Recycling problems

Figure 1.2 Process design starts with the reactor. The reactor design dictates the separation and recycle problem. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

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. 

GFRP offers savings of up to 30% in total car weight, at some increase in unit cost and considerable capital investment in new equipment. Recycling problems still have to be overcome. 

The material balance around the mixing point of a loop reactor is given by Equation (4.21) for the case of constant fluid density. How would you work a recycle problem with variable density Specifically, write the variable-density counterpart of Equation (4.21) and explain how you would use it. 

But, computational difficulties can arise due to the iterative methods used to solve recycle problems and obtain convergence. A major limitation of modular-sequential simulators is the inability to simulate the dynamic, time dependent, behaviour of a process. 

Gale, N.H. (1997). The isotopic composition of tin in some ancient metals and the recycling problem in metal provenancing. Archaeometry 39 71-82. 

This new solution to recycling problems uses the same polymer for the matrix and the reinforcement. 

Direct fluorination of organic compounds using fluorine is an important methodology when considering safety, environmental and recycling problems. Fluorine can be fed into the reactor from a cylinder for the direct fluorination procedure, but this method has economic and safety disadvantages. Electrochemical generation of fluorine has recently been adopted for fluorination even in the laboratory. It can be operated easily and safely, because operational difficulties have been improved. 

Scheme 6.7) is particularly attractive, as this species is inexpensive, nontoxic, and does not present regeneration and recycling problems. 

It is quite obvious that a two level optimisation formulation can be very expensive in terms of computation time. This is due to the fact that for any particular choice of R1 and xRi a complete solution (sub-optimal) of the two distillation tasks are required. The same is true for each gradient evaluation with respect to the decision variables (B7and xRj). Mujtaba (1989) proposed a faster one level dynamic optimisation formulation for the recycle problem which eliminates the requirement to calculate any sub-optimal or intermediate solution. In this formulation the total distillation time is minimised directly satisfying the separation requirements for the first distillation task as interior point constraints and for the second distillation task as final time constraints. It was found that the proposed formulation was much more robust and at least 5 times faster than the classical two level formulation. 

As a general rule, the combined feed to the reactor is a convenient stream to use as a basis of calculation for recycle problems when the stream composition is known. We will therefore temporarily ignore the specified methanol production rate, balance the flowchart for the assumed basis, and then scale the process to the required extent. In terms of the labeled variables, the problem statement will be solved by determining no, - oc, - sc, -Tsh. ip, and for the assumed basis, then scaling up no,... 

Since the overhead ternary azeotrope is heterogeneous, it may be opportunistically decanted, producing the exact composition in the organic layer previously assumed for the azeotropic entrainer as well as an aqueous layer as determined by the liquid-liquid tie line through the ternary azeotrope (Fig. 26). By mass balance it can be shown that the total amount of azeotropic entrainer required to be mixed into the column has not yet been generated by the decant, so the recycle problem is not quite completely solved. 

In the past, most simulation programs available to designers were of the sequential-modular type. They were simpler to develop than the equation-oriented programs and required only moderate computing power. The modules are processed sequentially, so essentially only the equations for a particular unit are in the computer memory at one time. Also, the process conditions, temperature, pressure, flow rate, etc., are fixed in time. With the sequential modular approach, computational difficulties can arise due to the iterative methods used to solve recycle problems and obtain convergence. A major limitation of sequential modular simulators is the inability to simulate the dynamic, time-dependent behavior of a process. 

Use the overall conversion and single-pass (once-through) conversion concepts to solve recycle problems involving reactors. 

Do not let recycle streams confuse you. The steps in the analysis and solution of material balance problems involving recycle are the same as described in Table 2.4. With a little practice in solving problems involving recycle, you should experience little difficulty in solving recycle problems in general. The essential point you should grasp with respect to recycle calculations in this chapter is that the processes such as shown in Fig. 2.4 or 2.16 are in the steady state. 

The strategy listed in Table 2.4 is the strategy to be used in solving recycle problems. You can make component and total material balances for each subsystem as discussed in Sec. 2.5, as well as component and total balances for the overall process. Not all of the equations so formulated will be independent, of course. Depending on the information available concerning the amount and composition of each stream, you can determine the amount and composition of the unknowns. If tie components are available, they often simplify the calculations. 

Now let us turn to recycle problems in which a chemical reaction occurs. Recall from Sec. 1.9 that not all of the limiting reactant necessarily reacts in a process. Do you remember the concept of conversion as discussed in Sec. 1.9 Two bases for conversion are used in describing a process examine Fig, 2.18. 

Recycle problems can become more complicated since there can be unknowns in the overall stoichiometry of the process and local unknowns involving the reactor and its feed streams. These problems are solved by using two material balances one an overall balance of the material entering and leaving the reactor complex the other a once-through, or local, balance about the reactor itself The latter balance includes recycle streams the former does not. 

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