Water System Design

CAB 3023: PROCESS PLANT DESIGN (JANUARY 2008 SEMESTER) CHAPTER 26

Water System Design REFERENCES Polley, G. T. and H. L. Polley. Design better water networks. Chemical Engineering Progress 96 (2000), 2: 47–52. Benkó, N., E. Rév, and Z. Fonyó. The use of nonlinear programming to optimal water allocations. Chemical Engineering Communications 178 (2000): 67–101. INTRODUCTION water reuse: reduces both volume of freshwater and volume of wastewater because the same water is used twice regeneration: any treatment process that regenerates quality of water such that it is acceptable for further use regeneration reuse: in addition to allowing a reduction in the water volume, also removes part of the contaminant load that would have to be otherwise removed in the final effluent treatment before discharge

• • • • • •

• • • • •

Figures 26.2c and 26.2d both show arrangements involving regeneration. Regeneration is a term used to describe any treatment process that regenerates the quality of water such that it is acceptable for further use. Figure 26.2c shows regeneration reuse in which the outlet water from Operation 2 is too contaminated to be used directly into Operation 3. A regeneration process between the two allows reuse to take place. Regeneration reuse reduces both the volume of the freshwater and the volume of the wastewater, as with reuse, but also removes part of the effluent load (i.e., kilograms of contaminant). The regeneration, in addition to allowing a reduction in the water volume, also removes part of the contaminant load that would have to be otherwise removed in the final effluent treatment before discharge. A third option is shown in Figure 26.2d where a regeneration process is used on the outlet water from the operations and the water is recycled. The distinction between the regeneration reuse shown in Figure 26.2c and the regeneration recycling shown in Figure 26.2d is that in regeneration reuse, the water only goes through any given operation once. Figure 26.2c shows that the water goes from Operation 2 to regeneration, then to Operation 3, and then discharge. By contrast, in Figure 26.2d, the water can go through the same operation many times. Regeneration recycling reduces the volume of freshwater and wastewater and also reduces the effluent load by virtue of the regeneration process taking up part of the required effluent treatment load.

CHAPTER 26

Water System Design CHAPTER 1. INTRODUCTION 1. Typical Water System and Effluent System Contaminated Stormwater

Operation1 RAW WATER TREATMENT

Freshwater

Operation2

Wastewater

Operation3

BFW TREATMENT

Steam

Steam System

Condensate Loss

Boiler Blowdown Ion Exchange Regeneration Evaporative Losses Cooling Tower Blowdown

§

Wastewater Treatment

2. Water-using operations include: reactors (vapour or liquid) extraction processes steam stripping steam ejectors equipment washing (e.g., water from cooling towers) hosing operations, etc. NOTE: There two sources of water: (1) process water and (2) utility water 3. Some trends:

Reuse Water and Apply Distributed Effluent Treatment

Discharge

Distributed Effluent Treatment System

(Water Using Units) Operation 1 (dirtier)

esuer

RAW WATER TREATMENT Freshwater (Water Using Units)

Freshwater

Treatment Plant

Discharge

Operation 2 Operation 3 (cleaner )

Treatment Plant

Contaminated Stormwater

Boiler Steam

BFW TREATMENT

Steam System

Condensate Loss

Boiler Blowdown Ion Exchange Regeneration Evaporative Losses

Wastewater Treatment Plant

Discharge

Discharge to sea

Cooling Tower Blowdown

Maximum level of contaminants

Water Reuse OPERATION 1

FRESHWATER

OPERATION 2

WASTEWATER

OPERATION 3

• •

Water reused in different operations Reduces freshwater and wastewater

Distributed Effluent Treatment • Increases opportunity for material recovery (through segregation) • Can significantly reduce effluent treatment costs Treatment processes can also be used to regenerate water for further use (NOTE: regenerate means treatment with the purpose of reusing or recyling the water) Regeneration can in principle be any reaction or separation process which removes contamination: • chemical oxidation • filtration • carbon adsorption • steam stripping, etc. Regeneration Reuse

OPERATION 1

FRESHWATER

OPERATION 2

REGENERATION

WASTEWATER

OPERATION 3

•

water regenerated to be reused in different operations (NOTE: water never sees the same operation twice)

Regeneration Recycling

OPERATION 1

FRESHWATER

OPERATION 2

REGENERATION

WASTEWATER

OPERATION 3

1.

Water can be recycled to processes in which it has been used previously—compare against regeneration reuse

Design Problem Summary 1.

Water system design maximum water reuse (what is the minimum feed water flowrate corresponding to maximum water reuse) minimize freshwater consumption and wastewater generation Distributed (NOT: centralized) effluent treatment treat only streams which need to be treated minimize treatment costs can help with material recovery Water system (i.e., water-using units) and effluent treatment (water-treatment units) system interact

2.

3.

WATER CONTAMINATION 1.

two reasons why we consider water contamination: discharge to environment is regulated

i.

Process 1

Process 1

Treatment Plant

Primary Treatment

Process 1

Cooling Tower Blowdown

Biological Treatment

Boiler Blowdown Discharge C ≤ C environmental law

ii. 2.

contamination limits water reuse (NOTE: contaminants = pollutants/solutes) What are some typical contaminants in water and effluent systems?

Two types of contaminants: aggregated contaminants: COD, BOD, suspended solids, dissolved solids single contaminants: sulphates, nitrates, metals e.g., mercury, iron, boron Oxygen Demand 1. Organic material oxidized by natural processes to stable end products: e.g.: CH4N2O + 9/2 O2 CO2 + 2H2O → urea oxygen carbon water dioxide

+

2NO3 nitrate

2. 3.

Oxygen used is biological oxygen demand (BOD) Three tests used: i. Biological Oxygen Demand (BOD5): oxidation with microorganisms over a 5-day period at 20°C ii. Chemical Oxygen Demand (COD): oxidation with acidic potassium dichromate iii. Total Oxygen Demand (TOD): oxygen in air stream at 900°C over catalyst 4. Relationship: BOD < COD < TOD for the same waste stream 5. Complete data on relationship: 6. Also, if analysis of waste stream is known, we can calculate Theoretical Oxygen Demand (ThOD) Example A process produces an aqueous waste stream containing 0.1 mol% acetone. Estimate the COD and BOD of the stream. Solution

First, calculate the theoretical oxygen demand from the equation that represents the overall oxidation of the acetone: (CH3)2CO acetone

+

4O2 oxygen

→

3CO2 carbon dioxide

+

3H2O water

Approximating the molar density of the waste stream to be that of pure water (i.e., 56 kmol/m3), then:  4 kmol O 2  theoretical oxygen demand = 0.1% × 56 ×   m3    32 kg O2  = 0.001× 56 × 4 ×   m3   kg O2 = 7.2 m3 thus: COD ≈ 7.2 kg/m3 BOD ≈ 7.2 × 0.35 = 2.5 kg/m3 Other contaminants/pollutants/solutes: • pH • suspended solids • heavy metals • halogenated organic • organic nitrogen • organic sulphur • nitrates • phosphates • toxicity • temperatures, etc. Summary 1. Water contamination dictates the effluent treatment and also limits reuse between operations 2. Wastewater streams are characterized by biological oxygen demand and many other measures (i.e., these are the contaminants)

WATER USE REPRESENTATION Water used for a wide variety of purposes: • reaction medium (vapour or liquid) • extraction processes • steam stripping • steam ejectors • equipment washing • hosing operations, etc. Do these operations have anything in common?

PROCESS Mass Transfer Water

fW CW,in

1. 2. 3.

C W,out

Process becomes less contaminated Water becomes more contaminated But we need to represent this quantitatively

PROCESS Mass Transfer Water

fW CW,in

CW,out

Concentration

Mass Transfer CW,out fW ter Wa

CW,in

Mass Load

∆mC = f W ∆c fW =

∆mC ∆mC = = slope ∆c Cout − Cin

Pinch analysis for energy: ∆H = CP∆T Traditional Approaches to Water Minimization 1. reduce water flowrate to extraction processes and reduce mass transfer driving forces 2. increasing the number of stages in extraction process that use water Reduce Water Flowrate Concentration

Reducing water flowrate

Cout,max Higher outlet concentration (hence less water )

Mass Load

Insight/Concept 1: Minimum flowrate of maximum outlet concentration limits ∆mC = f W ∆c fW =

∆mC 1 = ∆c slope of ∆mC vs. ∆c

slope ↓⇒ f W ↑ slope ↑⇒ f W ↓ Maximum outlet concentration depends on: maximum solubility corrosion limitations fouling limitations minimum mass transfer driving force minimum flowrate requirements maximum inlet concentration for downstream treatment Example To calculate mass load (pick-up of contaminants):

mass pick-up of contaminant = flowrate of water × concentration change ∆mC = f W × ∆c kg kg kg = × h h kg g t or = × ppm h h  t 1 kg  = ×  6  h 1× 10 kg 

Units

NOTE: Base concentration on flowrate of water, not flowrate of mixture: m mC ∆c = C NOT c = fW fW + mC Summary 1. Water-using processes can be represented on plot of concentration versus mass load 2. Traditional approaches to water minimization lower flowrate but the scope is limited by maximum outlet concentration

LECTURE 4

Targeting Maximum Water Reuse Keywords: Limiting water profile Limiting composite curve

Concentration

W a te rp

ro f il

e

Cout ,max

Cout

Mass Load

Maximum outlet concentration for each operation: • • •

hence minimum total water use—OR IS IT? Have we considered all options? Are there any other variables we have overlooked? Concentration

Cout ,max Cout Cin,max Mass Load

Alternative water profile (to the one shown here) uses more water but accepts slightly contaminated water. Limiting Water Profile Limiting water flowrate = flowrate required if specified mass of contaminant is picked up by water between max inlet and outlet concentrations If an operation has a maximum inlet contaminant concentration > 0 and it is fed by water with zero concentration,

Concentration Composite Curves Limiting composite curve Concentration CONCENTRATION COMPOSITE CURVE 800 LIMITING COMPOSITE CURVE Water supply line (gives minimum water flowrate = 91.11 t/h)

400

100 50 0

2

7

37

41

Mass load

s • • •

Represents a quantitative profile of the single-stream equivalent to the four separate streams Combined boundary between feasible and infeasible concentrations Water supply line that gives minimum water flowrate

Advantage of this approach: • Allows operations with different characteristics to be compared on a common basis (e.g., water used in an extraction process vs. a hosing operation) • Does not require a model of the operation to represent the mass transfer • Does not depend on any particular flow pattern (countercurrent vs. cocurrent) • Works on any type of water-using operation (e.g., firewater makeup, reactor medium, reactant in a reaction, cooling water makeup)

LECTURE 5

Design for Maximum Water Reuse