Semi-Batch Reactor

A semi-batch reactor is a variation of a batch reactor in which one reactant may be added intermittently or continuously to another reactant contained in a vessel as reaction proceeds Semi-batch reactors lie between batch and continuous reactors in terms of operation. Fig 1is a vertical reactor that can be configured to operate in semi-batch mode. One of the best reasons to use semi-batch reactors is to enhance selectivity in liquid-phase reactions.

A reactor in which some reactants are added to the reactor at the start of the batch, while others are fed intermittently or continuously during the course of the reaction is semi-batch reactor.

Fig 1 Semi-batch reactor

Mode of operation in semi-batch reactor

In a homogeneous liquid-phase reaction:

A+B = Product

Reactant A is initially charged to the vessel, and reactant B is added at a prescribed rate as reaction proceeds (Fig 2 — left). For a liquid phase reaction in which a gaseous product is formed, the gas is removed as the reaction proceeds (Fig 2 — right).

A semi-continuous reactor is a reactor for a multiphase reaction in which one phase flows continuously through a vessel containing a batch of another phase. The operation is thus unsteady-state with respect to the batch phase, and may be steady-state or unsteady-state with respect to the flowing phase, as in a fixed-bed catalytic reactor or a fixed bed gas-solid reactor respectively

Fig 2 Operations in semi-batch reactor

Usage Examples

Semi-batch reactors are not used as often as other reactor types. However, they can be used for many two-phase (i.e. solid/liquid) reactions. Also, semi-batch reactors are used when a reaction has many unwanted side reactions, or has a high heat of reaction. By limiting the introduction of reactants, potential problems are eliminated.

The semi-batch slurry reactor in Fig 3 is used in a lab for metallocene-catalyzed polymerization of alkenes.

Fig 3 Semi -batch slurry reactor

An exploded view of a semi-batch reactor is shown below in Fig 4, and is used to convert biomass feed stocks into usable products.

Fig 4 Exploded view of a semi-batch reactor

Mole Balance and Mass Balance in Semi-batch Reactor

A schematic diagram of this semi-batch reactor is shown in figure below. We shall consider the elementary liquid-phase reaction in which reactant B is slowly added to a well-mixed vat containing reactant A.

Fig 5 Schematic diagram of this semi-batch reactor

A mole balance on species A yields-

As the number of moles of A(NA) is the product of concentration of A(CA) and the volume(V) the above equation can be rewritten as-

Since the reactor is being filled, the volume, V, varies with time. The reactor volume at any time t can be found from an overall mass balance of all species:

For a constant-density system, ρ=ρ0, and

with the initial condition V= V0 at t=0, integrating for the case of constant volumetric flow rate v0 yields

Substituting Equation no. 4 into the right-hand side of Equation no. 2 and rearranging gives us

The balance on A [i.e., Equation no. 2] can be rewritten as

A mole balance on B that is fed to the reactor at a rate FB0 is

Rearranging

Substituting Equation no. 5 in terms of V and differentiating, the mole balance on B becomes

At time t = 0, the initial concentration of B in the vat is zero, CBi = 0. The concentration of B in the feed is CB0. If the reaction order is other than zero- or first-order, or if the reaction is non-isothermal, we must use numerical techniques to determine the conversion as a function of time.

Advantages of Semi-Batch Reactor

• In comparison with a batch reactor, the gradual or intermittent addition of a reactant in a semi-batch operation can result in improved control of the temperature; particularly for a highly exothermic reaction. This may be the case if the initial reaction rates are very high, which would result in uncontrollable temperature rises or gas evolution in the batch reactor.
• The concentration of a reactant can be kept relatively low (for the reactant introduced intermittently) or high (reactant charged initially) if either is advantageous for suppressing side reactions, thus improving product yield.
• The withdrawal of a product can result in higher conversion of a reactant, particularly if the reaction is equilibrium-limited.
• To react a gas with a liquid: this is inevitable when a batch reactor is desired but the gas is only sparingly soluble in the liquid.
• Good temperature control.
• Unwanted side-reactions minimized.
• Improved selectivity of a reaction.
• Better control of exothermic reactions.
• Product removal through a purge stream.

Disadvantages of Semi-Batch Reactor

• As in the case of a batch reactor, the production rate may be limited because of the cyclic nature of the operation.
• Also, as in the case of a batch reactor, the operating cost may be relatively high.
• The design or performance analysis is complicated because of the unsteady-state operation.
• Semi-continuous operation may require intricate piping and valving.
• High labor cost per unit product
• Large scale production difficult to achieve
• Reactor operations difficult to analyze

Contributors:

1. Rutuja Bhagwat
2. Suparshwa Chougule
3. Rushikesh Damame
4. Pratham Desale
5. Vaishnavi Gawande