Halogenated Solvent Recovery Coating process

A process to recover ethylene dichloride, EDC, and methylene dichloride, MDC, using activated carbon fibers, ACF, is discussed in this section. The gas at approximately 50°C, containing EDC and MDC is received from a painting process. It is pretreated by a filter to remove dusts and passed through a heat exchanger to cool it down to approximately 30-40°C. The pre-treated gas is fed to adsorption columns filled with ACF where the gas is cleaned sufficiently for it to be safely released to atmosphere.

Feed

Used solvent

Table 22.2.6. Brief specification of the cycloparaffins recovery unit

Item

Specification

Solvent

cycloparaffins

Capacity

70 l/min

Dimensions

1,050 x 533 x 1,355 mm

Weight

250 kg (approx.)

Steam pressure

6.0 MPa

Steam consumption

25 kg/h

Cooling water

<30°C, >2MPa, >20 l/min

Process Flow Steam Adsorption
Degassing unit Figure 22.2.9. Flow diagram of ACF solvent recovery process.
Process Flow Steam Adsorption
Figure 22.2.9 shows the process flow diagram. The process consists of two (or more) adsorption columns to ensure a continuous run of adsorption and desorption cycles. Figure 22.2.10 shows an industrial application of the system.

Item

Operational & performance data

Flow rate

100 m3/min, 20°C

Solvent

EDC MDC

46.7 l/h 23.3 l/h

Gas concentration Inlet Outlet

2,900 ppm 58 ppm

Adsorption efficiency

98%

ACF specific surface area

900 - 2,000 m2/g

Adsorption & desorption cycle

10 min

Table 22.2.7. Performance data for the The desorption is done by steam. The

ACF solvent recovery unit desorbed stream is fed to a series of two condensers in which both the steam and solvent are condensed. The condensate is fed to a decanter to separate water and solvent by gravity. Water containing approximately 1-2% solvent is fed to a degassing unit and the vaporized solvent is joined to the feed. The decanted solvent is refined, stored in a tank, analyzed, and adjusted to its appropriate EDC/MDC ratio. The refined solvent is recycled for painting.

Water from the degassing unit contains solvent in a concentration relative to the solvent's solubility. For example, if the solubility of perchloroethylenene in water is designated as 1, the relative solubility of EDC in water would be 58 and of MDC in water would be 88. In degassing at 60°C and 180 l/minx 30 minthe residual EDC content in water is brought below 0.01 ppm (the environmental standard limit is 0.04 ppm) but in the case of MDC it remains a problem to reach to the environmental standard of 0.2 ppm. A treatment by activated carbon may be one of the solutions. Table 22.2.7 shows performance data in the industrial application.

The performance of the ACF solvent recovery process can be summarized as follows: Since ACF has lager surface area and smaller bulk density than particulate activated carbon, PAC, ACF's filling density becomes 1/10 ofPAC's.

Advantages:

  • High recovery rate - The solvent recovery rate of the ACF process is higher than that of the PAC, especially in the case of lower boiling point solvents, such as, MDC, benzene, trichloroethane, etc.
  • High quality - ACF has low catalytic activity and a short adsorption and desorption cycle time (approx. 10 min). The solvent is less decomposed in the process. In the recovery of chlorine-containing solvents, it produces less acids as decomposition products. Thus it is less corrosive to the materials of construction. Also, the recovered solvent is of better quality.
  • Broader application - It is applicable to polymerizing monomers and high boiling point compounds.
  • Light weight, compact and safe - The process unit can be compact because of the short adsorption and desorption cycles and it is safer because there is less heat accumulation.
  • Energy savings - The process needs less steam consumption in the adsorption and desorption.

Disadvantages:

  • Less flexibility regarding capacity - ACF's filling volume cannot be readily changed.
  • Difficult in decreasing outlet temperature - The short cycle in adsorption and desorption makes the cooling process difficult to synchronize.
  • Expensive - ACF are more expensive than PAC, thus the unit modification cost is more expensive.
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