Table of Contents
Role of Chemical Engineering in Industry
Chemical engineering processes transform raw materials into valuable products using chemical reactions, separations, and energy transfer operations on a large scale. In this chapter, the focus is on what characterizes such processes in general and what distinguishes industrial, large-scale implementations from small-scale laboratory chemistry. The specific major processes listed in the subchapters (ammonia, nitric acid, sulfuric acid, etc.) will be treated there; here, we lay out the common principles and features that underlie them.
From Laboratory Reaction to Industrial Process
In the laboratory, a chemist typically thinks about:
- Which reaction yields the desired product?
- How to control reaction conditions (temperature, time, catalyst)?
- How to isolate and purify the product on a small scale?
In chemical engineering, the same reaction must be embedded into a complete process that:
- Converts raw materials (often impure) into products in large quantities.
- Runs continuously or in repeated batches over long periods.
- Uses energy, materials, and equipment economically and safely.
Key additional questions include:
- How to supply and remove heat at large scale?
- How to feed, mix, and transport reactants and products continuously?
- How to separate and recycle materials efficiently?
- How to minimize waste, emissions, and environmental impact?
The central task is to translate a chemically feasible reaction into a technically and economically viable process.
Typical Process Steps
Industrial chemical processes rarely consist of a single reaction step. They are integrated schemes made from recurring types of unit operations:
- Preparation of feedstocks
- Cleaning and drying gases (removal of dust, water, sulfur compounds, etc.).
- Preheating or cooling to the required reaction temperature.
- Adjusting pressure and composition (e.g. mixing gases in defined ratios).
- Chemical reaction
- Carried out in one or more reactors.
- Conditions (temperature, pressure, catalyst, residence time) optimized for rate and selectivity.
- May be operated continuously (constant flow) or batch-wise (charged, reacted, discharged).
- Separation and purification
- Removal of by-products and unreacted raw materials.
- Isolation of the desired product using methods such as distillation, absorption, extraction, filtration.
- Often includes concentration (e.g. concentrating acids, liquefying gases).
- Recycling and waste treatment
- Recovery and recycle of unreacted feedstock (e.g. unreacted gas streams).
- Treatment of off-gases, wastewater, and solid wastes to meet environmental standards.
- Heat recovery from hot streams for use elsewhere in the plant.
Many of the large-scale processes listed in the subchapters—like ammonia synthesis or sulfuric acid production—are classic examples combining these steps into an integrated, continuous process.
Continuous and Batch Operation
Chemical engineering distinguishes mainly between:
- Continuous processes
- Reactants are fed constantly; products are withdrawn continuously.
- Typically used for large-volume, commodity chemicals (ammonia, sulfuric acid, fuels).
- Advantages: steady operation, high throughput, easier heat integration, often more efficient.
- Requires accurate steady-state control of flow rates, temperatures, pressures, and compositions.
- Batch processes
- A reactor is filled with reactants, allowed to react, and then emptied.
- Common for specialty chemicals, pharmaceuticals, and smaller volumes where flexibility is important.
- Easier to change product and conditions from batch to batch.
- Often more labor- and time-intensive per unit of product.
Some industrial processes use semi-batch or combinations of continuous and batch steps.
Unit Operations and Process Design
Chemical engineering processes are built from standard building blocks called unit operations. These are generic tasks that appear in many different processes. Some important classes include:
- Reactors
- Vessels where the chemical transformation occurs.
- Designed to provide mixing, heat transfer, and sufficient contact time.
- Common types:
- Continuous stirred-tank reactors (CSTRs).
- Plug-flow reactors (tubular reactors).
- Fixed-bed and fluidized-bed catalytic reactors.
- Heat exchange equipment
- Heat exchangers to warm or cool fluids by transferring heat between streams.
- Furnaces or fired heaters to raise temperatures using combustion.
- Cooling towers to dissipate excess heat to the environment.
- Phase-contact equipment
- Distillation columns for separating components based on volatility.
- Absorbers and strippers for gas–liquid separations (e.g. removing a gas with a liquid solvent).
- Extractors for liquid–liquid separation based on different solubilities.
- Adsorbers using solid materials to selectively retain components.
- Mechanical separation
- Filtration, centrifugation, settling to separate solids from liquids or gases.
- Cyclones for removal of dust from gas streams.
Designing a process means selecting and connecting such units, sizing them, and specifying their operating conditions so that material and energy balances are fulfilled and the overall goal (conversion, purity, throughput) is met.
Material and Energy Balances at Process Scale
For industrial processes, conservation of mass and energy is central to design and assessment:
- Material (mass) balances
- Relate the flow of reactants, products, by-products, and recycle streams.
- Overall balance (steady state): input = output + accumulation (often 0) + loss.
- Applied to individual components: e.g. nitrogen atoms in ammonia synthesis, sulfur atoms in sulfuric acid production.
- Energy balances
- Account for:
- Enthalpy of feed and product streams.
- Heat released or absorbed by reactions.
- Heat exchange between process streams and environment.
- Work done by or on the system (e.g. compression, pumping).
- Crucial for designing reactors (e.g. controlling exothermic reactions) and heat exchangers.
- Enable heat integration: using excess heat from one part of the plant to supply another, reducing external fuel use.
These balances are the quantitative foundation of all the specific large-scale processes discussed in subsequent sections.
Process Conditions: Pressure, Temperature, Catalysts
While underlying chemical principles determine what is theoretically possible, industrial processes often operate at conditions quite different from those in the lab to reach practical performance:
- High pressures
- Increase concentrations of gaseous reactants and often shift equilibria.
- Require robust, thick-walled equipment and compressors.
- Widely used in processes such as ammonia synthesis and certain oxidation reactions.
- Elevated temperatures
- Increase reaction rates.
- Affect equilibrium positions—sometimes beneficial, sometimes not.
- Require careful heat removal for exothermic reactions to avoid runaway.
- Catalysts
- Essential in many industrial processes to achieve acceptable rates at feasible temperatures and pressures.
- Often solid, heterogeneous catalysts in fixed beds or fluidized beds.
- Must be accessible to reactants and stable under operating conditions; sometimes require special activation.
- Catalyst life, poisoning, and regeneration are important practical aspects.
The choice of conditions is always a compromise between kinetics, thermodynamics, materials of construction, safety, and economics.
Safety and Risk Management
Large chemical plants handle substantial amounts of reactive, often hazardous substances at high temperatures and pressures. Safety is therefore a central characteristic of chemical engineering processes:
- Containment and equipment integrity
- Appropriately designed and tested pressure vessels, piping, and valves.
- Relief systems and rupture disks to protect against overpressure.
- Corrosion-resistant materials selected for specific chemicals (e.g. strong acids, chlorine).
- Process control and monitoring
- Automated control systems to maintain safe operating ranges.
- Redundant measurement of key variables (pressure, temperature, flow, composition).
- Emergency shutdown procedures and interlocks.
- Hazard identification
- Systematic analysis of possible failure modes (e.g. leaks, runaway reactions, explosions).
- Safety distances and protective barriers for high-risk equipment.
- Operator and community protection
- Personal protective equipment and training.
- Gas detection systems and ventilation.
- Containment and treatment of accidental releases.
Many historical process accidents (e.g. involving ammonia-derived fertilizers, chlor-alkali plants, or petrochemical units) have shaped modern safety standards and regulations.
Environmental and Economic Considerations
Modern chemical engineering processes are not evaluated solely on technical feasibility. Environmental and economic criteria strongly influence design and operation:
- Resource efficiency
- High yields and selectivity reduce raw material consumption.
- Process integration and recycling decrease waste and feedstock use.
- Choice of raw materials (e.g. sulfur sources, fossil vs. alternative feeds) affects sustainability.
- Emission control
- Treatment of off-gases to remove pollutants (e.g. NOₓ, SO₂, dust).
- Wastewater purification before discharge.
- Minimizing solid waste and promoting by-product utilization where possible.
- Energy use and climate impact
- Fuel consumption in high-temperature processes and for compression.
- Opportunities for combined heat and power (CHP) systems.
- CO₂ emissions from fossil-based processes are important evaluation criteria.
- Economic viability
- Capital costs for construction of equipment and infrastructure.
- Operating costs: energy, raw materials, maintenance, labor, waste treatment.
- Market aspects: demand, product price, plant capacity, and location near resources or consumers.
These aspects fundamentally shape the design of the specific processes covered in the following subchapters.
Flow Diagrams and Process Representation
To understand and communicate complex chemical engineering processes, standardized diagram types are used:
- Block flow diagrams (BFDs)
- Show main process steps as simple blocks (e.g. “reactor,” “distillation”).
- Useful for an overview of how feedstocks become products.
- Process flow diagrams (PFDs)
- More detailed, showing major equipment, process streams, and conditions (flows, temperatures, pressures).
- Serve as the main reference for process design and analysis.
- Piping and instrumentation diagrams (P&IDs)
- Highly detailed: include all pipes, valves, instruments, and control loops.
- Used for construction, operation, and troubleshooting.
In studying the selected industrial processes, simplified diagrams help to visualize how the reaction steps, separations, and recycles are connected and how material and energy move through the plant.
Role of Catalysis and Process Integration
Across the specific industrial processes that follow, two unifying themes recur:
- Industrial catalysis
- Determines which reaction pathways are exploited industrially.
- Can make the difference between a purely academic reaction and a commercially viable process.
- Requires not only chemical knowledge but also engineering strategies for catalyst placement, contact with reactants, temperature control, and replacement.
- Process integration
- Combining multiple unit operations so that by-products or waste heat from one step become inputs to another.
- Example patterns:
- Using hot reaction gases to preheat incoming feed gases.
- Utilizing off-gas from one plant as raw material or fuel for another.
- Integration often happens at the scale of an entire chemical complex or industrial park.
When you look at the ammonia, nitric acid, sulfuric acid, chlor-alkali, aluminum, and petroleum processes in the following sections, you will repeatedly encounter these principles in concrete, technologically important forms.