Impurities separation system

Process trouble shooting. Analysis or separation system operation and malfunc tion, examination of composition profiles, and tracking of trace impurities with implications for corrosion and process specifications. 

It is sometimes desired to control some stream by varying an operating parameter. For example, in a reaction/separation system, if there is an impurity that must be purged, a common objective is to set the purge fraction so that the impurity concentration into the reactor is kept at some moderate value. Commercial packages contain procedures for... 

Consider the effect of every impurity or potential impurity in every feedstock, in the solvents in the ligand and in the catalyst. What harm, if any, will befall the product or catalyst if these impurities enter the process, and in particular, if they accumulate Is the product stable in both the reaction and separation system If byproducts form, what affect will they have on the process and catalyst life ... 

If an impurity in a liquid feed stream is also a byproduct or product component usually it is better to process the feed through the separation system. 

The goal of this case study is to illustrate some generic issues raised by the conceptual design of a large-scale process involving several reaction and separation sections interconnected in a complex plant with recycles. The study emphasizes the intricate effects of handling the removal of numerous impurities generated in different reactors by a common separation system, with implications on process dynamics and plantwide control. 

The plant simulation considers only a reduced number of units, but dynamically representative, as follows. Crude EDC from R1 and R3 are sent to washing/drying in the unit SO. Dissolved gases and very light impurities are removed in SI, and further in the distillation column S4, which is the exit point of the light impurities. After pretreatment, the crude EDC is sent to purification in the distillation column S2, which is the key unit of the separation system. This column receives crude EDC from three reactors. It is also the place where three large recycle loops cross. The top distillate of S2 should remove the light impurities mentioned above, while the purification of EDC from heavies is continued in the distillation columns S3 and S5. 

Practical aspects. Very low levels of electron-capturing impurities in the carrier gas (HjO, Oj, organic compounds) can substantially reduce the amount of available thermally excited electrons in the detector. This reduces the sensitivity drastically. A very clean chromatographic separation system is therefore required. 

The chemical reactor has a determinant role on both the material balance and the structure of the whole flowsheet. It is important to stress that the downstream levels in the Hierarchical Approach, as the separation system and heat integration, depend entirely on the composition of the reactor exit stream. However, a comprehensive kinetic model of the reaction network is hardly available at an early conceptual stage. To overcome this shortcoming, in a first attempt we may neglect the interaction between the reactor and the rest of the process, and use an analysis based on stoichiometry. A reliable quantitative relationship between the input and the output molar flow rates of components would be sufficient. This information is usually available from laboratory studies on chemistry. Kinetics requires much more effort, which may be justified only after proving that the process is feasible. Note that the detailed description of stoichiometry, taking into account the formation of sub-products and impurities is not a trivial task. The effort is necessary, because otherwise the separation system will be largely underestimated. 

The reaction system is the core of a chemical process. It is also the starting point in process design. By proper selection and design of the chemical reaction system, the modem process plants must cope with large flexibility with respect to production rate and selectivity. A greater attention should be paid to the formation of by-products and impurities. The selection and the design of the reactor system must be done in the context of interactions with the whole process, particularly with the separation system. 

The above simple analysis highlights an important issue in process dynamics the influence of positive and negative feedback on system s stability. Instability can occur in recycle systems due to positive feedback when the gain is larger than unity. We may give as example the recycle of energy developed by an exothermal reaction in an adiabatic PFR for feed preheating. Instability may occur because of the exponential increase in reaction rate with the temperature when this cannot be properly controlled (Bildea Dimian, 1998). Another example is the recycle of impurities in a plant with recycles, whose inventory cannot be kept at equilibrium by the separation system (Dimian et al., 2000). 

The quality of the intermediate DCE must fulfil strict purity specifications. Low impurity levels imply high energetic consumption, but higher impurity amounts are not desired for operation. The intermediate DCE is conditioned mainly in the distillation column S2. In the bottom product the concentration of the two bad impurities E and E must not exceed an upper limit, of 100 and 600 ppm, respectively, while the concentration of the good impurity E must be kept around optimal value of 2000 ppm. Because these impurities are implied in all three reaction systems through recycles that cross in the separation system, their inventory is a plantwide control problem. The problem is constraint by technological and environmental constraints, as mentioned. 

Chicken extract was obtained from whole chicken carcasses by heating the carcasses with water (Figure 22.3). Then, the extract was treated with ion exchange resin to remove acidic and neutral amino acids and proteins. The chicken extract after the treatment with the ion exchange resin contained 6.79 g/L of anserine and carnosine, and their purity was 60%-70%. Impurities contained in the extract after the treatment with the ion exchange resin were creatinine and sodium chloride. Concentrations of these impurities were 2.30 g/L and 0.85 g/L, respectively. The extract after the treatment with the ion exchange resin was used as material for membrane separation experiments. A bench-scale membrane separation unit supplied by DSS (Danish Separation System) was used in this study. 

Western-world bauxite production in 1988 totaled about 90 x 10 t, approximately 90% of which was refined to aluminum hydroxide by the Bayer process. Most of the hydroxide was then calcined to alumina and consumed in making aluminum metal. The balance, which constituted about 2.3 x 10 t in 1988 (Table 2), was consumed in production of abrasives (qv) adhesives (qv) calcium aluminate cement used in binding ceramics (qv) and refractories (qv) catalysts used in petrochemical processes and automobile catalytic converter systems (see Petroleum Exhaust control, automotive) ceramics that insulate electronic components such as semiconductors and spark plugs chemicals such as alum, aluminum halides, and zeoHte countertop materials for kitchens and baths cultured marble fire-retardant filler for acryhc and plastic materials used in automobile seats, carpet backing, and insulation wrap for wire and cable (see Flame retardants) paper (qv) cosmetics (qv) toothpaste manufacture refractory linings for furnaces and kilns and separation systems that remove impurities from Hquids and gases. 

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