Distillation: processes – separatory – With measuring – testing or inspecting – Of temperature or pressure
Reexamination Certificate
2000-03-30
2002-05-14
Manoharan, Virginia (Department: 1764)
Distillation: processes, separatory
With measuring, testing or inspecting
Of temperature or pressure
C196S111000, C202S160000, C202S189000, C203S003000, C203S099000, C203SDIG001, C203S047000, C568S699000, C568S913000
Reexamination Certificate
active
06387222
ABSTRACT:
The present invention relates to a process for the continuous separation of a relatively high-melting material from a starting mixture comprising a relatively high-melting material and a low-melting low boiler by distillation and a distillation apparatus for carrying out the process.
In general, not only the low boiler and the relatively high-melting material but also further components are present in such a starting mixture. The relatively high-melting material is usually to be isolated as intermediate high-boiling desired product from the starting mixture. In the customary purification of intermediate-boiling desired products by distillation to remove low- and high-boiling impurities, various process variants are customarily employed. In the simplest case, the feed mixture comprising the relatively high-melting material is separated into two fractions, namely a low-boiling top fraction and a high-boiling bottom fraction. In the separation of feed mixtures into more than two fractions, for example into low boilers, intermediate boilers and high boilers, two or more distillation columns have to be used for this process variant. In order to limit the outlay in terms of apparatus, the fractionation of multicomponent mixtures which consist of more than two components is also carried out using components which are suitable for taking off liquid or gaseous media at the side. However, the opportunities for using distillation columns with side offtakes are greatly restricted by the fact that the products taken off at the side offtakes are normally not completely pure. Products taken off at the side from the enrichment section of a distillation apparatus, which are usually in liquid form, contain proportions of low-boiling components which are normally separated off at the top. A similar situation applies to products taken off at the side in the stripping section, which are usually in vapor form and the side products contain proportions of the high boiler. When using such conventional side offtake columns, contaminated side products are virtually always obtained, so that the use of side offtake columns is unsuitable for isolating pure materials. Particularly for isolating intermediate-boiling pure materials from multicomponent mixtures, it is therefore generally necessary to use column arrangements which consist of at least two separate columns.
An advantageous alternative is given by dividing wall columns or thermally coupled distillation columns. The use of dividing wall columns makes it possible to isolate side products, i.e. intermediate-boiling components, in pure form from multicomponent mixtures. In the case of dividing wall columns, a dividing wall is installed in the middle region. This extends to above and below the feed point. On the other side opposite the feed point, there is a side offtake. The dividing wall is thus located between the side offtake and the feed point. In the column region which is divided by the dividing wall, transverse mixing of liquid and vapor streams is not possible. This reduces the total number of distillation columns required in the fractionation of multicomponent mixtures. This type of column is in principle a constructional simplification of thermally coupled distillation columns, but the latter involve higher capital costs. Compared to a combination of conventional distillation columns, dividing wall columns and thermally coupled columns offer advantages both in respect of energy consumption and capital costs, and are therefore preferably used in the industry. Information on dividing wall columns and thermally coupled distillation columns is given in EP-A-0 122 367, EP-B-0 126 288 and EP-B-0 133 510.
The fractional distillation of a multicomponent mixture which comprises an intermediate-boiling component having a melting point above ambient temperature is more complicated technically. In such a case, it is not readily possible to remove the heat of condensation at the top of one column by means of a cooler or by means of a condenser supplied with cooling water or backcooling water. The heat exchanger, i.e. an air cooler or a cooler operated using cooling water or backcooling water, would in such a case quickly become coated with crystals since the component having the high melting point would deposit. Such a crystal layer on a condenser is disadvantageous, since heat transfer is reduced and the condenser can therefore no longer fulfill its function as desired.
As a possible remedy, it is possible to employ a combination of two or more condensers which are alternated between the cooling mode and a phase in which the solid deposits are remelted and removed. This method of operation is complicated and requires a high level of automation.
The use of a secondary medium is widespread in the industry. Examples of suitable secondary media are thermostatted oil or hot water. The temperature of the secondary medium is set so that crystal formation on the heat exchanger surfaces is prevented. In general, such a secondary medium is in contact with a further cooling medium to which the heat can be transferred. Thus, two cooling circuits are required and this incurs increased capital costs.
A further disadvantage is blockages caused by the relatively high-melting component: the latter can, if it is not deposited on the condenser, deposit in down-stream waste gas lines. This causes undesirable pressure drops.
It is an object of the present invention to devise a process by means of which a relatively high-melting material, i.e. a relatively high-melting component, can be isolated from a starting mixture comprising a plurality of components. In this process, no solid should be deposited on the corresponding condenser used in the distillation. In addition, the high-melting material should be isolated from the corresponding starting mixture by means of an effective and energetically advantageous distillation process.
The achievement of this object starts out from a process for the continuous separation of a relatively high-melting material from a starting mixture comprising the relatively high-melting material and a low-melting low boiler, where the low boiler consists of one or more components having a boiling point lower than that of the relatively high-melting material, by distillation in a distillation apparatus configured either as a dividing wall column or as a system of thermally coupled distillation columns. The object of the invention is achieved by the distillation apparatus being fitted with a condenser located at the top above the offtake point for the relatively high-melting material, where that part of the surface of this condenser which is in contact with the interior of the distillation apparatus has a temperature lower than the melting point of the relatively high-melting material and the concentration of the low boiler at the top becomes so high that no deposition of the relatively high-melting material occurs in the condenser.
In this context, “no deposition” of the relatively high-melting material means that less than 1% by weight, preferably less than 0.001% by weight, of the relatively high-melting material fed into the column is deposited in solid form in the condenser. For the purposes of the present invention, a relatively high-melting material is a material which has a melting point higher than the average temperature prevailing on the surface of the condenser used. Conversely, a low-melting material has a melting point below the mean surface temperature of the condenser. Thus, a low-melting material cannot deposit as a solid on such a condenser. Low-melting and relatively high-melting are therefore relative properties. Depending on the distillation process, i.e. depending on the surface temperature of the condenser employed, the same material can be relatively high-melting or low-melting.
The present invention also provides a distillation apparatus which is configured either as a dividing wall column or as a system of thermally coupled distillation columns for carrying out the above process. In the distillation apparatus, a condenser
Kaibel Gerd
Tragut Christian
BASF - Aktiengesellschaft
Keil & Weinkauf
Manoharan Virginia
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