Impurities, reactants

The use of impure reactants may give a deep red syrup and delay considerably tbe final solidification they should, therefore, be redistilled in a vacuum before use. 

Percentage yields will always be less than 100%. Losses in mass occur through the transfer of the product from one flask to another and purification by recrystallisation results in some product remaining in the solution. A large reduction in yields can be due to side reactions occurring, incomplete reactions and sometimes impure reactants. 

The importance of catalyst stability is often underestimated not only in academia but also in many sectors of industry, notably in the fine chemicals industry, where high selectivities are the main objective (1). Catalyst deactivation is inevitable, but it can be retarded and some of its consequences avoided (2). Deactivation itself is a complex phenomenon. For instance, active sites might be poisoned by feed impurities, reactants, intermediates and products (3). Other causes of catalyst deactivation are particle sintering, metal and support leaching, attrition and deposition of inactive materials on the catalyst surface (4). Catalyst poisons are usually substances, whose interaction with the active surface sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface. Selective poisons are sometimes used intentionally to adjust the selectivity of a particular reaction (2). 

Reactants. The 1- and 2-butenes as well as the 1-pentene were 99.8% pure. The cis-2-pentene came from two different sources, one of them 97% pure (about 3% of Zrans-2-pentene impurity) and the other an A.P.I. Standard 99.8% pure. Data were obtained using the impure reactant. The results were then corrected by using the integrated first-order differential equations which give the time evolution of the system (3, 14). The validity of the correction was checked by making several runs using the pure reactant. 

The first inequality characterizes recycle systems with reactant inventory control based on self-regulation. It occurs because the separation section does not allow the reactant to leave the process. Consequently, for given reactant feed flow rate F0, large reactor volume V or fast kinetics k are necessary to consume the whole amount of reactant fed into the process, thus avoiding reactant accumulation. The above variables are grouped in the Damkohler number, which must exceed a critical value. Note that the factor z3 accounts for the degradation of the reactor s performance due to impure reactant recycle, while the factor (zo — z4) accounts for the reactant leaving the plant with the product stream. 

Another common cause of reduced yield is impure reactants. The theoretical yield is calculated based on the assumption that reactants are pure. You will learn about the effects of impure reactants on page 265. 

Often impure reactants are the cause of a percentage yield of less than 100%. Impurities cause the mass data to be incorrect. For example, suppose that you have 1.00 g of sodium chloride and you want to carry out a reaction with it. You think that the sodium chloride may have absorbed some water, so you do not know exactly how much pure sodium chloride you have. If you calculate a theoretical yield for your reaction based on... 

Such a feasibility constraint, characteristic to recycle systems, does not appear for stand-alone reactors. It can be explained by simple material balance reasons. The separation section does not allow the reactant to leave the process. Therefore, for a given a reactant input (Fa) either large reactor volume V) or fast kinetics ( CTref)) are necessary to consume entirely the reactant fed and avoid accumulation. These three variables are conveniently grouped in the plant Damkdhler number. The factor Zj accounts for the degradation of reactor performance due to impure reactant recycle. We note that a similar feasibility conditions also holds when the concentration of the reactant in the product stream is nonzero. Moreover, systems containing a purge stream of fixed flow rate have the same qualitative behaviour as the simple system described here. Finally, we remark that the condition 13.17 applies also to the system PFR -Separator - Recycle. 

So you predict that you ll get 699.5 grams of iron metal formed. What if, however, you carry out this reaction and only get 525.0 grams of iron metal formed There may be several reasons that you produce less than you expect, such as sloppy technique or impure reactants. It may also be quite likely that the reaction is an equilibrium reaction, and you ll never get 100 percent conversion from reactants to products. (Turn to Chapter 8 for details on equilibrium reactions.) Wouldn t it be nice if there was a way to label the efficiency of a particular reaction There is. It s called the percent yield. 

In Example 10.5 you found that burning 66.0 grams of CyHig will produce 203 grams of CO2. This is the theoretical yield (theo), the amount of product formed from the complete conversion of the given amount of reactant to product. Theoretical yield is always a calculated quantity, calculated by the principles of stoichiometry. In actual practice, factors such as impure reactants, incomplete reactions, and side reactions cause the actual yield (act) to be lower than the theoretical yield. The actual yield is a measured quantity, determined by experiment or experience. 

In almost any reaction, you re going to produce less of the product than you expected. You may produce less because most reactions are equilibrium reactions (see Chapter 8), because of sloppy technique or impure reactants, or because some other conditions come into play. Chemists can get an idea of the efficiency of a reaction by calculating the percent yield for the reaction using this equation ... 

The hydrogen in the vapor stream is a reactant and hence should be recycled to the reactor inlet (Fig. 4.8). The methane enters the process as a feed impurity and is also a byproduct from the primary reaction and must be removed from the process. The hydrogen-methane separation is likely to be expensive, but the methane can be removed from the process by means of a purge (see Fig. 4.8). 

The ratio of reactants had to be controlled very closely to suppress these impurities. Recovery of the acrylamide product from the acid process was the most expensive and difficult part of the process. Large scale production depended on two different methods. If soHd crystalline monomer was desired, the acrylamide sulfate was neutralized with ammonia to yield ammonium sulfate. The acrylamide crystallized on cooling, leaving ammonium sulfate, which had to be disposed of in some way. The second method of purification involved ion exclusion (68), which utilized a sulfonic acid ion-exchange resin and produced a dilute solution of acrylamide in water. A dilute sulfuric acid waste stream was again produced, and, in either case, the waste stream represented a... 

Loaded Adsorbents. Where highly efficient removal of a trace impurity is required it is sometimes effective to use an adsorbent preloaded with a reactant rather than rely on the forces of adsorption. Examples include the use of 2eohtes preloaded with bromine to trap traces of olefins as their more easily condensible bromides 2eohtes preloaded with iodine to trap mercury vapor, and activated carbon loaded with cupric chloride for removal of mercaptans. 

The alcoholysis reaction may be carried out either batchwise or continuously by treating the triglyceride with an excess of methanol for 30—60 min in a well-agitated reactor. The reactants are then allowed to settle and the glycerol [56-81-5] is recovered in methanol solution in the lower layer. The sodium methoxide and excess methanol are removed from the methyl ester, which then maybe fed directiy to the hydrogenolysis process. Alternatively, the ester may be distilled to remove unreacted material and other impurities, or fractionated into different cuts. Practionation of either the methyl ester or of the product following hydrogenolysis provides alcohols that have narrow carbon-chain distributions. 

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. 

Phase diagrams can be used to predict the reactions between refractories and various soHd, Hquid, and gaseous reactants. These diagrams are derived from phase equiHbria of relatively simple pure compounds. Real systems, however, are highly complex and may contain a large number of minor impurities that significantly affect equiHbria. Moreover, equiHbrium between the reacting phases in real refractory systems may not be reached in actual service conditions. In fact, the successful performance of a refractory may rely on the existence of nonequilibrium conditions, eg, environment (15—19). 

Essentially no waste products are formed ia the USP process if hydriodic acid and either sodium hydroxide or sodium carbonate are used as reactants. Water results from use of the former a mole equivalent quantity of carbon dioxide is produced from the latter reagents. Higher quaUty grades may require some purification steps which may result ia wastes from the treatment. Disposal of these impurities must then be carried out. 

Catalysts commonly lose activity in operation as a result of accumulation of materials from the reactant stream. Catalyst poisoning is a chemical phenomenon, A catalyst poison is a component such as a feed impurity that as a result of chemisorption, even in smaH amounts, causes the catalyst to lose a substantial fraction of its activity. For example, sulfur compounds in trace amounts poison metal catalysts. Arsenic and phosphoms compounds are also poisons for a number of catalysts. Sometimes the catalyst surface has such a strong affinity for a poison that it scavenges it with a high efficiency. The... 

Liquid-phase chlorination of butadiene in hydroxyhc or other polar solvents can be quite compHcated in kinetics and lead to extensive formation of by-products that involve the solvent. In nonpolar solvents the reaction can be either free radical or polar in nature (20). The free-radical process results in excessive losses to tetrachlorobutanes if near-stoichiometric ratios of reactants ate used or polymer if excess of butadiene is used. The "ionic" reaction, if a small amount of air is used to inhibit free radicals, can be quite slow in a highly purified system but is accelerated by small traces of practically any polar impurity. Pyridine, dipolar aptotic solvents, and oil-soluble ammonium chlorides have been used to improve the reaction (21). As a commercial process, the use of a solvent requites that the products must be separated from solvent as well as from each other and the excess butadiene which is used, but high yields of the desired products can be obtained without formation of polymer at higher butadiene to chlorine ratio. 

When the feedstock contains constant proportions of reactive impurities, the rate of decline also may depend on the concentration of the main reactant, thus ... 

Few mechanisms of liquid/liquid reactions have been established, although some related work such as on droplet sizes and power input has been done. Small contents of surface-ac tive and other impurities in reactants of commercial quality can distort a reac tor s predicted performance. Diffusivities in liquids are comparatively low, a factor of 10 less than in gases, so it is probable in most industrial examples that they are diffusion controllech One consequence is that L/L reactions may not be as temperature sensitive as ordinary chemical reactions, although the effec t of temperature rise on viscosity and droplet size can result in substantial rate increases. L/L reac tions will exhibit behavior of homogeneous reactions only when they are very slow, nonionic reactions being the most likely ones. On the whole, in the present state of the art, the design of L/L reactors must depend on scale-up from laboratoiy or pilot plant work. 

---