Extremes
Sizing beyond
limits
In separation technology, the
methodologies developed to scale the
results obtained in pilot equipment to
large industrial units are limited in their
validity. Engineers are sometimes
challenged with the task of constructing units of exceptionally large sizes—
sizes that are far beyond the usual limit
of confidence. The extrapolations
required are then based on extensive
experience, knowledge of the specific
separation technologies, and careful
attention to the customer expectations.
1 The successful scale-up of a large distillation complex led to additional
orders of similar magnitude.
14 Sulzer Technical Review 2/2014
When a chemical or pharmaceutical company contacts a provider of services and equipment for the separation and purification of its chemical products, it usually has completed thorough studies on its processing
capacity needs. The nameplate capacity of the plant
and the required product quality are determined by
existing market demands and projections for the future.
World-scale capacities are increasing in response to
rising demand and due to streamlined production capacities. With that, the scaling up from pilot test plants
to full industrial sizes is becoming more challenging.
The method the engineer uses to calculate and design
a piece of equipment of industrial size based on a test
at a smaller scale is called “scale-up.”1 Some equipment can be scaled directly from laboratory size to industrial size; others require the construction of a pilot
or semi-industrial-sized plant as an intermediary step.
The scale-up factor is defined as the size of the large
equipment or plant divided by the size of the pilot unit.
Consideration of typical scale-up factors limits the risk
of incorrect engineering, so that the plant finally produces according to the customer’s specifications. But
each step represents a substantial investment. At
Sulzer the scale-up is based on pilot tests realized for
the production rate at a scale of a few kilograms per
hour. The results allow the design of a plant that typically processes thousands of kilograms per hour. Because most of the equipment that Sulzer delivers to
Extremes
customers is of proprietary nature, this is a demanding
task that requires a solid theoretical basis and a profound understanding of the technologies. Based on
built-up experience, the Sulzer engineers are able to
provide a scaled-up design that is accurate enough to
supply the customer with guarantees on the plant
performance. Yet, from time to time, the customer’s
specifications ask for a capacity that is much larger
than anything designed before. The size may be so
large that the basis and even the method for the scaleup become questionable. These situations require
special skills to successfully predict the unusual complications that go with these giant steps.
Scaling up distillation columns
Distillation is a well-known and a very powerful separation technology. Sulzer has built and operated rectification towers of many possible sizes. Very slim, tall columns up to 90 meters high and less than 0.10 meter
in diameter are used to separate isotopes. Sulzer has
also designed and manufactured column sections of
more than 15 meters in diameter and less than 2 meters high for the wash bed of crude oil vacuum towers.
Some rectification towers operate at high pressure and
others at vacuum. The scale-up challenges have been
studied and described by many authors and are quite
well known.2 Nevertheless, new requirements from the
market, such as higher purity and higher
production rates, continuously push beyond the limits
of experience.
A customer required a purification unit for the monomer
MDI (methylene diphenyl diisocyanate), which is used
for the production of polyurethane. Although experience had been gained with piloting in a column of
0.10 meter diameter and industrial experience was
available for production up to 50 kilotons per year
(ktpa), the design for production of a few 100 ktpa,
combined with a very low operating pressure represented a challenge. Sulzer managed to ensure proper
liquid distribution over the whole diameter and to keep
the pressure drop to a minimum. In addition, the liquid
holdup had to be kept as small as possible to lower the
formation of side products. Another challenge was the
sizing of the equipment to condense the top vapor with
a duty of several megawatts at an absolute pressure of
a few millibars. The unit was successfully started and
has been running profitably for several years (Fig. 1).
Design of large liquid-liquid extraction systems
There are several industrial technologies for liquidliquid extraction.3 One of the most efficient setups is
the agitated contactor. The solution is based on a collection of agitated compartments arranged vertically in
a column. Each compartment is equipped with an
agitator and is separated by partition plates (Fig. 2, left).
The main challenges for the design of liquid-liquid
extraction systems are the changing physical properties along the column and the strong variation of the internal flow rates when a large amount of material is exchanged between the two liquid phases. The KühniTM
agitated column ECR can be designed for such conditions. Once the equilibrium diagram of the phases is
available, a test at pilot scale of 0.06 meter diameter is
run to validate the design. For a scale-up to a size
2 For this liquid-liquid extraction unit (left), Sulzer scaled the agitation system up to a diameter of almost 3 meters (right).
Sulzer Technical Review 2/2014 15
Extremes
above 1 meter in diameter, it is necessary to verify hydrodynamic parameters such as the flooding point using a pilot column of 0.15 meter diameter.
A customer wanted to produce food-grade phosphoric acid from a crude acid stream that contained several impurities such as sulfate and different metallic ions.
Sulzer already had several references for this application that used the agitated ECR columns. Usually industrial size columns process a few tons of feed per
Scale-up at Sulzer
Sulzer Chemtech’s Process Technology
business unit looks for the most economical
solution based on experience and process
simulation. Once the solution matches the
customer’s expectation, Sulzer offers a pilot
test to validate the concept with the actual
feed and to confirm the product concentrations specified. On this basis, Sulzer designs,
fabricates, and delivers the industrial-sized
equipment to the customer.
www.sulzer.com/Process-Technology
3 A large falling film crystallizer is transported to the customer production site.
16 Sulzer Technical Review 2/2014
hour. In this case, however, the feed was over 30 metric tons per hour, and the process required three very
large liquid-liquid extraction columns in series (Fig. 2,
right). In scaling up the agitation system to a diameter
of almost 3 meters, it was critical not to increase the
height of the agitated compartments too much in order
to minimize the overall height of the columns. On top
of this, the feed-to-solvent ratio, being above 25 to 1,
was exceptionally high, which added to the general
complications. An important part of the solution was to
adapt the geometry of the column internals along its
height to cope with the changing hydrodynamic conditions. The columns were delivered to the customer in
the USA and have been in operation for over 25 years.
Melt crystallization units with huge capacity
Crystallization is a very efficient separation technology.4
Melt crystallization is based on the thermodynamic
principle that impurities prefer to remain in the liquid
phase rather than in the crystal phase. It is used for
many applications, like paraffin deoiling and monomer
purification. It has been successfully applied in combination with distillation to obtain pure MDI and chloronitrobenzene, for example. In general, this purification
process is tested and optimized in a pilot plant. The
scale-up for the industrial unit is then a straightforward
Extremes
multiplication of the standard elements as applied in
the test center. The scale-up can be very large for applications such as acrylic acid or para-xylene. In these
applications, pilot tests with feed samples of 200–400
kilograms are used for designing plants with worldscale capacities over 100 ktpa (Fig. 3). Several references have validated this method.
Scaled-up technology for polymer devolatilization
Sulzer is a leading supplier of equipment for the removal of volatile components like solvents or residual
monomers from polymers. The first reference projects
with polystyrene were developed about 20 years ago
in Sulzer’s pilot test center. The underlying technology
is based on heating and flashing in one or more serialdevolatilization stages with a stripping agent optionally
injected into the melt. The residual monomer level can
typically be brought to the ppm level, for example, to
around 100 ppm in the case of polystyrene. Sulzer’s
expertise covers not only the physical properties of
systems that consist of volatiles and polymers, but also
the scale-up, which must account for critical parameters such as equipment dimensions, melt distributer
design, or residence time. The process has been
scaled up from piloting with feed streams of a few kilograms per hour to more than 100 000 kilograms per
“Scaling up process technology is
an exciting challenge because
large vessels and complex plants
are simply fascinating.”
Laurent Zuber, Chemtech Process Technology
hour, which corresponds to polymer production capacities up to 200 ktpa. The example of a large devolatilization vessel delivered to a customer in Asia is
shown in Fig. 4. Recently, this technology was developed further for the devolatilization of biopolymers like
polylactic acid (PLA) and the preconcentration of rubber solutions. This represents an economically interesting alternative to existing equipment like thin film
evaporators or steam stripping columns.
Challenges in complex modular plants
Distillation technology has limits, especially when encountering azeotropes. An azeotrope is a mixture of
two or more liquids that cannot be separated by simple distillation because, when boiling, the composition
4 Sulzer has scaled up its polymer degassing technology for production capacities up to 200 kilotons per year.
Sulzer Technical Review 2/2014 17
Extremes
of the vapor and the boiling liquid are exactly the same
(called azeotropic composition).5 There are different
ways to overcome this obstacle by combining different
unit operations—like distillation and membrane separation (for example, for ethanol drying), or distillation
and extraction (the extractive distillation like applied for
benzene/cyclohexane separation). In some cases, the
azeotropic composition depends strongly on the operating pressure, and the components can be separated
by pressure swing distillation. This method uses a
combination of two distillation columns that operate at
different pressure levels.
A customer producing an active ingredient of a blockbuster drug used ethyl acetate and cyclohexane as
solvents and needed to recycle these. The mother
liquor coming from the reaction also contained traces
of other compounds. So, a first large-scale batch
distillation was used to remove most of these unwanted components and obtain the solvent mixture that
needed to be recycled. The next step was a pressure
swing distillation. The first column, operating under
pressure, produced pure cyclohexane at the bottom
and the azeotropic mixture at its top. The top product
was processed in a second column which was operated under vacuum and produced pure ethyl acetate
at its bottom. The azeotropic mixture obtained at the
top of the second column was recycled back to the
first column.
The challenges in this project were not only the difficult
separation tasks, but also the delivery of the solvent recovery plant as completely preassembled skids (Fig. 5).
These only needed to be set up and tied in to the customer’s plant and utilities. The plant was delivered and
started up within a very short time, and there was
minimal interference to the existing production site.
The client has three such plants in operation, and all
have been running for more than eight years.
Contact: Laurent Zuber, Daniel Rytz
sulzertechnicalreview@sulzer.com
Literature
M. Zlokarnik, Scale-Up in Chemical Engineering,
2nd edition (Weinheim: Wiley-VCH, 2006).
2
H. Z. Kister, Distillation Design
(New York: McGraw-Hill, 1992), 405 and 554.
3
J. C. Godfrey and M. J. Slater, Liquid-Liquid Extraction Equipment (Chichester: John Wiley & Sons, 1994).
4
J. Ulrich and H. Glade, Melt Crystallization
(Aachen: Shaker Verlag, 2003), 76.
5
I. Smallwood, Solvent Recovery Book (London: Edward Arnold, Hodder & Stoughton, 1993), 94.
1
5 Sulzer delivered this solvent recovery plant as completely preassembled
skids.
18 Sulzer Technical Review 2/2014