Water and wastewater minimisation process integrationpinch technology and other optimisation techniques

Smith et al. (1994) presented a comprehensive overview of how to minimise both water and waste water. Water pinch analysis embedded in wastewater minimisation techniques offers simple methods and beneficial results when applied to water-using industries. Traditional approaches to water minimisation such as changing washing operations, when complemented with water pinch analysis methodology have been shown to achieve 30-60% fresh water savings in industrial applications, as stated by Smith and Petela (1991-92, 1994).

Smith (2005) has comprehensively described the technique that has been developed to minimise the amount of water that is used in water-using processes. He lists the main water-using operations as:

• reaction medium (vapour or liquid);

• extraction processes;

• steam stripping;

• steam ejectors for production of vacuum;

• equipment washing;

• hosing operations.

These are grouped together as they have the common feature of bringing water into contact with contaminants. The transfer of contaminant mass into the water increases the overall concentration of contaminant as shown in Fig. 5.1. The flowrate of water in the operation determines the concentration of contaminant exiting from the operation. If the flowrate of water is decreased, and the same mass transfer of contaminant is required, then the concentration of contaminant exiting from the operation is increased (Fig. 5.2). Consequently, if a water flowrate reduction is required, then the consequence will be an increase in contamination concentration at the outlet. This may not be permissible for the following reasons:

• maximum solubility;

• corrosion limitations;

• fouling limitations;

• minimum of mass transfer driving force;

• minimum flowrate requirements;

• maximum inlet concentration for downstream treatment.

Process

1 mass 1 transfer 1

f W

Y Y Y

Wate^

C W,IN

r r W,OUT

Fig. 5.1 Contaminant mass transfer in water. fW, water flowrate; CW, in, contaminant mass flow in; CWOUT, contaminant mass flow out (after CPI 2004 and 2005).

Fig. 5.2 Concentration of contaminant related to water flowrate (after CPI 2004

and 2005).

Fig. 5.2 Concentration of contaminant related to water flowrate (after CPI 2004

and 2005).

Assuming that all process operations use clean water, minimisation of water use can be achieved by reducing the flowrate to its minimum. However, as Smith (2005) emphasises, this loses the opportunity to reuse water, which can considerably reduce the overall amount of clean or fresh water used by the process operations.

In order to reuse water between operations, thereby further reducing the amount of clean water required by the water-using operations, some level of inlet concentration of contaminant has to be set. This is illustrated in Fig. 5.3. This figure shows a water-using profile where the contaminant

Fig. 5.3 Setting targets for contaminant concentration (after CPI 2004 and 2005).
Fig. 5.4 Limiting contaminant water profile (after CPI 2004 and 2005).

concentrations at the inlet and outlet have been set to their maximum values. This setting can be used to define the limiting water profile, which provides a boundary between feasible and infeasible concentrations. If the contamination concentration of a water profile is below that of the limiting water profile, then the concentration is feasible (Fig. 5.4). This feature can be used to recognise reuse opportunities and according to Smith (2005) has a number of advantages:

• Water-using operations that have different characteristics can be easily compared using a common basis for comparison.

• Calculation of the mass transfer of contaminant does not require a model of the operation.

• The flow pattern of the operation is not required for analysis.

Table 5.1 Problem data for four water-using operations (after Wang and Smith, 1994)

Operation number

Contaminant mass (kg/h)

Cin (ppm)

Cout (ppm)

fL (t/h)

1

2

0

100

20

2

5

50

100

100

3

30

50

800

40

4

4

400

800

10

• The methodology is applicable across the entire range of water-using operations.

Wang and Smith (1994) have used these principles to determine the amount of water required by a series of operations, employing water reuse, compared with these operations using fresh water. They use a simple example, making use of the limiting composite curve, for four water-using operations (Table 5.1). The table gives the maximum inlet and outlet concentrations (CIN and COUT, respectively) for a single contaminant for four operations; it also gives the limiting water flowrate (fL), which is the flowrate required by the operation if the mass of contaminant is taken up by the water between the inlet and outlet concentrations. It should be noted however, that if fresh water is available, and an operation has an inlet concentration greater than zero, then using uncontaminated fresh water would allow a lower flowrate than the limiting water flowrate for that operation. A simple analysis of the given problem, reveals that the total fresh water required by operations (assuming zero concentration of contaminant in the fresh water) is 112.5 t/h, with the four operations requiring 20, 50, 37.5 and 5 t/h, respectively. However, if reuse of water is allowed, then analysis, making use of the limiting composite curve, gives a targeted minimum flowrate of 90 t/h. The limiting composite curve of the four water-using operations is given in Fig. 5.5. The water supply line, satisfying the water-using operations represented by the limiting composite curve, has its origin at zero concentration, and lies below the composite. The slope of the line is such that it touches the composite at one point, known as the water pinch. Other water supply lines with the same origin could be drawn, but these would not touch the composite, and would indicate flowrates larger than the minimum. If the water supply line was drawn with a steeper slope, indicating a smaller flow-rate, then this line would cross the limiting composite curve, and would not be feasible.

Smith and Wang (1994), as well as other researchers (Kuo and Smith, 1998; Savulescu et al, 2005a,b) extended the methodology for calculating the minimum flowrate of water, including reuse, required to remove contaminants from water-using operations, also provided a methodology for the design of the water system. For the problem above, the final design of

20 t/h

Fig. 5.6 Final design of water treatment using water pinch methodology (after

CPI 2004 and 2005).

Fig. 5.6 Final design of water treatment using water pinch methodology (after

CPI 2004 and 2005).

the water operation is given in Fig. 5.6. This shows that of the original targeted amount of fresh water of 90 t/h, 20 t/h is fed to operation 1 and 50 t/h is fed to operation 2. The remaining 20 t/h is fed to operation 3, along with 20 t/h from operation 1. Of the original 50 t/h fed to operation 2, 5.7 t/h is fed to operation 4, while the remaining 44.3 t/h goes directly to waste water. However, the authors state that this design could be further evolved to produce alternative networks.

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