A significant portion of fine chemical production relies on batch processing using reactors. The lifecycle of a vessel—spanning selection, installation, charging, temperature control, clearing, and maintenance—integrates both equipment specifications and operational procedures.
I. Equipment (Questions 1–25)
1. What types of reactors are commonly used in fine chemicals? Categorizations include: by pressure (atmospheric vs. pressurized/jacketed or coiled); by material (carbon steel with lining, glass-lined, stainless steel [304/316L], special alloys, ceramic/graphite lining); by heat transfer method (jacketed, internal coil, external half-pipe jacket); and by function (reaction, crystallization, distillation, metering/dosing tanks). Selection primarily depends on medium corrosivity, temperature and pressure requirements, and explosion-proofing needs.
2. What are the application scope and limitations of glass-lined reactors? They are suitable for most inorganic acids, organic acids, organic solvents, and weak bases, with a typical temperature range of −20°C to 200°C (special formulations can reach 230°C). Limitations include: hydrofluoric acid and fluoride-ion-containing media; strong bases (NaOH, especially at high temperatures, corrodes the glass lining); phosphoric acid (at temperatures >180°C); conditions causing thermal shock to the glass layer (rapid cooling or heating); and impacts from hard objects.
3. For which media are 304 and 316L stainless steels suitable? 304 is resistant to dilute nitric acid, organic acids, and food-grade media, but is susceptible to chloride pitting. 316L, containing molybdenum, resists chloride ions, seawater, and various organic/inorganic acids, making it better suited for systems involving chlorine-containing solvents and salts. Neither grade withstands high-temperature concentrated nitric acid (a strong oxidant), hydrofluoric acid, or strong reducing acids (such as prolonged immersion in hydrochloric acid).
4. Under what operating conditions are Hastelloy and Monel alloys selected? Hastelloy (e.g., C-276) resists strong reducing acids (such as hydrochloric acid and high-temperature sulfuric acid), wet chlorine gas, and pitting; it is used in highly corrosive conditions where 316L fails. Monel resists hydrofluoric acid and seawater but is expensive and carries a risk of stress corrosion cracking. Generally, these special alloys are used only for critical equipment where they are indispensable (e.g., involving fluorine or strong reducing agents) to keep costs under control.
5. What are the pros and cons of jacketed versus coiled heat transfer methods? Jackets feature a simple structure, do not occupy internal vessel space, and are easy to maintain; however, they have low heat transfer coefficients and slow heating rates, making them less suitable for large vessels. Internal coils offer large heat transfer areas and high efficiency but take up internal volume, are prone to material buildup and scaling, and are difficult to clean. External half-pipe jackets offer a middle ground: they withstand high pressure, provide good heat transfer without occupying the internal cavity, and are often used in applications requiring rapid heat removal or vacuum operation.
6. What are the advantages of half-pipe jackets over full jackets? Half-pipes (semi-circular pipes) are welded to the vessel's outer wall to form a spiral channel. This design offers high pressure resistance (allowing for higher steam or heat transfer oil pressures), high heat transfer intensity per unit area, the ability to reduce vessel wall thickness, and more responsive heating and cooling; additionally, liquid is less likely to accumulate in the jacket. The downsides include stringent manufacturing and welding requirements and the need to verify local thermal stresses.
7. How to select an agitator type: paddle, anchor, turbine, or propeller? Use propeller or turbine types for low-viscosity homogeneous fluids (high circulation, strong dispersion); paddle or turbine types for medium viscosity; and anchor, frame, or ribbon types for high viscosity where wall-scraping is required. Gas-liquid reactions (e.g., hydrogenation) typically employ turbine impellers with baffles to enhance mass transfer; solid-liquid suspensions use propeller or paddle types, with rotational speed controlled to prevent settling.
8. What type of agitator should be selected for high-viscosity materials? Anchor, frame, ribbon, or permanent-magnet/dual-planetary mixers are suitable, emphasizing "wall-scraping and axial pushing" to prevent localized overheating and fouling/coking on the vessel walls. Extremely high viscosities (e.g., silicone rubber, late-stage resin polymerization) require low-speed, high-torque operation and a variable frequency drive (VFD). Agitation power requirements increase significantly with viscosity; therefore, the motor and gearbox should be sized with a sufficient safety margin.
9. What is an agitation "dead zone," and how can it be minimized? A dead zone is a stagnant area where agitation is ineffective and material does not flow; this leads to material accumulation, scaling, uneven reaction, or even decomposition. Countermeasures include: proper equipment selection; installing baffles to break up vortex formation; optimizing the number and placement of impeller blades; using combined impeller configurations (e.g., upper propeller and lower anchor); and maintaining the material fill level within the impeller's effective mixing zone.
10. How do you select a shaft seal: packed gland or mechanical seal? Packed glands feature a simple, low-cost design and allow for replacement under pressure, but they have higher leakage rates and require periodic tightening; they are suitable only for non-flammable, non-toxic fluids at atmospheric or low pressures. Mechanical seals (single or double-face) are mandatory for flammable, toxic, or valuable materials, or for processes requiring long, continuous operation. Mechanical seals offer minimal leakage and long service life but demand high installation precision and a proper flushing system.
11. How should the flush/barrier fluid for a double-face mechanical seal be formulated? A barrier fluid (isolation fluid) is introduced between the dual mechanical seal faces; its pressure must be 0.1–0.3 MPa higher than the internal vessel pressure to establish dual protection: blocking outward leakage and monitoring inward leakage. Clean inert oil or a process-compatible liquid is typically used, supported by a header tank/pump station, pressure interlocks, and liquid level alarms. In the event of seal failure, first restore pressure and locate the leak source; dry running is strictly prohibited.
12. Why are glass-lined vessel thermowells prone to breakage, and how can this be prevented? The glass lining is brittle; the thermowell (usually steel-lined glass or an internal thermocouple well) is susceptible to snapping due to agitator impact, thermal shock, or jamming caused by material caking. Broken fragments falling into the vessel can contaminate the product or damage the agitator. Protective measures include: maintaining sufficient liquid coverage, avoiding high temperatures in an empty vessel, installing impact-resistant supports, using thermocouples with protective sleeves, and exercising care during vessel cleaning.
13. What are the respective functions of manholes, handholes, sight glasses, and light ports? Manholes (DN450 or larger) allow personnel entry for maintenance and cleaning; handholes facilitate the charging of small items or inspections; sight glasses allow observation of liquid levels, reaction status, and crystallization; light ports accommodate explosion-proof lighting to illuminate the vessel interior. Selection must ensure sealing integrity and explosion-proof capabilities; sight glasses should be temperature- and pressure-resistant and anti-fogging (e.g., equipped with purge or heating systems).
14. How are electrical devices selected for hazardous (explosion-risk) areas (Ex d / Ex e / Ex ia)? Ex d (Flameproof/Explosion-proof): The enclosure withstands internal explosions without propagating the flame externally; Ex e (Increased Safety): Enhanced safety margins to prevent ignition sources; Ex ia (Intrinsically Safe): Energy limitation, suitable for Zone 0. Selection follows hazardous area classification (GB 50058): Zone 0 uses intrinsic safety or encapsulation; Zone 1 uses flameproof or intrinsic safety; Zone 2 allows increased safety. Motors, luminaires, switches, and junction boxes must all match the required protection level.
15. How are safety valves and rupture discs used in combination? Safety valves are resettable and capable of repeated actuation but carry risks of leakage and sticking; rupture discs are single-use, offer excellent sealing, and respond rapidly. A combined setup (safety valve and rupture disc in series, with the disc facing the vessel) balances sealing and reusability: the disc maintains the seal during normal operation, while in an overpressure event, the disc bursts first, followed by discharge through the safety valve, preventing medium crystallization from jamming the valve seat. Their discharge capacities must be matched.
16. How are rupture discs selected and their replacement cycles determined? Selection is based on medium corrosivity, temperature, burst pressure (typically 1.1–1.25 times the design pressure), and discharge area; attention must be paid to "burst pressure tolerance" and "fatigue life." Replacement cycles: follow manufacturer recommendations (usually 1–3 years) or replace early if the medium causes diaphragm embrittlement or corrosion; inspection is mandatory during major overhauls, and records must be kept for traceability.
17. How should vacuum receiver tanks, condensers, and water separators be configured? For vacuum distillation, a vacuum receiver tank follows the condenser (to buffer and collect distillate and prevent back-sucking); for reflux or dehydration reactions, a condenser combined with a water separator (Dean-Stark apparatus) is used for azeotropic dehydration; the entire system must ensure airtightness, and the vacuum level must match the condensation load. Condensing area must be calculated during configuration to avoid situations where material evaporates but cannot be condensed, leading to material carryover in the tail gas.
18. Why are upward-opening or downward-opening valves commonly used for vessel bottom discharge, and are ball valves suitable? Discharge valves must be resistant to particulates, capable of full opening for complete drainage, and resistant to clogging; upward-opening valves (plug lifts to open, not extending into the vessel) and downward-opening valves (plug extends downward to open) are suitable for slurries or high-viscosity materials. Standard ball valves are prone to material jamming, difficult to clean, and have large dead volumes; they are generally not used as primary discharge valves unless the medium is extremely clean. For systems containing particulates, priority should be given to specialized discharge valves or knife gate valves.
19. What is the role of metering tanks and dropping tanks (elevated tanks) in controlled feeding? These tanks control the feed rate via liquid level and flow meters, transforming "rapid dumping" into "slow dripping"; they are critical for managing exothermic reactions and preventing material ejection. They can be equipped with dual valves, sight glasses with scales, and temperature-control jackets (for low-temperature or thermally unstable materials), and can be interlocked with the DCS to ensure constant-speed feeding and automatic feed cutoff upon over-temperature.
20. What methods can be used to enhance heat transfer when the surface area is insufficient? Options include installing internal coils or external half-pipe jackets, increasing the temperature difference of the heat transfer medium (raising steam pressure or lowering cooling water temperature), increasing flow rate and turbulence (adding baffles or increasing flow velocity), switching to heat transfer oil or high-pressure hot water, or installing an external emergency cooling jacket or cold-source tank. The fundamental solution is to reduce heat generation on the process side (e.g., through dilution or staged feeding) rather than simply adding more heat transfer surface area.
21. Why are glass-lined vessels susceptible to thermal shock? What are the limits on heating/cooling rates? The glass lining and the steel substrate have different thermal expansion coefficients; rapid heating or cooling generates stress that can cause the glass lining to crack or spall ("pop off"). Generally, heating/cooling rates are restricted to a specific limit (commonly ≤80–100°C/h, depending on the manufacturer and wall thickness), and hot and cold media must not directly impinge on the same area in rapid succession. In winter, materials should be preheated with warm water before charging to avoid introducing frozen material into a hot vessel.
22. What is the typical range for the "volume utilization rate" of a reactor? It is usually 60%–80%, leaving sufficient vapor space to accommodate foam, reflux, and potential volume expansion. Higher rates are permissible for systems with high boiling points, low foaming tendencies, and atmospheric pressure operation; lower rates (even ≤50%) are required for systems involving gases, high foaming tendencies, risks of material surge/ejection, or vacuum distillation. Overfilling is a common cause of material ejection accidents.
23. How can the required heat transfer area and cooling capacity be estimated based on the heat release rate? First, determine the heat release power per unit volume, *q* (W/L), via calorimetry (e.g., reaction calorimetry using RC1); multiply this by the charge volume to obtain the total heat load, *Q*; then divide by the heat transfer coefficient, *K*, and the logarithmic mean temperature difference, Δ*Tm*, to calculate the required area: *A* = *Q* / (*K* · Δ*Tm*). Regarding cooling capacity, the requirement is: coolant flow rate × specific heat × temperature rise ≥ *Q*. During scale-up, *K* and Δ*Tm* often deteriorate; therefore, a safety margin must be factored in and calculations verified.
24. What are the applicable scenarios for repairing the inner lining of glass-lined reactors (using tantalum plugs or high-temperature enamel)? Small areas of enamel spalling can be temporarily sealed using tantalum plugs (via plug welding); this method offers corrosion resistance and rapid execution but is unsuitable for environments involving strong alkalis or fluorides and affects local surface smoothness. For damage covering large areas or critical locations, returning the vessel to the manufacturer for re-enameling is recommended. Post-repair, water-filling/pressure testing and corrosion resistance verification are mandatory; long-term operation with highly corrosive media while the vessel is damaged is strictly prohibited.
25. What are the criteria for extending the service life of aging stirred-tank reactors versus decommissioning them? Factors include wall thinning rate, corrosion rate, weld defects (via flaw detection), agitator shaft runout, seal lifespan, and jacket pressure-bearing capacity. Decommissioning is required if the vessel exceeds its design life or exhibits irreparable cracks, severe wall thinning, or material degradation (e.g., intergranular corrosion in stainless steel). Establish equipment records and conduct periodic inspections (e.g., in accordance with TSG regulations) to make repair-or-replace decisions based on data.
II. Operations (Questions 26–50)
26. Why is "first-reactor confirmation" or "process confirmation" performed in batch production? For every batch (especially the first reactor run after a product change, reactor switch, or maintenance), it is necessary to confirm that the equipment is in good condition, instruments are accurate, materials match specifications, and process parameters are within the validated operating window. This verifies that the current run can reproduce the quality of the previous successful batch. It serves as the first line of defense against batch-wide non-conformity and anomalies; results must be recorded in the batch record, and operations must halt immediately if an anomaly occurs.
27. How can dust dispersion and static electricity be prevented during the charging of solid materials? Use enclosed feeding methods (vacuum loading, or ton-bags with tight-seal interfaces), local negative-pressure exhaust, and wet feeding (adding materials after slurrying or dissolution); personnel must wear dust masks. Rapid powder flow generates static electricity; therefore, feed inlets, pipelines, and equipment must be bonded and grounded. High-speed charging and operations involving personnel carrying a static charge are prohibited.
28. Why must the rate of liquid addition be controlled, and how is it set? Addition often involves heat release or the formation of unstable intermediates; controlling the rate effectively controls the temperature and the accumulated concentration of reactants. The rate is calculated based on the heat release power versus cooling capacity (see Q23), with the "maximum allowable rate" determined during lab or pilot trials serving as the upper limit. Implementation in the plant uses a flow meter, control valve, and DCS; the system automatically reduces the rate or stops addition if the temperature exceeds limits.
29. How is the reactor heating rate determined, and what are the risks of excessive speed? The rate is determined by the target temperature, thermal sensitivity, heat release characteristics, and the equipment's heating capacity. Excessive speed can cause localized overheating, increased side reactions, or even trigger thermal runaway; for substances prone to decomposition, heating too rapidly past the decomposition onset temperature creates a hazard. Standard practice involves stepwise heating with holding periods (plateaus) and process validation of the temperature-time curve.
30. What is MTSR (Maximum Temperature of Synthesis Reaction), and how should it be monitored during operations? MTSR is the peak temperature the reaction mass would reach due to heat release if the cooling system failed. It must remain below the material's decomposition temperature (e.g., TD24); otherwise, a cooling failure would lead to a runaway reaction. Operational controls must ensure reliable cooling and prohibit arbitrary increases in feed concentration or temperature, thereby maintaining an adequate safety margin for the MTSR.
31. What is the purpose of reflux operations, and how is condenser load calculated? Reflux is used to maintain reaction temperature, recover solvents, and remove water or azeotropes. Condenser load is calculated as the evaporation rate multiplied by the latent heat of vaporization; the system must ensure complete condensation to prevent "venting" (vapor escaping from the top). Insufficient load can lead to material carryover in the exhaust, environmental pollution, or even ignition; heat transfer area and cooling water temperature must be calculated, with capacity increased or units connected in parallel if necessary.
32. Principles and operational essentials of the Dean-Stark apparatus? It utilizes azeotropic distillation to remove water generated during the reaction; after condensation, the mixture separates by density in the trap—the organic phase refluxes while the aqueous phase is drained—thereby "driving the equilibrium to the right via dehydration." Key points: select the correct solvent (forms an azeotrope with water and separates into layers), control the reflux ratio, prevent freezing/clogging, periodically drain water and observe separation to determine the endpoint, and replenish any lost organic phase.
33. Typical applications of negative-pressure (vacuum) distillation in fine chemicals? Used for purifying high-boiling or heat-sensitive materials (e.g., intermediates, fragrances, pharmaceutical intermediates) to lower boiling points and minimize decomposition. Operational essentials: ensure system airtightness, increase vacuum gradually, prevent back-sucking (using buffer tanks), ensure adequate condensation, break the vacuum with inert gas rather than air (to prevent oxidation/combustion/explosion), and monitor vacuum levels and distillation ranges.
34. How is the reaction endpoint determined (TLC/HPLC/pH/specific gravity/sampling)? Selection depends on the reaction type: TLC/HPLC (with visualization) is most accurate for tracking the disappearance of starting materials and product ratios; monitor pH/conductivity for acid-base neutralization; monitor refractive index/specific gravity/viscosity for condensation reactions; and observe crystal form and melting point for crystallization. Rely primarily on in-process control (IPC) analysis supplemented by sensory checks; define "endpoint criteria" in the SOP to avoid discharging the batch based merely on subjective experience.
35. How can sampling balance safety and representativeness? For high-temperature, high-pressure, or flammable systems, cool and depressurize the system and confirm safety before sampling; use closed-loop samplers or bottom-outlet valves to prevent splashing and volatilization. Representativeness: sample while the mixture is homogeneous (stirring), avoid dead zones, and follow "top-middle-bottom" protocols or designated sampling points. Sampling bottles must be labeled and anti-static, and operations should be conducted in a well-ventilated area.
36. What precautions are necessary for operations inside a reactor under oxygen-deficient or inert atmospheres? In nitrogen-blanketed environments where oxygen levels are low, personnel must—prior to entry—ensure gas purging, verify oxygen levels (≥19.5%), obtain a confined space entry permit, ensure ventilation, assign a dedicated monitor, and equip themselves with respiratory and emergency gear. Entry without protection is strictly prohibited, as is using pure nitrogen to "blast" personnel for cooling purposes. Inerting is intended to prevent fire and explosions; it does not guarantee that the space is safe for human entry.
37. What are the early warning signs of a reactor "runaway" (or material surge/eruption) and the corresponding emergency response? Warning signs: abnormal rise in temperature and pressure, violent reflux, rising foam levels, fluctuations in agitator motor current, and abnormal sounds or gas emissions. Emergency response: immediately stop feeding and heating, maximize cooling, relieve pressure if necessary (via safety valve or emergency vent to a safe location), add a reaction terminator, and evacuate personnel upwind. Follow the established emergency plan and do not approach the area blindly.
38. Why is reactor cleaning (clearing the line) between batches important? How is cross-contamination prevented? Residual material from the previous batch can react with the next batch, causing impurities, discoloration, or even hazards. Clearing procedure: drain completely → solvent/alkali wash/boil-out → rinse → inspect (visual + swab sampling if necessary) → hang "Cleaned" status tag. Perform "cleaning validation" when switching products; use specialized solvents for hard-to-dissolve substances; eliminate material mixing.
39. What are the effects of solvent recovery and reuse on the reaction, and how is quality controlled? Recovered solvents contain moisture, residues from previous batches, and heavy components, which can alter reaction rates and impurity profiles. Standards for recovered solvents (moisture, residue, color) must be established; reuse is permitted only after passing batch testing. Control the reuse ratio and refine the solvent if necessary. Indiscriminate reuse is a hidden source of quality fluctuations.
40. What are typical accidents caused by reversing the charging sequence? E.g., adding solid material before solvent (instead of solvent first) causes clumping and poor dissolution; adding alkali to concentrated acid (instead of diluting acid first) causes violent exotherms and material ejection; accidentally charging oxidizers and reducing agents (or incompatible materials) into the same reactor causes fire or explosion. Charging sequences must be documented in SOPs and verified by two people (especially for high-risk processes); "which container to add first" is a critical safety rule.
41. What are the cold sources and temperature control challenges for low-temperature reactions (below −10°C)? Common cold sources include chilled brine (CaCl₂/NaCl), ethylene glycol units, and liquid nitrogen/dry ice (for cryogenic temperatures). Challenges include: large temperature differences causing local freezing/blockages; increased material viscosity hindering mass transfer; temperature measurement lag; and refrigerant leaks. Temperature control relies on jackets, internal coils, and external baths; low-temperature alarms and backup units are required to prevent temperature spikes caused by cooling failure.
42. What are the key operational points and H₂ safety considerations for high-pressure reactors (hydrogenation reactors)? Hydrogen gas used for hydrogenation is flammable and explosive (explosive limits: 4%–75%). Key points: Purge air and solvent vapors with nitrogen until the atmosphere is non-flammable in the presence of H₂; perform leak testing; control temperature, pressure, and H₂ flow rate; prevent catalyst auto-ignition (e.g., Raney nickel can spontaneously ignite upon contact with air); and purge with inert gas after depressurization before discharging the product. Prohibit open flames and static electricity on-site; ensure gas detection and emergency ventilation systems are fully functional.
43. How is the scale-up effect evaluated for batch reactions (moving from lab-scale to pilot-scale to production-scale)? Focus on the "three transfers": heat transfer (specific surface area decreases upon scale-up, leading to potential heat accumulation), mass transfer (uneven agitation, increased mixing time), and fluid flow (dead zones, back-mixing). Use dimensionless numbers (mixing time, Nusselt number, Damköhler number) and pilot-scale data for extrapolation; conduct calorimetry and runaway reaction assessments for exothermic or high-risk reactions before scaling up, rather than relying on simple linear scale-up.
44. Emergency response for agitation stoppage or failure? Immediately stop heating and feed addition; determine the cause (power outage, motor trip, or mechanical jam). If the issue is short-term and recoverable, restart and verify operation; if it is long-term and the reaction is exothermic, initiate emergency cooling or termination procedures, and transfer materials or depressurize the vessel if necessary. Critical reactors should be equipped with dual power supplies (or emergency generators) and interlocks linked to agitation failure.
45. The "cool down – depressurize – terminate" three-step process for handling abnormal rises in reactor temperature and pressure? Step 1: Maximize cooling (activate jackets, internal coils, or emergency cooling sources; lower ambient temperature). Step 2: Controlled depressurization (vent via safety valves, rupture discs, or emergency vents to a safe containment system). Step 3: Add a terminator (e.g., polymerization inhibitor or quenching agent) to halt the reaction. Execute these steps in a tiered manner according to the emergency plan, prioritizing cooling and avoiding uncontrolled discharges.
46. Key operational points for aqueous/organic phase separation and decanting? Allow sufficient time for the two phases to separate clearly. Before draining, verify the interface (via sight glass, specific gravity, or sampling); drain the heavy phase (usually the aqueous phase) first, followed by the light phase, slowing down as the interface approaches to prevent phase entrainment. If emulsification occurs, use demulsifiers, apply heat, or allow for settling; do not force rapid separation, as this causes phase mixing. Operate separatory funnels or bottom-discharge valves steadily.
47. What are the key points for controlling speed during crystallization (cooling, anti-solvent, or evaporation)? Crystallization determines crystal form, particle size, and purity. For cooling crystallization, control the cooling rate (rapid cooling yields fine crystals or inclusions; slow cooling yields large crystals). For anti-solvent crystallization, control the addition rate and supersaturation level. For evaporative crystallization, control the evaporation intensity. Supersaturation must be "moderate" to avoid explosive nucleation. Agitation, seeding, and crystal aging time are all part of the process window.
48. How are centrifugation, filtration, and washing sequenced, and how is equipment cleaned? Reaction mixture → solid-liquid separation (filtration/centrifugation) → washing to remove mother liquor impurities → drying. During sequencing, prevent material crystallization and pipeline blockages, and avoid decomposition caused by prolonged retention of solids. Clean equipment (centrifuges, filter cloths, pipelines) for each batch according to the clearance SOP; do not allow hazardous materials (e.g., diazonium salt filter cakes) to accumulate—dispose of them promptly.
49. What key parameters should be recorded in the batch record? Batch number, material lot numbers and quantities, charging sequence and timing, temperature/pressure profiles, addition rates, agitation speeds, in-process control values (pH, specific gravity, etc.), sampling and test results, equipment IDs, operator/reviewer details, and deviations and their resolutions. Records must be real-time, accurate, and traceable; they serve as the "black box" for quality and safety.
50. What is "process discipline"? How should deviations from the process be handled? Process discipline means strictly adhering to approved SOPs and not altering parameters without authorization. Deviations (such as temperature excursions or incorrect material additions) must be immediately reported, recorded, and assessed for impact; products must be quarantined if necessary, and a deviation investigation (CAPA) initiated. "Making do" or cutting corners is strictly prohibited. Lax discipline is the root cause of accidents and quality issues.
