistance would limit their mass transfer in a scrubber, in reality these and similar compounds are liquid film controlled in a scrubber system. This is so because, although readily absorbed into water, the subsequent chemical reactions of these compounds in water are relatively slow, therefore, the liquid film resistance is the controlling factor when scrubbing these compounds from an air emission stream. In a stripping situation, the specie of concern is moving in the other direction, out of the liquid film into the gas film. Thus, in a stripping situation, the limiting factor to mass transfer is the ability of the molecule in question to break out of the liquid film to enter the gas phase. Thus, with very few exceptions, stripping is said to be liquid film controlled.

It is important to know the detailed relationship between the scrubbing process and the stripping process. The reader is referred to another chapter, “Gas Stripping,” of this handbook for a more detailed explanation of the stripping process than that given here. This chapter places emphasis on scrubbing process design and applications. Nevertheless, the reader should understand both the similarities and dissimilarities of the two processes.

For instance, if a packed tower reactor (Fig. 1b) or another reactor (Fig. 1a,c,d) is available, an environmental engineer may wish to use the same reactor both as a scrubbing process and a stripping process. In each instance, the scrubber or stripper will have two separate streams: (1) gas stream and (2) liquid stream.

It is a scrubbing process if (1) the gas stream is the target contaminated air emission stream from which one or more airborne pollutants (such as SO_{2}, H_{2}S, HAPs, VOCs, SVOCs, PM, heavy metals) will be removed by the reactor and (2) the liquid stream is the scrubbing solution (such as water with or without chemicals depending on the airborne pollutant(s) that need to be removed).

It is a stripping process if (1) the gas stream is the scrubbing agent (such as air with or without gaseous chemicals depending on the waterborne pollutants to be removed) and (2) the liquid stream contains the targeted pollutant (such as ammonia, chlorine, VOCs) that will be removed by the reactor. Normal instances of use of stripping towers is potable groundwater remediation, other contaminated groundwater treatment, or some other water pollution control need.

This discussion of the difference between scrubbing processes and stripping processes is more than an academic exercise. The optimum performance of a scrubber or a stripper tower most often depends on the correct selection of packing media with which to fill the tower. A given packing may perform better in promoting mass transfer in a scrubbing (gas film controls) process as opposed to promoting mass transfer in a stripping (liquid film controls) process. The opposite is true as well: A packing media may be better suited to enhancement of mass transfer in a stripping process and be less effective (less efficient, larger HTU value) in a scrubbing process.

2. The possibility of a combined wet scrubbing and gas stripping process is presented in the following discussion. A combined wet scrubbing and stripping process has been attempted by Wang and colleagues (48,49) for groundwater decontamination and reuse. The contaminated groundwater contains high concentrations of total hardness and volatile organic compounds (VOCs). An industrial plant near the contaminated site is discharging an air emission stream containing high concentration of carbon dioxide and is in need of additional industrial water supply.

It has been demonstrated by Wang et al.(48,49) in a small pilot-plant study that a combined wet scrubbing and stripping process system using the aeration or tray tower technology is technically feasible for achieving (1) reduction of CO_{2} from the air emission stream by scrubbing (i.e., groundwater is the scrubbing solution) and (2) reduction of VOCs by simultaneous stripping (i.e., the carbon dioxide gas is the stripping gas). Thus, the air emission stream and the groundwater stream treat each other. After treatment, the former is free from CO_{2}, whereas the latter is free from VOCs.

The treated groundwater that is free from both VOCs and hardness may be recycled for the in-plant application as the industrial water supply. The treated air emission stream free from CO_{2} is discharged into the ambient air.

The hardness in the groundwater contains mainly calcium bicarbonate, magnesium bicarbonate, magnesium sulfate, and calcium sulfate, which are to be removed. CO_{2} in the flue gas is reused as a chemical for hardness removal from the groundwater. Lime (calcium hydroxide or calcium oxide) and soda ash (sodium carbonate) are additional chemicals required for groundwater treatment as well as carbon dioxide gas stripping. The following are chemical reactions for the combined flue gas (air emission stream) and groundwater treatment in the combined wet scrubbing and stripping process system:

Contaminated flue gas → air +  CO_{2}

Contaminated groundwater → H_{2}O + VOCs + Ca(HCO_{3})_{2} + Mg(HCO_{3})_{2}

CO_{2} + Ca(OH)_{2} → CaCO_{3} (precipitate) +  H_{2}O

Ca(HCO_{3})_{2} + Ca(OH)_{2} → 2CaCO_{3} (precipitate) + 2H_{2}O

Mg(HCO_{3})_{2} + Ca(OH)_{2} → CaCO_{3} (precipitate) +  MgCO_{3} + 2H_{2}O

MgCO_{3} + Ca(OH)_{2} → Mg(OH)_{2} (precipitate) + CaCO_{3} (precipitate)

MgSO_{4} + Ca(OH)_{2} → Mg(OH)_{2} (precipitate) + CaSO_{4}

CaSO_{4} + Na_{2}CO_{3} → CaCO_{3} (precipitate) + Na_{2}SO_{4}

Air effluent → air + VOCs (to be removed by gas phase GAC)

Purified air → ambient environment

Purified groundwater (H_{2}O) → industrial water supply

The precipitates produced from the above chemical reactions occurred in the combined wet scrubbing and stripping process and must be further removed by one of the following water–solid separation processes (49,57), before the purified groundwater can be reused as an industrial water supply: (1) dissolved air flotation and filtration, (2) sedimentation and filtration, or (3) ultrafiltration or microfiltration.

The air effluent from the combined wet scrubbing and stripping process will contain air and stripped VOCs. Before the air effluent can be discharged into ambient environment, it must be further purified by gas-phase granular activated carbon (GAC) or an equivalent air pollution process.

More research on simultaneous air and water pollution by a combined wet scrubbing and stripping process system should be conducted aiming at water recycle, greenhouse gas reduction, and resource recovery (i.e., CO_{2} is a useful chemical for pH control, hardness precipitation, protein precipitation).