Solvent Recovery System for Flexible Packaging Industries
Activated Carbon adsorption, a new desorption technology with Inert Gas (nitrogen)
paper presented at the "International Schroeder Conferences on innovations in Plastic Based Packaging", Chicago - USA, by:
Andrea FORMIGONI
Managing Director (M.Sc. Chem. Eng.)
Alessandro PIROTTA
Sales Manager (Ph.D. Packaging and Paper Technology)
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A brief discussion about the new future VOC European directive based on two key points:
The optimization of SLA is the first target of a converting business, independently of the process chosen, whether solvent recovery or oxidation.
This principle is the key to the economy of the entire project as today it has been widely demonstrated. This investment will be applied to printing presses in use, because only in the past two years, manufacturers of rotogravure and flexo printing presses have proposed equipment complete of LEL controlling and automatic recycling of SLA.
The optimization of the printing inks, intended as solvent mixture, is the second target. It is necessary to seek a system of two or three main solvents, depending on the printing process, that may be considered "recoverable". A "recoverable" solvent system means being able to recycle the input solvent to obtain a pure solvent or a mixture of solvents of a purity similar to that of the ones purchased or effectively used in dilution.
The method most prevalent today for recovery of solvents is the use of activated carbon. The process is highly effective in removing the VOC's from the air and removal efficiencies of >99,5% are common. Once the carbon has reached "breakthrough" saturation, the process is reversed (desorption) in order to recover the "adsorbed" solvents.
The desorption method once universally used in solvent recovery process was steam desorption (DEC.RSV™). Steam by virtue of its high heat content and superior heat transfer characteristics has been always considered a very effective and economical desorption medium: this is especially true when dealing with water "insoluble" solvent (aliphatic - aromatic hydrocarbons). The solvents commonly used in the converting industry such as ethyl acetate and ethyl alcohol (or others alcohol) however present a big problem for the steam regeneration process. This is because these solvents are either partially or completely soluble in water.
Over the years various non condensing medium such as air, flue gases and nitrogen have used for desorbing activated carbon. The impracticality of complete condensation of the solvent vapors from the gas stream due to extremely low temperature required, necessitates the use of a recirculation system. These process differences aside, nitrogen gas is an ideal choice as desorption medium. Applying two different exclusive technologies DEC.RSG™ and DEC.MSU™ the results obtained were unbelievable: the recovered solvent/s are virtually anhydrous (typically <0.05% by weight of water) and acetic acid free (typically <0.04% by weight): DEC IMPIANTI has installed several tens of industrial units running since 1980's.
A simple batch or continuous atmospheric distillation system DEC.DIST-a™ (one or two columns) may be used to separate the solvents in the condensed mix, or in case of azeotropic mixtures (between ethyl acetate and alcohol) an additional pressure distillation column DEC.DIST-p™ will be used so to break it.
The recovery system requires in addition the optimization of the solvent system of the printing inks, an operation that European companies that recover today had no problem in carrying out: DEC IMPIANTI gives the right support for this intermediate step. The ROI curves as applied to Italian investment and interest costs, and the overall cost of recovery per 1 kg of solvent at Italian energy costs, show that the return on investment may require from 86 months for smaller installations to 45 months for medium-sized installations and 20 months for large ones. To conform to the E.U. standards means setting up the preliminary conditions for examining the advantages that the solvent recovery system offers to the company, even considering that the legislation forces to use the Best Available Technology (BAT). Note that through very simple distillation systems, the quality of recycled solvents is never inferior to the quality of "virgin" purchased solvents. All this has a total average recovery cost that ranges from 0,1 ÷ 0,25 EUR / kg of recovered solvent (depending on system configuration and local energy costs). This means amortizing substantial cost values equal to about 70÷80% of the purchasing value of the solvents. In conclusion, there is no technological or operative reason why the activated carbon solvent recovery system (nitrogen regeneration) should not be preferred over an oxidation system.
