Impurity accumulation prevention

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 The small stream may be withdrawn on flow control, flow-to-feed-ratio control, or flow-to-main-product-ratio control (Fig. 19.6a-c).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. 

The chlorination is mostly carried out in fluidized-bed reactors. Whereas the reaction is slightly exothermic, the heat generated during the reaction is not sufficient to maintain it. Thus, a small amount of oxygen is added to the mixture to react with the coke and to create the necessary amount of heat. To prevent any formation of HCl, all reactants entering the reactor must be completely dry. At the bottom of the chlorination furnace, chlorides of metal impurities present in the titanium source, such as magnesium, calcium, and zircon, accumulate. 

We consider the system of a gas-phase reactor and a condenser shown in Figure 4.1. The reactant A is fed at a molar flow rate Fo to the reactor, where a first-order irreversible reaction A —> B takes place with a reaction rate constant k. The reactor outlet stream is fed to a partial condenser that separates the light unconverted reactant A from the heavy product B. The gas phase, rich in A, is recycled to the reactor. A volatile inert impurity I is present in the feed stream in small quantities and a (small) purge stream P is used to prevent its accumulation in the recycle loop. 

Two control structures are shown in Fig. 2.11u and b. In both, the composition of component A in the product stream xSA is controlled by manipulating vapor boilup in the column. This prevents component A from leaving the system. Except for this small amount of A impurity in the product, all A that enters the system must be consumed in the reactor. This illustrates the point we made in Sec. 2.2 about the need to change conditions in the reactor so that the additional reactant is consumed and will not accumulate. 

Because the methane feed contains 1% nitrogen as an impurity, a portion of the recycle stream must be purged as shown in the flow diagram to prevent the accumulation of nitrogen in the system. The purge stream analyzes 5% nitrogen. 

Basic alkyl sulfonates, Ca(OH)S03R, are also used to contribute acidity neutralizing and dispersing activity to the oil. These detergents (dispersants), help to keep particulate impurities, such as carbon, dust, or metal fines (e.g., lead or other metal oxides and salts) and water or acid droplets suspended in the oil to prevent deposition on or attack of critical moving parts. At the appropriate oil change interval most of the accumulated suspension is removed from the engine with the spent oil. 

The catalyst is not sensitive to sulfur or other 0x0 poisons. Together with the simple but effective decanting operation which allows organic impurities and other byproducts to be removed at the very moment of separation, accumulation of activity-decreasing poisons in the catalyst solution is prevented. Therefore, no special pretreatment or even purification steps are necessary. The 0x0 catalyst HRh(CO)(TPPTS)3 is produced within the 0x0 reactor simply by reacting suitable Rh salts with TPPTS without any additional preformation step. Typical reaction conditions and compositions of crude products from the RCH/RP process based on a 14-year average are compiled in Table 3. 

Feed solution enters the downpipe before the suction of the circulating pump. Mother liquor and crystals are drawn off through a discharge pipe upstream from the feed inlet in the downpipe. Mother liquor is separated from the crystals in a continuous centrifuge the crystals are taken off as a product or for further processing, and the mother liquor is recycled to the downpipe. Some of the mother liquor is bled from the system by a pump to prevent accumulation of impurities. 

The contents of F2 are cooled with liquid nitrogen and F2 is attached to the apparatus (Fig. 3) while a slow nitrogen flow is passed through Si. Trap F4 contains a few drops of mercury and a small stirring bar, while F5 is an empty trap. High vacuum is applied to the system, and the valve S5 is closed. The Dewar flask is then removed from S2 and used to cool F4. Both [10B] Br3 and Br2 condense in F4. The temperature in F2 must not exceed 0° to prevent the vaporization of AlBr3. After all the volatile material has accumulated in F4, the valves S6 and S7 are closed and the contents in F4 warmed to room temperature. The liquid in F4 is now stirred for 30 min, and valve S7 is opened. Trap F5 is then cooled with dry ice/2-propanol, and valve S7 is opened. The impure [10B]Br3 is distilled at 0° from F4 into F5. This product may be further fractionated in a train of traps to remove traces of HBr. However, the [10B]Br3 obtained contains very little HBr, and another purification step is not necessary in most cases. The product is transferred into a vacuum storage flask. Yield 19.0 g (76%). ... 

The handling of impurities in a VCM plant has been investigated by Dimian et al. (2001) by means of computer simulation. It has been demonstrated that selective chemical conversion of intermediate impurities can be used to prevent their accumulation and the occurrence of snowball effects in the separation units. The separation of impurities can be properly handled by exploiting the interaction effects through recycles. More details are given in the Chapter 17 (Case Study 3). 

Gas-phase reactions may lead to gaseous by-products and impurities. Similarly, a gaseous reactant may contain light impurities that will pass through the reaction system or produce other impurities. As result, processes with gas recycles need the placement of one or several purges to prevent the accumulation of some gaseous components. The above observation may be extended to heavy components, for which exit points (bleeds) must be provided. 

Catalytic conversion. The most efficient way to prevent the accumulation of impurities that are difficult to remove is chemical conversion. Here we consider only two types of reactions catalytic combustion and hydrogenation. By catalytic combustion, a gaseous impurity is destroyed to COj, SOj, N2 and HjO. For safety reasons, the impurity concentration should be kept well below the lower explosion limit. By catalytic hydrogenation, unsaturated organic components are transformed in species easier to remove. High catalyst selectivity is required. 

Chemical conversion is an effective way to counteract the accumulation of impurities due to positive feedback. Also, changing the connectivity of units may be used to modify the effect of interactions, for example by preventing an excessive increase in recycles due to snowball effects. Effective plantwide control structures may imply controlled and manipulated variables belonging to different but dynamically neighbouring units. The methodology to evaluate the dynamic inventory of impurities consists of a combination of steady state and dynamic flowsheeting with controllability analysis. This is used to assess the best flowsheet alternative and propose subsequent design modifications of units. Case Study 3 in Chapter 17 will present this problem in more detail. 

Identify how chemical components enter, leave and are generated (or consumed) in the process. Accumulation of chemical components in recycle streams can be a major reason of failure in control. A preventing method consists in tracing the paths and evaluating inventories for reactants, products and inert, as well as for sub-products and main impurities. This accounting operation can be done by means of a component table where input, generation, output and accumulation region are noticed. 

To prevent the accumulation of I2, a side stream drawn from S2 is sent to the reactor Rl, where chlorination to heavies takes place. Because of constraint on li, the top distillate of S2 carries with a significant amount of DCE, which has to be recovered and recycled by the column S4. By recycling the bottom of S4 to the reactor Rl, some amounts of impurities ft and I2 are converted in heavies. This operation helps to reduce the accumulation of undesired impurities, particularly of I2, but affects the operation of the reactor Rl. 

Other requirements are placed on the carrier gas and the stationary phase. As indicated in the PTGC instrumentation list, the carrier gas must be dry to prevent the accumulation of water (and other volatile impurities) at the cool column inlet (before the start of a run) since this phenomenon will result in ghost peaks during the PTGC run. One conunon solution to this problem is to insert a 5A molecular sieve dryer in the gas line before the instrument. 

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