Separation by partial condensation

The term partial condensation is used to denote the liquefaction of a fraction froni a flow of vapour. It can be brought about deliberately by interposing a special condenser — a so-called dephlegmator — in the vapour line, and it also occurs as an undesired effect by loss of heat from an insufficiently insulated column (Fig. 170a). 

The flows of vapour leaving the column have to be in a particular ratio depending on the object of the distUlation. In industry it is common to make a dephlegmator produce the reflux and cool the distillate to be drawn off in a production condenser. The aim is to use the amplifying effect of the dephlegmator which is due to the partial condensation of the high-boiling components. Partial condensation is employed 

The performance depends on the nature of the mixture to be separated [84]. Another advantage is the low hold-up. In Table 44 two examples concerning mixtures of technical importance are given. 

The calculation of the enrichment caused by a dephlegmator can be carried out with a formula due to Fabuss  

Destinorm column head for partial condensation with capillary take-off tubes 

---

According to the discussion in Chapter 2.2, liquid mixtures containing different boiling components are partially separated by supplying heat to the system. Analogous to this, vapor mixtures with different condensing components are partially separated by partial condensation which is achieved by removing heat from the system. 

For example, for a vapor mixture Dp, with a mol fraction yp (of the lower-boiling key component), leaving an exothermic reaction cooled by vaporization, or a continuous flow evaporator, an amount R is separated by partial condensation (Fig. 2-85). The remaining vapor Dp, = Dp - R, leaving the partial condenser is then enriched with the lower-boiling mixture components, which gives yp > yp. The enrichment... 

The wax distillate and gas oil are to be separated by partial condensation in four units of condenser. Three of these are for wax distillate, and one for gas oil. The first of the three wax distillate condensers supplies hot reflux to the tower, but cold reflux is also used. 

Figure 11.8 Inproved first separation step of acrylonitrile by partial condensation. 

These models produced a zoned Earth with an early metallic core surrounded by silicate, without the need for a separate later stage of core formation. The application of condensation theory to the striking variations in the densities and compositions of the terrestrial planets, and how metal and silicate form in distinct reservoirs has been seen as problematic for some time. Heterogeneous accretion models require fast accretion and core formation if these processes reflect condensation in the nebula and such timescales can be tested with isotopic systems. The time-scales for planetary accretion now are known to be far too long for an origin by partial condensation from a hot nebular gas. Nevertheless, heterogeneous accretion models have become embedded in the textbooks in Earth sciences (e.g.. Brown and Mussett, 1981) and astronomy (e.g.. Seeds, 1996). 

Fractionation involves the separation of a liquid into products with a shorter boiling range using vaporization. In early refineries, this was done with a series of distillations where partly separated products were redistilled several times until the desired product was achieved. Next came fractionation by partial condensation. Here a mixture of vapor is condensed in portions by cooling the vapor to lower and lower temperatures. 

A process for separating nitrogen from natural gas is shown in Fig. 5.1-10. The separation is performed by partial condensation and partial evaporation The feed is cooled down and partially liquefied against the cold products. The two-phase system is flashed in a nozzle and, in turn, further cooled down. The two fractions after the nozzle are split in a vessel and heated up in separate product lines. The whole process can be simnlated with the eqnations presented before. Especially complex is the evalnation of the cooling down and heating np cnrves in the enthalpy/temperature diagram of Fig. 5.1-10. 

Fractionation. The term fractionation refers to the separation of a liquid mixture into several products of shorter boiling range by means of vaporization. In early refineries this separation was obtained, by a series of distillations in which, first, an inaccurate separation was made and then the partly separated products were redistilled several times, if necessary, until finally the product met market requirements. The next development was to fractionate by partial condensation (i.e., a mixture of vapor was condensed in portions by successively cooling the vapor to lower and lower temperatures). An invention of this nature was that of Hugh L. Allen,2 in which the vapor was successively cooled by means of a series of air-cooled condensers into several liquid products. 

Fractionation may be broadly defined as any method by which a liquid or vapor mixture may be separated into individual components by vaporization or condensation. The components may be pure compounds or if the original material is a complex mixture, the components may be products that are stUl mixtures but whose distillation range is limited by the fractionation process. In a more detailed way the various means of separation have been given special names. Distillation is usually considered to refer to a complete operation in which heating, vaporization, fractionation, condensation, and cooling are practiced. DephlegmaMon is a particular kind of fractionation in which a vapor mixture is separated into components by partial condensation. In this operation... 

Furthermore, it would appear in Fig. 17-16 that the hot reflux boils between exactly 753 and 890 F. Actually, the wax distillate and the hot reflux have different boiling ranges but much the same composition because of the very poor separation obtained by partial condensation. 

Separation of mixtures of condensable and non-condensable components. If a fluid mixture contains both condensable and noncondensable components, then a partial condensation followed by a simple phase separator often can give a food separation. This is essentially a single-stage distillation operation. It is a special case that deserves attention in some detail later. 

When a mixture contains components with a broad range of volatilities, either a partial condensation from the vapor phase or a partial vaporization from the liquid phase followed by a simple phase split often can produce an effective separation. This is in essence a single-stage distillation process. However, by its very nature, a single-stage separation does not produce pure products hence further separation of both liquid and vapor streams is often required. 

The output from the turbine might be superheated or partially condensed, as is the case in Fig. 6.32. If the exhaust steam is to be used for process heating, ideally it should be close to saturated conditions. If the exhaust steam is significantly superheated, it can be desuperheated by direct injection of boiler feedwater, which vaporizes and cools the steam. However, if saturated steam is fed to a steam main, with significant potential for heat losses from the main, then it is desirable to retain some superheat rather than desuperheat the steam to saturated conditions. If saturated steam is fed to the main, then heat losses will cause excessive condensation in the main, which is not desirable. On the other hand, if the exhaust steam from the turbine is partially condensed, the condensate is separated and the steam used for heating. 

The dephlegmator process recovers a substantially higher purity C2+ hydrocarbon product with 50—75% lower methane content than the conventional partial condensation process. The C2+ product from the cryogenic separation process can be compressed and further separated in a de-ethanizer column to provide a high purity C3+ (LPG) product and a mixed ethylene—ethane product with 10—15% methane. Additional refrigeration for the deethanization process can be provided by a package Freon, propane or propylene refrigeration system. 

In distillation operations, separation results from differences in vapor-and liquid-phase compositions arising from the partial vaporization of a hquid mixture or the partial condensation of a vapor mixture. The vapor phase becomes enriched in the more volatile components while the hquid phase is depleted of those same components. In many situations, however, the change in composition between the vapor and liquid phases in equihbrium becomes small (so-called pinched condition ), and a large number of successive partial vaporizations and partial condensations is required to achieve the desired separation. Alternatively, the vapor and liquid phases may have identical compositions, because of the formation of an azeotrope, and no separation by simple distillation is possible. 

When the sum of the partial pressures of the steam and the material distilled reach the system pressure, boiling begins and both components go overhead in the mol ratio of their partial pressures. Upon condensation of the overhead mixture, the condensate receiver will contain two layers that can be separated by gravity. 

---