Pros and cons of the batch reactor [BSTR]: A short review

Batch reactors are not ideal reactors because of backmixing issues. In chemical reaction engineering, the Plug-Flow-Reactor (PFR) and Continuous-Stirred-Tank-Reactor (CSTR) are ideal reactors. Three important requirements must be met by a reactor: 100% mixing, no dead volume, and no backmixing. In batch reactors, backmixing is a problem. Backmixing occurs when "product that has already formed" and should be exiting the reactor mixes with unreacted reactants, which in a batch process cannot happen until the batch is finished and thereby reducing the effective volume of the reactor.

Contents

-Introduction

-Description of batch reactor

-Residence time distribution in a batch reactor

-Batch reactor's key performance factors

-Pros and cons of batch reactor

-Backmixing

General

The heart of the chemical industry is the chemical reactor. The performance of the reactor drives all production and separation processes. There are several types of reactors, including homogeneous vs. heterogeneous, batch vs. continuous, ideal vs. non-ideal, and so on. Batch, CSTR, and PFR reactors are based on idealised assumptions. That is, in a batch, there is 100% mixing and no dead-volumes; similarly, in CSTRs, 100% perfect mixing is assumed. There is no back mixing in PFR, but 100% redial mixing is assumed. In an ideal PFR, temperature, pressure, concentration, and density are constant at any cross-section. Of course, none of these simplified assumptions are accurate. As a result, idealised reactor performance may predict far from true performance.

The basic goal of reactor design is to produce a specified product at a given rate from a given set of reactants.

Stirred tank reactors, also known as batch reactors, are simply vessels that hold the reactants and allow them to mix.

Batch reactors

A BSTR is a mechanically stirred vessel. It is first filled with substrate and catalyst to initiate the reaction, after which no material is removed until the reaction is stopped.

The main advantage of batch reactors is their versatility and multipurpose operation. One major disadvantage of batch reactors is backmixing.

The batch reactor has four major components.

1) The reactor container.

2) The medium of reaction

3) The headspace: the area above the medium that is empty. Changes in the liquid volume are accommodated by the headspace.

4) The agitator is essential because it allows for the mixing of several components as well as the introduction and removal of reaction heat.

Batch reactors are used in a variety of settings, ranging from small round bottom flasks in the lab to thousands of litres in manufacturing.

Residence time distribution RTD] of batch reactors

In an ideal plug-flow reactor, all the molecules of material leaving the reactor have been inside it for exactly the same amount of time. Similarly, in an ideal batch reactor, all the molecules of materials within the reactor have been inside it for an identical length of time. The time the molecules have spent in the reactor is the residence time of the molecules in the reactor. The idealized plug-flow and batch reactors are the only two classes of reactors in which all the molecules in the reactors have the same residence time. In all other reactor types, the various molecules the feed spend different times inside the reactor. There is no axial mixing in a plug-flow reactor, and this omission is reflected in the RTD.

Batch reactors: Key performance factors

Adaptability in reaction

Batch reactors are used in the production of nearly all chemicals because they can perform almost any reaction. This reactor's versatility allows it to accommodate three distinct reaction phases: liquid-liquid, gas-liquid, and solid-liquid reactions.

Scalability

Batch reactors range in size from microliters to hundreds of cubic meters. Scaling up processes is difficult. There is a lot of information available, but it is not as simple as transferring the reaction from a smaller to a larger vessel.

Reaction management

The primary scalability challenge is heat and mass transfer. The fluid velocity does not vary in direct proportion to the reactor volume.

Example

We added dye to a 4 mL vial in our demonstration, and the colour became uniform in less than 2 seconds. It took five times as long to mix the dye in the larger flask. The flask was 25 times larger than a vial, and the stirrer was 25 times larger. As a result of the much slower-moving fluid, mass transfer is much slower. The reactor, not the chemistry, may limit product quality.

Energy Conservation

Energy is obtained from a variety of sources. To begin, the reaction may require or consume heat. Heat is required by the reactor to maintain its temperature and compensate for heat loss (proportional to the external surface area). At the end of the reaction, the contents of the batch reactor are emptied and the entire process is restarted, resulting in the loss of all heat input. Using a large reactor necessitates a high energy requirement, as well as more energy to maintain a constant temperature.

Catalytic processes

In a heterogeneous catalyst reaction, the catalyst is added as a slurry - a suspension of small particles. However, several issues remain. The impeller may crush the catalyst particles during the reaction. It is frequently difficult to separate the catalyst from the product, the catalyst is rarely re-used. This leads to a short catalyst lifetime and a lot of waste.

Pros and cons of batch reactor: Summary

Batch reactors can be used for almost any reaction – their flexibility is excellent.

Although there some challenges, the scalability of batch reactors is good and there is a lot of scale-up experience.

Reaction control is rather limited – highly exothermic and fast reactions suffer from the limited heat and mass transfer rates. As a result, product quality suffers.

-Energy efficiency is also average due to the heat required every time at start-up. Large surfaces and difficult heat recovery lead to energy losses.

-The catalyst lifetime is moderate to poor because the (solid) catalyst separation is often challenging.

-The key advantage of batch reactors is flexibility and multipurpose operation – that is why these reactors are used in most chemical processes.

-Batch reactors are extremely versatile and can be used for almost any reaction.

-Despite some challenges, batch reactors have good scalability and a lot of scale-up experience.

-The limited heat and mass transfer rates make it difficult to control highly exothermic and fast reactions. As a result, the quality of the product suffers.

-Because of the heat required at start-up, energy efficiency is also average. Energy is lost due to the large surface area and difficult heat recovery.

-Because (solid) catalyst separation is frequently difficult, the catalyst lifetime is moderate to poor.

- One major disadvantage of batch reactors is backmixing.?

Backmixing

Backmixing occurs when a “product that has already formed” and should be exiting the reactor mixes with unreacted reactants but can’t do that in a batch process until the batch is over.?Backmixed product takes up space without contributing to the reaction. In addition, this also creates a barrier between two reactants molecules and increases the viscosity of the reaction medium. Both these effects have an adverse impact on the reaction rate.?

Credit: Google