Side streams, fractionating column

Figure 14.32 provides an indication of how well the model matches the plant data. The predicted product compositions appear to be quite good in general. The one major discrepancy between the plant data and the simulation results is the concentration of acetaldehyde in the side stream from column 1. This data point is not shown in Figure 14.32 since the model predicted no acetaldehyde in the sidestream (in contrast to the data) and could not be plotted in Figure 14.32 because it employs a log scale. In any event, there is no precise experimental measurement of the side stream composition the mole fractions of the side stream are, indeed, inferred from measurements of other streams. 

When a more delicate fractional vacuum-distillation is required, the flask and column shown in Fig. ii(b), p. 26, may be used, the side-arm of the column being fitted directly into receiver C (Fig. 14). A rubber stopper must then be used to fit the flask on to the fractionating column, and it should also carry a capillary tube leading to the bottom of the flask, to provide the usual fine stream of bubbles to prevent bumping. 

Several descriptions have been pubUshed of the continuous tar stills used in the CIS (9—11). These appear to be of the single-pass, atmospheric-pressure type, but are noteworthy in three respects the stills do not employ heat exchange and they incorporate a column having a bubble-cap fractionating section and a baffled enrichment section instead of the simple baffled-pitch flash chamber used in other designs. Both this column and the fractionation column, from which light oil and water overhead distillates, carboHc and naphthalene oil side streams, and a wash oil-base product are taken, are equipped with reboilers. 

The simplest unit employing vacuum fractionation is that designed by Canadian Badger for Dominion Tar and Chemical Company (now Rttgers VFT Inc.) at Hamilton, Ontario (13). In this plant, the tar is dehydrated in the usual manner by heat exchange and injection into a dehydrator. The dry tar is then heated under pressure in an oil-fired hehcal-tube heater and injected directly into the vacuum fractionating column from which a benzole fraction, overhead fraction, various oil fractions as side streams, and a pitch base product are taken. Some alterations were made to the plant in 1991, which allows some pitch properties to be controlled because pitch is the only product the distillate oils are used as fuel. 

A distillation column divides the feed stream components between the top and bottom streams, and any side streams. The product compositions are often known they may be specified, or fixed by process constraints, such as product specifications, effluent limits or an azeotropic composition. For a particular stream, 5 , the split-fraction coefficient is given by ... 

For a column with no side streams the fraction of the total feed flow going to the overheads is given by ... 

Sn = any side stream from the stage, qn = heat flow into, or removal from, the stage, n = any stage, numbered from the top of the column, z = mol fraction of component i in the feed stream (note, feed may be two-phase),... 

A mixture of water and ethyl alcohol containing 0.16 mole fraction alcohol is continuously distilled in a plate fractionating column to give a product containing 0.77 mole fraction alcohol and a waste of 0.02 mole fraction alcohol. It is proposed to withdraw 25 per cent of the alcohol in the entering stream as a side stream containing 0.50 mole fraction of alcohol. 

If AW AW the process of finding a linear-mixture basis can be tedious. Fortunately, however, in practical applications Nm is usually not greater than 2 or 3, and thus it is rarely necessary to search for more than one or two combinations of linearly independent columns for each reference vector. In the rare cases where A m > 3, the linear mixtures are often easy to identify. For example, in a tubular reactor with multiple side-injection streams, the side streams might all have the same inlet concentrations so that c(2) = = c(iVin). The stationary flow calculation would then require only AW = 1 mixture-fraction components to describe mixing between inlet 1 and the Nm — I side streams. In summary, as illustrated in Fig. 5.7, a turbulent reacting flow for which a linear-mixture basis exists can be completely described in terms of a transformed composition vector ipm( defined by... 

In operation, preheated feedstock meets a controlled stream of hot. regenerated catalyst. Vaporized oil and catalyst ascend in the riser, such that the catalyst particles are suspended in a dilute phase. Essentially all of the cracking occurs in the riser. The catalyst particles are separated from the cracked vapors at the end of the riser and the catalyst containing a coke deposit is relumed in the regenerator. The cracked vapors puss through one or more cyclones located in the upper portion of the reactor and proceed to Ihe fractionator (main column) thai produces the side streams indicated. 

In an information flow diagram, such as that shown in Figure 4.5b, each block represents a calculation module that is, the set of equations that relate the outlet stream component flows to the inlet flows. The basic function of most chemical processing units (unit operations) is to divide the inlet flow of a component between two or more outlet streams for example, a distillation column divides the components in the feed between the overhead and bottom product streams, and any side streams. It is therefore convenient, when setting up the equations describing a unit operation, to express the flow of any component in any outlet stream as a fraction of the flow of that component in the inlet stream. 

From the first column, commonly a tray column, a fraction roughly corresponding to the azeotrope is withdrawn as a liquid side stream and fed into a decanter 2, where it separates to form two liquid phases, a light phase rich in water, and a heavy phase rich in furfural. The light phase is refluxed into the first column, somewhat below the withdrawal point of the azeotrope. 

In 1992, a rather unusual furfural plant was built. With a front end according to the AGRIFURANE process described in chapter 10.2, the back end was designed as shown in Figure 112. The filtered reactor condensate containing 5 % furfural, 1.7 % acetic acid, 0.17 % formic acid, and various low boilers was introduced into an extraction tower 1 fed with chloroform at the top. On the way downwards, the heavy chloroform (density 1.498 g/cc at room temperature) picked up the furfural, and in view of the poor solubility of chloroform in water, it formed a chloroform/furfural extract at the bottom. This extract entered a distillation column 2 removing the chloroform as the head fraction. From a buffer tank 3, this chloroform was recycled to the extraction tower 1. The sump fraction of the distillation column 2 consisted of furfural, polymers, waxes, and some low boilers. This fraction was introduced into a distillation column 4, which yielded a head fraction of low boilers, a side stream of furfural, and a sump fraction of polymers and waxes. 

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