Micro process engineering
Encyclopedia
Micro process engineering is the science of conducting chemical or
physical processes (unit operation
s) inside small volumina,
typically inside channels with diameters of less than 1 mm
(microchannels) or other structures with sub-millimeter dimensions.
These processes are usually carried out in continuous
flow mode, as opposed to batch production
, allowing a throughput
high enough to make micro process engineering a tool for chemical
production. Micro process engineering is therefore not to be confused
with microchemistry, which deals with very small overall quantities of
matter.
The subfield of micro process engineering that deals with chemical
reactions, carried out in microstructured reactors or
"microreactor
s", is also known as
microreaction technology.
The unique advantages of microstructured reactors or microreactor
s are enhanced
heat transfer
due to the large surface area-to-volume ratio, and enhanced
mass transfer
. For example, the length scale of diffusion
processes is comparable to that of microchannels or even shorter, and efficient mixing
of reactants can be achieved during very short times (typically
milliseconds). The good heat transfer properties allow a precise
temperature control of reactions. For example, highly
exothermic reaction
s
can be conducted almost isothermally when the
microstructured reactor contains a second set of microchannels ("cooling
passage"), fluidically separated from the reaction channels ("reaction
passage"), through which a flow of cold fluid with sufficiently high
heat capacity
is maintained. It is also possible to change the temperature of
microstructured reactors very rapidly to intentionally achieve a non-isothermal behaviour.
process engineering device ("microstructured reactor") can contain many
thousands of such channels, and the overall size of a microstructured
reactor can be on the scale of meters. The objective of micro process
engineering is not primarily to miniaturize production plants,
but to increase yields
and selectivities of chemical reactions, thus
reducing the cost of chemical production. This goal can be achieved by
either using chemical reactions that cannot be conducted in larger
volumina, or by running chemical reactions at parameters (temperatures,
pressures, concentrations) that are inaccessible in larger volumina due
to safety constraints. For example, the detonation of the
stoichiometric mixture of two volume unit of hydrogen
gas and
one volume unit of oxygen
gas does not propagate in microchannels
with a sufficiently small diameter. This property is referred to as the
"intrinsic safety
" of microstructured reactors. The improvement of yields
and selectivities by using novel reactions or running reactions at more
extreme parameters is known as "process intensification".
when mechanical micromachining methods developed for the fabrication of
uranium
isotope separation
nozzles were first applied to the
manufacturing of compact heat exchanger
s at the
Karlsruhe (Nuclear) Research Center.
physical processes (unit operation
Unit operation
In chemical engineering and related fields, a unit operation is a basic step in a process.Unit operation involves bringing a physical change such as separation, crystallization, evaporation, filtration etc.. For example in milk processing, homogenization, pasteurization, chilling, and packaging are...
s) inside small volumina,
typically inside channels with diameters of less than 1 mm
(microchannels) or other structures with sub-millimeter dimensions.
These processes are usually carried out in continuous
flow mode, as opposed to batch production
Batch production
Batch production is a technique used in manufacturing, in which the object in question is created stage by stage over a series of workstations. Batch production is common in bakeries and in the manufacture of sports shoes, pharmaceutical ingredients , inks, paints and adhesives. In the manufacture...
, allowing a throughput
high enough to make micro process engineering a tool for chemical
production. Micro process engineering is therefore not to be confused
with microchemistry, which deals with very small overall quantities of
matter.
The subfield of micro process engineering that deals with chemical
reactions, carried out in microstructured reactors or
"microreactor
Microreactor
A microreactor or microstructured reactor or microchannel reactor is a device in which chemical reactions take place in a confinement with typical lateral dimensions below 1 mm;the most typical form of such confinement are microchannels...
s", is also known as
microreaction technology.
The unique advantages of microstructured reactors or microreactor
Microreactor
A microreactor or microstructured reactor or microchannel reactor is a device in which chemical reactions take place in a confinement with typical lateral dimensions below 1 mm;the most typical form of such confinement are microchannels...
s are enhanced
heat transfer
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
due to the large surface area-to-volume ratio, and enhanced
mass transfer
Mass transfer
Mass transfer is the net movement of mass from one location, usually meaning a stream, phase, fraction or component, to another. Mass transfer occurs in many processes, such as absorption, evaporation, adsorption, drying, precipitation, membrane filtration, and distillation. Mass transfer is used...
. For example, the length scale of diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
processes is comparable to that of microchannels or even shorter, and efficient mixing
of reactants can be achieved during very short times (typically
milliseconds). The good heat transfer properties allow a precise
temperature control of reactions. For example, highly
exothermic reaction
Exothermic reaction
An exothermic reaction is a chemical reaction that releases energy in the form of light or heat. It is the opposite of an endothermic reaction. Expressed in a chemical equation:-Overview:...
s
can be conducted almost isothermally when the
microstructured reactor contains a second set of microchannels ("cooling
passage"), fluidically separated from the reaction channels ("reaction
passage"), through which a flow of cold fluid with sufficiently high
heat capacity
Heat capacity
Heat capacity , or thermal capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount...
is maintained. It is also possible to change the temperature of
microstructured reactors very rapidly to intentionally achieve a non-isothermal behaviour.
Process Intensification
While the dimensions of the individual channels are small, a microprocess engineering device ("microstructured reactor") can contain many
thousands of such channels, and the overall size of a microstructured
reactor can be on the scale of meters. The objective of micro process
engineering is not primarily to miniaturize production plants,
but to increase yields
Yield (chemistry)
In chemistry, yield, also referred to as chemical yield and reaction yield, is the amount of product obtained in a chemical reaction. The absolute yield can be given as the weight in grams or in moles...
and selectivities of chemical reactions, thus
reducing the cost of chemical production. This goal can be achieved by
either using chemical reactions that cannot be conducted in larger
volumina, or by running chemical reactions at parameters (temperatures,
pressures, concentrations) that are inaccessible in larger volumina due
to safety constraints. For example, the detonation of the
stoichiometric mixture of two volume unit of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas and
one volume unit of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
gas does not propagate in microchannels
with a sufficiently small diameter. This property is referred to as the
"intrinsic safety
Intrinsic safety
Intrinsic safety is a protection technique for safe operation of electronic equipment in explosive atmospheres and under irregular operating conditions. The concept was developed for safe operation of process control instrumentation in hazardous areas, particularly North Sea gas platforms...
" of microstructured reactors. The improvement of yields
and selectivities by using novel reactions or running reactions at more
extreme parameters is known as "process intensification".
History
Historically, micro process engineering originated around the 1980s,when mechanical micromachining methods developed for the fabrication of
uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
isotope separation
Isotope separation
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. This is a crucial process in the manufacture of uranium fuel for nuclear power stations, and is...
nozzles were first applied to the
manufacturing of compact heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
s at the
Karlsruhe (Nuclear) Research Center.