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Drying of high moisture paddy

Drying remains the key problem. Martin Gummert (GTZ, Indonesia) declares that paddy drying is the problem in rice post-harvest. Drying notes Martin, is much more difficult compared to farm automation because use of mechanical dryers often requires adaptation of technology into an existing post-harvest system. Further, the drying process is a complicated process involving air and crop properties, which require a deep knowledge of the process. It is a challenge to engineers and as a result so many have been involved in seeking solutions that there are many prototypes of dryer which work technically in the labs. Martin adds that users of the newly designed dryers have not continued their use for various reasons. This is not due to lack of research on the topic; perhaps the problem analysis has not been satisfactory. IRRI, NARS, ACIAR projects have done a tremendous job of studying the drying behaviour of rice under diverse conditions and settings, promoting recent discussions around the question of where to do the drying: on the farm, with farmers' groups, at commercial mills, or at government rice marketing centres. Occasionally there is demand for certain dryers by farmers or private sector groups, without government subsidy. Cases include Dr Hien's flatbed dryers in Vietnam, Dr Somchart's fluid bed dryer in Thailand, the IRRI/GTZ-UAF low temperature SRR farm dryer in Vietnam, the recirculating Taiwan made dryers used by millers in the Philippines. The lesson seems to be that there is a place for both small scale and large-scale dryers. There are advantages to using dryers in the private sector; and, fitting the millers needs and modes of operation is more important to them than the "state-of-the-art" technology. The quest for an ultimate universal technology which seems so important in the scientific community, where each research group competes and promotes its own solution, is the wrong approach. Martin finds it more sensible to look at drying problems at all levels and circumstances, and identifies specific technical solutions for them within the systems context. This requires a database of different drying technologies.

Sun-drying. The Philippines has become notorious among its peers in post-harvest forums for its "highway dryers". The Filipino. observes with humor that they have the longest rice dryer in the world. Dr Arnold Elepano (UPLB, Philippines) reports that to get the farmers off the roads with their paddy, the government through NAPHIRE has been subsidizing the construction of multipurpose pavements principally for sun drying in every village. The most appropriate size has been found to be the size of a basketball court, with immense popularity among the youth in the villages.

There is nothing wrong with sun drying if done properly. Dr Virgilio Gayanilo (UPLB, Philippines) in post-doctoral work at IRRI presented how it should be done. Essentially, the paddy should be spread out in sufficiently thick layers, raking the grain to mix it continuously. This prevents the grain from being exposed to the strongest rays of the sun which cause sun-checking or fissuring. In the Philippines, mills pay labour charges of PHP5.00 (US$0.125)1 per bag of 44 kg paddy for sun drying in foul weather that might take more than a day. Sun drying is the cheapest method. Unfortunately sun drying of paddy is unreliable. The sun may not be available when it is most needed; if it rains for a week during harvest time the grain is likely to germinate, yellowed or rotten; when there is sun shine in the morning and the grain is spread out, a sudden rain storm can cause fissured grain; if the sun is hot, the workers prolong mixing the grain and the result again will be fissured grain. A miller interviewed in Laguna, Philippines, said that despite the greater expense of drying rice in their heated air dryers (estimated cost 5 times more), they could not afford to sun-dry anymore because of the damage to grain quality is more expensive for them.

1 Exchange rate used is Phil. Pesos 40 = US$1.00

Dwayne Sutter (Texas A & M) who has had long experience in the Philippines, says states sun drying of paddy is a major cause of lower head rice² yield. Sun drying was usually a single pass operation, which did not allow the grain to go through a tempering phase to relieve internal stresses. As a result, small fissures were formed. These defects are not visible to the naked eye but can be observed by shining a light through the grain. During years when they experienced late afternoon rains in the harvesting season, they correlated this weather pattern with the low head rice yields obtained. Dr Otto Kunze (Michigan State University, '64) found in his research that rehydration of the grains when exposed to high humidity condition causes numerous fissures. Dr Rusty Baustita working with Dr Bekki in Iwate University (Japan), observed the same phenomenon, except that the fissures started from the center of the grain. In each case, broken grains are found when the paddy is finally milled. Rusty states, broken grains, according to the Japanese, affect the taste.

² Head rice is equivalent to ¾ whole grains

Flatbed dryers. The flat bed dryer configuration is the most basic design. A review of the literature indicated that before World War II, grain in the Southern U.S. was dried in barns designed as flat bed dryers. A flat bed dryer is basically a perforated sheet floor above a plenum chamber, where the grain to be dried is placed about a foot deep. Heated air is forced through the grain mass from the plenum below the bed. The grain is loaded and unloaded from the drying bed manually. When the farm boys left to go to war, shrinking the labour force, U.S. agricultural engineers designed mechanical grain silos., This was the first application of column dryers loaded by bucket elevators.

During the early days of the miracle rice when drying the wet harvest crop first became a big problem, a two-ton flat bed batch dryer was designed and developed at the University of the Philippines in Los Baños. The design consisted of a 6 ft x 12 ft floor above a plenum, accommodating 18 inches depth of grain. An 18 inch fan was used which could deliver 3500 cubic feet of air per minute at 1-inch static pressure driven by a 5 HP engine. A vaporizer heated by a kerosene burner was designed to bring the drying air from ambient temperatures to a maximum of 110°F. Under these conditions the full 18 inches of grain was actively dried at a moisture removal rate of 1% per hour. Grain with initial moisture content of 24% wb decreased to 14% wb after 10 hours. There was a 1% moisture variance between the top and bottom layers, for instance the top layer was 14% wb and the bottom layer was 13% wb. If the fan was not calibrated to the correct rpm there was a larger moisture differential. This had to be remedied by mixing the grain at the mid-point of the drying procedure. To improve dryer operation, a simple plastic U-tube manometer was designed to gauge the static pressure in the plenum and adjust the engine to the correct speed.

The flat bed was widely introduced in Southeast Asia though a UNDP/FAO sponsored training programme. Today, versions of this dryer can still be found in some farmer coops or seed farms. It was intended to be used as a farm dryer, but some millers adopted it, later complaining about its small 2-ton capacity. Larger capacity dryers were built in the LPN complexes in Malaysia to augment the tonnage of the Cimbria dryers. Thailand produced their own flat bed dryer. The most successful applications were in Vietnam. Dr. Phan Hieu Hien developed several versions from 2 tons to 30 tons batch capacity. Dr. Hien adopted a very effective approach to facilitate the fabrication of the dryer in the villages. Local manufacturers were provided jigs to fabricate the axial flow fans, and the power tiller engines were harnessed to drive it. A step grate furnace was also developed to burn rice hull to heat the drying air instead of kerosene. A shop in Ho Chi Minh City responded to the demand and tooled up to fabricate the perforated metal sheets used for the flooring. The farmers were taught to build the grain bin using common red soil bricks. The dryer was so popular, almost every other household build a 6-ton batch dryer. In one village visited for this survey, a thatch roof over the dryer plus a working space was used. A large quantity of rice is produced in the Mekong delta, and the farmers are obliged to dry their paddy before they sell it to the mills in town; police regulations do not allow them to use their highways for sun drying.

The flat bed dryer is a very effective tool for teaching the rudiments of grain drying. The fan's performance curve can be plotted to explain air flow volume versus the resistance of the grain mass. The heating of the drying air and the drying process can be graphed on a psychometric chart. The effect of drying air temperature on drying rate and milling quality can be demonstrated. The fuel utilization efficiency to dry the moisture load can also be measured. Although it is easy to teach unskilled workers how to operate a dryer, effective and efficient utilization of dryers to produce good quality grain can only be done by trained technicians who understand the process.

Recirculating dryers from Japan or Taiwan. At first gland these dryers look like flimsy toys. These are silos available in 1,2 and 5 ton batch capacities. It contains a drying section (usually two columns) with a feed tank above it. It has unloading rollers and an auger at the bottom to direct the gain into the intake hopper of a bucket elevator that feeds the grain back into the top. The drying section is equipped with a suction fan. They drying air is heated directly (no heat exchanger) by a vaporizer with a kerosene burner. The heart of the operation is an in-line moisture meter that measures the moisture content of the grain as it posses through the drying section. After the dryer is loaded, fan started and burner ignited, the grain is recirculated through the dryer. The feed tank serves as a tempering bin. When the grain reaches 14% wb, the dryer is shut off and the grain is unloaded. Once the dryer is loaded and in operation the technicians can come back 8-10 hours later to unload the grain and start the next batch.

An increasing number of mills in the Philippines have invested in these models. Some have even installed multiples of these dryers. To know why these units were selling, flour millers were interviewed for this review. Their answers reveal: the local sales agent visited them with glossy brochures explaining the specifications; they got feedback from their fellow millers about the actual performance of the Taiwanese dryers; they were offered easy payment plans; investors in the unit discovered that they were easy to operate. These millers gained advantages of having drying capability in their rice business and bought additional units to meet their requirements. Maintenance needs were met by the sales agent's technicians, who visited regularly to replace worm out parts, usually the unloading augers, and to check on the in-line moisture meters. A 5 ton unit sells for PHP 500,000 or US$12,500, and the users reported two year pay back period. Their dryers enable them to purchase paddy in any weather, minimize labour problems, and produce better quality rice. These points support Martin Gummert's assertions. The best drying technology is one that meets user requirements: it is not necessarily the one most thermodynamically efficient. The dryer has to be actively promoted in the industry with a financing plan for easy acquisition. The dryer must be designed as modular units for smooth replacement of worn out parts. After sales service is the most important feature to be provided. The operation of the dryer must be fool proof.

In-store Drying. The University of New South Wales group has few positive things to say about the high temperature high airflow forced convection dryer that is favoured in Europe and the United States. These systems have made it commercially feasible to dry grain in deep bins, with low air volumes and very little supplemental heat. This dryer is used successfully in the Australian grain industry. The UNSW group of Drs. Robert Driscoll and George Srednicki were contracted by the Australian Centre for International Agriculture Research (ACIAR) to present this industry experience in Southeast Asia. Drs. Justin Tumambing at NAPHIRE, Philippines, and KMITT in Thailand, were the initial analysts. First they had to establish the thermophysical properties of the most common modern rice varieties in the Philippines and Thailand such as air-moisture balances with grain-moisture (note: grain-moisture equilibrium isotherms plotted against relative humidity). NAPHIRE and KMITT reported substantial differences between the southeast Asian varieties and the Australian varieties. Using pilot system dryers they determined the required airflows to dry the grain to 14% wb within a period of time. Rather than drying grain in a certain number of hours, time was reckoned in weeks. They concluded that the very high initial moistures of 24% wb and above require first stage drying to minimize grain loss. This yielded the two-stage drying strategy. First the high moisture grain had to be dried with a high temperature, high airflow forced convection system (such as the Louisiana State University {LSU} dryer), down to 18% wb. The more stable "dried" grain can then be loaded in bulk bins and slowly dried to 14% wb with the low airflow, low heat system.

NAPHIRE developed a carriage-mounted continuous flow recirculating silk with a batch capacity of ½ ton, to be used as a first stage dryer. It was assumed the dryer would be towed to the field for grain drying immediately after threshing. The 18% wb grain could then be brought in to the processing plant for final drying. The NAPHIRE strategy backfired when the government approved the idea and politicians endorsed the mobile flash-dryer. NAPHIRE was commissioned to implement the programme, which resulted in the manufacture of 2,000 flash-dryers. Units were produced by local shops funded by congressmen and later distributed to farm cooperatives. The farmers were unprepared and eventually furious. Coops were left with 18% wb grain with no equipment to complete the 2-step drying process. Information gathered in a survey conducted by Irene Villapando, Philippine Department of Agriculture consultant, indicated that NAPHIRE's dryer was not being used because of technical, economic and financial reasons. Further investigation prompted worldwide reactions. NAPHIRE's strategic error was to aggressively promote the first stage of a two-stage drying procedure.. It represented the risks faced in research and development.

In response to Justin, we confirmed that the two-stage drying technology remains a sound strategy, and that deep bin in-storage drying has sound merits. The unfortunate misstep was the promotion of the 1st stage without the second stage facility in-place.

Fluid Bed Drying. Over in Thailand at the King Mongkut Institute of Technology, Dr. Somchart Soponronnarit's group, with ACIAR and UNSW collaboration, were busy learning about in-store drying and developing a fluid bed dryer for the first stage. A Thai miller has successfully adopted his bulk warehouse for in-store drying. There may be others by now. The miller built air ducts flush with the floor to allow free movement of his payload to load and unload grain from the warehouse.

Dr. Somchart reports that there are already 100 units with 5 and 10 tons per hour capacity fluid bed drying units bought by private millers in Thailand, the Philippines, Indonesia, Malaysia and Taiwan. The device is used as a first stage dryer to quickly remove the surface moisture, followed by the second stage continuous flow forced convection for final drying or the in-store drying technology. Dr. Somchart joined the E-mail conference to ask for assistance from cereal chemists for an explanation of the increased head rice and artificial aging that occurs. Justin Tumambing who has also developed a fluid bed dryer at NAPHIRE responded. As suspected earlier, the higher head rice was caused by partially gelatinized starch generated by the very high temperatures used. The question posed is academic as the consumers are not complaining.

The Vietnamese (UAF)/GTZ/IRRI Low Cost SRR Dryer. IRRI is reported to have worked with eleven different models of drying hardware. The SRR dryer is yet one more in its inventory. The SRR dryer is an adaptation of the Kongskilde circular bin batch dryer It has a central perforated duct on its axis connected to a fan. The air is circulated by pushing or suction radially through the bin. The adaptation consists of scaling down the size to hold ½ ton of grain, and the bin is made of woven bamboo mat or screen wire mesh. A fan assembly and electrical heater is fitted at the top end of the duct. The dryer costs US$300. It is reported that there are some purchases of the dryer in Vietnam. It has been demonstrated in the Philippines, and acceptance reports are unclear. The unit is also being promoted in Bangladesh.

Conduction Heat Transfer Drying. Dr. Bill Chancellor sent us a design he worked on while he was at IRRI. It was a flat frying pan where the paddy is roasted. Fire is built underneath the pan, and to mix the grain a rake is pulled by a buffalo going round and round the pan. As far as it is known, this drying concept was not adopted by farmers. Other designs include: the rotary drum conduction heated dryer, an oscillating table dryer, a solar heat collector tent dryer and a reversible flow bed dryer.

Drying Technology Information: A Communication Problem. Results from the development and introduction of rice drying technology have been elusive. The need is recognized, but investment in dryers has been slow. The theory of drying, the heat and mass balance in the drying process, the understanding of how the rice grain dries without damaging its milling and the cooking and eating qualities of rice is well researched. The standard commercial process for drying rice is by using a forced convection system. Many physical configurations of this dryer have been developed. Drying the harvested crop was traditionally the responsibility of the farmers; efforts have been directed to develop dryers for farmers to use. The general concept is to create a low cost, easy to build, use and maintain dryer. In the Philippines, the simple flat bed batch dryer was designed and introduced in the 1970's. The technology was not accepted by the Filipino farmers. The basic complaint was the higher cost of the flat bed drying compared to sun drying. The same design was introduced in Vietnam. It was a hit with the farmers. In the village where it was first introduced, almost every other household built its own flat bed dryer. What is the explanation? We can only surmise that in the Philippines, where rice production cannot meet demands, there is a seller's market for wet paddy. Traders and millers have taken over the responsibility for drying; therefore the requirement is for plant dryers with larger capacities. In Vietnam there is a production surplus, a buyer's market, and the pressure is on the farmers to dry their paddy before they can sell it to mills. Farm dryers are therefore very much in demand. In Thailand, which has shifted to extensive use of locally manufactured rice combines, a parallel demand by traders and millers for large capacity dryers developed.

Drying of paddy in artificial or mechanical dryers costs 5 to 8 times more than sun drying according to a paper by Martin Gummert. This figure should be placed within the context of the total plant operation. A drying facility that is part of a processing plant enables the production of better quality milled rice that will sell at US$0.05 higher per kg. A processing plant with a drying facility allows the business to buy paddy even during prolonged periods of raining weather. A progressive entrepreneur must invest in a drying facility.

In the Philippines, the indispensability of plant dryers has not even become manifest in some research institutions and programmes continue to promote farm dryers. The demand for plant dryers by millers is being met with imported units. It was revealed in interviews that millers in the market for a dryer were not aware of any locally manufactured plant dryers available that met their requirements. This happens concurrent to efforts by local R & D institutions, which claim to have developed many designs.

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