Scaling the heights of plant design

Plant manufacturer, Airprotech, explores the steps involved in choosing the right technology in the manufacture of air purification plants.

Particular interest has been given to rotor concentrator applications when looking at industrial plant design inrecent years. This technology is the ideal solution to treat flow rates with low-pollutant concentrations that, if treated normally, require large plants with high operating and investment costs.

A plant designed and built for Swiss chemicals manufacturer, Osmopharm, provides a good example. Osmopharm is one of the largest Swiss pharmaceutical companies with over 100 plants operating in thirty countries. In its Bedano, Switzerland factory, Osmopharm needed to treat air from the tablet coating plant. The treatment plant is located on the factory roof.

During the design phase Airprotech put forward three designs, which included a biofiltration plant, a thermal regenerative oxidation plant and finally a VOC abatement system using preconcentration and thermal oxidation.

Biofiltration plant

The biofiltration plant package unit includes a centrifugal fan, a pre-scrubbing unit and a filtration unit. After the gases have been cooled and humidified, they pass through the bio filter where polluted compounds are absorbed on a vegetal filling substrate in which micro-organisms metabolise the pollutants contained in the gas.

Thermal regenerative oxidation plants (RCUC) are mainly based on the capacity of an inert material mass to accumulate heat and ‘give it back’ in a successive phase. Structurally, these plants are made of two inert masses which are used alternatively as ‘pre-heater’ or ‘recuperator’ of heat according to the air routing.

The VOC abatement system by preconcentration and thermal oxidation, has two fundamental components – a concentration unit and a thermal oxidation unit.

With the concentration unit, air to be treated, with low VOC concentration, is purified in the rotor concentrator and exhausts to atmosphere. Below the concentrator, the amount of VOC increases 10-times and the air flow is 10-times lower than the starting one. This application is economically viable because the oxidizer can be smaller and the operating costs are lower.

The concentrator is made of a zeolite rotor and the purified air is conveyed to the atmosphere by a fan through a stack. The zeolites rotor is the heart of the plant and is made of adsorbent and activated material incorporated in a rotating structure.

The adsorbent material is made of a ceramic structured support, strongly impregnated by hydrophobic zeolites, on which the solvents are adsorbed. The desorption air, which has been taken off before the concentrator, crosses first the rotor and then cools the zeolites sector just regenerated and therefore, after being heated up, passes through the desorption zone.

The highly concentrated outlet flow from the concentrator is sent to the oxidation unit where it is totally purified.

Heat losses

The package unit essentially consists of a centrifugal fan and a single body which contains the oxidation chamber, the heat exchanger and the burner. Externally the body is coated with an insulating material in order to limit the heat losses and to maintain the surface at a temperature of about 60°C. The oxidation chamber, which is in stainless steel to withstand high temperatures, is sized to guarantee a residence time of 0.6 seconds at the oxidation temperature of about 760°C.

A line burner complete with a gas train according to EN 746-2 regulations is installed in the oxidation chamber. The oxidation chamber is contained within a second chamber, which has a greater diameter and is the external body of the unit. The tubes of the heat exchanger are located in the cavity between the two chambers.

The polluted air from the concentrator is drawn in by the centrifugal fan installed before the oxidizer and, by means of a tube plate is conveyed into the tubes of the heat exchanger where it is preheated.

Polluted air

At the exit of the recuperator, the polluted air is fed into the line burner where the temperature is increased to the operating temperature. At this temperature VOCs are oxidised to CO2 and H2O with heat formation proportional to the solvent concentration.

In the oxidation chamber, the temperature is kept constant by means of a loop controlling the modulating valve of the gas control train.

The oxidation products leave the oxidation chamber, cross the cavity (equipped with special deflectors) between the two chambers and pass over the outside of the tubes of the heat exchanger, decreasing in temperature.

The heat of the oxidation products can be recovered by air/air and air/water exchangers. The purified air is discharged to atmosphere through an insulated stack.

From the analysis of the three plant types, the biofiltration plant (BF) has the disadvantage of needing uniform humidity, temperature and nourishment for the micro organisms. These parameters are extremely variable.

VOC abatement

By comparing the two other plants it was concluded that the regenerative thermal oxidation system (RCUC) represents an efficient solution both for VOC abatement and for investment and operating costs but not for fuel consumption.

Therefore the most favourable solution both for investment costs and for consumption is the rotor concentrator technology. As a result, Osmopharm has installed a recorder for the continuous monitoring of emissions at the stack. The system is extremely effective in terms of its efficiency level and the rotor concentrator’s capability of handling low-flow rates with low VOC concentrations (less than 1g/Nm3).

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