1984, Review-Selected Aspects of Crop Processing and Storage

J. agric. Engng Res. (1984) 30, 1-22

REVIEW PAPER

Selected Aspects of Crop Processing and Storage: A Review

F. W. BAKKER-ARKEMA*

Significant developments and technical trends of the past decade in the area of crop drying, processing and storage are reviewed. In particular, the effects of recent innovations in operational research, computer technology and microelectronics on the energy efficiency, product quality and process economics are assessed. The review concentrates on grain drying, grain storage, hay drying and biomass energy.

The sharp increase in energy prices during the 1970s and the requirement for improved product quality have stimulated innovations in grain drying. Significant developments have occurred in high-temperature drying (concurrent-flow and in-bin-counterflow drying) and in low- temperature drying (solar and chemical-assisted drying). Computer modelling has become a major tool in designing driers.

Controlled atmosphere storage is making a revival as a non-chemical grain preservation technique. High nitrogen and high CO2 content systems appear to be economical. Refrigerated aeration also has promise as an alternative method for grain preservation in warm climates.

In some parts of the world (e.g. the U.S.A.) hay is becoming of increasing importance as an animal feed. One-day hay harvesting is close to reality through the development of new mechanical and chemical conditioners.

Biomass energy has promise as a fossil fuel replacement. The technical feasibility of biomass furnaces has been proven, the economic feasibility remains in question.

1. Introduction

This review is limited to the major developments in the 1970s and early-1980s of post-harvest technology of grains, hay and a selected root crop (i.e. potatoes). Major emphasis is on the state-of-the-art of crop drying and storage.

Important recent developments in post-harvest technology have occurred in the areas of solar energy application, drier system design, process simulation, concurrent-flow dehydration, controlled atmosphere storage, biomass utilization, and other related topics such as product- quality modelling, microprocessor control and management optimization. Contributions to progress have been made worldwide, but the author has been forced to be very selective in his choice of material due to a limitation on length of the review. The paper is subdivided into five major parts, namely, grain drying, grain storage, hay storage, potato storage, and grain handling and cleaning.

2. Grain drying

2.1. Crop properties 2.1.1 Grain quality

The ultimate objective of post-harvest processing is to maintain the desired product qualities.

*Department of Agricuhural Engineering. Michigan State Universily. East Lansing, Michigan, U.S.A.

Received 16 September 1983; accepted in revised form 10 March 1984

Paper presented at AG ENG 84, Cambridge, U.K., I-5 April 1984

1 0021-8634/84/050001 + 22 S03.00/~ �9 1984 The British Society for Research in Agricultural Engineering

2 CROP P R O C E S S I N G AND ST O RA G E

NOTATION

c specific heat, J kg- 1 o C - l

hi 8 heat of desorption of water, J/kg h grain bed heat transfer coefficient, J m- 3 ~ 1 t time, s fit time increment, s x depth in bed from inlet, m fix depth increment, m G mass flow rate of air, kg m- 2 s- 1 H absolute humidity of air, kg/kg dry air M moisture content of grain, decimal dry basis, kg/kg dry matter T air temperature, ~

void ratio 0 grain temperature, ~ p density, kg/m 3

Subscripts a dry air g dry grain v water vapour w water (liquid)

It depends on the end-use of the product w~aich quality characteristic needs to be conserved" for wheat, baking quality is essential; for soybean, high oil recovery. In an exhaustive and excellent review on heat damage to cereal grains, Nellist 1 recommends a more systematic approach to the topic than has been the case in the past.

For optimum design and control of grain processing equipment it is necessary to quantify deterioration in grain quality. In the objective function the quality deterioration of the grain acts as a penalty function? Although much data on the subject can be found in the literature, modelling of the physical condition of a biological product is not well-developed, except for some empirical relationships for the breakage increase of maize, a for the moulding of maize* and for the baking quality of wheat. 1

A basic quantitative model of damage to seed viability has been proposed by Nellist. 5 The assumption is made that a pattern exists of seed death under steady state temperature and moisture content, which can be integrated to predict the change in physical condition during the non-steady state treatment in a grain drier. The Nellist model requires validation for the major cereal grains to allow its use in the optimum design and control of grain driers.

Different quality criteria have been used for process design. Ghaly and Taylor e monitored the micro-baking loaf volume, the turbidity and the viability during the thermal disinfestation of wheat. Ghaly and Sutherland ~ measured free-fatty acid content and oil yield during the high- temperature drying of soybeans. Mfihlbauer et alP considered lysine content and colour in evaluating maize dried at high temperatures. Other quality design criteria include head-yield of rice, a breakage susceptibility of maize, 1~ mould development of maize, lzla test weight of maize, 14 nutritive value ofbarley ~5 and uniformity of maize moisture content? e It is to be expected that models will be developed for the change of each of these quality criteria during processing.

2.1.2. Design parameters The knowledge of grain properties is essential for the design of grain drying equipment.

F. W. B A K K E R - A R K E M A

Without reliable values of such properties as moisture diffusivity and thermal conductivity, drier simulation studies are of questionable value. An excellent tabulation of available grain property values has been presented by Brook and FosterY ~ Equilibrium moisture content (e.m.c.) is a basic parameter in the drying of biological products. It determines the moisture content when the product is subjected to a certain ambient condition. Pixton and Howe TM reviewed the e.m.c, equations and concluded that the Pfost-Chung 17 equation best describes the experimental data for cereal grains in the range 20-9070.

The resistance to airflow through a bed of grain is an important parameter in the design of drying system. The Hukill and Shedd ~9 equation is acceptable for clean, non-packed grain. Haque et al. 2~ proposed a relationship for maize containing 0-50% broken corn and fine material in the mixture. The static pressure of the grain in a bin is affected by the method of loading the bin and by stirring; Bern et al. 2~ have quantified these effects for maize.

Reliable airflow--static pressure data are required for the accurate design of drying systems. The available experimental data have mainly been collected for in-bin drying systems and relate to relatively low airflow rates. The airflow range needs to be expanded to 04)5--ff07ma/s per square metre of screen area in order to allow proper design of high-temperature continuous flow driers.

Vemuganti et al. z2 determined hygroscopic, thermal and physical properties of 25 cereal grains, legumes and oil seeds; included are such fundamental properties as specific heat, bulk density and equilibrium moisture content; unfortunately, heat of vaporization values were not established. New values of thermal conductivity, thermal diffusivity and specific heat for six grains were reported by Scherer and Kutzbach 2a and M/ihlbauer and Scherer; 2+ respiration rates and dry matter losses were also established. 2s Moisture diffusion coefficients for rice were measured by Steffe and Singh; z6 drying rates for several maize hybrids by Stroshine et al. z7 Mechanical properties of maize as affected by moisture content and temperature were determined by Scherer and Kutzbach za and Paulsen et al.; 2a a+ review on the subject was presented by Kusterman and KutzbachY ~ Radiofrequency and microwave dielectric properties have been reported by Nelson 30 for maize and other seeds.

Optimal control of grain drying requires an accurate knowledge of grain property values. Many properties of common and uncommon grains remain to be determined, especially at high and low moisture contents and temperatures.

2.2. Drying models 2.2.1. Single grain kernels

The drying behaviour of a specific cereal grain is usually obtained by measuring the loss in weight of a thin layer of individual grain kernels under constant ambient conditions in a series of so-called "thin layer" drying tests. The drying equation of a particular grain can be empirical or be based on diffusion theory. Misra and Brooker ~ proposed a new empirical drying equation for maize; similar equations have recently been determined for rough rice" and for soybeans. 3a For several of the major grains (i.e. wheat, barley) the empirical drying rate equations need updating. 1~. ~ as

A diffusion-type drying equation is required when the moisture gradient in the grain kernels during (he drying process needs to be calculated. Steffe and Singh a developed a new diffusion-type model for rough medium-grain rice in which three different values of the diffusion coefficient (i.e. for the endosperm, bran and hull) are employed. Bakker-Arkema et alp used the Steffe model to evaluate the required tempering time in a multi-stage concurrent flow rice dryer. Diffusion-type drying rate equations need to be developed for most grains.

2.2.2. Psychrometrics Simulation of a deep-bed grain drying process requires a model of the thermodynamic

relationship between water vapour and dry air in order to calculate such psychrometric

C R O P P R O C E S S I N G A N D S T O R A G E

properties as the absolute humidity, specific volume and dry-bulb temperature of the mixture. Brooker et al. ~ summarized the theoretical and empirical relations among the psychrometric properties over the temperature range of interest in cereal grain drying. Bakker-Arkema et al. 2 published a package of subprograms in SI units which permits the calculation of the remaining properties, given any two properties uniquely describing a point on the psychrometric chart.

2.2.3. Deep-bed of kernels Thin layer drying and psychometric subprograms are essential parts of deep-bed grain drying

simulation models. There are four basic convective drier types, namely, fixed-bed, crossflow, concurrent flow and counterflow; each can be represented by a simulation model. Models for each have been described by Brooker et alY s Fixed-bed models have recently been reviewed by Sharp, x the other three drier models by Laws and Parry. aT

Bin and on-floor drying systems are in general fixed-bed driers. The drying air can be ambient (i.e. natural air drying), slightly heated (solar and low-temperature drying) or moderately heated (batch-in-bin drying). The drying process may continue for weeks or even months. Fixed-bed models are frequently used in operational research type studies ae and, thus, must be reasonably fast in computer time.

The fixed-bed drying models fall into two catagories: (1) empirical (or semi-empirical) and (2) theoretical. The first model leads to algebraic-type equations; the second to more complex partial differential equations (p.d.e.). The empirical models require less computer time, but are less accurate than the theoretical p.d.e, models. The best known of the empirical models is the one developed by Thompson et al. ae Although empirical drying models are still used in operational research type studies, ~ p.d.e, models are gaining ground because of their more-fundamental nature. Besides, digital computers are increasing in speed so that they will soon solve even extremely complicated drying models rapidly.

The p.d.e, fixed-bed grain drying model, developed from heat and mass balances on an elemental bed volume, has the following form :ae

Gou _ oM ou tgx - - p~ ~ - ep~ Ot ' ...(I)

dT O)-~t - h ( T - O) c,H) OT G(c, + c,H).-~-~-= pK.,(T- - p,,e(c,, + ..-~-, ...(2)

c30 0M Pz(Cz + ewM)~-i-= h ( T - O) + hf z ~ Ps, ...(3)

dM dt - thin-layer drying equation.

The model is solved by numerical integration employing an explicit finite-difference technique incorporating a variable time-step of about 1 s. The p.d.e, models for the crossflow, concurrent flow and counterflow driers are similar in form to the fixed-bed drying model presented in the previous paragraph. The models are presented in a general form in Laws and Parryg~ The crossflow model is solved similarly to the deep-bed model. The concurrent flow model (a system of ordinary differential equations) requires an implicit linear multi-step method. The counterftow drying model uses a search along with the concurrent flow drying subroutine:

2.3. Low-capacity drying In-storage grain drying includes natural air drying, solar drying, low-temperature drying, ammonia

drying, combination drying, dryeration, batch-in-bin drying and in-bin counterflow drying. Each of these drying techniques is energy-etticient compared with high-temperature drying and, thus, has been investigated extensively since the rapid increase in oil prices.

F . W. B A K K E R - A R K E M A 5

TABLE 1

Minimum airflow requirements (m s min- t t - t) for one location in each of the north central region states, assuming an initial moisture content of 24~o (w.b.) and a harvest date of 15 October. These values are based upon simulated drying results for n

10-year period. Increase these values by 50% for design purposes

Natural air* Solar dryingt Low-temperature:~

Location Nex t to worst Nex t to worst Nex t to worst

year year year

~hicago, IL indianapolis, IN Des Moines, IA Dodge City, KS Lansing, MI ~t Cloud, M N 2olumbia, M O Lincoln, NE Bismarck, N D Mansfield, O H Huron, SD Madison, WI

3"01 4"57 2"43 2"48 3"07 2"13 2"77 2"31 0'64 2"84 1"56 2"50

2"41 2"37 2"51 2"36 2"56 1"96 3"10 2"23 0"60 2"12 1"35 2"09

2"49 2"09 2"19 2"36 2"61 1 '94 2"44 2"00 0"60 2"06 1"26 1"97

*A I'I~ temperature rise due to drawing the air over the fan motor was assumed THeat supplementation includes the I'I~ from the fan plus a solar collector capable of

a 24 h average temperature rise of 1"7~ when collecting 1255 J/cruZ-day :12-8'C temperature rise from electrical heater and from the fan Source Pierce'and Thompson 4t

providing

2.3.1. Natural air and low-temperature drying Natural air and low-temperature drying are similar processes if grain is dried and stored in

the same storage, in on-floor or in-bin storage systems. The distinction is that no heat (except fan energy) is added in natural air drying while in the case of low-temperature drying, the air temperature is increased a few degrees (usually with electric heaters) to decrease the relative humidity to below 55~.

Natural air drying has been used for a long time to dry small grains harvested during the summer months at moisture contents below 18~ (w.b.). ~ Natural and low-temperature drying are now being adapted to dry maize harvested at much higher moisture contents. The r~pcommended minimum airflow rate and maximum bin depth depend on the initial moisture content of the maize and on the environmental conditions. Table 1 lists the minimum airflow rate for natural air and low-temperature drying of maize at 24~ m.c. in 12 north central states in the U.S.A.; the results are based on simulated drying experiments over a 10-year period?l Note the dramatic difference between a northern state (N. Dakota) and a more southerly state (Indiana). Due to the warmer climate during the harvest season, the southern maize needs to be dried more rapidly and, therefore, requires five times as much air than the maize in the north.

Mittal and Otten ~ demonstrated that for Canadian conditions (Ontario) natural air and low- temperature drying of maize is no more energy-efficient than conventional high-temperature drying unless a microprocessor control system is employed for the fan (and heater). M/ihlbauer et al? 2 compared ambient air and low-temperature drying systems under West German conditions for high moisture content [22~ (w.b.)] wheat and concluded that both systems are acceptable on-farm drying alternatives to conventional high-temperature driers. :

2.3.2. Solar drying In the past decade, much research has been conducted on the use of solar energy for in-bin

grain drying systems due to the energy crisis. The solar energy available at a particular location depends on the latitude, the angle above the horizontal of the collector, the time of day and year,

6 CROP PROCESSING AND STORAGE

and the local weather conditions.4a In many areas, solar energy is diffuse, intermittent and unpredictable; the price of solar collectors depends on availability of construction materials and labour costs. These factors have contributed to the wide diversity in opinion about the usefulness of solar grain drying systems.

Typical of the many experimental solar drying investigations is the 3-year study conducted in the mid-western U.S.A. by Kranzler et al.** with a plywood suspended-plate collector with polyethylene plastic film cover and absorber; they concluded that solar heat can replace a significant amount of the electrical resistance heat in a low-temperature maize drying system (20%), but the value of the energy savings does not equal the total cost of materials for the collector. Other experimental grain drying investigations have been reported by Johnson and Otten, 4s Foster and Peart ~ and Iowa State University. 47

Morey et al. 4 are among those who have used simulation to study the feasibility of drying maize using solar energy; they also concluded that solar heat does not appear to be economically feasible at any of the three U.S. locations under the prevailing U.S. fossil fuel price structure of the late-1970s.

Minimum airflow for in-bin solar maize drying as 12 U.S. locations of 249/0 (w.b.) maize is given in Table 1. 41 Similar data for solar wheat drying in Canada is given by Fraser and Muir. 4a The author has found this type of data very useful in the design of actual installations.

Misra et al. ~ considered summer storage of solar heat in water tanks and solar ponds and concluded that at the level of 1980 fuel prices the system is not economical for grain drying. Eckhoff and Okos s~ came to a similar conclusion for rockbeds.

The economics of high-temperature solar grain drying has been investigated by Loewer et al. 5~ They concluded that the method is technologically feasible, but will not become economically feasible until the U.S. prices of alternative fossil fuels become 2-3 times more expens.ive and/or more favourable tax policies are adopted (i.e. tax credits or low interest rates).

2.3.3. Trickle ammonia drying Trickle ammonia drying (t.a.d.) is a recently developed method for in-bin drying of maize for

animal feed. The concept of this on-farm drying system consists of drying high-moisture content maize at ambient temperatures with low airflows over a period of 1-2 months; intermittently a low level of anhydrous ammonia is injected into the airstream in order to suppress microbial (mould and bacteria) activity. The method of t.a.d, has been investigated extensively at the U.S. Department of Agriculture Laboratory in Peoria, Illinois; szSa field testing has recently been initiated, s*

The usual procedure for t.a.d, consists of drying 25-26~ maize at an airflow rate of 1-2 m 3 min- 1 t-1, applying initially 1 kg per 2000kg of maize (0.059/0). This treatment is followed by a similar NH3 injection in 3-4 d, and subsequently at weekly intervals until the grain has reached a safe storage level.

The t.a.d, process requires a conventional drying bin, a fan, an ammonia tank and a standard flowmeter. The investment and operating costs of the t.a.d, are low. Total drying costs are claimed to be less than 509/o of those of high-temperature driers ;54 energy consumption is comparable with high-temperature/ambient air drying. Little detectable odour or discolouration of the kernels results from t.a.d.; corrosion of bins and handling equipment is negligible.

Other chemicals have been tested in the trickle drying process as microbial inhibitors, such as formaldehyde and methylene-bis-propionate 52 and sulphur dioxide. 55

A general prediction model for the t.a.d, process has been developed by Hsieh et a L " The model ran simulate the treatment of grain with any fungicide by specifying the relevant properties for the grain and the chemical.57

The t.a.d, process is a recent technical development. The process is safe, energy efficient, minimizes deterioration of grain quality and can operate over a wide range of weather conditions. The process has promise as an on-farm drying system for feed maize.

F . W . B A K K E R - A R K E M A

2.3.4. In-bin counterflow drying In-bin counterflow grain drying is relatively new; the system consists of two bins, one of which

is a heated-air counterflow drier. Grain is loaded into the first bin and dried until the bottom 10cm layer has reached a moisture content of 16-5-18.5% (w.b.). The partially dried, hot grain is removed from the bottom by a tapered sweep auger and is transported to the second bin where final drying and cooling takes place. The air temperature in the first bin is between 70~ and 95~ depending on the type of product and its moisture content; the airflow rate is 10-30m a min-t t-1, depending on the grain depth. Ambient air is used in the second bin; the airflow is 3-10m a min -1 t -1, depending on the depth of grain in the bin. Removal o1~ the partially dried grain from the first to the second bin is intermittent; the cycle-time of the unload auger depends on the airflow rate, the grain moisture content and the air temperature. In-bin counterflow dryers have been investigated by Silva? a Baker et al. sa and Bridges et al. a~ Table 2 shows the simulated performance of a typical in-bin counterflow system as affected by loading procedure. The data indicates that load size and loading rate affect the throughput of the system because of the influence of grain depth on airflow rate.

In-bin counterflow grain driers have become popular on-farm driers in the U.S.A. because of the flexibility, automatic nature and excellent energy efficiency of the unitsP 0

T A B L E 2

P e r ~ r m a n c e o f a cont inuous in -b in m a i z e d r y m g s y s t e m a s a f f e c t e d b y i o a d i n g r a t e *

Analysis no.

No. of loads per

24 h

5 10 10

Load size, m 3

14"1 7"05 7'05

Arrival interval,

h

1"0 1-0 2"0

Grain driedt per

24 h, m 3

61"8 65"0 71"5

Drying rate, ma/h

2'61 2"72 3"11

Final depth,

m

0'73 0"64 0"43

*A 6"40 m drying bin with an initial grain depth of 0,46 m and a fan of 9.7 kW was used for each analysis tFor I I points of moisture removal [26-15% (w.b.)] at 71.1~ Source Bridges et al. ~9

2.3.5. Combination drying .~ Combination drying is defined as a system in which high-temperature, high-speed drying is

followed by in-bin low-temperature drying and cooling. 61 Combination drying was developed in the 1970s for maize in the U.S.A. to improve the energy-efficiency and the grain quality characteristics, and to increase the throughput of high-temperature drying systems.

The high-temperature, high-speed phase of combination drying can be a batch or a continuous flow drier. The maize is partially dried and subsequently moved hot to an ambient or low- temperature in-bin drying system. The low-temperature part of combination drying may require from 2d to 2 months, depending on the initial moisture content of the grain placed in the low-temperature bin and on the airflow rate and temperature of the drying air. In several states in the northern U.S.A., drying may have to be interrupted in the late-autumn due to excessively low temperatures, and be completed during the following spring.

The in-bin phase of combination drying may start at moisture contents ranging from 18% to 23%, depending on the airflow and temperature combination in the drying" bin. Kalchik et alp ~ compared four in-bin drying techniques as part of a combination drying system (see Table 3). Their data show that in a combination drying system, natural air drying is the most energy efficient, followed by low-temperature drying, in-bin drieration and in-bin counterflow drying. Note that the automatic batch drier requires twice as much energy as the natural air combination drying system.

C R O P P R O C E S S I N G A N D S T O R A G E

TABLE 3

Standardized energy consumption for five alternative combinat ion drying methods in Michigan, U~;.A. (43 ~ N latitude)

Drying technique

latural air :~ (26-23-15.5%) m.c.

,ow-temperature :~ (26-23-15-5%) m.c.

1-bin drierat ion :~ (26-20-15.5%) m.c.

In-bin eounterf low (26-18-15.5%) m.~

Automat ic batch (26-15.5%) m . c

Electricity,* k Wh/ha

343~

481"9

944

103"5

33"4

Propane, I/ha

72"9

72"9

t42"1

156"2

287"9

Energy efficiency,

kJ/kg

3227

3756

4140

4548

6589

Total energy, t propane equiv.

t/ha

121"6

141"1

155"2

171"0

292"6

*Based on 62-5 t. initial m.c. 26.0% (w.b.). final m.c. 15.5% 1"Based on 6,9 t/ha ~.Energy efficiency of high-temperature drying phase is 6228 kJ/Kg H:O

Kalchik et al. 6~ also found that combination drying systems produce improved grain quality. Extensive experimental results of four years of combination maize drying in the northern U.S.A. have been presented by Morey et al. sl. 62 They concluded that combination drying significantly reduces propane or natural gas requirements and results in a saving in the total energy requirements. Drieration is a special case of combination drying. Maize is dried in a high- temperature drier to 16--18% (w.b.) (sometimes to as high as 20-0%) and final dried at about 1-0 m 3 min-1 t-1 after 6-8 h of tempering:? Tempering is usually done in a special tempering bin to prevent condensation. In France a continuous flow high-volume drieration system has been developedY 3

2.3.6. Grain stirring In a grain bin, overdrying of the bottom layers and non-uniform drying of the grain mass are

common faults of in-bin drying systems. Grain stirrers are designed to overcome these problems and are in common use in the U.S.A. ~ A grain stirring system typically consists of one or more 51-mm augers suspended from the bin roof and sidewall and extending to near the bin floor. The augers lift the grain from near the bin floor towards the top of the grain while moving in a predetermined pattern through the bin. The horizontal travel rates vary from 2 to 10mm/s.

Stirring devices were originally developed for in-bin high-temperature batch driers. Recently, the units have been used in conjunction with in-bin low-temperature drying systems. 6s

2.3.7. H e a t pump drying The heat pump is an attractive energy source because it can deliver more energy as heat than

it consumes in electrical energy. The heat pump has been tested by a number of investigators on low-temperature, H solar-supplemented e7 and high-temperature~ grain drying systems.

Kutzbach e6 described a deep-bed wheat drying system with a heat pump as the air heater; the operating costs were 10% lower than for an equivalent oil heater. Hogan et a l ." conducted low- temperature maize drying with a heat pump over three seasons; Table 4 shows that the heat pump required only 60% of the energy needed by a resistance heater. Extensive data on the operational aspects of a heat pump drying system have been collected by Person et a lJ o

Several investigations have been conducted on solar supplemented heat pump driers, e? The heat pumps operated only at night and supplied about 50% of the energy for drying the grain;

F. W. B A K K E R - A R K E M A 9

TABLE 4

Energy used by the heat pump and resistance heaters in drying maize

Item Heat pump, kwh Resistance heat, kwh

Energy used by: Fan Heat source Grain stirrers Total

Energy used per kgwater removed

Source Hogan el al , n9

3813 2289

171 6273

0.557

3024 7091

136 10251

0'905

the electrical usage was 52~ of that required by conventional electrical heat low-temperature drying systems. -

A 60 kW heat pump was tested by Peart 6a as the sole heat source for a continuous flow high- temperature commercial crossflow maize drier; the energy efficiency was 1150 kJ/kg, although the throughput was limited to 2"25m3/h at 5~ points removal at a maximum drying air temperature of 80~

There appears to be little doubt that low-temperature electric heat pump drying is technically viable and saves significant amounts of energy. By using a fossil fuel engine instead of an electric motor to drive the heat pump, a further energy saving can be realized by utilizing the engine exhaust energyP a However, since the investment costs of heat pumps are high and the units are used for a limited number of hours annually, heat pump drying does not appear to be economically competitive with conventional drying techniques.

2.4. High-capacity drying Continuous flow driers fall into three categories: (1) crossflow, (2) mixed-flow and (3) concurrent/

counterflow. In the crossflow drier the drying air passes perpendicular to the direction of grain flow. In the mixed flow type, the drying air flows partially with the grain and partially against the grain by way of an alternate series of inlet and exhaust ducts. In the concurrent/counterflow drier, the drying air flows in the same direction as the grain (co-flow), while the cooling air flows in the opposite direction of the grain. The crossflow type is at present the most widely used system worldwide. In Western Europe and South America mixed flow driers are popular. The concurrent/counterflow drier is a new development and is at present only manufactured in the U.S.A.

2.4.1. Crossflow driers Crossflow driers have been on the market for several decades. '~ Recent research on this model

has been mainly concerned with improving the energy efficiency 71.7~ and the grain quality characteristics.le, a

In a conventional crossflow drier without air recirculation, some of the discharged air is only partly saturated. This condition leads to an energy consumption of 7000-9000 kJ/kg.S Recycling part of the drying air and all of the cooling air greatly decreases the energy requirements of crossflow driers. Lerew et al., 7a Meiering and Hoefkes, 7. Kuppinger ~1 and Pierce and Thompson ~2 all investigated different schemes of air-recirculation in crossflow driers.

Along with air recycling, reversal of the airflow direction is frequently incorporated in crossflow driers in order to offset a major disadvantage of this type of drier, the large moisture differential in the dried grain. A less expensive (although not quite as effective) solution to the problem of the large moisture gradient consists of placing one or more grain

10 C R O P P R O C E S S I N G A N D S T O R A G E

TABLE 5

Calculated energy requirements for the four driers operating under conditions which maintain grain quality and allow a grainllow of 48.5 kg of dry matter per hour per square metre of drier area

Drier type

Conventional erossflow

Reversed crossflow

Har t -Car te r Recireulating

drier

Total energy, kJ/kg of

H20 evap.

6940

702O 4890

4380

Drying air temp,

*C

68

68 65

66

Airflow rate,

ma/min- tra- a

42

41 58

51

Maximum av. grain temp, ~

6O

6O 6O

6O

Moisture differential,

% (w.b.)

5"0

1'9 1"3

1'1

Source Pierce and Thompson 7z

inverters in the grain column; these units turn the overheated grain at the air inlet side to the air exhaust side of the column. ~5 Crossflow maize driers without air-reversal or a grain inverter device have gradients across the column as large as 20~o for moisture content and 50~ for grain breakage, is

Pierce and Thompson 72 investigated four crossflow designs: (a) the conventional crossflow type, (b) the reversed airflow type, (c) the reversed airflow type with air recirculation of all of the cooling air and 50~o of the heating air (the so-called Hart--Carter design) and (d) the recirculating drier with re-use of the cooling air and the drying air from the second drying stage. All four driers are commercially available and they are compared in Table 5 in drying maize at the same drier capacity from 25~o to 15% (w.b.) under ambient conditions of 10~ and 50~ r.h.; the drier operating conditions are not the same because each maintains grain quality under different conditions. Note the potential for decreasing the energy consumption and the moisture differential by modifying the conventional crossflow drier.

Two new features have recently been added to the basic crossflow design, differential grain-speed and tempering.~6 Differential grain-speed refers to the design in some crossflow driers of moving grain closest to the air inlet side at a faster rate through the column than grain located at the air exhaust side; ~ in such designs grain-speed ratios are varied from 1:1 to 1:4. 77 Dual discharge rolls located under each drying column and rotating at different angular speeds are the essential feature of this design. Kuppinger n found that the moisture differential across a maize column was halved by changing the grain-speed ratio from 1 : 1 (a conventional crossflow drier) to 1:3; Bakker-Arkema et a!.77 showed that the optimum grain-speed ratio depends on the grain type and initial moisture content.

Tempering between drying passes or stages has been practised for years in drying rice. Duringtempering the temperature and moisture gradients within the individual kernels diminish~a 79 resulting in less fissuring and breakage. Tempering between stages in a crossflow drier also leads to an increase in the drier capacity, a~

A commercial crossflow grain drier incorporating differential-grain speed, tempering and air- recycling has been described and tested by Bakker-Arkema et al.;77 its energy efficiency was 3700 kJ/kg.

2.4.2. Mixed-flow driers Grain in mixed-flow driers is dried by a mixture of crossflow, concurrent flow and counterflow

actions. This is due to the rows of alternate inlet and exhaust air ducts over which the moist grain flows. Models of a mixed-flow drier, therefore, consist of a series of crossflow, concurrent flow, and counterflow subroutines, ao


The inlet air temperature in a mixed-flow drier can be higher than in crossflow models because the grain is not subjected to the high temperature for as long a period of time; as a result as much as 40% less air and energy is needed than in comparable crossflow driers> The energy-efficiency of mixed-flow driers with air recirculation has been reported to be 3500- 4000 kJ in drying high moisture content maize.a1 The material published in the open literature on mixed-flow grain drying during the past 10 years is sparse. An excellent treatise, however, has been published by Olesen.a2

2.4.3. Concurrent/counterflow driers Concurrent/counterflow drying was developed from a Swedish patent by Oholm an during

the 1970s and appears to have the potential to become the major grain drying technique of the 1980s. A concurrent/counterflow drier consists of one or more concurrent flow drying beds coupled to a counterflow cooling bed;~ in the multi-stage units a tempering zone separates two adjoining drying beds. 85

The maximum drying air temperature in a concurrent flow drier is not limited by the type or moisture content of the product to be dried since grain velocity is the determining parameter. Air temperatures as high as 500~ have been successfully used in drying maize without affecting product quality, ae The high air temperatures contribute to the high energy-efficiency and the low airflow requirement of the concurrent flow drier, aT

Grain dried in a concurrent flow drier is of uniform moisture content since each kernel has undergone the same drying, tempering and cooling treatment unlike grain dried in a crossflow or mixed-flow drier. The grain temperature is better controlled and the maximum temperature maintained for a shorter period of time in the concurrent flow drying section than in either of the two conventional driers. The counterflow cooler subjects the hot grain to a gentler cooling process than the other driers, since the largest temperature difference between the cooling air and the warm grain kernels is never more than 5--100C in a counterflow cooler. For these reasons concurrent flow dried grain is of higher quality than grain dried in other driers. ~. ~ as

Commercial concurrent flow driers are operating in the maize, rice, sorghum, soybean, sunflower and wheat processing industries.

The energy-efficiency of concurrent flow driers with and without air-recirculation ranges from 3000 to 3800 kJ/kg in drying a variety of grains.~ a

2.5. Maximizing energy-efficiency Minimizing the energy requirements has been one of the main criteria for drier designers in the

1970s and 1980s. Among the techniques employed are: (1) air-recirculation, (2) grain pre-heating, (3) drier staging, (4) grain tempering and (5) drier control.

Recirculation of part of the exhaust air from the heating and cooling sections of a grain drier improves the energy efficiency of the drier. Recycling of the air can be direct,~ with the use of a heat exchanger m or in conjunction with a heat pumpY 2 Some French driers recover the heat of condensation from the drier exhaust air. ea Energy savings from the re-use of exhaust air depend on the air-recycle design, the ambient conditions, and the drying air temperature; figures between 10~ and 40~ have been reported.. It seems probable that in the future air- recirculation will be a standard option for all high-temperature driers.

Grain pre-heating by counterflow convection has been proposed for high-temperature concurrent flow driers, a4 Although some improvement in the drying capacity results, grain quality deterioration may offset this advantage. Pre-heating by microwave heating is under active study in the U.S.A. and has promise if the cost can be reduced.

Staging of several drier modules allows the drying air to be varied in each stage. In addition to improving grain quality, staging results in an improvement in drying etticiency of 5-10~ .u


Fuel

Electricity Natural gas Bulk propane Bottle propane Diesel oil Coal Straw Wood

TABLE 6

Summar

Gross cost

3-55 p/kWh 27-2 p/therm 13.4 p/therm s kg 17 p/1 s s s

of fuel costs in the U.K. (1982)

Gross calorific

value

3"6 MJ/kWh 105-5 MJ/therm 26 MJ/I 50 MJ/kg 40 MJ/I 25600 MJ/t 16800 MJ/t 19800 MJ/t

Overall efficiency

(direct-fired),

100 98 98 98 95

75 (indirect) 65 70

Cost/useful MJ

0"98 p 0"26 p 0"53 p 0"93 p 0'45 p 0"31 p 0'18 p 0"22 p

Source Blakemant iI

Tempering of partially-dried grain between drying stages improves grain quality and also improves drying efficiency. 7g For a three-stage concurrent flow rice drier, two 1-hour tempering periods decreased the energy consumption by 5~. s3 More manufacturers of driers can be expected to incorporate tempering zones in their designs.

Optimization of drier operation can be achieved only by automatic control of the final moisture content so that over-drying is prevented. Drying exhaust temperature has traditionally been the control parameter for maintaining the desired product outlet moisture content. A microcomputer controller has been developed which maintains a constant average outlet temperature and moisture content on a concurrent flow drier. ~ An automatic controller based on continuous monitoring of the inlet and outlet grain moisture content has been patented for the Cimbria continuous flow mixed-flow drier, a5 A microprocessor controller for a low-temperature maize drying system has been developed. ~ Of further interest is the work by Fabian and Samu ~ on mixed-flow grain drier control, and by Schrader and ScharfS7 on the control of bulk- stored agricultural produce. The proper control strategy and microprocessor controller appear capable of saving 5-30~ in energy use compared with a non-automatic control system. Continued research on optimum controls, especially for multi-stage driers, can be expected.

2.6. Biomass furnaces Biomass in the form of straw, maize cobs or wood chips appears to have promise as a

replacement for petroleum based fossil fuels in grain drying. Table 6 shows that the fuel costs (1982 in the U.K.) of biomass fuels are attractive. Montalembert 9a reviewed the sources available for biomass energy.

Straw, in the form of small or big bales, is the main fuel used in Western Europe for biomass furnaces ;m in the U.S.A., dried maize cobs, husklage (cobs with husks) and wood chips appear to be major products. Large furnaces developing a thermal power of about 25 GJ/h are already in commercial use in France and Brazil.a3

Present research is centred on the improvement of the combustion efficiency of biomass furnaces, on the reduction of emissions and on the improvement of the operation and control. 100 Two main categories of combustion are found, direct combustion and gasification combustion.

In a direct combustion furnace, the energy from the burned biomass is transmitted to an airstream which is mixed with ambient air to give the required drying air temperature. Claar et al. ~o~ reviewed the different types of direct-combustion furnaces.

In a gasification combustion furnace, producer-gas is produced which subsequently can be burned in a conventional drier gas heater. Richey and Foster lo2 reviewed the different gasifier designs.

F. W. BAKKER-ARKEMA

T A B [ . E 7

Breakdown of costs for oil- and straw-fired systems in the U.K. (1982)

13

Capital cost--drier only (25 t/h), s Depreciation over 10 years amortised at 15~, s Maintenance (5~), s Total annual fixed costs, s Fuel to remove 2"5~ moisture from 10 000 t (6 MJ/kg water), s

Oil

40000 8000 2000

10000 7918

Straw

75000 15000

3750 18 750

3241

Additional annual fixed cost of straw furnace = s Annual savings in fuel oil costs= s Annual LOSS = s Source Blakemanl II �9

Barrett et a13 o3 compared direct-combustion and gasification-combustion furnaces and concluded that there are no significant differences in the heat recovery efficiency and particulate emission between the two types. However, the gasification units can operate at lower combustion temperatures (650-850~ than the direct-combustion types (1000-1100~ and produce a product (producer-gas) which replaces natural or LP gas directly.

In the direct-combustion system, the products of combustion pass directly through the grain and a part is retained through condensation and absorption by the grain. The quantity and types of combustion compounds depend on the fuel, the type of burner, the completeness of combustion, and the grain variety? o* Of particular concern are the polycyclic aromatic hydrocarbons (p.a.h.) because of their carcinogenic nature? ofi Anderson et al. 1~ tested maize dried with a biomass-fired direct-combustion furnace and did not find any p.a.h, residues. Mwaura ~0~ obtained similar results.

Many biomass furnaces available commercially appear to have been built by trial-and-error. Basic analyses are required of the effects of such parameters as biomass level, primary and secondary airflow, rate, burn time, etc., on the efficiency of a specific furnace before optimum design can be realized? ~176

Strehler ~.a~ and Milande ~~ presented design details o f European biomass installations. Mwaura et al? ~o described a number of commercial biomass furnaces built in the U.S.A. These papers stress the technical feasibility of biomass furnaces and ignore, for the most part, the question of economic feasibility. The main drawback of widespread adoption of biomass furnaces for drying installations is the high capital cost. In Table 7 an oil-fired system is compared with a straw-fired unit of the same capacity for 1982 U.K. conditions; 1~1 the additional capital cost results in a substantial annual loss for the biomass system. Mwaura et al. ~ o obtained similar results for biomass furnace drying systems under 1983 U.S. economic conditions. It appears that fossil fuel costs have to rise substantially over the next decade for biomass installations to become economically viable for grain drying.

3. Grain preservation

3.1. S t o r a g e Three recent books have been published on the subject of grain storage. 112-114 Such topics as

controlled atmosphere storage, and mould, mite and insect control are discussed in detail. Controlled atmosphere grain storage (c.a.g.s.) appears to be making a revival. Underground

grain storage has been practised for centuries to create an atmosphere lethal to stored grain pests; it is a form of c.a.g.s. Usually, c.a.g.s, is considered to consist of medium- or long-term storage in atmospheres consisting only of the major gaseous components of the earth's


atmosphere; that is, in mixtures of nitrogen, oxygen and CO2 The effect of c.a.g.s, on grain insects has been reviewed by Bailey and Banks, 115 the influence on storage moulds by Busta et al: TM

In Australia, c.a.g.s, in atmospheres high in CO2 is used on a commercial basis for insect control in wheat;lo? the economics of such systems are competitive with conventional storage systems, which require fumigation. In Italy, c.a.g.s, in high nitrogen atmospheres has been tested since the mid-1970s and is claimed to be economically competitive with conventional grain storage systems./TM It is obvious that c.a.g.s, has a role among future grain preservation techniques; however, it is not clear at this moment how large this role is.

Control of insects by fumigation~g continues to be practised successfully. Non-chemical methods include heat treatment e. 12o and radiation; lz~ neither method is currently very popular.

3.2. Aeration Grain stored for over a month requires aeration in order to prevent diffustion of moisture

through the bulk of the grain caused by temperature gradients. Thorpe lz2 presented an analysis of the diffusion mechanism based on the interstitial partial pressure of water vapour as the driving force; the theory allows the calculation of moisture accumulation in stored grain.

Movement of low volumes of air through the grain eliminates temperature differences and prevents moisture migration occurring in a grain bin. A number of recent investigations have been conducted on modelling the process of aeration. Marchant 123 used the finite element method to solve the linear airflow problem in two dimensions; the model was used to calculate the air pressure and airflow in a system of ducts? 2. The three-dimensional linear model for pressure and velocity distribution in a circular grain bin was solved by Lai us using the method of lines. Segerlind 126 developed the two-dimensional non-linear model and solved for the pressure pattern in a rectangular bin by the finite element technique. Finally, Khompis 127 solved by finite elements the three-dimensional non-linear airflow problem for a cylindrical grain store and presented three- dimensional pressure and velocity distributions. Each of these models allows a better determination of the position of the critical minimum air velocity regions in a grain store. An aeration model was used by Thorpe and Elder 128 to assess the effects of aeration on the persistence of chemical pesticides in bulk grain.

Refrigerated aeration, a new concept in insect disinfestation, has been applied successfully to a 1700 t wheat storage bin. 129 The system appears to have considerable merit as a non-chemical form of insect control due to the increasing resistance of insects to traditional chemicals.

4. Hay storage

The conservation of grasses and forage crops by haymaking is increasing in importance in many parts of the world? ao Minimizing dry matter losses is the major objective in the process of haymaking? 3~ New techniques have recently been developed to speed up by mechanical or chemical means the haymaking from a 2-4 d to a 1 d process in order to decrease the weather influences on hay quality.

The sources and the quantities of the dry matter loss during the conservation and utilization of hay have recently been reviewed? 3~,~32 The total overall loss for hay, field-cured under humid climatic conditions, frequently reaches 30~o of the initial dry matter; a minimum value of 15~ is a reasonable average value. The major sources of the loss are respiration, leaching, tedding and cutting, with respiration in the field and in storage making up about 10~ of the loss.

Speeding up the field-drying of hay greatly decreases the large respiration field losses. Recent developments in mechanical devices for increasing the field drying rate of hay include a lacerating rotor, TM a tandem roll conditioner la* and a macerator, l~s The macerator, consisting of a pair of serrated rollers running at different speeds, provides a high degree-of crushing and thereby, increases the drying rate of the hay fourfold. The mechanical conditioners allow under


favourable weather conditions 1 d hay harvesting although at the price of considerable losses due to fragmentation? 3s

An alternative to mechanical hay-conditioning is chemical treatment. While mechanical treatment of hay causes physical rupture of tissues, chemical conditioning reduces stomatal and cuticular resistance to water loss. Harris and Tullberg 137 investigated the stomatal pathway and a number of chemicals (including sodium azide) which inhibit closure of the leaf stomata. Some chemicals appear to affect the permeability of the plant cuticles of certain plants and, thus, their drying rates. Recent investigations by Tullberg and Angus ~38 and by Rotz et al? a~ have shown that a spray application of potassium carbonate can increase the drying rate of lucerne plants by 40Yo.

Losses in storage may occur due to moulding and heating when hay is stored improperly at moisture contents above 20~. Many chemicals have been tested as preservatives for 25-30~ moisture content hay; propionic acid and ammonium propionate at rates from 1.0~ to 2.0~ of forage wet weight have proven to be the most successful? 3g Other chemicals tested include acetic acid, formic acid and ammonium is obutyrate?*~ Properly, chemically-treated high-moisture hay maintains quality and feed efficiency comparable with barn-dried or heat-dried hay and is frequently better than field-cured hay.

The drying rate of a hay crop can be maximized by selecting a fast-drying variety and species. Owen and William ~4~ dried more than 100 varieties of grass from seven species at 25~ from harvest moisture content to 20~ and found a significant difference in drying rate among them; for example, perennial ryegrass took twice as long to dry as tall fescue. Jones and Prickett ~42 observed similar trends among grasses. The variation in drying rates may be accounted for by the differences in cuticle thickness and leaf-stem ratio.

The alternative to field-drying of hay and the use of chemical preservatives is barn hay- drying. For barn-drying, hay is wilted to at least 45~, baled (loose and chopped hay drying create handling problems) with a medium density baler, stacked to 5-6m, and dried to 20~o moisture content with 0"20-0.25m 3 s -~ m - 2 ambient air within 10d in a ventilated floor or centre duct system24a Basically, storage bale drying has not changed in the past 10 years.

Eimer et a l Y ~ developed a set of relationships for the appearance of the first mould on lucerne and grasses as a function of temperature, moisture content, relative humidity, and the degree of wilting; the empirical equations can be used to determine the optimum strategy for control systems of in-barn hay driers. The drying pattern of grass swaths in the field has been modelled by Clark and McDonald ;~45 the model should be useful in the analysis of hay-conditioning equipment.

5. Potato storage

Currently, potato storage research appears to be concentrated in the areas of modelling, storage structure design, ventilation and control. 1~ Each of these topics is discussed at length in a recent book on the topic? 47

Lerew and Bakker-Arkema ~4a developed a one-dimensional model for the storage environment of bulk stored potatoes; better prediction equations for weight-loss and heat-generation rate are necessary before the model can be employed for design. Brugger and Buelow ~4a extended the Lerew model to two dimensions. It is expected that fine-tuning of these potato storage models will soon lead to their use in the design and control of commercial potato storage structures.

Uniformity of air distribution is essential for successful potato storage. Cloud and Morey ~5~ analysed duct performance and recommended the use of ducts with a stepwise decrease in cross-section along the duct length. Statham ~51 suggested the use of tapered ducts.

McCarthy lsa expected microcomputer technology to contribute greatly to the control of potato storage. Hunter ~53 described a microprocessor monitoring and control system. Optimum use of such systems awaits the further development of natural and forced air convection models along with an optimum control strategy.

16 CROP P R O C E S S I N G AND STORAGE

6. Grain handling and cleaning

The technical advances made in the area of handling, cleaning and grading appear to be less than in the other subjects discussed in this review.

A fundamental analysis of screw conveyers for granular materials has been presented by Rademacher, lglss similar basic studies are needed for other materials handling equipment. Ross et al. Iss designed a constant-speed variable-capacity screw conveyor for grains. Reducing product damage remains a major objective of screw conveyor design.

Pneumatic conveying appears to be increasing in agriculture. The principles of such systems are reviewed in a recent article by Wirth) s~ Fayz and Hanna ~sa designed a pneumatic conveying system for soybeans by which header losses are minimized.

Holt lss and BulP eo presented an overview of the principles and practices of materials handling of different agricultural crops. Bucklin et al., le~ Benock et al. ~62 and Singley and Chaplin 183 developed materials-handling models of the operational research type.

Seed cleaning has recently been reviewed exhaustively by Welch et al. 1~ and gives screen openings for most seeds of agricultural importance. The principles of seed cleaning can be found in Brandenburg. ~6s New trends in seed cleaning have been discussed by Regge and Minaev) e6

7. Conclusions

Post-harvest technology has made significant progress in the past decade. This is particularly true for some areas in grain and hay processing.

Newly adopted grain drying techniques include solar-drying, combination-drying and concurrent-flow-drying. Each can save fossil-fuel energy and each can produce higher quality grain than that obtained with conventional high-temperature driers.

In warm climates, storage in a controlled atmosphere is now an alternative to conventional chemical methods of preservation. Along with refrigerated aeration, the method overcomes the problem of increasing resistance of grain pests to fumigants.

The development of fast-drying grass varieties and new mechanical and chemical hay conditioners, has made 1 d hay harvesting a realistic goal. Due also to the newly developed moist- hay preservatives, total dry matter hay losses will continue to decrease.

Biomass energy has shown promise as a fossil fuel substitute for drying and heating installations. Economic feasibility may, however, require multiple use under the present price and tax structure.

The effect of the contributions of operational research, microelectronics and optimum control is beginning to be felt in post-harvest processing. The full benefits should be realized in the next decade.

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ammonia process with the S02 process for drying high moisture corn. Trans. ASAE, 1982 25 (5) 1431-1434

so Hsieh, R. C.; Bakker-Arkema, F. W.; Cuppett, S. L.; Bagley, E. B. Ammonia-assisted grain draying in fixed-bed systems. Am. Soc. agric. Engrs, Paper No. 79-3563, 1979

57 Hsieh, R. C.; Bakker-Arkema, F. W. Influence of physical parameters on the ammonia grain drying process. Am. Soc. agric. Engrs, Paper No. 80-3075, 1980

so Silva, J.S. An engineering-economic comparison of five drying techniques '.for shelled corn on Michigan farms. Ph.D. Thesis, Michigan State University, East Lansing, 1980

so Baker, K. D.; Foster, G. H.; Tuite, J. Continuous counterflow bin drying with in-storage cooling. Am. Soc. agric. Engrs, Paper No. 81-3021, 1981

so Kalchik, S. J.; Silva, J. S.; Bakker-Arkema, F. W.; Miller, B. S. An engineering-economic comparison of five drying techniques for shelled corn on Michigan farms. Am. Soc. agric. Engrs, Paper No. 79-3518, 1979

61 Morey, R. V.; Gostafson, R. J.; Cloud, H. A.; Walter, K. L. Energy requirements for high-low temperature drying. Trans. ASAE, 1978 21 (3) 562-567

Morey, R. V.; Gustafson, R. J.; Cloud, H.A. Combination high-temperature, ambient air drying. Trans. ASAE, 1981 24 (2) 509-512

ea Lasseran, J. C. New developments in energy preservation for maize drying. Paper presented at the 7th World Cereal and Bread Congr., Prague, 28 June-2 July 1982


a4 Bern, C. J. Stir drying. In Proc. 1980 Grain Conditioning Conf. (Shove, G. C., Ed.), University of Illinois, Urbana, 1980

e5 Kalchik, S. J.; Silva, J. S.; Mwaura, E. N. ; Rodriguez, J. C.; Bakker-Arkema, F . W . On-farm corn drying comparison. Am. Soc. agric. Engrs, Paper No. 81-3011, 1981

aa Kutzbach, H. D. Heat pumps--their potential for drying. Agrartech. Ber., 1978 4 55-66 a7 Harrison, R. E.; Allen, W. H. A solar supplemented heat pump corn dryer. Am. Soc. agric. Engrs,

Paper No. 80-3023, 1980 as Peart, R. M. Low-energy grain drying with the heat pump. In Proc. 1982 Grain Conditioning Conf.

(Shove, G. C., Ed.), University of Illinois, Urbana, 1982 s9 Hogan, M. R.; Muller, R. E.; Foster, G. H.; Ayers, D. L. Development and testing of a heat pump

grain dryer. Am. Soc. agric. Engrs, Paper No. 79-3527, 1979 70 Person, N. K.; Jaska, E. H.; Newman, J. S. Peanut drying with heat pump systems. Am. Soc. agric.

Engrs, Paper No. 77-3536, 1977 71 Kuppinger, H. Investigation and improvement of crossflow drying of grain. Ph.D. Thesis, University of

Hohenheim, Stuttgart, 1980 72 Pierce, R. O.; Thompson, T. L. Energy use and performance related to crossflow dryer design. Trans.

ASAE, 1981 24 (1) 216-220 73 Lerew, L. E.; Bakker-Arkema, F. W.; Brook, R. C. Simulation of a commercial crossflow dryer:

the Hart-Carter model. Am. Soc. agric. Engrs, Paper No. 72-829, 1972 ?4 Meiering, A. G.; Hoetkes, H. J. Drier performance and energy use in corn drying. Can. agric. Engr,

1977 19 (1) 49-54 75 Otten, L.; Brown, R.; Anderson, K. A study of a commercial crossflow grain dryer. Can. agric. Engr,

1980 22 (2) 163-170 7a Rodriguez, J. C. Energy efficiency and grain quality characteristics of crossflow and concurrent flow

dryers. Ph.D. Thesis, Michigan State University, East Lansing, 1982 77 Bakker-Arkema, F. W.; Rodriguez, J. C.; Sehisler, I. P.; Westelaken, C. M. A new commercial

crossflow dryer with differential grain speed. Am. Soc. agric. Engrs, Paper No. 8-3007, 1982 7s Steffe, J. F.; Singh, R. P.; Bakshi, A. S. Influence of tempering times and cooling on rice milling yield.

Trans. ASAE, 1979 22 (5) 1214-1218 ?9 Ezeike, G. O.; Otten, L. Theoretical analysis of the tempering phase of a cyclic drying process.

Trans. ASAE, 1981 24 (6) 1590-1594 as Sokhansanj, S. Personal communication, Dept. agric. Engng, University of Saskatchewan, Saskatoon,

1982 Sl CNEEMA. Official tests of Law SCE112S continuous grain dryer with heat economiser. Rep. No. 2688,

Centre National d'Etudes et d'Experimentation de Machinisme Agricole, Antony, France, 1979 02 Olesen, H.F. Varmetekniske beregningerfor kornt~rrerier. Spec. Rep., Cimbria Unigrain Ltd., Thisted,

.. Denmark, 1980 as Oholm, G. V. Method and arrangement for drying grain and the like in drying silos provided with

direct heat supply. U.S. Patent No. 2,706,343. Washington, D.C., 1955 a4 Miihlbaner, W.; Kuppinger, H.; Isaacs, G. W. Development of a concurrent flow/counterflow

continuous dryer for grain and maize. Grundl. Landtech., 1979 29 (4) 131-140 a5 Brook, R. C.; Bakker-Arkema, F. W. Concurrent flow drying of food grains. In Drying '80, vol. 1

(Mujumdar, A. S., Ed.). Montreal: McGraw-Hill International, 1980 se Hall , G. E.; Anderson, R. J. Batch internal recycling dryer (BIRD). Am. Soc. agric. Engrs, Paper No.

80-3515, 1980 s7 lsaacs, G. W.; Miihlbauer, W. Possibilities and limits of energy saving in maize grain drying. Grundl.

Landtech., 1975 30 (9) 397--401 ea Bakker-Arkema, F. W.; Rodriguez, J. C.; Brook, R. C.; Hall, G. E. Grain quality and energy

efficiency of commercial grain dryers. Am. Soc. agric. Engrs, Paper No. 81-3019, 1981 aa Fontana, C.; Bakker-Arkema, F. W.; Westelaken, C. M. Concurrent flow vs crossflow drying of long-

grain rice. Am. Soc. agric. Engrs, Paper No. 82-3569, 1982 so Chandra, P.; Thompson, J. F.; Rumsey, T. R.; Singh, R. P. Potential of air recycling in an industrial

crossflow rice dryer. Am. Soc. agric. Engrs, Paper No. 81-6514, 1981 91 Sokhansanj, S.; Bakker-Arkema, F.W. Waste heat recovery in grain dryers. Trans ASAE, 1977 24 (5)

1317-1321 s2 Lai, F. S.; Foster, G.H. Improvement in grain-dryer fuel efficiency through heat recovery. Trans. ASAE,

1977 20 (2) 579-584


sa Fontana, C. Concurrent flow versus conventional drying of rice. Ph.D. Thesis, Michigan State University, East Lansing, 1983

Borsum, J. C.; Bgkker-Arkema, F. W.; Brook, R. C. Microprocessor control of drying processes. Am. Soc. agric. Engrs, Paper No. 82-6006, 1982

a50lesen, H. F. Automatic control arrangement for continuous drying plants. British Patent 152710. Br. Patent Office, London, 1978

m Fabiu, F.; Samu, M. Automatic control of the moisture content in grain dryers. GrundI. Landtech., 1980 30 (5) 174-178

sv Scfirnder, A.; Scharf, R. A method for the automatic control of hay drying and other agricultural bulk-stored material. Agratechn., 1982 32 (7) 323-325

m Montalembert, M. R. de Biomass resources for energy: the critical issues. Ceres, 1983 16 (1) 40-44 n Strehler, A. Experiences with straw furnaces. Part 2. Landtechnik, 1981 36 (4) 200-205

10o Orth, H. W.; Wilkens, D.; Meyer-Bothling, F. New investigations in the combustion of straw. Grundl. Landtech., 1982 32 (6) 195-199

lol Clasr, P. W.; Buehele, W, F.; Marley, S. J. Crop residue fired furnaces for drying grain. Am. Soc. agric. Engrs, Paper No. 81-1032, 1981

1or Richey, C. B.; Foster, G. H. A biomass gasification furnace. Am. Soc. agric. Engrs, Paper No. 80-3512, 1980

loo Barrett, J. R.; Jaeko, R. B.; Summer, H. R. Particulate emissions from biomass furnaces. Part 2. Characterizations. Am. Soc. agric. Engrs, Paper No. 81-3562, 1981

lo4 Hutt, W.; Winkler, E. Contamination of grain in drying with direct heating. Getrcide Mchl Brot, 1978 32 (10) 260-263

loo Joe, F. L.; Salemme, J.; Fazio, T. High performance liquid chromatography with fluorescence and ultraviolet detection of polynuclear aromatic hydrocarbons in barley malt. ]. analyt. Chem., 1982 65 (6) 261-266

lee Anderson , M. E.; Bern, C. J.; Baker, J. L. Corn drying with biomass combustion products. Am. Soc. agric. Engrs, Paper No. 83-3005, 1983

lo~ Mwaura, E. N. Biomass furnace design for grain drying installations. Ph.D. Thesis, Michigan State University, East Lansing 1984

loo Strehler, A. Problems with controlling solid fuel heaters. Landtechnik, 1983 38 (1) 10-1 l 1~ Milande, N. At Caussade in the S.W. maize is dried by cobs. Motoris. Technol. agric., 1981 25 (I) 39-40 11o Mwaura, E. N.; Guillaumon, M. H.; VanEe, G. R.; Bakker-Arkema, F. W. Corn drying with biomass

energy. Am. Soc. agric. Engrs, Paper No. 82-3520, 1982 111 Blakeman, R. Fuel saving in grain drying. Agric. Engr, 1982 37 (2) 60-67 112 Anderson, J. A.; AIcoek, A. W. (Eds). Storage of Cereal Grains and Their Products. St Paul: Am. Ass.

Cereal Chem., 1982 11a McLean, K. A. Drying and Storing Combinable Crops. Ipswich: Farming Press, 1980 114 Shebal, J. (Ed.) Controlled Atmosphere Storage of Grains. Amsterdam: Elsevier Scientific, 1980 11s Bailey, S. W.; Banks, H. J. A review of recent studies of the effects of controlled atmospheres on stored

product pests. In Controlled Atmosphere Storage of Grains (Shebal, J., Ed.). Amsterdam: Elsevier Scientific, 1980

11e Busta, F. F.;Smith, L. B.; Christensen, C.M. Microbiology of controlled atmosphere storage of grains-- an overview. In Controlled Atmosphere Storage of Grains (Shebal, J., Ed.). Amsterdam: Elsevier Scientific, 1980

117 B u k s , H. J. ; Annis, P. C.; Wiseman, J. R. Permanent sealing of 16,000-tonne capacity shed for fumigation or modified atmosphere storage of grain. CSIRO Auth. Divn Entom., Rep. No. 12. Canberra, Australia, 1979

11e Tranchino, L. Economic aspects of nitrogen storage of grains. In Controlled Atmosphere Storage of Grains (Shebal, J., Ed.). Amsterdam : Elsevier Scientific, 1980

11s Bowley, C. R.; Bell, C.H. Toxicity of twelve fumigants to three species of mites infesting grain. J. Stored Prod. Res., 1981 17 (2) 83-88

lzo Mittal, G .S . Thermal disinfestation of wheat with a solid heat transfer medium. Agric. Mech. in Asia, Aft., Lat. Am., 1981 12(3) 53-56

lm Kirklmtrie ' R. L.; Cagle, A. Controlling insects in bulk wheat with infrared radiation. J. Kans. ent. Soc., 1978 sz (3) 386-393

1,, Thorpe, G.R. Moisture diffusion through bulk grain. J. Stored Prod. Res., 1981 17 (1) 39-42

F. W. BAKKER-ARKEMA 21

123 Marchant, J .A. The prediction of airflows in crop drying systems by the finite element method. J. agric. Engng Res., 1976 21 (4) 417-429

124 Marchant, J. A.; Nellist, M. E. Air pressure and flow in ducted crop systems. 2. Systems of ducts. J. agric. Engng Res., 1977 22 (3) 311-319

12s Lai, F.S. Three-dimensional flow of air through non-linear grain beds. Trans. ASAE, 1980 23 (4) 729-734 128 Segerlind, L J. Solving the non-linear airflow equation. Am. Soc. agric. Engrs, Paper No. 82-3017, 1982 12~ Khompis, V. Three-dimensional finite element analysis of the flow field in cylindrical grain storages. Ph.D.

Thesis, Michigan State University, East Lansing, 1983 12e Thorpe, G. R.; Elder, W. B. Modelling the effects of aeration on the persistence of chemical pesticides

applied to stored bulk grain. J. Stored Prod. Res., 1982 18 (2) 103-104 12s Hunter, A. J.; Taylor, P. A. Refrigerated aeration for the preservation of bulk grain. J. Stored Prod. Res.,

1980 16 (2) 123-161 1so Rotz, C. A.; Thomas, J. W.; Johnson, T. R.; Herrington, D. A. Mechanical and chemical conditioning

to speed up alfalfa drying. Am. Soc. agric. Engrs, Paper No. 82-1036, 1982 131 Rees, D. V. A discussion of sources of dry matter loss during the process of haymaking. J. agric.

Engng Res., 1982 27 (6) 469-479 132 Wilkinson, J. M. Losses in the conservation and utilization of grass and forage crops. Ann. appl.

Biol., 1981 98 (2) 365-375 l as Klinner, W. E. A mowing and crop conditioning system for temperate climates. Trans. ASAE, 1976 19 (2)

237-241 la4 HeUwig, R. E.; Butler, J. L.; Monson, W. G.; Utley, P. R. A tandem roll mower-conditioner. Trans.

ASAE, 1977 20 (6) 1029-1032 1as Krutz, G. W.; Holt, D. A.; Miller, D. For fast field drying of forage crops. Agric. Engr, 1979 60 (8)

16-17 lar Khalilian, A.; Batchelder, D.; McLaughlir, G. Forage drying using hard crushing and binders. Trans.

ASAE, 1982 25 (5) 1225-1228 137 Harris, C. E.; Tullberg, J. N. Pathway of water loss from legumes and grasses cut for conservation.

Grass Forage Sci., 1980 35 (1) 1-11 13a Tuliberg, J. N.; Angus, D.E. The effect of potassium carbonate solution on the drying of lucerne. J. agric.

Sci., Camb., 1978 91 (3) 551-556 13a Charlick, R. H.; Hoiden, M. R.; Klinner, W. E.; Shepperson, G. The use of preservatives in haymaking.

J. agric. Engng Res., 1980 25 (1) 87-97 14o Nehrir, H.; Kjelgaard, W. L.; Anderson, P. M.; Long, T. A.; Hoffman, L. D.; Washko, J. ]3.;

Wilson, L. L.; Mueiler, J. P. Chemical additives and hay attributes. Trans. ASAE, 1978 21 (1) 217-221

141 Owen, I. G.; William, D. Differences between grass species and varieties in rate of drying at 25~ J. agric. Sci., Camb., 1983 100 (3) 629-636

142 Jones, L.; Prickett, J. The rate of water loss from cut grass of different species dried at 20~ Grass Forage Sci., 1981 36 (1) 17-23

1~ Anon. Hay Drying. Farm Electric Handbook. Stoneleigh, Kenilworth, Warwickshire, U.K.: Electricity Council, 1979

loA Elmer, M.; Hartmann, D.; Prigge, H. Molding as a criterion for the control of in-barn hay drying systems. Grundl. Landtech., 1981 31 (1) 29-36

1~ Clark, B. J.; McDonald, P. The drying pattern of grass swaths in the field. J. Br. Grassl. Soc., 1977 32 (2) 77-81

14a Burton, W. D. Recent developments in potato storage. Agric. Engr, 1977 32 (1) 20-21 147 Rastovski, A.; VanEs, A. (Eds). Storage of Potatoes. Wageningen: Pudoc, 1981 14a Lerew, L. E.; Bakker-Arkema, F. W. Storage of potatoes--a simulation model. Am. Soc. agric.

Engrs, Paper No. 78-4059, 1978 14a Brugger, M. F.; Buelow, F. H. Two-dimensional finite difference model of the transient environment

within a potato pile. Am. Soc. agric. Engrs, Paper No. 80-4522, 1980 15o Cloud, H. A.; Morey, R. V. Distribution duct performance for through ventilation of stored potatoes.

Trans. ASAE, 1980 23 (5) 1213-1218 1sl Statham, O. J. H. Better storage design. Agric. Engng J., 1983 38 (2) 46-48 152 McCarthy, T. T. The microprocessor in agriculture--two applications. Agric. Engr, 1980 35 (2)

40-41


lsa Hunter, J. Microprocessor managed monitoring and control of ~otato storages. Am. Soc. agric. Engrs, Paper No. 80-4523, 1980

1~ Rademaeher, J. J. C. On seed damage in grain augers. J. agric. Engng Res., 1981 26 (1) 87-96 l s s Rademacher, F. J. C. On possible flow back in vertical screw conveyors for cohesionless granular

materials. J. agric. Engng Res., 1981 26 (3) 225-250 1~ Ross, [. J.; White, G. M.; Loewer, O. J.; Wieman, D. M. Constant speed variable capacity screw

conveyor. Am. Soc. agric. Engrs, Paper No. 79-3515, 1979 is? Wirth, K, E. Principles of pneumatic conveying. Chemic Engng Technol. 1983 55 (2) 110-122 15a Fayz, J.; Hanna, M. A. .4 pneumatic conveying system for reducing soybean header loss. Trans. ASAE,

1979 22 (5) 962-966 l s s Holt, J. B. Materials handling in principle. Agric. Engr, 1978 33 (3) 67-68 1so Bull, D. A. Materials handling in practice. Agric. Engr, 1978 33 (3) 69-73 161 Bucklin, R. et al. Optimization of equipment and labor for seed processing and bagging systems. Trans.

ASAE, 1982 25 (4) 1041-1044 162 Benock, G.; Loewer, O. J.; Bridges, T.; Loewer, D. H. Grain flow restrictions in harvesting-delivery-

drying systems. Trans. ASAE, 1981 24 (5) 1151-1161 lea Singley, M. E.; Chaplin, R. V. Theflow of particulate materials. Trans. ASAE, 1982 25 (5) 1360-1366,

1373 1~ Welch, G. B.; Boyd, A. H.; Davila, S. Cleaning and conditioning seeds for marketing. In Handbook

of Transportation and Marketing in Agriculture, vol. II (Finney, E. E., Ed.). Boca Raton: CRC Press, 1981

165 Brandenburg, N. R. Principles and practice of seed cleaning. Seed Sci. Technol., 1977 5 (2) 173-186 1~ Regge, H.; Minaev, V. Grain precleaners and trends in their development. Agrartechn., 1979 29 (8) 349-

352

1984, Review-Selected Aspects of Crop Processing and Storage

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