1. Unit 2: An Overview of Chemical Process Technology
Introduction to the Chemical Industry for Technical Assistance Providers
Unit 2: An Overview of Chemical Process Technology

2. Outline of this Unit An introduction to process technology
organizing concepts
“unit operations” view of process technology
introduction to main process operations
Environmental considerations of unit ops

3. Learning Objectives
Gain an understanding of the “unit ops” view of process technology
Gain familiarity with key pieces of process equipment
Understand environmental implications of process equipment

4. Making Sense of Process Technology
From Compilation of Emission Factors, AP-42, Fifth Edition, Volume I Chapter 6:  Organic Chemical Process Industry – production of maleic anhydride
The process flow diagram (PFD) is used throughout chemical engineering process The complete process to produce a chemical product can be a very complex collection of process equipment. However, the PFD is always a set of the same basic process equipment, I.e., reactors, separators, heat exchangers and materials handling equipment.

5. Unit Operations or “Unit ops” Concept
Each chemical process can be broken down into a series of steps (operations)
Individual operations have common techniques – based on the same scientific principles
Underscores the common features of diverse processes
Crosses industry and
process lines
(From University of Utah Department of Chemical Engineering
The "unit operations" concept had been latent in the chemical engineering profession ever since George Davis had organized his original 12 lectures around the topic. However, it was Arthur Little who first recognized the potential of using "unit operations" to separate chemical engineering from other professions. While mechanical engineers focused on machinery, and industrial chemists concerned themselves with products, and applied chemists studied individual reactions, no one, before chemical engineers, had concentrated upon the underlying processes common to all chemical products, reactions, and machinery. The chemical engineer, utilizing the conceptual tool that was unit operations, could now claim to industrial territory by showing his or her uniqueness and worth to the American chemical manufacturer.
The strictly chemical aspects of processing are studied in a companion area of chemical engineering called reaction kinetics.
Chemical processes can be divided into discrete components known as units. A unit performs one operation in the process and is represented by a simple symbol, such as a square or circle, on a process flow sheet.
Chemical engineering is both an art and a science.

6. Behind the Complex Appearance, Chemical Manufacturing is Simple…
Raw materials are mixed and/or reacted to create useful products
These products are separated in one or more steps
Between each step, process streams may be heated or cooled to optimum temperatures
In some cases, products may be mechanically processed to convenient form for transport and use
“Chemical engineering has to do with industrial processes in which raw materials are changed or separated into useful products.” (McCabe, et al., Unit Operations of Chemical Engineering, Fifth Ed., McGraw-Hill, 1993.
The chemical engineer must develop, design, and engineer both the complete process and the equipment used; choose the proper raw materials; operate the plants efficiently, safely and economically; and see to it that products meet the requirements set by the customers. It is both an art and a science.

7. Introduction to Chemical Reactors

8. Reactor basics A + B  C (+ byproducts) (+ unreacted A & B)
Every reaction is governed by:
reaction stoichiometry
reaction equilibrium (maximum conversion)
rate of reaction

9. Trade-offs in Reactor Design
Want to maximize…
product throughput
conversion efficiency
selectivity
flexibility
process safety
“controllability”
Want to minimize
byproduct formation
energy use
downstream separations
physical complexity
capital cost

10. Some Common Reactor Types
Batch
Semi-batch
Continuous
Stirred tank
Packed bed
Fluidized bed
Electrolytic “cells”
Bioreactors
Note: Lots of info from next few slides is from EPA Office of Compliance Sector Notebook Project, Profile of the Organic Chemical industry, 2nd Ed., November 2002.
Electrolytic cell reactors are used primarily for inorganic chemical manufacturing, specifically chlorine, caustic soda, and hydrogen from brine. The three types of electrolysis processes are:
Mercury Cell
Diaphragm Cell
Membrane Cell

11. Batch Reactor
Reactants added to vessel and products emptied after completion of reaction
some reactants may be added continuously (“semi-batch”)
often referred to as “stirred tank reactor”
Agitator mechanism
Insulating jacket
Pipes & valves to control conditions
Primarily small-scale (e.g. specialty chem) and experimental processes
Reactant chemicals are added to the reaction vessel at the same time and the products are emptied completely when the reaction is completed.
The reactors are made of stainless steel or glass-lined carbon steel and range in size from 50 to several thousand gallons (U.S. EPA, 1993). Batch reactors, also called stirred tank reactors or autoclaves, have an agitator mechanism to mix the reactants, an insulating jacket, and the appropriate pipes and valves to control the reaction conditions.
SOCMA slide says that range is from “kilogram-scale” containers to units up to several hundred kilos in size.

12. Batch process characteristics
Not enough product demand to make continuously
More practical and feasible for multi-step synthesis
Can reduce overall process complexity
Allows chemists to maximize yield of desired compound, which can reduce waste
Easier to operate, maintain and repair
Can be adapted to multiple uses – important for facilities producing many different products (e.g. specialty)
Also, important subcategory of batch reactors is toll manufacturing: many organic chemicals require multi-step manufacturing processes. These steps often call for precise operating conditions, which in turn demand specialized equipment and trained employees.

13. Reactors in Series: A Battery of Stirred Tanks
Taken from H. Scott Fogler, Elements of Chemical Reaction Engineering, 1986, Prentice-Hall, p. 17, Figure 1-9.

14. Continuous Reactor
Reactants added and products removed at constant rate – constant volume in reactor
Continuous stirred tank – “CSTR” equipment similar to batch reactor
Pipe (tubular) reactor – tubing arranged in coil, jacketed in heat transfer fluid
Pipe reactor is a piece of tubing arranged in a coil or helix shape that is jacketed in a heat transfer fluid. Reactants enter one end of the pipe, and the materials mix under the turbulent flow and react as they pass through the system. Well suited for reactants that do not mix well, because the turbulence in the pipes causes all materials to mix thoroughly (Hocking, 1998).

15. Continuous Reactor Characteristics
Good for high production – used primarily for large-scale operations (>20 million pounds/yr product)
Usually dedicated to single product
Requires significant automation and capital expenditures
Continuous processes require a substantial amount of automation and capital expenditures,and the equipment generally must sbe dedicated to a single product. So, used primarily for large scale operations, such as those producing greater than 20 million pounds per year of product. (Hocking 1998) Between 4 and 20 million pounds of product, the choice of a batch or continuous process depends on the particular situation (product and other site-specific considerations).

16. Other Continuous Reactor Types
Packed bed
Tubular reactor packed with solid catalyst particles
Catalyst increases reaction rate and conversion
Fluidized bed
Combination of continuous stirred-tank and packed-bed
There is a wide variety of different combinations and configurations of reactors used in the industry today.

17. Reactors – potential wastes
Byproducts
Spent catalysts, salts, filter aids,etc
Waste (gas, liquid, solid) from reactivation of catalyst in fluidized bed
Discharge of fluidizing gas
Off-spec product
Cleaning waste
Vent gases from reactor charging
Contaminated cooling water
Fluidizing gas can be a Process chemical, air, or an inert gas.

18. Heat Transfer Equipment

19. Heat Transfer Operations
Needed to heat or cool reactants and/or products
control of process conditions
recovery of process heat
cooling (“quenching”) of reactants
to effect phase change
Can be stand-alone or integrated with other unit operation
reactor heat/cooling
distillation reboiler/condenser
May use either radiative or convective heat exchange

20. Shell and tube heat exchanger
From
January 1999 article from Chemical Processing magazine, “How to Compare Heat Exchangers,” by Jackson Ball, API Heat Transfer, Buffalo, NY.

21. Heat Exchange – Potential Wastes
Thermal degradation in process streams
Contaminated exchange fluid by process streams
Liquid waste from blowdown
Vapor and mist from cooling towers
Cleaning chemicals from maintenance

22. Separations Equipment

23. Separations
Most chemical reactions are not complete (some unreacted inputs remain)
Side reactions may result in one or more unwanted (or desired) byproducts
Separations needed to obtain purified product to be used by customers or downstream manufacturers

24. Distillation Separates liquids having differing boiling points
Can separate solutions where all components are appreciably volatile (fractionation)
Mixture heated to boiling of most volatile component (i.e. lowest boiling point), compound becomes gaseous, then condensed again in attached vessel.
Additional compounds can be isolated from mixture by increasing temperature to appropriate boiling point(s)
– excellent source for a review of distillation column operation and terminology.
Materials existing as gases at room temp. can be separated via distillation when they are refrigerated to a liquid and slowly warmed to their boiling points.
“Distillation is a method of separating the components of a solution which depends upon the distribution of the substances between a gas and a liquid phase, applied to cases where all components are present in both phases. Instead of introducing a new substance into the mixture in order to provide the second phase, as is done in gas absorption or desorption, the new phase is created from the original solution by vaporization or condensation.” (Treybal)

25. Distillation Column
Cross-sectional diagram from Jaeger Products, Inc.. Tower packing, trays, column internals,
PFD of distillation column is from The website states, “The copyright of this document and all locally linked documents belong to the author, Ming T. Tham, currently with the Department of Chemical and Process Engineering at the University of Newcastle upon Tyne, UK. You must not make use of these pages for commercial gain or publish them on your own web site without the express permission of the author. If you wish to include parts of the document in your publications, then an appropriate citation using the above information, would be most appreciated.” The copyright statement is at

26. Extraction
Separation of compounds based on differential solubilities in fluids such as water or organic solvents.
may also be done using supercritical fluids
requires that at least two distinct liquid phases be present
often requires that a second downstream separation be performed to recover the extraction solvent

27. Example of extraction process- 50/50 acetone/water mixture
From Elementary Principles of Chemical Processes Felder and Rousseau, Wiley & Sons, 1986., pp , Example 4.4-2, “An Extraction-Distillation Process.

28. Liquid-Liquid Extraction Unit
Courtesy Pressure Chemical Co. Pressure Chemical Co.
3419 Smallman Street
Pittsburgh, PA

29. Filtration Separates solids from liquids or gases
feedstock preparation
product or catalyst recovery
Slurry or mixture of liquid and suspended particles passed through porous barrier
Alternative form is centrifugation
Slurry placed in porous basket, spun rapidly and outward force pushes liquid through filter
Fluid reclaimed on outside of basket

30. Filtration Process
Treybal, Mass-Transfer Operations, p. 586,

31. Other Separations Processes
Gas-Liquid
Distillation (single stage=“flash”)
Evaporation
Gas Absorption
Liquid-Liquid
Liquid extraction
Product washing
Solid-fluid
Filtration
Adsorption and ion exchange
Crystallization
Drying
Leaching
Italics for the most common separation methods used in organic chemical manufacturing: distillation, extraction and filtration (next slides).

32. Separations - potential wastes
Distillation:
Overhead vapor contamination of contact or non-contact cooling water, steam jet condensate, etc… in distillation operations
Still bottoms
non-condensable gases
Filtration:
Filtrate, filter cake and filter presses from filtration processes
Extraction:
Vapor loss from headspace over extraction
Liquid or solid non-product phase
Additional process separation wastes include:
Wastewater and solid waste from regeneration of ion exchange membranes
Depending on the nature of the process solution being exchanged, the wastewater from regeneration might require secondary treatment, such as neutralization/precipitation, that would produce a sludge or solid waste requiring disposal.
Vapors, liquids and solids from drying crystallization, centrifugation,extraction
Vapors exiting the dryer can be captured by a condenser, absorber, or scrubber.

33. Materials Handling Equipment

34. Materials Handling Pipes, Valves and Connection
Pumps, compressors and steam jet ejectors
Storage tanks, containers, and vessels
Blending and milling (e.g., mix tanks, grinders)
Product preparation (e.g. Packaging stations)

35. Materials Handling – Potential Wastes
Leaks and spills
Airborne emissions through controlled vents (reactor)
Fugitives around seals, stirrer glands, pump and valve packing, piping flanges, joints, etc…
Contaminated exchange fluid from leaks into non-contact heating or cooling coils/pipes
Seal flushes
Maintenance Operations
Contaminated gas, steam or water from equipment flushing (cleaning)
Contaminated gaskets, packing, piping, filters, etc.
Paint stripping, welding, lubrication, etc…
No matter what unit operation you have, there are likely to be certain common waste types across all equipment.
Leaks during routine movement of gaseous, liquid, or even solid materials.
These leaks, particularly for gases, may be too small, or too remotely located, for easy location and repair.

36. Ancillary Equipment and Processes

37. Ancillary Equipment and Processes
Chemical loading and transportation
Maintenance activities (e.g., equipment cleaning)
Waste management
Vents/flares
Wastewater treatment/pretreatment
Hazardous and solid waste management
Laboratory activities
Office activities
All the other supporting “processes” or operations that allow the chemical production process to work.

38. Other Sources of Waste Chemical loading and transportation vent gases
spills
Maintenance activities (e.g., equipment cleaning)
cleaning fluids/solvent
drained material
Waste management
Vents/flares
Process water treatment/pretreatment
blowdown
treatment chemicals
Laboratory activities
sample wastes
lab reagents
Office activities

39. Unit Summary
Despite diversity of processes, underlying equipment and phenomenology is relatively simple
“Unit Ops” paradigm helps provide unifying framework for understanding process technology
Each process unit has characteristic waste and emission sources/causes
Emissions stem from both intrinsic and extrinsic causes