Recycles with Purges

As discussed in the previous section, a purge can be used to avoid the cost of separating a component from a recycle. Purges can, in principle, be used either with liquid or vapor (gas) recycles. However, purges are most often used to remove low-boiling components from vapor (gas) recycles. 

If the vapor from the phase split is either predominantly product or predominantly byproduct, then it can be removed from the process. If the vapor contains predominantly unconverted feed material, it is normally recycled to the reactor. If the vapor stream consists of a mixture of unconverted feed material, products and byproducts, then some separation of the vapor may be needed. The vapor from the phase split will be difficult to condense if the feed to the phase split has been cooled to cooling water temperature. If separation of the vapor is needed in such circumstances, one of the following methods can be used  

Absorption. Absorption was discussed in Chapter 10. If possible, a component that already exists in the flowsheet should be used as a solvent. Introducing an extraneous component into the flowsheet introduces additional complexity and the possibility of increased environmental and safety problems later in the design. 

Membrane separation. Membranes, as discussed in Chapter 10, separate gases by means of a pressure gradient across a membrane, typically 40 bar or greater. Some 

As an example, consider ammonia synthesis. In an ammonia synthesis loop, hydrogen and nitrogen are reacted to ammonia. The reactor effluent is partially condensed to separate ammonia as a liquid. Unreacted gaseous hydrogen and nitrogen are recycled to the reactor. A purge on the 

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The above example illustrates some important principles for the design of recycles with purges ... 

In what follows, we begin by introducing two examples of process systems with recycle and purge. First, we analyze the case of a reactor with gas effluent connected via a gas recycle stream to a condenser, and a purge stream used to remove the light impurity present in the feed. In the second case, the products of a liquid-phase reactor are separated by a distillation column. The bottoms of the column are recycled to the reactor, and the trace heavy impurity present in the feed stream is removed via a liquid purge stream. We show that, in both cases, the dynamics of the system is modeled by a system of stiff ODEs that can, potentially, exhibit a two-time-scale behavior. 

Figure 4.1 Process system with recycle and purge.

It is evident that the above models (Equations (4.5) and (4.14)) have terms of 0(1) and O(e) and are in a singularly perturbed form. This suggests, potentially, a two-time-scale behavior for the process systems with recycle and purge streams that they describe. In the next section, we will develop a generic modeling framework for such systems that captures this feature and allows a more general analysis of their dynamic behavior. 

Referring back to the theory introduced in Chapter 2, we can expect that the presence of terms of very different magnitudes (i.e., 0(1) and 0(e)) in the model (4.18) reflects a two-time-scale behavior in the dynamics of typical processes with recycle and purge. In what follows, we will show that this is indeed the case. Also, we will address the derivation of reduced-order models of the fast and slow dynamics, provide a physical interpretation of this dynamic behavior, and highlight its control implications. 

Figure 5.1 A generic reaction-separation process system with material recycle and purge.

MATERIAL BALANCE—LINKING RECYCLE AND PURGE WITH REACTION SELECTIVITY AND CONVERSION 2.10... 

Step 7. Methane is purged from the gas recycle loop to prevent it from accumulating, and its composition can be controlled with purge flow. Diphenyl is removed in the bottoms stream from the recycle column, where steam flow controls base level. Here we control composition (or temperature with the bottoms flow. The inventory of benzene is accounted for via temperature and overhead receiver level control in the product column. Toluene inventory is accounted for via level control in the recycle column overhead receiver. Purge flow and gas-loop pressure control account for hydrogen inventory. 

Recycle is a common feature of chemical processes. Its most common use is to send unused raw materials emerging from a process unit back to the unit. Overall system balances are usually (but not always) convenient starting points for analyzing process with recycle. A purge stream is withdrawn from a process when a species enters in the process feed and is completely recycled. If this species were not removed in the purge, it would keep accumulating in the process system and eventually lead to shutdown. 

The process is fed with three streams ethane, ethylene, and chlorine. The ethane and ethylene streams have the same molar flow rate, and the ratio of chlorine to ethane plus ethylene is 1.5. The ethane/ethylene stream also contains 1.5 percent acetylene and carbon dioxide. (For this problem, just use 1.5 percent carbon dioxide.) The feed streams are mixed with an ethylene recycle stream and go to the first reactor (chlorination reactor) where the ethane reacts with chlorine with a 95 percent conversion per pass. The product stream is cooled and ethyl chloride is condensed and separated. Assume that all the ethane and ethyl chloride go out in the condensate stream. The gases go to another reactor (hydrochlorination reactor) where the reaction with ethylene takes place with a 50 percent conversion per pass. The product stream is cooled to condense the ethyl chloride, and the gases (predominately ethylene and chlorine) are recycled. A purge or bleed stream takes off a fraction of the recycle stream (use 1 percent). Complete the mass balance for this process. 

The following manipulated variables are available toluene feed (Fi), hydrogen feed (F2), gas recycle (Fr), purge (Fp) and furnace duty (q h). Furnace duty may be used to control the reactor inlet temperature. The setpoint of this control loop may be coupled, in a cascade manner, with other variable from the previous list. 

Fresh acetic acid and a liquid acetic acid recycle stream are fed into a vaporizer along with a gas recycle stream and fresh ethylene feed. Oxygen is added after the vaporizer, and reactions occur in a gas-phase reactor. Reactor effluent is cooled and fed into a separator. Vapor from the separator is compressed and fed into an absorber to recover vinyl acetate. Recycle acetic acid is used as lean oil in the absorber. Exit gas from the absorber is sent to a CO2 removal unit, which produces a CO2 purge stream and gas recycle. Another purge stream is used to remove the small amount of ethane that is in the fresh ethylene makeup feed. 

For the ammonia process in Example 5.3, consider operation of the reactor at 932°F and 400 atm. Use a simulator to show how the product, recycle, and purge flow rates, and the mole fractions of argon and methane, vary with the purge-to-recyclc ratio. How do the power requirements for compression vary, assuming 3 atm pressure drop in the reactor and I atm pressure drop in the heat exchanger. 

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