Side-stream purification

High concentration races can be achieved by thoroughly purifying make up water, e.g. softening as already mentioned and even demineralization. 

Accordingly, water chat has undergone osmosis or demineraliratton can be used on sites with little available water. This can allow concentration rates of over eight with phosphate-d isp ersant- typ e conditioning. 

This treatment is carried out with a by-pass inside the system itself with, for example, in the same settler (Fig. 81)  

This allows high concentration rates ( 6), so blowdown can practically be eliminated (zero blowdown) and zinc-chromate conditioning becomes feasible. 

This type of purification is sometimes used in the United States. Side-scream chemical purification could be supplemented by other physical treatments such as evaporation, reverse osmosis or nanofilcration. The last two techniques use membranes and require careful precreatment to reduce fouling risks. 

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Decrease the amount of make up water necessary by increasing the concentration rate by more thorough purification of make up water, or side-stream purification (on a fraction of the circulating flow). 

Another solution is the use of a flash as pre-separator of lights from heavies, while the purification of the bulk component takes place in a second side-stream column. This device offers the simultaneous removal of lights and heavies. A practical example is the purification of phtalic anhydride (Dimian, 1997). 

The above flowsheet can be simplified tremendously by catalytic distillation. Figure 7.32 depicts a conceptual configuration. The RD column consists of a reactive zone at the top, and a distillation section at the bottom. The reaction mixture is sent to a purification column, from which ethylbenzene is obtained as top distillate. A side-stream containing PEB is sent to transalkylation for EB recovery. Obviously, the feasibility of this process depends largely on the availability of an active and selective catalyst. For zeolites the optimal operating conditions are about pressure around 3 MPa, temperature less than 200 °C, and reaction rate capable to give a space-time of 5 h" for almost complete ethylene conversion. 

Part of the stream is washed counter currently with a feed side stream in the vent absorber (9) for benzene recovery. The absorber overhead flows to the hydrogen purification unit (10) where hydrogen purity is increased to 90%+ so it can be recycled to the reactor. The stabilizer (11) removes light ends, mostly methane and ethane, from the flash drum liquid. The bottoms are sent to the benzene column (12) where high-purity benzene is produced overhead. The bottoms stream, containing unreacted toluene and heavier aromatics, is pumped to the recycle column (13). Toluene, Cg aromatics and diphenyl are distilled overhead and recycled to the reactor. A small purge stream prevents the heavy components from building up in the process. 

Even at these levels, the system must be run so as to prevent precipitation of silica (maximum solubility of around 140 to 160 mgT ). Additionally, conventional chemical purification of the make up water alone must be replaced by side-stream treatment of the main system water itself (Fig. 40), theoretically advantageous as to size and cost. This treatment provides for chemical precipitation of the calcic bicarbonate alkalinity and silica. 

These effluents are reused after purification when there is side-stream filtration on the system with an incorporated coagulant to retain colloidal matter and prevent exchanger fouling (see Chapter 3 Section 4= Recycling and Chapter 6 Treatment Systems). 

There are other column arrangements possible for simple distillation. Some of these are illustrated in Figure 12.2. Figure 12.2(a) illustrates distillation with a side stream It must be understood that in a typical distillation column, a side stream does not contain a pure conponent it contains a mixture. In certain petroleum refining operations, side streams are common because a mixture is the desired product. Sometimes a side stream is withdrawn because it contains a maximum concentration of a third component —for example, in the purification of argon from air. 

Case c shows a solution where the extra carbon dioxide is obtained by production of excess synthesis gas. The excess synthesis gas can be extracted as a side stream before the synthesis gas compressor and used as fuel in the reformer or exported as such or as hydrogen after purification. Alternatively, it can be compressed and passed through the synthesis loop which will then operate with a high purge rate and correspondingly low inert level. The actual solution will depend on the specific case, especially on the composition of the feedstock. Solutions of this type are used by the process schemes (see Sect. 6.5.3.2.3) using a stoichiometric amount of process air, such as the Kellogg, Topsoe and Uhde processes. 

The polymerization apparatus (see Fig. 3.2) consists of a 11 three-necked flask, fitted with stirrer, thermometer, gas inlet with tap, and gas outlet.On the inlet side the gas stream passes through three wash bottles one as a safety bottle (A), one for the purification of ethylene, filled with 30 ml of petroleum ether (bp 100-140 °C) and 5 ml of diethylaluminum chloride (B),and one filled with molecular sieves 5 A (C).The last of these dries the ethylene further and also serves to trap aluminum hydroxide carried over from B. On the outlet side there are two wash bottles the first is a safety bottle (D), and the second, (E), is filled with 50 ml of dry bis(2-hydroxyethyl) ether (diglycol), and isolates the apparatus from the external atmosphere. 

Purification of feeds. Reducing the concentration of impurities in the feed usually leads to reduced side reactions and less waste formation. This approach can also reduce the need for purges and vent streams. Feed impurities also often lead to degradation of solvents and catalysts. Care must be taken to select a purification approach that does not itself lead to more waste formation. 

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