Question

In a semi-batch reactor

• A. Rate of reaction can be controlled
• B. Maximum conversion can be controlled
• C. Both the reactants flow counter-currently
• D. Residence time is constant

Hint:

Semi-batch reactors allow control of reaction rate by adjusting the feed rate of reactants, making them ideal for exothermic or selectivity-sensitive reactions.

Answer 1

The Correct Option is A

Step 1: Understand a semi-batch reactor. 
A semi-batch reactor is a hybrid between a batch and a continuous reactor. Typically, one reactant is charged into the reactor initially (batch mode), and another reactant is fed continuously or intermittently while the reaction proceeds. Products may be removed continuously or at the end. 
This setup is often used to:
Control the reaction rate (e.g., for exothermic reactions to manage heat release),
Improve selectivity by controlling the concentration of reactants,
Handle reactions where one reactant needs to be added slowly (e.g., to avoid side reactions).
Step 2: Analyze the characteristics. 
Rate of reaction: In a semi-batch reactor, the feed rate of the continuously added reactant can be adjusted to control the reaction rate. For example, in an exothermic reaction, adding a reactant slowly prevents a rapid temperature rise, allowing better control of the reaction rate and safety.
Maximum conversion: Conversion depends on the reaction kinetics, stoichiometry, and operating conditions. While a semi-batch reactor can optimize conditions for high conversion, maximum conversion is not inherently controlled by the reactor type—it depends on the reaction equilibrium and extent.
Counter-current flow: Semi-batch reactors do not involve counter-current flow of reactants. One reactant is typically present in the reactor, and the other is fed in, often in a co-current manner or mixed within the reactor.
Residence time: Residence time in a semi-batch reactor is not constant because the volume of the reactor contents changes as reactants are added (and possibly products are removed). Residence time varies with time and depends on the feed and withdrawal rates. 
Step 3: Evaluate the options. 
(1) Rate of reaction can be controlled: Correct, as the feed rate of the continuously added reactant can be adjusted to control the reaction rate, a key advantage of semi-batch reactors. Correct.
(2) Maximum conversion can be controlled: Incorrect, as maximum conversion is determined by reaction thermodynamics and kinetics, not directly by the semi-batch operation (though conditions can be optimized). Incorrect.
(3) Both the reactants flow counter-currently: Incorrect, as semi-batch reactors do not typically involve counter-current flow; one reactant is usually charged initially, and the other is fed in. Incorrect.
(4) Residence time is constant: Incorrect, as residence time varies in a semi-batch reactor due to changing reactor volume with continuous addition of reactants. Incorrect.
Step 4: Select the correct answer. 
In a semi-batch reactor, the rate of reaction can be controlled, matching option (1).