Safety requirements

Adsorption systems including all the auxiliary and ancillary equipment needed for their operation are subject to the applicable safety legislation and the safety requirements of the trade associations and have to be operated in accordance with the respective regulations.2'19

Moreover, the manufacturer's instructions have to be observed to minimize safety risks. Activated carbon or its impurities catalyze the decomposition of some organics, such as ketones, aldehydes and esters. These exothermic decomposition reactions25'26 can generate localized hot spots and/or bed fires within an adsorber if the heat is allowed to build up. These hazards will crop up if the flow is low and the inlet concentrations are high, or if an adsorber is left dormant without being completely regenerated. To reduce the hazard of bed fires, the following procedures are usually recommended:

  • Adsorption of readily oxidizing solvents (e.g., cyclohexanone) require increased safety precautions. Instrumentation, including alarms, should be installed to monitor the temperature change across the adsorber bed and the outlet CO/CO2 concentrations. The instrumentation should signal the first signs of decomposition, so that any acceleration leading to a bed fire can be forestalled. Design parameters should be set so as to avoid high inlet concentrations and low flow. A minimum gas velocity of approximately 0.2 m/s should be maintained in the fixed-bed adsorber at all times to ensure proper heat dissipation.
  • A virgin bed should be steamed before the first adsorption cycle. Residual condensate will remove heat.
  • Loaded activated carbon beds require constant observation because of the risk of hot spot formation. The bed should never be left dormant unless it has been thoroughly regenerated.
  • After each desorption cycle, the activated carbon should be properly cooled before starting a new adsorption cycle.
  • Accumulation of carbon fines should be avoided due to the risk of local bed plugging which may lead to heat build up even with weakly exothermic reactions.
  • CO and temperature monitors with alarm function should be provided at the clean gas outlet.
  • The design should also minimize the possibility of explosion hazard
  • Measurement of the internal adsorber pressure during the desorption cycle is useful for monitoring the valve positions.
  • Measurement of the pressure loss across the adsorber provides information on particle abrasion and blockages in the support tray.
  • Adsorption systems must be electrically grounded. 22.1.4.3 Special process conditions 22.1.4.3.1 Selection of the adsorbent

The adsorption of solvents on activated carbon1-4 is controlled by the properties of both the carbon and the solvent and the contacting conditions. Generally, the following factors, which characterize the solvent-containing waste air stream, are to be considered when selecting the most well suited activated carbon quality for waste air cleaning: for solvent/waste air:

  • solvent type (aliphatic/aromatic/polar solvents)
  • concentration, partial pressure
  • molecular weight
  • density
  • boiling point, boiling range
  • critical temperature
  • desorbability
  • explosion limits
  • thermal and chemical stability
  • water solubility
  • adsorption temperature
  • adsorption pressure
  • solvent mixture composition
  • solvent concentration by components
  • humidity
  • impurities (e.g., dust) in the gas stream for the activated carbon type: General properties:
  • apparent density
  • particle size distribution
  • hardness
  • surface area
  • activity for CCl4/benzene
  • the pore volume distribution curve

For the selected solvent recovery task:

  • the form of the adsorption isotherm
  • the working capacity
  • and the steam consumption

The surface area and the pore size distribution are factors of primary importance in the adsorption process. In general, the greater the surface area, the higher the adsorption capacity will be. However, that surface area within the activated carbon must be accessible. At low concentration (small molecules), the surface area in the smallest pores, into which the solvent can enter, is the most efficient surface. With higher concentrations (larger molecules) the larger pores become more efficient. At higher concentrations, capillary condensation will take place within the pores and the total micropore volume will become the limiting factor. These molecules are retained at the surface in the liquid state, because of intermolecular or van der Waals forces. Figure 22.1.9 shows the relationship between maximum effective pore size and concentration for the adsorption of toluene according to the Kelvin theory.

It is evident that the most valuable information concerning the adsorption capacity of a given activated carbon is its adsorption isotherm for the solvent being adsorbed and its pore volume distribution curve. Figure 22.1.10 presents idealized toluene adsorption isotherms for three carbon types:

  • large pores predominant
  • medium pores predominant
  • small pores predominant

The adsorption lines intersect at different concentrations, depending on the pore size distribution of the carbon. The following applies to all types of activated carbon: As the toluene concentration in the exhaust air increases, the activated carbon load increases.

t i...

1 !

!

.

! :

i ! !

I 1

Toluene concentration [g/m3]

Toluene concentration [g/m3]

Figure 22.1.9. Relationship between maximum effective pore diameter and toluene concentration.

Relative saturation p/p.

Figure 22.1.10. Idealized toluene adsorption isotherms.

Relative saturation p/p.

Figure 22.1.10. Idealized toluene adsorption isotherms.

Concentration [g/in j

Figure 22.1.11. Adsorption isotherm of five typical solvents (After reference 13).

Concentration [g/in j

Figure 22.1.11. Adsorption isotherm of five typical solvents (After reference 13).

From the viewpoint of adsorption technology, high toluene concentration in the exhaust air is more economic than a low concentration. But to ensure safe operation, the toluene concentration should not exceed 40% of the lower explosive limit. There is, however, an optimum activated carbon type for each toluene concentration (Figure 21.1.10).

The pore structure of the activated carbon must be matched to the solvent and the solvent concentration for each waste air cleaning problem. Figure 22.1.11 shows Freundlich adsorption isotherms of five typical solvents as a function of the solvent concentrations in the gas stream. It is clear that different solvents are adsorbed at different rates according to the intensity of the interacting forces between solvent and activated carbon.

Physical-chemical properties of three typical activated carbons used in solvent recovery appear in Table 22.1.9.

Table 22.1.9. Activated carbon pellets for solvent recovery (After reference 13)

AC-Type

C 38/4

C 40/4

D 43/4

Appearance

cylindrically shaped

Bulk density (shaken), kg/m3

380+20

400+20

430+20

Moisture content, wt%

<5.0

<5.0

<5.0

Ash content, wt%

<6.0

<6.0

<5.0

Particle diameter, mm

4

4

4

Surface area (BET), m2/g

>1250

>1250

>1100

Carbon tetrachloride activity, wt%

80+3

75+3

67+3

Producers13 of activated carbon are in the best position to provide technical advice on

  • selecting the right activated carbon type
  • contributing to the technical and economical success of a solvent recovery plant

22.1.4.3.2 Air velocity and pressure drop

In solvent recovery systems, air velocity rates through the bed should be between 0.2 to 0.4 m/s. The length of the MTZ is directly proportional to the air velocity. Lower velocities (<0.2 m/s) would lead to better utilization of the adsorption capacity of the carbon, but there is a danger that the heat of adsorption not be carried away which would cause overheating and possibly ignition of the carbon bed.

The power to operate the blowers to move waste air through the system constitutes one of the major operating expenses of the system. Pressure drop (resistance to flow) across the system is a function of

  • air velocity
  • bed depth
  • activated carbon particle size

Small particle size activated carbon will produce a high pressure drop through the activated-carbon bed. Figure 22.1.12 compares the pressure drop of cylindrically-shaped activated carbon pellets with activated-carbon granulates. The activated carbon particle diameter must not be excessively large, because the long diffusion distances would delay adsorption and desorption. Commercially, cylindrical pellets with a particle diameter of 3 to 4 mm have been most efficient.

22.1.4.3.3 Effects of solvent-concentration, adsorption temperature and pressure

For safety reasons the concentration of combustible solvent vapors should be less than 50 % of the lower explosive limit. The adsorption capacity of adsorbents increases as the concentration of the solvents increases. But the length of the MTZ is proportional to the solvent concentration. Because of the adsorption heat, as the adsorption front moves through the

Solvent Concentration

Tcmpeiature=20°'

Figure 22.1.12. Pressure drop for various activated carbon types.

Tcmpeiature=20°'

Figure 22.1.12. Pressure drop for various activated carbon types.

Figure 22.1.13. Adsorption isotherm of tetrahydrofuran for several temperatures.

Concentration [g/m ]

Figure 22.1.13. Adsorption isotherm of tetrahydrofuran for several temperatures.

bed also a temperature front follows in the same direction. To deal with the adsorption heat the inlet solvent concentration is usually limited to about 50 g/m3.

The adsorption capacity of the adsorbent increases with pressure because the partial pressure of the solvent increases. An increase in adsorber temperature causes a reduction in adsorption capacity. Because the equilibrium capacity is lower at higher temperatures, the dynamic capacity (working capacity) of the activated carbon adsorber will also be lower. To enhance adsorption, the inlet temperature of the adsorber should be in the range of20-40°C. In Figure 22.1.13 the adsorption isotherms of tetrahydrofuran on activated carbon D43/3 for several temperatures are shown.13

Activated Carbon Adsorption Isotherm
Figure 22.1.15. Dynamic toluene adsorption from humidified air (after reference 24).

22.1.4.3.4 Influence of humidity

Activated carbon is basically hydrophobic; it adsorbs preferably organic solvents. The water adsorption isotherm (Figure 22.1.14) reflects its hydrophobic character. Below a relative gas humidity of about 40% co-adsorption of water can be neglected in most applications. However, higher humidity of the waste gases may affect the adsorption capacity of the activated carbon. Figure 22.1.15 demonstrates, using toluene as an example, how the relative humidity of the exhaust air influences the activated carbon loads.

Experimental work24 has shown that

  • relative humidity rates below 30% will neither reduce the adsorption capacity nor the adsorption time,
  • relative humidity rates above 70% substantially reduce the adsorption capacity,
  • the humidity of the gas will affect the adsorption capacity much more with low toluene concentrations than with high concentrations,
  • with toluene concentrations between 10 and 20 g/m3 the negative influence of the humidity is small.

The water content of the activated carbon after desorption may constitute another problem. The purification efficiency of each activated carbon is the better the less water is present after desorption. Unfortunately, the desorbed activated carbon in the vicinity of the adsorber walls usually contains high proportions of water (approx. 10 to 20%). With such a high water content it is difficult to remove all the water even when drying with hot-air over longer periods. The wet and poorly-regenerated activated carbons in these zones frequently lead to higher solvent concentrations in the purified air, and this is even at the beginning of the adsorption cycle. Some proposals for improvement include:

  • good insulation of the adsorber walls,
  • sufficient drying after each regeneration cycle,
  • cycle the desorption steam and the drying air counter-current to the direction of the adsorptive stream.

22.1.4.3.5 Interactions between solvents and activated carbon

The majority of solvents are effectively recovered by adsorption on activated carbon and, when this is the case, the operation of the plant is straightforward. But some solvents may decompose, react or polymerize when in contact with activated carbon during the adsorption step and the subsequent steam desorption.19'25'26

Chlorinated hydrocarbon solvents can undergo hydrolysis to varying degrees on carbon surfaces, resulting in the formation of hydrogen chloride. Each activated carbon particle which is in contact with a metal screen or other constructional component may then act as a potential galvanic cell in the presence of moisture and chloride ions.

Carbon disulfide can be catalytically oxidized to sulphur which remains on the internal surface of the activated carbon.

Esters such as ethyl acetate are particularly corrosive because their hydrolysis results in the formation of e.g. acetic acid.

Aldehydes and phenol or styrene will undergo some degree of polymerization when in contact with hot activated carbon.

During the adsorptive removal of ketones such as methyl ethyl ketone or cyclohexanone a reduced adsorption capacity has been measured. Corrosion problems were also apparent and in some cases even spontaneous ignition of the activated carbon occurred.

Activated carbon acts, in the presence of oxygen, as a catalyst during adsorption and even more frequently during desorption because of higher temperature.25 26 Figure 22.1.16 shows that the catalytic reactions occurring with cyclohexanone are predominantly of the oxidation type. Adipic acid is first formed from cyclohexanone. Adipic acid has a high boiling point (213°C at a 13 mbar vacuum). Further products identified were: cyclopentanone as a degradation product from adipic acid, phenol, toluene, dibenzofuran, aliphatic hydrocarbons and carbon dioxide. The high-boiling adipic acid cannot be desorbed from the activated carbon in a usual steam desorption. Consequently the life of the activated carbon becomes further reduced after each adsorption/desorption cycle.

Activated Carbon Reaction
Figure 22.1.16. Surface reaction of cyclohexanone on activated carbon (After reference 25).
Figure 22.1.17. Surface reaction of methyl ethyl ketone on activated carbon (After reference 25).

During adsorption of methyl ethyl ketone in the presence of oxygen, some catalytic surface reactions occur (Figure 22.1.17). The reaction products are acetic acid, di-acetyl, and presumably also di-acetyl peroxide. Di-acetyl has an intensive green coloration and a distinctive odor. Important for the heat balance of the adsorber is that all of the reactions are strongly exothermic.

Unlike the side products of cyclohexanone, acetic acid the reaction product of the MEK (corrosive!) will be removed from the activated carbon during steam desorption. Adsorption performance is thus maintained even after a number of cycles.

Operators of ketones recovery plants should adhere to certain rules and also take some precautions25

  • Adsorption temperature should not be higher than 30°C (the surface reaction rates increase exponentially with temperature).
  • The above requirement of low-adsorption temperatures includes:
  1. adequate cooling after the desorption step;
  2. a flow velocity of at least 0.2 m/s in order to remove the heat of adsorption. Since ketones will, even in an adsorbed state, oxidize on the activated carbon due to the presence of oxygen, these further precautions should be taken:
  • keep the adsorption/desorption cycles as short as possible,
  • desorb the loaded activated carbon immediately at the lowest possible temperatures,
  • desorb, cool and blanket the equipment with inert gas before an extended shut down.

22.1.4.3.6 Activated carbon service life

Activated carbon in solvent recovery service will have a useful service life of 1 to 10 years, depending on the attrition rate and reduction in adsorption capacity.

Attrition rates are usually less than 1-3% per year. The actual rate will depend on carbon hardness. Particle abrasion and the resulting bed compaction leads to an increased pressure loss after several years of service. After 3-5 years of service screening to its original size is necessary.

Adsorption capacity can be reduced by traces of certain high-boiling materials (resins, volatile organosilicone compounds) in the waste air which are not removed during the desorption cycle. Some solvents may decompose, react or polymerize when in contact with activated carbon and steam (Section 22.1.4.3.5).

This gradual loading will decrease the carbon's activity. While carbon can remain in service in a reduced capacity state, it represents a non-optimal operation of the system that results in

  • reduced amount of solvent removed per cycle
  • increased steam consumption
  • higher emissions
  • shorter adsorption cycles

Eventually, recovering capacity will diminish; it is more economical to replace carbon than continue its use in a deteriorated state. In replacing spent activated carbon there are two options:

  • Replacing with virgin carbon and disposal of the spent carbon.
  • Off-site reactivation of the spent carbon to about 95% of its virgin activity for about one-half of the cost of new carbon

Some activated carbon producers offer a complete service including carbon testing, off-site reactivation, transportation, adsorber-filling and carbon make-up.13 22.1.5 EXAMPLES FROM DIFFERENT INDUSTRIES

Some examples of solvent recovery systems in different industries show that reliable and trouble-free systems are available. 22.1.5.1 Rotogravure printing shops

Process principle

Adsorptive solvent recovery with steam desorption and condensation units with gravity separator and a stripper have become a standard practice in modern production plants. The solvents-laden air (toluene, xylene) is collected from emission points, e.g., rotogravure printing machines, drying ducts by means of a blower and passed through the recovery plant.

Design example30

In 1995, one of Europe's most modern printing shops in Dresden was equipped with an adsorptive solvent recovery system (Supersorbon® process (Figure 22.1.18.)).

Design data

(First stage of completion)

Exhaust air flow rate 240,000 m3/h

(expandable to 400,000 m3/h)

Solvent capacity 2,400 kg/h (toluene)

Exhaust air temperature 40°C

Toluene concentration in clean air max. 50 mg/m3

(half-hour mean)

Process Benzene Toluene Xylene
Figure 22.1.18. Supersorbon process for toluene recovery (After reference 30).

Solvent-laden air is exhausted at the three rotogravure printing presses by several fans operating in parallel and is routed in an upward flow through four adsorbers packed with Supersorbon® activated carbon. The solvent contained in the air is adsorbed on the activated carbon bed. Adsorption continues until breakthrough, when the full retentive capacity of the adsorbent for solvent vapors is used up.

The purity of the clean exhaust air complies with the regulation for emission level of less than 50mg/m3. Regeneration of the adsorbent takes place by desorbing the solvent with a countercurrent flow of steam. The mixture of water and toluene vapors is condensed and the toluene, being almost insoluble in water, is separated by reason of its different density in a gravity separator. The recovered toluene can be re-used in the printing presses without further treatment. Very small amounts of toluene dissolved in the wastewater are removed by stripping with air and returned to the adsorption system inlet together with the stripping air flow. The purified condensate from the steam is employed as make-up water in the cooling towers.

Operating experience

The adsorption system is distinguished by an extremely economical operation with a good ratio of energy consumption to toluene recovery. Solvent recovery rate is about 99.5 %.

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