FDE 211 Material & Energy Balances - ilgin.weebly.com 211 Material & Energy Balances Instructor: ... •Material and energy balances can be simple; ... independent material balance

FDE 211Material & Energy Balances

Instructor: Dr. Ilgin Paker Yikici

Fall 2015

•Process classification

•General mass balance equation

•Basic functions of Processes

•Process Flow Diagram

•Degree of Freedom Analysis

• Independent Equations

Agenda

• Industrial plants, manufacturing plants and waste treatment plants have at least two things in common:

• Energy usage

• Material flow

• Conservation of Mass. If one or more streams of material are flowing into a region of space, a process unit with boundaries, then material must be either flowing out of that region at the same rate or accumulating in the region.

• Measurement of Rate of Flow. Units must be correct all the time regardless of conditions such as mass/time or moles/time.

Flow streams

• Batch: In batch processes no material is transferred into or out of the system over the period of time of interest (e.g., heating a sealed bottle of milk in a water bath).

• Continuous: A material is transferred into and out of the system continuously (e.g., pumping liquid at a constant rate into a distillation column and removing the product streams from the top and bottom of the column).

• Semibatch: Any process that is neither batch nor continuous (e.g., slowly blending two liquids in a tank).

• Steady state: Process variables (i.e., T, P, V, flow rates) do not change with time.

• Transient: Process variables change with time.

Process Classification and Material Balance

•Classify the following processes as batch, continuous, semibatch, transient, or steady state.

a) A balloon is being filled with air at a steady rate.

Semibatch

b) A bottle of soft drink is taken from the refrigerator and left on the kitchen table.

Transient heat

Questions

•Material balances can be used to describe material quantities as they pass through a process operation (system). Such balances are statements on the conservation of mass.

• If no accumulation and generation occur in the system, what goes into a process is equal to what comes out.

•Material and energy balances can be simple; however, sometimes they can be very complicated.

• In all cases, the basic approach is the same. A balance on a conserved quantity (i.e., mass or energy) in a system may be written as:

Accumulation=(in − out) + (generation − consumption)

Material and Energy Balances

Input + generation – output – consumption = accumulation

General balance equation

a brief description is given of the most frequently used unit operations in chemical & food engineering processes. The explanation is focused on typical operations involving the transfer of mass through physical or chemical routes

•A splitter is used to divide the flow rate in a certain stream into two or more streams with different flow rates.

• In figure above, the composition of streams F1, F2, and F3 is the same since no operation is taking place between inlet and exit streams. There is only one independent material balance even in the case of a multicomponent system, since all compositions are equal. Mass flow rates F1, F2, and F3 may be different.

Splitter or Divider

•The mixing process has the following characteristics: There are two or more entering streams, and only one exit stream resulting from the blending of the incoming streams. •The streams can be in any

phase, that is, gas, liquid, or solid.

Mixer (Blender)

• Drying is a mass transfer process resulting in the removal of moisture by evaporation from a solid, semisolid, or liquid to produce a solid state.

• To achieve this operation, the dryer is supplied with a source of heat.

• Vapor is produced in the process. Resulting dried products are in solid phase.

• Dried solids may not be solvent free. Feed can be solid, slurry, or solution.

Dryer (Direct heating)

Dried Fruit

• Filtration is a technique used either to remove impurities from a liquid or to isolate a solid from a fluid.

• Filtration is commonly a mechanical or a physical operation that is used for the separation of solids from fluids (liquids or gases) by interposing a medium through which only the fluid can pass.

• Filtration can also be used to separate particles that are suspended in a fluid, where the latter can be a liquid, a gas, or a supercritical fluid.

Filter

•Depending on the application, either one or both of the components may be isolated. In the filtration process, filtrate, the exit liquid, is free of solids.

• The filtrate is saturated with soluble components. The filter cake remains with some liquid left out.

Filter

Rainwater Filtration

•Distillation is a method of separating chemical substances based on differences in their volatilities.

•Distillation usually forms part of a larger chemical process. In the distillation column, more volatile components are in the distillate, while less volatile components are in the bottoms.

• Separation is accomplished by boiling. However, perfect separation is not possible.

Distillation Column

• Each tray accomplishes a fraction of the separation task by transferring the more volatile species to the gas phase and the less volatile species to the liquid phase.

• Material and energy balances can be performed on an individual tray, the column, bottom reboiler, or top condenser, or the entire system.

Distillation Column

Distillation in Beverage Processing

• The process of evaporation is used in the different branches of the industry for food or chemicals processes, in which the concentration of the solutions is required.

• Theoretically, multiple-effect evaporators allow decreased consumption of energy for a concentration almost proportionally equal to the number of effects (evaporators).

• However, being expensive, evaporators require the reduction in the number of effects, in order to be cost-effective.

• The optimal number of effects is generally determined via calculations.

• The specifications of an evaporator are similar to those of a dryer, except that both process streams (feed and condensate) are liquids in the case of an evaporator.

Multieffect Evaporator

Multi-effect Evaporator

Milk Processing

• Humidifier is a device that increases the amount of moisture in indoor air or a stream of air. It operates by allowing water to evaporate from a pan or a wetted surface, or by circulating air through an air-washer compartment that contains moisture.

• Humidifier processes have the following characteristics: Feed gas is not saturated, liquid is evaporated in the process unit, and exit product may or may not be saturated.

Humidification

Gas humidifier

• A dehumidifier is a device that reduces the level of humidity in air or a gas stream.

• A dehumidification process has the following characteristics: Feed stream contains a condensable component and a noncondensablecomponent, and the condensate is a liquid with the condensable component only, such as water in air.

Dehumidification

A dehumidifier with internal cooling or heating coils

High-performance dehumidifiers provide an ideally balanced, constant ambient humidity level – essential for the ripening of cheese

Dehumidifiers in Food Industry

Preventing condensation, odor nuisance and the spread of germs –the perfect humidity control!

Dehumidifiers in Food Industry

• no condensation• less hygiene-related risks• the prevention of germ formation• quick drying of cut surfaces•more agreeable perceived coldness for the staff• significantly lower odor nuisance• quick drying after cleaning work• no slip hazard

• Closed system, no additional fresh air supply needed, temperature controlled

Benefits of Dehumidifiers in Food Industry

In the food production with deep-freeze sections, even in case of well-equipped air locks, it cannot be completely avoided that humid air flows into dry air zones. Due to the sub-zero temperatures prevailing there, the moisture first precipitates in form of condensate on walls, ceiling, floor, products and technology and then freezes.

Dehumidification in Food Industry

• Leaching is the removal of materials from solids by dissolving them. The chemical process industries use leaching, but the process is usually called extraction.

• Leaching of toxic materials into groundwater is a major health concern.

• Extraction processes have the following characteristics: Two liquid solvents must be immiscible and have different specific gravities, and at least one component is transferred from one solvent to the other by a difference in solubility.

• The process is often called liquid–liquid extraction.

• If one of the feed streams is a solid, the process is called leaching or liquid–solid extraction.

Leaching and Extraction

Extraction Column

• In gas absorption, a soluble component is absorbed by contact with a liquid phase in which the component is soluble. An absorber is often called a scrubber.

• This system is used for absorbing impurities from a gas stream of certain components such as hydrogen sulfide, carbon dioxide, and ammonia, using a suitable solvent.

• Absorption processes have the following characteristics: The purpose of the unit is to have the liquid absorb a component from the feed gas.

• The liquid stream flows down through the tower due to gravity, while the gas stream is pumped upward through the tower.

• No carrier gas is transferred to the liquid.

• Generally, no liquid solvent is transferred to the gas stream.

• Desorption is the same process as gas absorption except that the component transferred leaves the liquid phase and enters the gas phase.

Absorber

• In general, in an absorption tower (absorber), a gas is contacted with a liquid such that one or more components in the gas are transferred into the liquid.

• A stripping tower also involves a gas contacting a liquid, but components are transferred from the liquid into the gas.

Absorber

• A partial condenser partly condenses a vapor stream. Partial condensers have the following characteristics: Feed stream contains only condensable vapor components, and exit streams contain liquid, L, and vapor, V, which are in equilibrium. Condensation is caused by cooling or increasing pressure. Liquid and vapor emerging from the partial condenser are separated using a flash separator.

Partial Condenser

• Flash separator splits a liquid feed into vapor- and liquid-phase products.

• Flash units have the following characteristics: The process is the same as that of a partial condenser except that the feed is a liquid, and vaporization is caused by reducing the pressure or by heating.

• Vapor and liquid streams are in equilibrium.

Flash Separator

• Crystallizers are used in industry to achieve liquid–solid separation. The process for a crystallizer involves a crystallizer–filter combination so as to separate solid crystals from a solution.

• Solid crystals are formed in the unit by a change in temperature.

• Crystallization is capable of generating high purity products with a relatively low energy input

Crystallizer

• A chemical reactor carries out a chemical reaction that converts molecular species in the input (whereby a species loses its identity) to different molecular species in the output.

• A typical reactor has two reactant feed streams and a recycle stream.

Reactors

•Multiple exit streams are shown to remind you to watch for streams that separate because of their different phases. There are various types of reactors used in industry.

• The most common ones are the batch reactor, plug flow reactor (PFR), packed bed reactor (PBR), continuous stirred tank reactor (CSTR), and fluidized bed reactor.

Reactors

• The reactor content is loaded all at once and continuously mixed.

• The key characteristics of a batch reactor are unsteady-state operation (by definition) and spatial uniformity of concentration and temperature (perfectly mixed); that is, the reactor is a lumped parameter system.

Batch Reactor

• Batch operation is mainly used for small-scale production and is suitable for slow reactions.

• A batch reactor is mainly (not exclusively) for liquid-phase reactions with large charge-in/cleanup times.

Batch Reactors

• The key characteristics of a PFR are steady-state operation, variation of concentration and temperature on space, no mixing along the reactor but complete mixing in the radial direction of the reactor.

•PBR is suitable for fast reactions and mainly used for gas-phase reactions with difficult temperature control, and no moving parts.

Batch Reactors

• In Continuous stirred tank reactor (CSTR), inlet and outlet streams are continuously fed and removed, respectively. Fluidized bed reactors are sometimes treated as stirred tank reactors.

• The key characteristics of a CSTR are steady-state operation, good mixing leading to spatially uniform concentration and temperature, and the condition of the outlet stream being the same as the condition in the reactor. CSTR is used for liquid-phase reactions and is suitable for viscous liquids.

Continuous stirred tank reactor (CSTR)

•A PFD is a diagram commonly used in food and process engineering to describe the general flow of plant processes and equipment.

•PFD displays the relationship between major equipment of a plant facility and does not show minor details such as piping and control designations.

•Another commonly used term for a PFD is a flow sheet.

• In drawing the flowchart, one must know (or be able to determine) the total amount of the flow within the stream and composition of the stream.

Process Flow Diagram (PFD)

• Label what you do not know with variables.

•Major flow streams are represented by arrow lines directed from left to right in a diagram.

• Each stream line should have a specification indication as a minimum unit number and line number.

•PFDs are considered as preliminary drawings and are used to develop initial project estimates.

•A piping and instrumentation diagram, sometimes called process and instrumentation diagram (P&ID), is a diagram that shows the interconnection of process equipment and the instrumentation used to control the process.

PFDs

An amount of 100 kg/h of a mixture of 50% benzene and 50% toluene is separated in a distillation column. The distillate contains 90% benzene and the bottom stream composition is 95% toluene (compositions are in weight percent).

Draw and label the process flowchart, and specify vapor and liquid streams.

Labeling a PFD - Problem

•Known quantities: Inlet and exit stream compositions are known.

• Find: Draw the process flowchart.

•Analysis: Read the problem statement carefully, start with feed stream, and then connect the distillation column block.

• Two streams leave the distillation column, that is, top product (distillate) and bottom product.

• The following process flow sheet can be constructed like the figure in the next slide. Dashed lines indicate vapor streams

Solution

Separator PFD

Heat Exchanger

• Flash separator splits a liquid feed into vapor- and liquid-phase products.

• Flash units have the following characteristics: The process is the same as that of a partial condenser except that the feed is a liquid, and vaporization is caused by reducing the pressure or by heating.

• Vapor and liquid streams are in equilibrium.

Flash Separator

• Crystallizers are used in industry to achieve liquid–solid separation. The process for a crystallizer involves a crystallizer–filter combination so as to separate solid crystals from a solution.

• Solid crystals are formed in the unit by a change in temperature.

• Crystallization is capable of generating high purity products with a relatively low energy input

Crystallizer

Control Loops

• When attempting to solve a material balance problem, typical questions that may arise are: How many equations do I need, and where do these potentially come from? The DFA is used to address these questions.

• DFA is a highly useful tool for a systematic analysis of block flow diagrams. It provides a rapid means for assessing if a specific problem is “solvable,” that is, if the information available is sufficient, and provides a structured approach to decide on the order the equations must be solved.

• Basically, one simply counts the number of independent variables and the number of equations.

• To carry the analysis, you need to draw a flow diagram, label each stream with the components that are present in that stream, and make a list of additional information such as known flow rates, compositions, ratios, and conversions.

Degrees of Freedom Analysis

• First draw “balance boundaries,” the number of systems where you can write the material balance equation.

• There are three rules for drawing system boundaries: draw a boundary around each process unit, draw a boundary around junction points, and draw a boundary around the entire process (unless there is only one boundary).

• Second point has to do with how many equations you can write for each drawn boundary.

• You can write as many equations as there are unique components passing through the boundary.

• For a reacting system, the number of degrees of freedom (NDF) is defined as NDF = number of unknowns + number of independent reactions − number of independent material balance equations − number of useful auxiliary relations.

DFA

• The NDF can have three possible values, that is, if

•1. NDF = 0, the system is completely defined. You get a unique solution.

•2. NDF > 0, the system is under-defined (under-specified). There are an infinite number of solutions. More independent equations are needed.

•3. NDF < 0, the system is over-defined (over-specified). There are too many restrictions. Check if you have too many equations or too many restrictions. Over-defined problems cannot be solved to be consistent with all equations.

Possible Outcomes of DFA

•A set of equations are said to be independent, if you cannot derive one by adding and subtracting combinations of the others. Sources of equations that relate unknown process variables include

1. Material balances for a nonreactive process. Usually, but not always, the maximum number of independent equations that can be written equals the number of chemical species in the process.

2. Energy balances.

Independent Equations

3. Process specifications given in the problem statement.

4. Physical properties and laws, for example, density relation, gas law.

5. Physical constraints: mass or mole fractions must add to unity.

6. Stoichiometric relations for systems with chemical reactions.

Independent Equations

For example, the following set of equations derived from a material balance of a unit process is independent because we cannot derive any one by adding and/or subtracting combinations of the others:

m1+2m2+m3 = 100

2m1+m2-m3 = 200

m1+m2+2m3 = 500

Independent Equations

While the following set is not independent because we can obtain the second equation by dividing the third equation by a value of 2:

n1+2n2+n3 = 100

2n1+4n3 = 100

4n1+8n3 = 200

Independent Equations

Feed stream to a distillation column flows at a rate of 300 mol/h and contains 50 mol% of component A and 50 mol% of component B. The distillate flow is at a rate of 200 mol/h and contains 60 mol% of component A. Draw and label the process flowchart. Perform a DFA.

Binary Separation Process- Problem

•Known quantities: Feed and distillate stream flow rates and compositions.

• Find: Draw the PFD and perform a DFA.

•Analysis: The degree of freedom (DFA) analysis is provided in the table below;

Solution

Solution

The schematic of the of a binary distillation process flowchart

The number of unknowns is equal to the number of components. Since we have two components, two independent equations can be written: one is the overall material balance and the second is the component balance for one of either component.

Solution

•A feed stream flowing at a rate of 300 mol/h contains 20 mol% of components 1 and 80 mol% component 2. The distillate flow rate is 200 mol/h. Draw and label the process flowchart and perform DFA.

Binary Component Separation Process- Problem

• Known quantities: Feed flow rate and composition, distillate flow rate.

• Find: Draw and label the process flowchart, and perform DFA.

• Analysis: The process flow sheet is shown below:

Solution

•DFA is as follows;

•Conclusion?

•NDF is greater than 0 and, accordingly, the problem is under-specified. One extra piece of information is needed for the problem to be solvable.

Solution

A feed stream to a distillation column contains three components (A, B, and C). Component A’s mass flow rate is 100 kg/s, and the flow rates of components B and C are unknown. The distillate flow rate is 100 kg/s and contains 60 kg/s of component A and 40 kg/s of component B. It has been found that 40% of component A in the feed stream ends up in the bottom stream. The distillate and bottoms flow rates are equal. Draw and label the process flowchart, and perform DFA.

Multicomponent Separation Process- Problem

• Known quantities: Distillate components mass flow rates.

• Find: Draw and label the process flowchart and perform DFA.

• Analysis: The process flow sheet is shown on the right;

Solution

• The DFA is given below;

• In the feed stream, two unknowns are considered because the component flow rates are related; that is, their sum is equal to the feed flow rate, F. The same is applied for the bottom stream.

Solution

•A feed to a distillation column contains 60 kg/s of benzene (B) and 10 kg/s of toluene (T), and a small amount of xylene (X). The distillate contains pure benzene. The bottom stream flow rate is 100 kg/s. Hundred percent of toluene in the feed ends up in the bottom. Draw and label the process flowchart, and perform DFA.

Tertiary Component Separation Process- Problem

•A feed stream flows at a molar flow rate of 100 mol/h and contains three components (20% component A, 30% component B, and the balance, component C). Note that 80% of A in the feed and 50% of feed rate end up in the distillate. The bottom stream contains 10% A, 70% B, and 20% C (by moles). Draw and label the process flowchart and perform DFA.

Distillation Column- Problem

•An ethanol (E)–methanol (M) stream is fed at a rate of 1000 kg/h to be separated in a distillation column. The feed has 40% ethanol and the distillate has 90% methanol. The flow rate of the bottom product is 400 kg/h. Draw and label the PFD and perform DFA.

Binary Component Distillation Column- Problem

• Two hundred kilograms of wet leather is to be dried by heating in a dryer. The wet leather enters the drier with 1.5 g H2O per gram bone dry leather (BDL). The leather is to be dried to residual 20% moisture. Draw and label the process flowchart, and perform DFA.

Drying of Wet Solid Material- Problem

• A stream of ethanol–methanol mixture (40 wt% ethanol, 60 wt% methanol) is fed at a rate of 100 kg/h to a distillation column. The distillate has 90% methanol and the balance is ethanol. Eighty percent of methanol fed to the distillation column is to be recovered in the distillate. Draw and label the process flowchart, and perform DFA.

Binary Component Separation Process- Problem

•One hundred kilograms of wet slurry is to be dried by heating in a furnace. The wet slurry is placed in the furnace with 60% moisture and 40% dry solid (S). Note that 90% of the water is removed. Draw and label the process flowchart, and perform DFA.

Drying Process- Problem

Extraction mass balance problem