Recycle optimization

SYNAPTIC VESICLE TRAFFICKING IS A SPECIALIZED FORM OF REGULATED SECRETION AND RECYCLING OPTIMIZED FOR SPEED AND EFFICIENCY 158... 

The key result of the Hierarchical Approach is the development of the basic flowsheet structure, formed by Reactor-Separations-Recycles. This structure defines the material balance envelope. In this respect of highest importance is the behaviour of the reaction system, which should deliver a realistic image of the reaction mixture. Other constraints regarding the reactor operation, as molar ratio of reactants, or safety requirements, are determinant for the structure of recycles. Optimal conversion represents a complex optimisation problem between the valorisation of raw materials and the cost of reactor, separators and recycles. 

Badly, M. andTondeur, D. (1982) Recycle optimization in non-linear productive chromatography -1 Mixing recycle with fresh feed. Chem. Eng. Sci., 37,1199-1212. 

Figure 4.9 shows a plot of Eq. (4.12). As the purge fraction a is increased, the flow rate of purge increases, but the concentration of methane in the purge and recycle decreases. This variation (along with reactor conversion) is an important degree of freedom in the optimization of reaction and separation systems, as we shall see later. 

As the reactor conversion increases, the reactor volume increases and hence reactor capital cost increases. At the same time, the amount of unconverted feed needing to be separated decreases and hence the cost of recycling unconverted feed decreases, as shown in Fig. 8.1. Combining the reactor and recycle costs into a total cost indicates that there is an optimal reactor conversion. 

Now there are two global variables in the optimization. These are reactor conversion (as before) but now also the concentration of IMPURITY in the recycle. For each setting of the IMPURITY concentration in the recycle, a set of tradeoffs can be produced analogous to those shown in Figs. 8.3 and 8.4. 

Figure 8.6 shows the component costs combined to give a total cost which varies with both reactor conversion and recycle inert concentration. Each setting of the recycle inert concentration shows a cost profile with an optimal reactor conversion. As the recycle inert concentration is increased, the total cost initially decreases but then... 

When inerts are present in the recycle, the concentration is another important optimization variable, again influencing operations throughout the process. 

The overall inventory. In the preceding chapter, the optimization of reactor conversion was considered. As the conversion increased, the size (and cost) of the reactor increased, but that of separation, recycle, and heat exchanger network systems decreased. The same also tends to occur with the inventory of material in these systems. The inventory in the reactor increases with increasing conversion, but the inventory in the other systems decreases. Thus, in some processes, it is possible to optimize for minimum overall inventory. In the same way as reactor conversion can be varied to minimize the overall inventory, the recycle inert concentration also can be varied. 

It might be possible to reduce the inventory significantly by changing reactor conversion and recycle inert concentration without a large cost penalty if the cost optimization profiles are fairly flat. 

Economic tradeoffs. Interactions between the reactor and the rest of the process are extremely important. Reactor conversion is the most significant optimization variable because it tends to influence most operations through the process. Also, when inerts are present in the recycle, the concentration of inerts is another important optimization variable, again influencing operations throughout the process. ... 

In a single stage, without liquid recycle, the conversion can be optimized between 60 and 90%. The very paraffinic residue is used to make lubricant oil bases of high viscosity index in the range of 150 N to 350 N the residue can also be used as feedstock to steam cracking plants providing ethylene and propylene yields equal to those from paraffinic naphthas, or as additional feedstock to catalytic cracking units. 

There are, however, problems associated with pollution control. Whereas the sulfur may be removed downstream using suitable ancidaty controls, the sulfur may also be captured in the bed, thereby adding to the separations and recycle problems. Capture during combustion, however, is recognized as the ideal and is a source of optimism for fluidized combustion. 

Several variations of the above process are practiced. In the Sumitomo-Nippon Shokubai process, the effluent from the first-stage reactor containing methacrolein and methacrylic acid is fed directiy to the second-stage oxidation without isolation or purification (125,126). In this process, overall yields are maximized by optimizing selectivity to methacrolein plus methacrylic acid in the first stage. Conversion of isobutjiene or tert-huty alcohol must be high because no recycling of material is possible. In another variation, Asahi Chemical has reported the oxidative esterification of methacrolein directiy to MMA in 80% yield without isolation of the intermediate MAA (127,128). 

It is also important to study the interactions of papermaking additives (4) in the paper machine water system some additives act synergisticaHy, so that the performance of each is enhanced by the presence of the other. However, some additives have a negative impact on the performance of other additives, or on other desirable paper properties. Thus, optimization of the addition points and usage rates of the entire additive system is necessary in order to maximize performance of the chemical additives and the paper sheet properties, and to minimize cost and negative interactions both on the paper machine and in the white-water system. This is especially tme as unanticipated additives enter the wet end of the paper machine from recycled furnishes, including coated broke (5). 

Research Trends. Research since the late 1970s appears to have centered on further process modifications and optimization of conventional processes, reduction in pollution, evaluation of potentially more efficient processing technology, increased use of less expensive softwood sources to extend the available hardwood suppHes, and accommodation of recycled newspaper pulp (see Recycling, paper). 

Mineral acids are used as catalysts, usually in a concentration of 20— 40 wt % and temperatures of 30—60°C. An efficient surfactant, preferably one that is soluble in the acid-phase upon completion of the reaction, is needed to emulsify the a-pinene and acid. The surfactant can then be recycled with the acid. Phosphoric acid is the acid commonly used in the pine oil process. Its mild corrosion characteristics and its moderate strength make it more manageable, especially because the acid concentration is constandy changing in the process by the consumption of water. Phosphoric acid is also mild enough to prevent any significant dehydration of the alcohols formed in the process. Optimization of a process usually involves considerations of acid type and concentration, temperature, surfactant type and amount, and reaction time. The optimum process usually gives a maximum of alcohols with the minimum amount of hydrocarbons and cineoles. 

CSTBs—minimum volume of battery, maximum yield, optimal temperature for reversible reaction, minimum total cost, reactor volume with recycle, maximum profit for reversible reaction with recycle, and heat loss... 

---