Laboratory- and Industrial Immersion Lamps Energy Carriers For Photochemical Processes Author: Guenther H Peschl In recent years, the discipline of photochemistry has been greatly advanced in the chemical industry. No end to this development can be foreseen. Rather, further expansion is to be expected. Companies who supply photochemistry system parts can expect to see this in increasing inquiries. Particularly in the heavy chemical industry, more and more systems are being installed of greater and greater capacity. The precise product names are in very few cases made known to the suppliers. Mostly, they are known only in global terminolgy such as chlorination, bromination, photo-polymerisation, photo-oxygenation etc. General The word photochemistry indicates that use is made of radiation or light. That is, the chemical substance must absorb part of the irradiated wavelength spectrum and through this energy input either induce the reaction itself or free radicals to initiate a reaction. The effective spectrum of the substance must be matched with the emission spectrum of the radiation source. The emission spectrum of the radiation source is known. The effective spectrum of the substance must be established. This can be done either empirically by using different types and operations of emitters or by a photochemical analysis. Using this last method, the absorption spectrum is recorded. This indicates in which area of the spectrum there is absorption. However, it does not indicate that there is an energy conversion in this absorption range. The other method, the empirical evaluation, gives immediate information on the actual effect. An optimum approach is achieved by a combination of both methods. Radiation source suppliers will generally carry out absorption measurements with their radiation sources and the reaction substance. Selection of the Immersion Lamps The basic component of any immersion lamp is always the radiation source. Normally, mercury vapour low pressure and mercury vapour high pressure lamps are used. Mercury vapour, very high pressure lamps, as a rule, are not used in industrial photochemistry. Outside Europe, the high pressure lamp is known as a medium pressure lamp.The difference between mercury vapour high pressure lamps and low pressure lamps is not just in the construction and electrical power rating but mainly in the peak emission spectrum. While a low pressure lamp will emit predominantly at a wavelength of 254nm, a high pressure lamp emits over the complete mercury spectrum from 160nm to around 600nm. The individual spectral lines are of various intensities. The high pressure lamp is limited by the heat introduced in the form of a discharge current and there is a significant infra-red component in the emission. All high pressure lamps contain different metal halide dosings. (Figs. 1a and 1b) That is, apart from mercury, there is also a metal halide in the radiation source. In most cases, this is gallium iodide, thallium iodide, or cadmium iodide. Consequently, the metal lines of the metal halide also appear in the spectrum and these are characteristic of the metal just as the mercury lines are characteristic of mercury. Generally, it can be stated that because of the additional excitation of the metal lines, some mercury lines are smaller, so that in the energy balance the emitter has the same effectivity but at the concentration point of peak emission it is different. This difference can be used to improve certain chemical reactions. Other necessary components of an immersion lamp are: • Immersion tube • Cooling tube, and • Power supply Immersion tubes and cooling tubes can be manufactured in quartz glass or boron silicate glass. Quartz glass is permeable to the complete mercury radiation while boron silicate glass absorbs wavelengths below 300nm. Use can be made of this property to exclude attendant phenomena which appear below 300nm with some reactions. A cooling tube is not required for emitter operation if it is guaranteed that the ensuing heat can be removed from the substance or, alternatively, a mercury vapour low pressure lamp is not subjected to additional heat from a hot substance. For the electrical supply, impedance coils connected direct to the mains are used for low power lamps, while, for higher powers, scatter field transformers are necessary to generate the high voltages required to start and maintain the operation. In some cases there will be a need to regulate the lamp power. This can be done by means of impedance coils with various tappings connected in series either primary side or secondary side, or by means of transducer controls, which allow continuous control of the lamp power. With this, an additional choke is connected in series on the primary side and this is made more or less magnetised by a controllable d.c. current, so that the a.c resistance can be continuously varied. Laboratory Immersion Lamps There are three different designs in a product range. The laboratory immersion lamps comprise mercury vapour low pressure lamps of 15W power rating and mercury vapour high pressure lamps of 150W and 700W power rating. The immersion- and cooling-tubes are in all cases designed so that they can be inserted and have a standard ground joint of NS 29/32 or NS 45/40. High pressure or low pressure lamps can be inserted in these immersion- and cooling-tubes, as required. The 700W immersion lamp has a standard ground joint size of NS 45/50 for the immersion tube and NS 71/51 for the cooling tube. If there is a need for more play at the side of the lamp, to carry out empirical analysis, the 700W immersion lamp power supply is so designed that it is possible to regulate the lamp power. By means of a selector switch on the power supply, the power steps 500W, 600W and 700W can be set during operation. (Fig 3). This relates to a power increase of around 40%, from 500W. Consequently, it is possible either to establish the relationship between radiation flux and chemical transformation or to compensate in steps for the natural fall-off in radiation intensity during the lamp operating life. Compact Immersion Lamps Compact immersion lamps are mainly used in pilot plants or in industrial systems producing small product volumes.(Fig 4) This range consists of three high pressure types, with power ratings of 500W, 900W and 2000W. There is also a low pressure immersion lamp of 30W power rating. All designs in this range use exclusively quartz glass, i.e. the complete mercury line spectrum is passed. However, if the short wave UV radiation is not wanted, then a further tube in boron silicate glass must be used and this, at the same time, assumes the function of the cooling tube. As reaction vessels are individually designed, the installation of such a second tube is the customer’s responsibility. Typical reaction vessels can vary between 100 litre and 500 litre. Industrial Immersion Lamps In the Range In this range, mercury vapour high pressure lamps from 2 to 60kW and low pressure lamps up 150W are available. The latter, 150W lamp has an illuminated length of 150cm and is consequently the most powerful Hg low pressure lamp available for applications in photochemistry.In the high pressure lamp range, there are six models, each with different illuminated lengths: 2kW and 4kW with 1m illuminated length; 10, 20, and 40kW, each with 1.5m illuminated length; and 60kW with 2m illuminated length. Efforts are currently underway to expand the upper power limit from 60kW to 90kW. The immersion- and cooling tubes are available in conventional nominal widths in standard flanges. As in some cases, consideration must be given to reactors and other systems which are already in existence, the standard flange parts will not suffice and special flange designs must be manufactured. Generally, it can be stated that for lamps in the range, specially manufactured parts are necessary if there are not only changes in dimensions but also in lamp power ratings. With these lamp models, the type of operation must also be borne in mind, as this is often in explosion protected zones. Technical advice for these industrial immersion lamps is not simple, as all the parameters which have an effect on the total construction must be taken into consideration. These consist of process parameters, such as temperature, pressure, flow speed, mixing speeds etc, and the given technical parameters of the operation, such as what is the mains voltage, is there suitable coolant quality, is the nitrogen supply for explosion protection of the lamps via an on-site generated supply or is it via bottled nitrogen. The combination and consideration of these parameters seriously effect the functioning of the plant and, consequently, it is important to get in touch with an experienced manufacturer at the concept stage to take advantage of the valuable experience that such a manufacturer has gathered together over many years.     Radiation source with power and control cabinet                                       Immersion lamp with N2 circuit only