Cool solutions for business
30 May 2002 00:00 [Source: APC]
For many, the concept of cryogenics may conjure up images of an
excited lecturer shattering a rubber ball or crunching flowers
after being dipped into a bucket of liquid nitrogen. But to many
engineers, cryogenics is a very real subject and it is up to them
to discover new ways of using this potentially lucrative
technology. How can cryogenics help your business? Jon Trembley
reveals some practical solutions.
The dictionary definition of cryogenics is: the study of
extremely low temperatures: a branch of physics that studies the
causes, effects and utilisation of extremely low
temperatures.
At Air Products' Center of Excellence for Cryogenic Applications in
Allentown, Pennsylvania, US, the word utilisation means a great
deal. 'Very focused' is how Jon Trembley, a lead engineer with the
company, describes the facility's work. 'We are an R&D centre
for new applications with a strong commercial awareness.' While
much of the work in the Center of Excellence is in food freezing,
there are three other areas where cryogenics offers unique
processing advantages: cryogenic size reduction and recycling;
solvent recovery systems which reduce emissions to meet
increasingly stringent legislation; and reaction cooling in the
synthesis of organics.
VOC recovery
Companies spend a lot of time and money attempting to control
the release of volatile organic compounds (VOCs) to the atmosphere.
VOCs can cause ground-level ozone, smog and chlorinated compounds,
some of which are toxic to human health.
There are two ways to reduce emissions to the environment: either
change the way the process is operated or treat the resulting end
of pipe problem with VOC abatement technology. Often a combination
of the two is necessary to reach the increasingly stringent solvent
emission regulations being imposed.
The most common methods of removing VOCs are adsorption,
absorption, condensation and incineration. To choose the best
method a number of criteria must be considered:
- Process flow rate, temperature and pressure;
- Type, quantity and concentration of VOCs;
- Operational reliability;
- Technology's track record;
- Process operating cycle;
- Capital and operating costs.
Condensation of some organics is achievable with simple
refrigeration systems, down to a temperature of about
-40oF (-40oC), but most VOCs need much lower
temperatures than this and so the use of cryogenic coolants becomes
necessary. This technology is extremely versatile as the limit of
temperature is that of the coolant, liquid nitrogen, with a boiling
point of -320oF (-196oC). The system may
require more heat exchangers and more complex equipment, but
operating temperatures as low as -292oF
(-180oC) are possible.
Condensation using liquid nitrogen has been overlooked in the past
due to its perceived high operating costs. But now, with ever more
stringent legislation, the benefits of liquid nitrogen are becoming
increasingly attractive. Cryogenic systems actually have more
flexibility - temperatures can be easily adjusted if tighter limits
are required.
The process is simple, effective and has low capital cost. If the
nitrogen can be used again elsewhere on the plant, the effective
operating costs are vastly reduced. Although in principle nearly
all VOCs can be removed with cryogenics, a rule of thumb suggests
that it is most economic when flow rates are below 5000 Nm3/h
(Normal cubic metres per hour) or vapour concentrations above 5
g/Nm3 (Grams per Normal cubic metre).
In the US, the Clean Air Act Amendments have made the control of
VOC emissions an absolute necessity. Similar regulations are being
enforced in Europe (such as TA Luft in Germany and the
Environmental Protection Act in the UK) and other parts of the
world. Air Products is able to use its cryogenic technology,
CRYO-CONDAP, to reduce solvent emissions and meet the strictest of
these regulations, the German TA Luft standard concentrations of 20
mg/m3 (milligrams per cubic metre). With 70 Air Products
installations worldwide, this technology is becoming more and more
standard. And with regulations getting tighter every year, it is
definitely a technology for the future.
Cold chemistry
Another application is cold reaction chemistry. The future of
cryogenics in this field is bright and potentially very
profitable.
In the pharmaceutical and fine chemical industries, the purity of
chemical compounds is paramount. Any technology that can increase
reaction stereo selectivity is highly sought after. Organic
synthesis at low temperature is one method that can help to achieve
these aims; low temperature reagents such as n-butyl lithium or
Wittig reagents can be used. These reagents produce intermediates
at cold temperatures that after further processing lead to products
with greater regularity and better selectivity. However, n-butyl
lithium presents the engineer with a technical challenge. Although
it is especially helpful in the production of optically pure
isomers, at room temperature, it is a very unstable compound and so
in industrial applications there must be excellent cooling control.
Synthesis must take place at very low temperatures to ensure that
the reagent's stability is maintained and that the exotherm, which
can be released by the addition of such organometallic compounds,
is strictly controlled.
Typically, three main cooling methods can be used to keep
temperatures sufficiently low enough and control an exothermic
reaction: mechanical refrigeration, sublimation of carbon dioxide
and the evaporation of liquid nitrogen. Simple mechanical
refrigeration that works at the limit of its lower temperature
range is expensive. If the equipment's maximum potential is only
occasionally called on, it means that costly capital equipment
might be under utilised.
Sublimation of CO2 can reach temperatures as low as
-78oC (-108oF) at atmospheric pressure and as
such, is a viable option for controlling some cold chemistry
reactions. However, liquid nitrogen can achieve temperatures much
lower than this, giving a designing engineer more confidence in the
cooling system's control, and a chemist the full flexibility needed
when choosing which synthesis route to follow.
The most common usage of nitrogen on a chemical plant is as an
inert blanketing gas. If the evaporated liquid nitrogen from the
cooling process can be recovered and used elsewhere on the plant,
the running costs of the system can be dramatically lessened.
Liquid nitrogen is often portrayed as an expensive option. But when
improved reaction yields and selectivity, reduced unwanted
by-products and the relatively low capital costs involved are taken
into consideration, liquid nitrogen becomes an economically
attractive choice. Also nitrogen is a clean, dry, inert, non-toxic
and non-polluting fluid, making it extremely environmentally
friendly.
Direct injection
There are three main alternatives for cooling reaction vessels
with liquid nitrogen: directly, semidirectly and indirectly. Each
method has its advantages and disadvantages.
Direct injection of liquid nitrogen achieves maximum efficiency and
is cheap to install, but there is the chance of solvent
entrainment, foaming and local freezing. Semidirect heat transfer
takes place either via a coil inside the reactor or in a reactor
cooling jacket. The main benefits of this method include accurate
temperature control, the ability to re-use the nitrogen and
simplicity. The drawbacks include reduced efficiency, the take-up
of reactor volume and the expense of the cryogenic construction
materials. Indirect heat transfer is where the liquid nitrogen is
exchanged with a suitable heat transfer fluid in an external heat
exchanger. This approach has the most system flexibility; it has
accurate temperature control and large heat loads are possible.
Certainly there is financial incentive for companies such as Air
Products to develop their cooling technology.
Cryogenic size reduction
The historical perspective of the cryogenic recycling process
was that it is 'too expensive,' except very special fields. Today,
manufacturers are demanding material of less than 200 micron and
that the production process be economical. An independent German
research house recently carried out tests with Air Products'
cryogenic recycling system and traditional cryogenic systems.
Systems can make a big difference in the economics of a process.
Air Products' cryogenic recycling systems are designed to minimise
the amount of liquid nitrogen required by correct system sizing,
minimising losses and re-circulation/utilisation of cold nitrogen
gas.
In tests undertaken by Air Products, every case demonstrated that
the particle size achieved was >90% 100 micron rubber powder.
The specific costs of this process are lower than historical
results. However, it should be pointed out that the economics of
the process are variable and depend on the raw material, throughput
per hour, product size and size of equipment. It is not a necessary
requirement for all applications to have fine ground rubber;
sometimes 200 or 300 micron rubber powder can be successfully
re-used and producing larger particle sizes means that the nitrogen
cost decreases.
Manufacturing with fine ground powder (100 micron) can help
manufacturers use higher amounts of recycled rubber in formulations
and still maintain a high specification of physical
properties.
Grinding rubber at ambient temperatures produces highly temperature
stressed particles (500 micron) with a large specific surface area.
Sometimes this surface area is required because it will allow
reasonably good mechanical fixing in compounds.
Cryogenically recycled fine ground rubber powders actually have
even more surface area, and as they are smaller particles, they are
not so thermally stressed. That means that a very large number of
particles fit into the spaces in the rubber compound, and the
non-stressed surfaces are able to chemically interact with virgin
vulcanised particles.
Case Study Results
ACM Rubber Processor
Tests were done with various percentages (5 to 25%) of fine
ground ACM rubber in an original compound (type of seal). The
results showed:
- an increase in compression set
- negligible changes in other physical properties
EPDM rubber processor
The company wanted to run trials for grinding 400 micron rubber
under ambient conditions. They then went to a mill manufacturer to
buy a second system; the mill manufacturer offered to run a trial
with liquid nitrogen. The results showed:
- throughput increased enormously
- electrical power consumption
Jon Trembley is lead engineer in the Center of Excellence -
Cryogenic Applications at Air Products and Chemicals Inc,
Allentown, Pennsylvania, US.
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