Modern Drying Technology, Volume 5

Process Intensification

Evangelos Tsotsas, Arun S. Mujumdar

• Publisher: Wiley-VCH
• Year: 2014
• Print ISBN: 9783527315604
• eBook ISBN: 9783527651405
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

This five-volume series provides a comprehensive overview of all important aspects of modern drying technology, concentrating on the transfer of cutting-edge research results to industrial use.

Volume 5 is dedicated to process intensification by hybrid processes that combine convective or contact heat transfer with microwaves, ultrasound or radiation. Process intensification by more efficient choice, distribution, and flow of the drying medium - such as impinging jet drying, pulse combustion drying, superheated steam drying, drying in specially designed spouted beds - are thoroughly discussed.

Moreover, methods that favorably affect the process by changing the structure of the drying product, e.g. foaming, electroporation, are treated. Emphasis is placed on drying, including freeze-drying, of sensitive materials such as foods, biomaterials and pharmaceuticals.

Released Volumes of Modern Drying Technology:
* Volume 1: Computational Tools at Different Scales
ISBN 978-3-527-31556-7

* Volume 2: Experimental Techniques
ISBN 978-3-527-31557-4

* Volume 3: Product Quality and Formulation
ISBN 978-3-527-31558-1

* Volume 4: Energy Savings
ISBN 978-3-527-31559-8

* Set (Volume 1-5)
ISBN 978-3-527-31554-3

1

Impinging Jet Drying

Eckehard Specht

1.1 Application

Flat products such as tiles, tissue, paper, textiles and wood veneer are often dried using nozzle arrays (Mujumdar, 2007). Figure 1.1 illustrates the basic principle of the drying process using a nozzle array. Ambient air of temperature T_a is heated in a combustion chamber or in a heat exchanger to temperature T₀, requiring the energy

. The heated air is blown through the nozzle array in order to dry the product. These conditions result in the product temperature T_S. The evaporating mass flow has the enthalpy

. The nozzle array is characterized by the inner diameter d of the individual nozzle, by the pitch t between the nozzles, and the velocity w of the released gas.

Fig. 1.1 Drying using a nozzle array.

There are basically three possible designs of nozzle arrays which differ with regard to the spent flow of the air (Fig. 1.2). In a field of individual nozzles the air can flow unimpeded between almost all nozzles; however, in a hole channel the air can flow only between those above. In a perforated plate the air can only continue to flow laterally and then escape. Hole channels and perforated plates are easier to produce than single nozzles, as they only require holes to be perforated. However, the heat transfer is the highest for nozzle fields and the lowest for perforated plates, as will be subsequently shown.

Fig. 1.2 Types of nozzle arrays. (a) Single-nozzle array; (b) Hole channel; (c) Perforated plate.

For the design of the nozzle array the energy consumption needed for drying is essential. This is the energy for heating the air:

(1.1)

where T₀ and T_a represent the temperatures of the heated air and the environment, respectively, and c_P is the average specific heat capacity between these two temperatures. The density and the volume flow rate refer to the heated air. Volume flow rate depends on the discharge velocity w and the number n of the nozzles with a diameter of d:

(1.2)

The number of nozzles will depend on the nozzle pitch t and the surface area A of the material:

(1.3)

The air temperature is calculated from the condition that the transferred heat has to cover the enthalpy of vaporization and the enthalpy to heat the dry material flow from ambient temperature to the saturation temperature T_S:

(1.4)

In Eq. 1.4,

is the evaporation flux,

is the evaporation enthalpy, and c_s is the specific heat capacity of the material. The evaporating mass flux is obtained from the relationship for the mass transfer

(1.5)

Here, the influence of one-side diffusion is taken into consideration. The gas constant of the vapor is represented by R_v, the total pressure by p, the partial pressure of the vapor in the ambient air by p_a, and the saturated vapor pressure by p_S. Additionally, the analogy of the Nusselt and Sherwood function is applied to the ratio of the heat and mass transfer coefficients:

(1.6)

wherein for the exponent of the Prandtl number in the Nusselt function the value 0.4 was used. The saturation pressure is approximated from the equilibrium relationship

(1.7)

with the reference condition P₀, T₀, for example, P₀ = 1 bar, T₀ = 373 K. The minimum required energy is the enthalpy of vaporization of the water

(1.8)

The specific drying energy is the energy for heating the air

related to the enthalpy of evaporation. From the above equations results

(1.9)

The enthalpy to heat up the dry material was omitted for clarity purposes. Specific drying energy consumption according to Eq. 1.9 is dependent on the heat transfer coefficient. Particularly at high rates of evaporation, the specific energy consumption is lower with a high heat transfer coefficient. This strongly influences the drying rate and the size of the apparatus and, as a result, an increasing heat transfer coefficient increases the rate of drying which in turn allows a reduction in the size of the apparatus. The setting and the regulation of the heat transfer coefficient is therefore of great importance. The heat transfer of bodies in a crossflow is relatively low. In generating a high heat transfer, nozzle arrays are implemented wherein the jet emerging from the nozzles is perpendicular to the body. Such flows are called stagnation point flows. Nozzles may be either round or slot-shaped.

The fields of nozzles can be made from single nozzles, or hole channels, or from perforated plates with aligned or staggered arrangements, permitting a variety of geometric parameters. The heat transfer coefficient of nozzle arrays is therefore considered in more detail in the following.

1.2 Single Nozzle

First, an air jet emerging from a single nozzle is considered. In Fig. 1.3, the generated flow field of a nozzle is shown schematically. From the nozzle with the diameter d, the flow exits with the approximately constant speed w. The jet impinges the surface virtually unchanged with a constant velocity as long as the relative distance between the nozzle and the surface h/d is less than 6. At greater distances the core velocity decreases reduci...

Table of contents

• Cover
• Modern Drying Technology
• Title Page
• Copyright
• Series Preface
• Preface of Volume 5
• List of Contributors
• Recommended Notation
• EFCE Working Party on Drying: Address List
• Chapter 1: Impinging Jet Drying
• Chapter 2: Pulse Combustion Drying
• Chapter 3: Superheated Steam Drying of Foods and Biomaterials
• Chapter 4: Intensification of Fluidized-Bed Processes for Drying and Formulation
• Chapter 5: Intensification of Freeze-Drying for the Pharmaceutical and Food Industries
• Chapter 6: Drying of Foamed Materials
• Chapter 7: Process-Induced Minimization of Mass Transfer Barriers for Improved Drying
• Chapter 8: Drying Assisted by Power Ultrasound
• Chapter 9: Microwave-Assisted Drying of Foods – Equipment, Process and Product Quality
• Chapter 10: Infrared Drying
• Index