Fed-batch startups

Example 14.3 The initial portion of a reactor startup is usually fed-batch. Determine the fed-batch startup transient for an isothermal, constant-density... 

The steady-state conversion is achieved at 0.65 h compared with 2.78 h for the fed-batch startup. 

The initial portion of a reactor startup is usually fed batch. Determine the fed-batch startup transient for an isothermal, constant-density stirred tank reactor. Suppose the tank is initially empty and is filled at a constant rate go with fluid having concentration Oin. A first-order reaction begins immediately. Find the concentration within the tank, a, as a function of time f < ifuu-... 

The fed-batch scheme of Example 14.3 is one of many possible ways to start a CSTR. It is generally desired to begin continuous operation only when the vessel is full and when the concentration within the vessel has reached its steady-state value. This gives a bumpkss startup. The results of Example 14.3 show that a bumpless startup is possible for an isothermal, first-order reaction. Some reasoning will convince you that it is possible for any single, isothermal reaction. It is not generally possible for multiple reactions. 

Example 14.4 Compare the fed-batch and fast-fill-and-hold methods for achieving a bumpless startup. 

Suppose there are two parallel, first-order reactions in a steady-state CSTR. Show that neither the fed-batch nor fast-fill-and-hold strategies can achieve a bumpless startup if the reactions have different rate... 

Unsteady material and energy balances are formulated with the conservation law, Eq. (7-68). The sink term of a material balance is and the accumulation term is the time derivative of the content of reactant in the vessel, or 3(V C )/3t, where both and depend on the time. An unsteady condition in the sense used in this section always has an accumulation term. This sense of unsteadiness excludes the batch reactor where conditions do change with time but are taken account of in the sink term. Startup and shutdown periods of batch reactors, however, are classified as unsteady their equations are developed in the Batch Reactors subsection. For a semibatch operation in which some of the reactants are preloaded and the others are fed in gradually, equations are developed in Example 11, following. 

---