Treatment of gas emissions coming from flexible packaging printing

When a new rotogravure printing machine was installed, for production purposes, the customer considered the possibility of using ink diluted with a mixture of solvents instead of commonly used single solvent-based inks.

Accordingly, the gas emissions coming from the rotogravure printing machine and containing the mixture of used solvents cannot be sent to the currently installed solvent recovery plant as it was designed to recover ethyl acetate with a small percentage of ethyl alcohol.

Therefore not being able to send the mixture of various solvents to the solvent recovery plant means that the data relative to the emission to be treated must be analysed:

DESCRIPTIONDATA
Emission flow rate50.000 Nm³/h
VOS concentration5-8 g/Nm³
VOS typeVarious solvents

The applicable technologies orbit around thermal oxidation. Specifically the following can be valid for this application:

  • catalytic oxidation plant
  • recuperative thermal oxidation plant
  • regenerative thermal oxidation plant

Description of the project

The choice of proposing and installing a regenerative thermal oxidizer led to the development of a design for a 5-tower machine.

The air stream coming from the rotogravure printing machine, drawn by a specific extraction fan, crosses the first two towers containing the honeycomb ceramic packing bodies heated during the previous cycle from the bottom upwards. This way the air stream is heated up to a temperature as close to the reaction temperature as possible (750-800°C, based on the substances being abated), thanks to the thermal exchange with the ceramic mass and combustion of the VOS.

It then goes to the combustion chamber for the required permanence time at a set temperature controlled by an auxiliary burner.

These conditions guarantee the complete oxidation of the organic substances in the emission being treated.

Leaving the combustion chamber, the hot and purified gases pass through the third and fourth tower from the top downwards, yielding the heat they contain and thus heating the ceramic material contained in them.

The reversal of the streams at scheduled and automatically adjusted intervals recovers the heat of the second towers and returns to the initial conditions.

A fifth tower is necessary so that, before each stream reversal, the gas volume contained and not yet treated can be cyclically purged.

Innovative aspects of the project

Ceramic material

Unlike the more commonly used saddle shape, a honeycomb component was used as ceramic material for the heat recovery system.

The larger specific surface and the enhanced thermal recovery efficiency reduce pressure drops and therefore power consumption by the extraction fan, as well as reduce the volume necessary and thereby the weight and overall dimensions of the plant.

Distribution valves

These are globe valves designed and developed to guarantee long-lasting tightness reliability and less need for maintenance.

Towers

Considering the flow rate to be treated, it was decided to use the 5-tower solution to make the plant more compact and to reduce the size of the various components.

A 3-tower solution would have meant 3 equal towers (one for heating, one for cooling and one for purging) while the 5-tower solution allows having only one purging tower and namely 50% of the heating or cooling units.

Heat recovery

By installing a hot by-pass which connects the combustion chamber and the emission stack, it is not only possible to control the excess heat but also, using an appropriate heat exchanger, to recover it.

In this case, an exchanger was installed to heat the diathermic oil with a capacity of over 1200 kWh, namely sized according to the heat available with an inlet concentration of 5 g/Nm³.

Intake air flow rate adjustment

An electronic vacuum controller installed on the plant inlet pipe makes it possible to adjust fan speed, by means of an inverter, and therefore the intake air flow rate based on production requirements.

Plant supervision

A sophisticated supervision system, managed by an industrial PC, manages and views the operating conditions of the plant on graphical screens. Furthermore, by reading appropriate parameters, it is possible to assess elements such as plant efficiency, utility consumption, recovered heat, etc.

Analyser

The FID analyser is capable of continuously controlling and recording the value of the treated emissions as well as the plant inlet concentration, useful to verify the recovered heat.

Specific objectives of the order, specific issues addressed

The company’s goal is to fulfil legal standards without creating new problems to the existing and operating solvent recovery system. The customer therefore wanted to keep the pollutant emissions below the legal limits together with reduced management costs of the plant and even to recover more energy useful for their company.

Abatement efficiency

Following the most stringent European guidelines, this system can guarantee the abatement of pollutants at a maximum outlet value < 20 mg/Nm³ (which with an inlet 5 g/Nm³ equal to an efficiency level of higher than 99.5 %). Actually the value obtained is even better, in the vicinity of 5 mg/Nm³.