Corrosion, Heat Exchangers, and Material Balance

• Heat exchangers are thermodynamic equipment in automotive, chemical, process engineering, industrial heating, cooling, and heat-recovery processes.
• Heat transfer typically occurs under steady state operating conditions.
• Steady state heat transfer stops thermal energy from being stored in the heat exchanger.
• In agro-food industries, heat exchange occurs under transient state operation.
• The exchange between the heat transfer fluids uses a conductive element to store part of the thermal energy in the heat exchanger.
• Thermal energy can exist in sensible, latent, or chemical forms.
• Heat exchangers are common in the food industry but can be expensive and energy-consuming.
• Integrated heating systems achieve approximately 95% energy efficiency.
• Integrated heating systems achieve 46.56% second-law efficiency relative to conventional electric boiler systems.
• Heat transfer is key to food preservation, regulating chemical reactions, texture, and properties.
• Water as a structural element greatly affects food stability.
• Evaporation increases heat-transfer efficiency, solidifying food solids.
• New designs aim to increase efficiency and minimize thermal degradation by decomposing products through thin tubes or heat transfer surfaces.

Feedstock

• The soft drink industry uses a variety of feedstock ingredients, each affecting flavor, appearance, and stability.
• Understanding these components is vital for optimizing moisture removal processes in heat exchanger dryers while ensuring the quality of the beverage.

Water

• Water is the primary solvent in soft drink formulations.
• High purity of water is needed, meeting standards for potable water, with turbidity below 1 NTU and coliform counts under 500 CFU/mL
• Before use, water undergoes rigorous purification processes like filtration and reverse osmosis to ensure safety and flavor maintenance

Sugars and Sweeteners

• Sugars and sweeteners provide key sweetness and mouthfeel in soft drinks.
• Sucrose is a common natural sugar usually found in crystalline form, with solubility of approximately 2000 g/L in water at room temperature.
• High-fructose corn syrup (HFCS), especially HFCS 55, is a liquid sweetener with about 55% fructose and 42% glucose, with a viscosity of approximately 1.5–2.5 cP.
• Artificial sweeteners like aspartame are used in low-calorie beverages.
• Aspartame is highly concentrated, around 200 times sweeter than sucrose, and is used at levels of 0.1% to 0.5% in formulations.

Flavorings and Extracts

• Flavorings and extracts make the distinctive taste of soft drinks.
• Natural fruit extracts, such as those from oranges and lemons, are commonly used in liquid and concentrated forms.
• Natural fruit extracts can be heat sensitive, and their volatile compounds degrade above 70°C.
• Careful temperature control is needed during drying.
• Synthetic flavorings like ethyl maltol are utilized for their sweetness, typically used at concentrations of 0.1% to 0.5%.

Acids and Stabilizers

• Acids and stabilizers significantly enhance flavor and product stability.
• Citric acid is frequently added to provide tartness, ranging from 0.1% to 0.5%.
• Preservatives like sodium benzoate are incorporated to stop microbial growth, typically at levels of no more than 0.1%.
• Acids and stabilizers are crucial for extending shelf life and maintaining quality during storage.

Colors and Preservatives

• Coloring agents enhance the visual appeal of soft drinks, with both natural and synthetic options.
• Natural colors commonly used, such as beet juice concentrate, at concentrations of 0.1% to 0.5%.
• Synthetic dyes, like Red 40, are usually incorporated at levels of about 0.02%.
• Preservatives prevent spoiling and ensure the product remains safe and appealing.

Carbonation

• Carbonation defines many soft drinks, giving them effervescence that consumers expect.
• Carbon dioxide (CO2) is infused to achieve a concentration of approximately 2.5 to 3.0 volumes of CO2, with infusion pressures from 2.5 to 4.0 bar.

Operational Conditions

• Temperature needs to be 60°C to 130°C: Optimal for moisture evaporation without degrading sensitive ingredients.
• Inlet Humidity needs to be >80% for fruit extracts: Initial moisture content of feedstock affecting ingredient type.
• Outlet Humidity needs to be 1% to 5%: Target moisture level is key for product stability and extending shelf life.
• Operating Pressure needs to be at Atmospheric or vacuum: Standard atmospheric pressure; vacuum for heat-sensitive materials
• Flow Rate is Variable: Adjust to ensure residence time for drying, preventing product damage.
• Energy Input is Steam, hot water, or gas: Must be controlled to maintain drying temperatures consistently.
• Drying Duration needs to be 30 minutes to several hours: Time needed depends on feedstock type.

Operational Type: Batch or Continuous

• Batch operation provides flexibility, quality control, ease of operation, and adaptability.
• Batch systems create a range of products that require high standards of quality and safety.

Material Construction

• Materials used in constructing heat exchanger dryers for the soft drink industry are critical to performance, durability, and safety.
• Key factors are hygiene, resistance to corrosion, and thermal transfer qualities.
• Suitable materials include: Stainless Steel, Aluminum, Carbon Steel, Glass-Lined Steel, and Composite Materials.

Properties of stainless steel

• Types of stainless steel include Type 316
• Type 316 stainless steel has ~580 MPa Tensile Strength and ~290 MPa Yield Strength

Properties of Aluminum

• Aluminum has Tensile Strength of ~70-700 MPa depending on alloy.

Mechanical properties

• These properties of the materials affect the effectiveness and longevity of heat exchanger dryers
• Materials exhibit mechanical characteristics that determine their suitability for applications, ensuring efficiency, safety, and product quality.
• Type 304 stainless steel has a tensile strength of approximately 520 MPa.
• Type 316 offers even greater strength at around 580 MPa.
• The yield strength for Type 304 is about 210 MPa, and for Type 316, it is approximately 290 MPa.
• These high strength values help stainless steel's to durability under high temperatures and pressures.
• Stainless steel exhibits elongation of 40-50%, allowing it to deform without breaking.
• Its hardness, measured by Rockwell B, typically ranges from 70 to 90, enhancing its wear resistance.
• The tensile strength of aluminum alloys varies significantly, from about 70 MPa to 700 MPa, depending on the specific alloy used.
• Aluminum exhibits a yield strength of around 30-500 MPa and an elongation of 10-30%.
• Aluminum's hardness on the Rockwell B scale is between 40 and 100.
• Carbon steel’s tensile strength ranges from 370 MPa to 700 MPa, with yield strengths between 250 MPa and 450 MPa.
• Carbon steel also has an elongation of about 20-30%, for a reasonable degree of ductility.
• Protective coatings are needed to prevent rust and degradation in humid environments.
• The tensile strength of the underlying steel, ranging from 370 MPa to 700 MPa.
• Composites tensile strength can vary from 50 MPa to over 700 MPa.

Corrosion Resistance

• Type 316 stainless steel has superior resistance, particularly against chlorides and acidic environments, making it for processing fruit juices and carbonated beverages.
• Aluminum forms a protective aluminum oxide layer that offers moderate corrosion resistance.
• Aluminum can suffer suffers localized corrosion if the oxide layer is compromised.
• Carbon Steel has low inherent resistance, and is prone to rusting when open to moisture: Protective coatings like epoxy or galvanization are needed to enhance corrosion resistance.
• Glass-Lined Steel has a glass lining provides resistance to acids and alkalis, preventing chemical attack.
• Composites have excellent resistance, with many composites being suitable to withstand harsh chemicals and moisture.

Design factors of Heat Exchangers

• Design heat exchanger dryers to effectively remove moisture: while maintaining product quality.
• Use Type 316 stainless steel or similar materials to withstand harsh conditions.
• Ensure Durable materials can handle long-term operation and cleaning protocols.
• Feedstock Preparation: Establish protocols for uniformIntroduction
• Drying Chamber Configuration: Design is needed for airflow and thorough moisture removal.
• Monitor corrosion rates during operation: Ensure they remain within reasonable limits
• Prevent Pitting corrosion: Evaluate the potential for pitting based on material composition and operating conditions.

Heat Transfer Calculations

• The heat transfer rate can be calculated using the formula: Q = m.Cp.ΔT
• Q is Heat transfer rate
• m is Mass flow rate of the fluid
• Cp is Specific heat capacity of the fluid
• ΔT is Temperature difference between the inlet and outlet

Heat Exchanger Area Calculation

• LMTD = ΔΤ 1-ΔΤ2 / ln( ΔΤ1/ΔΤ2 )
• Heat exchanger area uses the formula: A=Q/ U•ΔTlm
• U is Overall heat transfer coefficient
• Corrosion Rate Solution The corrosion rate can be calculated using the formula: Corrosion Rate = K×W/D×A×T
• K = Constant
• W = Weight loss
• D = Density of the material
• A = Surface area exposed to corrosion
• T = Time of exposure

Rationale for a Safety Factor of 4.0

• Corrosion Allowance, corrosion rates vary based on environmental conditions.

Determining Corrosion Allowance

• Needs corrosion rate and lifespan of of the equipment

Pitting Corrosion Limit

• Occurs in stainless steel, and can be influenced by alloy composition.
• Follows a formula: CPT = 0.1 × Cr +0.5 × Mo + N × 0

Estimating the Maximum Allowable Corrosion Rate

• To ensure the equipment remains functional over its intended lifespan a maximum allowable corrosion rate must be determined.
• Requires Initial thickness of the material and expected service life

Heuristic

• A problem-solving strategy used when information is unavailable or impractical to obtain which relies on experiences, intuition or common sense.

Design Heuristics

• These points provide guidelines for dimensions, materials, that enhance operational efficiency.

Operational Heuristics

• Managing flow rates, temperatures, and parameters, they exist to maximize efficiency and conserve energy.

Maintenance Heuristics

• Are crucial for ensuring longevity and the Heat Exchangers.
• Routine Inspections: Regular checks for fouling, leaks, every 6 to 12 months.
• Cleaning Protocols: cleaning schedules to address fouling.
• Performance Monitoring: Use sensors to consistently monitor parameters.

Material Balance Equation

• Mass Flow Rate and Initial Moisture Content are needed
• Output Stream (Dried Product), Mass Flow Rate, Initial Moisture Content

Material Balance Equation

• It involves the equation: m1.(1-w1)=mout. (1-Wout)

Overall heat transfer

• Follow this equation: Q=U•A•ΔT lm

Numerical Techniques

• Finite Difference Method (FDM): the equation allows for equation solving.
• Finite Element Method (FEM): system has a good structure.
• Computational Fluid Dynamics (CFD): equations enable equations to be solved within exchangers.

Heat Transfer Rate (Q)

• Calculated using the overall heat transfer equation:

Applications in Heat Exchangers

• For fluid a and b flow in and out of the exchangers, involving their mass, the temperature, and the balance of the fluid.

Concepts of Heat Transfer

• Conduction: transfer via solid material.
• Convection: transfer between surfaces and fluid.
• Radiation: transfer via heat from electromagnetic waves.