GATE Agricultural Engineering Agricultural Process Engineering Syllabus

Agricultural Process Engineering syllabus — full chapter and topic list

Expand any chapter to see its topics and sub-topics. This is the whole examinable outline for Agricultural Process Engineering in GATE Agricultural Engineering, not a summary of it.

1. 1

   Engineering properties of agriculture produce

   5 topics

   • Physical properties
   • Thermal properties
   • Frictional properties
   • Rheological properties
   • Electrical properties
2. 2

   Evaporation and Drying

   5 topics

   • Concentration and drying of liquid foods
     • Evaporators
     • Tray dryers
     • Drum dryers
     • Spray dryers
   • Hydrothermal treatments
   • Drying and milling of cereals, pulses and oilseeds
   • Drying kinetics
   • Psychrometry
     • Properties of air-water vapor mixture
3. 3

   Size Reduction and Material Handling

   9 topics

   • Mechanics and energy requirement in size reduction of agriculture produce
   • Particle size analysis for comminuted solids
   • Size separation by screening
   • Fluidization of granular solids
     • Pneumatic conveying
     • Bucket conveying
     • Screw conveying
     • Belt conveying
   • Cleaning and grading
   • Effectiveness of separation
   • Centrifugal separation of solids, liquids and gases
   • Homogenization
   • Filtration and membrane separation
4. 4

   Processing of Agriculture Produce

   4 topics

   • Processing of seeds
   • Processing of spices
   • Processing of fruits and vegetables
   • Value addition of agriculture produce
5. 5

   Storage Systems

   4 topics

   • Controlled and modified atmosphere storage
   • Perishable food storage
   • Godowns, bins and grain silos
   • Packaging material and machines

Agricultural Process Engineering flashcards for GATE Agricultural Engineering

22 of 61 cards from the Agricultural Process Engineering deck — real questions with worked answers.

1. Define bulk density and how it differs from true (particle) density for agricultural produce.

   Bulk density $\rho_{b}$ is the mass of a bulk material divided by the total volume it occupies (including inter-granular air spaces): $\rho_{b} = \frac{m}{V_{bulk}}$. True/particle density $\rho_{t}$ uses only the solid volume of the particles (excluding voids). Thus $\rho_{b} < \rho_{t}$.

2. Write the formula for porosity of a granular agricultural material in terms of bulk and true density.

   $$\varepsilon = \left(1 - \frac{\rho_{b}}{\rho_{t}}\right) \times 100\%$$ where $\rho_{b}$ is bulk density and $\rho_{t}$ is true density. Porosity is the fraction of total volume occupied by inter-granular voids.

3. Define sphericity and give its formula for a grain.

   Sphericity $\phi$ expresses how close a particle's shape is to a sphere: $$\phi = \frac{\left(d_{a}\, d_{b}\, d_{c}\right)^{1/3}}{d_{a}}$$ where $d_{a}$ is the longest (major) dimension and $d_a,d_b,d_c$ are the three principal axes. Equivalently $\phi = \frac{d_{e}}{d_{c}}$ (diameter of equivalent-volume sphere over diameter of smallest circumscribing sphere). For a perfect sphere $\phi = 1$.

4. What is the geometric mean diameter of a grain with principal dimensions $a$, $b$, $c$?

   $$D_{g} = (a\,b\,c)^{1/3}$$ It is the equivalent diameter of a sphere having the same volume product of the three mutually perpendicular axes.

5. Define the coefficient of static friction and the angle of friction for a grain on a surface.

   The coefficient of static friction $\mu = \tan\phi$, where $\phi$ is the angle of friction. At the point of incipient sliding the frictional force equals $\mu N$ ($N$ = normal force). $\phi$ is the inclination of the surface at which the grain just begins to slide.

6. Differentiate the angle of repose (static vs dynamic) for granular agricultural materials.

   The angle of repose is the angle the free surface of a heaped bulk material makes with the horizontal. The static (emptying/filling) angle of repose is formed when grain is poured into a heap; the dynamic angle of repose is formed when grain flows out of an orifice (during emptying). The dynamic angle is generally smaller than the static angle.

7. Define specific heat and give the units used in food/grain thermal property work.

   Specific heat $c_{p}$ is the heat required to raise the temperature of unit mass by one degree: $Q = m\,c_{p}\,\Delta T$. Units: $\mathrm{J\,kg^{-1}\,K^{-1}}$ (or $\mathrm{kJ\,kg^{-1}\,^{\circ}C^{-1}}$).

8. State Siebel's equation for the specific heat of a food above its freezing point.

   Above freezing: $$c_{p} = 0.837 + 3.349\,X_{w}\quad (\mathrm{kJ\,kg^{-1}\,K^{-1}})$$ where $X_{w}$ is the mass fraction of water. Below freezing the second coefficient is reduced (ice term): $c_{p} = 0.837 + 1.256\,X_{w}$.

9. Define thermal conductivity and thermal diffusivity, and give the relation between them.

   Thermal conductivity $k$ measures the rate of heat conduction (Fourier's law $q = -k\,\frac{dT}{dx}$, units $\mathrm{W\,m^{-1}\,K^{-1}}$). Thermal diffusivity $\alpha$ measures how fast temperature changes propagate: $$\alpha = \frac{k}{\rho\,c_{p}}\quad (\mathrm{m^{2}\,s^{-1}})$$

10. What is the latent heat of vaporization of water at $100\,^{\circ}\mathrm{C}$, a key value in drying and evaporation calculations?

    $\lambda \approx 2257\ \mathrm{kJ\,kg^{-1}}$ (about $2.26\ \mathrm{MJ\,kg^{-1}}$) at $100\,^{\circ}\mathrm{C}$ and atmospheric pressure.

11. Define the dielectric constant and dielectric loss factor relevant to electrical heating of agricultural produce.

    The complex permittivity is $\varepsilon^{*} = \varepsilon' - j\varepsilon''$. The dielectric constant $\varepsilon'$ is the material's ability to store electrical energy; the dielectric loss factor $\varepsilon''$ governs conversion of electrical energy into heat (dielectric/microwave heating). The loss tangent is $\tan\delta = \frac{\varepsilon''}{\varepsilon'}$.

12. Give the equation for power dissipated per unit volume during dielectric (microwave/RF) heating.

    $$P = 2\pi f\,\varepsilon_{0}\,\varepsilon''\,E^{2}$$ where $f$ is frequency, $\varepsilon_{0}=8.854\times10^{-12}\ \mathrm{F\,m^{-1}}$, $\varepsilon''$ is the dielectric loss factor and $E$ is the electric field strength. Power absorbed rises with frequency, loss factor and the square of field strength.

13. Classify the rheological behaviour of fluid foods using the power-law (Ostwald-de Waele) model.

    $$\tau = K\,\dot{\gamma}^{\,n}$$ where $\tau$ = shear stress, $\dot{\gamma}$ = shear rate, $K$ = consistency index, $n$ = flow behaviour index. $n=1$: Newtonian; $n<1$: pseudoplastic (shear-thinning); $n>1$: dilatant (shear-thickening).

14. Distinguish a Bingham plastic fluid from a Newtonian fluid rheologically.

    A Newtonian fluid obeys $\tau = \mu\,\dot{\gamma}$ and flows at any nonzero stress. A Bingham plastic requires a finite yield stress $\tau_{0}$ before flow begins: $$\tau = \tau_{0} + \mu_{p}\,\dot{\gamma}\quad (\tau > \tau_{0})$$ where $\mu_{p}$ is the plastic viscosity. Examples: tomato paste, ketchup, chocolate melt.

15. Define apparent viscosity for a non-Newtonian fluid.

    Apparent viscosity is the ratio of shear stress to shear rate at a given shear rate: $$\eta_{app} = \frac{\tau}{\dot{\gamma}} = K\,\dot{\gamma}^{\,n-1}$$ (for a power-law fluid). It is not constant but varies with shear rate, unlike the true viscosity of a Newtonian fluid.

16. What is thixotropy, and how does it differ from rheopexy?

    Thixotropy is a time-dependent decrease in apparent viscosity under constant shear, with recovery on rest (e.g. starch gels, mayonnaise). Rheopexy (negative thixotropy) is a time-dependent increase in viscosity under constant shear. Both are reversible time effects, in contrast to permanent shear breakdown.

17. List the main purposes of concentrating liquid foods before drying.

    Concentration removes part of the water to: reduce mass/volume (cheaper transport and storage), lower water activity for preservation, reduce energy needed for subsequent drying, and improve product properties. It is usually achieved by evaporation, freeze concentration, or membrane (reverse osmosis) concentration.

18. Write the steady-state mass balance for total solids in a single-effect evaporator concentrating a feed.

    For feed $F$ at solids fraction $x_{F}$ giving product $P$ at $x_{P}$ and vapour $V$: Total: $F = P + V$. Solids: $F\,x_{F} = P\,x_{P}$ (solids leave only in product). Thus $P = \dfrac{F\,x_{F}}{x_{P}}$ and $V = F - P$.

19. Define the steam economy of an evaporator.

    Steam economy is the mass of water vapour evaporated per unit mass of heating (live) steam supplied: $$\text{Steam economy} = \frac{\text{kg water evaporated}}{\text{kg steam used}}$$ A single-effect evaporator has economy $\lesssim 1$; an $N$-effect system has economy approaching $N$.

20. How does the number of effects relate to steam economy in multiple-effect evaporation?

    In an $N$-effect evaporator the vapour from one effect becomes the heating medium for the next, so roughly $1\ \mathrm{kg}$ of steam evaporates about $N\ \mathrm{kg}$ of water. Steam economy $\approx N$ (slightly less due to losses), while the total heat-transfer area and capital cost rise roughly in proportion to $N$.

21. What is boiling-point elevation (BPE) in evaporators and why does it matter?

    BPE is the increase in the boiling point of a solution above that of pure water at the same pressure, caused by dissolved solids (a colligative effect). It reduces the effective temperature-difference driving force $\Delta T$ available for heat transfer, lowering evaporator capacity. It is accounted for using Dühring's rule.

22. Why are falling-film and rising-film evaporators preferred for heat-sensitive liquid foods?

    They give thin liquid films with short residence times and high heat-transfer coefficients, allowing low-temperature operation (often under vacuum). This minimizes thermal damage, fouling and quality loss in heat-sensitive foods such as fruit juices and milk.

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