Low Temperature Thermal Desalination (LTTD): new sustainable desalination process. S. Kathiroli and Purnima Jalihal

Transcription

1 Int. J. Nuclear Desalination, Vol. 3, No. 1, Low Temperature Thermal Desalination (LTTD): new sustainable desalination process Marco Rognoni* Saline Water Specialists S.r.l. (SWS) Largo Buffoni, Gallarate (VA), Italy marco.rognoni@swsonweb.com *Corresponding author S. Kathiroli and Purnima Jalihal National Institute of Ocean Technology Chennai , India kathir@niot.res.in purnima@niot.res.in Abstract: Water is a scarce resource, and new collection and production facilities are predicted as necessary in the coming years. Sea water desalination shall be a key technology for the production of fresh water in many areas of the world, and great efforts are now in progress to increase the reliability of the desalination processes and to reduce the relevant investment and operation costs. In this effort, new conceptual desalination process and schemes have been implemented with remarkable innovative content by desalination engineers worldwide. This paper is focused on a quite innovative and inexpensive new process. This process was originally tested by the former SOWIT in Italy and is now redesigned on independent parameters by the National Institute of Ocean Technology (NIOT) in India, for the same specific installations on islands of the Indian Ocean. Keywords: desalination; low temperature thermal desalination; LTTD; flash; solar energy. Reference to this paper should be made as follows: Rognoni, M., Kathiroli, S. and Jalihal, P. (2008) Low Temperature Thermal Desalination (LTTD): new sustainable desalination process, Int. J. Nuclear Desalination, Vol. 3, No. 1, pp Biographical notes: M. Rognoni graduated in Chemical Engineering in 1969 from the Politecnico of Milano (Italy) and completed his PhD in After experience in various countries in North Africa and the Middle East, he has researched desalination processes since the early 1980s. Formerly a Technical Manager and MD of SOWIT, he is now Chairman of Saline Water Specialists (SWS) based in Italy and SWS & GB based in India, both active in the design and realisation of thermal desalination plants (MSF, MED, MVC). He designed and supervised over 30 plants now in successful operation all over the world. He is a member of EDS and InDA, and author of several scientific publications and of patents applicable to desalination plants, including the SED system. Copyright 2008 Inderscience Enterprises Ltd.

2 70 M. Rognoni, S. Kathiroli and P. Jalihal S. Kathiroli got his PhD from the University of Liverpool, UK, in He is a Specialist in coastal and offshore structures, with several years of experience in difficult field technology demonstration projects. He is currently the Director of NIOT. Purnima Jalihal got her PhD in Civil Engineering from Duke University, USA, in Her areas of interest are the dynamics of structures and renewable energies. She is currently heading the group for desalination and ocean renewable energies in NIOT. 1 Process Any distillation process of desalination requires huge amounts of energy as necessary to evaporate the sea water and to condensate the vapour. However, this energy can be supplied at rather low temperature, and therefore the process of evaporation can be suited appropriately to recover waste heat or naturally available heat. Two types of temperatures are necessary to work with: 1 higher temperature for the evaporation of the sea water 2 lower temperature for the condensation of the vapour. With a T > 7 C some reasonable efficiency in desalination can be achieved. This T is available in a few situations, with large quantities of energy and flows involved in the process. Figure 1 Conceptual process scheme (see online version for colours)

3 Low Temperature Thermal Desalination (LTTD) 71 In the previous experiments of SOWIT, the two temperatures were provided by the ambient sea water (cold level) and by the cooling water rejected by the turbine condensers (warm level) in the power plant of Piombino (Italy). With a T = 8 C, the plant designed by SOWIT has been proving a production rate of approximately 500 t/d with excellent regularity since over 14 years. In the pilot plant of National Institute of Ocean Technology (NIOT) and in the present design for the Indian islands, the T of over 12 C is ensured by the sea water naturally available at different sea depths, more details of which are provided in Section 3.0, and outlined in this process flow diagram. In both the cases, the warm water is flashed from its temperature T 1 down to the I (T I 1 T 1 ) reduced temperature T, 1 thus producing the specific flow of vapour F = cp H where H is the evaporation heat at the working condition. I The vapour at can be condensed by a cooling sea water flow at the cold water T 1 temperature T2, raised up to I T 2 : I T T I influences the extension of the condensation surface and therefore the 2 1 investment cost of the plant T T I influences the capacity of the plant, and therefore the productivity of the 1 1 investment T T I influences the flow of the necessary cooling water and therefore the running 2 2 costs intended as pumping energy. The experiments of both SOWIT and NIOT evidence that for T 1 T 2 > 7 C, inexpensive desalinated water can be produced either in a power plant and in shore installation. The higher the T, the more efficient and less expensive is the plant. In the case of the Indian islands, the heat and mass datas of the project are the following, referring to 1 m 3 /h production rate: INDIA (project in progress) Flow (m 3 /h) Temp. ( C) Energy (kw) Flows in warm sea water cooling sea water Flows out brine cooling sea water distillate In the case of Piombino by SOWIT the data recorded since 1992 have been copied here, referring to the average production of 20 t/h.

4 72 M. Rognoni, S. Kathiroli and P. Jalihal Piombino (since 1992) Flow (m 3 /h) Temp. ( C) Energy (kw) Flows in warm sea water cooling sea water Flow out brine cooling sea water distillate (average) The reliability of the process is proven by over 14 years of operation in Piombino. During this period: The hourly production fluctuated naturally (no regulation) between 12 and 25 t/h according, to the load of the power plant and to the relevant T thus available. No ordinary nor extraordinary maintenance was necessary since start-up (the plant was never stopped). No acid cleaning was necessary and no significant reduction of the productivity has been recorded according to the very low working temperature. No chemical additivation was necessary, because the water (both cold and warm) has already been additivated according to the requirement of the turbine condensers (chlorine, antiscalant and corrosion inhibitor, dosed as requested for the operation for the turbine condensers). No extra attention to the plant was necessary, and the alarms (repeated by the local PLC to the control room of the power plant) never requested for any intervention nor for any regulation during 14 years of industrial operation. The energy consumption for the auxiliary equipment (vacuum system and pumps) was recorded as follows for the production of up to 25 t/h: a electricity (axial pumps and distillate pump) = 68 kw b MP steam (three stages vacuum system) = 700 kg/h 0 10 bar g. The quality of the distillate was recorded excellent in any seasonal and load conditions, with the conductivity of the distillate fluctuating between 0.5 and 1 µ S/cm 2 according to the ambient temperature. The conductivity is seemed to be mostly due to the adsorption of CO 2 during the condensation (as detectable by the ph level). The actual TDS is estimated to be in the range of 0.1 to 0.2 ppm, suitable for make-up to boilers after thermal deaeration, without any chemical polishing (ion-exchange on mixed beds). The reliability is further confirmed by NIOT in its installation at Kavaratti island, now in continuous operation after over one year.

5 Low Temperature Thermal Desalination (LTTD) 73 2 Project criticalities Two main criticalities have to be considered by properly experienced designers, as referred to the conceptual scheme and to the H&M balance outlined in Section 1. Both criticalities originated from the large flows of warm and cold water to be processed for the production of 1 m 3 of distillate. 2.1 Pumping energy Three pumps are to be installed P1 warm water pump The flash drum is to be installed as a little above 10 m elevation above sea-level as necessary for the rejection of the brine by gravity. The pump head is therefore necessary to cover the difference between the actual elevation and the piezometric level of the vacuum suctioned water (approximately 2 m w.c.). Power absorption is approximately 1.2 kw P2 cold water pump The condenser is supposed to be installed below an elevation of 10 m above sea-level and therefore the system is fully siphoned of water. The head of the pump equals the friction losses in the circuit (approximately 3 m w.c.). Power absorption is approximately 1.8 kw. Contingencies for tides are to be considered as applicable Distillate pump The power absorption is negligible, as necessary to extract 1 m 3 /h of distillate from vacuum to the user requirement. Approximately = 0.01 kw. The typical total power absorption is therefore evaluated as 3 kw/m 3 /h of production, that is the very lowest achieved by desalination technologies, provided that the elevation of the desalination plant is properly optimised. 2.2 Vacuum system The vacuum system is quite critical, because a large flow of warm water is to be deaerated, thus generating a large flow of incondensable gases. For the production of 1 m 3 /h of distillate, the following flows of incondensable gases are predicted to be vented out of 170 m 3 /h of warm sea water: Deadsorbed 0 2 Deadsorbed N 2 CO 2 from the dissociation of bicarbonates Air leakages from gaskets (approx.) Total 1.2 kg/h 2.7 kg/h 6 kg/h 2.1 kg/h 12 kg/h

6 74 M. Rognoni, S. Kathiroli and P. Jalihal From the condensation pressure of 28 mbar (23 C) to the atmosphere, a three-stage system is to be considered, based on any of the following systems or on a combination of them: sea water eductors liquid ring pumps, which may take advantage of a small portion of very cold water available for condensation steam ejectors, where steam is available from other sources as in power plants. The most convenient system is to be studied case by case, depending on the available facilities at site. In the experiment of Piombino, SOWIT installed three-stage steam ejectors, with a total consumption of approximately 50 kg/steam per 1 t/distillate (equivalent GOR = 20 of the traditional evaporative processes). 3 NIOT experience The NIOT is the technical arm of the Ministery of Earth Sciences, and is involved in developing technology for utilising ocean resources in an eco-friendly manner. As a part of its research activities, the NIOT has established a desalination plant at Kavaratti in May The plant has since been generating water continuously and has been extremely helpful to the people of Kavaratti. The plant runs on the principle of Low Temperature Thermal Desalination (LTTD) that involves evaporating warm surface water (at about 28 C) in a flash chamber maintained under vacuum (at about 25 mbar) and consequently liquefying the resulting vapour in a condenser. The coolant water for the condenser is drawn using the thermal gradient available in the ocean, namely, using the feature of reduction of ocean water temperature with an increase in depth, a long pipe is deployed in the ocean to draw the cold water (at about 14 C) from a depth of about m. Figure 2 shows a preliminary sketch of the process, with nominal values for temperature and pressure. Figure 2 A rough sketch of the LTTD process

7 Low Temperature Thermal Desalination (LTTD) 75 Figure 3 gives a general depth profile near the islands. The actual erection site for the plant on the islands will be decided after a detailed bathymetric survey that comes under the purview of the NIOT. Figure 3 Plant layout with a typical island that bathymetry A pilot plant had been operated continuously by the NIOT, thus assessing the design parameters. A small plant rated 1000m 3 /day has been in operation at the Kavaratti island since One large plant rated 1000 m 3 /day was then constructed and installed on a barge and commissioned in While the plant yielded desired fresh water for two weeks, a flexible joint in the cold water supply was damaged during rough ocean weather. The problem is being rectified and the barge-mounted desalination plant is planned to be recommissioned in December The two-week trials have been encouraging and already basic work is afoot to scale up the plant by a factor of 10. The next larger desalination plant of capacity is planned to be installed by December In tropical countries like India, It is possible to get a temperature difference of 16 degrees round the year. In parallel, the NIOT is working on power generation using this temperature difference, which should be sufficient to run the desalination plant without external fuel. 4 Challenges in implementation 4.1 Marine installation The most critical component of the Indian land-based LTTD plant is the long cold water pipe since it carries the cold water from the deep sea for the condensation of the vapour. In the island where this method has been implemented already, the deep water is available within 500 m from the shore. A 600 m long pipe of high-density polyethylene

8 76 M. Rognoni, S. Kathiroli and P. Jalihal was deployed in an unusual configuration such that its bottom end is at around 350 m water depth to draw cold water. Also the island is made up of coral rock hence the civil structures were very difficult to construct. In general the offshore-related activities for the drawing of cold water are very difficult and challenging. The most complex task is the towing and deployment of the long, cold water pipe. The NIOT has now successfully implemented this task a few times and continuous pumping of cold water has been demonstrated for over a year. One added advantage of bringing up the deep sea cold water is that it is very rich in nutrients and contributes to significant growth in mariculture. The configuration of the plant on the island is shown in Figure 4. Figure 4 Architectural view (see online version for colours) 4.2 Power plant installation The installation inside the coastal power plant is also considered quite interesting suitable to produce high quality distillate for make up to the boilers. This process minimises the running costs, the troubleshooting and the maintenance requirement with remarkable advantages over any other technology (either membranes or evaporation). 4.3 Process improvement Despite several tests in India and the long-running experience in Italy, the process is still subject to scientific review and remarkable improvements are now in progress. The main achievements expected are the following: new arrangement of cheap construction materials as allowed by the very low working temperature remarkable savings in the duty of the vacuum system through a two-stage deaeration (first stage in an expansion chamber) where the warm water is partially deaerated (at an intermediate vacuum level) before flashing at very low pressure.

9 Low Temperature Thermal Desalination (LTTD) 77 5 Advantages of the process The process described herein ensures remarkable advantages: No primary energy input is requested, because it is entirely recovered from the waste (e.g., Piombino power plant) or from the sea (NIOT design). Only the energy for the auxiliary systems is to be provided (pumps and vacuum). The very low working temperatures ensure a valuable protection against the risks of scaling and corrosion. The life of the plant is predicted to be very long and trouble-free. The plant runs very smoothly without any control system. The plant naturally produces according to the actual instantaneous T. The quality of the distillate is extremely pure, as achieved in the very low-density status of the demisted vapour. TDS less than 0.2 ppm was experimented on in Piombino. Therefore the high quality can be appreciated for industrial services and can be credited for relevant high value. The investment cost is lower than for any industrial plant of the same capacity constructed according to the traditional technologies MSF-MED-TVC-MVC. Because of the low working temperature, lower grade materials can be used. The request for maintenance is extremely reduced, having experimented over hours of continuous run without any stop. 6 Size and capacities The high ratio of sea water flow/distillate is a limit for the maximum size achievable by this technology. The Piombino plant is rated 500 t/d as mentioned earlier, with the production fluctuating between 12 and 25 t/h according to the load of the power plant. Figure 5 Piombino low temperature flash desalination unit rated 500 t/d (see online version for colours)

10 78 M. Rognoni, S. Kathiroli and P. Jalihal However, this size can be well exceeded. A new plant was recently commissioned in Chennai, (India) by the NIOT with a fresh-water generation capacity of 1000 t/d. As no deep sea cold water was available near the site, the entire plant was housed on a floating barge, which was moored 40 km offshore in a water depth of 1000 metres. The total production of over 1000 t/d per module now can be afforded based on proven design parameters, both in power plants and in onshore/offshore installations. The scientific cooperation between the scientists of the NIOT and the engineers of SWS is now backing the plan for the implementation of a m 3 /day plant to be installed on a floating barge and moored offshore in the Indian Sea. 7 Conclusion The progress of the state of the art in desalination is quite remarkable and ensures proper applications in specific local conditions, at sustainable costs with very limited environmental impact and with user-friendly running procedures.