Assessment of cleaner process options: a case study from petroleum refining.
Weston, Neil ; Clift, Roland ; Basson, Lauren 等
INTRODUCTION
The Paradigm Shift in Chemical Processing
Over roughly the last two decades, there has been a shift in both
thinking and practice away from the "clean-up" or
"end-of-pipe" technology approach which had replaced the
"dilute-and-disperse" approach to emissions from process
plants, to approaches to process selection, design and operation which
have the objective of avoiding rather than managing emissions and wastes
(e.g. Clift and Longley, 1995; Allen and Rosselot, 1997; Sikdar et al.,
1999). In North America, this new paradigm is generally termed Pollution
Prevention, defined as "source reduction and other practices that
reduce or eliminate the creation of pollutants through increased
efficiency in the use of raw materials, energy, water or other
resources; or protection of natural resources by conservation"
(Sikdar et al., 1999). In Europe, the term Clean Technology is more
commonly applied, defined as "a means of providing a human benefit
which, overall, uses less resources and causes less damage than
alternative means with which it is economically competitive"
(Clift, 1995). In this context, "overall" means over the whole
life cycle of a product or a process; that is, over the whole supply
chain of materials and energy from the "cradle" of primary
resource to the "grave" of long-term managed disposal. This
attention to complete life cycles is also implicit in Pollution
Prevention and other conceptual approaches such as Cleaner Production
(see Clift, 2001).
The shift in assessing environmental performance away from the
individual process to a broader system boundary encompassing complete
life cycles requires, inter alia, new approaches to identifying best
available technology (BAT) for any product, process or service (e.g.
Geldermann et al., 1999; Nicholas et al., 2000). The parameters
describing environmental performance form a set of categorically
distinct environmental impacts; any attempt to reduce them to a single
metric such as damage cost is unsatisfactory for a number of reasons,
one of which is that aggregation to a single objective hides essential
information about the origins and nature of the impacts. Consideration
of complete life cycles may require trade-offs to be recognized between
impacts arising in different parts of the life cycle. In turn, this
means that defining the boundary delineating the product or process
system is critical. To deal systematically with trade-offs between
different environmental impacts and economic costs, process selection,
design and operation must use multi-objective optimization (see e.g.
Clift, 2006) and decision-structuring and analysis approaches
appropriate for complex contexts (see e.g. Basson and Petrie, 2007).
This complication and complexity mean that simple decision rules may not
be universally applicable and may even be misleading. This applies even
to simple waste hierarchies--"Prevention, Minimization, Recycling,
Disposal" (e.g. Finnveden et al., 2005) or "Elimination,
Source Reduction, Recycling, Treatment, Disposal" (e.g. Crittenden
and Kolaczkowski, 1992; Balm et al., 1997)--which are contested for some
wastes; the debate over whether paper waste should be recycled as a
recoverable material or burned as a renewable biofuel is a well-known
example (e.g. Ekvall and Finnveden, 2000; Arena et al., 2004; Hart et
al., 2005).
Three levels of change have been recognized in modifying a process
to produce a particular product by a cleaner route (Allen, 1997):
1. Minimize arisings of waste and effluent and consumption of
energy within an essentially unchanged process;
2. Modify the process or technology to use the same materials but
more efficiently and with less energy;
3. Redesign the process completely and change input materials.
Substitution of the product by another way of delivering the same
function is a more radical step (Clift, 2001), not considered in this
paper. Process change at level l is essentially incremental,
corresponding to changes in process configuration or operation which are
low-risk and for which the return is readily calculable. Level 2 changes
are usually matters of routine process engineering; some financial risk
may be involved, so that return on capital is of concern, but there is
limited uncertainty. Changes at level 3 usually involve significant
capital investment, with uncertainty over financial performance and
possibly over the environmental impacts of changing the supply chain of
materials and energy.
These levels of change are also associated with different barriers
and drivers, and therefore require different approaches to
decision-making (e.g. Basson, 2004; Mitchell et al., 2004). Incremental,
level 1 changes are matters of routine process evolution. Level 2
changes can be assessed in terms of simple parameters such as payback time or return on capital but these approaches must be adapted to
include consideration of environmental impacts unless these impacts are
"internalized" into the economic calculations, for example, as
emission charges or traded emission permits. Changes at level 3 are more
contentious and risky, so that more sophisticated approaches to
decision-making are required (see e.g. Wrisberg and Udo de Haes, 2002).
The barriers to adoption of new cleaner technologies have been the
subject of several careful studies (e.g. Christie et al., 1995; Clayton
et al., 1999) which dispel the neo-classical economic myth that a
technology which meets demand more cheaply and complies with regulations
will necessarily be adopted. A general conclusion is that technological
developments do not proceed by smooth linear progression; rather,
"lock-in" to past technologies and investments constrains the
paths which are available for future developments (e.g. Arthur, 1989;
Stirling, 1998). Occasionally major technological shifts do occur, as
the development of ammonia processes illustrates, but the more usual
pattern of development is by changes at levels 1 and 2--incremental
changes or small steps, but guided by the longer term and larger scale
changes driven by changes in public expectations as well as market
conditions.
Sustainability Considerations
The agenda which is embodied in the term "sustainable
development" requires assessment of the social benefits of a
product or service as well as its economic and environmental performance
(see e.g. IChemE, 2002; Mitchell et al., 2004). Although established
models of sustainability group technological and economic factors
together, and it is accepted that many technological hurdles may be
overcome with sufficient economic resources, technical and economic
considerations are subdivided here for pragmatic reasons. The following
considerations are relevant in assessing the cleaner process options
discussed below. There are almost always trade-offs to be made between
different aspects of performance, and this is one of the principal
reasons why sustainability concerns introduce the need for structured
decision-making processes (Elghali et al., 2008).
Technological
An important consideration in assessing feasibility is time taken
to implement the change. This is especially true in the process
industries where shutdowns must be scheduled to allow intrusive
modifications to be carried out. Any modification requiring a shutdown
must therefore be assessed in terms of its complexity so that a decision
can be made as to whether it is a viable for completion within any
forthcoming scheduled shutdown. Furthermore, reliability of any process
modification is an important concern: a business is unlikely to
implement a process change unless there is a high degree of confidence
in the ability of the modified process to perform its function reliably.
Modification time and reliability are treated here as technological
concerns.
Economic
Economic considerations address the financial implications of
carrying out a proposed modification. Unless a modification is forced by
external factors (such as new regulations), a business will require some
financial incentive for carrying out the work. Economic metrics used for
assessing economic performance are routine: for example, net present
value (NPV), which accounts for all the savings over a given timescale
that will result from implementing the modification, and payback, which
calculates the length of time required to recover the cost of installing
the modification. These metrics show in principle that any size project
is economically feasible if the cost savings from making the
modification justify the expenditure.
Environmental
Environmental performance covers both use of materials or energy
and environmental emissions to land, water or air. Qualitative
considerations can guide identification of the most significant factors
in assessing process options. However, full consideration of options
requires quantitative analysis of the whole supply chain of materials
and energy (see The Paradigm Shift in Chemical Processing Section above)
to ensure that process changes do not merely displace environmental
impacts to some other place in the life cycle.
Social
It is also necessary to consider the impact of process changes on
different social groups affected, including local residents, the
workforce and the general public. Performing this high level qualitative
assessment of social factors indicates the extent to which different
social groups are affected by the options being considered and thereby
aids decision makers in determining the appropriate level of involvement
in a deliberative decision-making process.
System Boundaries--There Is no such Place as "Away"
The discussion above introduced the importance of approaching
environmental performance on a life cycle basis and of attention to
strict definition of system boundaries. The appropriate tool for such
analysis is Life Cycle Assessment (LCA). While the intention here is not
to go into the details of LCA (1), it is necessary to introduce some of
the considerations in drawing system boundaries for LCA. The system must
include the full supply chain from primary resource extraction through
to waste management: wastes cannot simply be "sent away". It
is useful to make a pragmatic distinction, illustrated in Figure 1,
between (Clift et al., 1999, 2000):
1. Foreground: the set of processes whose selection or mode of
operation is affected directly by the change being considered;
2. Background: all other processes which interact with the
Foreground, usually by supplying or receiving materials or energy.
The total Life Cycle Inventory for the product produced by the
system in Figure 1 consists of:
1. Direct Burdens emitted from the Foreground processes
2. plus Indirect Burdens arising from the supply chains of
materials and energy provided to the Foreground
3. minus Avoided Burdens associated with activities in the
Background displaced by materials or energy recovered from the
Foreground processes.
This distinction was originally introduced to help in compilation
of the Life Cycle Inventory (Nord, 1995). Foreground processes are
identifiable and can be represented by plant-specific primary data
whereas Background processes are not specifically identified and are
therefore represented by generic average data. If a transfer between the
Foreground and Background takes place through a homogeneous market, then
the precise source of the materials or energy is unknown and so the
supply chain is necessarily in the Background.
[FIGURE 1 OMITTED]
The distinction between Foreground and Background systems requires
careful consideration when assessing process changes at the different
levels outlined above; treating emissions from known processes as direct
emissions is of particular value in determining the specific
consequences of modifications to the system. (2) Processes whose
selection or design may be changed substantively must be treated as part
of the Foreground system, because the nature of the environmental
impacts associated with the process may be changed. Where only the
amount of material (or energy) sent to or output from a process is
affected, the scale but not the nature of the environmental impacts is
changed; these processes can be considered part of the Background
system. However, if the exchange is with a specific identifiable
process, then that process can be represented by specific data; in
effect, it can then be treated as part of the Foreground. As a specific
example considered in the case study set out in this paper, treating
waste on-site rather than exporting it for management elsewhere means
that part of the waste treatment moves from the Background into the
Foreground.
Aim and Outline of Paper
This paper describes developments in a specific process at a UK oil
refinery, to provide an illustration of the methodological and practical
challenges associated with the introduction of cleaner technologies. The
section Existing Alkylation Process and Waste Management below
introduces the process--alkylation--concentrating on the management of
fluoridic waste arising from HF used as catalyst. The ensuing section
Waste Reduction and Management Section introduces possible technological
changes which could avoid, reduce or enable partial recycling of the
waste, identifying the barriers and drivers associated with each option
and focusing particularly on the way in which the system boundaries must
be drawn to show the trade-offs to be considered in assessing them and
examines how quantitative LCA should be carried out to compare the full
environmental implications of the alternative changes. Discussion and
Conclusions Sections discuss the more general conclusions to be drawn
from this case study.
EXISTING ALKYLATION PROCESS AND WASTE MANAGEMENT
Alkylation is an operation in petroleum refining which produces a
high value blend for transport fuels, alkylate, with a high octane
rating; it contains no aromatic compounds or olefins and is
"essentially free of sulphur and other impurities"
(Hommeltoft, 2003). As fuel specifications have become more stringent
and additives, primarily lead compounds, have been phased out, alkylate
has become an increasingly important refinery product (Dunham, 2005).
Alkylate is formed by the following overall liquid-phase reaction; see
Albright (2003) for a fuller discussion.
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.]
The detailed mechanism is still under study, especially amongst
researchers seeking to establish alternative catalysts (e.g. Hommeltoft,
2003; Kane and Romanow, 2003).
An acidic catalyst is used to obviate the use of high temperature
and pressure. Two processes are in use worldwide, using respectively
hydrogen fluoride (HF) and sulphuric acid as catalyst (Wood et al.,
2001; Nieto et al., 2007); globally, total outputs from the two
processes are almost equal (DuPont STRATCO, 2006). There are relatively
few configurations of alkylation units, the primary differences being
between the major licensors: UOP and ConocoPhillips (previously
Phillips) for HF units, and DuPont/STRATCO and ExxonMobil for
[H.sub.2]S[O.sub.4] units.
Hydrofluoric Acid (HF) Alkylation
The refinery which is the focus of this case study uses the HF
process. As a highly reactive compound, HF brings significant safety
hazards that must be carefully managed. Losses of fluorine through
various process streams exacerbate the safety hazards as they require
increased deliveries of fresh HF and removal of spent fluoridic
compounds which are wastes and must be treated prior to disposal. This
section gives an overview of the alkylation unit to establish the main
process characteristics and objectives and to identify the causes of
fluorine losses. Waste Management Systems Section considers the current
strategies at the refinery to deal with the fluorine-containing wastes,
including the additional material and energy resources consumed by the
waste treatment systems and the eventual form of the final waste.
[FIGURE 2 OMITTED]
Alkylation process
The system under consideration here is a Phillips-type HF
alkylation unit, shown schematically in Figure 2; it has been in
operation for nearly 30 years. The principal inputs to the process are
liquid olefins and iso-butane (iC4). The olefins are distilled (A) to
remove water and impurities, primarily ethane, which form by-products in
the alkylation reaction while the iC4 is dried (B). The fresh olefin and
iso-butane streams are mixed with recycled iso-butane and then contacted
with HF acid. The reaction is exothermic and occurs instantaneously in
the riser entering the acid settler (D); the mixture contains excess
iso-butane so that the olefins react to completion. The hydrocarbon
reaction products and HF catalyst are immiscible; they are allowed to
settle and separate in the acid settler (D). The heavier acid layer
contains free HF, water, organic fluorides (produced by incomplete
reaction) and a group of heavy hydrocarbon by-products known as conjunct
polymers or, more commonly, acid soluble oils (ASO) (Miron and Lee,
1963; Albright et al., 1988; Berenblyum et al., 2002). Make-up anhydrous HF (<0.01 wt% water) from the batch HF storage drum (C) is mixed with
this stream (3); it is cooled and all but a small proportion is recycled
to the reaction loop.
Controlling acid strength is important in operation of the
alkylation unit for a number of reasons discussed below; therefore an
acid regeneration loop is required to remove impurities and return clean
acid. A small proportion (<0.1%) of the bottom stream from the acid
settler (D) is partially vaporized before entering the acid re-run
column (E) ; the liquid, comprising about 10% of this stream, contains
ASO, water and HF. A hot stream of iso-butane vapour enters towards the
bottom of the re-run column to strip HF from the liquid for return as a
vapour to the acid settler (D). Water, some of the heavier organic
fluorides and the ASO pass to the bottom of the acid re-run column (E)
and are drained from the process. The acid re-run column is normally
operated to maintain 1-2 wt% water in the acid loop; this level favours
a high octane rating of the alkylate product (Hutson and Hays, 1977)
while higher water concentration can lead to accelerated corrosion. Due
to inefficient stripping and the formation of an azeotrope at 38.26 wt%
HF in water (Munter et al., 1947), a significant part of the fluorine
losses are in the form of HF in the acid re-run bottoms; this is
considered in greater detail below.
The liquid hydrocarbon stream from the top of the acid settler (D)
to the downstream separation columns (F & J) consists of alkylate,
propane, n-butane, un-reacted iso-butane and some organic fluorides.
Part goes to the depropaniser (F) to separate propane from iso- and
normal butane. Propane vapour leaves the top of the depropaniser for
further separation (G) and subsequent treatment. A two-phase side-stream
from the depropaniser, containing a high proportion of iso-butane, is
condensed and recycled to the reaction stage while a proportion is
vaporized and sent to the acid re-run column. The liquid bottom stream
from the depropaniser, mainly normal butane, and the rest of the top
stream from the acid settler pass to the iso-stripper (J) where iso- and
normal butane are separated from the alkylate, which leaves the bottom
of the column as liquid product. This contains some (in the order of
ppm) organic fluorides but is not defluorinated. (4) Iso-butane vapour
from the top of the iso-stripper is condensed and joins the recycle
stream from the depropaniser (F). A small two-phase side stream from the
iso-stripper, containing primarily normal butane, goes for further
treatment.
The normal butane and propane product streams require treatment to
remove organic fluorides and trace HE This is carried out from vapour in
the defluorinators (H and K) and from the liquid phase in the potassium
hydroxide treaters (I and L). In each defluorinator, a bed of activated
alumina is used to decompose the organic fluorides into olefins and free
HF, which reacts with the alumina to form aluminium trifluoride
(Al[F.sub.3]) (Blachman, 2000); this constitutes one of the main
fluoridic wastes. In the KOH treaters, residual traces of HF react with
solid KOH to yield water and potassium fluoride (KF) which is a further
fluoridic waste. Units H, I, and K have two columns in parallel to
enable one bed to be replaced while the other is on-line. Depending on
specification, some of the n-butane may be added to the alkylate
product, by-passing the KOH geater (L). A single column is used for L,
replenished while the n-butane by-passes this treatment.
Not shown in Figure 2 but vital to the safe operation of the unit
(and relevant to later discussion regarding fluorine losses) is the acid
relief system whose primary function is to receive process fluids when
disturbances cause an overpressure in the system. It is also used when a
vessel is vented to create an inert atmosphere for safety reasons; this
occurs immediately prior to refilling the acid feed storage drum (C),
with smaller quantities vented when a defluorinator or KOH greater is
replenished. Fluids entering the relief system are lost from the
process, neutralized, and burned at the flare.
Fluorine losses
Fluorine is lost from the process as organic fluorides which leave
with the product streams and as HF (plus traces of organic fluorides) in
the bottom stream from the acid re-run column, limited by the water/HF
azeotrope. In total, approximately 0.2-0.9 kg of HF are lost per cubic
metre of alkylate produced. (5) Table 1 summarizes the routes by which
fluorine is lost from the process with indicative figures for the
contribution of each route. The main process parameters that lead to the
losses will be introduced briefly here. The eventual fate of the losses
is discussed in Waste Management Systems Section.
Re-run column bottoms. As described above, the re-run column
maintains acid quality by removing water and acid soluble oils (ASO)
using a hot stream of iso-butane vapour to strip HF and return it to the
process. Approximately half the lost fluorine leaves in the stream from
the bottom of this column (see Table 1). The three main factors
determining this loss are:
i. water entering the alkylation process;
ii. ASO formation; and
iii. incomplete stripping in the re-run column.
The amount of water leaving the process via the re-run column and
removing HF as an azeotropic mixture is equal to that entering the
process from the dryers (A & B). A certain level of water in the
process (~1-2 wt%) is favourable to achieving a high octane rating of
the alkylate product (see above); this level is therefore a fixed
operating parameter, controlled by the operation of the re-run column.
The polymerization reaction forming ASO is complex and unavoidable.
As for water, some ASO in the process (~4-5 wt%) is favourable to a high
octane rating of the alkylate and so operators seek to maintain it at
this level. ASO represents the largest component of the fluids leaving
the re-run column bottoms and the rate of its formation determines the
rate at which fluids are drained from the re-run column; therefore the
greater the rate of ASO formation, the greater the HF losses in the
re-run column bottoms. ASO formation can be reduced by removing
impurities in the olefin feed, a process which is already carried out at
the refinery.
The stripping efficiency of the re-run column, measured by the
proportion of HF recovered from the re-run column feed, is a function of
its design, which determines phase-contacting efficiency, and operating
conditions. Increasing temperature improves recovery but the operating
temperature is limited by the allowable working temperature of the
equipment.
Losses via product streams. The next most significant fluorine
losses (see Table 1) occur in the form of HF and organic fluorides in
the propane and butane product streams. Organic fluorides, which cause
much the larger loss, are present in these streams because they have
similar volatilities to the products; they are only present as a few ppm
but this is significant because the product flows are large. Organic
fluorides are formed in the alkylation reactions and require further
contact with acid to progress the reaction towards the desired products.
The availability of "acid sites" (i.e., protons) in the
reaction solution, determined by the acid strength, is therefore a key
influence on the residual concentration of organic fluorides. The acid
strength in operation is determined by a number of factors including
alkylate yield, quality and the discharge rates that can be effectively
handled by the acid re-run column.
Losses to the relief system. Major disturbances that could lead to
overpressure are avoided as a matter of course on any process unit since
they create safety hazards and losses of valuable material. Minor
disturbances, which cause momentary overpressures, occur more
frequently; their causes can be complex so that they are difficult to
avoid completely. Fluorine losses to the relief system, which are less
significant than the other losses (see Table 1), occur whenever the HF
storage drum is purged with nitrogen to remove HF to make the vessel
safe for manually refilling with fresh HF, with smaller losses
associated with replenishing the beds of alumina and KOH. Therefore
losses to the relief system depend primarily on the total rate of HF
loss and make-up.
[FIGURE 3 OMITTED]
Waste Management Systems
Three waste management systems are used to treat the fluorine
losses:
* defluorinators: treat organic fluorides in propane and butane
product streams;
* KOH treaters: treat free HF in propane and butane product
streams;
* neutralization with sodium hydroxide (NaOH): treats all HF losses
from the re-run column and relief system.
Referring to Table 1, approximately 25 % of the losses are treated
by the defluorinators and KOH treaters while approximately 55 are
neutralized by NaOH; the remaining 20% is lost as organic fluorides in
the alkylate product. This paper concentrates on the wastes treated by
NaOH because they account for more than half the losses, are the most
problematic for the refinery and offer the greatest potential for
process improvements.
Defluorinators
Once approximately 60% of the defluorinator bed has been converted
to Al[F.sub.3], it is considered 'spent' meaning that it is no
longer effective at removing organic fluorides. The bed must then be
removed and replaced with fresh [Al.sub.2][O.sub.3]. Treatment of the
spent defluorinator bed presents a waste management problem. This
problem has been addressed by seeking uses for the Al[F.sub.3]. A
partnership has developed with a company producing aluminium
electrolytically from alumina ([Al.sub.2][O.sub.3]); Al[F.sub.3] is
added to their process as a flux. The partner company collects the spent
defluorinator beds and replenishes the defluorinator with fresh
[Al.sub.2][O.sub.3].
KOH treaters
Water produced by the reaction between KOH and HF dissolves the
other reaction product, KF, with some KOH. The liquid collects as a
slurry at the bottom of the KOH treaters; it is drained off and sent for
treatment at the refinery's waste-water treatment plant.
Neutralization with sodium hydroxide
The third treatment route for wastes from the alkylation unit is
also the most problematic in terms of the volume of waste generated,
additional materials required, manual operations involved and cost to
the refinery. The current management system for wastes from the acid
re-run column and relief system is shown schematically in Figure 3.
These wastes are first treated at the refinery in separate batch wash
vessels by neutralization with NaOH to produce sodium fluoride (NaF) and
water. For the acid-rerun bottoms, the process also causes Acid Soluble
Oils (ASO) to form a separate phase, removed from the top of the vessel
for use elsewhere on the refinery as a fuel oil. NaOH is delivered as 50
wt% solution in water but is diluted to 4.5 wt%, using local river
water, to prevent precipitation of NaF in the process (Science Lab,
2005). The two streams of spent fluoridic caustic (SFC) are blended for
transport.
SFC is a hazardous and odorous slurry requiring further treatment
before it can be disposed to landfill. It contains approximately 95 %
water, NaF, unreacted NaOH and a variety of other impurities including
mercaptans. Currently, it is transported approximately 400 km to a
specialist waste management company where it is converted into a form
suitable for final disposal. It is reacted with hydrogen peroxide ([H.sub.2][O.sub.2]) over an iron catalyst to remove odours, primarily
mercaptans, and then mixed with other waste streams to precipitate the
NaF along with other waste components. The precipitate is sent to
controlled land-fill while the supernatant water is sufficiently clean
to be discharged to sewer.
WASTE REDUCTION AND MANAGEMENT
Preliminary Identification of Alternatives Table 2 summarizes
alternatives identified so far at the refinery to avoid, reduce or
manage fluoridic wastes, classified by the levels of change introduced
in The Paradigm Shift in Chemical Processing Section. The objective of
each possible change is also summarized in Table 2, categorized according to the appropriate tier in the waste hierarchy (see The
Paradigm Shift in Chemical Processing Section). It is notable that the
level of the change in the waste hierarchy does not correlate with level
in Allen's (1997) categorization of process changes; the waste
hierarchy is useful in identifying possible process changes whereas the
level of change provides a guide to the analysis needed to assess the
change. The different process modifications are also associated with
different drivers and barriers (see Appendix A).
[FIGURE 4 OMITTED]
Figure 4 summarizes level 1 changes, that is, incremental changes
to reduce waste arisings by (a) changing operating parameters to reduce
by-product formation or (b) concentrating the waste prior to transport
to off-site treatment. Level 2 changes, that is, process modifications,
are shown in Figure 5. As for the level 1 changes, these may be designed
to reduce waste formation (c--equipment modification to improve the
efficiency of separation in the re-run column) and to achieve more
efficient management of the waste (d--regeneration of caustic and
production of calcium fluoride as a saleable product). Figure G shows
possible changes at level 3, that is substantial changes to the process
or inputs to (e) regenerate the HF catalyst, (f) replace NaOH by KOH to
reduce the volume of waste exported, and (g) eliminate HF completely by
changing to a different catalyst.
[FIGURE 5 OMITTED]
Qualitative analysis (Table 3) shows that alternative (c) is
clearly attractive; in fact, this modification is scheduled for
implementation (see below). For the other possible modifications,
quantitative evaluation is necessary considering the implications of
these changes for the whole material life cycle, not restricted to
refinery operations, that is, using Life Cycle Assessment. However, as
introduced in the section on Sustainability Considerations above, this
requires careful distinction between the Foreground and Background (sub)
systems. Figures 4 to G show the Foreground and Background systems
needed for this evaluation.
For both waste reduction and waste management, the higher level
changes generally require broader Foreground systems. For changes at
level 1, the Foreground is confined to the specific operation modified.
Shifting from level 1 to 2 moves waste management into the
Foreground--waste management is viewed as an additional process step, to
be considered as part of the "design space" for process
optimization. Some of these steps (e.g. acid regeneration, off-site
by-product processing) may be operated by parties other than the
refinery. In these cases, life-cycle based optimization would require
discussion and possibly active collaboration with these parties, but may
provide opportunities for economic, environmental and social benefits
(see Introduction). This type of analysis helps to identify the
stakeholders who will need to be considered and possibly involved in the
decision, thereby paving the way for a systematic deliberative decision
making process, such as that presented by Basson and Petrie (2007).
Furthermore, identification of the key issues associated with each
alternative can act as a precursor to risk assessments, such as the
process environmental risk assessment (PERA) approach presented by
Sharratt and Choong (2002).
[FIGURE 6 OMITTED]
We now consider these options in more detail to examine what
decisions can be made based upon the qualitative assessment summarized
in Table 2 and Figures 4 to G and the issues raised for the refinery in
selecting a preferred alternative. In particular, the following are
considered:
* What decisions can be made based upon the initial qualitative
assessment;
* What dilemmas are presented within and between each dimension of
sustainability.
Level 1 Changes
Superficially, incremental changes or additions to the process
should be the most straightforward ways to reduce waste, not requiring a
shutdown. Assessment of such changes must take a life cycle perspective
but it suffices to limit the Foreground to the process itself (see
Figure 4). However, process design and operation are usually already
optimized within this restricted system boundary, so that incremental
changes may require the most detailed quantitative analysis and
justification.
Operational changes (a)
The factors affecting operation were outlined in Existing
Alkylation Process and Waste Management above. Making adjustments to the
operating conditions may appear to be the simplest way of reducing HF
losses. However, the alkylation unit was designed to produce alkylate
and has been operated to do this safely and at minimal cost.
Environmental impacts are not normally included in optimizing plant
operation, except where they are "internalized" through
emission taxes, traded permits or feedstock costs. It is possible to use
process modelling to make the trade-offs between economic and
environmental performance explicit (e.g. Clift, 2006) but this analysis
has yet to be carried out in this case.
Waste concentration (b)
Some of the impacts associated with off-site waste management arise
from road transport of the waste; the waste, resource use and emissions
are obvious, while the disruption associated with heavy traffic
movements can dominate the social impacts. Reducing the volume of waste
to be transported is therefore a driver to concentrate the spent
fluoridic caustic (SFC) on-site by evaporation, to form slurry or
perhaps even a dry solid. Given that the acid re-run bottoms is already
neutralized in batches (see Neutralization with Sodium Hydroxide above),
this modification should be possible without shutting down the
alkylation process itself. However, this would required installation of
new processing equipment (for which space may not be available) and also
increase energy demand. Thus, although a rather simple process change in
concept, it may be complex to evaluate and difficult or even impossible
to implement.
Level 2 Changes
Design changes to alkylation unit (c)
A significant driver was that a shutdown was coming up, making
internal changes to the alkylation unit feasible. This introduced a time
constraint: it was not feasible to perform a full assessment of all the
alternatives in time for the turnaround, so priority was given to
alternatives which depended upon the unit being shutdown. Although this
decision was made at an early stage, it is well supported by the
qualitative analysis. Tackling the problem where it originates, the
alkylation unit, offers the opportunity to reduce HF losses, thereby
reducing both HF consumption and the downstream treatment needed.
HF losses from the re-run column can be reduced by modifying the
column internals to improve stripping efficiency. The internals
currently in use consist of four baffles or "shed-decks":
sloped trays extending from the edge to the centre of the vessel causing
the liquid to cascade towards the bottom of the vessel. Such phase
contacting devices are relatively inefficient; they are used with fluids
which tend to "foul" but this is not the case here.
The alternative internals include horizontal trays with fixed
valves (no moving parts) and a "chimney tray" to encourage
disengagement of gases from the acid feed; these are more efficient than
the baffles currently used. Detailed design calculations showed that the
improved stripping efficiency should reduce HF losses from the re-run
column by about 32% (and overall HF losses by ~16%, losses from the
re-run column accounting for approximately 50% of the losses). Savings
from waste treatment and input materials mean that this project should
achieve payback in less than 1 year.
Re-use of waste (d)
Moving along the waste hierarchy, re-use of the waste
("open-loop recycling", following LCA parlance) for another
beneficial purpose or direct recycling within the process
("closed-loop recycling") should be considered after source
reduction. Given the way the process operates, option (d) could in
principle be implemented without a process shutdown. However, it differs
from the level 1 options in the way LCA must be applied: converting the
waste into an input to another process requires the waste processing to
be brought into the Foreground with the Background extended to include
the other process, as shown in Figure 5, to allow for the Avoided
Burdens (see Sustainability Considerations Section).
In this specific case, there is a possibility of reacting the SFC
with calcium hydroxide to regenerate the caustic soda, for example using
the "HARD TAU reaction (Veolia, 2008),
2NaF + Ca[(OH).sub.2] [right arrow] Ca[F.sub.2] [down arrow] +
2NaOH
Calcium fluoride, precipitated by this reaction, could be used as a
flux in metallurgical operations (Kirk and Othmer, 1994). Therefore
economic credits would arise from any sales of this material, along with
environmental "credits" corresponding to the Avoided Burdens;
that is, the production of fresh calcium fluoride displaced. Imports of
caustic soda should also be lower, reducing the Indirect Environmental
Burdens (see Figure 1). Like the use of aluminium oxide and production
of aluminium fluoride in the product defluorinators (see Defluorinators
Section) this would be an example of Industrial Symbiosis: mutually
beneficial cooperation between different businesses to reduce net use of
materials and energy (see e.g. Clift, 2001).
The viability of re-using the waste in this way depends not just on
economic assessment but also on technical viability: whether
impurities--primarily those, including arsenic, entering with HF
catalyst--would build up in the process or whether they would leave with
the calcium fluoride. (6)
Level 3 Changes
Level 3 changes, involving substantial redesign of the process
and/or changes in inputs, typically require the system assessed to be
much broader (see Figure 6) so that the process itself ceases to be the
sole focus for evaluation of technical, economic, environmental, and
social performance.
Regeneration of HF catalyst (e)
Option (e) includes option (d) but would treat the precipitated
calcium fluoride with acid to regenerate HF for recycling to the process
as alkylation catalyst. Again, given that catalyst introduction and
waste treatment are batch operations, this could in principle be
implemented without a shutdown.
Qualitative considerations show obvious environmental and social
benefits from this change, including those associated with greatly
reduced transportation of HE Whether the change is technically viable
and whether the trade-offs of capital and operating cost against
improved environmental and social performance justify the change will
need to be assessed by the same kind of process modelling as option (a)
but with the extended system boundary shown in Figure 6 (e). This also
illustrates a case where the economic measures can change over time:
increasing landfill costs and charges will progressively favour this
kind of process modification.
Neutralization with KOH (f)
A further possibility is to neutralize the acid re-run column
bottoms with potassium hydroxide rather than sodium hydroxide. This has
the benefit of greatly reducing the volume of waste to be transported
and processed off-site, because KF has much higher solubility in water
than NaF so that the resulting spent fluoridic waste can be much more
concentrated. Compared to the other options, it is simple to implement.
(7) It also introduces the possibility of common treatment of KF along
with the waste from the existing KOH treaters. However, KOH is a less
widely available commodity than NaOH--it is more expensive, and the
Indirect Burdens associated with production and supply may differ from
those associated with conventional caustic soda. Both the Foreground and
Background for LCA must therefore be expanded, as shown in Figure 6 (f).
Change of catalyst (g)
A more radical change would be to use a different catalyst
altogether. Currently the only proven alternative is sulphuric acid (see
Existing Alkylation Process and Waste Management Section) and the
trade-offs are such that there is no general preference for one process
over the other (Chapin et al., 1985; Dunham, 2005). Therefore this
options remains for longer-term consideration, probably dependent on
commercialization of one of the less hazardous processes which are being
developed.
Assessment of any new process will need to use the system shown in
Figure 6(g). Catalyst production is treated as part of the Background
because both HF and sulphuric acid are bulk commodities so that any
change in use for this alkylation plant would represent no more than a
marginal change in demand. This approach is common practice in both
"attributional" and "consequential" LCAs (see
Baumann and Tillman, 2004).
DISCUSSION
Decision Structuring
The case study shows that decisions over process modifications to
improve sustainability (or, more accurately, to reduce unsustainability)
must inevitably be taken in the face of significant uncertainties: over
future technological developments, over costs (including the costs of
waste treatment and disposal), over environmental impacts and their
relative significance and over the significance attached to social
impacts by different groups (including the media and general public).
Companies like that operating the refinery which provides the case study
for this paper already have procedures in place to make decisions over
process changes, particularly where they involve significant capital
expenditure. However, decisions involving trade-offs between
incommensurable "costs" and "benefits" with
considerable uncertainties require approaches to decision structuring
which are unfamiliar in the process sector (Mitchell et al., 2004;
Basson and Petrie, 2007; Elghali et al., 2008). The case study
introduced here is being used as one of the vehicles to explore the use
of new decision processes incorporating results from LCA.
Life Cycle Assessment
The use of LCA to evaluate the environmental performance of
processes is becoming more routine. However, LCAs are notoriously
time-consuming, requiring much detailed data collection. The pragmatic
distinction between Foreground and Background sub-systems (Figure 1) is
intended to provide a rational approach to limiting the amount of
detailed work needed. The purpose of the case study discussed in this
paper is to show how the LCA system boundaries must be tailored to the
process and modifications being considered. In general, higher level
changes require the sub-systems to be broader, but this must be
considered on a case-by-case basis. Furthermore, the level of detail
required in the process analysis does not necessarily correspond to the
breadth required of the LCA: option (a), for example, needs the simplest
LCA but a detailed process model.
Existing software packages for LCA are generally designed for
application to products and services, rather than to chemical processes.
There is therefore a recognized need for LCA packages which can be
linked readily to process simulation packages. The case study introduced
here shows why the Foreground and Background will have to be defined by
the user even when such LCA tools become generally available. They must
also be designed to show uncertainties explicitly, a feature which will
differ from conventional process simulation. The next stages in this
work will entail life cycle assessment of some of the options in Tables
2 and 3, both to assess their merits and trade-offs and to inform the
use of LCA in the company operating the refinery.
CONCLUSIONS
Assessment of the sustainability of processes requires a range of
considerations--environmental and social as well as technical and
economic--which go beyond those familiar to chemical engineers. The case
explored in this paper, cleaner production approaches to reduce fluorine
use and waste in the alkylation process at a specific UK oil refinery,
illustrates the need to broaden the issues and trade-offs considered in
assessing processing and waste management options. Furthermore, it is
necessary to extend the boundaries of the system analyzed beyond the
immediate process to consider the complete life cycles of materials and
energy, but the way in which this is best done depends on the level and
nature of the change considered. Allen's (1997) classification of
process changes provides a useful framework, and the case study shows
how it can be used to guide definition of system boundaries for life
cycle assessment of the environmental consequences of a process change.
However, the level of process change is not related to the level of
objective in the waste management hierarchy and is not a guide to the
level of details and complexity needed in process analysis. Furthermore,
the (financial) scale of a project is not a guide to the extent or
significance of its environmental consequences, underlining the need for
systematic sustainability assessment.
ACKNOWLEDGEMENTS
Neil Weston is registered on an Engineering Doctorate Programme at
the University of Surrey; financial support from the UK Engineering and
Physical Sciences Research Council (EPSRC) is gratefully acknowledged.
Manuscript received January 9, 2008; revised manuscript received
February 18, 2008; accepted for publication February 22, 2008.
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(1) For a complete exposition of LCA and its uses see, for example,
Baumann and Tillman (2004).
(2) This kind of assessment is sometimes termed a
"consequential" LCA; see, for example, Rebitzer et al. (2004).
(3) Some impurities, including SOz, [H.sub.2]S[O.sub.4],
[H.sub.2]Si[F.sub.6], and As, enter with the HF but are not discussed in
this paper.
(4) Alkylate is defluorinated when used in aviation fuel, but this
is not carried out at this particular refinery.
(5) Converted from Eastman et al. (2001) who state that HF losses
between 0.08 and 0.3 lb/bbl of alkylate have been observed on operating
units.
(6) Arsenic levels are sufficiently low that they are unlikely to
render the calcium fluoride unusable, but this must also be verified.
(7) It could also be followed by further processing of the waste as
in options (d) and (e), but in that case the principal advantage of
substituting NaOH by KOH is no longer present.
Neil Weston, [1,2] Roland Clift, [1] * Lauren Basson, Andrew Pouton
[2] and Neil White [2]
[1.] Centre for Environmental Strategy, University of Surrey,
Guildford GU2 7XH, United Kingdom
[2.] Chevron Limited, Pembroke Plant, Pembrokeshire SA71 SSJ,
United Kingdom
* Author to whom correspondence may be addressed. E-mail address:
rcliftaa sarrey.ac.ak Can. J. Chem. Eng. 86:302-315, 2008
Table 1. Routes by which fluorine is lost from the process
Source of fluorine How it is lost Unit (Fi- Approx. pro-
loss gure 2) portion (%)
HF in azeotrope with Extracted via re-run E 5
water column bottoms
Free HF Entrained in re-run E 45
column bottoms
Free HF and organic Extracted from H & I 10
fluorides propane product
Free HF & organic Extracted from K & L 15
fluorides butane product
Organic fluorides Entrained in J 20
alkylate product
Free HF and organic Relief system (Distri- 5
fluorides buted)
Table 2. Waste reduction and management alternatives and level process
of change required
Alternative Description
(a) Make operational changes Change operational parameters to reduce
generation of organic fluorides, ASO
generation and water fed to the unit
(b) Remove water Evaporate water from neutralized
solution to reduce volume of material
sent for waste disposal
(c) Make design changes (to Make structural changes to the acid
alkylation unit) regeneration section of the alkylation
unit to improve stripping efficiency in
the re-run column
(d) Feedstock to another React fluoridic waste with limestone to
process form calcium fluoride (Ca[F.sub.2]) as a
co-product. Also may enable recycling of
caustic
(e) Regenerate HF As (d) but with (Ca[F.sub.2]) co-product
acidified to regenerate HF for use in
the process or elsewhere
(f) Alternative neutraliza- Replace sodium hydroxide with potassium
tion substance (KOH) hydroxide, which produces a fluoride
(KF) with greater solubility in water
therefore allowing a greater
concentration to be used resulting in a
lower waste volume
(g) Alternative catalyst Avoid generation of fluoridic wastes by
removing HF from the unit and replacing
with an alternative catalyst
(Hommeltoft, 2003; Nieto et al., 2007)
Alternative Objective Level of change
(a) Make operational changes Reduction 1
(b) Remove water Improved 1
treatment
(c) Make design changes (to Reduction 2
alkylation unit)
(d) Feedstock to another Recycling 2
process
(e) Regenerate HF Recycling 3
(f) Alternative neutraliza- Reduction 3
tion substance (KOH)
(g) Alternative catalyst Elimination 3
Table 3. Drivers and barriers for process alternatives
Alternative Technological Economic Environmental
Drivers Barriers Drivers
(a) Make Easiest change Have to Reducing
operational to make. Offers operate within waste means
changes potential to design limits less waste
save on all of plant so disposal
types of large costs--as these
fluoridic improvements costs are
wastes may not be gradually
achievable increasing
then maybe
this will
become
profitable
(b) Remove Simple Need to find Reduces waste
water technology. space for disposal costs
Can be carried additional
out while unit equipment
is in operation
(c) Make Possible Intrusive Reducing
design modifications modifications waste means
changes (to vary in their require the less waste
alkylation complexity but unit to be disposal costs,
unit) they are all shutdown and chemical
proven in costs. Also
industry and reduces scope
feasibly for any
implemented downstream
waste
treatment
strategies
therefore
reducing their
cost
(d) Feedstock Does not Requires Eliminates
to another involve Ca[F.sub.2] to waste disposal
process additional be high enough costs and may
treatment quality for provide a
equipment further use small sales
revenue
(e) Regene- Proven HF Save money
rate HF technology is regeneration on HF imports
commercially is something and potentially
ready. Off-site that would not eliminate
facilities "normally" be waste disposal
exist which done at an oil costs. Using an
already carry refinery. May off-site
out this pro- not be large contractor
cess (e.g. HF enough flow to would mean
manufacturers). make on-site no capital
Potential to facility costs of
combine flows feasible. regeneration
with other Relies upon equipment
refiners in a being able to
common achieve good
regeneration quality
process. Ca[F.sub.2]
Modification
may be carried
out while unit
in operation
(f) Alterna- Simple Need to Compared to
tive neutra- modification establish all previously
lization already in hazards of mentioned
substance commercial modifying alternatives
(KOH) use, which equipment this is a
offers large while unit is relatively
reduction in in operation cheap way to
waste volume. reduce waste
Potentially
carried out
while unit is
in operation.
Possible
precursor to
other
alternatives
(g) Alterna- Less Only sulphuric Savings will
tive catalyst hazardous acid ([H.sub.2] result from
technologies S[O.sub.4]) has reduced
are being been proven material
developed and over long term consumption
some are industrial use and waste
already being and this brings disposal. Other
commercially its own hazards. benefits, such
applied Chapin et al. as improved
(Hommeltoft, (1985) suggest octane rating
2003; Nieto that although of the
et al., 2007). [H.sub.2]S alkylate, may
Some [O.sub.4] has occur as a
modifications benefits in result of the
are more easily terms of safety, modification
installed as an the choice therefore
additive to HF between HF providing
alkylation and [H.sub.2]S further (and
processes [O.sub.4] is perhaps more
(Wood et al., dependent on significant)
2001) many factors financial
many of which incentive
are refinery
process and
site specific.
For a
discussion on
the factors
affecting the
choice of
catalyst and
the relative
merits of the
two catalysts
see, for
example,
Chapin et al.
(1985) and
Dunham
(2005)
Alternative Social
Barriers Drivers Barriers
(a) Make Current Reducing waste at Any affect on
operational operations the source reduces alkylate
changes generally set HF, caustic and production
up to optimize water efficiency or
profitability of consumption and quality will
the unit amount of waste lead to greater
therefore any transport and demand for
changes are disposal feed products,
likely to which would
increase costs offset
or reduce environmental
value of benefits of
product waste
reduction
(b) Remove Would require Transport of dry Produces a
water capital solids reduces risk waste water
expenditure of release. stream.
which must be Reduced transport Increases
justified by emissions energy
cost savings demands
which may
offset reduced
emissions
from transport.
Still requires
disposal of
hazardous
waste to
landfill
(c) Make Some more Reducing waste at Life-cycle
design complex the source reduces emissions
changes (to modifications HF, caustic and associated
alkylation may be water with the
unit) expensive consumption and fabrication of
compared to amount of waste new
the savings transport and equipment
achieved. disposal
Delays
restarting the
unit damages
financial gains
(d) Feedstock No further Avoids sending Importing
to another economic waste to landfill, limestone may
process expense to and reduces increase
water removal emissions due to overall
technologies Ca[F.sub.2] emissions
(described production
above) elsewhere
(e) Regene- Building an Reducing caustic Importing
rate HF on-site facility imports (by limestone may
would incur recycling) may increase
capital costs. offset increased overall
Cost of emissions from emissions.
sulphuric acid limestone imports. Leads to the
imports Closes the loop generation of
for HF losses. gypsum,
Eliminates which is
hazardous waste already in
products. Using abundant
off-site supply and
regeneration could therefore
process raises become a
possibility of waste product.
partnerships with Introducing
other refineries [H.sub.2]S
achieving [O.sub.4]
synergistic brings new
benefits environmental
hazards
(f) Alterna- Would require Reduced waste so Source of KOH
tive neutra- capital less transport and may have a
lization expenditure chemical large impact on
substance which must be demands. Creates life-cycle
(KOH) justified by possibility for a emissions
cost savings combined
treatment route
with HF losses to
KOH treaters
(g) Alterna- Redesigning Complete Alternative
tive catalyst the alkylation elimination of the catalysts may
unit to operate life-cycle result in other
with an emissions related types of waste,
alternative to the current which may be
catalyst would treatment route more hazardous
incur large for fluoridic or greater in
costs, which losses. Also volume. Large
would not be remove the risks scale
recovered by of environmental modifications
the benefits of impacts from HF to the alkyla-
waste release tion unit would
prevention result in
alone associated
emissions and
current
equipment
would have to
be
decommissioned
Alternative Drivers Barriers
(a) Make Reducing waste at Refinery
operational source reduces would not
changes manual handling make any
operations and modifications
transport of that might
hazardous increase safety
material risks of the
unit
(b) Remove Transport of dry May introduce
water solids is less laborious
hazardous and solids removal
therefore more task
acceptable to the
general public
(c) Make Reducing waste at Refinery
design source reduces would not
changes (to manual handling make any
alkylation operations and modifications
unit) transport of that might
hazardous increase safety
material risks of the
unit
(d) Feedstock Does not require Same as for
to another additional water removal
process transport or option
manual handling
operations of
hazardous
materials
(e) Regene- An onsite facility Introduces
rate HF would reduce HF another highly
transport and hazardous
manual handling substance
operations (sulphuric
acid) to the
site requiring
more manual
handling
operations and
transport
(f) Alterna- Reduce manual Effectively an
tive neutra- handling end-of-pipe
lization operations and solution which
substance transport of fresh does not
(KOH) and spent caustic reduce HF
therefore reduced losses from the
safety risks and unit and
road use therefore
transport of HF
would remain
the same
(g) Alterna- Replacement with Replacing with
tive catalyst a less hazardous [H.sub.2]S[O.sub.4]
catalyst would be would increase acid
favourable to consumption
plant safety and therefore
would receive leading to
support from the increased
general public transport of
hazardous
substances by
road and
through local
towns. Despite
its hazards
there have
been relatively
few incidents
resulting from
exposure to HF
(Wood et al.,
2001)