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Question 3 (2015)

Figure 3-a shows an organic chemical production process. A large amount of water is consumed as reactor
feed, as well as washing water (in washer 1 and 2). The dissolved solid (DS) content is taken as the main
characteristic in evaluating the water reuse/recycle opportunity within the process. The limiting flowrates and
DS concentrations (ppm) are given in Figure 3-a. The initial water source is a high grade (HG) demineralized
water at 10 ppm.

Figure 3-a: Organic chemical production

a. From Figure 3-a, extract the limiting water flow rates (t/hr) and concentrations (ppm), noting any water losses
or water gain throughout the process. (4 marks)
Operation Process Mass Load CIN (ppm) COUT (ppm) Limiting flow (t/hr)
(g/hr)
1 Water Feed 600 60 100 15

2 Wash Water 1 1000 50 150 10

3 Wash Water 2 720 10 100 8

4 (gain) Dryer WW - - 55 4
(gain)

b. Draw the water composite curve on the graph paper provided. Assuming that HG demin water is the only water
source available in the plant, target the minimum HG demin water supply (t/hr) and state the pinch load (g/hr)
and pinch concentration (ppm). Draw the water supply line (considering water losses and gains, if any) on the
composite curve. (5 marks)
Pinch load, mPINCH = 1820 g/hr
Pinch conc, CPINCH = 100 ppm
HG demin water at 10 ppm, FHG = 18.22 t/hr

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 160

140

120

100 1820, 100

80

60
819.9, 55

40

20
0, 10
0
0 500 1000 1500 2000 2500
Composite Curve Water Suppply Line

*Note*
Slope of water supply line for interval 10-55ppm = 18.22
Slope of water supply line for interval 55-95ppm = (18.22 + 4) = 22.22 (due to water gain)

c. HG demin water is actually produced from a water treatment plant. The water treatment plant is shown in Figure
3-c below. Town water (TW) is supplied at 500 ppm, which undergoes a two stage reverse osmosis process,
producing low grade (LG) at 50 ppm at the first stage and HG demin water at the 2nd stage. Based on Figure 3-
c, there is the option of using TW and/or LG demin water as alternative water sources.
LG demin water
Town water HG demin water
50 ppm Reverse Osmosis 10 ppm
500 ppm Reverse Osmosis
Stage 1 Stage 2

LG demin water reject HG demin water reject

Figure 3-c: Water treatment plant

Determine which alternative water source(s) (TW and/or LG) are viable for the chemical plant to help in further reducing
the HG demin water flowrate. Explain your answer.
Only LG and HG demin water viable. TW at 500 ppm is above pinch (not viable)

Given the utility costs for TW, LG demin water and HG demin water are $0.10/ton, $0.3/ton and $0.4/ton. Target the
lowest cost supply of the water to the process based on your answers to parts b and c.
(4 marks)

Conclusion: Using single source (HG only) is more cost effective ($7.29/hr) compared to a mixture of sources (LG
+ HG) ($8.72/hr)

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 d. Design a water network for the organic chemical plant that achieves the lowest cost obtained in part (c).
(5 marks)
Most cost effective network utilizes HG water only (there are more than one possible design)
FHG = 18.22t/hr 30.8ppm
10 ppm 100 ppm 150 ppm
F = 4.66t/hr (10ppm)
F = 8.66t/hr
1

F = 10t/hr
2
F = 5.55t/hr
F = 4.45t/hr

F = 8t/hr
3

F = 4t/hr
55 ppm
WW = 8.21t/hr WW = 10t/hr

e. There are two main types of water using unit operations known as either fixed load or fixed flowrate type. Choose
the correct statement (2 marks)
(i) For a fixed flowrate type unit operation, the inlet and outlet flowrates should be equal.
(ii) For a fixed load type unit operation, we could feed the unit with flowrate less than or equal to the
limiting flowrate. (correct)
(iii) Cooling tower make-up and boiler blowdown are examples of fixed load type unit operations.
(iv) Limiting water profile is a useful tool to target the minimum fresh water requirement for a process plant.
It uses the same targeting steps for both fixed load and fixed flowrate unit operation.

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Question 4 (2015)

An industrial plant generates the following effluent streams as shown in Table 4. Each effluent stream would
require the removal of dissolved solids (DS) prior to discharge. A wastewater treatment plant (WWTP) is to
be set up to treat the effluent streams. The DS discharge limit is set at 20 ppm.

Table 4: Effluent data for an industrial plant

Stream Effluent Concentration (ppm) Flow rate (t/hr)
No.
1 Low grade demin water reject 216.65 11.71
2 High grade demin water reject 143 3.43
3 Wastewater 1 95 17.89
4 Waste wash water 2 150 10

a. Draw the limiting effluent composite curve on the graph paper provided indicating clearly the pinch concentration
and treatment line. (4 marks)

250

5366.4115, 216.65
200
Concentration (ppm)

4585.94, 150
150
4433.97, 143

100 3227.25, 95

50

0, 20
0
-1000 0 1000 2000 3000 4000 5000 6000
WW Treatment
Mass Load (g/h)

b. Assuming the wastewater treatment plant has a removal ratio (RR) of 90%, determine the minimum treatment
flowrate (t/hr), mass load removed by the WWTP (ppm), and the mass load discharged (ppm) to the environment.
(4 marks)
Treatment flow = 40.25 t/hr

c. Represent the wastewater treatment process as a conventional flowsheet. (3 marks)

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 11.71 t/hr
Stream 1 (LG reject) WWT Plant

3.43 t/hr 2.78 t/hr

Stream 2 (HG reject)

10 t/hr 15.11 t/hr

Stream 4 (WW2)

Stream 3 (WW1)

d. Assuming the industrial plant which generates the wastewater has a water supply pinch at 100 ppm, can the high
grade demin water reject be reused directly in the industrial plant? Explain your answer briefly. If your answer is No,
calculate the minimum RR required, for stream 2, before one could consider regeneration/reuse in the industrial plant
(you may present the answer in a range, RR > 0.7 etc.)
(4 marks)
No, discharge of HG demin water concentration is higher than pinch location at 100 ppm
In a regen unit;
DS inlet to regen = 143 ppm
DS outlet from regen = 95 ppm or lower (for possible reuse below pinch)

Removal ratio;
RR = 1 – (95/143) = 0.336
-------------------------------------------- End of this part of Question 4 ------------------------------------------

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e. Assume we have a generic water composite curve for an existing plant per Figure 4-e, which uses water supply at 0
ppm.

Figure 4-e: Generic Water Composite Curve

Now, the new management would like to consider the following streams as alternative water supplies. Determine if
they should or should not be considered for DIRECT reuse/recycle in the existing process represented in Figure 4-e.
(2 marks)

(i) Low Grade demin water plant reject (1000 ppm) (No)
(ii) High Grade demin water plant reject (150 ppm) (Yes)
(iii) Boiler blowdown (300 ppm) (Yes)
(iv) Cooling Water blowdown (1500 ppm) (No)
(v) Rain water (100 ppm) (Yes)

f. A mixed effluent stream contains the following contaminants. Fill the last column with the suitable treatment method
for each of the contaminant. The available treatment methods are:
(3 marks)
 Dissolved air flotation
 Steam stripping at a pH of 10
 Steam stripping at a pH of 5
 Wet oxidation
 Reverse osmosis
 Activated carbon
 Strainer/screen

Stream Contaminant Treatment Method

A Leaves and Twigs Strainer screen
B Ammonia Steam high pH
C Oil (similar to dirty motor oil) DAF
D Phenol Activated carbon
E Phosphates Reverse Osmosis
F Halogenated Organics Wet Oxidation

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Question 4 (2016)

You have recently conducted a retrofitting project to minimize the fresh water consumption in a
petroleum refinery plant. Your boss is very happy about the amount of water that is able to be saved.
Despite having to spend some investment cost to implement your proposal, he/she is convinced by
your professional economical analysis that the pay back is only two years.

Due to the reduction of fresh water consumption, the amount of wastewater is also greatly reduced
from 1500 t/h to only 750 t/h. Following are the wastewater streams that are being generated in the
plant after your modification. The concentrations refer to dissolved solids.

Table 4: Wastewater Stream Generation in Petroleum Refinery Plant

Wastewater (WW) streams Flowrates (t/h) Concentration (ppm)

WW 1 200 400
WW 2 50 100
WW 3 500 800

(a) Target the minimum treatment flowrate for wastewater streams in Table 4. Using a recovery ratio
RR = 0.95, and a maximum discharge concentration Ce = 50 ppm. [5 marks]

900 Effluent Composite Curve

800
Concentration (ppm)

700
600
500
400
247,500, 400
300
200
100
-23,553,
0 0
-50000 0 50000 100000 150000 200000 250000 300000 350000 400000 450000
Mass Load (g/h)

Min Treatment Flow = 677.63 t/hr

(b) The cooling tower is a 20 years old system with low energy efficiency. Therefore, you suspect it
has been causing the recently high electricity consumption in the plant. Noting the fact that you
have just retrofitted the plant, you need a strong proposal to convince your boss again to spend
more money. A ‘strong’ retrofitting proposal should have the least investment cost, but gives
maximum utility savings.

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 Assuming the cooling tower has a maximum makeup concentration of 100 ppm dissolved solids.
Is there an opportunity of achieving simultaneous electricity and water saving after improving the
performance of cooling tower? (Yes/No). Explain your answer (you do not need to do water
targeting) [3 marks]

Yes. We can implement process changes for reduced water consumption. Since the cleanest
wastewater is at 100ppm, it means 100 ppm is the pinch concentration. Therefore by improving
the cooling tower performance we would definitely see fresh water reduction.

(c) Assuming you have answered Yes to part B. List down one measure to improve the cooling tower
performance. Include assumption when necessary. [4 marks]

Assuming evaporative cooling tower.

Decreasing the heat duty on evaporative cooling tower circuit by better energy efficiency to
decrease the evaporation from the cooling tower, and hence decrease the makeup water
requirement.

Other possible process changes include improving control of cooling tower blowdown to
increase the life cycle of concentrations and reduce the cooling tower blowdown rate.

(d) There is a plan to produce a new type of chemical which could potentially generate a new
wastewater stream. Since no budget is allocated for expanding the current wastewater treatment
facility. Determine the maximum flowrate of the new wastewater stream that the plant could
handle, remembering that the original waste water treatment plant had a capacity of 1500 t/h.
Calculate the respective maximum concentration of the new wastewater stream. [3 marks]

the maximum flowrate of the new wastewater stream = 750 t/h.

The respective maximum wastewater concentration will be 1353 ppm

(e) Your boss would like to consider changing to a cheaper filter in the wastewater treatment plant
(cheaper means lower efficiency). What is the minimum RR of the wastewater treatment plant
could operate using the existing infrastructure? Use wastewater stream data in Table 4 and Ce =
50 ppm. [3 marks]

R ≈ 92%

(f) Discuss at least two methods of treating waste water to reduce the concentration of dissolved
solids. [2 marks]

1. Reverse Osmosis (Energy intensive, requires clean water with only dissolved solids present,
so suspended solids require micro filtration – produces very clean water).
2. Ion exchange / demineralization (changes the composition of the dissolved solids without
reducing the concentration)
3. Flash evaporation (effective if waste energy is available, but there are corrosion and fouling
issues)

All methods produce a brine stream that still needs to be disposed of.

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