Membranes for Industrial Wastewater Recovery Episode 9 pptx

Relaxing the inlet and outlet concentration It is most often assumed, in the first instance, that all processes are fed with pure water, such that the minimum water usage is obtained th

Rueraqe P e r n e a t e f l u x - 14.6 GFD, Permeate flow- 400.85 GPH

Recouery Permeate Feed Feed f e e d

Figure 4.6 Cnlculatcdarray water quality and hydraulicdata (ROSA, FilrnTrc)

4.1.4 Cost calculation

Some RO software packages (Koch, Hydranautics) include a provision for costing, which extracts such cost items as capital costs, pumping energy demand, membrane replacement and chemical dosing, automatically from the design file Pumping energy costs, for example, relate directly to the system hydraulics Other specific data, such as labour costs, anti-scalant and cleaning chemical unit prices, amortisation, overheads and maintenance, must be entered by the user Figure 4.8 illustrates the CostPro software (Koch), which permits fairly comprehensive costing of the RO design Care must be taken when using such packages, since the cost can be sensitive to assumptions made about such things as membrane life, period of capital amortisation and interest rate On the other hand, the CostPro software allows a direct cost comparison of two candidate designs

Cost is also very dependent on the scale of operation Higher specific costs, i.e cost prices per m 3 permeate product, result from smaller plants due to the

relatively high capital and maintenance costs, the latter pertaining to labour A

more useful basis for specific cost may thus be the cost per unit time, which is inversely related to the amortisation period and increases with increasing interest rate Given the sensitivity of costs to maintenance requirements, cleaning frequency, membrane and plant life, interest rates and residual value, it

is prudent to compute costs based on a range of assumed values for these parameters which are, of course, largely beyond the designer's control

System design aids 1 8 3

l o 5 4252 4 28.2 1187 144.7 113 4117 5 25.2 1325 142.1

7

Figure 4.7 Sraling indices (Argo Analyzer, Betz-Dearborn)

4.1.5 Overview

Whilst RO CAD packages have become increasingly sophisticated and also more

user friendly, more versatile and more widely available over the years (Table 4.3), it should be stressed that they cannot provide a n alternative to pilot trails Indeed, all suppliers emphasise that their software is intended to expedite design, rather than provide absolute values for the key process performance parameters with respect to the system hydraulics and permeate product water quality None

of the existing design packages are able to predict organic fouling or equate this, even by approximation, to some measurable feedwater quality determinant such

as TOC It is such determinants that have a n impact upon crucial parameters as the pretreatment requirement, frequency and nature of membrane cleaning and membrane life On the other hand, these CAD packages provide the option of designing a complete RO plant under what could be considered optimum conditions of zero permanent fouling In practice a more conservative process design and/or operation may well be called for

One obvious constraint on the use of commercial software packages is their limitation both to reverse osmosis and the suppliers' own products Several generic RO, and more recently NF, design software packages have been developed over the years by academic and independent research groups

worldwide An example of a pragmatic MS Visual Basicm-based software

approach to handle the design and cost calculations for single pass cross-flow, feed and bleed and semi dead-end filtration systems, applicable to all types of

184 Membranes for Industrial Wastewater Recovery and Re-use

-

Total Caatal I f.46O.OO0 00 1 265 o[)o OD Euro f m*3 Perm 1 D 180463 1 D 166820

"- - -

KOCH Membrane System, ~ I C 9 29 $1 41E22002

Figure 4.8 Cost analysis (Costpro, Koch-FZuidSystems)

membranes, is that of Vito Three (p-version) programs have been written, creating an interactive environment with predefined dialogue windows and allowing the user to handle in a systematic way the numerous filtration parameters (Brauns et al., 2002) The software allows the calculation of both the

design of the installation and its global cost or cost per unit permeate product The absence of accurate universal membrane filtration models, with the obvious exception of RO, is compensated for by enabling the input of basic filtration

values, such as, for example, permeate flux or feed pressure, from a datasheet As

a result the software user is able to implement in a suitable spreadsheet a preferred calculation (model) or extrapolation method based on real experimental data to produce basic filtration input values that later can be imported into the Visual Basic@ program This pragmatic approach allows the user to provide the appropriate basic filtration data, and is totally flexible with regards to membrane process and technology On the other hand, it relies on available hydraulic data (i.e flux vs TMP correlations, pressure loss data, etc.) to

be usable

Given the current widespread activity in membrane process modelling, it

seems likely that more CAD packages for porous membrane processes will be

System design aids 18 5

Table 4.3 Summary of RO CAD package capability

- Parameter

Fluid Systems Film-Tec Hydrdnautics Osmonics

specs

Y

Membrane specs

Y Membrane specs

Y Membrane

Y Membrane specs

processes through a consideration of the operating conditions and the

membrane and feedwater matrix characteristics is largely limited to model, single-component feedwaters

186 Membranes for Industrial Wastewater Recovery and Re-use

membrane filtration systems Desalination and Water Reuse, 12( l ) , 40-44

4.2 Water pinch analysis

4.2.1 Introduction

As is apparent from the example industrial sectors discussed in Chapter 2, the use of water within and its discharge from a n industrial site can be very complex Minimising water consumption is obviously desirable on environmental and economic grounds, but it becomes very difficult to determine the extent to which this can be done without endangering product quality The complexity is compounded not only by ignorance of water volumes involved, but also of water quality with respect to key pollutants The latter can relate both to effluents generated and process water quality demanded by specific processes Such basic issues have to be addressed ahead of any assessment of water purification technologies, since such purification may not even be necessary Moreover, the problem of assessment of water demand, discharge and quality is generally compounded by the disparate knowledge base for individual water-consuming processes throughout the company Clearly a n overview is required encompassing all unit operations demanding and discharging water throughout any one site This information can then be used to form the basis of what

is referred to as pinch analysis which, when applied to water systems, is usually referred to as water pinch

In the following sections the water pinch concept is discussed in detail, and practical methods for its application outlined Water pinch provides a means of determining the theoretical minimal overall water consumption for a site with different processes, as well as indicating the requirements of a purification technology based on a specific scenario Although the concepts of water pinch might seem relatively simple, applying them under real process constraints demands a comprehensive knowledge of the overall industrial process to which it

is applied Moreover, as many parameters are involved and interrelationships between the various process streams can be very complex, it is apparent that dedicated software is necessary to expedite the application of the methodology, The latter is widely available although, as with the reverse osmosis software discussed in Section 4.1, the software is merely a tool rather than providing a process solution

Sgstem 7

4.2.2 Water pinch: the history

The pinch concept was developed at the Department of Process Integration at the IJniversity of Manchester Institute of Science and Technology (UMIST), UK, in

1 9 7 0 as a method to reduce energy demand by recovering or transferring heat

by empIoying heat exchangers a t critical junctures of a process This pinch concept was then taken and applied to processes using water The fundamental theoretical formulations for the application of the pinch concept to wastewater problems were amongst others pioneered by El-Halwagi and co-workers (1 992,

1995), Smith and co-workers (1991a,b, 1994, 1996) including Kuo and Smith (1997, 1998), Alva-Argaez et al (1998a,b) and Wang and Smith (1994a,b,

1995) A large number ofstudies applying the concept ofwater pinch have since

been performed Mathematical programming approaches have been formulated (e.g Rossiter and Nath, 1995; Doyle and Smith, 1997), and several software tools are now available (Section 4.2.3) Two such software tools are

WaterTargetB , a commercial software provided by Linnhoff March - a division of KBC Process Technology Limited - and Water, a package provided by UMIST to the members of its own research consortium In the following sections, these software tools will also be referred to as the LM and UMIST software WaterTarget@ is a software suite comprising WaterTrackerm and WaterPinchm’ Whereas the latter is the heart of the program for defining the optimal water network, the first is used to set up the water balance In the following sections, the program will be referred to as WaterTarget ”, although Waterpinch@ is the part used and elaborated on The graphical plot used in WaterTarget tL does not represent concentration versus mass load, but concentration versus flow rate This methodology and the interpretation of the curves generated can be found in Buehner and Rossiter (1 996)

4 2 3 Methodology

Every problem definition begins with accurately identifying every unit operation using and producing water, including processes as well as utility operations such as steam production The existing water network is thus obtained and, for given measured flow rates, the water balance can be checked The accuracy of this balance determines to a great extent the result and usefulness of a pinch analysis Indeed, a substantial imbalance of water would strongly indicate either unaccounted for water-consuming unit operations, leaks, and/or a n ignorance of flow rates through some or all of the

selected units A water pinch analysis can only uscfully procccd if the imbalance is less than 10%

Relaxing the inlet and outlet concentration

It is most often assumed, in the first instance, that all processes are fed with pure water, such that the minimum water usage is obtained through summing the flow rates through all the units It is necessary to stipulate, for all units, the maximum inlet and outlet concentrations for the different curltarninants of

Membranes for Industrial Wastewater Recovery and Re-use

interest A contaminant, in this context, is defined as any property of the water that prevents its direct reuse, and can thus include heat content (i.e temperature) as well as the usual physicochemical attributes such as suspended solids, acidity and hardness Increasing the allowable influent concentration results, in general, in a n increased effluent concentration In most cases basic rules can be applied to determine the maximum allowable effluent concentration and, as such, the maximum allowable influent concentrations, based around such fundamental properties as mass transfer, solubility of scalants, corrosivity and (organic) fouling This relaxing of concentrations allows flow rates to be

determined that are most appropriate for efficient water use

Non-fixed flowrate approach: both inlet and outlet

Concentration are allowed to attain their physico-chemical

The limiting water flow rate concept (limiting water line)

An important difference can be observed between two basic precepts concerning flow rate The flow rate may either be fixed at some value or can be assumed to take on some limiting value whereby the maximum allowable inlet and outlet concentrations are obtained according assumptions or measurements made

based on deterioration of water quality through a unit In Fig 4.9 these

approaches are presented for a simple single-contaminant case The physicochemical properties of the process and of the equipment allow a maximum inlet and effluent concentration of 50 ppm and 150 ppm respectively However, when increasing the inlet concentration up to 50 ppm, and keeping to the existing flow rate of 2 t/h, the maximum effluent concentration is not reached for this fixed flow rate Permitting a variable flow rate for the process considered allows the maximum effluent concentration to be reached The water pinch methodology was initially presented as a problem without flow rate

max

System design aids 1 89

constraints (Wang and Smith, 1994a) and later extended to fixed flow rate

(Wang and Smith, 1995) since this situation is most common in process industries However, the original methodology can be extended to processes with flow rate constraints

The limiting composite h e

Consider a n example scheme with three processes (P1 to P3) and two contaminants (C1 and C2) It is assumed, for all processes, that the limiting flow rates and the initial flow rates are thc same for the relaxed case, and that the flows have been optimised Data for the example are provided in Table 4.4

The pinch methodology begins with the construction of the limiting composite

line To this purpose separate limiting water profiles (solid, fine lines) are plotted

from the data in Table 4.4 as a concentration versus mass load diagrams for both

contaminants (Figs 4.10 and 4.1 1) From Fig 4.10 four concentration intervals can be distinguished (0-25; 25-50; 50-100; 100-150 mg/l) for C 1 , whereas only two intervals can be distinguished for C 2 (Pig 4.1 1) a t 0-50 and 100-1 5 0

mg/l In each concentration interval, a line is then constructed between the point at lowest mass load and concentration and the highest, such that limiting composite curves are obtained for both contaminants (solid, thick lines) For both contaminants the minimum flow rate through the overall scheme can now

be determined by drawing a line that at no point bisects the limiting composite line This line is called the water supply line, the inverse of the slope being the

overall lirnitingflow rate In this case, for both contaminants the water supply

line only touches the composite curve a t the end point This point called the

pinch point

From Figs 4.10 and 4.11 it is observed that two limiting flow rates are obtained: 2.67 t/h for C 1 and 2.33 t/h for C 2 In general, the theory of water pinch is presented in articles and textbooks as a single-contaminant case Indeed,

it is not possible to consider more than one contaminant since concentration

shifting, as proposed by Wang and Smith (1994a) is required when targetting a

multiple contaminant case This entails a very lengthy and complex procedure Although a two-contaminant case has been assumed to illustrate the method

Table 4.4 Water data for example 1 after relaxation of the contaminants

190 Membranes for Industrial Wastewater Recovery and Re-use

’ Water supply line for contaminant two,

revealing a flowrate of 2,33 rih

/

Mass load [th]

F i g u r e 4 1 7 Constructionofthe watersupply linejorcontaminant 2

and its application, nearly all practical cases concern multiple-contaminant matrices It should be stressed that, although applicable for this case, it is not generally the case that the overall target flow rate, based on all contaminants in

a multiple-contaminant system, corresponds to the highest value flow rate observed when constructing the water supply lines

System desigri 1 9 1

4.2.4 Computed solutions

For the simple scenario given in Table 4.4, implementation of both LM and UMIST software provides the same limiting flow rate, corresponding to the theoretical limit However, the networks proposed by the two different software

packages differ (Figs 4.12 and 4.13) Indeed, by allowing a variable flow rate

through the process, the UMIST software Water, which allows both approaches (Le fixed and non-fixed flow rates) projects flow rates lower then the limiting ones Using the LM WaterTarget'Q software, based on flow rates fixed at the limiting value, the individual flow rates in the processes remain the same To achieve the same target flow rate, but constraining to a fixed process flow rate, WaterTargetD proposes a network where part of the effluent of Process 2 and 3 is

recycled and used as influent for those processes (Fig 4.12) As such the overall

flow rates through all the individual processes remains constant The network proposed by Water looks the same as the WaterTargetR one, although providing

no recycling and thus allowing a reduced flow rate through the third process This example shows that whether or not the flow rate through the process is constrained a t a fixed value, the target minimum flow rate remains the same and both software tools provide a possible water network achieving the target Moreover, when the results of the optimisation indicate recycling of the outlet back to the inlet of the process, this suggests the possibility of lowering the flow rate through the process

Since the two approaches result in a different network, the inlet and outlet concentrations of the individual processes will also differ (Table 4.5) By not constraining the individual process flow rates, lower inlet concentrations from the Water solution are obtained compared to those from the fixed flow rate approach of WaterTarget" However, since the final effluent is produced by Process 3 , the effluent concentration of this process is the same for both

192 Membranes f o r lndustrial Wastewater Recovery and Re-use

2 t n l

Process 1 0.33 t/h

2.67 tib

Figure 4 I 3 Networkproposed b y W a t e r to achieve the targetfor example 1

Table 4.5 Inlet and outlet concentrations for the processes

Further decreasing the minimum flow rate

determining the minimum flow rate, taking into account the prevailing constraints The task of any engineer dealing with water saving must now be to search for ways to further reduce the water consumption The graphical presentation provides a n aid in attaining this goal (The graphical plot used in WaterTargetO does not represent concentration versus mass load, but concentration versus flow rate This methodology and the interpretation of the curves generated can be found in Buehner and Rossiter (1996).)

A decrease in the overall water usage implies an increase in the slope of the water supply line To achieve this for a single-contaminant scenario it can be

observed from Fig 4.10 that the maximum effluent concentration of Process 3

should be allowed to increase since, provided the inlet concentration is unchanged, the slope of the limiting water line for this process will increase when not constraining the flow rate As such, the slope of the dotted line,

representing the water supply line, will increase resulting in a lower target For the example presented, however, the gain is only minor Indeed, one

System design aids

immediately observes that the slope of the limiting composite curve can only increase to the point at which the line touches the composite curve at the point (125, 50) As such, the effluent concentration can only be allowed to increase up

to 1 6 0 ppm, resulting in a n overall target of 2.5 t/h The limiting composite curve for the new problem is presented along with the water supply line and the pinch point in Fig 4.14 It is observed that further increasing the effluent

concentration of Process 3 provides no further advantage

Whereas Water provides a graphical presentation of the composite curves, thus allowing the engineer to tackle the problem graphically, WaterTarget provides a visual representation Performing a so-called inlet sensitivity analysis, WaterTargetS pinpoints those processes and contaminants that may be adjusted

so as to achieve a n improved target The result of this sensitivity analysis, which essentially corresponds to the graphical analysis described previously, is presented as a bar chart of cost saving in $/h per unit ppm concentration change for each contaminant (Fig 4.15) For the example given in this section, the software advises the engineer that it is beneficial to increase the inlet concentration of Contaminant 1 in Process 3 For this simple case, the bar chart

is perhaps not necessary For a more complex case, however, it provides a more accessible illustration of the system data Indeed, whilst the theory is only valid for a single-contaminant problem, the sensitivity analysis applies to multiple- contaminant cases

As already observed, the different approaches of fixed and non-fixed flow rates provide different solutions To permit a n increased effluent concentration from Process 3 and maintain a constant flow rate through the process, the inlet concentration must be allowed to increase for the fixed flow rate approach If