Material balances recycle streams

The entrainer recovery column takes the distillate stream, from the azeo-column and separates it into a bottoms stream of pure water, and a ternary distillate stream for recycle to column 2. The overall material balance line for column 3 is shown in Figure 19b. This sequence was one of two original continuous processes disclosed in 1915 (106). More recendy, it has been appHed to other azeotropic separations (38,107,108). 

The application of a selective pyrolysis process to the recovery of chemicals from waste PU foam is described. The reaction conditions are controlled so that target products can be collected directly from the waste stream in high yields. Molecular beam mass spectrometry is used in small-scale experiments to analyse the reaction products in real time, enabling the effects of process parameters such as temperature, catalysts and co-reagents to be quickly screened. Fixed bed and fluidised bed reactors are used to provide products for conventional chemical analysis to determine material balances and to test the concept under larger scale conditions. Results are presented for the recycling of PU foams from vehicle seats and refrigerators. 12 refs. 

Without recycle, the material balances on a series of processing steps can be carried out sequentially, taking each unit in turn the calculated flows out of one unit become the feeds to the next. If a recycle stream is present, then at the point where the recycle is returned the flow will not be known as it will depend on downstream flows not yet calculated. Without knowing the recycle flow, the sequence of calculations cannot be continued to the point where the recycle flow can be determined. 

Material balance calculations on processes with by-pass streams are similar to those involving recycle, except that the stream is fed forward instead of backward. This usually makes the calculations easier than with recycle. 

Hint start the material balance at the reactor inlet (after the addition of the recycle streams) and use a basis of 100 kmol/h benzene at this point. 

Plug flow reactor (PFR) with recycle. The recycle reactor is characterized by a non-zero value of R, that is the ratio between the mass flow rate of the recycled stream and the feeding rate Q. The material balance reads for this case as... 

A recycle PFR, operating at steady-state for the reaction A +. . - products, is shown schematically in Figure 15.6, together with associated streams and terminology. At the split point S, the exit stream is divided into the recycle stream (flow rate RqJ and the product stream (flow rate q,), both at the exit concentration cA1. At the mixing point M, the recycle stream joins the fresh feed stream (flow rate q0, concentration cAo) to form the stream actually entering the reactor (flow rate (1 + R)q0, concentration ca o)-The inlet concentration c Ao may be related to cAo, cA1, and R by a material balance for A around M ... 

A yeast is cultivated in a CSTR with partial recycle of the product. Effluent from the reactor has concentrations Cx and Cs and goes to a separator where a product stream, F, has concentrations 0.3CX and Cs. The recycle stream, R, likewise has concentration Cs and a material balance on X as Recycle = (F+R)CX -0.3FCX = (0.7F+R)CX = RCxr (1)... 

From a computational viewpoint, the presence of recycle streams is one of the impediments in the sequential solution of a flowsheeting problem. Without recycle streams, the flow of information would proceed in a forward direction, and the cal-culational sequence for the modules could easily be determined from the precedence order analysis outlined earlier. With recycle streams present, large groups of modules have to be solved simultaneously, defeating the concept of a sequential solution module by module. For example, in Figure 15.8, you cannot make a material balance on the reactor without knowing the information in stream S6, but you have to carry out the computations for the cooler module first to evaluate S6, which in turn depends on the separator module, which in turn depends on the reactor module. Partitioning identifies those collections of modules that have to be solved simultaneously (termed maximal cyclical subsystems, loops, or irreducible nets). 

Separator no. 2 returns 80% of the unreacted chlorine to the reactor and separator no. 3 returns 90% of the benzene. Both recycle streams are pure. Fresh chlorine is charged at such a rate that the weight ratio of chlorine to benzene in the total charge remains 0.82. The amounts of other streams are found by material balances and are shown in parentheses on the sketch per 100 lbs of fresh benzene to the system. 

The operation of the system outlined in Fig. S.61 is analysed by taking material balances over the fermenter vessel. It is assumed that in this idealised case, there is no biochemical reaction or growth occurring in the separator, so that the substrate concentration S in the entering stream is the same as that in the clarified liquid effluent stream, in the recycle stream and in the exit biomass rich stream. The material balance then becomes ... 

In this case the material balance for the substrate about the point of the mixing of the fresh feed and the recycle stream gives ... 

If all cells are recycled back into the fermenter, the cell concentration will increase continuously with time and a steady state will never be reached. Therefore, to operate a CSTF with recycling in a steady-state mode, we need to have a bleeding stream, as shown in Figure 6.19. The material balance for cells in the fermenter with a cell recycling unit is... 

It should be noted that actual flow rates of the streams going in and out of the filter unit do not matter as far as overall material balance is concerned. For a steady-state CSTF with cell recycling and a sterile feed,... 

The flow rate of the recycle stream, R, required to achieve complete conversion at the process outlet can be computed as a function of the reaction rate constant and the reactor volume from the steady-state material balance equations ... 

Note that the above model of the fast dynamics involves only the large recycle and internal flow rates u1, and does not involve the small feed/product flow rates us. Examining the material balance equations in (3.1), it is intuitive that the flow rates of the internal streams do not affect the total holdup of any component in the process, and that total holdups are affected only by the flow rates us of the... 

The input/output structure defines the material balance boundary of the flowsheet Often it is referred as the inside battery limit envelope. A golden rule requires that the total mass flow of all components entering the process must be equal with the total mass flow of all components leaving it. It should be kept in mind that the recycles affect only the internal process streams, but not the input/output material balance. 

In the RSR approach the chemical reactor is the key unit, designed and simulated in terms of productivity, stability and flexibility. From the systemic viewpoint the key issue is the quality and dynamics of flows entering the reactor and less how they have been produced. Obviously, these flows include fresh reactants and recycle streams. The dynamics of flows must respect the overall material balance at steady state, as well as the process constraints. For this reason, the chemical-reactor analysis should be based on a kinetic model. 

Input-output analysis focuses on the overall structure of the flowsheet, and the recycle streams are not considered here in as much as they do not appear in the overall flow streams. Hence, this structure is essentially an overall mass balance for the entire process. A simplified representation of the input-output structure is shown in Fig 6.5. The main purpose of this analysis is to identify the amount of raw materials used, useful products, and waste formed. 

MATERIAL BALANCE—CHEMICAL REACTION AND A RECYCLE STREAM INVOLVED 2.6... 

MATERIAL BALANCE—CHEMICAL REACTION A RECYCLE STREAM INVOLVED... 

Once we have fixed a flow in each recycle loop, we then determine what valve should be used to control each inventory variable. This is the material balance step in the Buckley procedure. Inventories include all liquid levels (except for surge volume in certain liquid recycle streams) and gas pressures. An inventory variable should typically be controlled with the manipulated variable that has the largest effect on it within that unit (Richardson rule). Because we have fixed a flow in each recycle loop, our choice of available valves has been reduced for inventory control in some units. Sometimes this actually eliminates the obvious choice for inventory control for that unit. This constraint forces us to look outside the immediate vicinity of the holdup we are considering. 

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