Distillation: processes – separatory – And recovering heat by indirect heat exchange – Utilizing recovered heat for heating the distillation zone
Reexamination Certificate
1998-06-02
2001-07-03
Manoharan, Virginia (Department: 1764)
Distillation: processes, separatory
And recovering heat by indirect heat exchange
Utilizing recovered heat for heating the distillation zone
C203S049000, C203S098000, C203S099000, C203SDIG001, C423S387000
Reexamination Certificate
active
06254735
ABSTRACT:
The present invention relates to a process for separating middle boilers from a mixture of low, middle and high boilers which is separated into a fraction containing low and middle boilers and a fraction containing low and high boilers.
A problem frequently encountered in the chemical industry is the need to separate the middle boilers in pure form or with only traces of low boilers from a liquid multi-substance mixture consisting of a low- (L), middle- (M) and high-boiler (H) fraction.
To do this it is possible to employ the known distillation methods, for example those described in Ullmann's Encyclopedia of Industrial Chemistry Vol. B3, page 4-46 et seq. A common feature of the known distillation methods is that the high boilers are drawn off at the bottom in pure form, or possibly with residual traces of middle boilers, and that the middle-boiling component is separated off at the top of the column at temperatures determined largely by the concentration of the high-boiling component and its boiling temperature. Furthermore, with the known methods it is not possible to co-separate a mixture of low and middle boilers while at the same time separating out a mixture of low and high boilers which is free from middle boilers. In many cases, however, this would be desirable, especially if low and high boilers are to be put to some further conjoint use (sale, recovery, disposal).
Page 4-48 of the abovementioned publication describes the use of side columns for separating middle boilers from the mixture of low, middle and high boilers (L,M,H mixture). In this case too, low and high boilers are always separated. The same applies to the directly or indirectly coupled columns describe on pages 4-62 and 4-63 in the abovementioned publication. In all of these cases it is ultimately necessary to separate the middle-boiling component from the high-boiling component by means of distillation, which in every case requires boiling temperatures which are at least equal to that of the middle-boiling component and, in extreme cases, are close to the boiling temperature of the high-boiling component and are therefore very high. This is particularly the case if the middle-boiling component must be separated completely from the high-boiling component. Such high temperatures may arise that, even in the caseof relatively heat-stable substances, decomposition or chemical conversion (polymerization etc.) of the substances involved may occur. For this reason, complex distillation techniques, for example gentle distillations carried out under reduced-pressure conditions (thin-film evaporators, molecular jet distillation, etc.) are often necessary for separation tasks of this kind. Such distillation techniques have the disadvantage that the through-puts are extremely low. This leads to high capital investment and product costs, which may mean that a distillative separation which per se is advantageous may not be economic to carry out.
Also known are special techniques for separating difficult-to-separate liquid mixtures. Special techniques are only relevant if they are more cost-effective or when other, common techniques have failed. They are frequently employed with substances whose capacity to withstand thermal stress is limited, ie. if the boiling point is above or close to the decomposition temperature. A known method of separating components of low volatility from mixtures comprising immiscible components is that of carrier-gas distillation. The basis for this method is that, in a mixture of immiscible substances, each substance behaves as if the other was not there; in other words, at a given temperature, each substance possesses a partial pressure which—independently of the composition of the mixture—is equal to the vapor pressure of the substance concerned. Consequently, the pressure over such a mixture is equal to the sum of the vapor pressures of the individual components. A known example of this is the water/bromobenzene system. The mixture boils at 95° C., whereas the pure substances boil at 100° C. (water) and 156° C. (bromobenzene). Carrier-gas distillation is suitable in particular for separating immiscible components of relatively high boiling point (eg. glycerol), for separating substances which polymerize or decompose even before reaching the boiling point (fatty acids), and for separating substances which are very difficult to handle and for which direct heating to the boiling point may be hazardous (eg. turpentine).
The best-known example of carrier-gas distillation is steam distillation, ie. where steam is the carrier gas. It is extensively employed, for example, in the petroleum-processing industry, for removing light hydrocarbons from absorber oils; in the coal industry, for the steam distillation of hydrocarbon cuts from the coal distillation operation; for separating off turpentine from resins in the rubber industry; and in preparative organic chemistry. Steam distillation is a special form of azeotropic or extractive distillation, as described in the abovementioned publication on pages 4-50 to 4-52. The technical effect of this process is based on the finding that, by adding a substitute substance (an entrainer), the azeotropic point is surpassed and, consequently, the desired concentration above the azeotropic point is achieved.
All of these techniques have the disadvantage that an additive (entrainer) is introduced into the system which is to be distilled, and this entrainer then has to be separated off from the system again by way of an additional process step.
A further known method of removing relatively high-boiling substances from a mixture of substances is stripping. Stripping has the disadvantage that it always produces only a highly dilute solution of the high- boiling component or middle-boiling component in the stripping medium, thereby necessitating a laborious and costly separation. In general, the process is only economical when the products can be separated by phase separation, ie. when the mixture of substances exhibits a miscibility gap.
It is an object of the present invention, therefore, to provide a simple and gentle process for separating a middle-boiling component or a fraction comprising low and middle boilers from a mixture which includes low, middle and high boilers.
We have found that this object can be achieved, surprisingly, if the abovementioned mixture in a column is treated in the bottom with low-boiler vapor.
The present invention consequently provides a process for separating a fraction containing low and middle boilers (L,M fraction) and a fraction containing low and high boilers (L,H fraction) from a homogeneous mixture comprising low, middle and high boilers (L,M,H mixture), which comprises treating the L,M,H mixture in a column with low-boiler vapor and separating it into an L,M fraction and an L,H fraction. The middle-boiling component accumulates in the low-boiler vapor, so that the L,M fraction can be recovered above the infeed site of the mixture, and the L,H fraction is obtained in the liquid phase.
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Ullmann's Enc. of Ind. Chem., vol. B3, pp. 4-46, 1990.
Perry's Chem. Eng. Handbook, Section 13, pp. 5-10, 1984.
BASF - Aktiengesellschaft
Keil & Weinkauf
Manoharan Virginia
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