NAMP
Program
for North American Mobility in Higher Education
PIECE
NAMP
Module 8
Introduction to Process
Integration
Tier III
Introducing
Process integration
for Integration
Environmental Control in Engineering Curricula
Module
8 – Introduction
to Process
PIECE
1
NAMP
PIECE
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Module 8 – Introduction to Process Integration
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Table of contents
Project Summary
Participating institutions
Module creators
Module Structure & Purpose
Tier III
Statement of Intent
The Kraft Pulping Process
Kraft Process Flowsheet
Wastewater Treatment in the Kraft Pulping Process
Energy in the Kraft Pulping Process
Question 1
Question 2
Question 3
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Project Summary
Objectives
Create web-based modules to assist universities to address
the introduction to Process Integration into engineering
curricula
Make these modules widely available in each of the
participating countries
Participating institutions
Two universities in each of the three countries (Canada,
Mexico and the USA)
Two research institutes in different industry sectors:
petroleum (Mexico) and pulp and paper (Canada)
Each of the six universities has sponsored 7 exchange
students during the period of the grant subsidised in part by
each of the three countries’ governments
Module 8 – Introduction to Process Integration
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NAMP integration for Environmental Control in Engineering Curricula
Process
Paprican
PIECE
PIECE
École
Polytechnique de
Montréal
Universidad
Autónoma de San
Luis Potosí
University of
Ottawa
Universidad de
Guanajuato
North Carolina
State University
Instituto
Mexicano del
Petróleo
Program
forIntroduction
North American
Mobility
in Higher Education
Module
8–
to Process
Integration
University of
Texas A&M
NAMP
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PIECE
Module 8
This module was created by:
Carlos Alberto Miranda Alvarez
Paul Stuart
From
Host Institution
Host director
Martin Picon-Nuñez
Jean-Martin Brault
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Structure of Module 8
What is the structure of this module?
All modules are divided into 3 tiers, each with a specific
goal:
Tier I: Background Information
Tier II: Case Study Applications
Tier III: Open-Ended Design Problem
These tiers are intended to be completed in that particular
order. Students are quizzed at various points to measure
their degree of understanding, before proceeding to the
next level. Each tier contains a statement of intent at the
beginning and a quiz at the end.
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Purpose of Module 8
What is the purpose of this module?
It is the intent of this module to cover the basic aspects
of Process Integration Methods and Tools, and to
place Process Integration into a broad perspective. It
is identified as a pre-requisite for other modules related
to the learning of Process Integration.
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Tier III
Open-ended problem
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Tier III Statement of intent
The goal of this tier is to solve a real-life application of
Process Integration, in which the student must interpret
the results obtained from a range of Process Integration
tools. At the end of Tier III, the student should be able
to identify the following:
Benefits of the use of Process Integration tools
Potential cost saving opportunities from the use of
Process Integration tools
Environmental impact reduction resulting from the
application of Process Integration tools
How the application of Process Integration tools
can be used to obtain an operable process
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Tier III – Problem Statement
The Kraft pulping process
The basic features of a Kraft pulping process are shown on the next slide. Wood chips
(containing 50% water) are conveyed from a surge hopper to a presteaming unit to
facilitate subsequent impregnation of the chips with chemicals. A high-pressure feeder
transfers the chips from the presteaming vessel to the digester. In the digester, the wood
chips are “cooked” using white liquor (a mixture of cooking chemicals including NaOH,
Na2S, Na2CO3 and water) to solubilize the lignin in the wood chips. In the cooking process,
methanol is produced. Following digestion of the lignin, the cooking chemicals are washed
out of the pulp. A countercurrent multistage washing unit is utilized to minimize the
carryover of chemicals with the pulp. The residual chemicals from the pulping process are
called the weak black liquor. The black liquor contains sodium salts (hydroxide, sulphide,
carbonate, chloride, sulphite and sulphate), dissolved lignin, methanol and water. Before
the outlet to the digester is fed to the washers, the cooked pulp and liquor are passed to a
blow tank where the pulp is separated from the weak black liquor which is fed to a
recovery system for conversion to white liquor. The first step in recovery is concentration of
the weak black liquor via multiple effect evaporators. The concentrated solution is sprayed
in a recovery furnace. The evaporation process results in the generation of a large amount
of combined condensate which is classified as a wastewater stream and of gaseous waste
whose primary pollutant is H2S. The smelt from the furnace is dissolved in water to form
green liquor which is reacted with lime (CaO) to produce white liquor and calcium
carbonate “mud”. The recovered white liquor is mixed with make-up materials and recycled
to the digester. The calcium carbonate mud is thermally decomposed in a kiln to produce
lime which is used in the causticizing reaction. There are several gaseous wastes emitted
from the process, some of which can be used for steam generation or cogeneration.
Reference: El-Halwagi, M. M., Pollution Prevention through Process Integration: Systematic Design Tools. Academic Press, 1997.
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Tier III – Problem Statement
Gaseous Waste
Reference: El-Halwagi, M. M., Pollution Prevention through Process Integration: Systematic Design Tools. Academic Press, 1997.
Evaporators
Strong
Black
Liquor
Multiple Effect
Wood
Chips
Steam
Weak
Black
Liquor
Recovered
White Liquor
Digester
Off
Gas
Gases
Pulp to
further
processing
Steam
Blow Tank
Condensate
Lime
Flue
Gas
Recovery
Washers
Slaking &
Causticizing
Furnace
Air
Smelt
Off
Gas
Water
Lime Kiln
Dissolving
Tank
Green
Liquor
Module 8 – Introduction to Process Integration
Settling &
Calcium
Carbonate
Filtration
Gases
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Tier III – Problem Statement
Wastewater treatment in the Kraft pulping process
Pulp and paper mills employ high levels of fresh water that lead to the generation of a significant
amount of aqueous effluent. Therefore, the objective of optimizing water usage and wastewater
discharge presents a major challenge to the industry. Due to the direct contact of water with
various species, the aqueous streams are laden with various compounds including methanol,
non-process elements and organic and inorganic species. Methanol is classified as a high priority
pollutant for the pulping industry. In addition, it may provide a source of revenue if properly
recovered.
Methanol can be found in most wastewater streams of the Kraft pulping process particularly in
the condensate leaving the multiple effect evaporators and the condensers used to condensate
the steam from the presteaming unit before the wood chips are taken to the digester. All the
wastewater streams are treated using biotreatment and then discharged to the river. Any stream
discharged to the river should not have a methanol composition which exceeds 15 ppmw. The
following information is available for the biotreatment facility:
•
•
•
acceptable methanol composition entering biotreatment < 1.000 ppmw
average outlet methanol composition = 15 ppmw
biotreatment operating cost = 0.11*M + 0.0013*G where M is the mass load (kg/h) of
methanol and G is the flowrate of wastewater (kg/h)
Reference: El-Halwagi, M. M., Pollution Prevention through Process Integration: Systematic Design Tools. Academic Press, 1997.
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Tier III – Problem Statement
Wastewater treatment in the Kraft pulping process (2)
The amount of methanol in the wastewater could be reduced using air stripping and recovered
from aqueous streams to provide methanol sales that are higher than recovery costs. The
flowrate of air is determined as follows:
L = 0.5*ƒ*G
Where L and G are the mass flowrates (kg/h) of air and wastewater, respectively, and ƒ is the
fractional mass removal of methanol from water by stripping. The operating cost for air stripping
is given by the following relationship:
Operating Cost (US$/h) = 0.003*L (kg air/h)
This cost includes air compression and methanol condensation.
The wastewater treatment plant operator also has problems predicting when the treatment
process will go from one operating regime to another or when the process will produce water
with above permitted limits concentrations of methanol and other pollutants. He disposes of the
treatment facility´s last three years of operating data but does not know how to interpret such
amounts of information.
Reference: El-Halwagi, M. M., Pollution Prevention through Process Integration: Systematic Design Tools. Academic Press, 1997.
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Tier III – Problem Statement
Wastewater treatment in the Kraft pulping process (3)
Along with methanol as one of the main pollutants found in Kraft pulp mill aqueous effluents,
other organic and inorganic compounds are found. These include chloroform, formaldehyde,
phenol and others, depending on the mill and process used. Phenol is of concern primarily
because of its toxicity, oxygen depletion and turbidity. In addition, phenol can cause
objectionable taste and odour in fish flesh and potable water.
Several techniques can be used to separate phenol. Three external technologies are here
considered for the removal of phenol. These processes include adsorption using activated
carbon, ion exchange using a polymeric resin and stripping using air.
The operating costs for each method comprise cost of make-up and cost of regeneration. For
activated carbon, steam is used to regenerate the mass-separating agent while caustic soda
(NaOH) is used for the regeneration of the ion exchange resin. In the case of air stripping, the
gaseous stream leaving the mass-exchange unit cannot be discharged to the atmosphere owing
to air-quality regulations. Hence, the air leaving the separation unit is fed to a phenol-recovery
unit in which a refrigerant is used to condense phenol. The operating cost related to each
technology is thus 0.737 US$, 1.150 US$ and 2.069 US$ per kg of removed phenol for activated
carbon, ion exchange resin and air stripping respectively.
Reference: El-Halwagi, M. M., Pollution Prevention through Process Integration: Systematic Design Tools. Academic Press, 1997.
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Tier III – Problem Statement
Energy in the Kraft pulping process
The Kraft pulping process is a very energy-intensive process: electricity end-uses common to all
pulp and paper mills include pumping, air-handling, and lighting. In addition, steam needs and
the large number of process streams makes this sector of the industry a good candidate for
improved heat integration. Black liquor concentration is usually the biggest single steam using
operation in a Kraft pulp mill. Evaporators installed in the 1960s and 1970s were built with four
or five effects, whereas most Kraft mills today use five or six effect evaporators, with a
concentrator to further increase solids content. Firing the recovery boiler with the black liquor at
higher solids content improves overall boiler performance and is a general trend in the industry.
To counter this energy consumption problem, a Kraft pulp mill uses biomass. In fact, in addition
to being the feedstock for pulp and paper production, biomass is also a major energy resource
for the industry. The industry also has access to residues of pulpwood harvesting, some of
which can be removed from the forest on a sustainable basis. All black liquor and most mill
residues are used at mill sites to fuel cogeneration systems, providing steam and electricity for
on-site use. Cogeneration also known as Combined Heat and Power (CHP) is the simultaneous
production of electricity and useful heat from the same fuel or energy. A typical cogeneration
system consists of an engine, steam turbine, or combustion turbine that drives an electrical
generator. A waste heat exchanger recovers waste heat from the engine and/or exhaust gas to
produce hot water or steam.
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Tier III – Problem Statement
Energy in the Kraft pulping process (2)
Cogeneration produces a given amount of electric power and process heat with 10% to 30%
less fuel than it takes to produce the electricity and process heat separately. Facilities with
cogeneration systems use them to produce their own electricity, and use the unused excess
(waste) heat for process steam, hot water heating, space heating, and other thermal needs.
They may also use excess process heat to produce steam for electricity production. In the
chemical recovery, steam plant and cogeneration areas, pulping liquor solids, purchased and
self-generated woodwaste, primary clarifier sludge from the wastewater treatment plant, and
knots are burned to recover cooking chemicals and to produce energy. Spent pulping liquors
account for over 70% of the biomass-derived fuels used in the pulp and paper industry today.
In the recovery process, the resulting strong black liquor from the evaporators is sprayed into
the recovery boiler where the organic content in the liquor is burned, releasing energy and
producing steam for use in the mill. Upon combustion, the inorganic portion of the strong black
liquor produces a flue gas.
The electricity-to-heat production ratio for a conventional back-pressure steam turbine
cogeneration system ranges from 40-60 kWh/GJ, which is relatively well-matched to the steam
and electricity needs at older Kraft mills. Much higher electricity-to-heat ratios are possible using
biomass and black liquor cogeneration technologies based on gas turbines rather than steam
turbines. Commercially-aimed development of technologies for converting black liquor or
biomass residues into combustible fuel gas is ongoing, along with the cleanup systems that
would be needed to enable use of the gas in gas turbine cycles.
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Tier III – Questions
Question 1. Wastewater treatment in the Kraft pulping process
Which Process Integration tools could be used to address all the issues presented
in the methanol related slides? Define the steps in the methodology you would
use to answer the following points:
(A) Methanol minimization in the wastewater streams as well as reduced water usage
and reduced wastewater discharge
(B) Trade-off between minimization of operating costs related to the elements stated
in (A) and benefits resulting from the recuperation of methanol
(C) Interpretation and use of process operating data to help the treatment plant
operator obtain better control of the operation of the wastewater treatment plant
Question 2. Wastewater treatment in the Kraft pulping process (2)
Using your knowledge of Process Integration tools, describe the methodology that
could be used to choose the best mass-separating agent to treat the waste streams
of phenol in this Kraft pulp and paper mill.
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Tier III – Questions
Question 3. Energy in the Kraft pulping process
With the knowledge of Process Integration acquired over the last two tiers,
propose a methodology that would help identify the energy savings possibilities as
well as the potential for cogeneration in a Kraft pulp mill. Elaborate on each of the
steps taken to conduct such a study and remember to include in your proposition
the impact of your solution on the environment.
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End of Tier III
This is the end of Module 8. Please submit your report to your
professor for grading.
We are always interested in suggestions on how to improve the
course. You may contact us as http://process-integration.tamu.edu/
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