essence of a discrete process.
The only batch process in the plant is the manufacture of the time delay powder. The rest of the book will provide many examples of batch processes.
Other Process Examples
Consider a plant that converts scrap baling wire from hay and cotton bales into high quality iron oxide for magnetic tape. This process is referred to as a “bucket and paddle”
operation because it is simple, but it is also highly profitable. There are only a few buckets, so each must be charged with bits of baling wire and secret ingredients, processed according to a secret procedure, and emptied so the next batch can begin.
This is a classic example of batch processing: a vessel is filled, sequentially processed to chemically transform the added materials, and emptied before another cycle can begin.
A continuous plant may also have sequenced equipment, such as the switch condensers that extract waxy hydrocarbons from the process gas stream. A switch condenser runs for a while with process gas flowing over tubes of chilled water that congeal the waxy product. Then the process gas is diverted to another condenser, and the chilled water is replaced with steam to melt the wax, which drains into a product pipe. The unit is switched back to cold water, and process gas once again flows through it. There is a sequence of operations, but it is designed to allow continuous flow through a set of process equipment. Other examples are gas dryers and centrifugal separators that must be stopped in order to remove product. The result is a continuous flow of product and the sequence never varies, so the processes are continuous.
The procedure for starting up or shutting down continuous processes is definitely sequential, probably has chemical changes, and does not have a continuous output.
But no oil refinery engineer would call startup a batch process. Startup is viewed as a temporary aberration in the life of a continuous process.
The point is that the three categories of manufacturing processes are convenient divi- sions that make it possible to break a large problem into smaller parts. That is the
engineering use of categories. Marketing uses them to be able to say, “Mr. Customer, what you have there is a batch process. What we have is far and away the best batch control system available, and you need it.” This bold assertion can be used to bypass all sorts of pesky little details.
The following sections will explain how to distinguish a batch process from other processes with a high probability of being correct.
Process Properties
The following table presents some properties of processes, in an attempt to find one or two properties that always characterize the correct process. There are other processes in a plant, such as transportation and storage, but they will not be described here.
Table 1-1 Process Properties
Input materials are mixed or chemically changed
Chemical change transforms the ingredients into materials that have a dif- ferent chemistry. Mixing or blending transforms the ingredients into a product that is homogeneous, not discrete. A discrete process may cause a chemical change such as oxidation or flame hardening, but the change occurs in a discrete part. A continuous process usually changes chemistry, but it may only cause a physical separation.
Input materials are only changed physically
Physical change transforms one or more physical properties of the material, such as shape or hardness, or it adds other physical parts to make a different part. Some chemical change may occur, but that is not the principle reason for the process. Separation is a physical change, such as liquid from solid or indi- vidual hydrocarbon gases from natural gas. Discrete separation is called “sorting.”
Property Discrete Batch Continuous
Input materials are mixed or chemically changed Maybe Yes Maybe Input materials are only changed physically Yes No Maybe Input materials must be fed continuously Maybe No Yes Product materials leave while materials are coming in Yes No Yes A sequence of operations is required for normal production Yes Yes Maybe
A recipe is required Yes Yes Yes
One or more production vessels require a mixing agitator No Yes Maybe Output is multiple objects that can be individually labeled Yes No No
Output is an unbroken stream of product No No Yes
Output requires that the production vessel(s) be emptied Yes Yes No
Input materials must be fed continuously
The law of conservation of mass requires that a process with a continuous output have a continuous feed. The transition from discrete barges or trucks or rail cars to continuous feed is done with a feed storage tank. Filling bottles is a discrete process that may be able to take a continuous feed. Such a process requires multiple filling nozzles, arranged so that a new bottle starts filling as a full bottle stops.
Product materials leave while materials are coming in
Continuous and discrete processes may be thought of as assembly lines. Input materials go through a series of operations and become exiting product while more material is coming in. Only a batch process contains all of the material that is being added and stops adding material before product is withdrawn.
A sequence of operations is required for normal production
A set of procedures performed in a prescribed order that may depend on current conditions is a sequence of operations. A discrete process example is the following:
If part is in place, lower drill to surface of part.
Feed drill to specified depth.
Raise drill and signal completion.
A batch process example:
If reactor is ready, add a measured amount of ingredient A.
Start agitator and heat to reaction temperature.
Add ingredient B until reaction is complete.
Cool to a stable temperature.
Stop agitator and wait for permission to dump.
A continuous process example:
If time is up, divert process stream to dryer A for specified time.
Heat and vent dryer B.
Cool dryer B.
Divert process stream to dryer B and repeat cycle with dryers reversed.
A recipe is required
All processes require a specification that defines how the product is to be made, including the quantities of raw materials and the specific processing sequence. The difference between the recipes of the three process types lies in the quantity and complexity of the details. A continuous process may have only a few specified products and does not call the specifications “recipes.” A discrete recipe may be called a “bill of materials.” The number of products that a discrete process can make is limited by the degree of specialization of the machinery. A wire folder cannot make washers. Almost anything that can
be homogenized can be made with a bucket and paddle, so the number of products possible from a batch process may be much larger than for either a continuous or discrete process. True, there are batch processes that are spe- cialized by their physical limits or auxiliary equipment, but a reactor that has a heat exchanger and agitator is able to make more diverse products than the other processes.
One or more production vessels require a mixing agitator
It is difficult to induce chemical change without some form of mixer. Few dis- crete processes want to mix parts. A continuous process usually relies on flow to do the mixing but may require the help of an agitator.
Output is multiple objects that can be individually labeled
The objects may look identical, but each could have a unique label. The size of the label is not a consideration.
Output is an unbroken stream of product
The output of a bottling line or a pill press may seem continuous because of its speed, but the output is composed of discrete objects. A stream can be gas or liquid or an extrusion of solid material.
Output requires that the production vessel(s) be emptied
The “vessel” in a discrete process is the fixture that holds the part being processed. It has to be emptied so a new part can be inserted and processed.
Continuous distillation must not empty the tower during processing. About the only thing that gets emptied in an operating continuous process is one of a set of devices that sequentially receive the process stream, perhaps to dry it or to separate a crystal slurry from solvent in several centrifuges. A batch reactor is filled with reactants until the required processing is complete; then the contents are transferred for further processing or packaging.
Properties for Process Classification
The table presented in the preceding section shows just three properties that are not ambiguous:
Table 1-2 Three Unambiguous Properties
Property Discrete Batch Continuous
Output is multiple objects that can be individually labeled Yes No No
Output is an unbroken stream of product No No Yes
Product materials leave while materials are coming in Yes No Yes
The different processes each have one defining property:
• A discrete process has an output of discrete objects.
• A continuous process has an output that is a stream of something that is homo- geneous.
• A batch process does not have product materials leaving while materials are coming in.
The other properties in the earlier table may help you to resolve an ambiguity between two processes. The following table shows two properties that each distinguish a batch from a continuous process:
Table 1-3 Batch vs. Continuous Processes
Batch Process Definition
The word batchcomes from the Old English word for “bake,” so one of the definitions for batch is “all the loaves of bread baked at the same time” (www.cogsci.princeton.edu /cgi-bin/webwn). The present-day general definition (from the same source) is “a col- lection of things or persons to be handled together.” The following definition of batch comes from www.bridgefieldgroup.com/glos1.htm, which provides education in enterprise-level matters such as enterprise resource planning (ERP) and supply chain management (this is not an endorsement):
1) In production, a lot or given quantity processed at the same time with the same process parameters. A batch may consist of more than one item number, but all items are considered to have the same characteristics for purposes of trace- ability.
2) In data processing, a set of files or records gathered and processed together instead of one at a time.
Bridgefield Group’s first definition is directly related to the Old English meaning, which could apply to either discrete or batch processing as defined in this book. The second definition refers to the world of data processing, where it means a collection of data, as shown in the following definitions:
An accumulation of data or programs intended for processing in a group or
“batch,” typically at a later time, and scheduled by the computer—as opposed to “on-line,” immediate processing. [www.aits.uillinois.edu/live/site.xml?doc- ument=Glossary/glossaryb.html]
Property Batch Continuous
Input materials must be fed continuously No Yes
Output requires the production vessel(s) to be emptied Yes No
A group of statements processed as an entity. Execution of DOS batch files (such as AUTOEXEC.BAT) and SQL statements are examples of a batch process. [www.pace.ch/cours/glossary.htm]
The following definition of batch is specific to the glass industry, but it is typical of other industry definitions:
The mixture of raw materials (often silica, soda or potash, and lime) that is melted in a pot or tank to make glass. Cullet is added to help the melting process. [www.cmog.org/index.asp?pageID=687HPZ]
The ANSI/ISA-88.01 standard definition of batch avoids the industry-specific or baking definitions by stating that batchmeans “the production of a batch.” Specifi- cally, ANSI/ISA-88.01 definition 3.5 reads:
1) The material that is being produced or that has been produced by a single execution of a batch process.
2) An entity that represents the production of a material at any point in the process.
A Google search defines entityas “that which is perceived or known or inferred to have its own distinct existence (living or nonliving).” That means that entitycan refer to the actual material or to a column in a database structure. Also, an entity has a unique name that differentiates it from other entities.
The words batchand lotare sometimes used interchangeably. It is common practice to say, “a batch of [discrete loaves of] bread” or to call a batch of material a lot, or to give it a lot number. The following definition equates batch and lot:
In manufacturing production, a quantity scheduled to be produced together;
also called a lot. [www.risnews.com/Glossary/glossary.html]
The word lotis defined in ANSI/ISA-88.01 section 3.28 as “a unique amount of mate- rial having a set of common traits.” This means that a lot is “An identifiable quantity of material or discrete objects having a common set of characteristics.”
A batch of product material is almost always homogeneous because it has been thor- oughly stirred. A single sample taken from anywhere in the mass of the material represents the properties of the entire mass. Discrete objects have to be grouped into lots to discover the probable properties of any object in the lot. A random sample of objects in the lot is measured to get the statistical properties of the entire lot. If several batches with unique IDs are blended together, then the blend becomes a lot and receives a lot number. An example is a drum of plastic pellets obtained by mixing the output of four pelletizing extruders, each with its own polymerization reactor. The drum is given a lot number and not a batch ID.
Batch Process
A variety of definitions of batchprocess can be found. None of them are completely correct, but it will be instructive to see why they are not.
1) A process for carrying out a reaction in which the reactants are fed in dis- crete and successive charges. [http://www.eere.energy.gov/consumerinfo/
energyglossary.html]
An 88 batch is not always the product of a reaction. Paint may be mixed in batches.
Reactants may be fed simultaneously, but not while product is withdrawn.
2) Any process on which operations are carried out on a limited number of arti- cles, as opposed to continuous process. [www.windmill.co.uk/glossary.html]
Unfortunately, this definition is not any different from that of a discrete process. Per- haps this idea of “batch” is related to baking.
3) Process in which no new material is fed into or removed from reactor vessel.
[www.westp2net.org/hazwaste/app/glossary.html]
This seems too restrictive. Polymerization reactions commonly add monomer during the reaction. We will clear this up in the discussion of semi-batch operation in the next section.
4) The feed is charged into the system at the beginning of the process, and the products are removed all at once some time later. No mass crosses the system boundaries between the time the feed is charged and the time the product is removed. [www.eng.usouthal.edu/huddleston/che_glossary.htm]
This is not unlike definition 3, but the system boundaries could be extended to cover dosing or monomer feed.
Standard Definition
We now have enough information to examine the standard definition of a batch man- ufacturing process. In the ANSI/ISA-88.01 standard, definition 3.7 of a batch process reads:
“A process that leads to the production of finite quantities of material by subjecting quantities of input materials to an ordered set of processing activities over a finite period of time using one or more pieces of equipment.”
You can see how difficult it is to capture an accurate definition of a batch process in one sentence. We can analyze this definition further as follows:
A [manufacturing] process that leads to [results in] the production of finite quantities of material...
The finite quantity is measured in volume or weight, not as a count of objects.
It is not a rate measurement like liters per minute. The volume is usually a large fraction of the volume of the processing vessel.
... bysubjecting [measured] quantities of input materials [added at appropriate times]
to an ordered set [sequence] of processing activities over a finite period of time...
“Finite period” does not refer to the time it takes to produce a product. It means that each item in the sequence takes its own distinct amount of time.
At least one of the items is always on the “critical path” that determines how long it takes to make a batch. Each item is either a timed wait of known dura- tion or a process activity that has a deadline. If the deadline is exceeded, the operator is warned that the sequence might take forever if nothing is done.
The same thing is true of a discrete process. Only a batch process does not withdraw product while it is adding material.
... using one or more pieces of [process] equipment.
All processes use process equipment. The sentence was already too long to add that the batch of material being processed has exclusive use of the required processing equipment. That is, the processing vessel can hold zero or one batch of material, and the vessel’s auxiliary equipment is dedicated to processing that material. A batch process may require a series of production vessels, but each vessel holds no more than one batch at a time.
Semi-Batch Process
Batch processes predate the SP88 committee work by many years, as the Internet defi- nitions dissected above indicate. One of them was:
“Process in which no new material is fed into or removed from reactor vessel.”
This definition is used in academia because it was formalized shortly after the first batch of anything was made. Now it is common to add a material to a reactor while it is reacting, so academia introduced the idea of a “semi-batch process.” An Internet search yields five thousand references to semi-batchin academic papers, but no common definition. How about this one:
Process in which new material is fed into or removed from the reactor vessel during reaction.
This definition states that the material is fed orremoved. An example of feeding during a batch process is the addition of monomer to a polymerization reaction, which is limited by the rate at which heat can be removed. An example of the removal of material during a batch process is batch distillation, where material is removed as distillate until it does not meet specifications or until there is nothing left to remove.
These actions are allowed by the 88 definition.
Allowing material to be both added to andremoved from the reactor means that the process can start like a batch process and then go into continuous reaction as reac- tants are fed in and product is withdrawn. This goes on until the supply for one of the reactants runs out, and then the outlet valve is closed and it finishes like a batch process. This blurs the distinction between batch and continuous processing. In fact, any continuous process is semi-batch in that sense. The difference lies in how long the process runs before the supply of a reactant runs out.
This book will not discuss semi-batch processes further because the orvariant is con- tained in the 88 definition. The andvariant can be treated as a batch process that is interrupted by a continuous interval when both feed and product valves are open.
Alternatively, it can be treated as a discontinuous process that is interrupted by some batch processing.
Auxiliary Processes
Discrete, batch, and continuous are the three manufacturing processes in any plant.
They all use equipment to accomplish their functions, but they are not the only users of equipment. Transportation, storage, and utilities enable manufacturing processes to operate, but they are distributed among processes so the equipment on which transportation, storage, and utilities rely is usually not contained within a process boundary. Each has its own set of details that should be separated from process details when boundaries are drawn.
A plant’s storage facility uses vessels to maintain the integrity of its contents, in the same way that a boat is a vessel that separates cargo from the sea or a tank separates its contents from the ground.
The transportation function uses pipes and pumps to move liquids and gases from one place to another. The materials that cannot be transported by pipe are moved by conveyor belts or vehicles. Pipe routes are changed by using block valves or manual coupling stations. Vehicles that are constrained by rails or roads have routing prob- lems similar to those of piping systems. Other vehicles find their own way from source to destination.
A plant’s utilities may provide common materials like water, or may source or sink energy, or they may convert waste products into something that can leave the plant. A boiler house is a utility in which fuel is converted into steam at one or more pressures as required by the processes. Lower steam pressures may be obtained by using turbines that turn electrical generators. A utility may burn fuel to heat water or oil to various pressures and temperatures. Refrigeration compressors may be used to cool a heat- transfer medium in order to remove energy from a process. A distillation column may be required to separate recyclable material such as solvent from a wastewater stream.
Storage, transportation, and utilities are found on almost all process diagrams. Each has its own set of details that must be contained within its own boundaries.