Wasteless processing seems to be the buzzword in todays world of industrialization as it guarantees a healthy and pollutant free world. Hence, recycling of possible components of a chemical system at the right place, at the right time and in the right proportion at the right conditions is a welcome step towards achieving this goal. This also results in maximum utilization of the so- called critical components of a system, attainment of the highest possible yield out of a reaction, accompanies the minimum production of unwanted wastes, assures non-wastage of precious components, etc. Undoubtedly, the design of wasteless processing, starts with recycling and ends with providing a healthy life to all.
In chemical engineering, any given system constitutes plenty of features. And, when a few of these features are combined to yield a better output, the complication of design grows exponentially. Most often, the unit operations involved in these systems are generally unique in nature. But while functioning in coordination with one another, these operations become inter-related. In such a scenario, it is a welcome move to bring in an inter-relationship between these operations as long as the effects are easily manageable and the yield is positive.
At one stage or the other of such a system, this inter-relationship also brings in an interaction criterion. Similar interaction and inter-relation can be detected not only in the unit operations but also equipment items that are used to carry out the said operations. This leads to a knitted network of the equipment chain. This again calls for the unity of the action they are subjected to by virtue of the purpose. The number of functions thus getting performed is only adding up to the complexity of the basic functioning. All these are digestible as long as the end results are going to be promising. Unfortunately, this is not the case always. Quite often, one may face aspects that are competitive to the basic function intended for, when the system was originally designed. This will tend to render the system with a temporary brake effect. However, in majority of instances, these minor hurdles can be better handled via thoughtful innovative or sometimes tricky application of principles that will resist or remove or alter the effects.
The functioning of any chemical engineering system can be depicted by means of process operators, as these process operators interact between themselves to complete the system.
Mixing, chemical conversion and separation are examples of basic process operators that make a system. In order to ensure, enhance, retard or control the functioning of these process operators, many secondary process operators are employed. Also known as auxiliary process operators, some of the examples include:
· Compression
· Expansion
· Heating
· Cooling
· Phase transformation, etc
Representation
Flow diagrams are used to conceptualize the way process operators function in any chemical engineering system design. These can be further classified as:
· Process flow diagram
· Energy flow diagram
These diagrams, also known as generalized models help in presenting each element as a collection of several unit operations, while trying to expose the physical and chemical highlights of the system as a whole. Adopting and incorporating either, any or a combination of the following criteria can further enhance the efficiency of a system:
· Performance improvement of the basic process operators Introducing variances between the process flows
· Incorporating additional basic process equipment at appropriate places
· Incorporating additional auxiliary process factors at appropriate places
· Incorporating additional process flows at appropriate places
· Relieving the unwanted intermediate produce at their earliest possible retention time
· Incorporating a process de-routing at the onset of an intermediate produce
· Returning the intermediate produce back into the system at appropriate places etc
In fact, the list may be endless, as the possibilities become numerous. These are due to the ways and means available for interconnecting the process operators and subsystems of a system.
The flow scheme can be classified based on how the component streams are designed to continue in a pre-defined path. These include:
· Series flow scheme: If the stream of components leaves a given equipment only to enter as the input to the one next to it, this is termed as a series flow scheme. A by-pass flow system is also considered to be an extension to this simple scheme.
· Parallel flow scheme: A parallel flow scheme is one in which a single feed is diverted to produce two or more intermediate produce, but with an aim of combining all these to get a single output product only.
· Cross flow scheme: A cross flow scheme is flow streams of intermediate products. These are generally a combination of one or more of the series or parallel or cross or recycle flow schema.
· Recycle scheme: A recycle scheme speaks about the usage of a return process stream from one equipment to another, or from one stage of equipment to another of the same equipment.
Structure
If one draws a box around a given complete process, the net result will be only the feed streams and product streams. This representation looks too simple to make clear the intricacies of such an attempt in practice. This representation will aid as a major source in understanding the variables incorporated in design, that may affect either slightly or adversely the overall material balances of the process or system as a separate identity. The necessity and importance of the material balances also need to be over emphasised. This is due to the fact that the material balances form the dominant part and serves the purpose of any system flow diagram. Moreover, it is not worth spending much time in investigating the variables appearing in the design, especially where the cost of the products or by-products is much less than that of the raw materials. Consideration of any such input - output structure of the flow sheet and the decisions that affect this structure take priority before consideration of recycle system designs.
Any flow sheet can be simplified successively via better designs. This leads to a smoother and efficient method for solving inherent, derived or anticipated problems in the system even before the project implementation begins. Besides, process and system limitations can be envisaged and defined with great clarity on the drawing board itself. It also aids in developing systematic procedures without giving due consideration to any of the procedural details. However, the underlying structure could differ with the type of process.
Scheme
Recycling process involves the usage of a return process stream. If sets of reactions take place in a system, and that too at different temperatures or pressures or in the presence of different catalysts or accelerators, it implies the use of a number of reactors and complicated process flow schemes. Here, the purpose is to extract a majority of the work out of every single component and step of the process, before discarding anything as a waste. Thus, recycling helps wasteless processing to a great extent. However, it has to be introduced in the right fashion, at the right place and at right process conditions.
At least in a majority of cases, it is possible to associate a number of reaction steps with a reaction vessel. This opens the gateway to associate the number of feed streams with the particular reaction vessel that is meant to be the container for the designed reaction to take place.
In any process, the decision to incorporate recycling must be taken after strong and thorough consideration of the fact that no two components must be separated and then allowed to remix at the inlet of a reaction vessel.
One way of setting up a wasteless flow scheme that is closed circuit in nature and highly effective in end-results is using recycling. This process returns some of the stream from the exits to the process at different entry points, and back. Besides resulting in overall conversion of the product, this also ensures the complete utilization of the source materials and energy being. In addition, the condition under which the conversion proceeds towards completion also improves.
Classification
It is essential to clearly distinguish between the different recycling systems, viz: liquid and gaseous streams. The gaseous recycling requires expensive compressors, while liquid recycle systems requires only pumps (with the exception of the high-head, high-volume ones), whose costs are relatively lesser than that of compressors, furnaces or distillation units.
The usage of recycling, as far as chemical engineering is considered, is focused to serve only very few major causes. For example:
· In cases where energy is to be recuperated. Hence, if a better and to-the- maximum-extent utilization of the energy fed to the system is solicited, recycling comes handy
· In cases where the scope of utilization of inert solvents in a single or multiple reactions in the system is designed to the maximum advantage of the system
· Usage of catalysts in reactions in the system is required to the maximum extent
· When the cost of catalysts or any of the components used in the reactions is prohibitively costly
· Emissions to the outside has to be restricted or minimised to the maximum possible extent in any given system, as they are objectionable
· In cases where the operating conditions are optimized.
