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Module 4: Liquid Characteristics - Types of Liquids
Practice Problems
Objective
- Describe the properties of the four different types of liquids that are commonly encountered in air pollution control systems and emission sampling.
Molecule-to-molecule attraction occurs when the freely moving molecules of a gas are compressed or cooled to form a liquid. These weak interactions, called Van der Waal forces, are sufficient to keep the molecules spaced closely together. Accordingly, liquids are practically incompressible. Unlike gases, liquid volume does not change appreciably when pressure is applied.
While liquids remain at a relatively constant volume, they do not maintain shape. The interactions between the molecules are too weak. Although closely spaced, individual molecules can move freely around from position to position within the liquid.
Due to this molecular motion, diffusion can occur in liquids just as it occurs in gases. However, due to the close spacing between the molecules in the liquid phase, diffusion is slowed by the large number of collisions between the molecules.
Water is the liquid most commonly used in air pollution control and emission testing applications. However, the water is generally not pure. The water may be mixed with one or more other liquids that dissolve in water. It may contain dissolved solids, or it may have solid particles suspended in the liquid phase. These materials present in water can substantially alter the physical and chemical properties of the resulting liquid. It is generally useful to categorize the liquid according to the type and form of the other materials present in water. There are no consistent terms for these categories. Throughout Basic Concepts in Environmental Sciences, the following terms will be used:
- City water
- Aqueous liquids
- Slurries
- Ionic solutions
There are no consistent definitions for the categories listed above. This lesson provides the most common definitions used in air pollution control.
City water is water with very low levels of dissolved or suspended solids. Throughout these modules, city water will be defined as water that meets U.S. EPA drinking water standards. The concentrations of materials such as sodium, calcium, magnesium, iron, and chlorine are very low. The amount of suspended solids is also very low. Therefore, city water looks clear with zero percent turbidity.
City water is used in air pollution control applications primarily as "make-up." This is the liquid that must be added to an air pollution control system on a regular basis to account for the losses due to water evaporation and water contained in the liquid or solid waste streams leaving the air pollution control system. City water is also used in evaporative cooling towers, which cool hot combustion gases prior to entering the air pollution control system.
The term "aqueous" indicates that the main component of the liquid is water. However, the liquid could have significant concentrations of other dissolved liquids and/or low-to-moderate levels of solids. For example, the liquid stream that recirculates through a wet scrubber system is an aqueous liquid. Due to the capture of contaminants within the liquid, the concentrations of dissolved and suspended solids are much higher than normal drinking water.
The concentration of suspended solids is usually below 2% by weight and the concentration of dissolved solids is below 15% by weight for an aqueous solution. Above these concentrations, a liquid is usually classified as either a slurry or ionic solution. As stated earlier however, there is no universally accepted definition of aqueous liquids, slurries, and ionic solutions.
If the suspended solids levels are high, the turbidity is usually very high. Aqueous liquids with suspended solids levels in the 0.2 to 2% by weight range can have turbidities that approach 100%. In other words, it is difficult to see through a sample of the liquid.
Aqueous liquid with high dissolved solids levels, but without any suspended solids, remain essentially as clear as pure water. The dissolved species in the liquid phase do not scatter light to cause turbidity.
Aqueous liquids can also have substantial quantities of dissolved liquids. There are a number of liquids that dissolve very readily in water. For example, isopropyl alcohol is essentially infinitely soluble in water. In some emission testing systems, isopropyl alcohol and water solutions are used in impingers for treating the sample gas stream prior to pollutant capture.
Aqueous liquid is the most common type of liquid in air pollution control and emission testing situations. Throughout the remainder of these modules, aqueous liquids will be termed simply "liquids."
Slurries are liquids that have a very high level of suspended solids. In air pollution control applications, slurries of alkaline solids and water are prepared for use in acid gas scrubbing systems. The slurries have suspended solids concentrations that vary from as low as 2% by weight to more than 30% by weight of the liquid.
Some air pollution control systems create solutions with very high concentrations of dissolved solids. Many of the dissolved species are ions, which have either a positive or negative charge. Due to their high concentration of charged species, these ionic or electrolytic solutions have properties that are different from those of city water, aqueous mixtures, or slurries. (These modules will use the term ionic rather than electrolytic when discussing these solutions.) Ionic solutions have at least 15% ionic dissolved species by weight.
One common example of an ionic solution is the scrubbing liquid in a dual alkali scrubber. In this type of system, the concentration of dissolved species can exceed 15% by weight.
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#1 - How would you classify the liquids shown at Points A - H in the
scrubber system shown below?
- City water
- Aqueous liquid
- Slurry
- Ionic solution
- City water
Practice Problems
Types of Liquids
- Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.