Ignition impossible

Published: 1-Jun-2007

Dust caused by the transfer of pharmaceutical powders can be toxic and prone to explode. Chris Broadbent, director of Dec UK Ltd, describes a new, safer system for charging reactors with powder

Dust caused by the transfer of pharmaceutical powders can be toxic and prone to explode. Chris Broadbent, director of Dec UK Ltd, describes a new, safer system for charging reactors with powder

The hazards of dust explosions encompass all segments of industry. Some of the more common materials presenting a dust explosion hazard are drugs, dyes, fertilizers, foods, grain, insecticides, metals, paper, plastics and rubber. Explosions have occurred in buildings as a result of dust on the floor, on ledges and in equipment such as bins, blenders, collectors, conveyors, dryers, grinders, mixers, pneumatic transport, and storage facilities.

A new solution to the problematic and often hazardous operations of powder transfer and powder charging in process industries is the patented Powder Transfer System (PTS) developed by Dietrich Engineering Consultants (Dec). The system is capable of automatically transferring toxic, explosive or other types of powder into closed vessels, containing dangerous vapours and liquids, at any temperature.

The PTS is designed for pressure and is installed directly onto existing process equipment, predominantly reactor vessels. By using a source of absolute vacuum and pressure, the PTS can transfer powders in the same way that liquids can be conveyed, over large distances (horizontal and vertical), in a variety of volumes, and directly into vessels in a safe and contained manner. This means that a powder room on the ground floor can be used to charge reactors large distances and several floors up.

The PTS works for all powders regardless of their characteristics, even those that are extremely fine, lumpy or even solvent wet, as during the transfer of such materials the PTS will not modify the homogeneity of the powder.

Full vacuum sucks powder from the storage container (drums, bags, FIBC's or hoppers) usually via a simple stainless steel lance held by the operator, and conveys it into the PTS cylindrical chamber. A flat filter membrane separates the chamber from the vacuum line preventing powder reaching the vacuum pump. When the chamber is filled with powder the vacuum valve closes and pressurised nitrogen or other inert gas is supplied to the chamber.

Once an overpressure is created within the chamber the powder is pneumatically discharged into the designated receptacle as soon as the outlet valve is opened. The overpressure also prevents gas and vapour that may already be present within the reactor from rising back up into the PTS chamber. Furthermore, the source of pressure also serves to clean the filter membrane at the top of the chamber (reverse jet) after every cycle of the PTS, thus ensuring each cycle performs under optimum conditions.

The PTS uses pneumatic logic to power the entire process, and forms a physical barrier between the designated receptacle and the PTS chamber, reducing the risk of explosive atmospheres developing without the need for isolation valves.

The system excludes most sources of ignition owing to the earthed, conductive components, and the lack of moving parts that could produce incendive mechanical sparks. Its pneumatic operation does not require a source of electricity or motors obviating electrical sparks and hot surfaces that could also be ignition sources.

Electro-static discharge remains the only potential ignition source, however this is avoided, due to the plug flow of powder in the dense conveying phase through the hose into the PTS chamber. The usual ratio of powder to air is more than 100:1, which is well above the upper explosion limit of most powders. Due to the low conveying velocity it has been calculated and measured that insufficient energy is created to form an electrostatic discharge and powders with an MIE (minimum energy required for ignition) below 1 milijoule can be safely conveyed.1

It is amazing that in 2007, there is still open manway charging of powders happening in a huge number of fine chemical and pharmaceutical plants. Operators are forced to wear heavy, hot and restrictive air suits or hoods, which only protect them from inhalation of toxic powders and do nothing to protect them from the potential explosion hazards.

Oxygen enrichment of recipient vessels which increases the potential for explosion is an inherent problem of most powder transfer systems, the only exception is the PTS. A combination of the plug flow conveying, vacuum within the PTS chamber and inertion. by nitrogen ensures that the oxygen concentration in the reactor remains below the limiting oxygen concentration at which explosions are possible.

The closed system also ensures that the ATEX delimitation of zones does not apply to processes using the PTS system unlike those utilising gravity based methods. Problems associated with gravity charging - including the formation of explosive atmospheres, the presence of ignition sources, bridging within the powder in a chute, or cleansing and validating such systems - are not inherent within the PTS.

In addition to the safe transfer of explosive powders and the reduction in operator manual handling, the transfer of highly toxic materials can also be achieved with the PTS. As it is a completely closed system it offers protection to the operators, product and the environment. A number of different devices are available at the other end of the hose, such as a mini glovebox (DCS- Drum Containment System) for transfer of toxic powders from drums, or suction hoppers with integrated lump breakers for discharge from FIBC's or IBC's. The PTS can also be used to link existing equipment together and avoid intermediate storage. For example, a pressure filter dryer or centrifuge can be discharged via PTS and charged directly into drums using a dosing device.

ATEX Exemption

Historically, operations where powders charged into reactors have resulted most conspicuously in fires and explosions, the risks are increased significantly where flammable solvents are also present within the process. A large proportion of such operations are still carried out manually, thus exposing the personnel involved to immense safety risks.

With or without the presence of flammable gases or vapours, the MIE of the powder and the method of transfer can create the risk of an explosion. In order to increase the safety of these processes, the transfer of powders should be carried out in closed systems, the recipient vessel should be made inert, every precaution should be taken during and after the transfer to maintain the lowest possible oxygen concentrations within the reactor and the systems should be separated by a physical barrier.

Most gravity based transfer systems offer overall poor levels of safety, and risks are further compounded by the nature of the material being transferred and the process conditions. This is true to such an extent that an operation that is considered safe under one set of parameters can be de-stabilised by changing one small aspect of the system. The PTS system does not use gravity, eliminates oxygen from the powder, has a physical barrier between itself and the reactor during operation and provides a safe solution for powder transfer independent of the nature of the powder and the process parameters.

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