I85PREP
⌘ K
PRES 2 study materials
PRES 2 study resources

PRES 2 practice questions

One question at a time. Build your PRES 2 understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Pulp mill processes and fiber line

In a kraft pulp mill, what is the primary purpose of the white liquor applied in the digester?

  1. To cool the chip column and stop the delignification reaction
  2. To dissolve lignin by supplying hydroxide and hydrosulfide ions Correct answer.
  3. To precipitate dissolved lignin onto the fiber surface
  4. To bleach residual lignin and adjust final brightness

Explanation

White liquor contains sodium hydroxide and sodium sulfide; the hydroxide and hydrosulfide ions attack and fragment lignin so it becomes soluble in the cooking liquor. Bleaching and cooling are downstream or secondary functions, and lignin is removed, not precipitated onto fibers.

Question 2Pulp mill processes and fiber line

Which cooking variable is normally expressed as the H-factor in a kraft batch digester?

  1. The combined effect of time and temperature on delignification Correct answer.
  2. The liquor-to-wood ratio inside the vessel
  3. The sulfide-to-alkali ratio of the white liquor
  4. The total alkali charge relative to wood mass

Explanation

The H-factor integrates cooking time and temperature into a single number representing the extent of delignification, allowing cooks at different temperature profiles to be compared. Alkali charge, liquor-to-wood ratio, and sulfidity are separate variables.

Question 3Pulp mill processes and fiber line

What does the kappa number of a pulp sample primarily indicate?

  1. The brightness of the unbleached pulp
  2. The residual lignin content of the pulp Correct answer.
  3. The average fiber length of the pulp
  4. The carbohydrate yield of the cook

Explanation

The kappa number measures how much permanganate the pulp consumes, which correlates strongly with residual lignin. It is used to judge how far delignification has progressed and to set bleaching chemical charges.

Question 4Pulp mill processes and fiber line

In a continuous Kamyr-type digester, what is the function of the impregnation zone?

  1. To allow liquor to penetrate chips before the main cook Correct answer.
  2. To wash black liquor from the cooked pulp
  3. To screen oversized chips out of the feed
  4. To cool the pulp before the blow line

Explanation

The impregnation zone gives chips time to absorb cooking liquor at moderate temperature so that delignification proceeds evenly in the cooking zone. Washing, cooling, and chip screening occur elsewhere in the system.

Question 5Pulp mill processes and fiber line

What is black liquor in a kraft mill?

  1. The spent cooking liquor containing dissolved lignin and wood solids Correct answer.
  2. The white liquor after it has been oxidized with air
  3. The filtrate from the final bleaching stage
  4. The condensate collected from the evaporator surface condenser

Explanation

Black liquor is the spent cooking liquor leaving the digester, containing dissolved lignin, hemicelluloses, and residual cooking chemicals. It is concentrated and burned in the recovery boiler to regenerate chemicals and recover energy.

Question 6Pulp mill processes and fiber line

What is the main purpose of brownstock washing after cooking?

  1. To add bleaching chemicals to the pulp
  2. To increase the kappa number of the pulp
  3. To refine the fibers to a target freeness
  4. To remove dissolved lignin and residual cooking chemicals from the fiber Correct answer.

Explanation

Washing displaces the black liquor trapped in the pulp so that dissolved lignin and cooking chemicals do not carry over to bleaching. Good washing reduces bleach chemical demand and improves chemical recovery.

Question 7Pulp mill processes and fiber line

Which unit operation converts green liquor into white liquor in the kraft recovery cycle?

  1. Calcining
  2. Slaking
  3. Evaporation
  4. Causticizing Correct answer.

Explanation

Causticizing reacts green liquor (sodium carbonate) with slaked lime to form sodium hydroxide, producing white liquor. Evaporation concentrates black liquor, calcining regenerates lime, and slaking is only the lime-hydration step within causticizing.

Question 8Pulp mill processes and fiber line

What is sulfidity in kraft cooking liquor?

  1. The fraction of sodium sulfide sulfur relative to total active alkali sulfur Correct answer.
  2. The pressure used in the impregnation zone
  3. The concentration of dissolved lignin in black liquor
  4. The ratio of liquor volume to wood mass in the digester

Explanation

Sulfidity is defined as the sulfide sulfur expressed as a percentage of the total alkali sulfur (sulfide plus hydroxide expressed on the same basis). It affects delignification rate, yield, and odor emissions.

Question 9Pulp mill processes and fiber line

A mill cooks a batch with a high effective alkali charge but holds the H-factor constant. What is the most likely outcome?

  1. Lower kappa number but reduced pulp yield from carbohydrate degradation Correct answer.
  2. Higher yield because alkali protects cellulose from attack
  3. No change in kappa or yield because H-factor alone controls the cook
  4. Higher kappa number because alkali inhibits delignification

Explanation

At a fixed H-factor, raising effective alkali accelerates delignification and lowers kappa, but excess alkali also attacks hemicellulose and cellulose, peeling carbohydrates and reducing yield. Alkali does not inhibit delignification, and H-factor alone does not determine yield.

Question 10Pulp mill processes and fiber line

A mill observes that pulp kappa number is rising while cooking temperature and time are unchanged. Which change in white liquor quality would most directly explain this?

  1. An increase in liquor-to-wood ratio
  2. A drop in effective alkali concentration Correct answer.
  3. A higher chip moisture content
  4. A rise in digester pressure

Explanation

Lower effective alkali means less reagent available to delignify the wood, so residual lignin and kappa rise at constant time and temperature. Pressure alone does not drive delignification, and chip moisture is normally accounted for in charge calculations.

Question 11Pulp mill processes and fiber line

Why is a high sulfidity in white liquor often limited despite its benefit to delignification?

  1. It increases reduced sulfur emissions and can lower pulp yield at very high levels Correct answer.
  2. It causes excessive lime consumption in the kiln
  3. It prevents the causticizing reaction from forming white liquor
  4. It raises the kappa number of the pulp

Explanation

Higher sulfidity accelerates delignification and protects carbohydrates up to a point, but it also increases malodorous reduced sulfur emissions and can reduce yield and viscosity at very high levels. Mills therefore balance sulfidity against environmental and quality constraints.

Question 12Pulp mill processes and fiber line

In a brownstock washing line, what does the displacement ratio measure?

  1. The consistency of pulp entering the first washer
  2. How effectively fresh wash liquor displaces dissolved solids from the pulp Correct answer.
  3. The ratio of wash water flow to pulp production rate
  4. The amount of foam generated on the washer surface

Explanation

The displacement ratio compares the actual reduction in dissolved solids to the maximum reduction achievable if fresh liquor fully displaced the liquor in the pulp. It is a key indicator of washer performance.

Question 13Pulp mill processes and fiber line

A mill wants to reduce bleach plant chemical consumption without changing kappa number. Which action most directly supports this goal?

  1. Reducing the wash water temperature to zero
  2. Improving brownstock washing efficiency to lower carryover of dissolved lignin and COD Correct answer.
  3. Increasing the digester temperature to raise kappa
  4. Adding more white liquor to the digester

Explanation

Better washing removes dissolved lignin and oxidizable material that would otherwise consume bleaching chemicals. Raising kappa or adding white liquor does not reduce bleach demand, and eliminating wash water would worsen carryover.

Question 14Pulp mill processes and fiber line

In the recovery cycle, what happens to the smelt produced in the recovery boiler?

  1. It is dissolved in water to form green liquor Correct answer.
  2. It is burned in the lime kiln as fuel
  3. It is dried and fed back to the digester as chips
  4. It is filtered and sold as tall oil

Explanation

Smelt from the recovery boiler, mainly sodium carbonate and sodium sulfide, is dissolved in water or weak wash to form green liquor. Green liquor is then causticized to regenerate white liquor.

Question 15Pulp mill processes and fiber line

What is the role of the lime kiln in a kraft mill?

  1. Calcining lime mud to regenerate quicklime for causticizing Correct answer.
  2. Concentrating black liquor before combustion
  3. Burning hog fuel to generate steam for the digester
  4. Recovering turpentine from chip digestion

Explanation

The lime kiln heats calcium carbonate lime mud to drive off carbon dioxide, producing calcium oxide (quicklime) that is slaked and reused in causticizing. Evaporation and recovery boiler duties are separate.

Question 16Pulp mill processes and fiber line

A continuous digester shows uneven kappa across the blow line. Which condition is the most plausible cause?

  1. A low lime mud solids content in the causticizing plant
  2. Excessive wash water flow on the brownstock washers
  3. A high sulfidity in the white liquor
  4. Poor chip impregnation leading to non-uniform liquor penetration Correct answer.

Explanation

Uneven impregnation causes chips to cook at different rates, producing variable residual lignin and kappa at blow. Washer flow, sulfidity, and lime mud solids affect other parts of the process and would not typically create digester kappa variation.

Question 17Pulp mill processes and fiber line

What is the primary purpose of oxygen delignification between cooking and bleaching?

  1. To replace the causticizing plant in the recovery cycle
  2. To refine the pulp to a target freeness
  3. To extend delignification with an oxidative stage that reduces bleach chemical demand Correct answer.
  4. To increase the kappa number before bleaching

Explanation

Oxygen delignification uses alkaline oxygen to remove additional lignin after cooking, lowering the kappa entering the bleach plant and reducing chlorine-based or other bleach chemical use. It does not refine fibers or replace recausticizing.

Question 18Pulp mill processes and fiber line

A mill measures a lower-than-expected pulp yield after a cook with normal kappa. Which explanation is most consistent with kraft chemistry?

  1. Too little alkali prevented lignin removal
  2. Excess alkali caused carbohydrate peeling and loss of hemicellulose Correct answer.
  3. The digester pressure was too low to compact chips
  4. The wash water was too cold to dissolve lignin

Explanation

Alkali attacks carbohydrates through peeling reactions, and an excess charge at normal kappa can reduce yield by degrading hemicellulose and cellulose. Insufficient alkali would raise kappa, and cold wash water or low pressure does not explain yield loss at normal kappa.

Question 19Pulp mill processes and fiber line

Which measurement best indicates the efficiency of a causticizing plant?

  1. The displacement ratio of the last washer
  2. Causticizing efficiency, the conversion of sodium carbonate to sodium hydroxide Correct answer.
  3. The sulfidity of the green liquor
  4. The kappa number of the brownstock

Explanation

Causticizing efficiency expresses how completely sodium carbonate in green liquor is converted to sodium hydroxide. Kappa, displacement ratio, and sulfidity describe cooking, washing, and liquor composition rather than the causticizing conversion.

Question 20Pulp mill processes and fiber line

A mill reduces the liquor-to-wood ratio in a batch digester while keeping alkali charge per ton of wood and H-factor constant. What is the most likely effect?

  1. No effect on the cook because only H-factor matters
  2. A large drop in kappa because delignification becomes more selective
  3. A significant rise in pulp yield from reduced hemicellulose loss
  4. Higher dissolved solids concentration and potentially poorer liquor distribution in the chip column Correct answer.

Explanation

Lowering liquor-to-wood ratio concentrates dissolved solids and reduces the volume of liquor available to penetrate and circulate around chips, which can cause uneven cooking. Alkali charge per ton and H-factor are unchanged, so large kappa or yield shifts are not expected.

Question 21Pulp mill processes and fiber line

Why can excessively high cooking temperature be detrimental even when the target H-factor is reached?

  1. It raises the causticizing efficiency above the practical limit
  2. It accelerates alkaline hydrolysis of cellulose, lowering pulp viscosity and strength Correct answer.
  3. It increases the sulfide content of the white liquor
  4. It prevents lignin from dissolving in the cooking liquor

Explanation

At high temperature the rate of carbohydrate chain cleavage by alkaline hydrolysis rises faster than delignification benefits, reducing pulp viscosity and strength. Reaching the same H-factor with a hotter, shorter cook can therefore give a weaker pulp.

Question 22Pulp mill processes and fiber line

A mill switches to a wood furnish with higher lignin content while keeping the same effective alkali charge and H-factor. What is the most likely consequence?

  1. Lower kappa because denser wood cooks faster
  2. Unchanged kappa because H-factor fixes delignification
  3. Lower bleach chemical demand because lignin is easier to oxidize
  4. Higher kappa at blow because more lignin must be removed Correct answer.

Explanation

At the same alkali charge and H-factor, a furnish with more lignin leaves more residual lignin, so kappa rises. Higher kappa typically increases downstream bleach chemical demand rather than reducing it.

Question 23Pulp mill processes and fiber line

In a two-vessel continuous digester with an impregnation vessel, why is the transfer circulation between vessels important?

  1. It regulates the solids content of the black liquor sent to evaporation
  2. It sets the final kappa of the pulp independently of the cooking zone
  3. It determines the causticizing efficiency of the white liquor
  4. It controls the temperature and liquor strength the chips experience during impregnation Correct answer.

Explanation

The transfer circulation carries heated liquor and chemicals to the impregnation vessel, controlling the temperature and liquor strength during penetration before the cook. Final kappa is set mainly in the cooking zone, and causticizing and evaporation are separate operations.

Question 24Pulp mill processes and fiber line

A mill wants to lower the kappa entering bleaching while protecting pulp strength. Which combination is most consistent with fiber line practice?

  1. A large increase in sulfidity beyond normal limits
  2. Moderate extended cooking followed by oxygen delignification Correct answer.
  3. Eliminating brownstock washing to retain dissolved lignin
  4. Very high cooking temperature with a short H-factor

Explanation

Extended cooking to a moderate kappa followed by oxygen delignification removes lignin in two gentler stages, limiting carbohydrate degradation and preserving strength. Excessive temperature or extreme sulfidity risks viscosity and yield, and skipping washing increases carryover.

Question 25Pulp mill processes and fiber line

What is the function of the blow tank or blow line after a batch digester?

  1. To release digester pressure and cool the pulp before washing Correct answer.
  2. To cook the chips to the final kappa
  3. To calcine lime mud for reuse
  4. To regenerate white liquor from green liquor

Explanation

At blow, the digester pressure is released, which cools the pulp and discharges it to the blow tank before washing. Cooking is already complete, and liquor regeneration and lime calcining occur in the recovery area.

Question 26Pulp mill processes and fiber line

In a kraft pulp mill, which cooking liquor stream is recovered from the digester blow and then concentrated before being burned in the recovery boiler?

  1. Weak black liquor Correct answer.
  2. Tall oil soap
  3. Green liquor
  4. White liquor

Explanation

Weak black liquor is the spent cooking liquor separated from the pulp after cooking; it contains dissolved lignin and spent chemicals and is concentrated in evaporators before firing in the recovery boiler. White liquor is fresh cooking chemical, green liquor is the dissolved smelt, and tall oil soap is a by-product skimmed from black liquor.

Question 27Pulp mill processes and fiber line

What is the main function of a chip bin and steaming vessel ahead of a continuous digester?

  1. To screen out oversized chips before they enter the digester
  2. To remove air and preheat chips so liquor impregnation is uniform Correct answer.
  3. To cool the chips so the cook proceeds slowly and evenly
  4. To add white liquor and begin bulk delignification

Explanation

Steaming displaces air from the chip voids and warms the chips, which improves liquor penetration and gives a more uniform cook. Air left in the chips can block impregnation and cause kappa variation. Screening and alkali addition occur elsewhere in the fiber line.

Question 28Pulp mill processes and fiber line

In a kraft mill, what is the primary product of the causticizing reaction between green liquor and slaked lime?

  1. Sodium carbonate, which is recausticized directly
  2. Sodium sulfide, giving white liquor
  3. Sodium hydroxide, giving white liquor Correct answer.
  4. Calcium carbonate, which is the cooking chemical

Explanation

Green liquor contains sodium carbonate and sodium sulfide; adding slaked lime converts the carbonate to sodium hydroxide, producing white liquor. Calcium carbonate is the lime mud by-product, which is later calcined back to lime in the kiln.

Question 29Pulp mill processes and fiber line

Which pulp property is most directly improved by gentle, low-consistency mixing during oxygen delignification compared with severe high-shear conditions?

  1. Kappa number reduction beyond the delignification ceiling
  2. Viscosity retention, reflecting less cellulose chain damage Correct answer.
  3. Bulk, resulting from less fiber collapse
  4. Brightness gain from removal of residual lignin chromophores

Explanation

Oxygen delignification generates radicals that attack both lignin and cellulose; excessive shear and localized high alkali or temperature accelerate cellulose degradation and lower viscosity. Gentle mixing distributes chemicals and oxygen evenly while limiting this strength loss.

Question 30Pulp mill processes and fiber line

A mill raises the white liquor sulfidity from 28% to 38% while holding effective alkali and H-factor constant. What is the most likely change in the cook?

  1. Faster delignification and slightly lower kappa at the same H-factor Correct answer.
  2. Higher pulp yield because sulfide protects carbohydrates
  3. Lower residual alkali in the black liquor
  4. A large increase in rejects from incomplete impregnation

Explanation

Higher sulfidity accelerates the lignin-removing action of hydrosulfide, so delignification proceeds faster and kappa tends to fall at fixed H-factor. Yield gains from sulfide are modest and not guaranteed, and the effect on impregnation or residual alkali is secondary.

Question 31Pulp mill processes and fiber line

A brownstock washing line shows a high dilution factor but a low displacement ratio. Which condition best explains this combination?

  1. Excessive pulp consistency entering the washer
  2. Poor liquor displacement in the wash zone despite ample dilution water Correct answer.
  3. High black liquor solids leaving the digester
  4. Too little wash water relative to the pulp flow

Explanation

Dilution factor reflects how much wash water is applied relative to the liquor in the pulp, while displacement ratio measures how effectively that water displaces the liquor. High dilution with low displacement points to poor contact or channeling in the wash zone rather than a water shortage.

Question 32Pulp mill processes and fiber line

In a bleach plant, which sequence position most commonly applies hydrogen peroxide to reinforce brightness and prevent brightness reversion?

  1. The final alkaline peroxide stage before storage Correct answer.
  2. The first acidic chlorine dioxide stage
  3. The oxygen delignification stage
  4. The chelation stage before peroxide

Explanation

A final peroxide stage raises brightness and stabilizes it by oxidizing residual chromophores that would otherwise cause reversion during storage. Chelation removes metals ahead of peroxide, and chlorine dioxide stages are typically acidic delignification or brightening steps.

Question 33Pulp mill processes and fiber line

A mill observes that the lime kiln is producing lime with high residual carbonate. Which operating change is most likely to correct this?

  1. Lowering the excess oxygen in the kiln atmosphere
  2. Raising the kiln burning-zone temperature to complete calcination Correct answer.
  3. Increasing the lime mud moisture entering the kiln
  4. Reducing the kiln rotation speed to lengthen residence time

Explanation

Residual carbonate indicates incomplete calcination, so raising the burning-zone temperature drives the endothermic calcination reaction closer to completion. Wet mud and insufficient residence time also matter, but temperature is the direct driver of carbonate conversion.

Question 34Pulp mill processes and fiber line

Which measurement is the most direct indicator of the degree of delignification achieved in the digester?

  1. Black liquor solids concentration
  2. Kappa number of the washed pulp Correct answer.
  3. Sulfidity of the white liquor
  4. Residual alkali in the black liquor

Explanation

Kappa number measures the lignin remaining in the pulp, so it is the standard indicator of how far delignification has progressed. Sulfidity describes liquor composition, residual alkali shows chemical consumption, and solids concentration reflects evaporation duty.

Question 35Pulp mill processes and fiber line

A mill wants to reduce the carryover of black liquor solids into the bleach plant without adding washing stages. Which action is most consistent with that goal?

  1. Improving the final wash stage displacement efficiency Correct answer.
  2. Increasing the dilution factor uniformly across all washers
  3. Raising the pulp consistency in the last washer
  4. Reducing the wash water temperature in the final stage

Explanation

Carryover is the dissolved solids leaving with the pulp, so the final washer's displacement efficiency directly controls it. Increasing dilution everywhere or raising consistency does not necessarily improve the last-stage removal of liquor from the fiber mat.

Question 36Pulp mill processes and fiber line

Which condition in a continuous digester most likely causes a higher-than-normal kappa at the blow line while cooking temperature is stable?

  1. Increased white liquor sulfidity
  2. Longer cooking-zone residence time
  3. Higher chip moisture content
  4. Reduced impregnation time in the upstream vessel Correct answer.

Explanation

Insufficient impregnation leaves chips with uneven liquor penetration, so some fibers remain undercooked and raise the average kappa. Sulfidity changes and moisture shifts affect the cook but do not specifically create the undercooked fraction seen with short impregnation.

Question 37Pulp mill processes and fiber line

What is the main purpose of the blow valve and blow line arrangement at the discharge of a batch digester?

  1. To separate knots and shives from the cooked pulp
  2. To add dilution water and adjust consistency for washing
  3. To release the cooked pulp and liquor rapidly into the blow tank at reduced pressure Correct answer.
  4. To cool the pulp before it enters the brownstock washers

Explanation

The blow valve opens to discharge the digester contents under pressure into the blow tank, where the pressure drop defibrates the softened chips and releases steam and liquor. Knot removal and consistency adjustment occur downstream in screening and dilution equipment.

Question 38Pulp mill processes and fiber line

A mill seeks to lower kappa entering bleaching while protecting pulp strength. Which combination of changes is most consistent with that objective?

  1. Higher cooking temperature with a shorter H-factor
  2. Higher sulfidity with a reduced effective alkali charge
  3. Increased bleaching chemical charge with no cooking change
  4. Moderately extended cooking with a controlled oxygen delignification stage Correct answer.

Explanation

Extending delignification moderately and following it with oxygen delignification removes more lignin before bleaching while keeping selectivity acceptable. Raising temperature while cutting H-factor or cutting alkali tends to increase carbohydrate damage or leave more lignin.

Question 39Pulp mill processes and fiber line

A batch digester cook shows normal kappa but a lower screened yield than expected. Which explanation is most consistent with these results?

  1. A lower effective alkali charge than target
  2. A shorter impregnation time before cooking
  3. Higher lignin content in the wood furnish
  4. Increased carbohydrate degradation producing fines and dissolved material Correct answer.

Explanation

If kappa is on target, delignification reached the intended point, so yield loss must come from carbohydrate breakdown into fines and dissolved solids that are lost in screening and washing. Higher lignin or lower alkali would typically raise kappa rather than leave it normal.

Question 40Pulp mill processes and fiber line

A mill reduces the liquor-to-wood ratio in a batch digester while keeping alkali per ton of wood and H-factor constant. What is the most likely consequence?

  1. Higher dissolved solids concentration and a steeper alkali gradient across the chip Correct answer.
  2. Reduced carbohydrate degradation from lower heat exposure
  3. A higher sulfidity in the cooking liquor
  4. A lower kappa number at the same H-factor

Explanation

A lower liquor-to-wood ratio concentrates dissolved solids and reduces the liquor volume available to carry alkali into the chips, creating a steeper alkali gradient. Kappa and sulfidity are not directly improved by this change, and carbohydrate degradation is not reduced simply by using less liquor.

Question 41Reactor and digester equipment design

In a continuously stirred tank reactor (CSTR) used for anaerobic digestion, what is the primary purpose of the mechanical mixer?

  1. To maintain solids in suspension and promote contact between substrate and biomass Correct answer.
  2. To compress the biogas headspace and drive gas flow
  3. To raise the internal temperature above the thermophilic range
  4. To increase the hydraulic retention time of the feed slurry

Explanation

Mixing in a CSTR keeps solids suspended and ensures intimate contact between the incoming substrate and the microbial population, which supports stable conversion. It also helps release entrained biogas and equalize temperature and pH. Mixing does not change retention time or create pressure.

Question 42Reactor and digester equipment design

Which reactor configuration keeps a high concentration of biomass in the reactor by retaining it while allowing treated liquid to pass through?

  1. Anaerobic membrane bioreactor Correct answer.
  2. Plug-flow reactor without recycle
  3. Batch digester with no mixing
  4. Open anaerobic lagoon

Explanation

An anaerobic membrane bioreactor uses a membrane to separate solids and biomass from the permeate, so biomass is retained while clarified liquid exits. This decouples solids retention time from hydraulic retention time. The other configurations do not selectively retain biomass.

Question 43Reactor and digester equipment design

What is the function of the gas holder in a fixed-dome biogas plant?

  1. To heat the digester contents during winter
  2. To grind and homogenize the incoming feedstock
  3. To remove hydrogen sulfide from the raw biogas
  4. To store gas at variable volume while maintaining delivery pressure Correct answer.

Explanation

A gas holder stores biogas and accommodates fluctuations in production and demand while maintaining a usable delivery pressure. In fixed-dome designs the dome itself acts as the gas storage volume. Desulfurization, heating, and feedstock conditioning are separate functions.

Question 44Reactor and digester equipment design

In a plug-flow anaerobic digester, what geometric feature is essential to approximate plug-flow behavior?

  1. A shallow uncovered basin with no walls
  2. A high length-to-width ratio with baffles or partitions Correct answer.
  3. A vertical cylinder with a single bottom inlet
  4. A perfectly spherical tank with central mixing

Explanation

Plug-flow behavior requires a long, narrow flow path so that material advances as a moving front with limited back-mixing. Baffles or internal partitions increase the effective path length. Spherical, shallow, or single-inlet cylindrical tanks do not create this flow pattern.

Question 45Reactor and digester equipment design

Which parameter directly determines the hydraulic retention time of a completely mixed digester?

  1. The ratio of methane to carbon dioxide in the headspace
  2. The ratio of biogas production to organic loading
  3. The ratio of volatile solids to total solids
  4. The ratio of working volume to influent flow rate Correct answer.

Explanation

Hydraulic retention time equals the working (active) volume divided by the volumetric feed rate. It is a purely hydraulic relationship and is independent of gas composition or solids ratios. Organic loading and gas quality are separate performance parameters.

Question 46Reactor and digester equipment design

What is the main role of a heat exchanger in a mesophilic anaerobic digester?

  1. To maintain the contents within the optimal temperature band for the microbial consortium Correct answer.
  2. To add oxygen for aerobic polishing of the effluent
  3. To increase the pressure of the biogas before combustion
  4. To separate grit and heavy inert material from the sludge

Explanation

Heating maintains the digester within the mesophilic range, typically around 35 to 38 degrees Celsius, where the microbial community is most productive and stable. Temperature fluctuations can upset digestion. Pressure boosting, grit removal, and aeration are handled by other equipment.

Question 47Reactor and digester equipment design

In a two-stage anaerobic digestion system, what is typically accomplished in the first (hydrolysis/acidogenesis) stage?

  1. Conversion of complex polymers into soluble acids and alcohols Correct answer.
  2. Removal of ammonia by stripping
  3. Conversion of acetate into methane by methanogens
  4. Dewatering of the digested sludge

Explanation

The first stage favors hydrolytic and acidogenic organisms that break down polymers into soluble monomers and then into volatile fatty acids and alcohols. Methanogenesis occurs mainly in the second stage under controlled conditions. Ammonia stripping and dewatering are downstream operations.

Question 48Reactor and digester equipment design

Which component protects a digester from over-pressurization in the event of a downstream gas blockage?

  1. A condensate trap at the low point of the gas line
  2. A pressure relief valve or rupture disc on the gas line Correct answer.
  3. A gas flow meter at the boiler inlet
  4. A flame arrestor on the gas outlet

Explanation

A pressure relief valve or rupture disc opens when gas pressure exceeds a set limit, venting gas safely and protecting the digester structure. Flame arrestors prevent flame propagation, condensate traps remove water, and flow meters measure volume; none of these relieve overpressure.

Question 49Reactor and digester equipment design

A completely mixed digester has a working volume of 2000 cubic meters and receives 100 cubic meters of feed per day. What is the hydraulic retention time?

  1. 2 days
  2. 20 days Correct answer.
  3. 200 days
  4. 0.05 days

Explanation

Hydraulic retention time equals working volume divided by feed flow rate, so 2000 divided by 100 gives 20 days. This is a standard calculation for sizing and operating continuously fed digesters. The other values result from inverting or misplacing the terms.

Question 50Reactor and digester equipment design

A digester is fed 50 cubic meters per day of slurry at 6 percent total solids. What is the approximate total solids loading rate in kilograms per day, assuming a slurry density of 1000 kilograms per cubic meter?

  1. 3000 kilograms per day Correct answer.
  2. 30000 kilograms per day
  3. 600 kilograms per day
  4. 300 kilograms per day

Explanation

The mass of slurry is 50 cubic meters times 1000 kilograms per cubic meter, equal to 50000 kilograms per day. Six percent of that is 3000 kilograms of total solids per day. The other values misapply the percentage or the density.