INTRODUCTION

The scientific landscape on continuous and batch microwave irradiation applications in chemical processing is growing, with a number of unique chemical applications namely (Saifuddin and Chua, 2006), waste oil/water emulsions produced in metallurgical industries (Abdurahman et al., 2006) demulsification of water-in-oil emulsions (Nour et al., 2010a, b; Nour and Yunus, 2006) crude oil demulsification, similarly the progress of microwave drying adaptation at industrial scale has been relatively slow, Chou and Chua (2001) identified one such reason high initial capital investment and low energy efficiency when compared with conventional drying technologies.

Traditionally dryer selection has been practiced as an art based prior experience and vendors’ recommendations (Mujumdar and Devahastin, 2000). However, there are a number of key parameters required for dryer selection, for example dryer throughput; properties of the material being handled physical geometry, physical, chemical and biochemical properties of the feed; including moisture variability, drying characteristics of the material drying kinetics; flow of the material to and from the dryer; operations and product safety, fire hazard and explosion hazards, Product qualities, Recovery problems; mode of process control, effluents environmental regulations, type and cost of fuel supply. Facilities available at the site of the proposed installation; space availability.

In analyzing the economic viability of a microwave curing system, the costs can be divided into capital and operating costs and the maintenance costs can be broken down further to magnetron replacement, general maintenance, energy and floor space costs. However, from an operations perspective the prime aim is to factor in tube replacement cost and energy cost in the consumables.

To expand the discussion on the economic viability of microwave drying, it is necessary to incorporate energy conservation as a feature of the microwave system offering. By and large, industrial microwave equipment is not luxurious; the capital costs associated with a microwave power generator are approximately $1000 per kilowatt. Together with specially designed cavity and conveyor system, the total purchase price of a microwave system is around $1500-5500 per kW depending on the system power (Mujumdar and Zhonghua, 2008), frequency and complexity. Summary of dayers classification is shown in Table 1.

Maintenance costs of industrial microwave systems are moderate and, once established, they are very robust Metaxas (Metaxas and Meredith, 1983) with the only "consumable" being the magnetron. The microwave generator, magnetrons usually last significantly longer than 5000 h. Replacement of 1 kW magnetron would cost about $750 AUD. Typically the, magnetron is typically about 65% efficient and a 1.2 kW unit would draw 1.8 kW from the mains. It is therefore estimated, on the assumption that a typical electrical unit costs reference price of 11 cent per kilowatt hour, that running cost for 6,000 h would be approximately $1188.

A number of noteworthy publications exist on this topic for example the investigation by Damskey et al. (1969) included the procedure for a computer simulation for microwave paper dryers. Program options evaluated the economic feasibility of microwave equipment.

Table 1:	Summary of dryers classification extracted from (Mujumdar and Devahastin, 2000)

The profitability of paper-making machines with microwave drying relative to those with conventional steam heat drying was computed by assuming that microwave drying procedures provided a sufficient level of moisture profile.

The increased drying capacity of the process resulted in higher machine speeds and therefore profitably. Also a feasibility and decision analysis procedure for microwave and conventional heating systems based on quantitative comparisons between two heating methods was reported by Ishii (1972).

Finally, Schiffmann (1987) investigated the economic analysis of microwave drying systems, with an emphasis on large projects and included budgeting and costing for acquisition or costing of large manufacturing infrastructure.

We had also reviewed a host of literature on various case studies that did also include important information that applied to any general cost-benefit analysis in microwave drying microwave drying. In relation to the specific information required for performing decision analysis studies. The investigation by Schiffmann (1987) included a procedure for the development of a cost model. It concluded that industrial microwave systems vary between $4000 and $ 7000 per kilowatt.

An additional economic analysis into factors affecting both initial capital initial and operating costs was conducted by Disman (1966) who found that the major cost-determining factor is the efficiency of the microwave applicator. Relations between magnetron costs, life, efficiency and the cost of prime power were derived.

Efficiency and reliability were two major economic considerations, where applicator efficiency was calculated by determining the ratio between the effective heating power delivered to the material being processed and microwave power supplied to the applicator.

This investigation also analyzed the total system cost, where it was divided into applicator cost, power supply, tube costs and other engineering and miscellaneous costs. It was assumed that the initial cost of a microwave system, at several hundred-kilowatt power levels, lies between $750 and 1250 per kilowatt of microwave power generated.

This evaluation revealed several aspects that related to the feasibility analysis undertaken in this research. For example, for effective cost estimates of the system, the availability of reliable historical cost data on similar items or products was vital. Without data sources of adequate quality, the accuracy of the cost estimate would be sacrificed (Graves, 2000). Hence it is sensible to consider cost-indexes.

The presented indexes are used for the purpose of adjusting an item's cost from one period to another. The reasons for an item are cost to change continuously included inflation, changing technology and changing the availability of materials and labor. These issues are important for a technological device and the development methodology of microwave curing of corrugated board adhesive that may rely on improving technology, materials and labor. Theoretically any potential saving in microwave application would very much depend on the attributes and the advantages of the industrial processing system as well as the actual quality improvements which can be achieved (Disman, 1996).

Therefore, justifying the purchase of a microwave processing unit to achieve energy savings is based on the fact that conventional steam plant remains inefficient and the microwave capital cost are repaid after a number of years through savings in energy usage.

CASE STUDY

In this study, the intrices of microwave costs models are illustrated through a case study.

Case study 1 corrugator: This case study examines a typical corrugator capable of running up to speeds of 300 m min^-1. The production line is fitted with a steam hot platen, also known as hotplate dryer which consists of a number of heating plates, N_EX and the total area of the heating surface A_S.

A gas-fired boiler would supply the steam in the hotplate section as shown in Fig. 1.Typically for a medium sized corrugator converting plant the heating duty of the boiler would be in the range of 2500 kW similarly with an optimum efficiency of 80%. Figure 2 illustrates the trend in steam consumption in relation to production speed as well as liner grade or density. This graph was plotted using real data adopted from Nylund (1998).

Any increases in the production rate would require additional energy as shown in Fig. 2.

Fig. 1:	Boiler arrangement supplying steam to platen heat exchanger

Fig. 2:	Steam consumption versus production speed

Fig. 3:	Production rate optimization pathway

However, there are three options available in order to accommodate the increases within the limits of the existing equipment; increasing steam supply temperature, increasing steam temperature by super heating or increasing surface heating area which requires more energy input from the unit.

The limitation of current equipment for production rate increases dependent on an array of influencing factors, the essential are listed in Fig. 3.

Selection of appropriate drying conditions is necessary for minimizing thermal stress (Ramesh et al., 2001) corrugated wrap or cupping. Heat is the first fundamental drying parameter however the influence of the various process parameters and pre-treatments like inlet air temperature, dew Point wet and dry-bulb, air velocity mass flow rate, moisture content, bulk density, residence time. Another technique to achieve temperature increase is by increasing steam pressure inside the steam platen in Fig. 1. However, naturally the steam platens have inherent limitations for example practical limitations to the maximum pressure that can be used.

Disadvantages of steam platens: Although steam platens remain in usage drying corrugated board, there are several associated disadvantages with this drying system. The overall floor size and volume of the drying section is a major contributor to the size of the plant and the length of the production floor. Now-a-days, typical machinery occupies a linear space of about 120 m and total factory building, of about 140 meters in length (Jones and Metaxas, 1988). The heating platens system necessitates extensive length in order to ensure sufficient curing of the adhesive bond as the board passes over the platens at speeds of up to 300 to 400 m min^-1. Steam platens are typically made of heavy cast iron, in the manner of a pressure vessel, in order to contain the high-pressure saturated steam at temperature of 185°C and pressure of 1138 kPa.

A steam chest has a flat upper web-supporting surface and a required width sufficient to support the full width of the travelling web. This width is typically in the order of 500 to 600 mm. Another feature of the steam platen is that they have thick walls. The heavy walled construction of these steam chests requires sufficient time to bring them up to temperature on start-up. As the heat is drawn at a rapid rate by the travelling web on the upper surface of the heat platen, the surface temperature may also drop significantly; the heavy walls retard the conduction of replacement of heat. Therefore, the platen runs noticeably cooler than the steam temperature.

The lower effective operating temperature requires the use of a larger number of steam chests in order to provide more efficient heat transfer. An additional method to compensate for this temperature reduction is to reduce the operating speed in order to cure the corrugated board sufficiently. In addition, the edges of the chests are flanged, so the steel is thicker at the extremities of the full-width web and for this, loss of heat in the edges limits the running speed. The centre of the board may adhere well but the edges may have only green bonded.

Another problem which is also related to the inefficiency of the heat transfer through the steam platen casting during operation, is the transverse bowing of the upper surface. In this situation transverse concave bowing of the steam chest across the path of the travelling web is a common occurrence. Consequently, uniform load distribution on the web, is disrupted, by the hold down belt and the transverse riding ballast rollers sandwiching the belt downwards and against the upper surface. The result of the previously mentioned operating efficiencies is uneven curing of the adhesive zones, poor or no adhesion and crushing of the lateral edges of the web.

Finally, the heavy mass of cast iron steam platen results in high heat retention and slow cool down rate, often requiring intricate process control systems to manage the heating rate and avoid excess heating. Another drawback of steam platen corrugated board is the "pressure lines" which result from the pressure applied by the pressure roll at the single facer. This is compounded by "wash boarding" which is the effect of the adhesive, when the drying process is drawing the liner out of its linearity. These effects make the board unsuitable for high quality printing on that side. Further factors to be alleviated include paper shrinkage, cardboard wrapage and print scuffing.