Файл: Курсовая работа по дисциплине Процессы и аппараты химической технологии на тему Adsorption.docx

Modern use of adsorbents began with the discovery of the decoloring effect of charcoal on solutions, followed by the invention of a charcoal cartridge for personal protection during the First World War.

With the increasing availability of different types of adsorbents nowadays, and the more recent interests in biotechnology and green technology, there has been a great expansion in the applications of adsorption in many areas. A partial list of adsorption applications follows. [8]

For liquid-phase adsorption

• Decoloring, drying, or degumming of petroleum products

• Removing of dissolved organic species from water supplies

• Removing odor, taste, and color from water supplies

• Advanced treatment of waste water (domestic and industrial)

• Decoloring of crude sugar syrup and vegetable oils

• Recovery and concentration of proteins, pharmaceuticals, and bio-compounds from dilute suspensions

• Bulk separation of paraffin and isoparaffins

For gas-phase adsorption

• Recovering organic solvent vapors

• Dehydration of gases

• Removing toxic agents and odor for personal protection

• Air separation

• Separating normal paraffins from isoparaffin aromatics

• CO2 capture for addressing climate change [2]

Applications of Adsorption

1) Air pollution masks:

These consist of silica gel or activated charcoal powder, when dust or smoke are paused through them, those particles get adsorbed on the surface of these materials.

2) Separation of noble gases by Dewar’s flask process:

A mixture of noble gases of Ne, Ar, Kr is passed through Dewar’s flask in presence of heated coconut charcoal. Argon and Krypton gels adsorbed leaving Neon.

3) Purification of water:

By the addition of alum stone to the water, impurities get adsorbed on the alum and water gets purified.

4) Removal of moisture and humidity:

Moisture in the air is removed by placing silica gel on which water molecular gets adsorbed.

5) Adsorption chromatography:

It is used to separate pigments and hormones.

6) Ion exchange method:

In this method of removing the hardness of water, calcium and magnesium ions get adsorbed on the surface of ion exchange resin.

4 ADSORPTION PROCESSES IN WATER TREATMENT

4.1 Introduction to adsorption processes in water treatment

Adsorption processes are widely used in water treatment. Depending on the adsorbent type applied, organic substances as well as inorganic ions can be removed from the aqueous phase. Activated carbon is the most important engineered adsorbent applied in water treatment. It is widely used to remove organic substances from different types of water such as drinking water, wastewater, groundwater, landfill leachate, swimming-pool water, and aquarium water. Other adsorbents are less often applied. Their application is restricted to special adsorbates or types of water. [10]

Table 4.1 Adsorption processes in water treatment

Application field	Objective	Adsorbent
1	2	3
Drinking water treatment	Removal of dissolved organic matter	Activated carbon
	Removal of organic micropollutants	Activated carbon
	Removal of arsenic	Aluminum oxide, iron hydroxide
Urban wastewater treatment	Removal of phosphate	Aluminum oxide, iron hydroxide
Urban wastewater treatment	Removal of micropollutants	Activated carbon
Industrial wastewater treatment	Removal or recycling of specific chemicals	Activated carbon, polymeric adsorbents

Continuing of table 4.1

1	2	3
Swimming-pool water treatment	Removal of organic substances	Activated carbon
Groundwater remediation	Removal of organic substances	Activated carbon
Treatment of landfill leachate	Removal of organic substances	Activated carbon
Aquarium water treatment	Removal of organic substances	Activated carbon

4.2 Adsorption in drinking water treatment

For nearly 100 years, adsorption processes with activated carbon as adsorbent have been used in drinking water treatment to remove organic solutes. At the beginning, taste and odor compounds were the main target solutes, whereas later the application of activated carbon was proved to be efficient for removal of a wide range of further organic micropollutants, such as phenols, chlorinated hydrocarbons, pesticides, pharmaceuticals, personal care products, corrosion inhibitors, and so on. Since NOM (natural organic matter) is present in all raw waters and often not totally removed by upstream processes, it is always adsorbed together with the organic micropollutants. Since activated carbon is not very selective in view of the adsorption of organic substances, the competitive NOM adsorption and the resulting capacity loss for micropollutants cannot be avoided. The competition effect is often relatively strong not least due to the different concentration levels of DOC (dissolved organic carbon) and micropollutants. The typical DOC concentrations in raw waters are in the lower mg/l range, whereas the concentrations of organic micropollutants are in the μg/l range. On the other hand, the NOM removal also has a positive aspect. NOM is known as a precursor for the formation of disinfection by-products (DBPs) during the final disinfection with chlorine or chlorine dioxide. Therefore, removal of NOM during the adsorption process helps to reduce the formation of DBPs. [12]

Activated carbon is applied as powdered activated carbon (PAC) in slurry reactors or as granular activated carbon (GAC) in fixed-bed adsorbers. [11]

In recent years, the problem of arsenic in drinking water has increasingly attracted public and scientific interest. In accordance with the recommendations of the World Health Organization (WHO), many countries have reduced their limiting value for arsenic in drinking water until 10 μg/l. As a consequence, a number of water works, in particular in areas with high geogenic arsenic concentrations in groundwater and surface water have to upgrade their technologies by introducing an additional arsenic removal process. Adsorption processes with oxidic adsorbents such as ferric hydroxide or aluminum oxide have been proved to remove arsenate very efficiently. The same adsorbents are also expected to remove anionic uranium and selenium species. [14]

4.3 Adsorption in wastewater treatment

The conventional wastewater treatment process includes mechanical and biological treatment (primary and secondary treatment). In order to further increase the effluent quality and to protect the receiving environment, more and more often a tertiary treatment step is introduced into the treatment train. A main objective of the tertiary treatment is to remove nutrients, which are responsible for eutrophication of lakes and rivers. To remove the problematic phosphate, a number of different processes are in use (e.g., biological and precipitation processes). Adsorption of phosphate onto ferric hydroxide or aluminum oxide is an interesting alternative in particular for smaller decentralized treatment plants. A further aspect is that adsorption allows for recycling the phosphate, which is a valuable raw material, for instance, for fertilizer production. [13]

In recent years, the focus has been directed to persistent micropollutants, which are not degraded during the activated sludge process. To avoid their input in water bodies, additional treatment steps are in discussion and in some cases already realized. Besides membrane and oxidation processes, adsorption onto activated carbon is considered a promising additional treatment process because its suitability to remove organic substances is well known from drinking water treatment. As in drinking water treatment, the micropollutant adsorption is influenced by competition effects, in this case between the micropollutants and the effluent organic matter. [15]

In industrial wastewater treatment, adsorption processes are also an interesting alternative, in particular for removal or recycling of organic substances. If the treatment objective is only removal of organics from the wastewater, activated carbon is an appropriate adsorbent. On the other hand, if the focus is more on the recycling of valuable chemicals, alternative adsorbents (e.g., polymeric adsorbents), which allow an easier desorption (e.g., by solvents), can be used. [14]

4.4 Adsorption in hybrid processes in water treatment

Adsorbents can also be used in other water treatment processes to support these processes by synergistic effects. Mainly activated carbon, in particular PAC, is used in these hybrid processes. As in other activated carbon applications, the target compounds of the removal processes are organic substances. [11]

Addition of PAC to the activated sludge process is a measure that has been well known for a long time. Here, activated carbon increases the removal efficiency by adsorbing substances that are not biodegradable or inhibit biological processes. Furthermore, activated carbon provides an attachment surface for the microorganisms. The high biomass concentration at the carbon surface allows for an enhanced degrading of initially adsorbed substances. Due to the biological degradation of the adsorbed species, the activated carbon is permanently regenerated during the process. This effect is referred to as bio regeneration. In summary, activated carbon acts as a kind of buffer against substances that would disturb the biodegradation process due to their toxicity or high concentrations. Therefore, this process is in particular suitable for the treatment of highly contaminated industrial wastewaters or landfill leachates. [13]

The combination of activated carbon adsorption with membrane processes is a current development in water treatment. In particular, the application of PAC in ultrafiltration (UF) and nanofiltration (NF) processes is under discussion and in some cases already implemented. Ultrafiltration membranes are able to remove particles and large molecules from water. By adding PAC to the membrane system, dissolved low-molecular weight organic substances can be adsorbed and removed together with the PAC and other particles. As an additional effect, a reduction of membrane fouling can be expected because the concentration of organic matter is decreased by adsorption. Addition of PAC to nanofiltration systems is also proposed, although nanofiltration itself is able to remove dissolved substances, including small molecules. Nevertheless, a number of benefits of an NF/PAC hybrid process can be expected. The high solute concentrations on the concentrate side of the membrane provide favorable conditions for adsorption so that high adsorbent loadings can be achieved. Furthermore, the removal of organics on the concentrate side by adsorption decreases the organic membrane fouling. Additionally, abrasion caused by the activated carbon particles reduces the coating of the membrane surface. [15]

UF/PAC or NF/PAC hybrid processes can be used for different purposes, such as drinking water treatment, wastewater treatment, landfill leachate treatment, or groundwater remediation. [11]

5 ADSORBENTS FOR WATER TREATMENT

5.1 Introduction to adsorbent classification for water treatment

Adsorbents are either derived from natural sources or from industrial production or activation processes when used to treat water. Natural zeolites, clay minerals, oxides, and biopolymers are examples of common natural adsorbents. Engineered adsorbents can be divided into carbonaceous, polymeric, oxidic, and zeolite molecular sieve adsorbents. The most frequently used adsorbents in water treatment are activated carbons, which are made from carbonaceous material by chemical activation or gas activation. Although polymeric adsorbents made by copolymerizing nonpolar or weakly polar monomers exhibit adsorption properties comparable to activated carbons, a wider application has not yet been possible due to high material costs and expensive regeneration. Adsorbents with stronger hydrophilic surface properties include oxides and zeolites. Therefore, their preferred field of application is the removal of polar compounds, particularly ionic ones. [16]

In general, the highest adsorption capacities are found in engineered adsorbents. They exhibit nearly constant qualities and are produced with strict quality control. Most of the time, recommendations for application can be derived from scientific studies and information provided by producers because the adsorption behavior towards a wide range of adsorbates is well understood. Engineered adsorbents, on the other hand, are frequently very expensive. Natural and other inexpensive adsorbents, in contrast, have much lower adsorption capacities and more variable properties. Because of their low cost, they might be interesting, but most studies on LCAs are restricted to very specific applications, and there isn't enough data to generalize the experiences and make a final judgment. [18]