Putting protection through the mill

With higher toxicity pharmaceutical products requiring more stringent standards of containment, the protection of mill systems against the consequences of a powder explosion are increasingly important. George Tunnicliffe and Martin Thomson from Kemutec consider the issues

The selection of explosion protection methods is often made at a late stage in the mill system design when many other parameters have already been fixed. However, as the method of protection can be highly interactive with other process requirements, it is necessary to have a good understanding of the relative merits of the various methods of explosion protection and to consider these at an early stage in the design process in order to achieve an optimum overall design for the system.

The purpose of this paper is to examine practical, cost-effective, solutions to a variety of mill applications where the need is to provide total protection against the risk of a powder explosion while maintaining effective operational control over product emissions.

increased potency

The surge in demand for containment in the pharmaceutical industry, and indeed in other industries too, is a direct reflection of the increased potency of modern drugs and the demands of ever more stringent legislation. This has challenged the pharmaceutical companies and, in turn, their equipment manufacturers to meet the criteria. Here we consider the implications for manufacturers of size reduction equipment where impact, air-flow, temperature, high speed rotating components, bearings and seals are all design considerations in addition to the hazards of the product being processed.

As pharmaceutical compounds become more potent, acceptable Operator Exposure Levels (OELs) are reduced. The trend towards finer Particle Size Distributions (PSDs) makes effective containment more difficult. Finer PSDs also make products more of an explosion risk.

In addition, legislation is getting tougher: the ATEX regulations came into effect on 1 July 2003 in the European Union. Despite all this, there are still the commercial pressures to ensure that equipment costs are kept under control while meeting these higher standards.

particular problems

Key factors influencing the containment solution for any process include: the toxicity and associated OEL levels of the materials to be handled; their PSDs, both before and after processing; the pressure at which the process takes place; any change in product characteristics after processing; and the explosion risk characteristics of the product.

Milling systems pose all these problems and particularly at the system endpoints, i.e. the material infeed and discharge points, and also at the mill air/gas intake and exhaust vent.

The provision of explosion protection for a milling operation can add significantly to the costs, size and complexity of the system. Poor selection of the protection method will waste money and compromise the system's operational efficiency. It is important to have a good understanding of the relative merits of explosion protection methods in relation to the process requirements.

The four recognised ways of dealing with the explosion risk in a milling system are venting, suppression, inerting and containment. Although the principle of suppression does not compromise the containment requirements of a system, it is not widely used in the pharmaceutical industry. This is due mainly to the amount of clean-up required in the system after a suppressant discharge and because it involves the introduction of materials that may be detrimental to the product whose value may be considerable. The two methods preferred by the pharma industry are inerting and containment.

low oxygen level

Inerting systems, figure 1, use an inert gas such as nitrogen or argon to reduce the oxygen level to below the limit that will support combustion. Their main advantages are the complete elimination of the possibility of an explosion, the ability to site them anywhere and their suitability for low Minimum Ignition Energy (MIE) materials. Disadvantages include higher system cost, ongoing cost of the inerting gas, and the need to protect operatives against hazards such as asphyxiation and 'cryo burns', if the gas is generated from liquid. Safe venting of the inert gas must also be catered for, which may be problematic.

Containment systems (figure 2) are constructed to contain the maximum pressure rise during an explosion. Such systems can be sited anywhere, do not compromise the containment OELs in the event of an explosion and have negligible maintenance requirements. On the other hand, the initial cost of manufacture is higher and explosions can take place, which may not be suited for use with sensitive, low MIE materials.

Kemutec has asked itself a number of questions: 'Can we utilise the "best bits" from the standard traditional methods? Can we produce compact mill designs to aid containment issues - e.g. use gloveboxes? Does this need to incur additional expense?'

One possible solution is the Closed Loop Milling System. Closed loop systems can be based upon the containment principle but are more compact and cheaper than traditional containment designs. The mill process gas is recirculated around the system, which eliminates the need for filters, slam shut valves, etc. These systems offer easier cleaning with less chance of cross contamination.

Closed loop mill systems have many benefits, but they may still not be suitable if the product has a low MIE and is prone to dust explosions. A system was needed to cater for this class of materials, hence the inert/closed loop hybrid system was developed.

This offers many of the advantages of the closed loop method, such as compactness and cleanability, with the additional advantages that the inert processing atmosphere enables its use with even the most sensitive materials. The systems do require the use of filtration, but because only 'top up' volumes of gas are being vented, these can be small 'throwaway' units. Also they do not require construction to withstand pressure shock containment.

For the ultimate in containment, milling systems can be enclosed in gloveboxes. Closed loop & inerted closed loop systems are ideally suited for this as their design requires a smaller enclosure with fewer penetrations through the walls. Where required, the gas flow through the enclosure can be chilled to limit the temperature rise of the mill.