Created at

1

• Program for North American Mobility in Higher
  Education (NAMP)
• Introducing Process Integration for Environmental
  Control in Engineering Curricula (PIECE)

Module 8 Introduction to Process Integration
Tier III
2
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

3
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.

4
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.

5
Tier IIIOpen-ended problem
6
Tier III Objective

• 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

7
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.
8
Tier III Problem Statement
Reference El-Halwagi, M. M., Pollution
Prevention through Process Integration
Systematic Design Tools. Academic Press, 1997.
9
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 lt 1.000 ppmw
• average outlet methanol composition 15 ppmw
• biotreatment operating cost 0.11M 0.0013G
  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.
10
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 0.5ƒG 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)
0.003L (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
facilitys 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.
11
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.
12
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.
13
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.
14
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
• Methanol minimization in the wastewater streams
  as well as reduced water usage and reduced
  wastewater discharge
• Trade-off between minimization of operating costs
  related to the elements stated in (A) and
  benefits resulting from the recuperation of
  methanol
• 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.
15
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.
16
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/