Using the benefits

The author: Markus Zimmermann Sales and Proposal, Dürr Systems EES

The air pollution control specialists Environmental and Energy Systems (EES), a business unit of the Dürr machinery and plant manufacturing group, prefers to apply catalytic meth-ods when treating air pollutants in the chemical processing industry. The classic area of application concerns abatement systems used for PTA production facilities, calcination processes (catalyst manufacturing) and the manufacture of maleic anhydride, olefines, natural rubber and acrylic.

As part of the drive to improve cost efficiency, treating smaller exhaust air streams and lower VOC loads is taking over as the predominant trend, for example in the chemical processing, petrochemical and pharmaceutical manufacturing industries. Since solvents remain a cost factor, the quantity of solvents discharged with the exhaust air stream should be as low as possible. Advanced production processes increasingly consider this issue in their material efficiency calculations.

Process parameters such as flow rate, temperature, particulates in the contaminated air and the VOC species are among the important standards for selecting the appropriate procedure. To choose the right catalyst, it is vital to know the type of VOC (aromatic, oxygenated, halogenated hydrocarbons or alcanes) that has to be removed. No effective catalyst is currently available for short-chain alcane methane. The aim is, however, that future catalyst types will be capable of removing this substance. In order to be better placed to offer the most suitable catalyst, Dürr is partnering with catalyst manufacturers such as Haldor Topsoe to make beneficial use of their wide-ranging portfolios.

Catalyst poisoning can de-activate the catalyst within a short operating time. The catalyst material may have to be replaced in order to keep emission levels within specified limits. The active substances of the catalysts and the catalyst poisons have not changed significantly in the past few decades. Substances such as silicon, phosphorus, sulphur halogens, and metals – in particular germanium, arsenic, selenium and tellurium – are the materials most often cited as causing poisoning. These substances can be tolerated in low concentrations, but defined limits must not be exceeded.

The last important criterion for selecting the proper abatement method is the VOC concentration itself. Catalytic methods differ from comparable thermal methods in that they can be auto thermal at lower concentrations. This means that no additional energy needs to be supplied to pre-heat the contaminated air sufficiently when it flows through the catalyst. If the VOC concentration is too high, the air has to be diluted before it passes through the abatement system. However, certain limits apply. If the treated air stream is diluted too much, the air flow rate increases disproportionately, leading to higher costs for the individual equipment units and the catalyst.

The benefits of catalytic abatement methods ultimately derive from the operating costs saved for primary energies. VOCs can be removed without any additional energy providing their concentrations are low. As a result, fuel consumption, CO2 and NOx decrease in comparison to other thermal methods.

In order to purify the flue gases from facilities producing purified terephthalic acid – PTA for short – and other applications exhausted at high pressure, Dürr applies the Ecopure HPX abatement system as standard. The high-pressure PTA flue gas typically contains methyl acetate, methyl bromide and carbon monoxide, which can be treated directly without prior decompression.

The flue gas from the process is heated to the required catalyst operating temperature by a combination of heat exchangers that operate on steam or preheated air as well as by a purified, air-heated, air-to-air heat exchanger. After the pollutants on the catalyst have been converted, the purified flue gas is cooled and the heat is recovered in the air-to-air heat exchanger. The purified, pressurised flue gas is then routed to an expander for energy recovery and used for pneumatic conveying or further treatment of hydrobromic acid. Dürr is currently engineering, fabricating and installing an Ecopure HPX for a PTA production site in China. This HPX system has a treatment capacity of 440,000 kg/h.

Ecopure LPX systems are in principle suitable for all catalytic applications. In an Ecopure LPX system, the conversion of VOCs takes place at pressures slightly above or below ambient. The exhaust air from the production facility is typically heated by an air-to-air heat exchanger. The air is then usually heated further to the necessary catalyst operating temperature by a burner, thermal oil or steam. The heat generated during the VOC conversion process is transferred from the production facility to the untreated exhaust air. If the VOC concentration is so high that more heat is generated than required, the excess heat can be ducted through a heat exchanger bypass directly into the stack without being used. Dürr’s most recent Ecopure LPX system is currently being commissioned at a facility owned by Sauer Continence, a customer of Dürr’s in Germany. This very compact system is designed for a capacity of 3000 Nm³/h.

If exhaust air from a production facility has a very low VOC concentration, a more efficient heat exchanger material has to be used than the recuperative heat exchanger systems featured in the Ecopure LPX model. Dürr’s Ecopure RCO (regenerative catalytic oxidiser) is the right system for this purpose. In this system, the process air is heated up in a regenerative manner via ceramic heat exchange media before the VOCs are converted by the catalyst. After treatment, the heat in the purified air is recovered again in the ceramic media. The stored heat is then transferred back to the contaminated air. To ensure that this principle works reliably, the Ecopure RCO has at least two ceramic heat exchange media beds, which are interconnected by a combustion chamber and alternately transfer and absorb the energy.

Ecopure SCR systems for the selective catalytic reduction of NOx catalytically convert nitrogen oxides contained in flue gas. As this is not a suitable system type for converting VOCs, it is not described here in further detail and is only mentioned for the sake of completeness.

With this broad range of VOC removal systems, Dürr is flexible in its ability to offer customers pollution control solutions for a wide variety of production processes. Dürr’s Ecopure systems not only provide effective and efficient solu-tions to compliance issues, but also offer low operating costs and energy consumption.

Online-Info: www.cpp-net.com/2111442