# Mass and energy balance

Fabio Bocchi October 14, As a technical support engineer, one of the most common technical questions I receive is: Here, I will demonstrate how to perform these calculations in COMSOL Multiphysics and introduce some of the predefined variables available for postprocessing the energy rate terms of the energy balance equation. The heat sink is made of aluminum, shaped with a cluster of pillars for cooling, and mounted on a chip that is made of a silica glass material. Retention Time and Other Expressions for.

Dedicated to helping students and faculty use more active learning methods in their engineering courses. Chapter 7 – Energy and Energy Balances Similar to mass balances studied previously, a balance on energy is crucial to solving many problems. _____ System A “system” is an object or a collection of objects that an analysis is done on. The system has a definite . Chapter 4 Mass and Energy Balances In this chapter we will apply the conservation of mass and conservation of energy laws to open systems or control volumes of interest.

A number of terms are used to describe the average period spent in a given reactor. The terms retention time, detention time, and residence time are all used to refer tothe average period spent in the reactor.

This parameter has units of time. As discussed above, for a plug flow reactor the retention time is actually the time spent in the reactor. However, for a CSTR the retention time is the average period spent in the reactor.

The reciprocal of the retention time,has units of inverse timethe same units as a first-order rate constant. This value is sometimes referred to as the exchange rate. For the CSTR in example 2. Again, we can calculate the retention times as. This assumption is not far off for retention times significantly longer than one year. This means that pollutant concentrations can increase in Lake Ontario much more quickly than they can in Lake Michigan, but it also means that concentrations will drop much more quickly in Lake Ontario if a pollutant source is eliminated.

Energy Balance Modern society is dependent on the use of energy. Such use requires transformations in the form of energy and control of energy flows.

## How to Calculate Mass Conservation and Energy Balance | COMSOL Blog

For example, when coal is burned at a power plant, the chemical energy present in the coal is converted to heat, which is then converted in the plant's generators to electrical energy. Eventually, the electrical energy is converted back into heat for warmth or used to turn motors.

However, energy flows and transformation are also the cause of environmental problems. We can keep track of the movement of energy and changes in its form using energy balances, which are analogous to the mass balances we discussed in the previous section.

We can do this because of the law of conservation of energy which states that energy can neither be produced nor destroyed. Conservation of energy is sometimes referred to as the first law of thermodynamics.

As long as we consider all the possible forms of energy, there is no term in energy balances which is analogous to the chemical reaction term in mass balances.

## What is Energy Balance?

That is, we can treat energy as a conservative substance. Forms of Energy The forms of energy can be divided two types: Energy which is a part of the molecular structure or organization of a given substance is internal.

Energy which results from the location or motion of the substance is external. Examples of external energy include gravitational potential energy and kinetic energy. Gravitational potential energy is the energy gained when a mass is moved to a higher location above the earth.

Kinetic energy is the energy which results from the movement of objects. When a rock thrown off of a cliff accelerates toward the ground, the sum of kinetic and potential energy is conserved neglecting friction as it falls it loses potential energy, but increases in speed, gaining kinetic energy.

Examples of some common forms of energy are given in Table 4. Heat is a form of internal energy. It results from the random motions of atoms. Heat is thus really a form of kinetic energy, although it is considered separately. When you heat a pot of water, you are adding energy to the water.

Heat capacity is a property of a given material. Chemical internal energy reflects the energy in the chemical bonds of a substance.

This form of energy is composed of two parts: The strengths of the atomic bonds in the substance. When chemical reactions occur, if the sum of the energies of the products is less than that for the reactants, a reduction in chemical internal energy has occurred.

As a result of the conservation of energy, this leftover energy must show up in a different form. Usually, the energy is released as heat. This fact is used to our advantage when we burn fuel.

The energy in the bonds between molecules. This energy depends on the phase of the materialwhether it is a solid, liquid, or gas. The energy required to change phases is known as the latent heat.Write the mass and energy balances and equilibrium expressions for any stage in a column Balances around distillation stages/column The first step in doing solving distillation column problems is to .

A mass balance, also called a material balance, is an application of conservation of mass to the analysis of physical systems. By accounting for material entering and leaving a system, mass flows can be identified which might have been unknown, or difficult to measure without this technique. ﻿Energy Balance complete by __UY_HUYNH____ Energy Balance Activity ____ out of 25 points This assignment will give you the opportunity to apply some of the principles discussed about Energy Balance.

General Energy Balance m1 E1 cj1 m0 E0 cj0 V Q W in which the hat indicates an energy per unit mass. 2/ Work Term It is convenient to split the work term into three parts: Wf, the work done by the ﬂow streams while moving material into and out of the reactor, Ws, the shaft work.

1 Chapter 1 Environmental Transport and Fate – Mass & Energy Balances Benoit Cushman-Roisin Thayer School of Engineering Dartmouth College To extract useful quantitative informati on from a system, it is necessary. 1 Chapter 1 Environmental Transport and Fate – Mass & Energy Balances Benoit Cushman-Roisin Thayer School of Engineering Dartmouth College To extract useful quantitative informati on from a system, it is necessary.

Mass/Energy Balances - LearnChemE - Educational Resources for Engineering Courses