406610 SULZER TECHNICAL REVIEW 3/2002

HANS KEISTDANIEL DNNENBERGER

SULZER CHEMTECH

Polymerization a chemical reaction by which many like molecules combine to form a chain is of great technical importance by the manufactureof numerous plastics. Sulzer Chemtech offers aneconomic and reliable process, which is alsoemployed now in the manufacture of polycarbonatefor the respectively required degasification.

Degasification of Polymer in the Vacuum Evaporator

daily and therefore produced in-dustrially in large quantities.

Low Residual MonomerContentWhereas an initiation reactioncauses the monomers to grow andform long molecular chains duringthe course of manufacture, a chain-terminating reaction ends it. As a

Medicinal packings, yoghurttubs, interior linings of refrig-

erators, plastic lenses for specta-cles (Fig. 1), compact discs, auto-mobile parts or drinking cups arejust a few of the numerous prod-ucts that are made of polycarbon-ate or polystyrene. Polymeric plas-tics are the raw material for innu-merable articles that are required

SULZER TECHNICAL REVIEW 3/2002 11

0682 2068-2

tion and the desired characteristicsof the product, Sulzer Chemtechcan supply single- or multi-stagedegasification units.The first stage of a Devoli processcomprises a static mixer/heat ex-changer and an expansion vesselfor the vacuum degasification. Theheat exchanger with integratedmixing elements (Fig. 2) providesfor a rapid and uniform heating ofthe polymer, which is suppliedfrom the reaction stage. Localoverheating, which could destroythe polymer chains, is thus avoid-ed. The simultaneous mixing andheating reduces the residence timeof the polymer and also preventsundesired heat reactions.The actual separation of the mate-rial by means of degasificationtakes place in the downstream ex-

of the highly viscous polymermelts, Sulzer Chemtech has devel-oped the devolatilization processDevoli, with which an end prod-uct with an extremely low contentof residual monomer and solventcan be attained. In addition, theenergy requirement of the Sulzerprocess is much less than that ofcompetitive methods. Further-more, the gentle handling of theprocess contributes appreciably tothe retention of the original poly-mer quality.

Devolatilization by Means of Static InstallationsIn the main, the devolatilizationprocess is based on a vacuumevaporation in static installations.Depending on the residual mon-omer content after the polymeriza-

2 The heat transfer in this heat exchanger with SMXL mixer internalsis four to eight times greater than in a normal pipe. The detrimentaltemperature differences in the polymer melts are leveled out quicker.

result, a proportion of the non-bonded monomers is left behind,which impairs the quality of theend product and therefore has tobe kept as low as possible. In themajority of cases, the product ofthe polymerization has the follow-ing composition:

Polymer (desired product)Admixtures of unbonded mon-omerOligomer from only a fewbonded moleculesSolventOther impurities

The proportion of undesired sub-stances is mostly highly volatile,but firstly by temperatures atwhich the quality of the polymerwould be greatly impaired. To re-move the undesired proportions

1 The plastic lenses of spectacles are frequently made of polycarbonate.The cost-effective and highly efficientDevoli degasification technique canthus be applied for the manufacture of this polymer.

12 SULZER TECHNICAL REVIEW 3/2002

1 510

3 4 3

7

8

9

76 6

2

1

2

3

4

6

7

8

9

5

the two important criteria ener-gy consumption and the exploita-tion of raw material are also con-vincing. The efficient heating ofthe polymer in the mixer/heat ex-changer and the small number ofpumps that require motive energyreduce the energy consumption ofthe complete system. If water isused as a stripping agent, theprocess can be operated as a closedcycle, and there is no need for anexternal waste-water treatmentplant.

Many Years ExperienceThe realization of the simple prin-ciple in a large-scale, complexchemical plant is by no meanscommonplace and necessitates awealth of know-how and experi-ence. With its knowledge of themixer technology, Sulzer Chem-tech has been able to turn a simpleidea into a successful process. Thekey components of the system are

nologies in many aspects. The cap-ital investment and operatingcosts are less than those for com-petitive systems, a fact which isconfirmed by a case study for amodel installation with a produc-tion capacity of 25 000 tons peryear (Fig. 4).The vacuum degasification also of-fers major advantages from thetechnical standpoint, because mostpolymers can be purified with thismethod. Since the employment ofcentrifugal pumps is largely dis-pensed with, the polymer is onlysubject to low shear forces duringthe degasification process. The res-idence times are also short and ex-tend over a narrow spectrum, andso it can be ensured that there willbe no change in the product char-acteristics. This is an important ad-vantage especially by the manu-facture of elastomers.The characteristics of the Sulzerprocess especially with regard to

pansion vessel (Fig. 3). With the so-called flash evaporation, the poly-mer is expanded abruptly, whichcauses the highly volatile part consisting of monomer, oligomerand solvent to evaporate. Thesolvent and monomer are separat-ed from this vapor in a rectificationcolumn and returned to the reac-tion again, while the oligomer is ofno further use.If there is call for a polymer withexceptional purity which is nec-essary in the case of polystyrenefor foodstuff packings the resid-ual monomer content of the endproduct can be reduced to lessthan 100 ppm (especially throughthe use of stripping agents, such asCO2, water or nitrogen) in furtherstages.

Low CostsThe process developed by SulzerChemtech is clearly superior toother possible alternative tech-

Polymer with residual monomer and other pollutants

Static mixer/heat exchanger

Devolatilization vessel

Static mixer

Degassed polymer melts

Rectification column

Monomer + solvent for recycling

Oligomer

Vacuum pump

Stripping agent

3 The installations for the evaporation degasificationhardly require any moving mechanical parts, e.g.

centrifugal pumps. This enhances the availability ofthe installations and keeps the maintenance

costs at a low level.

First stage Final stage

SULZER TECHNICAL REVIEW 3/2002 13

0

20

40

60

80

100%

0

20

40

60

80

100%

% %

% %

% %

% %

% %

products that could be degassedwith Devoli. As a part of conver-sion projects, existing plants canbe re-vamped for the applicationof this energy-efficient and eco-nomical technology.

Apart from being employed forthe manufacture of polystyrene,various Devoli plants from Sulzerhave been operating for manyyears, and now, with the produc-tion of polycarbonate (Fig. 5), theexperts from Sulzer Chemtech seeeven greater potential for theprocess with other polymers. Poly-ethylene, polyvinyl acetate, orelastomers are just a few of the

5 Expansion vessel of aSulzer Devoli plant on itsway to a customer.

monomer and solvent from thepolycarbonate. The process pa-rameters were adapted to the cus-tomers requirements in a pilotplant, so that there would be nounexpected complications whenthe plant is put into operation.

4 In comparison with a two-stage, thin-layer evaporation extruder stage(production capacity 25000 t/year), the Sulzer concept for polymerdevolatilization requires only 60% of the capital investment and 40% of the operating costs.

the mixer/heat exchanger, the ex-pansion vessel and the static mix-er. These components are manu-factured by Sulzer and adapted tothe specific substance characteris-tics of the product material bymeans of their respective internals.In particular, the use of strippingagents places very stringent re-quirements on the constructor.Conclusive laboratory tests alsocontribute to the manufacture ofpolymers of top quality and puri-ty.

Polycarbonate Devolatilizationin IranThe Devoli process from SulzerChemtech has been successfullyemployed for the manufacture ofpolystyrene with a quality suitablefor foodstuffs for many years. Thetechnology has now found a fur-ther field of application on its wayto industrial application, namelythe manufacture of polycarbonate.The engineers from Sulzer Chem-tech have designed the devolatili-zation section of a polycarbonateinstallation for the Iranian Khou-zestan Petrochemical Company.From 2003 onwards, the colorlessplastic will be produced on an in-dustrial scale in the Bandar ImanPetrochemical Zone in the south-west of Iran. The original intentionwas to employ conventional de-gasification technology at thisplant. Following the recommenda-tions made by experts from SulzerChemtech, it was decided to re-consider the possibility of employ-ing vacuum degasification. Theadvantages of this process were soevident that the planning had to berevised accordingly. Sulzer wasawarded the contract for the plan-ning, engineering, and supply ofthe units for the removal of

CONTACTSulzer Chemtech AGHans KeistPostfach 65CH-8404 WinterthurSwitzerlandPhone +41(0)52-262 68 14Fax +41(0)52-262 03 88E-mail hans.keist@sulzer.com

Capital investment costs Annual operating costs

Thin-layer evaporation with extruder stage

Sulzer Devoli process