Vacuum Technology in the Chemical Industry E-Book
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- Herausgeber: Wiley-VCH
- Kategorie: Fachliteratur
- Sprache: Englisch
Based on the very successful German edition and a seminar held by the German Engineers` Association (VDI) on a regular basis for years now, this English edition has been thoroughly updated and revised to reflect the latest developments. It supplies in particular the special aspects of vacuum technology, applied vacuum pump types and vacuum engineering in the chemical, pharmaceutical and process industry application-segments. The text includes chapters dedicated to latest European regulations for operating in hazardous zones with vacuum systems, methods for process pressure control and regulation and leak detection.
All of the authors work or did work at a selection of the most important German companies involved in vacuum technology, and their expertise is disseminated here for engineers working in vacuum technology, chemical process design, plant operation, and mechanical engineering.
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Veröffentlichungsjahr: 2015
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Table of Contents
Cover
Related Titles
Title Page
Copyright
List of Contributors
Chapter 1: Fundamentals of Vacuum Technology
1.1 Introduction
1.2 Fundamentals of Vacuum Technology
References
Chapter 2: Condensation under Vacuum
2.1 What Is Condensation?
2.2 Condensation under Vacuum without Inert Gases
2.3 Condensation with Inert Gases
2.4 Saturated Inert Gas–Vapour Mixtures
2.5 Vapour–Liquid Equilibrium
2.6 Types of Condensers
2.7 Heat Transfer and Condensation Temperature in a Surface Condenser
2.8 Vacuum Control in Condensers
2.9 Installation of Condensers
2.10 Special Condenser Types
Further Reading
Chapter 3: Liquid Ring Vacuum Pumps in Industrial Process Applications
3.1 Design and Functional Principle of Liquid Ring Vacuum Pumps
3.2 Operating Behaviour and Design of Liquid Ring Vacuum Pumps
3.3 Vibration and Noise Emission with Liquid Ring Vacuum Pumps
3.4 Selection of Suitable Liquid Ring Vacuum Pumps
3.5 Process Connection and Plant Construction
3.6 Main Damage Symptoms
3.7 Table of Symbols
Chapter 4: Steam Jet Vacuum Pumps
4.1 Design and Function of a Jet Pump
4.2 Operating Behaviour and Characteristic
4.3 Control of Jet Compressors
4.4 Multi-Stage Steam Jet Vacuum Pumps
4.5 Comparison of Steam, Air and Other Motive Media
Further Reading
Chapter 5: Mechanical Vacuum Pumps
5.1 Introduction
5.2 The Different Types of Mechanical Vacuum Pumps
5.3 When Using Various Vacuum Pump Designs in the Chemical or Pharmaceutical Process Industry, the Following Must Be Observed
References
Chapter 6: Basics of the Explosion Protection and Safety-Technical Requirements on Vacuum Pumps for Manufacturers and Operating Companies
6.1 Introduction
6.2 Explosion Protection
6.3 Directive 99/92/EC
6.4 Directive 94/9/EC
6.5 Summary
References
Further Reading
Chapter 7: Measurement Methods for Gross and Fine Vacuum
7.1 Pressure Units and Vacuum Ranges
7.2 Directly and Indirectly Measuring Vacuum Gauges and Their Measurement Ranges
7.3 Hydrostatic Manometers
7.4 Mechanical and Electromechanical Vacuum Gauges
References
Further Reading
Chapter 8: Leak Detection Methods
8.1 Definition of Leakage Rates
8.2 Acceptable Leakage Rate of Chemical Plants
8.3 Methods of Leak Detection
8.4 Helium as a Tracer Gas
8.5 Leak Detection with Helium Leak Detector
8.6 Leak Detection of Systems in the Medium-Vacuum Range
8.7 Leak Detection on Systems in the Rough Vacuum Range
8.8 Leak Detection and Signal Response Time
8.9 Properties and Specifications of Helium Leak Detectors
8.10 Helium Leak Detection in Industrial Rough Vacuum Applications without Need of a Mass Spectrometer
References
Further Reading
European Standards
Chapter 9: Vacuum Crystallisation
9.1 Introduction
9.2 Crystallisation Theory for Practice
9.3 Types of Crystallisers
9.4 Periphery
9.5 Process Particularities
9.6 Example – Crystallisation of Sodium Chloride
References
Chapter 10: Why Evaporation under Vacuum?
Summary
10.1 Introduction
10.2 Thermodynamics of Evaporation
10.3 Pressure/Vacuum Evaporation Comparison
10.4 Possibility of Vapour Utilization
Further Reading
Chapter 11: Evaporators for Coarse Vacuum
Summary
11.1 Introduction
11.2 Criteria for the Selection of an Evaporator
11.3 Evaporator Types
Further Reading
Chapter 12: Basics of Drying Technology
12.1 Basics of Solids–Liquid Separation Technology
12.2 Basics of Drying Technology
12.3 Discontinuous Vacuum Drying
12.4 Continuous Vacuum Drying
12.5 Dryer Designs
Reference
Chapter 13: Vacuum Technology Bed
13.1 Introduction to Fluidized Bed Technology
13.2 Vacuum Fluidized Bed Technology
References
Chapter 14: Pharmaceutical Freeze-Drying Systems
14.1 General Information
14.2
Phases
of a Freeze-Drying
Process
14.3 Production Freeze-Drying Systems
14.4 Final Comments
Further Reading
Chapter 15: Short Path and Molecular Distillation
15.1 Introduction
15.2 Some History
15.3 Outlook
References
Chapter 16: Rectification Under Vacuum
16.1 Fundamentals of Distillation and Rectification
16.2 Rectification under Vacuum Conditions
16.3 Vacuum Rectification Design
16.4 Structured Packings for Vacuum Rectification
References
Chapter 17: Vacuum Conveying of Powders and Bulk Materials
17.1 Introduction
17.2 Basic Theory
17.3 Principle Function and Design of a Vacuum Conveying System
17.4 Continuous Vacuum Conveying
17.5 Reactor- and Stirring Vessel Loading in the Chemical Industry
17.6 Conveying, Weighing, Dosing and Big-Bag Filling and Discharging
17.7 Application Parameters
References
Chapter 18: Vacuum Filtration – System and Equipment Technology, Range and Examples of Applications, Designs
18.1 Vacuum Filtration, a Mechanical Separation Process
18.2 Design of an Industrial Vacuum Filter Station
18.3 Methods of Continuous Vacuum Filtration, Types of Design and Examples of Application
References
Index
EULA
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Guide
Cover
Table of Contents
Chapter 1: Fundamentals of Vacuum Technology
List of Illustrations
Figure 1.1
Figure 1.2
Figure 1.3
Figure 1.4
Figure 1.5
Figure 1.6
Figure 2.1
Figure 2.2
Figure 2.3
Figure 2.4
Figure 2.5
Figure 2.6
Figure 2.7
Figure 2.8
Figure 2.9
Figure 2.10
Figure 2.11
Figure 2.12
Figure 2.13
Figure 2.14
Figure 2.15
Figure 2.16
Figure 2.17
Figure 3.1
Figure 3.2
Figure 3.3
Figure 3.4
Figure 3.5
Figure 3.6
Figure 3.7
Figure 3.8
Figure 3.9
Figure 3.11
Figure 3.10
Figure 3.12
Figure 3.13
Figure 3.14
Figure 3.15
Figure 3.16
Figure 3.17
Figure 3.18
Figure 3.19
Figure 3.20
Figure 3.22
Figure 3.23
Figure 3.24
Figure 3.25
Figure 3.26
Figure 3.27
Figure 3.28
Figure 3.29
Figure 3.30
Figure 3.31
Figure 3.32
Figure 3.33
Figure 3.34
Figure 3.35
Figure 3.36
Figure 3.37
Figure 3.38
Figure 3.39
Figure 3.40
Figure 3.41
Figure 3.42
Figure 3.43
Figure 3.44
Figure 4.1
Figure 4.2
Figure 4.3
Figure 4.4
Figure 4.5
Figure 4.6
Figure 4.7
Figure 4.8
Figure 4.9
Figure 4.10
Figure 4.11
Figure 4.12
Figure 4.13
Figure 4.14
Figure 4.15
Figure 4.16
Figure 4.17
Figure 4.18
Figure 4.19
Figure 5.1
Figure 5.2
Figure 5.3
Figure 5.4
Figure 5.5
Figure 5.6
Figure 5.7
Figure 5.8
Figure 5.9
Figure 5.10
Figure 5.11
Figure 5.12
Figure 5.13
Figure 5.14
Figure 5.15
Figure 5.16
Figure 5.17
Figure 5.18
Figure 5.19
Figure 5.20
Figure 5.21
Figure 5.22
Figure 5.23
Figure 5.24
Figure 6.1
Figure 6.2
Figure 6.3
Figure 6.4
Figure 6.5
Figure 6.6
Figure 6.7
Figure 6.8
Figure 6.9
Figure 6.10
Figure 6.11
Figure 7.1
Figure 7.2
Figure 7.3
Figure 7.4
Figure 7.5
Figure 7.6
Figure 7.7
Figure 8.1
Figure 8.2
Figure 8.3
Figure 8.4
Figure 9.1
Figure 9.2
Figure 9.3
Figure 9.4
Figure 9.5
Figure 9.6
Figure 9.7
Figure 9.8
Figure 9.9
Figure 9.10
Figure 9.15
Figure 9.11
Figure 9.12
Figure 9.13
Figure 9.14
Figure 9.16
Figure 9.17
Figure 9.18
Figure 9.19
Figure 9.20
Figure 9.21
Figure 9.22
Figure 9.23
Figure 9.24
Figure 9.25
Figure 9.26
Figure 9.27
Figure 10.1
Figure 10.2
Figure 10.3
Figure 10.4
Figure 10.5
Figure 10.6
Figure 10.7
Figure 11.1
Figure 11.2
Figure 11.3
Figure 11.4
Figure 11.5
Figure 11.6
Figure 11.7
Figure 11.8
Figure 11.9
Figure 11.10
Figure 11.11
Figure 12.1
Figure 12.2
Figure 12.3
Figure 12.4
Figure 12.5
Figure 12.6
Figure 12.7
Figure 12.8
Figure 12.9
Figure 12.10
Figure 12.11
Figure 12.12
Figure 12.13
Figure 12.14
Figure 12.15
Figure 12.16
Figure 12.17
Figure 12.18
Figure 12.19
Figure 12.20
Figure 12.21
Figure 13.1
Scheme 13.1
Scheme 13.2
Scheme 13.3
Figure 13.2
Figure 14.1
Figure 14.2
Figure 14.3
Figure 14.4
Figure 14.5
Figure 14.6
Figure 14.7
Figure 14.8
Figure 14.9
Figure 14.10
Figure 14.11
Figure 14.12
Figure 14.13
Figure 14.14
Figure 14.15
Figure 14.16
Figure 14.17
Figure 15.1
Figure 15.2
Figure 15.3
Figure 15.4
Figure 15.5
Figure 15.6
Figure 15.7
Figure 15.8
Figure 15.9
Figure 15.10
Figure 15.11
Figure 15.12
Figure 16.1
Figure 16.2
Figure 16.3
Figure 16.4
Figure 16.5
Figure 16.6
Figure 17.1
Figure 17.2
Figure 17.3
Figure 17.4
Figure 17.5
Figure 17.6
Figure 17.7
Figure 17.8
Figure 17.9
Figure 17.10
Figure 17.11
Figure 17.12
Figure 17.13
Figure 17.14
Figure 17.15
Figure 17.16
Figure 17.18
Figure 17.17
Figure 18.1
Figure 18.2
Figure 18.3
Figure 18.4
Figure 18.5
Figure 18.6
Figure 18.7
Figure 18.8
Figure 18.9
Figure 18.10
Figure 18.11
Figure 18.12
Figure 18.13
Figure 18.14
Figure 18.15
Figure 18.16
Figure 18.17
Figure 18.18
Figure 18.19
Figure 18.20
Figure 18.21
Figure 18.22
Figure 18.23
Figure 18.24
Figure 18.25
Figure 18.26
Figure 18.27
Figure 18.28
Figure 18.29
Figure 18.30
Figure 18.31
Figure 18.32
Figure 18.33
List of Tables
Table 1.1
Table 2.1
Table 5.1
Table 5.2
Table 6.1
Table 6.2
Table 6.3
Table 6.4
Table 6.5
Table 6.6
Table 8.1
Table 8.2
Table 8.3
Table 13.1
Table 15.1
Table 16.1
Table 16.2
Table 16.3
Table 17.1
Table 17.2
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Edited by Wolfgang Jorisch
Vacuum Technology in the Chemical Industry
Editor
Dr. Wolfgang Jorisch
Kreisbahnstrasse 66
52511 Geilenkirchen
Germany
All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
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List of Contributors
Daniel Bethge
GIG Karasek GmbH
Research and Development
Neusiedlerstraße 15-19
Gloggnitz-Stuppach
Austria
Harald Grave
Forschung und Entwicklung
GAE Wiegand GmbH
Andreas Hofer Street 3
Karlsruhe
Germany
Werner Große Bley
INFICON GmbH
Bonner Strasse 498
Köln
Germany
Pierre Hähre
Speck Pumps GmbH & Co KG
Vacuum Technology Department
Regensburger Ring 6-8
Roth
Germany
Hartmut Härtel
IBExU Institut für Sicherheitstechnik GmbH
An-Institut der Bergakademie Freiberg
Fuchsmühlenweg 7
Freiberg
Germany
Manfred Heldner
Hüttengarten 23
Bornheim-Widdig
Germany
Guenter Hofmann
GEA Messo PT Duisburg
Friedrich-Ebert-Street 134
Duisburg
Germany
Thorsten Hugen
Julius Montz GmbH
Hofstraße 82
Hilden
Germany
Michael Jacob
Glatt Ingenieurstechnik GmbH
Nordstr 12
Weimar
Germany
Wolfgang Jorisch
IVPT Industrielle Vakuumprozeßtechnik
Kreisbahnstrasse 66
Geilenkirchen
Germany
Gregor Klinke
GEA WIEGAND
Department Research and Development
Andreas-Hofer-Straße 3
Karlsruhe
Germany
Jürgen Oess
SiCor Engineering Partners Gbr
Burgunderweg 5
Bönnigheim
Germany
Thomas Ramme
Volkmann GmbH
Schloitweg 17
Soest
Germany
Franz Tomasko
FLSmidth Wiesbaden GmbH
Am Klingenweg 4a
Walluf
Germany
1Fundamentals of Vacuum Technology
Wolfgang Jorisch
1.1 Introduction
Vacuum technology is being used widely in many chemistry applications. Here it is not used in the same way as in physics applications. In physics applications, it is the objective to perform experiments in volumes (vacuum chambers) which are as pure as possible, that is, which contain as few particles as possible as these particles generally impair the physical process.
Vacuum technology is used in the area of chemistry applications for the purpose of performing basic thermal and mechanical operations to reprocess reaction products under conditions which preserve the product. Typical applications for thermal separating processes in a vacuum are distillation, drying or sublimation at reduced pressures as well as applications which accelerate the reaction itself when reaction products from the reaction mixture need to be removed for the purpose of shifting the equilibrium in the desired direction, for example. An example of this is the process of esterification.
A mechanical process performed in a vacuum is that of vacuum filtration where the pressure difference created between vacuum and atmospheric pressure is utilised as the driving force for the filtration process.
The planning process engineer or the consulting engineer of a chemical plant not only faces questions how to properly dimension a vacuum system so as to comply with the demanded process specifications, but he needs to solve in a satisfactory way, problems relating to operating costs which shall be as low as possible and questions as to the minimisation of emissions in the discharged air and waste water. The wide variety of vacuum pumps used in the area of chemistry technology reflects this. The responsible planning engineer or plant chemist will have to select, in consideration of the process engineering questions which differ from process to process, vacuum generators which promise to offer the best possible solution for the specific case.
For this reason, this book covers besides vacuum process engineering fundamentals, above all also the different types of vacuum pumps.
1.2 Fundamentals of Vacuum Technology
Also for the vacuum technology used in chemistry applications, the underlying fundamental laws of physics apply.
The standard DIN 28400 Part 1 defines the vacuum state as
Vacuum is the state of a gas, the particle number density of which is below that of the atmosphere at the Earth's surface. Since the particle number density is within certain limits time and location dependent, it is not possible to state a general upper limit for the vacuum. Here the gas particles exert a pressure on all bodies which surround them, this pressure being the result of their temperature dependent motion. The pressure is defined as a force per unit of area, with the unit of measurement being the Pascal.
In the area of vacuum process engineering, frequently not Pascal is used as the unit of measurement for pressures but instead also the allowed unit ‘bar’ or ‘mbar’.
Owing to the differing behaviour of the particles within the considered volume (particle number density) which is dependent on the number of particles which are present, different pressure (vacuum) ranges have been defined with respect to their flow characteristics, for example:
Pressure range (mbar)
Rough vacuum
Medium vacuum
High vacuum
Ultrahigh vacuum
Remark:
New definition of beginning rough vacuum:
Lowest pressure on Earth surface
300 mbar (Mount Everest)
The pressure
When gas particles impinge on a wall (surface) they are subjected to a change in impulse, whereby they transfer an impulse to this wall. This impulse is the cause for the pressure exerted on the wall:
since the force is equivalent to the change in impulse of over time:
p = pressure; F = force; A = area; m = mass; v = velocity; and t = time.
When now considering a surface onto which particles impinge from a hemisphere and when integrating the transferred impulse over time, then one obtains for ideal gases
= particle number density; k = Boltzmann constant and T = absolute temperature.
That is, the exerted pressure is only dependent on the number of gas molecules n in the volume but is not dependent on the type of gas [1].
This statement ultimately confirms also Dalton's Law, which states that the total pressure of a gas atmosphere is equal to the sum of all partial pressures of this gas mixture:
or
= total pressure; = particle number density of all gas particles (types); and = particle number density of the particle type i.
Given as an example is in the following the composition of air at atmospheric pressure [2] (Table 1.1).
Table 1.1 Composition of atmospheric air
Constituent
Volume share (%)
Partial pressure (mbar)
Nitrogen
—
78.09
—
780.9
Oxygen
—
20.95
—
209.5
Argon
—
0.93
—
9.3
Carbon dioxide
—
0.03
—
3.10
−1
Water vapour
2.3
—
—
Remainder: noble gases, hydrogen, methane, ozone, and so on.
1.2.1 Fundamentals of Gas Kinetics
The individual molecules or gas particles contained in a volume are in constant motion and collide with each other. In doing so, the particles change their velocity each time they collide. From a statistical point of view, all velocities are possible but with differing probability.
Maxwell and Boltzmann found the following relationship for the velocity distribution of the gas particles [3]:
=
mass of each particle
=
temperature in Kelvin
=
number of particles
=
Boltzmann constant
=
velocity of the particles.
Figure 1.1 depicts the velocity distribution between velocity and based on the example of air at 0, 25 and 400 °C [3].
Figure 1.1 Velocity distribution of air molecules (nitrogen and oxygen) at 0, 25 and at 400 °C.
From this, it is apparent that there does not exist a molecule with a velocity of ‘zero’ or with an infinitely high velocity. The location of the most probable velocity (maximum, ) is a function of the mean gas temperature. Moreover, the molecule velocity depends on the molar mass. The most likely velocity can be stated through
and the arithmetic mean velocity can be stated through
Figure 1.2 depicts the velocity of the gas molecules as a function of the type of gas.
Figure 1.2 Gas molecule velocity as a function of the type of gas.
1.2.1.1 Mean Free Path
The fact that the molecules move at different velocities allows for the conclusion that they will move within a specific unit of time over a different distance (free path) before colliding with another particle. The mean free path , resulting from the kinetic gas theory is
where = collision radius in [mm] of an ideal point-like particle and is the particle number density (number of molecules per volume).
Since the particle number density, as already derived, depends on the pressure, also the mean free path of the gas molecules is pressure dependent (at constant temperature), the product of the prevailing pressure and the mean free path is, at a given temperature, a constant (gas-type dependent).
For nitrogen at 20 °C, this product amounts to , that is at a prevailing pressure of there results at a temperature of 20 °C a mean free path of 6.5 mm [4].
Moreover, kinetic gas theory states the distribution of the mean free paths as follows:
=
number of molecules in the volume
=
number of the molecules which have a free path of
.
From this equation, it can be derived that 63% of all collisions occur after a path of whereas 37% of the collisions occur after a path of .
Only 0.6% have a free path exceeding .
1.2.2 Equation of State for Ideal Gases
In the year 1662, Robert Boyle demonstrated that the volume of a gas, provided it is maintained at a constant temperature, is inversely proportional to the pressure:
In 1787, the French chemist Charles found the law according to which the volume of a gas is proportional to its temperature when maintaining it at a constant pressure:
Also, the French physicist Amontons discovered that the pressure of a gas is proportional to its temperature when maintaining it at a constant volume:
It is apparent for formal reasons that at also the volume and the pressure of the ideal gas disappear. From this it can be concluded that the temperature represents a physical limit for the temperature (absolute zero).
The laws of Charles and Amontons can be combined as
and
One then obtains an equation of state which contains all state variables (p, V, T):
The Italian physicist Avogadro discovered in 1811, that the pressure of a gas is proportional to the number of molecules present:
At a ‘standard temperature’ of (0 °C) and a ‘standard pressure’ of (101 300 Pa) 1 mol of a gas always contains particles taking up the volume of 22.4136 l (molar volume).
When inserting for the ‘standard pressure ’, for the ‘standard temperature ,’ and for the ‘molar volume ’, then one obtains the ‘universal or molar gas constant R’ for ideal gases:
or
Since the molar volume can also be expressed as the volume of the gas divided by the number of moles ():
one obtains from Eqs. (1.20) and (1.22) the universal equation of state for ideal gases:
This equation of state which in effect only applies to ‘ideal gases’ may in a vacuum be applied with good accuracy also to real gases and vapours, because in vacuum the interactions between the particles reduce owing to the increasing free path. For this reason, this equation of state can be used to calculate pumped gas, respectively vapour quantities as they occur in connection with chemical engineering processes.
1.2.3 Flow of Gases through Pipes in a Vacuum
The flow conditions within a gas can be described through the Knudsen number. Flows through pipes are characterised through the Reynolds number. At relatively high pressures in a rough vacuum, a viscous flow is present which may either be laminar or turbulent. For a viscous flow, a mutual influence between the flowing particles is typical. In the turbulent range, the molecules behave chaotically. The Knudsen number is defined as follows:
A viscous gas flow is characterised through a Knudsen number of < 0.01.
The pipe diameter is here much bigger than the mean free path of the molecules, and the gas flow is characterised by constant collisions amongst the particles.
The boundary between a continuous flow and a turbulent flow can be characterised through the Reynolds number (Re):
=
gas density in kg/m
3
=
viscosity the gas
=
flow velocity
=
pipe diameter.
The following applies to the flow through pipes:
When the non-dimensional Reynolds number attains values of over 2200, then one speaks of a turbulent flow, in the case of values below 1200 one then speaks of a laminar flow. In the range in between, either turbulent or laminar flow conditions can be present.
When the mean free path
of the molecules is equal to the pipe diameter
or exceeds it, then there results a Knudsen number of >1 and when the Reynolds number is <1200 then the behavior of the gas molecules becomes more and more individual, that is, the flow changes to a ‘molecular flow’.
1.2.3.1 Gas Throughput and Conductance
The term gas throughput is defined as a quantity of gas flowing through a given area per unit of time.
The gas quantity Q corresponds here to the volume of gas occurring per unit of time multiplied with the prevailing pressure p and Eq. (1.29):
The SI unit for the gas throughput is Pa m3 s−1.
The gas throughput corresponds to an energy passing through an area per unit of time [3]
From Eq. (1.28) one then obtains the gas throughput as a mass flow:
= molar mass of the gas.
The transported gas quantity , which flows through a pipe is in the case of a continuous flow dependent on the prevailing pressure difference between inlet and outlet.
When taking the ratio between the gas quantity being throughput and the prevailing pressure difference across the pipe, then one obtains a quantity which can be designated as ‘conductance’ of this pipe:
Equation (1.31) is also termed ‘Ohm's Law’ of vacuum technology. When now considering the flow of a gas through an orifice (very short pipe) one obtains a rather complicated dependency of the gas being throughput as a function of the pressure difference ahead of, and after the orifice. Let the pressure ahead of the orifice be the atmospheric standard pressure, whereas the pressure is being reduced behind the orifice. Through the reduction in ‘flow out pressure’ the gas quantity being put through will increase until it attains a maximum. One now speaks of a critical pressure ratio, the gas then flows through the orifice at the speed of sound.
The gas quantity now passing through the orifice can be calculated from [3]
where , = area of the orifice, = pressure ahead of the orifice, = pressure downstream of the orifice, = heat capacity at constant pressure, respectively constant volume.
is a reduction factor since the gas which passes at a high velocity through the orifice, is subject to a volume contraction downstream of the orifice (‘vena contracta’).
For thin orifices, has a value of approximately 0.85.
When after attaining the speed of sound, the flow out pressure is reduced further, then the gas throughput will not increase further. Under these conditions the following applies
and one speaks of the critical expansion ratio.
The gas flow which is then being put through becomes
And is thus independent of the flow out pressure .
Equation (1.33) represents the critical gas flow. For air, .
And blocking takes place at . Figure 1.3 clarifies the flow under blocking conditions.
Figure 1.3 Gas flow under blocking conditions after attaining the critical pressure ratio.
These flow resistance considerations are important to vacuum systems when through these, gas flows are restricted in connection with pump-down processes or venting processes or when measuring leaks, for example.
1.2.3.2 Flow through Long, Round Pipes
The most well-known mathematical approach describing a viscous flow through long, straight pipes is through the so-called Hagen–Poiseuille equation:
The conductance of a cylindrical pipe for air at 0 °C is stated in litres per second whereby diameter d and pipe length l are entered in centimetres and the pressure p is entered in millibars as follows [4]:
This special solution is valid, provided the following additional conditions are fulfilled:
The flow profile in the pipe is fully present (location independent, applies to long pipes).
The flow conditions are laminar, that is,
and
.
The ratio between the actual gas velocity and speed of sound is <0.3.
A comprehensive description of the flow conditions in gas flows also through conducting sections exhibiting different geometries under different pressure and flow conditions can be found in [2, 3].
Analogously to ‘Ohm's Law’ the conductance changes when connecting pipes, valves or equipment like condensers as follows:
Conductances add for a parallel connection:
Conductances add reciprocally for a series connection:
Figure 1.4 states the conductances for pipes of commonly used nominal widths for a laminar flow [5].
Figure 1.4 Conductance values of pipes of common nominal widths with a circular cross-section (apply to a laminar flow (p = 1 mbar), flowing medium is air).
If in the pipe, bends and components like valves are present then a correction is introduced by assuming a longer length of the pipe (‘equivalent pipe lengths’).
In Figure 1.5 the equivalent pipe lengths of valves and pipe bends are stated. As to the conductance values for equipment it will be necessary to ask the manufacturers for such information. As a rule, all vacuum lines should be as short as possible and have a diameter which is as large as possible (same dimension as the intake port on the vacuum pump used).
Figure 1.5 Equivalent pipe lengths in metres for components.
In the case of a molecular flow, the conductance becomes independent of the mean pressure difference across the length of the pipe [4]:
1.2.4 Vacuum Pumps Overview
The standard DIN 28400 Part 2 provides an overview of commonly used vacuum pump types.
However, in the area of chemical process engineering only pumps of the following general types are used:
Positive displacement pumps:
diaphragm vacuum pumps, liquid ring vacuum pumps, rotary vane and rotary piston vacuum pumps as well as Roots vacuum pumps (dry compressing claw and screw pumps are not yet mentioned here but also belong to this group of pumps).
Kinetic vacuum pumps:
vapour jet pumps, gas ejector pumps and liquid jet pumps.
Adsorption pumps:
the condenser.
The specific design types of different vacuum pumps, their special characteristics and application in the area of chemical process engineering are covered in the following chapters of this book.
The areas of application for these vacuum pumps are all within the rough and medium vacuum range, the principal vacuum range for chemistry processes. Only the short path and molecular distillation processes rely also high-vacuum pumps like the diffusion pump or even the turbomolecular pump (kinetic gas pumps). High-vacuum pumps are not covered in this book. For these refer to [2, 3].
It is the task of all vacuum pumps outlined above to generate a vacuum in a process engineering plant, for example, and to provide enough pumping speed. A pumping speed is in the end only a volume which passes per unit of time through an area.
Or the pumping speed of a vacuum pump corresponds to the gas throughput , which passes through the area of the intake port of a vacuum pump, for example within a certain time (), divided by the prevailing pressure.
The pumping speed of a vacuum pump operating in the rough and medium vacuum range is commonly stated in .
Figure 1.6 depicts the so-called pumping speed diagram of a mechanical rotary vane vacuum pump. The pumping speed is graphed as a function of the intake pressure.
Figure 1.6 Pumping speed diagram based on the example of a rotary vane vacuum pump.
The evacuation process for a vacuum chamber, like that of a distillation column, is effected with a rough vacuum pump, regardless of type, as follows:
The gas quantity pumped out per unit of time from a constant distillation column volume flows at the prevailing pressure in the distillation column into the intake port of the vacuum pump. The corresponding gas quantity is equal to the effective pumping speed of the vacuum pump, respectively the pump system connected to the vacuum chamber (due to low conductance values of the vacuum line, the pumping speed of the vacuum pump may suffer throttling), multiplied by the prevailing pressure:
The change in pressure within the distillation column is proportional to the pumpdown time:
By integrating the relative change in column pressure over time, one obtains the relationship between pumpdown time , the attained pressure p in the column and the initial pressure [6]:
respectively
This relationship applies to a pressure range, in which the pumping speed of the vacuum pump, respectively the pump system is almost constant (see Figure 1.6).
For vacuum applications in the area of chemical process engineering, the question as to a specific pumpdown time rarely arises. Instead process and leakage gas flows need to be pumped away by a vacuum pump to such an extent that a demanded operating pressure is reliably attained and maintained.
2Condensation under Vacuum
Harald Grave
2.1 What Is Condensation?
The origin of the word ‘condensation’ is from the Latin ‘condensare’. This means ‘to seal up, to compact’. In this chapter, it stands for the compression and the congregation of molecules during the transition from the gaseous to the liquid state. For a physicist, this is the transition to a molecular motion with less energy. Therefore, condensation is combined with a release of energy. This energy is emitted as the condensation heat, which has to be dissipated. The reverse process is evaporation. Each liquid has a vapour pressure that increases with an increase in temperature; the relationship between the saturation temperature Ts and vapour pressure pv of a substance is shown in the vapour pressure diagram. The vapour pressure curves for water and some solvents for a range from 1 mbar to 100 mbar are shown in Figure 2.1.
Figure 2.1 Vapour pressure diagram for some substances. (1) Hexane, (2) benzene, (3) ethanol, (4) water, (5) octane, (6) propanoic acid, (7) dodecane, (8) hexadecane, and (9) nonadecane.
Here, the region to the right of the vapour pressure curve represents the gaseous state. To the left side of the curve, the substance is liquid or solid. In the condition described by the curve liquid and saturated vapour coexist in equilibrium. When the temperature of an unsaturated (superheated) vapour is reduced, condensation starts at the saturation temperature (dew point temperature) corresponding to the actual vapour pressure.
The heat of condensation – which is equal to the heat of evaporation – has to be drawn off during the condensation. The specific heat of condensation depends on temperature; for example, for water at ϑ = 25 °C the heat of condensation is about 10% higher than at ϑ = 100 °C. For some substances, the heat of condensation is given in Table 2.1.
Table 2.1 Molar mass M and specific condensation enthalpy r at 1013 mbar pressure for some substances
Substance
Molar mass (kmol kg
−1
)
Heat of condensation (kJ kg
−1
)
Acetone
58.08
523
Benzene
78.11
394
Ethanol
46.07
846
Hexane
86.18
335
Octane
114.23
301
Water
18.02
2257
If the vapour is not saturated – according to the condition given by the vapour pressure diagram – but has a higher temperature, that is, the vapour is superheated, then the superheat also has to be drawn off.
2.2 Condensation under Vacuum without Inert Gases
One can imagine that there is some liquid in a container. This liquid will be evaporated. A weightless, movable piston lies on the liquid surface, as represented in Figure 2.2. If the liquid is water and this is heated, as in Figure 2.3, then the water evaporates at 100 °C and the piston moves upward, so that the evaporation process can take place at constant pressure (1000 mbar) and at constant temperature (100 °C).
Figure 2.2 Container with weightless piston.
Figure 2.3 Boiling water and steam at atmospheric pressure.
The space between piston and water surface is filled with steam at atmospheric pressure. Then, the heating is stopped, the piston is fixed at its actual position, and the container is put into a cooling tank, as shown in Figure 2.4. Here, the complete equipment is cooled to 20 °C. The major part of the steam will condense and, if the equipment is sealed, a vacuum of 24 mbar can be measured. For the assumptions made, a vacuum is generated by condensation.
Figure 2.4 Steam and water in phase equilibrium.
2.3 Condensation with Inert Gases
Starting from the condition in Figure 2.3, suppose that a certain amount of air (a noncondensable gas and in the following known as inert gas) was also included in the container before the cooling. With a total pressure of 1000 mbar, for example, a water vapour partial pressure of 950 mbar is assumed. If one cools down to 20 °C with a constant volume, the mass of air remains the same and the partial pressure of air decreases from 50 to 40 mbar by the cooling from 100 to 20 °C. This partial pressure has to be added to the water vapour partial pressure of 24 mbar after water vapour condensation. So a vacuum of 64 mbar is created in this case. The remaining air–vapour mixture is saturated with water vapour, because the water vapour partial pressure is equal to the saturation pressure at the actual temperature of 20 °C. Instead of water, one can imagine any arbitrary liquids. Depending on the vapour pressure diagram for this media, other pressures will be reached.
Often condensable vapours are emitted continuously in vacuum processes, that is, at almost constant mass flow rates. In such a vapour flow, a certain inert gas fraction is normally contained, this can, for example, be a leakage air stream escaping into the vacuum equipment, which should, of course, be kept to a minimum. If this mixture flows along a suitable cooled surface at constant pressure, here, that is, under vacuum, then vapour will condense. The inert gas percentage, which was small at the beginning of the condensation process, will become larger with progressive condensation. Then, at the end of the condensation process, the remaining inert gas–vapour mixture must be removed. This is done via extraction by means of a suitable vacuum pump. The vapour portion, with which, at the end of the condensation surface, the inert gas is saturated, is naturally lesser with a reduction in temperature. Therefore, it is appropriate to extract at the coldest section of the condenser, that is, where the coolant is supplied. This is the reason why the coolant is usually taken in counterflow to the inert gas–vapour mixture that is to be condensed (Figure 2.5).
Figure 2.5 Counterflow of vapour and coolant in a surface condenser.
2.4 Saturated Inert Gas–Vapour Mixtures
An inert gas–vapour mixture saturated with condensing vapours has to be extracted independent of the condenser design. If there is only one kind of vapour, then the saturation quantity can be calculated easily with the equation:
M is the molecular weight, p is the partial pressure, and the mass flow rate. The index ‘I’ stands for the inert gas and ‘V’ for vapour.
An example can best illustrate the influence the condensation conditions have on the specification of the vacuum pump.
Steam is to condense at a vacuum of 60 mbar. This corresponds to a saturated steam temperature of 36 °C. The steam to be condensed contains 10 kg h−1 of air. The condenser should be suitable to cool the air, together with the included steam down to 30 °C. The question is now, how much steam–air mixture has still to be extracted by a vacuum pump. The steam partial pressure of a saturated mixture with 30 °C is pv = 42.4 mbar (= saturated steam pressure at 30 °C). The partial pressure of the inert gas is then the difference between the total pressure and the steam partial pressure.
Thus, the saturation quantity of steam is calculated with Equation 2.1:
This means, the vacuum pump must extract 15 kg h−1 steam and the 10 kg h−1 air from 60 mbar. It can be seen that the quantities of saturation steam are considerable in vacuum. The importance of good cooling of the outlet flow from a condenser is shown by the fact that for the above example a decrease in the outlet temperature from 30 to 25 °C would mean a reduction of the steam mass flow rate to only 7 kg h−1.
However, the conditions are normally not as simple as in this example. Usually, mixtures of different kinds of vapours are condensed, and the individual components have different vapour pressure diagrams. The components can be perfectly soluble into each other, partly soluble, or insoluble.
2.5 Vapour–Liquid Equilibrium
For the example of a water and air mixture, as specified earlier, simple conditions also result, as the liquid phase comprises only one component. For a mixture with several components, the equilibrium must be determined, which means that the mole and/or mass fraction of the different components in both gas and liquid phases have to be calculated from appropriate sets of equations. For the gaseous phase, the sum of the partial pressures still results in the total pressure. This is described by the equation:
where pi is the partial pressure of the component i, yi is the mole fraction of the component i in the gaseous phase, and p is the total pressure.
The correlation to the liquid phase for an ideal solution is given by the equation:
where xi is the mol fraction of the component i in the liquid phase and psi is the vapour pressure of the pure component i.
From Equations 2.2 and 2.3, the above-mentioned Equation 2.1 can be derived easily for the system steam–air if one neglects the solubility of air in water and sets the mole fraction of water in the liquid phase . If more than one component is contained in the liquid phase, then Equations 2.2 and 2.3 have to be solved iterative.
For many substances, the partial pressure over a diluted solution cannot be described with the ideal model of Equation 2.3. A method of resolution is the introduction of the activity coefficient γ. This coefficient depends on the substances that are concerned, their concentration, and the temperature:
For an ideal solution, γ = 1. Some substances have very large activity coefficients, which means that only very little condensate will form together with water. One can consider this fact by considering that these substances are not soluble in water, for example, this is true for oil.
It is possible for several liquid phases to exist, each reacting with a common vapour phase as there are no other liquid phases. An example makes this clearer.
Example
Water vapour and a substance X that is insoluble in water should condense. The molecular weight of X is 80 kg kmol−1, the pressure is 217 mbar, and the temperature is 36 °C. The vapour is also mixed with an air flow rate of 10 kg h−1.
The partial pressure of water vapour at 36 °C is mbar and the vapour pressure of X should be pA = 71.6 mbar. Thus, the partial pressure of the inert gas is the remaining difference to the total pressure:
With this data, the saturation mass flow rate of water vapour and for X can be calculated. One can do this with Equation 2.1, because only one liquid component has to be considered in each case:
If less than 23 kg h−1 of substance X flow into the condenser, then there will be no liquid phase of X. In this case, X together with the air has to be regarded as not condensable. For example, an inert gas flow of 10 kg h−1 air + 10 kg h−1 X = 20 kg h−1, with a mean molecular weight of 42.6 kg h−1, would give a saturation vapour flow of
2.6 Types of Condensers
In principle, this can be simplified to the difference between direct-contact condensers and surface condensers. The first type is characterised by mixing the coolant with the vapour to be condensed; in the second type, surfaces separate coolant and vapour. Figure 2.6 shows different designs of direct-contact condensers.
Figure 2.6 Different types of direct-contact condensers: upper row, counterflow; lower row, co-current flow.
Often these condensers have internal structures to create a better distribution of the coolant, so that the liquid surface is as large as possible. In spray condensers, nozzles are used for a good distribution of the coolant.
Direct-contact condensers are mostly built as counterflow condensers. There are, however, cases where one uses co-current flow, for example, if the inert gas–vapour mixture should not be under-cooled to too great a degree, then the components of the extracted product will form solids; here the co-current principle can offer advantages. Since with co-current condensers the vapours flow from top to bottom, with the force of gravity and in same direction as the coolant, the pressure losses are lesser and higher flow rates can be realised or smaller cross sections are sufficient for the duty. However, in co-current condensers, the mixture that has to be extracted is in contact with the heated coolant, the outlet temperature is higher than that of counterflow condensers; therefore, a larger mass flow rate has to be extracted. Sometimes, a co-current condenser is used for the main condensation to allow the overall dimensions to be as small as possible, and downstream of a counterflow condenser is used for the under-cooling of the exhaust mixture.
The advantages of a direct-contact condensation are the low purchase price and the best possible utilisation of the coolant. With direct-contact condensers, one can raise the temperature of the coolant nearly to the boiling temperature of the condensate without the need to build an extremely large condenser. This is because the heat transfer coefficients for such a direct heat transfer are very high. Therefore, a smaller cooling water flow rate is needed than with surface condensers. Furthermore, their insensitivity to fouling, high operating reliability, and easy maintenance are other advantages.
The biggest disadvantage of a mixing condenser is the fact that the vapour condensate is mixed with the coolant. This mixing is only acceptable when the condensate is harmless and is to be discarded after use. For all other applications, surface condensers have to be used.
With surface condensers, the condensation surface is usually formed by tubes, in which the coolant (normally cooling water) flows at speeds from 0.4 to 2 m s−1. Vapour flows usually around the pipes, as with this construction sufficient cross sections for the large volume flow of the vapour can be realised. As an example of such a condenser, Figure 2.7 shows a tubular type condenser. For condensation surfaces up to certain square metres, this type is common in vacuum engineering. It allows both high cooling water velocities and large steam cross sections to be achieved simultaneously.
Figure 2.7 Tubular type condenser: (1) vapour inlet, (2) outlet for not condensed flow, (3) condensate outlet, (4) coolant inlet, and (5) coolant outlet.
Figure 2.8 shows a horizontal tubular type condenser with fixed bank of tubes, where the condensation takes place around the pipes and the cooling water flows in the tubes. A disadvantage of this design is the inherent difficulty in cleaning the condensation area.
Figure 2.8 Tubular type surface condenser with fixed tubes: cooling water flows in the tubes; vapour flow in the shell room around the tubes.
Figure 2.9 represents a surface condenser with fixed vertical tubes. Here, to allow for better cleaning, the vapour condenses inside the tubes, while the cooling water is led around the tubes. With this arrangement, there is poor cleaning on the cooling water side. With this design, the vapour enters the tubes with a very high velocity. This causes a good distribution and a thin condensate film. But care must be taken to ensure that sonic velocity is not reached at the inlet to the tubes. It should also be noted that how by means of baffles, the flow velocity of the coolant is increased, in order to increase the heat transfer.
Figure 2.9 Surface condenser with fixed tubes. Condensation in the tubes, cooling water around the tubes. (1) Vapour inlet, (2) outlet for not condensed vapour and gas, (3) coolant inlet, (4) coolant outlet, and (5) condensate outlet.
