Pasteurizing section

The propylene fractionator operates at a pressure of 1.8 to 2.0 MPa with more than 160 trays required for a high purity propylene product. Often a two-tower design is employed when polymer grade (99.5%+) is required. A pasteurization section may also be used when high purity is required. The bottoms product contains mainly propane that can be recycled to the cracking heaters or used as fuel. Typical tower dimensions and internals for a 450,000 t/yr ethylene plant with naphtha feed are summarized in Table 7. 

Distillation with vapor product. When a partial condenser is used, the flash drum plays the role of a vapor/liquid separator. In the setup known as a stabilizer there is only vapor distillate, while the liquid is returned as reflux. The column has a pasteurization section when a gaseous stream leaves at the top, while the... 

When the feed to a distillation column contains a small amount of impurities that are much more volatile than the desired distillate, it is possible to separate the volatile impurities from the distillate by removing the distillate as a liquid side stream from a stage located several stages below the top stage. As shown below, this additional top section of stages is referred to as a pasteurizing section. 

In many side-drawoff columns, the side draw is very small compared to the other product streams. This is a common situation when the side stream serves to remove an intermediate impurity (Sec. 13.7). In many other side-drawoff columns, distillate is withdrawn as a side product a few trays below the top of the column, leaving an upper "pasteurizing section for separating light ends as a small vent stream. Common examples are an ethylene plant Cg splitter, where small quantities of methane are "pasteurized out of the ethylene product, and an alcohol still, where "heads are pasteurized out of the alcohol product. The reverse situation has the bottom product drawn as a vapor side product a few trays above the bottom, leaving a small heavy end stream to exit from the bottom. 

In all these situations, the two prominent products are MB-controlled using one of the normal schemes (Fig. 16.4), as if the small stream does not exist. There is little incentive to tightly control the composition of the small stream, and it is often assigned a non-MB control. The small stream may be withdrawn on flow control, flow-to-feed-ratio control, or flow-to-main-product-ratio control (Fig. 19.6a-c). A generous flow or ratio setting is usually fixed as a means of positively preventing impurity accumulation. If this leads to excessive product losses, the small flow can be manipulated by a temperature (or composition) controller in the pasteurizing section. For simplicity. Fig. 19.6 shows the small stream to be on flow control, but the discussion below also applies when the small steam is temperature- or ratio-controlled as described above. 

In the distillation of products from naturally occurring raw materials, the feed may contain a small percentage of very low-boiling compounds that are not desired in the distillate product. A separate column could be used to stabilize the feed by removal of these very low boilers before distillation in the main column. This stabilizer would require additional energy to operate the reboiler and condenser. However, it may be possible to install a pasteurization section at the top of the main column to remove these very low boilers. 

However, another packed bed can be installed above the rectifying section as a pasteurization section to separate C-12 alcohol from C-16 alcohol. Because less than 4% of the feed is removed as distillate at the column top, the reflux liquid returned to the column is about 24 times this distillate flow. At this high reflux ratio in the pasteurizing bed, only a few theoretical stages are needed to make the desired separation. 

Below the pasteurization section, the product is removed as a liquid sidedraw. This product contains only 0.8 wt% C-12 alcohol and represents 95.5% recovery of the C-16 alcohol present in the feed. The total energy input for the operation of this column is less than that required for a more conventional arrangement of two columns operated in series. 

The crude feed initially is dehydrated and then separated from the pitch residue. The first fractionating column removes the rosin acids from the feed as a bottom product. A crude fatty acid product is removed from this column below a pasteurization section. The top distillate contains all the 014 acids and lower boiling compounds in the feed. The principal separation in this column is between C-18 fatty acids and C-20 rosin acids. 

The C-2 splitter normally operates at a pressure of 260 to 320 psia with a top temperature of —30° to — 10°F. The C-2 splitter typically consists of two columns, operated in series, that are equipped with a total of 120 to 160 actual trays. The feed enters the first column that has a reboiler at its base. The overhead vapor from this column goes to the bottom of the second column. Liquid from the bottom of the second column is pumped to the top of the first column. The upper 8 to 11 actual trays in the second column serve as a pasteurization section that removes the small amount of methane present in the feed. The ethylene product is withdrawn as a liquid sidestream below the pasteurization section. Any propylene in the feed leaves in the bottoms from the first column. 

The polymer grade product is 98.4% to 99.6% propylene, with propane as the major impurity. The distillate also contains the small amount of ethane present in the feed however, a pasteurization section usually is not required to meet the product specifications. Chemical grade has a wide range of purities, but typically contains around 93% propylene. Normally, about 98% of the propylene in the feed is recovered in the product. 

On the outlet of the holder tube, the FDV directs the pasteurized product to the regenerator and then to the final cooling section (forward flow). Alternatively, if the product is below the temperature of pasteurization, it is diverted back to the balance tank (diverted flow). The FDV is controlled by the safety thermal-limit recorder. 

A homogenizer or rotary positive pump may be used as a timing or metering pump to provide a positive, fixed flow through the pasteurization system (Fig. 6). The pump is placed ahead of the heater and the holding section. Various control drives assure that the pasteurized side of the heat exchanger is at a higher (7 kPa (1 psi)) pressure than the opposite side. 

Because the WAO process also aims to reduce sludge volume we will spend more time describing this process under the section dealing with Volume Reduction. The other thermal sludge conditioning method is best-known as sludge pasteurization, and deserves more than just a brief overview. 

The process of fermentation gives off heat, and the tanks may need to be cooled with chilled water coils, with jackets, or by cooling the cellar in which the tanks are located. When fermentation is complete, many beers are now pasteurized, in the same manner as milk (see Section 17.1). The beer is then cooled to just above freezing, filtered and left to age . Before final bottling, kegging or canning it will undergo a fine filtration to improve the clarity. 

Transfer the sample to the column with a Pasteur pipet and drain the solvent to the top of the sodium sulfate layer. Rinse the round-bottom flask three times with 3-mL portions of hexane-ethyl acetate (10 1, v/v), adding these rinses sequentially to the column and draining the solvent to the top of the sodium sulfate layer before the next addition. Discard the accumulated eluate and place a 100-mL round-bottom flask under the column. Elute the residues with 28 mL of hexane-ethyl acetate (10 1, v/v). Evaporate the column eluate just to dryness by rotary evaporation under reduced pressure in a <40 °C water-bath. Reconstitute the sample in 2.0 mL of toluene for analysis (Section 6.2). 

All of the threonine stereoisomers 19-22 are chiral substances that is, they are not identical with their mirror images. However, it is important to recognize that not all diastereomers are chiral. To illustrate this point, we return to the tartaric acids mentioned previously in connection with Pasteur s discoveries (Section 5-1C). 

The crystallization procedure employed by Pasteur for his classical resolution of ( )-tartaric acid (Section 5-1C) has been successful only in a very few cases. This procedure depends on the formation of individual crystals of each enantiomer. Thus if the crystallization of sodium ammonium tartrate is carried out below 27°, the usual racemate salt does not form a mixture of crystals of the (+) and (—) salts forms instead. The two different kinds of crystals, which are related as an object to its mirror image, can be separated manually with the aid of a microscope and subsequently may be converted to the tartaric acid enantiomers by strong acid. A variation on this method of resolution is the seeding of a saturated solution of a racemic mixture with crystals of one pure enantiomer in the hope of causing crystallization of just that one enantiomer, thereby leaving the other in solution. Unfortunately, very few practical resolutions have been achieved in this way. 

Introduction. The importance of microorganisms in certain citrus products should not be underestimated. Especially vulnerable are citrus concentrates, pasteurized citrus juices, citrus juice from concentrate and chilled citrus sections. 

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