Process Design Interview Questions

A Process Flow Diagram (PFD) is a schematic representation of a process showing the major equipment, process streams, and basic control philosophy. It contains: major equipment with names and numbers, process piping (no utility piping), stream flow rates, compositions, temperatures and pressures, major control loops, and stream data tables. PFDs are used for process understanding, design calculations, and communication between disciplines. They differ from P&IDs by showing less instrumentation detail and excluding utility systems, minor equipment, and detailed piping.

A Piping and Instrumentation Diagram (P&ID) is a detailed engineering drawing showing all piping, equipment, instrumentation, and control systems. Key differences from PFD: includes all piping (process and utility), shows all valves (manual, control, safety), displays complete instrumentation with tag numbers, includes equipment details (nozzles, internals), shows electrical/signal connections, and uses standardized symbols (ISA). P&IDs are used for construction, operation, and maintenance. They are the primary documents for plant detailed engineering, providing information needed for procurement, installation, and commissioning.

A material balance accounts for all material entering, leaving, and accumulating in a system: Input + Generation = Output + Consumption + Accumulation. For steady-state without reaction: Input = Output. Material balances are fundamental because they: determine equipment sizes (flow rates, volumes), establish stream compositions, identify recycle requirements, validate process feasibility, and form the basis for energy balances. They apply to overall process and individual units. Proper material balance closure (mass in equals mass out) is essential for reliable design; closure errors indicate calculation mistakes or missing streams.

Standard equipment numbering follows the format: Area-Type-Sequence (e.g., 100-P-101). Area number indicates plant section (100, 200, etc.). Type letter indicates equipment category: P=pump, V=vessel, T=tower/column, E=heat exchanger, C=compressor, R=reactor, F=furnace, D=drum, TK=tank. Sequence number is unique within area and type. Additional suffixes: A/B for parallel equipment, R for spare. This system provides: quick equipment identification, location indication, consistent referencing across documents, and maintenance tracking. Companies may have variations, but the principle of area-type-sequence is standard.

Design pressure is the pressure used for equipment mechanical design, typically set at operating pressure plus a margin (10-15% or minimum 25 psi above normal operating). Design temperature is the metal temperature used for material selection and thickness calculation, set at maximum operating temperature plus margin (usually 25-50 degrees F). These values account for: process upsets, safety valve settings, startup/shutdown conditions, and normal variations. Higher design conditions increase equipment cost and weight. Values must consider all operating cases including abnormal conditions. ASME codes specify minimum requirements for vessels and piping.

ISA S5.1 defines standard instrumentation symbols. Tag format: First letter = measured variable (F=flow, P=pressure, T=temperature, L=level), succeeding letters = function (I=indicator, C=controller, V=valve, A=alarm, T=transmitter). Examples: FIC=flow indicating controller, LT=level transmitter, PSV=pressure safety valve, TI=temperature indicator. Symbols: circle=field-mounted, square with diagonal=DCS/PLC function, triangle=shared display, dashed line=software link. Instrument lines: solid line=process connection, dashed=electrical signal, dash-dot-dash=pneumatic signal. Understanding these symbols is essential for reading and creating P&IDs.

An energy balance accounts for all energy entering, leaving, and stored in a system: Energy In + Heat Added = Energy Out + Work Done + Accumulation. For steady-state: sum of inlet enthalpies + Q = sum of outlet enthalpies + W. Energy balances are linked to material balances because: flow rates from material balance multiply with specific enthalpies, compositions affect enthalpy calculations, and phase changes determined by material balance affect latent heats. Energy balances are used to size heat exchangers, heaters, coolers, and calculate utility requirements. Always complete material balance first as it provides the flow rates needed for energy balance.

A pressure relief valve (PRV) or pressure safety valve (PSV) is a device that automatically releases fluid when system pressure exceeds a set value. Purpose: protect equipment from overpressure that could cause rupture, protect personnel from equipment failure, meet regulatory requirements (ASME, API), and prevent environmental releases from equipment failure. Key specifications: set pressure (relief opening point), overpressure (allowable above set pressure, typically 10%), and relieving capacity (sized for worst-case scenario). Relief scenarios include: fire, blocked outlet, cooling failure, tube rupture, and runaway reaction. Every pressurized system must have adequate overpressure protection.

Block valves (isolation valves) are manual valves used to isolate equipment or sections of piping for maintenance, safety, or operational purposes. Common types: gate valves (full open/close, low pressure drop), ball valves (quick operation, tight shutoff), and globe valves (also for throttling). Placement rules: at equipment nozzles (inlet and outlet), at battery limits, around control valves (for bypass and maintenance), at branch connections, at process unit boundaries, and upstream of relief devices. Double block and bleed for hazardous isolation. Proper valve placement enables safe maintenance without full plant shutdown.

Vessel sizing depends on function and process requirements. Key factors: residence time (volume = flow rate x time), phase separation (settling velocity and area requirement), surge capacity (buffer for downstream upset), liquid level ranges (normal, high, low), vapor space (for disengagement), and internals requirements (distributors, demisters). Design considerations: L/D ratio (typically 2-4 for horizontal, 2-5 for vertical), nozzle sizes (velocity limits), mechanical constraints (thickness, weight), and access for maintenance. Process engineer provides functional requirements; mechanical engineer completes detailed design. Always document design basis for future reference.

Globe valve: most common control valve, good throttling capability, uses linear or equal-percentage trim. Ball valve: high capacity, tight shutoff, good for on-off and moderate throttling. Butterfly valve: compact, light weight, good for large lines and moderate control. Three-way valve: mixing or diverting service. Eccentric plug: handles slurries and viscous fluids. Selection factors: pressure drop available, required rangeability, fluid characteristics, shutoff class, and cost. Trim characteristics: linear (flow proportional to stroke), equal percentage (same percentage change per stroke increment), and quick opening (high flow at low stroke).

Centrifugal pump sizing requires: flow rate (normal, maximum, minimum operating ranges), total dynamic head (TDH = discharge pressure - suction pressure + friction losses + elevation), fluid properties (density, viscosity, vapor pressure), and NPSH available vs. required. Process engineer provides: operating conditions, fluid properties, piping arrangement for NPSH calculation, and control scheme. Pump engineer selects: pump type, impeller size, driver power, and materials. Key outputs: pump curve, efficiency, power consumption, and NPSH margin. Always include margin (10-20% on head) for uncertainties and future capacity.

HAZOP (Hazard and Operability Study) is a systematic technique for identifying hazards and operability problems in process designs. Method: apply guide words (No, More, Less, Reverse, etc.) to process parameters (flow, pressure, temperature, etc.) at each node to identify deviations, their causes and consequences. Team includes: process engineer, operations, instrumentation, safety, and mechanical. Timing: typically conducted at P&ID completion stage, before detailed engineering. HAZOP identifies: hazards requiring additional safeguards, operability issues affecting reliability, and design improvements. Results documented in HAZOP worksheets with recommendations tracked to completion.

Shell and tube: most common, versatile, handles high pressures and temperatures, TEMA standards. Plate heat exchangers: compact, high efficiency, good for clean fluids, easily expandable. Air coolers (fin-fan): uses air for cooling, no cooling water needed, higher capital but lower operating cost. Double pipe: simple, small duties, easy maintenance. Spiral: handles slurries and fouling fluids. Plate-fin: compact, cryogenic applications. Selection factors: temperature and pressure, fouling tendency, material compatibility, space constraints, and cost. Shell and tube dominates in refineries and chemical plants; plates common in food and pharmaceutical industries.

Capital cost (CAPEX) is the one-time investment to build the facility: equipment purchase, installation, piping, instrumentation, civil/structural, electrical, buildings, and project services (engineering, construction management). Operating cost (OPEX) is ongoing expense to run the plant: raw materials, utilities (steam, power, cooling water), labor, maintenance, catalysts and chemicals, and overhead. Design decisions affect both: larger equipment increases CAPEX but may reduce OPEX (better efficiency). Total cost evaluation uses: payback period, NPV (net present value), or IRR (internal rate of return) to balance CAPEX and OPEX over project life.

Process simulation software (Aspen Plus, HYSYS, PRO/II) performs rigorous material and energy balances. Usage in design: conceptual design (flow scheme development, option screening), basic engineering (equipment sizing, operating conditions optimization), detailed engineering (design case verification, control philosophy), and operation (troubleshooting, debottlenecking). Key capabilities: thermodynamic property calculation, unit operation models, reaction kinetics, economic evaluation, and optimization. Best practices: validate against plant data, document assumptions, use appropriate thermodynamic models, and maintain model revision control. Simulation output feeds into equipment sizing, P&ID development, and safety studies.

Distillation column sizing involves: (1) Thermodynamic modeling - select VLE model, validate against data. (2) Stage calculation - determine minimum stages (Fenske), minimum reflux (Underwood), actual stages (rule: 1.2-1.5 x minimum at 1.2-1.5 x minimum reflux). (3) Feed location - optimize for energy. (4) Hydraulic design - column diameter from flooding correlation (Fair or similar), tray or packing selection based on system. (5) Internals design - tray spacing (typically 24 inch), downcomers, weirs for trays; HETP and bed depth for packing. (6) Condenser and reboiler sizing. (7) Control scheme development. Simulation provides heat duties and profiles; vendor finalizes mechanical details.

Relief system sizing: (1) Identify scenarios - fire, blocked outlet, cooling failure, runaway reaction, tube rupture, control valve failure, thermal expansion. (2) Calculate relief load for each scenario - mass flow rate and required orifice area using API 520/521. (3) Select controlling scenario (largest relief rate). (4) Size relief device - orifice area from API equations considering back pressure. (5) Design inlet piping - pressure drop less than 3% of set pressure. (6) Design outlet piping - calculate back pressure, ensure below device limits. (7) Disposal system - flare, scrubber, or atmosphere depending on fluid. Document all scenarios and calculations for future verification and regulatory compliance.

Line sizing criteria balance velocity, pressure drop, and economics. Velocity limits: liquid 1-3 m/s (avoid erosion), gas 20-40 m/s (avoid noise/vibration), two-phase uses erosional velocity formula. Pressure drop: typically 0.1-0.5 psi/100ft for liquids, higher acceptable for gases. Economic optimization: larger pipe = lower operating cost (less pumping) but higher capital. Specific applications: pump suction - low velocity, minimize losses for NPSH; relief headers - API 520 back pressure limits; gravity flow - adequate slope and head. Line size affects: control valve sizing (pressure drop available), pump head, and compressor discharge pressure.

Control philosophy development: (1) Define control objectives - safety, product quality, throughput, efficiency. (2) Identify key variables - manipulated and controlled. (3) Develop regulatory control scheme - level, flow, pressure, temperature controllers with appropriate pairing. (4) Address interactions - decoupling or proper loop tuning. (5) Define operating modes - startup, normal, turndown, shutdown. (6) Safety interlocks - ESD triggers and responses. (7) Advanced control - cascades, ratio control, feed-forward where beneficial. (8) Document in control narrative. Key principles: close level loops with flow, close pressure with vent/make-up, pair fast variables with slow, and provide adequate rangeability for all operating cases.

Heat exchanger sizing steps: (1) Define thermal duty - from energy balance, heat curves for phase change. (2) Assign fluids to shell/tube side - corrosive to tube, high pressure to tube, dirty to shell (easier cleaning), condensing usually shell side. (3) Estimate LMTD - log mean temperature difference with correction factor F. (4) Assume overall U based on fluid types (tables available). (5) Calculate preliminary area: Q = U x A x LMTD. (6) Select tube parameters - diameter, length, pitch, material. (7) Calculate individual heat transfer coefficients - tube side (Sieder-Tate), shell side (Kern or Bell-Delaware). (8) Iterate U and area. (9) Check pressure drops against allowable. (10) Specify TEMA type and mechanical details.

Separator design: (1) Define inlet conditions - flow rates, compositions, pressure, temperature. (2) Flash calculation to determine L/V split. (3) Vapor disengagement - size for droplet settling using Souders-Brown correlation: V_max = K x sqrt((rho_L - rho_V)/rho_V). K depends on demister type and pressure. (4) Liquid residence time - typically 2-5 minutes for process control, longer for three-phase. (5) Surge volume - based on upset duration. (6) Select orientation - vertical for high L/V ratio, horizontal for low L/V or three-phase. (7) Set level ranges - LLLL, LLL, NLL, HLL, HHLL with appropriate spans. (8) Size nozzles for inlet momentum and outlet velocity limits. Add internals as needed (inlet device, demister, vortex breaker).

Compressor selection factors: Flow rate - centrifugal for high flow (>1000 cfm), reciprocating for low flow; positive displacement for accurate metering. Pressure ratio - single stage centrifugal up to 3:1, reciprocating handles higher ratios. Gas properties - molecular weight affects centrifugal head, corrosive gases need special materials or diaphragm type. Efficiency requirements - centrifugal 70-80%, reciprocating 75-90% at design point. Turndown - centrifugal limited to 70% (surge), reciprocating more flexible. Reliability - centrifugal fewer moving parts, reciprocating needs more maintenance. Other factors: driver availability, space, noise, pulsation dampening needs. Detailed selection involves head/flow calculations and vendor curves.

Recycle loop considerations: Purge requirements - prevent buildup of inerts or byproducts; calculate purge rate from material balance at steady state. Accumulation dynamics - recycle takes time to reach steady state, affects startup. Snowball effect - small changes in feed can cause large swings in recycle; stabilize with appropriate control. Equipment sizing - base on total flow including recycle, not fresh feed. Heat integration - recycle streams may need heating/cooling. Composition tracking - trace components may accumulate to problematic levels. Simulation - converge recycle loops using tear streams with appropriate convergence method. Control - typically control fresh feed, let recycle find its level, or use total flow control.

Main utility systems: Steam - multiple pressure levels (HP, MP, LP), used for heating, driving turbines; sized from steam balance. Cooling water - removes process heat, recirculated with cooling towers; sized for total duty. Instrument air - clean, dry air for pneumatic instruments; typically 6-7 barg. Nitrogen - inert blanketing, purging; from plant or cryogenic supply. Plant air - general service, not instrument quality. Fuel gas - fired heaters, flares. Power - purchased or generated; emergency generation for critical loads. Each utility requires: generation/supply capacity, distribution network, control and monitoring, and consumption tracking. Utility balances ensure adequate supply for all operating cases.

Equipment datasheets capture all design requirements for procurement. Contents vary by equipment type but include: Equipment identification (tag, service, location). Process conditions (normal, design, maximum, minimum). Fluid properties (composition, physical properties). Performance requirements (duty, efficiency, capacity). Mechanical requirements (materials, corrosion allowance, design codes). Connections (nozzle sizes, ratings, orientations). Instrumentation requirements (connections, protection). Utility requirements (power, steam, cooling water). Special requirements (noise, vibration, layout). Datasheets are prepared by process engineer (process requirements), completed by mechanical engineer (mechanical details), and used by procurement for bidding. Standard forms ensure complete information transfer.

Process optimization methods: Parametric studies - vary one parameter to find optimum (e.g., reflux ratio vs. stages). Design of experiments (DOE) - systematic exploration of multiple variables. Pinch analysis - optimize heat integration. Economic optimization - minimize total cost (CAPEX + OPEX present value). Mathematical optimization - use simulation optimizer with objective function (e.g., minimize steam consumption subject to product spec). Common optimization variables: operating pressures, reflux ratios, reaction temperatures, recycle ratios. Constraints: product specifications, equipment limits, safety requirements. Tools: Excel solver, Aspen optimizer, GAMS for complex models. Always verify optimum is robust to parameter uncertainties.

Safety Integrity Level (SIL) rates the required reliability of a Safety Instrumented Function (SIF). Levels 1-4 with increasing reliability (SIL 4 = highest). Determination methods: Risk graph - qualitative assessment of consequence, exposure, avoidance possibility. LOPA (Layer of Protection Analysis) - quantitative, calculates risk reduction required based on initiating event frequency and consequence severity. Independent Protection Layers (IPLs) are credited. SIL requirement = risk gap not covered by non-SIS protections. Higher SIL requires: redundancy, more frequent testing, rigorous design. SIL 3+ is rare in process industry. IEC 61511 provides guidance for process sector. SIL assignment affects instrumentation specification, voting logic, and lifecycle management costs.

Battery limit defines the boundary of a process unit for design purposes. Battery limit conditions specify stream parameters at the boundary: composition, flow rate, temperature, pressure, and phase. Considerations: Coordination - agree conditions with upstream/downstream units early. Margin - include margin for variations (typically 10% on flow). Specifications - clearly state guaranteed vs. expected values. Turndown - specify range of acceptable conditions. Utilities - define available utility conditions (steam pressure, cooling water temperature). Quality - specify impurity limits. Documentation - battery limit conditions appear on PFD and in process description. Changes to battery limits can have significant project impact, so establish and freeze early in design.

Reactor sizing factors: Reaction kinetics - rate expression determines volume needed for conversion. Residence time - varies from seconds (fast reactions) to hours (slow reactions). Heat management - exothermic reactions need cooling capacity, endothermic need heating. Mixing requirements - fast reactions need good micromixing. Phase contacting - multiphase reactions need appropriate interfacial area. Catalyst considerations - loading, activity decay, regeneration. Selectivity - operating conditions to maximize desired product. Pressure effects - affects equilibrium and reaction rate. Safety - runaway potential, containment requirements. Scale-up - pilot data correlation to full scale. Mechanical constraints - maximum vessel size, L/D ratio, internals. Output: reactor volume, dimensions, heat transfer area, catalyst quantity.

Flare system components: Collection system - headers from relief valves (separate hot/cold if needed). Knockout drum - remove liquids before flare. Seal drum/molecular seal - prevent flashback. Flare stack - elevated or ground flare. Flare tip - smokeless operation, pilot system. Design considerations: Header sizing - API 521, consider choked flow, back pressure limits. Knockout drum - sized for largest liquid case, residence time 20-30 seconds. Purge gas - continuous flow to prevent air ingress. Steam/air injection - for smokeless combustion. Radiation - maintain safe distance from equipment and personnel (API 521 F-factor). Height - typically 20-100m depending on radiation and dispersion requirements. Pilots - redundancy for reliability.

Turndown ratio is the ratio of maximum to minimum operating capacity. Important because: Operating flexibility - plants rarely run at exactly design capacity. Startup/shutdown - need to operate at low rates. Market variations - adjust production to demand. Process stability - maintain control at reduced rates. Equipment considerations: Heat exchangers - reduced flow affects velocity and fouling. Distillation - trays may weep, packing may maldistribute. Pumps - minimum flow to prevent damage. Control valves - sized for rangeability, typically 50:1. Compressors - surge limits minimum flow. Furnaces - minimum firing for stable combustion. Design practice: specify turndown requirement (typically 50-70% of design), verify equipment operates satisfactorily, and include recirculation or spillback where needed.

Emergency Shutdown System rapidly brings process to a safe state during emergency. Components: sensors (high pressure, toxic gas, fire detection), logic solver (safety PLC or relay-based), and final elements (shutdown valves, trip signals). Design basis: define ESD levels (unit, plant, area), identify critical equipment requiring trip, determine required response time, and specify failure position (fail-safe). Logic: typically hardwired for fastest response, use voting (1oo2, 2oo3) for reliability vs. spurious trip balance. Key principles: independent from process control system, de-energize to trip (fail-safe), bypass management procedures, and regular testing. Standards: IEC 61511 for process safety systems. Documentation includes cause and effect matrix and ESD logic diagrams.

Equipment cost estimation methods: Vendor quotes - most accurate but time-consuming. Capacity factoring - scale from known cost using exponent: Cost_2 = Cost_1 x (Capacity_2/Capacity_1)^n where n is typically 0.6 for many equipment types. Cost curves - published correlations (Peters & Timmerhaus, Towler & Sinnott). Software - ICARUS, Aspen Economic Analyzer. Historical data - company cost databases adjusted for time and location. Adjustment factors: material factor (SS vs. CS), pressure factor, location factor (Gulf Coast index), and inflation (CEPCI or similar). Equipment cost is typically 40-60% of total installed cost. Factor methods (Lang factor, Hand factors) estimate total plant cost from equipment cost. Accuracy: early estimate +/- 30-50%, detailed +/- 10-15%.

Plot plan factors: Safety spacing - API 2510 and company standards for distances between hazardous equipment. Fire protection - access for firefighting, firebreaks. Process flow - minimize long pipe runs, logical equipment arrangement. Operability - access for operation, sample points, instrumentation. Maintenance - crane access, tube pull space, valve access. Expansion - space for future additions. Environmental - emissions control, drainage, containment. Prevailing wind - locate hazardous areas downwind of control room and occupied buildings. Utilities - efficient routing of power, steam, cooling water. Constructability - modular fabrication, heavy lift requirements. Area classification - electrical hazardous area zones. Typical arrangement: process units in center, tanks on periphery, control room upwind, flare remote.

Inherently safer design eliminates or reduces hazards rather than controlling them. Principles: Minimize - reduce hazardous inventory (smaller reactors, just-in-time delivery). Substitute - use less hazardous materials (water-based vs. solvent-based). Moderate - use less hazardous conditions (lower temperatures, pressures, dilution). Simplify - eliminate complexity that can lead to errors (fewer interconnections, passive vs. active safeguards). Application: ISD hierarchy considers inherent solutions before add-on safety systems. Examples: continuous vs. batch processing (smaller inventory), using safer reaction routes, designing for safe containment vs. relying on relief systems. HAZOP and LOPA should evaluate ISD opportunities. While often more expensive initially, ISD reduces lifecycle costs and risks.

Dynamic simulation models time-dependent behavior. Applications: Control system design - tune controllers, test cascade/feedforward strategies before commissioning. Startup/shutdown - develop procedures, identify constraints and timing. Relief system verification - confirm transient loads don't exceed relief capacity. Safety system testing - verify ESD logic and timing. Training simulators - operator training on realistic model. Upset analysis - evaluate system response to disturbances. Debottlenecking - identify dynamic limitations. Tools: Aspen Dynamics, HYSYS Dynamics, gPROMS. Considerations: requires good steady-state model first, need equipment volumes and dynamics, control system details, and validated against plant where possible. More effort than steady-state but essential for complex or safety-critical applications.

Multicomponent distillation alternatives: Direct sequence - separate lightest component first, then next lightest. Indirect sequence - separate heaviest first. Complex configurations - side draws, dividing wall columns, thermally coupled. Evaluation criteria: Energy consumption - compare condenser and reboiler duties. Capital cost - number of columns, total stages, heat exchangers. Product specifications - achievability with each scheme. Operability - control complexity, startup difficulty. Selection methods: Heuristic rules (remove dominant component first, do difficult separations last). Shortcut methods (Underwood equations for minimum vapor). Optimization - formulate as MINLP problem. Dividing wall columns save capital and energy (up to 30%) but are more complex to operate. Always verify with rigorous simulation.

Heat integration methodology: (1) Data extraction - identify all streams needing heating/cooling with temperatures, duties, and heat capacity flow rates. (2) Targeting - composite curves and grand composite curve determine minimum utilities at chosen delta_T_min. (3) Select delta_T_min - smaller gives more recovery but higher area; optimize based on economics. (4) Pinch analysis - identify pinch point, design above and below pinch separately. (5) Network design - match streams, design heat exchangers, consider practical constraints (control, safety, operation). (6) Utilities - place heaters above pinch, coolers below pinch. Tools: Aspen Energy Analyzer, SPRINT. Practical considerations: don't cross the pinch, avoid transferring heat across pinch, consider startup flexibility, and maintain some cross-exchangers for feed-effluent heat recovery.

Modular design packages equipment into transportable modules built offsite. Considerations: Transport limits - modules sized for road/ship transport (typically <15m wide, <35m long, <500 ton). Equipment arrangement - maximize density within module envelope. Interface management - pipe connections, electrical, instrument signals at module boundaries clearly defined. Structural design - withstand transport loads, lifting stresses. Piping design - minimize field welding, pre-test in shop. Plot plan - modules arranged for crane access and hook-up. Schedule - parallel fabrication reduces project duration. Cost trade-off - higher engineering cost offset by lower field labor, faster schedule. Best for: remote locations, repetitive designs, and skilled labor shortage areas. Documentation must clearly distinguish shop vs. field scope.

Debottlenecking methodology: (1) Define target - capacity increase, quality improvement, efficiency gain. (2) Data gathering - actual operating data, equipment capacities, hydraulic surveys. (3) Model validation - update simulation to match current operation. (4) Bottleneck identification - systematic screening of each equipment: heat exchangers (clean coefficients vs. fouled), columns (approach to flooding), reactors (conversion, temperature limits), pumps/compressors (head, power), relief systems (relieving capacity). (5) Solution development - operational changes first (controls, procedures), then minor modifications, finally major changes. (6) Economic evaluation - cost vs. benefit. (7) Implementation - manage risk during modifications. Common bottlenecks: heat transfer (fouling), column hydraulics, compressor limits, relief capacity. Low-cost gains often possible through optimization before capital investment.

Revamp design modifies existing plant; grassroots builds new. Revamp challenges: Equipment constraints - must work with existing equipment, sizes, materials. Documentation - original drawings may be incomplete or inaccurate; field verification essential. Tie-ins - minimize shutdown time, plan hot/cold cuts carefully. Relief systems - verify existing capacity for new conditions. Utilities - check available capacity for increased loads. Constructability - work in operating plant, limited space, confined areas. Operating constraints - often must maintain production during modification. HAZOP scope - focus on changes but consider interaction with existing. Advantages of revamp: lower capital (utilize existing equipment), faster implementation, lower risk (known site conditions). Grassroots advantages: optimal design without constraints, clean documentation, easier construction. Revamps often compromise technical optimum for practical implementation.

MPC design steps: (1) Define scope - identify controlled and manipulated variables, constraints. (2) Dynamic modeling - step testing to identify process gains and dynamics, typically 1-3 hours per MV-CV pair. (3) Model identification - fit first or second order plus dead time models. (4) Controller configuration - set horizons (control, prediction), move suppression, constraint handling. (5) Commissioning - implement in simulation first, then gradual rollout with close monitoring. (6) Tuning - adjust weights to balance competing objectives. Benefits: handles interactions, constraints, dead time, and optimizes operation. Common applications: distillation (maximize throughput at product specs), furnaces (minimize fuel with tube constraints), compressors (surge avoidance). Challenges: model maintenance, operator acceptance, software costs. Typical payback 6-18 months for well-selected applications.

Two-phase flow challenges: Flow regime identification - stratified, slug, annular, mist flow have different characteristics; use Baker chart or similar. Pressure drop - higher than single-phase; methods include Lockhart-Martinelli, Beggs and Brill for pipes; must consider elevation effects. Slug flow - cyclic surging, structural vibration, control upsets; avoid or mitigate with slug catchers. Line sizing - consider maximum velocity (erosional), minimum velocity (liquid accumulation). Slug catcher sizing - accumulate liquid from slug while maintaining continuous flow to downstream. Control - level control on separators challenged by slugging. Safety - relief sizing for two-phase more complex (HEM, omega method). Special considerations: risers (liquid holdup), downcomers (flooding), heat exchangers (maldistribution). Use specialized software (OLGA, PIPEPHASE) for complex systems.

CUI prevention design: Material selection - use corrosion-resistant alloys for susceptible temperature ranges (0-175C for carbon steel under insulation). Coating - thermal spray aluminum (TSA), epoxy-phenolic coatings under insulation. Insulation selection - closed cell (foam glass) prevents water ingress better than open cell. Weather barrier - high integrity jacketing, proper sealing at terminations and penetrations. Design details - avoid water traps, slope horizontal runs, seal penetrations. Drainage - provide weep holes at low points. Inspection accessibility - removable cladding sections at high-risk locations. Temperature screening - identify equipment in CUI temperature range for enhanced protection. Process side considerations - intermittent operation increases risk (thermal cycling). Inspection program - use thermography, neutron backscatter, or targeted insulation removal for monitoring.

Value engineering systematically improves value (function/cost ratio). Process: Assemble cross-functional team. Function analysis - identify primary and secondary functions of each element. Brainstorm alternatives - different ways to achieve same function. Evaluate alternatives - technical feasibility, cost, schedule, risk. Select and develop best options. Present recommendations with quantified savings. Areas for focus: equipment simplification (combine functions), material optimization (right material for service), standardization (reduce variety), packaging (modular design), plot optimization (reduce piping), process simplification (eliminate unnecessary steps). Timing: most effective in early design when flexibility exists. Target: typically 10-20% cost reduction achievable. Documentation important - record rejected alternatives to avoid re-evaluation. Success requires management commitment and proper incentives.

PSM elements affecting design: Process Hazard Analysis - HAZOP, What-If studies identify design changes needed. Mechanical Integrity - design for inspectability, specify correct materials. Management of Change - design modifications require MOC review. Pre-startup Safety Review - verify design matches drawings, procedures ready. Operating Procedures - design must support safe operation. Process Safety Information - complete documentation of design basis. PSI requirements: chemical hazards, technology basis, equipment design limits. Design implications: inherent safety considerations, adequate instrumentation for safe operation, maintainability, compliance with codes/standards, clear design basis documentation. PSM compliance is not optional - OSHA requirements for facilities with threshold quantities of hazardous chemicals. Design engineer must understand PSM interface requirements.

Scale-up principles: Geometric similarity - maintain ratios (L/D, internals dimensions) where important. Kinematic similarity - same velocity ratios, flow patterns. Dynamic similarity - match dimensionless numbers (Re, Nu, Da) where governing. Heat transfer scaling - surface/volume ratio decreases; may need internal cooling. Mixing scaling - maintain power per volume or tip speed depending on application. Reaction scaling - residence time distribution, heat/mass transfer effects. Not everything scales linearly - identify critical parameters. Scale-up factors: surface area scales as L^2, volume as L^3. Pilot requirements: operate at commercial conditions where possible, include all process steps, measure everything. Risk mitigation: intermediate scale (demo plant), parallel trains, conservative design margins. Documentation of scale-up basis essential for troubleshooting.

FEED deliverables: Process - PFDs (final), P&IDs (issued for design), heat and material balances, process description, equipment datasheets, utility summary, relief/flare study. Mechanical - equipment specifications, vessel drawings (outline), piping specifications. Electrical - load list, single line diagrams. Instrumentation - instrument index, control narrative, cause and effect. Civil - plot plan, soil investigation report. Cost - Class 3 estimate (+/- 10-15%), project execution plan. Schedule - detailed project schedule. HSE - HAZOP reports, environmental permits. FEED quality directly impacts project success - incomplete FEED leads to changes during detailed engineering. Typical FEED effort is 10-15% of total engineering hours. Gate review before proceeding to detailed engineering should confirm complete scope definition and cost certainty.

Technical risk assessment: Identification - technology maturity, scale-up uncertainty, novel equipment, challenging materials, tight specifications. Quantification - probability of occurrence, impact on cost/schedule, combine for risk ranking. Categories: Technology risk - unproven processes, first-of-a-kind equipment. Performance risk - will equipment meet specifications? Integration risk - interfaces between systems. Schedule risk - long-lead equipment, permit timing. Mitigation strategies: Additional testing - pilot work, prototype testing. Design margin - conservative sizing, material selection. Redundancy - spare equipment, backup systems. Phasing - implement in stages, allow learning. Fallback options - alternative designs if primary fails. Tracking - risk register maintained throughout project with regular review. Risk response should be proportional to risk level - major risks need active mitigation, minor risks can be accepted with monitoring.