Book summary “Chemical engineering design”: summary

Introduction

You face the complex challenge of bridging vague customer needs and predicted financial outcomes when designing a chemical plant. You must translate initial chemical concepts into actionable specifications for safe and economical process facilities. Gavin P. Towler provides a comprehensive methodology for managing this transition from project inception through detailed engineering.

Chemical engineering design bridges vaguely defined customer needs and predicted financial outcomes by creating actionable specifications for safe, economical process plants.

The book guides you through project organization, flowsheet development, energy management, process simulation, cost estimation, plant safety, and equipment design. How do you structure a project and draw process flow diagrams? How do you apply pinch technology and estimate capital costs? How do you prevent catastrophic failures and select reliable equipment?

1. Process Design and Project Organization

Chemical engineering design bridges customer needs and financial outcomes through structured project phases, standard documentation, and design margins. Design is a creative and iterative activity. You must balance external constraints like physical laws and regulations with internal constraints like material choices and process conditions. Multiple plausible designs usually exist for any single problem.

Industrial projects are categorized by novelty. Plant modifications and additions account for about half of design activity. New production capacity and established process sales account for 45 percent. Novel processes developed from laboratory research account for less than 5 percent.

Projects proceed through two main phases:

• Phase 1 is process design, moving from initial process selection to issuing process flowsheets and equipment specification, handled mainly by chemical engineers.
• Phase 2 is plant design, involving detailed mechanical, structural, civil, and electrical design handled by multidisciplinary specialist groups.

Effective communication across these specialist groups requires standardized documentation. You organize project documentation into general correspondence, calculation sheets, drawings, specification sheets, health safety and environmental information, and purchase orders. Every document receives a code number for easy retrieval. A techno-economic analysis report summarizes preliminary technical and economic evaluations for new products or process technologies.

Engineering codes and standards provide uniform rules for materials, testing, preferred sizes, design methods, and safety practices. Standards ensure interchangeability and streamline component selection, utilizing bodies like the American National Standards Institute, the American Petroleum Institute, the American Society for Testing Materials, the American Society of Mechanical Engineers, and the Tubular Exchanger Manufacturers Association.

You incorporate design factors to account for uncertainties in data and approximations in calculations. Structural design typically uses a factor of around 4 on tensile strength or about 2.5 on 0.1 percent proof stress. Process design usually increases average stream flows by typically 10 percent to provide operational flexibility.

Successful product development requires identifying customer requirements, market segments, and the distinction between proximate and ultimate customers. Market research must capture latent needs and willingness to pay.

UOP developed a renewable jet fuel based on hydrotreated vegetable oils that had to meet all standard ASTM specifications for jet fuels.

You can uncover customer requirements by asking specific questions:

• When and why do you use this product?
• What do you like about the existing products?
• What do you dislike about the existing products?
• What issues do you consider when purchasing the product?
• What improvements would you make to the product?

2. Flowsheet Development and Process Synthesis

Flowsheet development requires precise PFD preparation, careful analysis of continuous and batch options, and rigorous conceptual synthesis steps.

The process flow diagram, or PFD, serves as the primary engineering document showing equipment arrangement, stream connections, flow rates, compositions, and operating conditions. You rely on this document for specialist design groups, operating manuals, operator training, plant start-up performance comparisons, and future plant revamps. A complete process flowsheet must contain specific essential items including all process equipment, control valves, stream compositions, total flow rates, temperatures, pressures, and enthalpies. Flowsheet layout must mirror proposed plant geometry with numbered streams, tabulated data, and three to four significant figures of precision. You must never leave small or trace flows blank or set to zero if they have catalytic or materials impacts; instead, you should mark them as TRACE or indicate them in ppm.

A generalized chemical manufacturing process consists of six sequential stages: raw material storage, feed preparation, reaction, product separation, purification, and product storage. Continuous processes are more economical for large-scale production with high plant utilization, whereas batch processes are preferred for small-volume, high-value specialty products needing flexibility. You assume 8000 operating hours per year in a typical design basis with a 90 to 95% typical plant attainment or operating rate. Reactor performance is governed by conversion, selectivity, and overall yield, where low single-pass conversion is often accepted to maximize selectivity and avoid unwanted byproducts.

Acetylene generated via the carbide process contains arsenic and sulfur, requiring scrubbing with concentrated sulfuric acid before reacting with hydrochloric acid to form dichloroethane. In vinyl chloride manufacture via dichloroethane pyrolysis, reactor conversion is restricted to 55% to minimize carbon formation that fouls reactor tubes. In ammonia production where conversion is limited to 15%, an incoming feed containing 0.2% argon requires a purge rate of 4 moles per 100 moles of feed to hold the argon in the recycle stream below 5.0%.

Process synthesis for novel flowflows follows a structured 7-step hierarchy integrating economic assessments with research yield targets, where 80 or more of the cost of production is feedstock cost less credits for viable byproducts.

A rigorous walkthrough review of the Process Flow Diagram by a multi-disciplinary team is vital to catch missing equipment and verify design assumptions. You apply a specific review procedure during this walkthrough:

• Pin a large printout of the PFD to a wall for group markup.
• Provide heat and material balance printouts for reviewers if not on the PFD.
• Deliver a brief overview of process chemistry and block flow diagrams for complex processes.
• Conduct a sequential stream walkthrough from storage through all operations, explaining design criteria and exit conditions.
• Challenge assumptions and identify missing equipment through targeted questioning.
• Mark up immediate corrections and document follow-up action items.
• Highlight completed streams on the PFD to ensure none are overlooked.
• Reconvene after modifications are made if substantial changes are required.

A feed section walkthrough of technical grade A leaving a floating roof storage tank T101 via line 101, pumped by centrifugal pump 101, regulated by control valve FCV100, and heated in steam heater E101 demonstrates the level of scrutiny applied against parameters such as a -5 °C winter low ambient temperature, 30 °C summer high ambient temperature, 10 bar gauge pump discharge pressure, 1.3 bar control valve design pressure drop, 180 °C reaction temperature, 240 °C high pressure steam temperature, 0.7 bar heat exchanger pressure drop allowance, and 8 bar gauge reactor pressure.

3. Energy Management and Pinch Technology

Effective energy management utilizes utility systems, waste stream combustion, and pinch technology to minimize total energy requirements.

Conventional power stations waste exhaust steam in condensers, whereas gas-turbine cogeneration plants pair a gas turbine with a heat recovery steam generator to raise steam, reaching high overall thermal efficiencies. You can achieve 70% to 80% overall thermal efficiency for cogeneration plants compared to 30% to 40% thermal efficiency obtained from conventional power stations. Wholesale electricity prices average $0.06/kWh in North America.

Fired heaters operating with both radiant and convective sections achieve higher efficiency than those utilizing the radiant section alone. Using both sections of a fired heater raises the efficiency to about 0.85, whereas relying solely on the radiant section drops the efficiency to about 0.6.

Example 3.1 demonstrates calculating the annual fuel cost for a 4 MW process duty using natural gas priced at $3.20/MMBtu, resulting in an annual cost of $411,500 at 0.85 efficiency versus $582,600 at 0.6 efficiency over 8000 operating hours.

Industrial steam systems utilize multiple pressure tiers to supply high-temperature heating, drive turbines, and facilitate energy recovery. High pressure steam can be expanded through let-down valves or back-pressure turbines into medium and low pressure steam mains, generating shaft work credits and allowing precise temperature control through condensation. Typical systems use 40 bar high pressure steam, 20 bar medium pressure steam, and 3 bar low pressure steam, priced using $6/MMBtu fuel and $0.05/kWh electricity costs.

Cooling towers reject heat via partial water evaporation, and their lowest possible temperature is constrained by the ambient air wet bulb temperature. Cooling towers operate with a temperature approach of at least 2.8 degrees Celsius above the inlet air wet bulb temperature, utilizing blowdown streams to prevent solid accumulation and make-up streams to compensate for evaporation. Design conditions can feature a 35 degrees Celsius dry bulb temperature with 80% humidity, establishing a 32 degrees Celsius wet bulb temperature.

Vapor compression refrigeration systems provide cooling below ambient temperatures, and their performance is evaluated using the coefficient of performance relative to Carnot efficiency. Real refrigeration cycles operate at a fraction of the ideal reverse Carnot cycle efficiency, typically achieving about 0.6 times the Carnot COP for simple cycles or up to 0.9 for complex arrangements. Example 3.3 estimates the annual operating cost for a 1.2 MW condenser duty at -5 degrees Celsius, using cooling water at 40 degrees Celsius, an 80% Carnot efficiency, and electricity at $0.06/kWh.

Combustible process waste gases can be utilized as low-grade fuels for steam raising, and their economic value depends on their quantified calorific value and supply continuity. The calorific value of a vent gas stream is calculated by summing the products of component molar fractions and their respective standard heats of combustion. Example 3.4 calculates the vent-gas calorific value for an oxyhydrochlorination process producing dichloroethane, utilizing component heats of combustion such as 67.6 kcal/mol for carbon monoxide, 372.8 kcal/mol for ethylene, 337.2 kcal/mol for ethane, and -1084.4 kJ for dichloroethane, compared against the 37 MJ/m3 calorific value for natural gas.

Pinch technology identifies the thermodynamic bottleneck in a network of process streams, representing a distinct break where heat should not be transferred across for minimum energy requirements. Bodo Linnhoff and his collaborators developed pinch technology at ICI, Union Carbide, and the University of Manchester. The method plots system temperatures versus heat transferred to reveal a pinch between hot and cold stream curves, dividing the process into two independent thermodynamic regions: a heat sink above the pinch and a heat source below.

The problem table method is a numerical algorithm that eliminates the need to sketch composite curves for identifying pinch temperatures and minimum utility requirements. The procedure converts actual stream temperatures into interval temperatures by adjusting for half the minimum temperature difference, ranks them, carries out a heat balance for each temperature interval, and cascades the surplus heat downward to find the minimum hot and cold utility requirements. You execute the procedure through strict sequential rules:

• Convert actual stream temperatures into interval temperatures by subtracting half the minimum temperature difference from hot stream temperatures and adding half to cold stream temperatures.
• Note any duplicated interval temperatures.
• Rank the interval temperatures in order of magnitude, showing duplicated temperatures only once.
• Carry out a heat balance for the streams falling within each temperature interval.
• Cascade the heat surplus from one interval to the next down the column of interval temperatures.
• Introduce just enough heat to the top of the cascade to eliminate all negative values.

Designing a heat-exchanger network for maximum energy recovery requires splitting the problem at the pinch and strictly matching streams adjacent to the pinch according to their heat capacity flow rates. Above the pinch, hot stream heat capacities must be less than or equal to cold stream heat capacities, and no cooling utilities should be used. Below the pinch, hot stream heat capacities must be greater than or equal to cold stream heat capacities, and no heating utilities should be used. You follow explicit network design guidelines:

• Divide the problem at the pinch.
• Design away from the pinch.
• Above the pinch match streams adjacent to the pinch, meeting the restriction CPh <= CPc.
• Below the pinch match streams adjacent to the pinch, meeting the restriction CPh >= CPc.
• If the stream matching criteria cannot be satisfied, split a stream.
• Maximize the exchanger heat loads.
• Supply external heating only above the pinch and external cooling only below the pinch.

Process simulation models must carefully handle stream mixing, phase changes, and heats of mixing to avoid severe energy balance errors. Engineers should avoid mixing streams at disparate temperatures or pressures, segment phase-change exchangers into distinct sub-models to prevent internal temperature pinches, account for heats of mixing in acid-base systems, and incorporate design margins. You apply specific modeling checks:

• Avoid mixing streams at very different temperatures.
• Avoid mixing streams at different pressures.
• Segment heat exchangers to avoid internal pinches during phase changes.
• Check for heat of mixing when acids or bases are mixed with water.
• Remember to allow for process inefficiency and design margins.

The minimum number of exchangers in a network dictates capital cost, and loops within a maximum energy recovery network indicate opportunities to reduce exchanger counts by allowing some heat to cross the pinch. For simple networks, the minimum number of exchangers is the total number of streams minus one, expressed as Zmin = N' - 1. For complex networks involving loops and independent branches, you apply a systematic revision procedure:

• Start with the design for maximum heat recovery.
• Identify loops that cross the pinch.
• Starting with the loop with the least heat load, break the loops by adding or subtracting heat.
• Check that the specified minimum temperature difference has not been violated.
• Estimate the capital and operating costs, and the total annual cost.
• Repeat the loop breaking and network revision to find the lowest cost design.
• Consider the safety, operability, and maintenance aspects of the proposed design.

4. Process Simulation and Optimization

Process simulation models complex chemical systems using thermodynamic property packages, rigorous unit operations, and recycle convergence algorithms.

Process simulation programs are divided into sequential-modular and simultaneous types, each with distinct capabilities regarding recycle convergence and dynamic simulation. Sequential-modular programs solve modules stepwise, which can cause convergence difficulties with recycles and prevent dynamic simulation. Simultaneous programs solve all equations together, eliminating recycle convergence issues and enabling unsteady-state modeling for start-ups and safety studies at the cost of higher computing power.

Choosing the correct number and type of components is critical for building an accurate process simulation model that converges successfully. Models generally solve more efficiently with fewer than about 40 components. Using too few components leads to inadequate predictions of reactor and separation performance, while using too many makes convergence difficult in systems with multiple recycles.

• Always include any component that has a specified limit in any of the products if that component is present in any of the feeds or could be formed in the process.
• Always include any component that has a specified limit in any of the feeds.
• Always include components that are expected to be formed in side reactions or consecutive reactions.
• Always include any compounds that are expected to be present and are known to have significant health, safety, or environmental concerns.
• Usually include any compound that might be present at a mass or mole fraction greater than 2% in any stream in the process.
• Do not include isomers unless the process specifically requires distinction between isomers.

Phase-equilibrium models utilize equations of state to model vapor-phase nonideality and activity coefficient models to handle liquid-phase nonideality. Vapor-liquid equilibria are evaluated via distribution coefficients combining fugacity coefficients from equations of state with liquid activity coefficient models and Poynting pressure corrections.

Simulators provide various options including Conversion, Equilibrium, Gibbs, CSTR, PFR, and Yield Shift reactors. Early design stages often use simple conversion or equilibrium models when kinetics are unknown, while detailed designs require specific kinetic models or custom user models.

Example 4.4 uses a Gibbs reactor to analyze the equilibrium distribution of 22 hydrocarbon species with five carbons in heavy oil cracking across various temperatures at 200 kPa operating pressure.

Distillation columns are modeled using a hierarchical approach, starting with fast shortcut models to establish initial estimates for rigorous stage-by-stage calculations. Shortcut models utilize Fenske-Underwood-Gilliland methods based on constant relative volatility and key component recoveries. These results initialize rigorous models which solve mass and energy balances stage by stage and handle nonideal liquid phases.

• Estimate the minimum reflux and number of stages using shortcut models.
• Set the actual reflux ratio between 1.05 and 1.25 times the minimum reflux ratio, commonly 1.15 times Rmin.
• Transfer the shortcut results, temperature profiles, and pressure gradients to initialize the rigorous model.

Examples 4.6 and 4.7 design a distillation column to separate 225 metric tons per hour of feed containing an equimolar mixture of benzene, toluene, ethylbenzene, paraxylene, and orthoxylene, using a 250 metric tons per hour equipment design basis with a safety factor to yield a 2.130 minimum reflux ratio, a 2.45 actual reflux ratio, 16.4 minimum stages rounded up to 17, 39 actual trays required, and a 4.42 m column diameter.

5. Materials of Construction and Cost Estimation

Plant profitability depends on selecting corrosion-resistant materials and accurately estimating capital investments and production costs.

The overriding considerations for chemical plants are high temperature strength and corrosion resistance. Mild steel features 450 N/mm² tensile strength at 25 °C falling to 210 N/mm² at 500 °C, and 200,000 N/mm² Young's modulus falling to 150,000 N/mm² at 500 °C.

You must evaluate metallic corrosion operating via electrochemical cells requiring an anode, a cathode, an electrolyte conducting medium, and an electrical circuit completion. A minimum chromium content of 12 percent is required to impart corrosion resistance in oxidizing conditions, while nickel improves resistance in non-oxidizing environments. Ferritic stainless steels contain 13-20% chromium, austenitic grades contain 18-20% chromium and at least 7% nickel, and martensitic grades contain 10-12% chromium with 0.2 to 0.4% carbon. Type 304 stainless steel exhibits a typical thermal conductivity of 16 W/m°C at 100 °C, whereas mild steel reaches 60 W/m°C.

You must follow these selection and evaluation procedures:

• Evaluate mechanical properties including tensile strength, elastic modulus, fracture resistance, and wear resistance.
• Assess corrosion rates through published corrosion data guides and laboratory coupon tests under simulated plant conditions.
• Identify acceptable corrosion rates using penetration rate thresholds to decide between frequent equipment replacement or upgrading to corrosion-resistant alloys.
• Electrically insulate dissimilar metals in contact or increase anodic material thickness to mitigate galvanic corrosion.
• Apply post-weld heat treatment, use low carbon grades, or add stabilizing elements like titanium or niobium to prevent intergranular weld decay in stainless steels.
• Calculate cost rating factors incorporating cost per unit mass, density, and maximum allowable stress when comparing material economic efficiencies.
• Specify high mechanical surface finishes via abrasive blasting and polishing in food, pharmaceutical, and textile processes to ensure hygiene and prevent snagging or iron staining.

Fixed capital investment comprises inside battery limits plant costs, offsite infrastructure modifications, engineering costs, and contingency charges. Offsite costs typically range from 20 to 50 percent of inside battery limits costs for typical chemical projects, using an initial 40 percent figure when site details are unknown. AACE International classifies estimates into five types. Class 5 order of magnitude estimates provide a ±30-50% accuracy band, Class 4 preliminary estimates yield ±30% accuracy, Class 3 definitive estimates achieve ±10-15%, Class 2 detailed estimates reach ±5-10%, and Class 1 check estimates yield ±5-10%.

You can rapidly generate order-of-magnitude capital cost estimates using capacity scaling exponents. The six-tenths rule uses an average 0.6 exponent, while mechanical or solids-handling plants use 0.8 to 0.9, petrochemical processes use 0.7, and specialty chemical or pharmaceutical plants use 0.4 to 0.5. The chemical industry features typical turnover ratios between 1.0 and 1.25.

You must apply these estimation practices:

• Calculate plant capital cost from a known base plant cost and capacity ratio using the scaling exponent equation C2 equals C1 multiplied by the ratio of S2 to S1 raised to power n.
• Compute unit production capital cost by dividing total plant cost by plant capacity to demonstrate economies of scale.
• Estimate plant cost using Bridgewater's method by substituting plant capacity, reactor conversion, and number of functional units into empirical logarithmic equations.
• Estimate plant cost via the payback method by assuming a recovery period of three to five years against gross annual profit.
• Estimate purchased equipment cost by substituting equipment size parameters and empirical cost constants into the power-law equation Ce equals a multiplied by S to the power n.
• Perform a detailed cost estimate by establishing an itemized work breakdown structure of parts, fabrication steps, machine time costs, and labor hours.

Project production costs divide into variable costs and fixed costs. Maintenance requires 3% to 5% of inside battery limits investment, while property taxes and insurance require 1% to 2% of inside battery limits fixed capital. Aspen Process Economic Analyzer applies equipment design allowances of 15% for new and unproven processes, 10% for new processes, 7% for redesigned processes, 5% for licensed processes, and 3% for proven processes. Raw materials represent 80% to 90% of the total cost of production.

You must follow these screening and financial evaluation steps:

• Prepare material and energy balances; draw up preliminary flowflows; size major equipment items and select materials of construction.
• Estimate the purchased cost of the major equipment items.
• Calculate the inside battery limits installed capital cost using specific factors and materials corrections.
• Calculate the offsite, engineering, and contingency costs.
• Sum inside battery limits, offsite, engineering, and contingency costs to get fixed capital investment.
• Estimate working capital as 10% to 20% of fixed capital investment.
• Add fixed and working capital to get total investment required.
• Assess whether by-product is produced in stoichiometric quantities or side reactions.
• Determine if by-product requires purification to meet market specifications.
• Calculate whether net benefit of recovery exceeds $200,000 per year including avoided disposal costs.
• Review CEO letter describing operations, acquisitions, divestitures, and plans.
• Inspect the balance sheet for assets, liabilities, and stockholder equity.
• Examine the income statement for revenues, cost of goods sold, and net income.
• Analyze the cash-flow statement for operating, investing, and financing cash flows.
• Read independent auditor comments and health, safety, and environmental performance reports.
• Determine the initial cost of the fixed capital investment and subtract land value.
• Identify the applicable class life and recovery period from IRS publication 946.
• Apply the half-year convention assuming property is acquired midyear.
• Calculate annual depreciation charges using straight-line, declining-balance, or MACRS formulas.
• Add noncash depreciation charges back to net income after taxes to compute total operating cash flow.
• Establish the project life and schedule of cash flows for each year including construction and start-up.
• Calculate the discount factor using the chosen cost of capital interest rate.
• Multiply annual cash flows by their corresponding discount factors to find present values.
• Sum all present values to compute the net present value.
• Adjust the interest rate via trial-and-error or spreadsheet goal seek until the net present value equals zero to find the discounted cash-flow rate of return.
• Select the interest rate based on the cost of capital and the service life in years.
• Calculate the annual capital charge ratio using principal, interest rate, and term.
• Multiply the initial capital expense by the annual capital charge ratio to find the annualized capital cost.
• Add operating costs to the annualized capital cost to determine the total annualized cost of production.
• Establish a base case using the most probable values for investment and cash flows.
• Define percentage variation ranges for key parameters such as sales price, feed cost, and fixed capital.
• Adjust each parameter individually within its error range to plot its impact on net present value or discounted cash-flow rate of return.
• Estimate upper and lower bounds for cost items to compute means and standard deviations via statistical formulas.
• Determine required contingency charges based on desired confidence levels.

US corporations face a 35% top marginal rate of federal income tax for incomes greater than $18,333,333. Chemical manufacture carries a 9.5 years class life and a 5 years recovery period under the Modified Accelerated Cost Recovery System. A carbon steel heat exchanger costing $140,000 with a 5-year life yields an annualized cost of $38,780, whereas a type 304 stainless steel exchanger costing $182,000 with a 10-year life yields an annualized cost of $32,214 using a 12% cost of capital. An adipic acid plant project yielded a discounted cash-flow rate of return of 11.2% over 15 years of production. Sensitivity analysis on this facility demonstrated that economics were more sensitive to capital investment and gross profit variations than to construction schedule delays, using a ±20% variation range for sales price and production rate, -10% to +30% for feed cost, and -20% to +50% for capital investments.

6. Process Safety and Loss Prevention

Process safety management integrates safety layers, hazard identification methods like HAZOP, and pressure relief systems to prevent catastrophic failures.

Safety is structured in layers where each upper layer activates if the lower levels fail.

• The base layer is inherently safe and sound equipment design.
• Above this sit the basic process control system, automated alarms to warn operators, trip systems for emergency shutdown, pressure relief systems to handle overpressure, on-site emergency response, and finally local community emergency services.

Flammability hazards are characterized by flash point, autoignition temperature, and flammability limits specifying the lower and upper concentration bounds in air. For instance, hydrogen has lower and upper flammability limits of 4.1% and 74.2% by volume in air, whereas gasoline specifies limits of 1.3% and 7.0% by volume in air. Incompatible materials can undergo spontaneous decomposition, rapid exothermic reactions, or release toxic gases upon contact with moisture or other chemicals.

At the BP Texas City refinery on March 23, 2005, an explosion killed 15 people and injured over 170 due to multiple failures in supervision, operating procedures, and training, including holding a safety meeting in the control room during startup.

Hazard Analysis and Relief

A Hazard and Operability Study systematically examines process designs vessel by vessel and line by line using specific guide words to identify operational deviations. You can perform this analysis by following these steps:

• Divide the process documentation into manageable sections, such as individual vessels and lines.
• Define the design intention for each specific line or vessel.
• Systematically apply standard guide words to generate potential deviations.
• Evaluate whether each deviation has a realistic cause.
• Determine the physical and operational consequences of meaningful deviations.
• Identify resulting hazards and record necessary design modifications or safeguards.

The Dow Fire and Explosion Index evaluates the inherent hazards of process units based on material properties and general or special process hazards. The resulting index classifies the degree of hazard across several ranges:

• 1-60 indicates a light degree of hazard range for the Fire and Explosion Index.
• 61-96 represents a moderate degree of hazard range.
• 97-127 covers the intermediate degree of hazard range.
• 128-158 designates a heavy degree of hazard range.
• 159 and above defines a severe degree of hazard range.

Pressure relief devices must be fitted to all ASME Section VIII pressure vessels to prevent catastrophic failure from overpressure by providing a mechanical means of relief. The primary pressure relief device must have a set pressure not exceeding the maximum allowable working pressure of the vessel and must prevent the pressure from rising more than 10% or 3 psi above it.

7. Equipment Design and Selection

Detailed equipment design requires applying mechanical codes to pressure vessels, sizing separation columns, and optimizing heat exchangers.

You must ensure pressure vessel design and fabrication complies with national codes and standards like the ASME Boiler and Pressure Vessel Code to guarantee structural safety. Most refinery vessels fall under Section VIII, divided into Division 1 for general rules, Division 2 for restrictive rules allowing thinner walls, and Division 3 for high pressures.

• Consult the latest edition of the national design code during detailed design.
• Select the appropriate division of Section VIII based on pressure, material, and inspection requirements.

You apply safety factors to specified minimum tensile and yield strengths to establish maximum allowable stress. Welded joint efficiencies reduce allowable stress values based on radiographic examination levels.

• Apply safety factors to specified minimum tensile and yield strengths to establish maximum allowable stress.
• Select joint efficiency factors based on weld categories and radiography inspection levels.

You determine minimum wall thickness for thin-walled vessels using membrane stress analysis factoring in internal pressure, diameter, allowable stress, and joint efficiency. High-pressure processes above 200 bar require specialized construction like compound walls or autofrettage.

• Apply ASME Section VIII Division 2 alternative rules for pressures above 2000 psia to achieve thinner walls.
• Utilize shrink-fitted compound cylinders or multilayer construction for high-pressure compressor barrels and reactors.
• Employ autofrettage by overpressurizing monobloc vessels hydraulically to create residual compressive stresses in the inner wall.

You design fixed-bed catalytic reactors by following a sequential sizing process.

• From the space velocity determine the bed volume of catalyst required.
• Assume the bed volume is contained in the tangent-to-tangent section of a pressure vessel with an aspect ratio between 1:1 and 4:1.
• Check the pressure drop using the Ergun equation; if too high, use a lower aspect ratio.
• If multiple reheats or quenches are needed, make allowance for internals between the beds.
• Design the pressure vessel using standard mechanical methods.

You use Kern's method to estimate shell-side heat-transfer coefficients and pressure drops in tubular exchangers.

• Calculate the area for cross-flow As for the hypothetical row of tubes at the shell equator.
• Calculate the shell-side mass velocity Gs and the linear velocity us.
• Calculate the shell-side equivalent diameter for square or triangular pitch arrangements.
• Calculate the shell-side Reynolds number.
• Read the value of jh from the appropriate figure for the selected baffle cut and tube arrangement, and calculate the shell-side heat-transfer coefficient hs.
• Read the friction factor from the friction factor figure and calculate the shell-side pressure drop.

10 Key Ideas

1. Chemical engineering design bridges vaguely defined customer needs and predicted financial outcomes by creating actionable specifications for safe, economical process plants.
2. Flowsheet development requires precise PFD preparation, careful analysis of continuous and batch options, and rigorous conceptual synthesis steps.
3. Effective energy management utilizes utility systems, waste stream combustion, and pinch technology to minimize total energy requirements.
4. Process simulation models complex chemical systems using thermodynamic property packages, rigorous unit operations, and recycle convergence algorithms.
5. Plant profitability depends on selecting corrosion-resistant materials and accurately estimating capital investments and production costs.
6. Safety is structured in layers where each upper layer activates if the lower levels fail.
7. A Hazard and Operability Study systematically examines process designs vessel by vessel and line by line using specific guide words to identify operational deviations.
8. The Dow Fire and Explosion Index evaluates the inherent hazards of process units based on material properties and general or special process hazards.
9. Pressure relief devices must be fitted to all ASME Section VIII pressure vessels to prevent catastrophic failure from overpressure by providing a mechanical means of relief.
10. Detailed equipment design requires applying mechanical codes to pressure vessels, sizing separation columns, and optimizing heat exchangers.