li MSF

Presented at the EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperationbetween Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the EuropeanDesalination Society and the University of Montpellier II, Montpellier, France, 2125 May 2006.

0011-9164/07/$ See front matter 2007 Elsevier B.V. All rights reserved

Desalination 206 (2007) 633652

Libyan Petroleum Institute experience in evaluation ofdesalination plants in the Libyan oil sector

Salah Al-Hengari*, Mohamed El-Bousiffi, Walid El-MoudirLibyan Petroleum Institute, PO Box: 6431, Tripoli, Libya

Tel. +218 (21) 483-6821/23; Fax: +218 (21) 483-0031; email: [email protected]

Received 22 May 2006; Accepted 5 July 2006

Abstract

A study by the Libyan Petroleum Institute of Libya (LPI) evaluated the performance of desalination at fourLibyan oil-processing companies for two years of operation. The performance criteria were performance ratios,conversion ratios, availability factors and thermal efficiencies. Mass and energy balances of the units were checked.Construction materials used with regard to corrosion resistance and mechanical strength were checked. Cost analysiswas conducted for each of the desalination units. The cost of unit product of distillate water was compared withinternational costs of water. Problems facing the desalination units were discussed and an attempt made to suggestsolutions.

Keywords: Desalination plants in Libyan Oil sector; MEDTVC; MSF; Plant data evaluation; Performance factors;Modelling; Corrosion; Economic evaluation; Troubleshooting

1. Introduction

Libya is a large landscape country in NorthAfrica with a total area of 1.5 M km2. More thanninety percent of the country suffers from a desertclimate and only a strip along the country coast

*Corresponding author.

on the Mediterranean Sea coast has a seasonalclimate. The population and the industrial powerare continually growing. As a result, natural watersupplies are under pressure to fulfill current andfuture requirements of both domestic and indus-trial use.

Libya is an oil producing country whose eco-nomy depends on the oil and gas industry. The oil

S. Al-Hengari et al. / Desalination 206 (2007) 633652634

and gas revenues account for more than 90% ofthe countrys income. The oil and gas facilitiesare scattered all over the country, on-shore andoff-shore, in which water demands for quality andquantity are very high. These facilities include oiland gas fields, pipelines and terminal ports of oiland gas shipping, refineries, gas processing plantsand chemical complexes. The desalination indus-try provides sufficient quantities of water withspecific requirements for both industrial anddomestic consumption in those scattered indus-trial sites. Therefore, evaluation of the desali-nation industry is critical.

2. Libyan oil industry

The Libyan oil sector is managed by theNational Oil Company (NOC) through a numberof local and international oil companies andorganizations which can be divided into fourcategories as shown in Fig. 1. The oil sector wasthe first in Libya to utilize desalination. The firstunit was installed in Mersa Al-Brega in 1964 witha production capacity of 757 m3/day. It is esti-mated that the installed capacity in the oil sectoris 15% of total installed capacity in the country.The oil sector has built a number of desalinationplants using different desalination technologies.For example: C In locations near the sea, thermal desalination

technologies are used for seawater where thereare also energy sources.

C On-shore locations (oil production fields andprocessing sites), non-thermal processes arepreferred when brackish water exists andelectric power in available.The research and development arm of the

National Oil Corporation (NOC) is the LibyanPetroleum Institute (LPI) founded in 1977 toundertake the responsibility of technical andscientific support needed by the upstream anddownstream Libyan oil and gas industry (Fig. 1).LPI works in close cooperation with NOC and

other operating oil and gas companies to providetechnical services and consulting in areas includ-ing desalination. Therefore, LPI conducted thestudy of Performance Evaluation to review thecurrent situation of the desalination industry atfour industrial sites.

LPI also supports the oil sector by providingtraining programmes. Three training programshave been organized and two others are plannedas follows:

Courses given:C Desalination Technologies (2 weeks)

Prof. M. Darwish, Prof. H. El-Dessouky,Dr. Hassan Fath

C Desalination Operation and Maintenance(1 week)Dr. Hasan Fath

C Intensive Course on Industrial Water Treat-ment (2 weeks)Dr. M. El-Bousiffi

Future CoursesC Intensive Course on Water Desalination

TechnologiesC Thermal Desalination Processes: Modeling

and Economic Analysis

3. Performance evaluation of desalinationplants

Objectives of this study were to survey andevaluate desalination plants currently in operationin four industrial sites, each run by a different oilcompany in order to give a picture of the desali-nation industry and its reliability for the Libyanoil sector.

Operating problems are studied and appro-priate solutions recommended to enhance theirperformance and to minimize their operatingcosts. It will also be helpful for future planningand better utilization of desalination processes interms of production capacity, energy utilization,

S. Al-Hengari et al. / Desalination 206 (2007) 633652 635

Fig. 1. Libyan oil sector (NOC and its affiliates).

Table 1Desalination plants in this study

Industrialsites

Number ofunits

MSF-BR MSF-OT TVC Date start operation (Old/New units)

Total installedproduction

1 12 7 2 3 1964/1989 22,708 m3/day2 5 5 1983/1997 40,000 m3/day3 2 2 1999 750 m3/day4 7 5 2 1974/1995 4,500 m3/dayTotal 26 15 7 8 1964/1995 68,000 m3/day

maintenance activities, process reliability (e.g.planning and forced shutdowns), the product costand process suitability. Twenty-six MSF andMEE-TVC desalination units were studied(Table 1). Mediterranean seawater composition isshown in Table 2.

3.1.Methodology and stages of evaluation

The study covers two years of operation foreach unit. Each site consists of a number ofdesalination units each of which was evaluatedseparately and then was evaluated as part of the

site in the following stages (Fig. 2):C Stage 1 Site visits. Collection of operating

data and relevant information C Stage 2 Analysis of the data for evaluation

of their performance. performance ratios, con-version ratios and availability factors

C Stage 3 Checking of the mass and energybalances of the units

C Stage 4 Investigation of the suitability ofmaterials of construction with regard to corro-sion resistance and mechanical strength

C Stage 5 Cost analysis to determine theeconomy of each of the desalination units by


Fig. 2. Sequences of evaluation stages.

Table 2Comparison between reference seawater and Mediter-ranean seawater

Referenceseawater

Mediterraneanseawater

Anions: Chloride, Cl!

19,360 22,010

Sulphate, SO4!2 2,702 3,264 Bicarbonate,HCO3! 142 173 Bromide, Br! 66 Trace Fluoride, F! 1 Iodide, I! 1 Trace Sodium, Na+ 10,768 12,144 Magnesium, Mg+2 1,288 1,464Cations: Calcium, Ca+2

408 480

Potassium, K+ 388 453.6 Strontium, Sr+2 14 Silicon, Si+ 4 TraceTotal dissolvedsolids (ppm)

35,168 40,460

determining the cost of unit product of distil-late water and comparing it with standard(commercial) cost of water today

C Stage 6 Troubleshooting the problems facingthe desalination units

STAGE 1 DATA COLLECTION

Process specification sheets, process diagramsand technical/operation process manuals havebeen prepared (Fig. 2): site visits, communicationwith plant management and operators andunderstanding of process design and operation.

The evaluation team prepared the followingforms:C General information C Operating data C Cost data C Planning shutdown informationC Forced shutdown informationC Chemcials consumptionC Corrosion information


STAGE 2 PERFORMANCE EVALUATIONThis stage is based on evaluating the operating

data of each unit and comparing the actualoperating data with design operating data.Calculations are made to detect and estimate anydeviation in the data for each item of the plant.C Performance factors MSF and MED

technologies:Water production capacity (actual vs. normal

load): it shows the potential capability of unit toproduce water in comparison with the expectedproduction at that operating condition. Due to ageof some units or operational problems, fullproduction capacity will be gradually decreasedleading to increase the product cost.

Steam consumption capacity (actual vs.normal load): The energy consumption as steamis a factor which will show if the unit consumesthe energy at normal rate or there is a deviation(plus or negative). Deviation may show the sys-tem is not in full balance in terms of productionand consumption. Its deviation usually showspresence of a problem such as fouling. C Availability Factor (AF) = total running

hours/total period of evaluation:The availability factor is related directly to the

reliability of a process to stay online for produc-tion over a certain period of time. Reduction ofthis factor can show the failure of the process,losses of planned production capacity anddeterioration of its product cost. The referencevalue of 90% of an operating year is usually used.It means that the plant is operating over a periodof 3650.9 days in the calendar year. The restperiod is used for planning shut-down for normalmaintenance and repairs activities (i.e. 5 weeks).In some cases, the plant was shut down once inevery two years. C Performance Ratio (PR) = production

capacity/steam consumptionIt is a factor used to describe how much water

if produced over how much steam is consumed.Reduction of this factor can increase energy

consumption and deteriora-tion of its productcost. The reference value for this factor is usuallygiven by plant specifications. C Capacity Factor (CF) = actual production/

design productionThis factor is another description for the total

production of a plant. If the plant works normallyat normal operating conditions, this factor wouldbe 1. However, the plant may not be able toproduce normal production due to problems (i.e.fouling in heat exchangers) or it becomes veryold (i.e. heat transfer area is not fully available).C Recovery Ratio (RR) = production capa-

city/seawater feed capacityThis ratio can give an indication of how much

water can be recovered from seawater feed to aplant. It works as an indication of a problem suchas change of seawater salinity or seawatertemperature variation. C Plant Factor (PF) = AF CF

From this, both the availability factor andcapacity factor influence on the plant behaviorcan be concluded. Cost of production can bechanged considerably if this factor is dropped forany reason (see the economic analysis section).The reference value of PF is usually 90 % (AF =0.9 and CF = 1).

Despite the fact that the above-mentionedfactors can help in assessing the performancebehavior of a plant, the actual performanceanalysis factors are much more than only theseseven factors. The plant data could be evaluatedin a different manner such as back calculations orprocess simulation.

An Excel spreadsheet system was built toevaluate MED-TVC units. For each unit, twotables were prepared: Table 3 for the monthlyoperating data and Table 4 for performance calcu-lations. Fig. 3 gives an example of the form foroperating data. The top line is for the design(reference) data while the last line representseither the total or average value. The spreadsheetautomatically detects any wrong data entered


Fig. 3. Monthly operating data sheet.

Table 3Monthly operating data sheet (MEDTVC)

Col. Information

1 Months of the year2 Number of hours per month3 Number of operating hours per month*4 Distillate water produced*5 Steam consumption*6 Brine blowdown*7 Seawater feed consumption*8 Salinity of seawater* 9 Salinity of brine blowdown*10 Brine blowdown temperature* 11 Top brine temperature (TBT)*12 Seawater temperature*13 Distillate temperature*

*Form by plant operators [1]

by coloring them in red. For example, a pro-duction capacity is 1,500 m3/day for a unit with adesign capacity of 1,000 m3/day.

Deviation in columns 15, 16 and 17 is definedas follows:

Reference value - Actual valueDeviation = 100Reference value

Table 4Performance sheet (performance factors and deviation)

Col. Information

1 Months of the year2 Performance Ratio (PR)3 Recovery Ratio (RR)4 Availability Factor (AF)5 Capacity Factor (CF)6 Plant Factor (PF)7 Actual antiscalent consumption 8 Calculated antiscalent consumption 9 Actual antifoam consumption 10 Calculated antifoam consumption 11 Calculated steam consumption (based on design

values)12 Calculated seawater consumption (based on design

values)13 Calculated brine blowdown (based on design

values)14 Extra steam consumption 15 Calculated deviation of steam consumption16 Calculated deviation of seawater consumption17 Calculated deviation of brine blowdown18 Deviation of steam consumption from designed

value19 Deviation of seawater consumption from designed

value20 Deviation of brine blowdown from designed value


Table 5Design vs. operating performance values

Design (reference) Operating Notice

Production, m3/day 1,000 866Performance ratio (PR) 5.55 4.0Availability factor (AF) 90% 98% Forced shutdown Recovery Ratio (RR) 0.321 0.224Capacity Factor (CF) 100% 86.6%Plant Factor (PF) 90% 84.86% Acceptable Product quality, ppm 3 20 Acceptable

while deviation in 18, 19, 20 is

Calculated Value - Actual ValueDeviation 100Actual Value

=

The graphs produced demonstrate the comparisonbetween reference and actual operating values.These graphs were divided into three categories:C production and consumption (seawater

feed, distillate, brine blowdown, and steamconsumption)

C performance factors (PR, RR, AF, CF, PF)C consumption of chemicals (antifoam and

antiscalent)

3.2. Example of plant data evaluation

A MEDTVC unit is used to demonstrate theutilization of Excel in the evaluation. The unit hasa nominal capacity of 1,000 m3/day and theevaluation period will cover only one year ofoperation. Table 5 presents design and operatinginformation and values of the unit during a calen-dar year. The unit had already passed plannedmaintenance works in the previous year ofoperation; therefore, there were no planned shut-down for this unit set-up. The only shutdownswere forced shutdowns. Figs. 4 and 5 show theproduction profile and performance ratio over ayear of operation respectively.

STAGE 3 PROCESS ENGINEERING: CHECK MASSAND ENERGY BALANCE

Modelling and simulation of desalination pro-cesses is one approach of evaluation by checkingmass and energy balance. For each technologycovered by the study, specific process modelthrough Excel spreadsheets or Fortran programswere developed and used in the evaluation. Forexample, the following Excel spreadsheets wereprepared for specific units to carry out processmodelling of MSFOT, MSF-BR, and MEDTVC.

Stage by stage (MSF) or effect to effect(MED) calculations were done. Mathematicalmodels of El-Dessouky and Ettouney [11] wereused. They have given a set of equations whichdescribes the material and energy balance for anumber of desalination processes. These modelscan estimate production rate, temperature,pressure, and brine concentration profiles. Thesemodels were found to be reasonably accurate in areal operating situation.

Besides all programming utilization, theHysys process simulator was used for simulationof a MED pure process or specific process com-ponents (i.e. heat exchangers or condensers). Thissimulator has shortage comings in considering thethermodynamic losses in the seawaterdesalination.


Fig. 4. Distillate water production for MED-TVC unit.

Fig. 5. Performance ratio for MED-TVC unit.


Fig. 6. Simulation results for MST-OT (summarized plant sheet).

Fig. 7. Temperature profiles inside the unit (seawater, distillate, flashed brine).


El-Dessouky and Ettouney [11] designed spe-cific software for designing thermal desalinationprocesses. It has been found that this software hassome limitations such as the number of stages oreffects for processes (i.e. number of MED process$4) or range of top brine temperature and steamtemperature.

MSFOT demonstration unit: The calcu-lation methodology can be started either from thecooled section of the MSF plant (i.e. or in the hotsection, i.e. the brine heater which is used in thesimulation. A small temperature variation hasbeen noted between calculated and specificdesign. Finally, the results of production rate andtemperature profiles are given, and they are veryclose to design values.

Temperature profiles for seawater cooling,flashed brine and distillate (see Fig. 7):C Distillate production profile C Brine blowdown profileC Heat transfer coefficient (clean and dirty)C Pressure profile C Heat load profile C Concentration factor vs. CaSO4 solubility and

stage temperature

The demonstration unit information is shownin Fig. 6. The software of thermal desalination ofEl-Dessouky and Ettouney [11] failed in carryingout the calculation with appearance of an errormessage on a number of stages although thenumber of stages covered by software is 940stages. This simply showed that the softwarecould not handle a simulation of a differentnumber of stages.

STAGE 4 PERFORMANCE EVALUATION:CORROSION RESISTANCE

Desalination processing is a corrosive envi-ronment (presence of CO2, high temperature andsalinity). Therefore, all types of corrosion mightbe experienced in a single unit. The corrosionproblems will enhance failure of unit components

such as tubes and pumps, in which they mighteventually lead to take plant out of service formaintenance and reduce its productivity andavailability. Distillate water produce cost willincrease considerably.

The evaluation is based on questionnaires andon site observations from both the plant operatorsand the evaluation team. Questionnaires cover thefollowing points:C Description of materials used in construction

for heat exchanger tubes, desalination cham-ber, tube sheets, etc.

C Types of corrosion observed and measurestaken to limit or prevent corrosion.

C Which part of the unit suffered most fromcorrosion and what are the causes

C Performance of the replaced parts with regardto corrosion

C What type of scale and corrosion inhibitorsare used and how efficient they are? (eitherthrough chemicals or others technologies suchcathode protection)

C How often does the unit have to be stoppeddue to major corrosion problems?

C Have you observed any environmental corro-sion in the units?

C Have you observed any corrosion problems inseawater intake pipeline? Explain.

C Any failure problems exhibited by materials(metals and non-metals) used in the con-struction of the unit.During the study, certain corrosion problems

were noticed. For example, some MSF unitssuffer from:C Corrosion of steel structures as the volume of

steel construction exposed to a cor-rosiveenvironment is very high although there wereprotection systems applied (i.e. cathodeprotection).

C In some cases, the shells of plant designedfrom C.S. in which it was badly cor-roded insidewalls and bottom of the unit stages.General or localized corrosion are noticed atvarious locations as varying temperature.


C Corrosion of non-condensable gases ventinglines and equipments (i.e. direct condenser)due to CO2 attacks.

C Erosion corrosion of condenser tubes due tosmall sand particles escaped with the seawaterintake.

C The brine heater is exposed to the highestplant temperature (i.e. 110EC). Experience hasshown that the tubing in this section has thehighest failure rates. These tubes were exam-ined and shown that they were failed byscaling or due to damage during mechanicalde-scaling rather than corrosion failures.

C The internal cement lining damage and bitu-men wrapping of buried seawater pipelines arealso damaged. Fiberglass pipes failed due toexposure to direct sunlight.

C Scale precipitation was observed in which it isleading to tube blockage and unit shutdownand the mechanical cleaning of hard scalemight cause tube damage.

Reducing corrosion problems will improve theunit life, availability and productivity as well asreduce the product cost. Therefore, recommen-dations were given to minimize corrosionproblems. Samples of these recommendations forMSF units are given as follows: C To minimize evaporator damage or failure,

cladding all internal surfaces with 316L stain-less steel is necessary (for old units).

C Complete water drainage and care is neededduring shutdown to avoid leaving chloride-containing water with access to oxygen insidethe units.

C Avoid mechanical cleaning of the scale toprevent mechanical damage by minimizing thescale formation conditions.

C Painting with special marine coating materialsto prevent atmospheric corrosion of carbonsteel components is essential required.

C Seawater intake should be improved to elimi-nate fin sands and seaweed.

C Outlet oxygen content of seawater should be

measured more frequently and it should be inrange of 0.018 mg/L.

STAGE 5 ECONOMIC ANALYSES

The objective of this part of evaluation is topresent elements of desalination economics inorder to estimate unit product cost, $/m3 of water.A convenient method for estimating the cost ofdesalted water is used. This section outlinescomponents of fixed cost and operating cost(Fig. 8) in which costs are usually calculated onannual or monthly basis. The estimation of theunit product can be given as follows:

Unit product cost = fixed costs + operating costs

To construct a reference basis for water costcomparison for each unit, we assume two cases,one for design (or reference) case and another foroperation (real) case. This will be discussed in thecoming example in this section.

Fixed costs: Fixed charges are predominantlythe result of capital costs. They consist of amorti-zation and interest to recover the installed cost ofthe plant, i.e. the capital cost of the plant is afixed cost to be paid annually for repayment ofthe loan required for financing the project. Theamount of these payments will depend on thetotal cost of the installation, the applicable

Fig. 8. Unit product cost items and factors.


Table 6Capital recovery factor (fr)

Interest rate, r, % per year

N (year) 5 7.5 10

10 0.128 0.146 0.16320 0.08 0.100 0.117

interest rate and amortization period. The capitalrecovery factor (fr) is given as

(1 )(1 ) 1 (1 ) 1

N

r N N

r r rf rr r

+= + =+ + where r is the interest rate, the second term in theequation [r/(1+r)N!1] is the depreciation factorand N is the amortization period which equal tonumber of annual payments. The capital recoveryfactor fr is calculated for a selected value of r andN as given in Table 6. The depreciation takes intoaccount the timing of returned money by includ-ing the interest rate. If the installed cost for theplant is $X, then the annual capital cost would beX fr ($/yr).

The other fixed costs are due to charges forproperty taxes and insurance. These are usuallyon the order of 1 or 2% of the installed cost of theplant. The plant factor (PF=AFCF) are theannual capital costs:C AF influences water cost in rigorous estimate.

Forced and planned outage of a desalinationunit for maintenance and repair reduces itsavailability for production. Fixed charges arepaid whether the plant is operating or not.

C CF can affect the annual payments if the pro-duction of plant is lower than design values.

An example to illustrate the influence of bothAF and CF on the annual fixed cost is shown inTable 7 for three cases. A plant has a capacity of1000 m3/day and the plant total annual fixed cost

Table 7Illustration of influence of availability and capacityfactors

Case 1 Case 2 Case 3

Capacity factor 1 1/0.7 0.7Availability factor 0.9 0.7/1 0.7Plant factor 0.9 0.7 0.49Specific annual fixed cost, $/m3

0.553 0.711 1.015

(interest and zmortization + tax and insurance)equals $181,680/yr:

Specific annual fixed cost = annual fixed cost/

(plant capacity 365 AFCF)

Operating costs:1. Steam cost (energy cost): Energy cost

makes up the greatest part of the total productioncost of water. The cost of energy per productwater varies directly with price of fuel andinversely with performance ratio, PR. Low PRmeans high steam consumption and hence highfuel con-sumption. The steam cost can vary from$23.5/ton [12].

The exhausted thermal sources of energy canbe used such as low grid of saturated steam; thissource energy will save substantial part of costs.It has been found in some desalination plantswhich were covered by this study that they areeither using steam which produced as by-productor as waste energy evaporators. This reduces thesteam cost considerably (Table 8).

2. Power cost: Power consumption depends onthe size of the plant and mode of desalination.MED plants consume less power than MSF plantswhich require higher power consumption forrecycle pumps, however, MED plants do not. Onthe other hand, MEDTVC units consume muchless power than MEDMVC units; the later arepower intensive process. In fact, power may costless especially on-site generation. Generally


Table 8Steam prices for four industrial sites

Industrial sites Steam cost ($/m/t)

1234

1.7655.0754.1664.92

speaking, for estimation purposes power cost canbe taken as $0.037/kWh for Libya.

3. Labour cost: Labour costs include perma-nent staff for operation, routine maintenance andadministration [i.e. $9001250/(person/month)].

4. Chemical treatment cost: All desalinationplants are usually designed to operate on anantiscale program with periodic acid cleaning.Multi-effect distillate (MED) units consumefewer chemicals since they use less feed seawatercompare with MSF plants. Costs of chemicalsused in desalination units vary due to many fac-tors such as location of plant, its size, chemicalsexport from outside the country or produce withinthe industrial site.

5. Maintenance costs: These include spareparts and manpower for non-routine maintenance.There are usually estimated on the basis of thelabour experience, plant size and age, and loca-tion. This can be considered as a fixed costamounting yearly to 11.5% of the total installedcost of the plant.

6. Overhead costs: These costs are used inorder to fulfill any money flow shortage for aproject. In many cases, it is difficult to estimate a

real value because it usually comes from unex-pected failures or extra hours of maintenance andrepair jobs. It can be taken as 100% of labourcost.

7. Unit product cost: Internationally waterproduction costs tend to be in the range of $14/m3 depending on technology applied, size of theunit, capital cost, operating costs and otherfactors. In Libya, the estimation of the cost of onecubic meter of desalted water is around $13/m3.Lower figures can be achieved through use of acheaper source of energy. Average range of pro-duct cost of water in the four industrial sites isshown in Table 9.

Desalination Economic Analysis Spreadsheets,(DEAS): Excel spreadsheet developed at LPI. Awell known economic methodology to conductthe economic analysis was used [6, 11]. However,this package is still under development to acquiremore information and to conduct further calcu-lations (Fig. 9).

DEAS can be used by plant engineers andoperates in order to save time and to obtainaccurate results. It has two routes calculationcapabilities. The first route (or option) can beused for estimating unit product cost of desaltedwater (Fig. 10). The second route can be used toconduct a feasibility study of installing a newplant or review an existing desalination unit. Onlythe first route (Route I) will be presented in thispaper.

The spreadsheet of Route I consists of threetables and one figure. First, Table 10 gives cost ofproduction at design (reference) conditions (AF =

Table 9Average product cost and plant factor

Industrial sites Site 1 Site 2 Site 3 Site 4

Production installed capacity 22,700 40,000 750 4,500Plant factor 6189 2084 50 67Specific cost ($/m3 water) 1.402.50 1.562.65 5.70 2.005.00


Fig. 9. Desalination Economic Analysis Spreadsheets.

Fig. 10. Route I of water cost salculations.

Table 10Cost data general information

Mode of desalination MEDTVCDesign capacity (m3/day) 946Hours actually operated 8611Installed costs, $ 1,179,066.00Steam consumption, $/yr 118,942.20Power consumption, kW/yr 484,421.90Operating labour costs, $/month 1,967.188Cost of repairs and maintenance, $ 89,606.25Cost of chemicals, $ 4,407.813

0.9, CF = 1). This table shows the reference costvalues. Another table (not shown) gives the cost

of production for actual (selected) conditions.Fig. 11 shows a comparison between cost atdesign and actual (selected) conditions. Fig. 12 isa graphical representation of data given inFig. 11. To demonstrate DEAS, the followingcase study of an MEDTVC unit which wastaken from the study will be examined.

STAGE 6 TECHNICAL PROBLEMS ANDTROUBLE-SHOOTING

Desalination units covered by this study showthat they are facing some technical difficulties.These difficulties will lead to reduce the plantavailability and productivity in which it will


Fig. 11. Comparison between design (reference) and actual operating load.

Fig. 12. Sharing percentage of cost elements on the total product cost.


eventually lead to increase the product cost. Somemain problems facing the units are as follows:C Scaling problems are still experienced in some

units despite the antiscalants used. Some unitsstill use old methods of scale prevention inwhich they need evaluation and search foralternatives.

C Corrosion (as already been given in Stage 4):C Internal and external corrosion attacks due to

CO2 attacks (i.e. failure of ventilation system)and leakage of rain water through thermalinsulator.

C Erosion corrosions for tubes due to fineparticles and corrosion rust.

C Sand and seaweed due to limited capacity ofintake system and/or failures in seawaterfiltration stages.

C Ball cleaning system failure due to thermalexpansion between scale and ball materials.

C Costly and long duration jobs on checking ofleakage locations.

C Decrease in productivity and performance dueto drop in seawater temperature.

Case study of MEDTVC unit: The variationof seawater temperature leads to drop in theproduction of a MEDTVC unit. Its productioncapacity is 375 m3/day. Fig. 13 shows the processdiagram for this unit.

Extra steam was consumed to compensate theloss of production leading to drop the perfor-mance ratio and increase the product cost. Thisextra steam might lead to deteriorate the steamejector due to erosion corrosion. It was reportedthat the seawater temperature can drop as low as11EC and usually takes the average trend shownin Fig. 15. The performance ratio lost about 20%of its value. Fig. 14 shows the performance ratio

Fig. 13. Process diagram of MEDTVC.


Fig. 14. Performance ratio for MEDTVC.

Fig. 15. Average monthly seawater temperature wariation (EC).


Fig. 16. Current installation for preheating seawater.

Fig. 17. Suggested installation for preheating seawater.

over a period of 14 months operation. Fig. 16shows the current preheating installation in theunit.

To overcome this problem and keep steadyproduction and operation, a modified configu-ration has been suggested for preheating seawaterfeed to the unit by utilizing a part of brineblowdown. This solution might operate only in

cold months and whenever there is a need for it(Fig. 17).

4. Outcome of the study

Some outcomes of the study and recom-mendations are listed as follows:


1. Good inspection and scheduled mainten-ance for plants from seawater intake to productoutlet is very essential to maintain proper ope-ration and avoid production loss.

2. It is not advisable to keep very old units inoperation and keeping them online only willincrease maintenance costs.

3. For aged desalination units, it is advisableto reduce production rates to within 90 % of thedesign capacity, at the same time increasing unitsavailability to an average of 85%.

4. Continuous evaluation of plant operatingconditions (temperature, TDS, pressure) andobtaining plant data should be made on regularbasis. This will help operators to monitor plantperformance on a regular basis.

5. The use of additional steam during the coldseason (winter operations) can be avoided byutilization of hot brine blowdown in heatingcoming seawater feed to a unit.

6. Continuous and careful monitoring ofcorrosion problems are required.

7. Accumulation of non-condensable gasesappears to be a contributing factor to corrosionand should be investigated.

8. The proper maintenance of all measuringand control instruments is very important. Insome cases, some units measuring devices haveeither been missing or out of order.

9. Due to the effect of feed salinity on the unitperformance, it is advisable to conduct a regularbasis record of seawater salinity.

10. Conserving energy by insulating exposedparts of the plant or even building a hangar ora shed to accommodate the small units isrecommended.

11. Evaluation of the chemical treatment andchemical cleaning methods are needed withrespect to cost, impact on environment andefficiency as well as other non chemical methods(i.e. ball cleaning system).

12. It is advisable to perform overall unit andequipment mass and energy balances to locate theareas of major loss and maloperation. The results

could be compared with design and guaranteedvalues.

13. The operation of the venting system at fullload (due to air leakage) is not advisable. Thisleads to product loss (vapour withdrawal) andmore steam consumption at the ejectors.

14. Detailed economic evaluation is to beconducted of all desalination units to determinethe actual cost of water.

15. The oil sector has gained good experiencein running MSF and MEDTVC desalinationunits. Therefore and for future planning, it isrecommended to bring the same technologies.

16. Selection of process will be based on thefeasibility study.

References

[1] M. Snussi, M. El-bousiffi, F. Agram and S. Al-Hengari, Performance evaluation of a desalinationunit at Brega petrochemical complex Sirte OilCompany, Energy and Water Desalination Inter-national Conference, Tripoli, Libya, 2000.

[2] Study of performance evaluation of desalinationplants in the oil sector, Industrial ResearchDepartment, Libyan Petroleum Institute, 20002003.

[3] W. El-Moudir, M. El-Bousiffi and S. Al-Hengari,Performance evaluation of a small size TVCdesalination plant, Desalination, 165 (2004) 269279.

[4] S. Al-Hengari, M. El-Bousiffi and W. El-Moudir,Performance analysis of MSF desalination unit,Desalination, 182 (2005) 7385.

[5] S. Al-Hengari, M. El-Bousiffi and W. El-Moudir,Polymeric membranes in water desalination units,International Conference and Exposition on Poly-mers and Their Further Applications, Tripoli, Libya,2003.

[6] W. El-Moudir M.A. El-Bousiffi and S.R. Al-Hengari, Economic analysis for thermal desalinationprocesses, Petroleum Research Centre Conference onTechnology of Oil and Gas (TOG2004), Tripoli,Libya, 2004.

[7] R. Bakish, Practice of Desalination, Noyes DataCorp., USA, 1973.


[8] A. Porteous, Saline Water Distillation Processes,Longman, UK, 1975.

[9] M.S. Peters and K.D. Timmerhaus, Plant Design andEconomic for Chemical Engeers, McGraw-HillChemical Engineering Series, New York, 1980.

[10] A. Hassan Khan, Desalination Processes and Multi-stage Flash Distillation Practice, Vol. 1, ElsevierScience, the Netherlands, 1985.

[11] H.T. El-Dessouky and H.M. Ettouney, Fundamentalsof Salt Water Desalination, Elsevier Science, theNetherlands, 2002.

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