Recycle and Bypass

It is rare that a chemical reaction A B proceeds to completion in a reactor. No matter how little A is present in the feed or how long the reaction mixture remains in the reactor, some A is normally found in the product. 

Unfortunately, you have to pay for all the reactant fed to a process, not just the fraction that reacts, and any A that leaves with the product therefore represents wasted resources. Suppose, however, you could find a way to separate most or all of the unconsumed reactant from the product stream. You could then sell the resulting relatively pure product and recycle the unconsumed reactant back to the reactor. You would, of course, have to pay for the separation and recycle equipment, but you would compensate for this cost by having to purchase less fresh reactant and being able to sell the purified product at a higher price. 

Fresh air containing 4.00 mole% water vapor is to be cooled and dehumidified to a water content of l.70mole% H2O. A stream of fresh air is combined with a recycle stream of previously dehumidified air and passed through the cooler. The blended stream entering the unit contains 2.30 mole% H O. In the air conditioner, some of the water in the feed stream is condensed and removed as liquid. A fraction of the dehumidified air leaving the cooler is recycled and the remainder is delivered to a 

SOLUTION The labeled flowchart for this process, including the assumed basis of calculation, is shown below. 

Dashed lines depict the four subsystems about which balances might be written—the overall process, the recycle-fresh feed mixing point, the air conditioner, and the recycle-product gas splitting point. The quantities to be determined are i, n, and n.  

---

P14-1b Make up and solve an original problem. The guidelines are given in Problem P4-1. However, make up a problem in reverse by first choosing a model system such as a CSTR in parallel with a CSTR and PFR [with the PFR modeled as four small CSTRs in series Figure P14-l(a)] or a CSTR with recycle and bypass [Figure P14-l(b)]. Write tracer mass balances and use an ODE solver to predict the effluent concentrations. In fact, you could build up an arsenal cf tracer curves for different model systems to compare against real reactor RTD data. In this way you could deduce which model best describes the real reactor. 

Explain what recycle and bypassing involve by means of words and also by a diagram. 

Figure E5.6 An example system containing recycle and bypass streams, precedence order ...

Do we include recycle and bypass streams If so, where are they placed within the structure ... 

Recycle and bypass from one CSTR with all other CSTRs. 

A distribution network (DN) block The DN block coordinates the distribution of inlet and effluent streams throughout the network via mixing, splitting, recycling, and bypassing operations. The purpose of the DN blocks acts as switchboard for all streams in the system. This leads to a system of linear constraints when the concentrations in the ROP block are specified beforehand. 

The brine system should be fully operational, on recycle and bypassing saturation and the electrolyzers, at a rate equivalent to that required for operation at 2-3 kA m. The brine temperature should be about 50°C at the cell room. Concentration should be as specified for normal operation (typically 300 gpl), and quality should meet all specifications. Analytical routines for the brine system should be in full operation by this time. 

Here we understand by complex colunms a countercurrent cascade without branching of flows, without recycles and bypasses, which, in contrast to simple columns, contains more than two sections. The complex colunm is a column with several inputs and/or outputs of flows. The column of extractive distillation with two inputs of flows - feed input and entrainer input - is an example of a complex column. 

Detailed flow at the pass level, including recycles and bypasses... 

It is important to be able to recognize recycle and bypass streams in chemical processes. When identifying recycle and bypass streams, we look for flow loops in the PFD. Any time we can identify a flow loop, we have either a recycle or a bypass stream. The direction of the streams, as indicated by the direction of the arrow heads, determines whether the loop contains a recycle or a bypass. The following tactics are applied to flow loops ... 

It is worth noting that certain pieces of equipment normally contain recycle streams. In particular, distillation columns very often have top and bottoms product reflux streams, which are essentially recycle loops. When identifying recycle loops, we can easily determine which loops contain reflux streams and which do not. Example 5.3 illustrates the procedure for identifying recycle and bypass streams in the toluene hydrodealkylation PFD. 

For the toluene hydrodeallgdation PFD given in Figure E5.1. identify all recycle and bypass streams. 

Identify the main recycle and bypass streams for the following ... 

---