Question 1

Flight conveyor sizing: derive the capacity in tons/hour in terms of width W (m), depth D (m), belt/flight speed V (m/s), and bulk density ρ (kg/m^3). Which option matches the standard form?

Accepted Answer

Introduction / Context: Designing a flight (scraper) conveyor requires a quick estimate of its mass throughput. The capacity depends on the cross-sectional area of material carried, its bulk density, and the conveying velocity. This question reinforces the basic capacity relation and the unit conversion factor used in plant calculations. Given Data / Assumptions: Cross-section approximated as W * D for filled section. Speed V is uniform along the conveyor. Bulk density ρ is constant and expressed in kg/m^3. Desired capacity unit: tons/hour. Concept / Approach: Mass flow rate m_dot equals volumetric flow rate times bulk density. Volumetric flow rate for a prismatic cross-section is area * velocity. With SI units, m_dot is in kg/s. Converting kg/s to tons/hour introduces the factor 3.6 because 1 kg/s equals 3.6 t/h (1000 kg per ton and 3600 s per hour). Step-by-Step Solution: Cross-sectional area A = W * D (m^2).Volumetric rate Q = A * V = W * D * V (m^3/s).Mass rate m_dot = Q * ρ = W * D * V * ρ (kg/s).Convert to tons/hour: Capacity = 3.6 * W * D * V * ρ (t/h). Verification / Alternative check: Dimensional analysis: [W][D] gives m^2; multiplied by V (m/s) → m^3/s; times ρ (kg/m^3) → kg/s. Multiply by 3.6 → t/h. The expression matches plant datasheets for flight/drag conveyors under full cross-section loading. Why Other Options Are Wrong: Options omitting ρ cannot yield mass capacity; those omitting either W or D do not represent area. Common Pitfalls: Using loose fill depth rather than effective cross-section; forgetting de-rating for slope or return flights; confusing metric ton with short ton (use consistent units). Final Answer: 3.6 W.D.V.ρ

Question 2

Comminution energetics: under otherwise comparable conditions, energy consumption per unit mass in a crusher decreases with an increase in which of the following?

Accepted Answer

Introduction / Context: Power and energy in size reduction operations are governed by empirical laws (Kick, Bond, Rittinger) and equipment characteristics. Understanding how energy per unit mass shifts with product size, feed size, and machine capacity helps in selecting stages and predicting costs. Given Data / Assumptions: Similar ore and operating conditions. Comparison is on specific energy (kWh/ton), not absolute power. Reduction ratio effects are considered where stated. Concept / Approach: For a fixed feed size, making a coarser product (larger product size) requires less new surface area, hence less energy (Rittinger emphasizes surface creation). At constant reduction ratio, increasing feed size generally favors Kick’s law, indicating energy proportional to the size ratio log, making specific energy less sensitive and often lower per ton at larger absolute sizes. Higher-capacity machines tend to have better mechanical efficiency and lower specific energy because of scale effects and reduced idle/overhead losses per unit throughput. Step-by-Step Solution: Option (a): Coarser product → less surface area created → lower energy per ton.Option (b): Larger machines/throughputs → better utilization → lower kWh/ton.Option (c): At constant reduction ratio, larger absolute feed sizes generally reduce specific energy (Kick regime) compared with fine grinding (Rittinger regime).Therefore, (d) all three are valid trends. Verification / Alternative check: Plant data commonly show higher kWh/ton in fine crushing and grinding steps than in primary/secondary stages, reflecting increased surface area creation demands. Why Other Options Are Wrong: Choosing only one factor understates the multi-parameter nature of comminution energetics; all listed factors contribute to lower specific energy. Common Pitfalls: Confusing installed motor power (kW) with specific energy (kWh/ton); larger machines may draw more kW but process far more tons, reducing kWh/ton. Final Answer: all (a), (b) & (c)

Question 3

Size reduction of fibrous materials (wood, asbestos, mica): which disintegrator type is exemplified by the cage mill?

Accepted Answer

Introduction / Context: Different solids fracture differently. Fibrous materials such as wood, asbestos, and mica resist simple compressive breakage and respond better to shear and impact that tear fibers apart. Disintegrators are designed to produce such actions and provide high-speed shattering and shearing rather than pure compressive crushing. Given Data / Assumptions: Material class: fibrous, tough, sheet-like (mica). Goal: disintegration rather than coarse lumpy crushing. Examples of equipment provided in options. Concept / Approach: A cage mill (disintegrator) uses concentric cages rotating in opposite directions to impart repeated impact and shear. The high peripheral speeds and multiple passes create rapid size reduction suitable for fibrous and friable feeds. In contrast, a Blake jaw crusher and Bradford breaker rely on compressive breakage and are best for hard, brittle, or friable lumps (e.g., coal). A stamp mill is a heavy impact device suited to brittle ore; it is not optimized for fibrous tearing. Step-by-Step Solution: Identify mechanism needed: shear/impact for fibers.Match equipment: disintegrator with high-speed impacts → cage mill.Select "cage mill" from the options. Verification / Alternative check: Process handbooks list the cage mill among typical "disintegrators" for fibrous or flaky materials where tearing and shattering are preferable to simple compression. Why Other Options Are Wrong: Blake jaw crusher: compressive, for coarse primary reduction of hard rocks.Stamp mill: heavy drop-weight impacts, not efficient for fibrous materials.Bradford breaker: rotary breaker for coal sizing and de-shaling, not a fiber disintegrator. Common Pitfalls: Assuming any "crusher" suits fibers; mechanism matters—fibers require tearing and repeated impacts. Final Answer: cage mill

Question 4

Definition check: the term "angle of nip" is specifically associated with which type of crusher operation?

Accepted Answer

Introduction / Context: The angle of nip is a fundamental concept in roll crushing. It determines whether a particle will be drawn into the gap between rolls and nipped for crushing, or whether it will simply ride along the surface and not be reduced. Understanding this definition helps in selecting roll diameters and setting gaps for desired size reduction. Given Data / Assumptions: Two-roll crusher with parallel, counter-rotating rolls. Angle between roll faces at the point where a particle is just captured. Friction and particle shape influence the effective nip. Concept / Approach: The angle of nip (commonly denoted 2α) is defined for roll crushers as the included angle between the roll faces at the location where a particle just enters the crushing zone. For successful nipping, the frictional forces must be sufficient to draw the particle in; excessive smoothness or too large a particle relative to the gap reduces nipping probability. Step-by-Step Solution: Identify the concept: "nip" refers to capture between opposing surfaces.Associate with equipment: classical with roll crushers, not with jaw or gyratory mechanisms.Choose "roll" as the correct answer. Verification / Alternative check: Design equations for roll crushers explicitly include the angle of nip when relating feed size, product size, and roll diameter. Why Other Options Are Wrong: Jaw/gyratory: while they grip material, the term "angle of nip" is not the standard design parameter used; jaw crushers focus on "set" and "throw," gyratories on "gape" and "mantle angle." Common Pitfalls: Using nip-angle formulae for non-roll machines; each crusher family has its own governing geometry. Final Answer: roll

Question 5

Vacuum filtration applicability: for which duty is a vacuum filter most suitable in liquid–solid separation?

Accepted Answer

Introduction / Context: Vacuum filters (e.g., rotary drum, disc, belt) are workhorses for continuous liquid–solid separation in mineral processing, chemicals, and wastewater treatment. Choosing them correctly requires understanding their advantages and limitations with respect to liquid properties and cake behavior. Given Data / Assumptions: Aqueous or low-volatility slurries. Moderate viscosity liquids and vacuum levels typical of plant equipment. Goal: remove suspended fines and produce a filter cake. Concept / Approach: Vacuum filtration applies a pressure differential by lowering pressure on the filtrate side. It is effective for slurries containing fine solids where forming a permeable cake is feasible. High vapor-pressure liquids tend to flash under vacuum, causing gas breakthrough and poor operation. Very high viscosity liquids restrict flow severely and are better handled by pressure filters or centrifuges. Step-by-Step Solution: Match vacuum filters to duty: continuous removal of fines and cake washing/drying.Eliminate unsuitable cases: high vapor pressure (flashing) and very viscous liquids (excessive resistance).Choose "removal of fines from liquid" as the proper application. Verification / Alternative check: Vendors specify maximum filtrate viscosities and caution against volatile solvents unless controlled with condensers/seals. Why Other Options Are Wrong: High vapor pressure: vacuum induces flashing and unstable operation.Very high viscosity: leads to very low flux; pressure filters are preferred. Common Pitfalls: Attempting vacuum filtration of solvent-rich slurries without vapor handling; overlooking precoat/filter aid methods that can transform marginal slurries into filterable ones. Final Answer: removal of fines from liquid

Question 6

Grinding of talc (to avoid iron contamination): which mill is commonly selected for mineral talc?

Accepted Answer

Introduction / Context: Talc is soft and often used in applications where color and purity are critical (fillers, cosmetics). Grinding equipment must therefore limit metallic contamination, especially iron, which can discolor the product. Mill choice balances fineness needs, contamination risk, and energy efficiency. Given Data / Assumptions: Talc requires fine grinding but not necessarily ultra-fine jet milling for all grades. Minimizing iron pick-up during comminution is important. Wet or dry grinding routes may be used. Concept / Approach: Pebble mills are essentially ball mills that use non-metallic pebbles (e.g., flint) as grinding media and can be lined with ceramic. This greatly reduces metal contamination compared with steel media and liners. While ring-roll mills and tube/compartment ball mills can grind talc, they typically use metallic grinding elements, increasing iron contamination risk unless specially lined. Step-by-Step Solution: Identify constraint: low contamination required → non-metallic media preferred.Select equipment: pebble mill with flint/ceramic media suits talc.Choose "Pebble" as the correct option. Verification / Alternative check: Process texts recommend ceramic-lined, non-ferrous media mills for color-sensitive minerals such as talc, kaolin, and feldspar. Why Other Options Are Wrong: Tube/compartment: commonly steel media unless ceramic-lined; higher contamination risk.Ring-roll: metal contact surfaces and classifier wear can introduce contaminants. Common Pitfalls: Equating "soft mineral" with "any mill"; purity requirements often dominate equipment choice over raw grindability. Final Answer: Pebble

Question 7

Particle size statistics: the Sauter mean diameter (often used in mass transfer) is identical to which named mean?

Accepted Answer

Introduction / Context: Different particle size "means" emphasize different physical phenomena. The Sauter mean diameter d32 is central in interfacial processes (e.g., droplet evaporation, gas–liquid mass transfer) because it weights particles by surface area per unit volume. Recognizing its equivalence to the volume–surface mean helps in choosing the right metric for design correlations. Given Data / Assumptions: A polydisperse particle or droplet population. Interest in surface-area-related transport. Standard nomenclature for statistical means. Concept / Approach: The Sauter mean diameter d32 is defined so that the total surface area-to-volume ratio of the real distribution equals 6/d32 (spherical assumption). Algebraically, d32 = Σ(n_i d_i^3)/Σ(n_i d_i^2), the ratio of the third to the second moment of size distribution. This is exactly the volume-surface mean because the numerator represents a volume-type weighting and the denominator a surface-type weighting. Step-by-Step Solution: Recall definition: d32 emphasizes particles by contribution to surface area for a given volume.Match to named mean: volume–surface mean is the same construction.Select "volume-surface" as the correct option. Verification / Alternative check: Mass transfer correlations (e.g., k_L a) often employ d32 because a ∝ 6/d32 for spherical dispersions, directly linking size to interfacial area per unit volume. Why Other Options Are Wrong: Arithmetic, geometric, and mass means do not preserve surface-to-volume equivalence and would mispredict interfacial area for polydisperse systems. Common Pitfalls: Confusing number mean or volume mean with Sauter mean; using the wrong mean can produce large errors in predicted rates. Final Answer: volume-surface

Question 8

Hydraulic/equivalent diameter for a 3 cm × 6 cm non-circular duct: using the standard equivalent-diameter relation for rectangular channels, what is De (cm)?

Accepted Answer

Introduction / Context: For non-circular ducts, an equivalent diameter is used to adapt circular correlations for pressure drop or heat/mass transfer. Several "equivalent" definitions exist; for rectangular ducts a common practice is to use a De that preserves hydraulic behavior within a practical accuracy band. Given Data / Assumptions: Duct cross-section a × b = 3 cm × 6 cm. Standard equivalent-diameter relation for rectangular channels used in many handbooks. Goal: compute De in centimeters. Concept / Approach: A frequently used equivalent diameter for rectangular ducts is De = 4ab/(a + b). Substituting a = 3 cm and b = 6 cm gives De = 4 * 3 * 6 / (3 + 6) = 72 / 9 = 8 cm. (Note: some references define the hydraulic diameter as Dh = 4A/P = 2ab/(a + b); ensure the definition required by the question—here, the listed options match De = 4ab/(a + b).) Step-by-Step Solution: Choose De definition: 4ab/(a + b).Insert numbers: De = 4 * 3 * 6 / 9 = 8 cm.Match to options: 8 cm. Verification / Alternative check: Using Dh = 4A/P = 2ab/(a + b) would yield 4 cm; since 4 cm is not among the options, the question clearly uses the 4ab/(a + b) convention common in some screening/flow references. Why Other Options Are Wrong: 20, 12, 2 cm are inconsistent with either 4ab/(a + b) or 2ab/(a + b) and therefore incorrect. Common Pitfalls: Mixing different "equivalent" definitions; always match the convention used in the correlation or textbook. Final Answer: 8

Question 9

Agitation power in the turbulent regime: during fully turbulent mixing in a baffled tank, power consumption is primarily proportional to which liquid property?

Accepted Answer

Introduction / Context: In mixing, the dimensionless power number Np relates impeller power to liquid properties, impeller speed, and diameter via P = Np * ρ * N^3 * D^5 for baffled tanks. Recognizing which properties matter in the turbulent regime guides scale-up and energy estimates. Given Data / Assumptions: Baffled tank, radial/turbine or pitched-blade impeller. High Reynolds number based on impeller diameter (fully turbulent). Constant Np in the turbulent asymptote. Concept / Approach: At high Reynolds number, viscous effects are negligible in the global power correlation; Np tends to a constant for a given geometry. The power draw scales linearly with fluid density ρ and with N^3 D^5. Surface tension and thermal conductivity do not feature in the standard mechanical power equation for turbulent agitation. Step-by-Step Solution: Use P = Np * ρ * N^3 * D^5 (turbulent regime).Holding geometry and speed fixed, P ∝ ρ.Therefore, density is the relevant property. Verification / Alternative check: Experimental power curves show plateau Np at high Re, confirming independence from viscosity in this regime; density shifts absolute power linearly. Why Other Options Are Wrong: Viscosity: important at low Re; negligible for fully turbulent power draw.Surface tension/thermal conductivity: not in the mechanical power correlation. Common Pitfalls: Applying laminar correlations (P ∝ μ) in turbulent scale-up; mixing regimes must be identified correctly before calculations. Final Answer: density

Question 10

Cutting vs grinding: which of the following is not classified as a cutting machine in solids processing?

Accepted Answer

Introduction / Context: Solids size reduction equipment can be grouped by the dominant mechanism: cutting, compression, impact, or attrition. Correct classification aids in selecting equipment for specific materials (e.g., fibrous, ductile, brittle) and desired product forms (flakes, granules, powders). Given Data / Assumptions: "Cutting" implies blade-based separation (shear) producing chips or slices. Tube mills are tumbling grinders with media, not blades. Dicers, knife cutters, slitters use edges to shear material. Concept / Approach: Dicers, knife cutters, and slitters employ sharp edges and controlled clearances to produce precise cuts, common in food and polymer film processing. Tube mills (a form of ball/rod mill) perform comminution by cascading media that apply impact and attrition, producing powders instead of clean cuts. Step-by-Step Solution: Identify cutting machines: dicer, knife cutter, slitter.Recognize tube mill: grinding device using media and rotation.Hence, "Tube mills" is the non-cutting option. Verification / Alternative check: Tubular mills are widely used in cement and mineral industries for powder production, confirming they are grinding, not cutting, machines. Why Other Options Are Wrong: Dicers/knife cutters/slitters clearly rely on blade shear mechanisms and are standard cutting equipment. Common Pitfalls: Assuming any size reduction is "cutting;" many unit operations rely on impact or abrasion rather than shear. Final Answer: Tube mills

Question 11

Vertical transport of bulk materials: which option lists equipment capable of truly vertical conveying in plants?

Accepted Answer

Introduction / Context: Bulk solids handling requires moving materials not only horizontally but also vertically between floors or process stages. Selecting the right vertical conveying equipment improves reliability, containment, and footprint. Given Data / Assumptions: Pneumatic conveyors can be arranged as vertical risers. Bucket elevators are classic vertical lifts for granular solids. Apron conveyors are primarily horizontal/inclined with mechanical flights. Concept / Approach: Bucket elevators use buckets attached to a belt or chain to lift bulk solids vertically, discharging by centrifugal or gravity action. Pneumatic conveyors move solids suspended in air through pipelines; vertical risers are common in dilute or dense phase systems. Apron conveyors, although robust, are not intended for true vertical lifts. Step-by-Step Solution: Identify equipment with vertical capability: bucket elevators and pneumatic conveyors.Exclude apron conveyor: mainly horizontal or modest incline.Select "both(b)&(c)" as the correct choice. Verification / Alternative check: Vendor datasheets specify vertical lift heights for bucket elevators and pressure/vacuum risers for pneumatic systems. Why Other Options Are Wrong: Apron conveyors are heavy-duty horizontal/low-incline devices; not suitable for vertical transport. Common Pitfalls: Confusing inclined conveyors with vertical lifts; vertical conveying imposes distinct design constraints (seal, slip, backflow). Final Answer: both(b)&(c)

Question 12

Rubber compounding practice: which mixer is standard for intensive internal mixing of rubber with fillers and additives?

Accepted Answer

Introduction / Context: Rubber compounding requires high shear and temperature control to incorporate fillers (e.g., carbon black, silica), oils, and curatives uniformly into the rubber matrix. The industry-standard mixer provides intensive internal mixing under controlled pressure and rotor speed. Given Data / Assumptions: Task: internal mixing of rubber, not simple blending. High viscosity and elasticity of the compound. Need for uniform dispersion and distribution of fillers. Concept / Approach: The Banbury internal mixer (a tangential-rotor, enclosed chamber mixer) is the classic choice for rubber compounding. It exerts high shear, allows ram pressure control, and manages heat via jacketed chambers and rotor cooling. Mixer–extruders, muller mixers, and pug mills are useful in other industries (plastics, foundry sands, clays) but are not the standard for rubber compounding intensity and dispersion quality. Step-by-Step Solution: Identify requirement: intensive internal mixing with high shear and pressure.Match equipment: Banbury internal mixer meets these needs.Select "banbury internal mixer." Verification / Alternative check: Rubber plant flowsheets commonly show Banbury mixing followed by two-roll mills for sheet-out and cooling prior to extrusion or calendering. Why Other Options Are Wrong: Mixer–extruder: combines mixing with extrusion but not a replacement for Banbury in primary compounding.Muller and pug mill: suitable for granular or clay-like materials, not high-elasticity rubber. Common Pitfalls: Confusing two-roll mills (finishing and sheet-out) with internal mixing; both are used sequentially but serve different purposes. Final Answer: banbury internal mixer

Question 13

Basket centrifuge scaling: if the basket radius is halved and the rotational speed (rpm) is doubled, what happens to (i) rim linear speed and (ii) centrifugal force?

Accepted Answer

Introduction / Context: Understanding how geometric and kinematic changes affect centrifuge performance is crucial for scale-up and troubleshooting. Two key relationships are the rim linear speed v = 2 * π * N * r and the centrifugal (centripetal) acceleration a_c = ω^2 * r, which governs the "g-force" driving separation. Given Data / Assumptions: Initial radius r and rotational speed N (rpm). New radius r/2 and new speed 2N. Rigid basket; focus on kinematics and acceleration, not throughput complexities. Concept / Approach: Translate rpm to angular speed ω = 2πN (in rad/s). The rim linear speed is v = ω * r. The centrifugal acceleration is proportional to ω^2 * r and the corresponding "g" level determines filtration/drainage rate capabilities for a given material system. Step-by-Step Solution: Original v0 = 2πN * r; after change v1 = 2π*(2N)(r/2) = 2πN * r → unchanged.Original a0 ∝ (2πN)^2 * r; after change a1 ∝ (2π(2N))^2 * (r/2) = 4*(2πN)^2 * (r/2) = 2*(2πN)^2 * r → doubled.Therefore, rim speed remains the same, but centrifugal force (per unit mass) doubles. Verification / Alternative check: Express in dimensionless "g" terms: G = ω^2 * r / g; halving r and doubling N gives G_new = 2 * G_old. Why Other Options Are Wrong: Rim speed doubled/halved: incorrect; it remains unchanged.Capacity increased: not guaranteed; capacity depends on feed properties, drainage path, cake behavior, and area, not G alone. Common Pitfalls: Confusing linear speed with centrifugal acceleration; they scale differently when r and N change simultaneously. Final Answer: centrifugal force is doubled.

Question 14

Tyler (Taylor) standard screen scale: the ratio of open area (aperture area) of one screen to the next smaller screen is approximately

Accepted Answer

Introduction / Context: Standard screen series (Tyler/Taylor) follow a geometric progression of aperture sizes so that test results and equipment selection are comparable. Understanding the step ratio between adjacent screens helps in interpreting cumulative size distributions and designing screening stages. Given Data / Assumptions: The Tyler (often called Taylor) series uses the square root of 2 progression for aperture size. Open area of a single aperture scales with the square of the linear aperture dimension. Concept / Approach: In the Tyler series, each successive aperture size differs by a factor of √2 in linear dimension. Since area ∝ (size)^2, the ratio of aperture areas between successive screens is (√2)^2 = 2. Therefore, the open area of one aperture relative to the next smaller is doubled, assuming square openings of equal wire diameter effects ignored for this conceptual question. Step-by-Step Solution: Linear step = √2 between adjacent sizes.Area step = (√2)^2 = 2.Hence, area ratio ≈ 2. Verification / Alternative check: Tyler standard sieve scale lists sizes where mesh numbers increase by √2 in terms of opening changes, consistent with area doubling between steps. Why Other Options Are Wrong: 1 or 1.5 do not match the geometric progression definition; "none" is unnecessary because 2 is the established value. Common Pitfalls: Confusing linear aperture ratio with area ratio; remember area scales with the square of linear dimension. Final Answer: 2

Question 15

What is not achieved by liquid agitation in standard tank agitators? Identify the exception.

Accepted Answer

Introduction / Context: Agitators perform several tasks: blending, heat transfer enhancement, gas–liquid contact, suspending solids, and emulsification. However, some tasks require specific conditions or chemistry; simply stirring is insufficient to overcome fundamental thermodynamics such as immiscibility without a dispersive mechanism or surfactants. Given Data / Assumptions: Baffled tanks with standard impellers (e.g., Rushton, PBT). No chemical reaction that changes miscibility. "Blending" is taken to mean achieving a single homogeneous phase. Concept / Approach: Agitation can disperse gas into a liquid (forming bubbles) and suspend solids by turbulent eddies. It can also disperse one immiscible liquid into another as droplets, creating an emulsion when aided by interfacial tension modifiers (surfactants). But "blending" immiscible liquids into a single, truly homogeneous phase is not accomplished by agitation alone; thermodynamic immiscibility persists, and once shear stops, the phases separate. Step-by-Step Solution: List typical accomplishments: gas dispersion, solids suspension, and emulsification.Identify the exception: creating a single-phase mixture from immiscible liquids is not possible by agitation.Therefore, choose "Blending of immiscible liquids." Verification / Alternative check: Process design texts distinguish between blending (miscible) and emulsification (immiscible). For immiscible systems, coalescence and separation occur without stabilizers or continuous shear. Why Other Options Are Wrong: Gas dispersion, emulsification, and solids suspension are canonical roles of agitators when operated at appropriate speeds and with suitable impellers. Common Pitfalls: Equating "well mixed" with thermodynamic homogeneity; immiscible systems remain two-phase despite good mixing. Final Answer: Blending of immiscible liquids.

Question 16

Filter-cake structure: in a compressible cake formed during filtration, where is the porosity highest across the cake thickness?

Accepted Answer

Introduction / Context: As a cake forms on a filter medium, pressure drops across the cake thickness. If the cake is compressible, its structure (porosity and permeability) is not uniform; it varies with the local effective pressure. This spatial variation strongly influences filtration rate and washing efficiency. Given Data / Assumptions: Cake compressibility is significant. Pressure is highest at the medium surface and decreases toward the slurry side. Steady filtration under constant pressure or constant rate conditions. Concept / Approach: Compressible cakes compact more where the applied stress is greatest. Because the maximum pressure occurs near the filter medium, pores there are most compressed (lowest porosity). Moving toward the upstream slurry interface, the local pressure is smaller and the cake is looser (higher porosity). Thus, porosity is highest at the upstream side and lowest at the medium side, giving a porosity gradient through the cake. Step-by-Step Solution: Recognize pressure gradient across cake thickness.Compressibility implies porosity decreases with increasing local pressure.Therefore, porosity is highest upstream (low pressure) and lowest at the medium (high pressure). Verification / Alternative check: Experimental washing data often show poorer wash efficiency near the medium due to tighter, lower-porosity layers. Why Other Options Are Wrong: Maximum at medium: contradicts compressibility behavior.Uniform porosity: valid only for incompressible cakes. Common Pitfalls: Assuming cake properties are constant; design models frequently incorporate a compressibility coefficient to reflect this gradient. Final Answer: maximum porosity at the upstream side

Question 17

Filter medium resistance significance: during which part of a filtration cycle does the medium resistance dominate compared with cake resistance?

Accepted Answer

Introduction / Context: In cake filtration, total resistance comprises the clean medium resistance and the growing cake resistance. Knowing when each component dominates helps in interpreting dV/dt curves and in deciding on precoats or filter aids. Given Data / Assumptions: Cake builds on a porous septum (cloth, screen, leaf). Initially, no cake is present. Darcy-type flow in laminar regime. Concept / Approach: At the start of filtration, the cake thickness is near zero; the only significant resistance is that of the clean medium (plus any precoat if applied). As solids deposit, cake resistance rapidly increases and soon becomes the dominant term. Late in the cycle, cake resistance is much larger than medium resistance, which becomes relatively negligible. Step-by-Step Solution: At t ≈ 0: Rcake ≈ 0 → Rtotal ≈ Rmedium.As time proceeds: Rcake grows with filtrate volume → dominates.Therefore, medium resistance is important during the early stages. Verification / Alternative check: Plotting 1/(filtration rate) vs. filtrate volume often shows an intercept associated with medium resistance and a slope associated with cake resistance. Why Other Options Are Wrong: Final stages/entire process: contradict the growth of cake resistance; only early on does the medium govern flow. Common Pitfalls: Forgetting that precoats intentionally increase initial resistance to capture fines and protect the cloth; "early stages" then refer to "through precoat" conditions. Final Answer: early stages.

Question 18

Specific surface shape factor (spherical particles): which expression is constant for spheres and equals 6?

Accepted Answer

Introduction / Context: Shape factors relate measurable geometry (surface area A, volume V, and characteristic diameter D) to capture how particle shape affects transport and reaction. For spheres, certain ratios become constants, simplifying correlations in drying, reaction kinetics, and mass transfer. Given Data / Assumptions: Spherical particle with diameter D. Surface area A = π D^2 and volume V = π D^3 / 6. Define a "shape factor" that is constant for spheres. Concept / Approach: Compute A/V for a sphere: A/V = (π D^2) / (π D^3 / 6) = 6 / D. Multiplying by D gives (A * D) / V = 6, a constant independent of size for perfect spheres. Many correlations use AD/V as a dimensionless indicator of compactness, equalling 6 for spheres and differing for non-spherical shapes. Step-by-Step Solution: Start with sphere geometry.Derive A/V = 6/D.Multiply both sides by D: AD/V = 6 (constant).Therefore, AD/V is the desired expression. Verification / Alternative check: Check units: A (L^2) * D (L) / V (L^3) → dimensionless, appropriate for a shape factor. Why Other Options Are Wrong: A/V alone equals 6/D, which varies with particle size; D/V and other permutations are not constant and lack standard physical meaning as shape constants. Common Pitfalls: Confusing specific surface (A/V or A/mass) with shape factors; the former depends on size, the latter on geometry. Final Answer: AD/V

Question 19

Power number–Reynolds number relation: for a turbine-impeller, baffled tank operating at low Reynolds number (laminar regime), how does the power number Np vary with NRe?

Accepted Answer

Introduction / Context: The power number Np characterizes impeller power draw: P = Np * ρ * N^3 * D^5. Its dependence on the impeller Reynolds number NRe = ρ * N * D^2 / μ reveals the mixing regime. At low NRe (laminar), viscous effects dominate; at high NRe (turbulent), Np approaches a constant that depends on geometry. Given Data / Assumptions: Turbine impeller in a baffled tank. Low Reynolds number (laminar flow). Newtonian liquid. Concept / Approach: In the laminar regime, dimensional analysis and experiments show P ∝ μ * N^2 * D^3 (for a given geometry). Rearranging the general power correlation gives Np = P / (ρ * N^3 * D^5). Substituting the laminar expression for P yields Np ∝ (μ / (ρ * N * D^2)) = 1 / NRe. Thus, as NRe increases (approaching transition), Np decreases inversely until it levels off in turbulence. Step-by-Step Solution: Start with P_laminar ∝ μ * N^2 * D^3.Compute Np = P / (ρ * N^3 * D^5) ∝ (μ / (ρ * N * D^2)) = 1/NRe.Therefore, Np ∝ 1/NRe in laminar mixing. Verification / Alternative check: Power curves in mixing handbooks plot Np vs NRe and show a straight inverse relationship at low NRe, transitioning to a constant plateau at high NRe. Why Other Options Are Wrong: Np ∝ NRe or constant contradicts laminar scaling; a constant Np applies only in the turbulent asymptote. Common Pitfalls: Using turbulent power numbers for viscous scale-up; always check Reynolds number range before applying Np values. Final Answer: Np ∝ 1/NRe

Question 20

Wet grinding vs. dry grinding in a revolving mill
In industrial comminution practice, which effect is typically observed when grinding is carried out wet (with a slurry) instead of dry in the same revolving mill and for the same target fineness?

Accepted Answer

Introduction / Context: Plant engineers often must decide between wet and dry grinding modes for ball, rod, or tube mills. This choice affects power draw, liner life, product handling, and—crucially—throughput. Understanding the dominant trends allows better sizing and operating policies without exhaustive pilot testing. Given Data / Assumptions: The same mill geometry and media are assumed in both cases. Target product fineness and material properties are comparable. Wet grinding means a slurry with appropriate solids percentage; dry grinding means negligible moisture. Concept / Approach: In wet grinding, the presence of water forms a slurry that reduces interparticle adhesion, improves transport of fines out of the breakage zone, and cushions impacts, which collectively reduces overgrinding. Slurry transport also enhances the effective classification inside the mill, permitting a higher solids throughput at the same fineness. Consequently, mills operated wet typically achieve higher capacity. Additional known effects are reduced liner and lifter wear (water lubricates and transports chips) and easier size classification. Conversely, wet product handling can be more complex (pumps, thickeners, dryers) though this is outside the specific capacity comparison. Step-by-Step Solution: Hold fineness constant to compare modes fairly.Note that slurry transport reduces cushioning of fines accumulation and accelerates removal of fines.Higher breakage efficiency at the same grate/classifier conditions translates to higher tonnage → increased capacity.Therefore, select the option stating that capacity increases in wet operation. Verification / Alternative check: Standard mill performance curves and plant reports show greater tonnage in wet grinding at comparable product size (e.g., cement raw mills and ore grinding circuits). Pilot tests further confirm higher net production rates due to improved classification and reduced dust-related losses. Why Other Options Are Wrong: Gives less wear: often true, but the question asks for what is typically observed; capacity increase is the most universal and decisive operational effect. Requires more energy: for the same fineness, specific energy is usually similar or lower in wet grinding, not higher. Complicates handling: may be true operationally, but it is not the primary mill performance effect. All equally true: not all statements hold equally across materials; capacity increase is the most consistent trend. Common Pitfalls: Comparing at different fineness targets or solids percentages; conclusions must be drawn at equal product size to be meaningful. Final Answer: Increases the mill's throughput (capacity) at a given fineness

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