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Chapter 8: Problem 18

Describe the pros and cons of direct drying using flue gas and steam.

Short Answer

Expert verified
Answer: Direct drying using flue gas is energy efficient and cost-effective by recovering waste heat from combustion processes. However, it may contribute to air pollution due to harmful pollutants and cause corrosion to machinery. On the other hand, steam provides a clean and consistent heat transfer over a broad temperature range, making it environmentally friendly. Nonetheless, it has higher energy consumption and a risk of condensation, which may lead to uneven drying and potential damage to the material.

Step by step solution

01

Introduction

To begin the analysis, it's necessary to understand what direct drying is and how flue gas and steam are used in this process. Direct drying is a process where the material being dried comes in direct contact with a stream of heated gas, in this case, either fluke gas or steam.
02

Direct Drying using Flue Gas: Pros

Flue gas is the exhaust gas generated during combustion processes. Using flue gas for direct drying offers a few advantages: 1. Energy efficiency: Drying with flue gas recovers the waste heat produced during combustion, reducing energy waste and overall energy consumption. 2. Cost-effective: Utilizing waste flue gas for drying purposes can result in lower energy costs because of the repurposing of otherwise lost energy.
03

Direct Drying using Flue Gas: Cons

Despite its advantages, flue gas also has a few drawbacks when used for direct drying: 1. Air pollution: Flue gas contains harmful pollutants, like sulfur dioxide and nitrogen oxides. If not properly treated before use, the drying process can inadvertently cause environmental harm. 2. Corrosion: High levels of sulfur and other corrosive chemicals in flue gas can damage machinery and materials, requiring regular maintenance and vigilance on equipment.
04

Direct Drying using Steam: Pros

Steam is another option for direct drying, and it comes with its own set of advantages: 1. Clean process: Steam does not contain harmful pollutants, eliminating the risk of air pollution and making steam-based direct drying an environmentally friendly choice. 2. Consistent heat transfer: Steam provides a steady stream of heat, ensuring a uniform drying process. 3. Broad temperature range: With proper equipment, steam can be used in a wide range of temperatures, making it suitable for various drying applications.
05

Direct Drying using Steam: Cons

However, steam is not without its drawbacks: 1. Energy consumption: Generating steam requires a source of energy, such as the use of a boiler, that may result in higher energy costs when compared to using waste flue gas. 2. Risk of condensation: During the steam drying process, steam can condense on the drying surface, leading to an uneven drying process and causing potential damage to the material being dried. Overall, both flue gas and steam come with their respective pros and cons when used for direct drying. The choice between these two options depends on the specific application and goals, along with a careful consideration of the environmental impact, energy efficiency, and cost-effectiveness.

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Most popular questions from this chapter

A certain type of straw (with 15 wt\% moisture content) is submerged in water at a mass ratio of water to straw of 4 . The straw initially contains \(0.8 \mathrm{wt} \% \mathrm{Cl}\) (ar basis). When taken out of the water, the straw is very wet with a moisture content of \(80 \mathrm{wt} \%\) but a \(\mathrm{Cl}\) content of only \(0.03 \mathrm{wt} \%\). What is the partitioning coefficient of \(\mathrm{Cl}\) when this is defined as the mass fraction of \(\mathrm{Cl}\) in the water phase divided by the \(\mathrm{Cl}\) mass fraction in the wet biomass before submerging? What do you assume?

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