Report - 08.03.07

					            Environmental Engineering Project


              Supervisor: Prof. Andrew Livingston


                            Group M




                            Contents

            Executive Summary     -   Page 1
             Problem Definition   -   Page 1
Environmental Effect Assessment   -   Page 2
                       Option 1   -   Page
                       Option 2   -   Page
                       Option 3   -   Page
                       Option 4   -   Page
                     References   -   Page
Executive Summary:
The main aim of this project was to perform a detailed analysis of the best available technologies
(BAT) for minimizing the levels of pollution emitted in a chemical process involving the
manufacture of methyl benzophenone. Following a risk assessment based on model world
calculations it was possible to choose amongst four different treatment processes, namely
Aerobic Treatment and Sludge Incineration, Adsorption Treatment, Air Stripping and Flue Gas
Desulphurization using a Spray Tower Wet Scrubbing process.
        The aerobic treatment of the toluene involves injecting oxygen into the wastewater
causing the toluene to be broken down into carbon dioxide, water and activated sludge. This is a
cost effective process with a high amount of toluene being removed. The disadvantages include
the sludge discharge, which can be removed by incineration, and the small amount of CO2
produced. The adsorption treatment involves toluene being removed by activated carbon by
adsorption. It is the most common method used and is highly efficient. It is however more costly
than other methods and the left over carbon can cause waste problems. Steam stripping is
another option for removing toluene. The contaminants of the wastewater are more soluble in
steam and therefore are transferred from the wastewater. The efficiency of this method is over
99% and it also allows toluene to be recycled back to the reactor. The final method allows the
treatment of the SO2 in the waste gas stream by Flue gas desulphurisation. Using limestone,
oxidisation occurs and gypsum and CO2 are produced. A higher efficiency can be reached if a
wet scrubbing process using spray towers is applied. The performance rate can reach up to 99%
and it is overall an effective way of managing the SO2 produced. These four methods provide
effective treatment of the waste.

Problem Definition:
        The objective of this project is to conduct an evaluation of the best available techniques
for controlling pollution in a chemical process. The process in question is the production of
methyl benzophenone from the reaction of toluene with benzoyl chloride. The problem
considers two separate reaction paths for the formation of methyl benzophenone; the different
types of pollution being emitted will different environmental impacts. In both production
methods there is a total batch production of 5.88 tons of methylbenzophenone based on a 100%
yield on benzoyl chloride. The plant will produce 500 batches a year and will operate for 7500
hours every year.
        The first of these two reaction paths uses a traditional batch process. It involves the use
of aluminium trichloride in a Friedel-Crafts acylation reaction and produces some waste products
in addition to the desired product. A by-product of the reaction is in fact gaseous HCl. This can
be absorbed into water and implemented elsewhere in the plant so it is not necessary to assess the
environmental risk associated to the formation of this. The other waste product from this path is
the AlCl3 which will be de-complexed into water at a concentration of 30wt% where it will
contain some of the excess toluene not utilized in the reaction. This toluene has a concentration
of 500mg.L-1 of AlCl3 solution. This contaminated AlCl3 stream is trickled into the site
wastewater and fed into a nearby river estuary. The levels of toluene in our AlCl3 solution are
too high and it has been recommended that process modifications should be implemented to
reduce the level of toluene in this stream. If this reaction path were to be used, the stream would
need to be treated so that the toluene levels were no larger than 1mg.L-1.
        An alternative production path is to use an alternative catalyst in the reaction instead of
the AlCl3 based Friedel-Crafts acylation reaction. Although, this catalysed reaction uses the


                                               -1-
same products and similar reaction condition as the first option outlined above, there are no
traces of toluene in the product stream. In this reaction path, the catalyst would need to be
regenerated, at a negligible cost. The regeneration process produces a nitrogen stream of
300m3.h-1 containing 20g.m-3 of SO2 and would be released as a flue gas. However, the level of
sulphur dioxide released is far too high and if the process was to be applied, the level1 would
need to be reduced to 50μg.m-3.
         Following an initial risk assessment, it was possible to choose four waste treatment
options that are best suited to the process based on initial calculations on the fate of the pollution,
the severity of the pollutant and the financial cost to IC Chemicals. Three options will focus on
the first reaction path and thus on the wastewater treatment of toluene in the 33 wt % solution of
AlCl3. The remaining option will focus on reducing the amount of SO2 to the recommended level
by designing a flue-gas desulphurization treatment. The four options will then be narrowed
down to a detailed analysis on the two best techniques, finally deciding on the optimum reaction
path for the process and BAT (Best Available Technology) for controlling the plant pollution
based on a comparison between treatment cost and environmental benefits.

Environmental Effect Assessment:
Model World Analysis:
        The risk due to the release of chemicals into the environment can be assessed by using
unit world models, which assume that chemicals are completely non-reactive and accumulate in
the environment. They may, however, be broken down to less harmful species. This model
estimates how chemicals are distributed within the environment among the air, water and biota
phases.
        There are two methods available for this assessment. The first method is the Level I
analysis which considers a fixed quantity of a chemical emitted into the model world and its
partitioning between phases at steady state. The second method is the Level II analysis and this
considers continuous emissions of chemicals to the model world and rates at which chemicals are
removed from the model world by transport (advection) and decay (reaction).
        The problem definition states that a solution containing 500 mg L-1 of toluene is currently
                                                                                3 -1
being trickled into the site waste water with an overall flowrate of 1000 m d which is in turn
discharged into the nearby river estuary. The total flowrate of toluene discharged into the
environment was calculated to be 10.64kg d-1. The model world calculations assume that the soil
is inert and equilibrium between phases is instantaneous. As there is a continuous emission of
toluene into the model world, Level II analysis is chosen for the fate calculation.
        The equilibrium concentrations of toluene (in mol m-3) for air (Cg), water (Cl), and biota
(Cb) phases can be calculated using the following equations:

         H i Cli
C gi            Eqn. 1 ;             Cbi  BCFi   b  Cli Eqn 2;
          RT
                                 
                                 mi
C li                                                             Eqn 3.
       Vl / t l  k lVl  V g H i / t g RT  k giV g H i / RT
          
where m i is the molar flow rate of component i ; H i is the Henry’s Law constant (Pa.m3mol-1)




                                                            -2-
                           BCFi is the bioconcentration factor of component i (L.kg-1) ;  b is the density of biota
                          (kg.m-3) ; Vi is the volume of phase i (m3) ; k ij is the decay rate of the component j in
                          phase i ; R is the ideal gas law constant ; T is the temperature (K)

Using all the data given in the problem statement, the results are tabulated as follows,
                          Table 1, equilibrium concentrations of toluene in each phase
                            mol m-3       Cl               Ca                 Cb
                            Toluene       1.28x10-9        3.49x10-10         1.16x10-7

In the alternative reaction mechanism, the catalyst must be regenerated. This process produces a
                                                                -3
nitrogen stream containing SO2 at a concentration of 20 g m . Although sulphur dioxide is
considered to be a harmful chemical within the environment, no model world calculations can be
performed as there is no accumulation in any phase because it rapidly reacts to form acid rain.

The exposure of this chemical to humans is assessed by the chronic daily intake and is calculated
by the following equation:
                                              Total dose per day(mg  d 1 )
                chronic daily i ntake  CDI                                            Eqn. 4
                                                    Body weight(kg)
The concentration of chemicals in the model world compartment is known, and the EPA-
recommended standard values for the daily intake are also known. Therefore, CDIs are
calculated and tabulated as below:

                          Table 2, Chronic Daily Intake for different intake route
                            mg kg-1 d-1       Air Breath        Water Ingested               Fish Consumed          Total
                            Toluene           9.16x10-7         3.37x10-9                    8.88x10-8              9.26x10-6

The value calculated above for toluene consumptions is much lower than the acceptable daily
intake, RfD, for toluene, which is 0.3 mg kg-1 d-1. Hence, it can be considered to be of minimal
risk in causing acute effects in humans.

Plume Modelling:
                                                                                           This diagram represents a plume model
                        3.0E+04
                                                                                          for the concentration of SO2 released
                                                                                          from a flue gas stack for different stack
                        2.5E+04
 -1




                                                                                          heights. Plume modeling enables an
 Concentration / µg L




                        2.0E+04
                                                                                          analysis of ground level concentrations
                                                                      150 m               of the pollutant from a distance x from
                                                                      200 m
                        1.5E+04                                       250 m               the stack from which it has been
                                                                      300 m               released. For the calculations made, it
                        1.0E+04                                                           was assumed that there was a surface
                                                                                          wind velocity of 5 ms-1 in a slight day
                        5.0E+03                                                           solar insolation (class C for slightly
                                                                                          unstable atmosphere). The graph shows
                        0.0E+00                                                           that it is not permissible to release this
                                  0       5           10         15             20

                                      Distance from Base of Stack / km
                                                                              -3-
        level of SO2 even using a 300m stack, thus a flue-gas treatment would be mandatory.
Fig 1. Graph representing the concentration of SO2 a ground level for different stack heights



Waste Water Purification:
                Option 1 – Aerobic Treatment & Sludge Incineration:
Process Description: Toluene is soluble in water and the ratio of the BOD and COD values
(1.2) for toluene show that the compound was well degradable1, making aerobic waste treatment
a suitable solution for minimizing waste from this path. Firstly, the waste solution will need to
be treated with calcium carbonate (CaCO3) to neutralize the solution and dispose of the AlCl3
toxins in the stream.
                                                    Process Schematics:
                                                    Aerobic waste water treatment involves the
                                                    injection of oxygen, in the form of air or pure
                                                    oxygen, into the waste water stream so that, in
                                                    the presence of the dissolved oxygen, the
                                                    toluene in the stream will break down into
                                                    carbon dioxide, water and activated sludge. As
                                                    one of the most common aerobic biological
                                                    treatment techniques, the complete-mix
                                        activated sludge process will be used for the stream
Fig. 2. Tower biology schematic         treatment and due to the large degradation efficiency
needed a tower biology (see Figure 1) will be used for the aeration chamber. In this tower
biology the waste water stream and the air/oxygen stream are injected through the bottom of the
tower. The CO2 formed exits through the top of the tower and the purified effluent and excess
sludge exit through the side of the tower. The excess sludge from the aerobic treatment will then
be incinerated using a fluidized-bed incinerator where the sludge will be incinerated at a
temperature of 760-820°C and a pressure of 20-35kPa.
Process Cost: Aerobic waste treatment is a relatively cost effective method with an initial
instillation capital cost of FIM 15-20 million(2) which equates to $3.27-4.36 million using present
day exchange rates(3). A rough estimate for the annual operating cost is 2% of the capital cost(2)
therefore including a annual cost for CaCO3 of $265000 based on a per pound cost of $0.0535(4)
the total annual operating cost will be $330000-352000. Compared with the aerobic treatment
process the cost of the incineration is negligible.
Environmental Impacts: Both the neutralization of the AlCl3 using CaCO3 and the aerobic
treatment itself give off a CO2 byproduct, an analysis of the levels of CO2 emitted will follow.
The main by-product from the aerobic treatment is the activated sludge which, when incinerated,
may give off gaseous emissions and odours that may need to treated before release into the
environment.
 Table 3: Advantages and disadvantages of aerobic treatment/incineration process

 Process         Advantages                                                    Disadvantages
 CaCO3           Efficient method of neutralization                           CO2 by-product
 neutralization  Cheaper than using NaOH
 Aerobic waste  Cost effective method of waster treatment                     Large sludge discharge



                                                    -4-
 treatment         Achieves high degradation efficiency
 Incineration      Cheap method of sludge disposal removing                Possible gaseous emissions
                    aerobic disadvantage                                     and odours from sludge

                                 Option 2- Adsorption Treatment

                                                          Process Description: Adsorption is a
   500mgL-1                                               separation process involving a sparingly
   Toluene                                                soluble material adsorbing to a solid surface
   solution
                                                          of activated carbon (1a). This process
                                                          allows for the adsorption of small amounts
                                                          of solute (1a), which is useful for the
                       ADSORPTION                         removal of toluene from the AlCl3 solution,
                          COLUMN
                                                          as the toluene content in the solution is very
                         With beds of
                       activated carbon                   small, namely 500 mg L-1.                More
                                                          specifically, Granular Activated Carbon
                                                          (GAC) can be used to remove organic
                                                          contaminants (2a) and is the most
                                                          commonly used in water treatment options
                                             <1mgL-1      of adsorption (3a). Granular Solid Filters
                                             Toluene      are included upstream of the GAC to ensure
                                             solution     removal of any possible solids present after
Figure 3. Adsorption Column for Removal of Toluene       adsorption (3a).      A schematic of the
adsorption process is included in Fig XXX.

Table 4. Advantages and disadvantages of the adsorption process [2a[
Advantages                   Disadvantages
High removal efficiency      Activated carbon would need to be thermally
Requires low equipment space regenerated after saturation, which would require a
                             high amount of energy for temperatures of up to 750-
                             1000oC.
Use of automated systems     Disposal of the carbon after it can no longer be
                             regenerated would also cause a waste problem as it
                             would need to be incinerated
Toluene adsorbed to the      This process is relatively more expensive compared
activated can be recovered.  to other separation processes

Process Cost: For flowrates of up to 14m3/hr, the capital cost of installing a column is £30,000,
however the stream flow rate of the defined process is below this, so the installation cost will be
marginally less. The operating costs which include the cost of the GAC and its regeneration is
£3400 per tonne of GAC, where the plant would be designed to need regeneration biannually.
However the basic cost of regeneration is between £1000-2000 (2a).
Environmental Impacts: The major environmental impact involved in GAC adsorption is the
regeneration of the GAC, which releases a series of waste chemicals into the air and/or water




                                                   -5-
which would in turn need to be further treated. Furthermore, once the GAC can no longer be
regenerated, it must be disposed of as chemical waste or incinerated (2a).




Option 3- Steam Stripping
Process Description: The principle of this process is to bring the waste water in contact with a
high flowrate of steam in order to remove volatile contaminants, using the fact that the
contaminants are more soluble in steam than water (higher temperature). Air can also be used in
stripping processes, however in this case, as it is required to reduce the concentration of toluene
in the waste water stream to 1mg L-1, the more efficient steam stripping is preferred. The
following diagram shows the basic structure of a steam stripping process.
Process Schematics:
      Waste Water out
                                                                                              Waste Water
                                                                                              (with some
                                                                                             contaminant)
                                                                                             recycled back
                                                    Stripping                                to the column
                 Heat                               Column          Condenser
               Recovery                             (usually
                                                     packed
 Waste                                              column)
 Water in                                                                Phase
                                                                        Separator




                                                                               Contaminant



                                                                Steam


Figx [2]: General flow diagram of a steam stripping process

Environmental Impacts: The contaminant in the case of this project is toluene, a valuable
reactant, therefore it should be recycled back to the reactor and this is in fact an advantage of
steam stripping. Other advantages include high removal efficiency (typically over 99%) and no
extra contaminant or waste gas introduced. Some issues that stripping columns would have are
fouling in the column and the cleaning associated. Also, if the contaminant is not to be recycled,
it will have to be further treated.

Process Cost: Typical capital cost for a column which allows 50m3 of waste water flowrate (into
the column) is $400,000-600,000 [2] and operating cost of about $60 per m3 [2] of water treated.
The operating cost would mainly come from the steam. Detailed costs and design are pending on
further analysis.



                                                        -6-
Waste Gas Purification
Option 4 - FGD: Limestone with Forced Oxidation using Spray Tower Wet Scrubbers

                                                                    Process Description: Water
                                                                    slurry     (of     approx.10%
                                                                    limestone) is used in the
                                                                    LSFO       process1.    It    is
                                                                    introduced into the gas stream
                                                                    via a series of spray nozzles
                                                                    located at the top of the spray
                                                                    tower. The gaseous mixture of
                                                                    N2/SO2 enters near the bottom
                                                                    and flows counter-current to
                                                                    the liquid. The calcium
                                                                    sulphite slurry is 100%
                                                                    oxidized      following     the
                                                                    addition       of      bubbling
                                                                    compressed air1 (see Eqn 1).
                                                                    This forms calcium sulphate
                                                                    (or gypsum) which is treated
Figure 4. A schematic diagram of the FGD treatment process

further to form crystals and the high gypsum content allows for the disposal of de-watered waste
without further treatments. Gypsum has commercial value and this should be considered in the
overall economics of this treatment process.

                SO2 + CaCO3 + 1/2O2 + 2H2O = CaSO4.2H2O + CO2               Eqn. 1

Spray towers offer the best design against the problem of plugging2, a major drawback with most
other wet scrubbing applications. While its simple and open structure prevents the slurry from
plugging the nozzles of the tower, other wet scrubbers such as impingement plate scrubbers are
frequently subject to plugging in the small holes of their plates. Spray towers are also designed to
provide a sufficiently high residence time, so that enough time is allowed for the SO2 gas
particles to absorb into the water slurry. These towers experience lower pressure drops than other
wet scrubbers which implies lower operating costs and easier maintenance. This treatment is
known to have a performance rate of 99%3. Generally, spray towers require lower capital costs
as well as relatively low energy consumption. Disadvantages of this treatment include that it has
a lower mass transfer efficiency which is not believed to be a major problem for the removal of
highly soluble gases.

Process Cost: Spray towers have the lowest capital and annual operating costs which amount to
about $1300 to $30,300 per 1000 Nm3/hr. 3



                                                 -7-
Environmental Impacts: The residue in this FGD process is gypsum which has a high purity of
>99%3 and can be directly sold in the commodities market. It would be necessary to monitor the
emissions of CO2 as well as ensuring that any release of unreacted SO2 is limited to 50 μm m-3.4
References:

Chris
(1) -   IPPC H1 – Environmental Assessment and Appraisal of BAT – Environment         Agency –
        Version 6 July 2003
(2) -
(2) - Reference Document on Best Available Technique in Common Waste Water and Waste
      Gas Treatment February 2003 – European Commission – Page 146
(3) - www.ratesfx.com/rates/rates-converter.html -
(4) - www.micronmetals.com/calcium_carbonate.html

References for the Waste Gas:

(1) - http://www.worldbank.org/html/fpd/em/power/EA/mitigatn/aqsowet.stm
(2) - http://www.epa.gov/eogapti1/module6/sulfur/control/control.htm#1
(3) - IPPC – Reference Document on Best Available Techniques in Common Waste Water and
       Waste Gas Treatment / Management Systems in the Chemical Sector – February 2003
(4) - IPPC H1 - Environmental Assessment and Appraisal of BAT – Environment Agency –
       Version 6 July 2003


References for Steam Stripping

(1) - Jose L Bravo, Design Steam Strippers for Water Treatment, Chemical Engineering
Progress, December 1994, AlChE publication
(2) - Prof. Andrew Livingston, Environmental Engineering Lecture Notes, 2006-2007
(3) - Reference Document on Best Available Technique in Common Waste Water and Waste
Gas Treatment February 2003 – European Commission – pp 158

References for Adsorption

(1a) – Prof. Andrew Livingston, Environmental Engineering Lecture Notes, 2006-2007, pp 83
(2a) - Reference Document on Best Available Technique in Common Waste Water and Waste
Gas Treatment February 2003 – European Commission – pp 110-115
(3a) - Mark H. Stenzel, Remove Organics by Activated Carbon Adsorption, Chemical
        Engineering Progress, April 1993, AlChE publication




                                             -8-

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:7
posted:12/1/2011
language:English
pages:9