Combining Distillation and Crystallization

MANFRED STEPANSKI Methylene diphenyl diisocyanate – in short MDI – PETER FÄSSLER is an important raw material in the manufacture of SULZER CHEMTECH polyurethanes. Sulzer Chemtech has developed a process which, in comparison with conventional processes and through the combination of distillation and crystallization, enables MDI to be manu- factured with appreciably lower capital investment and operating costs, as well as with a reduced consumption of energy.

Polyurethanes are out-and- Combining Successful out multipurpose plastics. Processes They are used primarily in the MDI results from a reaction of manufacture of rigid and flexible methylene dianiline with phos- foams. Due to their diverse appli- gene. The reaction forms a mix- cation possibilities, polyurethanes ture of monomeric MDI-isomers have a wide range of usage in the (Fig. 2) and components of higher automotive and electrical indus- molecular weight. Monomeric tries, for thermal and cold in- MDI is isolated from this mixture sulation, by the construction of in a number of purification stages. technical components and in the Conventional processes separate furniture industry. Nevertheless, the isomers by means of either dis- polyurethanes are more than just tillation or melt crystallization. foam. They can also be used as Sulzer has developed a hybrid coatings and adhesives, as well as process that combines the two for the manufacture of highly technologies (Fig. 3): After the re- stressed rollers and drums (Fig. 1). action, the crude MDI is distilled

14 SULZER TECHNICAL REVIEW 4/2002 4073 1 The majority of the wheels for in-line skates, microscooters or kickboards are made of polyurethane. Their manufacture requires MDI, which can be brought to the necessary purity with a Sulzer Chemtech process.

High Throughput and High Yield standard pumps and valves. At the The distillation unit for the iso- same time, it is so flexible that the meric separation consists of a frac- composition of the end product tionating column with three or can be tailor-made to meet specific four beds equipped with highly ef- product requirements by control of ficient structured packings. The the cooling and heating cycles design criterion for the whole sys- (Fig. 4). tem is low pressure drop and min- imum residence times. Thanks to Scale-up in Large Plants © BASF Polyurethane the high throughput quantities, Sulzer Chemtech has conducted one distillation column is suffi- numerous pilot tests for the design cient for large processing units of this new process on a large in- having capacities of up to 160 000 dustrial scale. These tests for MDI- first to remove higher-boiling con- tons per year, whereas two distillation cannot answer all ques- taminants and readily volatile columns would be necessary with tions concerning the behavior components. A product with more the application of conventional of participant substances. Sulzer than 60% 2,4’-MDI and a pre-con- technology. Chemtech, however, has acquired centrated 4,4’-MDI are obtained in The not readily volatile bottom decade-long experience with the the isomeric distillation unit. The fractions from the two distillation distillation and crystallization of 4,4’-MDI is then concentrated to a stages are a marketable product MDI in large-scale plants and purity of more than 99% in the with a high proportion of MDI- can therefore exclude almost any downstream static melt crystal- polymers. Residual monomers are scale-up uncertainty by the trans- lization unit. separated from the bottom product in the recycling unit and re- 2 Isomers are chemical compounds, which are Distillation with turned to the distillation process composed of the same kind and number of atoms, but have different physical or chemical properties. Low Pressure Drop again. With this step, the yield of In the figure: 2,4’-MDI (above) and 4,4’-MDI (below). Successful Sulzer technology is MDI-monomers is markedly in- Chemical Elements: employed in all process stages. creased. C N O H The distillation column for crude MDI is equipped with flow-opti- Separation without Solvent mized MellapakPlus packings, fa- The pre-concentrated 4,4’-MDI cilitating high product throughput from the isomeric distillation unit with exceptionally low pressure is routed continuously to the drop and lower hold-up of the liq- melt crystallization unit (see also uid phase (see also STR 3/1999, STR 2/2002, p. 6). In this step, the p. 24, as well as the advertisement feed product is crystallized by on the back cover of this issue). As means of cyclic cooling and heat- a result of direct condensation, the ing to separate polymers or other distilled MDI is quickly cooled to isomers. Static melt crystallization storage temperature or processed ensures excellent product quality further directly in the isomeric rec- and high color stability of the 4,4’- tification column. The very short MDI-isomer. The process is very residence time at high temperature robust, since no equipment with ensures smooth distillation. moving parts is used, except for

SULZER TECHNICAL REVIEW 4/2002 15 3 Flow of material by the new hybrid Lights process for the purification and separation of MDI-isomers. If one of the 2,4’-MDI process units is already available in (> 60%) 4,4’-MDI an existing plant, the capacity can be 4,4’-MDI Crude MDI Isomer (> 99%) expanded by means of a retrofit. crystallization distillation unit distillation unit 4,4’-MDI (> 95%) unit

4,4’-MDI Recycle 4,4’-MDI (~ 80%) (50–80%)

Residue Residue Residue processing Recycle

fer of test results to a large plant. Further Fields of Application with highly efficient internals, the Feedback from industrial practice The combination of distillation plant size needed for a specific and numerous reports from pilot and crystallization not only en- performance is reduced along with tests facilitate process simulations ables producers to utilize the ad- decreases in capital investment for a wide spectrum of possible vantages of the respective separa- costs for new plants. These advan- feedstocks. With this experience, tion process to the full, but also tages will bring Sulzer Chemtech plant design can be adapted very eliminates the disadvantages. The further application potential in accurately and reliably to the cus- results are good yields with high numerous branches of the organ- tomer’s specifications. product purity. High flexibility en- ic chemical industry, especially The worldwide capacity for MDI ables purity to be adapted easily to where stringent requirements on is about 3.1 million tons per year market demands. In comparison product quality and low energy and rising with annual rates of 4 to with conventional processes, opti- demand exist. 5%. The new hybrid process from mized interfacing of components Sulzer Chemtech will have an im- and goal-oriented control of the portant role in this market. The heat flows in the plant reduce con- CONTACT FOR EUROPE first contract has been received sumption of high-pressure steam, Sulzer Chemtech AG from Asia with the plant to be cooling water, and electricity. And Manfred Stepanski commissioned in mid-2003. since the columns are equipped Industriestrasse 8 CH-9470 Buchs Switzerland Phone +41 (0)81-755 45 27 Melting Fax +41 (0)81-755 45 00

Crystallization E-mail manfred.stepanski Sweating @sulzer.com

4 A product purity CONTACT FOR ASIA of more than 99.5% Draining Sulzer Chemtech Pte Ltd.

is realized in the Temperature Peter Fässler crystallization stage. 11 Tuas Avenue 18 Any desired purity of Singapore 638895 the 4,4’-MDI can be Phone +65 68 63 7527 achieved through further sweating or Fax +65 68 61 1518 by repeating the Phase 1Phase 2 Phase 3 Phase 4 E-mail peter.w.faessler whole cycle. Time @sulzer.com

16 SULZER TECHNICAL REVIEW 4/2002