Environmental Issues in Chemical Engineering Tutorial Questions and Solutions PDF

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Environmental Issues in Chemical Engineering Tutorial Questions and Solutions...

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Tutorial 1 – Environmental regulations and standards Q1 What does the acronym IPPC stand for (i.e., give the full name)? Briefly explain the meaning of each of these four words, in the context Industrial Emission Directives and derived national regulations. Q2 List the categories of prescribed substances under the IPPC regime, in the broadest possible sense. Then give a detailed list of the types of prescribed substances regarding emissions to water, and for each very briefly explain why their release into the environment is subject to controls. Q3 What types of conditions are included in the application for a permit to operate under the Industrial Emissions Directive and the national regulations that implement it? Q4 In an application for a permit to operate under the IPPC regime, what are the two main constraints that will ultimately determine limits for emission to air? (Hint: One describes what should be achieved, and the other what can be done) Q5 List and very briefly describe the types of Best Available Techniques that would be recommended for use on an oil refinery site for the control of SOx emission to air.

Tutorial 2 – Control of pollutants in air Q1 What are the options for controlling NOx emissions from a refinery site? Q2 How would you deal with a gas effluent rich in hydrogen sulphide? Q3 Discuss the origin of ozone as a pollutant and remedial actions. Q4 Discuss remedial action taken by the international community and industry to preserve stratospheric ozone while still allowing the availability of refrigerants and propellants for personal, commercial and industrial use. Q5 List five strategies to reduce carbon dioxide emissions, very briefly describing the mechanism that make them potentially effective and at least one obstacle or drawback.

ENVIRONMENTAL ISSUES IN CHEMICAL ENGINEERING - Tutorial 3 Q1 – a bit of practice with basic conversions and mathematical expressions a) Convert 1 ppm by volume in a gas mixture to a mol fraction b) What is the concentration in ppmv (ppm by volume) in the flue gas resulting from the combustion of coal containing 1% w/w of sulphur at an equivalence ratio of 0.9 in air? c) Remind yourself of the following expressions: [exp{x}]a = …? exp{x}exp{y} =…? ln[exp{x}] =…? 1/exp{x} = … ? exp{x+ln(a)}=…? Q2 Explain how the vertical stability of the atmosphere impacts on the dispersion in the atmosphere of pollutants emitted from a stack. In your answer, distinguish between the effects on the motion of the plume in the vicinity of the stack, and the overall dispersion of the pollutants. Q3 Sketch the vertical temperature profile (z on the vertical axis, T on the horizontal axis) corresponding to the situations described in figures 1, 2 and 3 for the dispersion of a plume on a day with fairly weak wind, making sure that you clearly label your diagrams. What is the stability class for each of these, and is it good for dispersion?

Figure 1

Figure 3

Figure 2

Tutorial 4 – Atmospheric dispersion – part 2

Q1 A sheet is provided giving some of the equations for dispersion of instantaneous and continuous atmospheric discharges. List at least two limitations of the analysis leading to these equations. Q2

5 Nm3/s of a waste stream from a natural gas processing plant is produced in an incinerator dealing with the H2S contaminated tail gas from the sulphur recovery unit. The emitted gas contains 700 ppmv SO2 (this being the maximum that BATs will let you get away with!), the balance being mostly nitrogen with some CO2 and water vapour and residual oxygen. For the purpose of the exercise, you can assume that the average molecular weight of the gas is comparable to that of air. The temperature of the emitted gas is 650 oC 1. The height of the stack is 20 m and the diameter of its tip is 1.5 m. For modelling the plume, you may assume that the wind speed is 2 m/s, the ambient temperature 10 oC and the environmental temperature gradient of the atmosphere is about  0.8 oC / 100m (negative). Use Holland’s correlation to estimate the effective height of dispersion. a) What is the maximum concentration of SO2 on the ground? Is the AQS of 175 g/Nm3 breached anywhere on the ground? b) If the combustor somehow failed to ignite or its flames went off (not a good place to be at all!), what would be the consequences regarding the nature and concentration of the most significant pollutant at ground level, and the hazards in the vicinity of the combustor and further away? Support your answer with figures. (Hints: you can assume to a good approximation that the total number of moles of gas emitted is unchanged. If relevant, you may also refer to data from lecture 10 or other reliable source to help with your argument).

1

The operator is not keen on recovering the heat, because the combustor is also used for disposing of sudden releases from relief valves as well as purge streams from start-ups or shut-downs of sections of the plants. Unobstructed operation is preferable!

Tutorial 5 – Atmospheric dispersion – part 3 Q1 The gas stream from an HCl scrubber consists of 0.5 m3s-1 of air at a temperature of 288 K and a pressure of 1.013 x 105 Nm-2. Its residual volume fraction of HCl is 0.001. It is discharged from a stack 60m high into a wind of speed 1.0 ms-1.

i.

The factory fence is 300m distant from the stack in a downwind direction. Find the ground-level HCl concentration at this point and compare it with a permitted maximum of 100 ppb (by volume).

ii.

Show that the point of maximum ground-level HCl concentration is about 1500 m downwind of the stack and that the HCl concentration at this point is about 30 ppb.

iii.

During an incident involving a malfunction of the flow control for the water supply line to the scrubber, the concentration of HCl in the gas stream reaches 1% vol for twelve hours, in air at a total flowrate of 0.5 m3s-1. a) Draw a ground level contour for the 100 ppb limit if treating the release as continuous. Comment. b) How long will it take for the toxic load at the position found in question ii) to reach the SLOT for HCl? Please comment.

Data: R = 8314 J kmol-1 K-1 MW of HCl = 36.5 kg kmol-1 SLOT for HCl is 2.37·104 ppm·min

Environmental Issues in Chemical Engineering Tutorial 6 Q1 Define the terms Biological Oxygen Demand and Chemical Oxygen Demand. How is COD related to the concentration of a pollutant? Why is BOD never greater than COD? What situation is indicated if the COD measured is more than twice the BOD?

Q2 (We’ll skip this during the tutorial, but please practice in your own time) Discuss five of the main factors affecting water quality in an industrial environment, giving in each case the most common sources of the pollution and possible means of treatment. Q3 2500 kg/day of BOD is released into a watercourse with a mean cross-section for water flow of 10 m2 and a mean water velocity of 0.15 m s-1. (a) Use the Streeter-Phelps oxygen sag model to predict the dissolved oxygen concentration 10 km downstream of the discharge if its value upstream of the discharge point was 6.0 mg/litre. (b) At what downstream point would the oxygen level in the water be a minimum, and what would this value be? (c) Would that value be of concern, and why?

Streeter-Phelps Oxygen Sag Model Please mind the meaning of D in this context. D

k1 BOD 0  k 1t e  e  k 2t k 2  k1





Take the values of k1 and k2 as 0.2 day-1 and 0.3 day-1, respectively.

Environmental Issues in Chemical Engineering Tutorial 7 Q1 An effluent is discharged from a pharmaceutical plant that has an isopropanol content of 20 ppm by weight. Estimate the COD contributed by that pollutant.

Q2 You are designing a chemical plant that will release a compound A into a water effluent. What factors and sources of information would you take into consideration when proposing a specification for the maximum concentration of A into the effluent? The specification will be used in an application for a permit to operate made to SEPA.

Tutorial 8 – Activated Sludge Digester

Q1 A chemical plant produces an effluent with flow rate F = 4000 m3 day-1, BOD so = 500 mg l-1. It is required to reduce this BOD by 95% before discharge into a nearby river. (i)

Write a material balance for the sludge around the aerator. Use it to estimate the required volume, V, of the aerator, given the following: - A concentration ratio K of 3 for the clarifier - A recycle ratio  = 0.3 - A sludge age s = 5 days Some definitions: K = Xr / Xa = 3, where Xa is the concentration of sludge in the aerator and Xr that in the recycled sludge.  s is defined as Xa /rX, with rX = net growth rate per unit volume after discounting the cell death rate.

(ii)

The yield of biomass with respect to substrate, Y, is defined as 𝑌 =

𝜇𝑋𝑎 −𝑟𝑆

, in which the

numerator is the actual growth rate in the digester (mg biomass / l / day); rS is the rate of BOD consumption (mg/l/day); and the specific growth rate is  = (1/s + ke). You are given Y = 0.4 and ke = 0.03 day-1 (specific death rate constant). Write a steady state material balance around the whole plant on the BOD, and use it to find the value of Xa. (iii)

What is the flowrate of sludge pumped out of the process (‘wasted sludge’), and the equivalent mass of dry sludge produced per day? You are given Xe = 30 mg ml-1.

Q2 Comment on the pros and cons of the following options for denitrification in an activated sludge plant: -

Non-aerated zone at the start of the aerobic digestion process Non-aerated zone at the end of the aerobic digestion process

How do you get the best of both configurations, and why does it work?

Environmental Issues in Chemical Engineering 3 - Tutorial No. 9

Q1 The following figure refers to an USBR anaerobic digester, for which the specifications and performance at its rated operating conditions (1 atm, 20oC) are described here:  

   

The influent has a flowrate F of 800 m3/day, with so = 500 mg/L of BOD. The treated water must contain less than 25 mg/L of BOD (it is then recycled back into a primary settler followed by activated sludge treatment). We assume Xe = 0. The sludge out at the bottom of the digester has concentration Xws = 40 g/L The BOD load rate is 1800 mg / L/ day. The sludge age Xa / rX is 45 days. The net yield of biomass against BOD consumption is Yobs = rX / (–rS) = 0.08 POWER

HEAT

Engine + Generator

Biogas G

F, so Xo = 0

USBR V sa Xa

(1-) F, sa Xe= 0 F, sws = sa Xws=40

a) Find the volume V of the digester, as well as the rate of sludge withdrawal  F and the concentration of sludge in the bioreactor Xa. b) What would be the impact of a change in operating temperature on the performance of an existing USBR anaerobic digester, or its required volume?

Q2 List the options available when dealing with solid wastes, by order of preference. Q3 What options are available for minimising harmful emissions from incinerators?

Tutorial 1 – Environmental regulations and standards - SOLUTIONS Q1 What does the acronym IPPC stand for (i.e., give the full name)? Briefly explain the meaning of each of these four words, in the context Industrial Emission Directives and derived national regulations. Solutions: IPPC = Integrated Pollution Prevention and Control ‘Integrated as taking a holistic approach, on several levels. For example, not just looking at one medium separately (e.g. air or) but all of them together (air, water, solid wastes) and also considering cross-medium pollution effects (e.g. transferring pollution from air to water). Also several points covered by the next keywords. Also includes all aspect related to life cycle (e.g. energy efficiency, water consumption, maintenance, decommissioning, regular reviews) ‘Pollution’ covers not just polluting substances, but also noise, heat and vibrations, even light. ‘Prevention’ includes avoiding processes that pollute, and when not possible looking at minimising the amounts involved, and reusing, recycling, and recovering in that order, and as a last resort mitigate (with emission limits). ‘Control’, again in the broadest sense with a very holistic approach. It includes: environmental management systems; methods of measurement (including instrumentation and frequency of sampling) and level of monitoring (where?); training of personnel; incident and accident reporting; safe operating procedures, including for start-ups and shutdowns; calendar of inspection and reviews, accident prevention (link with safety), improvement plans… as well as modelling of consequences of accidental releases (recall that some of the data gathered as part of an application to the Health and Safety Executive may be used in your application for a permit)

Q2 List the categories of prescribed substances under the IPPC regime, in the broadest possible sense. Then give a detailed list of the types of prescribed substances regarding emissions to water, and for each very briefly explain why their release into the environment is subject to controls. Chemicals, but also noise, heat and vibrations, even light. In water, the following chemicals are prescribed: •

• •

Organohalogen compounds and compounds that are organohalogen precursors in water (e.g. bleach). Many of these are known carcinogenic or PoPs (see below). Organophosphorus and organotin compounds: Generally harmful to animals as they Attach the nervous system. Substances that are carcinogenic or mutagenic or proven to affect reproduction in or via the aquatic environment

Tutorial 1 – Environmental regulations and standards - SOLUTIONS •

• • • •

•

Persistent hydrocarbons and persistent and bioaccumulable organic toxic substances (‘POPs’). Non-biodegradable and fat soluble and often potentially harmful, they tend to accumulate up the food chain up, and as a result we consume them when we eat. Cyanides; metals; Arsenic; and the compounds of these. Poisonous. Biocides and plant health products. May disrupt ecosystems. Materials in suspension. May obstruct daylight in aquatic ecosystems, thus killing off plants and the animals that depend on them. Eutrophication agents (esp. nitrates and phosphates): stimulate unsustainable growth of plants, leading to depletion of dissolved oxygen (respiration at night!) and subsequent fish kill etc. Substances which compromise the oxygen balance (and can be measured using BOD, COD, etc.). for the reason outlined above for eutrophication agents.

Missing from this list given in your note but explicitly mentioned in the IED were also all the substances listed in Annex X of the Water Framework Directive (you won’t find it in the Directive itself, as it took a while to get agreed, but in the Annex II of Directive 2008/105/EC, which lists a number of organic substances and heavy metals that are known environmental pollutants. However, it seems that most if not all of the substances included in this Annex X already are within the more general list given above. (Full title is DIRECTIVE 2008/105/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 16 December 2008 on environmental quality standards in the field of water policy, amending and subsequently repealing Council Directives 82/176/EEC, 83/513/EEC, 84/156/EEC, 84/491/EEC, 86/280/EEC and amending Directive 2000/60/EC of the European Parliament and of the Council) During the tutorial on water treatment a couple of years ago, someone asked, “where is ammonia?” Incredibly, nowhere to be seen either in the IED list of prescribed substances or Annex X. However, it can be considered to be an eutrophication agent since it gets oxidised to nitrate, and it has a direct effect on BOD by getting reduced to nitrates (in fact it used to be that nitrification affected the reliability of BOD measurements before improved techniques were devised in the 1960’s. (Siddiqi R.H., Speece R.E., Engelbrecht R.S. and Schmidt J.W., Journal (Water Pollution Control Federation), Vol. 39 (4), pp. 579-589, 1967). And there are standards for its concentration in water, e.g. as laid out in The Scotland River Basin District Directions 2009.

Q3 What types of conditions are included in the application for a permit to operate under the Industrial Emissions Directive and the national regulations that implement it? Permits are issued with conditions, including • • •

Emission limits Methods of measurement, and level of environmental monitoring Training of personnel

Tutorial 1 – Environmental regulations and standards - SOLUTIONS • • • • • • • •

Maintenance Abnormal occurrences Accident prevention Reporting Improvement plans. Variations can be initiated by either party, with right of appeal. Regular reviews are specified. End of life decommissioning

Q4 In an application for a permit to operate under the IPPC regime, what are the two main constraints that will ultimately determine limits for emission to air? (Hint: One describes what should be achieved, and the other what can be done) These factors are -

-

Limits that are explicitly sets by the IED or other derived or relevant regulations, and the current state of the local environment with respect to established Environmental Quality Standards (and sometimes also emission targets that were agreed at national level, e.g. for air pollutants like acid gases) The use of Best Available Techniques

Q5 List and very briefly describe the types of Best Available Techniques that would be recommended for use on an oil refinery site for the control of SOx emission to air. On an oil refinery, sources of emissions of sulphur will include: - the energy systems (typically, the burning of residues, raw gases, natural gas etc. to provide power and steam, most likely in Combined Heat and Power (CHP) plants), - gases produced during refining, which will contain hydrogen sulphide and light organosulphur compounds (e.g. from desulphurization, catalytic cracking, visbreaking and coking). BATs to deal with these will include For gases produced during refining, typically, H2S sent to Sulphur Recovery Unit (SRU), e.g. multistage Claus plant, followed by further tail gas treatment (e.g. absorption). This will enable recovery of sulphur > 99.5%. Waste gas is then incinerated, resulting in some emission of SO2, which (depending on its concentration) may require scrubbing in the same manner as gases form combustion processes. For combustion processes: “End of pipe” Flue-Gas Desulphurization (FGD) relying on absorption using e.g. CaCO3 or Ca(OH)2 slurry (either wet, or semi-dry), to treat the flue gases from CHPs and other off-gases. The byproduct of this is gypsum (calcium sulphate). “Wet’ FGD processes are comparable to traditional absorption processes, and tend to first produce CaSO3 dissolved in water if pH is controlled and kept acidic enough (depending on the design, it may be important to prevent uncontrolled precipitation of

Tutorial 1 – Environmental regulations and standards - SOLUTIONS the sulphite in the upstream part of the process, and this may require the use of a buffering acid, e.g. formic acid). Usually, aeration is then required to oxidize the CaSO3 to CaSO4 (gypsum) which readily precipitates. “Spray” (or “semi-dry”) FGD processes does not try to avoid the premature precipitation of the sulphite. Instead by spraying a concentrated solution of lime, the liquid inventories are kept low, the use of buffer acid is avoided and there is no need to dispose of a waste liquid stream. It may also be possible to use “Dry” sorbent FGD, especially if the fuel is a solid add especially if the furnace is a fluidized bed. In this case, the CaCO3 or MgCO3 sorbent is blended with the pulverised fuel and the reaction occurs during combustion. The sulphat...