Integrated Framework to Measure Sustainability of Desalination

Team Develops First Index to Account for the Sustainability Performance of Desalination Technologies

In water-scarce urban environments like those of the UAE, desalination technologies play a central role in transforming plentiful saline and brackish water to create freshwater that meets the population’s needs. In the UAE, natural gas-powered thermal desalination is estimated to produce around 80% of the country’s domestic water.

However, desalination is not an entirely benign process, with associated economic, environmental and social impacts. This makes ensuring that desalination does not harm the very environments and populations that they are meant to help support an ongoing challenges. In response to this need, a Khalifa University research team has collaborated with both international and regional experts to develop the first universal integrated framework to assess the sustainability of desalination technologies.

“As far as we could find out, there was no unified sustainability metric to measure the sustainability of a desalination plant in the UAE. That is why we decided to formulate a comprehensive framework for the UAE, to generate a sustainability index that takes into account the four factors of sustainability, which are environmental, social, technical, and economical,” explained Dr. Faisal AlMarzooqi, Assistant Professor of Chemical and Environmental Engineering at Khalifa University.

A paper on the framework titled “An integrated framework for sustainability assessment of seawater desalination” was recently published in journal Desalination, co-authored by research associate Yazan Ibrahim, Dr. AlMarzooqi, Professor of Chemical and Environmental Engineering Dr. Hassan A. Arafat, and Professor of Engineering Systems and Management Dr. Toufic Mezher, all from Khalifa University.

“What makes desalination a different and more urgent challenge than ever before, is the rapid evolution of this region in its social, environmental, and economic contexts. This led to a significant dependence on desalination as a reliable freshwater alternative due to the geographical and geological structure of the UAE that limit the number of natural water resources,” Ibrahim shared.

The framework developed by the team combines different desalination-related sub-factors and covers the four sustainability factors. It took a unique methodological approach to integrate the different framework components to be able to assess the sustainability of any desalination technology worldwide. The framework consists of three levels, the first being the goal sought to be reached, the
second level being the main sustainability factors and the third being the sub-factors assigned to each factor.  The framework was then demonstrated by assessing the sustainability of the three main desalination technologies in the UAE, which are multi-stage flash distillation (MSF), multiple-effect distillation (MED), and seawater reverse osmosis (SWRO).

“SWRO, which is a membranes-based process, is the most widely adopted technology worldwide, with a global share of around 68% in 2018. It is characterized with low environmental impacts, low cost, reduced land use, and ease of operation. On the other hand, MED and MSF, which are thermally-based technologies, are known for their reliability and robustness as well as their high environmental footprint. Therefore, the challenge for sustainable desalination today lies in the ability to find a tradeoff between the economic, social, and environmental aspects of these technologies,” Yazan explained.

Overall, the three main sustainability factors were environmental, techno-economic and social, each of which had 5-6 desalination-related sub-factors, which were selected from published literature and expert opinion on the topics. The technical factor demonstrated the technically feasible of the technology. This is closely related to the economic factor. Therefore, the team decided to combine those two factors into one representative factor namely techno-economic. Some of the sub-factors included water extraction and discharged brine impacts in the environmental factor, quality of produced water and scaling and fouling propensity in the techno-economic factor, and technology safety and level of noise in the social factor.

When the framework was applied to the three major types of desalination technologies used in the UAE, SWRO was found to be the most sustainable technology followed by MED and MSF.

“This was due to the unique local conditions and parameters of the UAE – like the relatively low price of natural gas and the relatively higher weightage of environmental impact. That is why it is important to calculate the sustainability of a technology in a way that is specific to its local application. In the future if new technologies emerge, these too can be added to the index and framework,” Dr. Al Marzooqi explained.

The team is now working on the technological aspects of sustainable desalination and hope that opportunities are generated in the near future to further develop sustainability indices.

“Till date, the economics and efficiency of sustainable desalination technologies are not able to fully replace traditional desalination technologies. Sustainable desalination technologies are still awaiting a technological breakthrough to give it a competitive advantage against traditional desalination technologies. This research will serve as a performance metric for sustainable desalination. This will benefit the UAE and the world by enabling the government and regulatory bodies in measuring the
current sustainability of desalination plants and setting future targets which will help in achieving other sustainability related targets such as climate change and other,” Dr. Arafat added.

And though the team’s framework was developed to test the sustainability of desalination technologies in the UAE, it can be universally applied to other desalination technologies and/or other countries.

Their research has also been presented through two conference presentations – one at the International Desalination Workshop that was held in Busan, South Korea in November 2017, and another at the Desalination for the Environment Conference of the European Desalination Society that was held in September 2018 in Athens, Greece.

Zarina Khan
Senior Editor
17 December 2018

Cooling Amine Solvent Using Vortex Tubes

Team Demonstrates Energy and Cost Savings Potential for Acid Gas Enrichment Units

A collaborative project at the Khalifa University Center for Catalysis and Separation has explored how to improve the sustainability of the acid gas enrichment (AGE) process in natural gas processing plants operating in hot countries, to reduce their carbon footprint and improve energy efficiency.

When natural gas contains containing significant amounts of hydrogen sulfide and carbon dioxide, it is considered ‘sour gas’ and has to undergo processes that remove the acidic components through a process called ‘gas sweetening’.

Gas sweetening units produce a by-product known as ‘acid gas’ besides the main product named ‘sweet gas’. Acid gas, which is a mixture of H2S and CO2 predominately, is processed further in sulfur recovery units to prevent the emission of sulfur species and recover the elemental sulfur. If the acid gas contains low concentrations of H2S, an AGE unit is employed to enrich the H2S content of the acid gas. AGE units also produce a CO2-rich stream besides the enriched acid gas. In hot climates like in the UAE, high ambient temperature leads to AGE operation with hotter solvents, which results in higher energy consumption in the regeneration section of the plant. In order to reduce this inefficiency, the team considered the use of a scheme for cooling the solvent within an AGE unit, to reduce the operational energy.

The team was composed of Khalifa University Associate Professor Dr. Abdallah S. Berrouk, Assistant Professor Dr. Yasser F. AlWahedi, Research Engineer Satyadileep Dara, and Chemical Engineering alumna Aisha A. AlHammadi, along with Abdulla Al Shaiba from Al Yasat Petroleum Operations Company Ltd and Fadi Al Khasawneh from the Abu Dhabi National Oil Company.  

“We looked to integrate a Ranque-Hilsch vortex tube (RHVT) within the acid gas enrichment unit to decrease its energy consumption while enhancing the purity of the resulting gas product,” Dara explained. He was the lead author on a recently published paper in the Journal of Cleaner Production titled ‘Carbon footprint reduction of acid gas enrichment units in hot climates: A techno-economic simulation study’.

A RHVT is a mechanical device that separates a compressed gas into hot and cold streams. Requiring no moving parts, electricity, or Freon, it instead leverages principles of physics to separate the gases into a hot end that can reach temperatures of 200 °C and a cold end that can reach −50 °C, making it an energy-efficient cooling tool. RHVTs are often used in to cool cutting tools that heat up during use.

This potential solution to reduce the energy waste of AGE was inspired by the team’s knowledge of the UAE’s Mirfa plant.

“We were aware that the Mirfa plant produced high pressured nitrogen as a by-product of the air separation unit in the same plant complex, and realized that integrating a nitrogen-fed RHVT was the best option to reduce energy wastage, given the available resources and resulting economics,” Dara shared.

In the team’s proposed solution, the high-pressure nitrogen enters the RHVT and is separated into hotter and colder streams. The latter is then mixed with ambient air in an air-nitrogen mixer to provide a coolant stream at sufficiently lower temperatures, such that it cools down the lean solvent to the desired levels. Lower lean solvent temperature in turn results in significant reduction in energy consumption and higher product purities.

The solution they proposed was tested and validated in process simulator ProMax, which found that at the optimal temperature, their proposed RHVT solution can achieve 13 kg/s in steam savings (equivalent to 40% reduction in total steam rate). This reduced energy consumption leads to an annual carbon dioxide footprint reduction of 83.7 million kg, which is equal to a 40% reduction in the plant’s total carbon dioxide footprint. Economically, the evaluated annual energy savings translate to USD11.2 million.

The team believes that the solution they have hit upon can be utilized in sour gas processing plants in hot climates, all of which struggle with reducing energy wastage due to the high temperatures of the solvents.

“Hot climate regions like that of the Gulf would benefit significantly from the proposed scheme, since it results in a coolant stream that is not readily available in hot regions due to the high ambient temperature. And while our project used pressurized nitrogen from a specific facility, in fact any high pressure stream can be used as the working fluid for the RHVT, like compressed ambient air. Regardless what gas is used, we have demonstrated that the integration of RHVT can help a natural gas processing plant operating in hot climate achieve increased operational efficiency in terms of product quality and energy consumption,” Dr. AlWahedi added.

Following their simulation based work, the team are now doing laboratory-scale tests to assess the performance of RHVT to provide a quantitative prediction of levels of cooling achieved using the RHVT.

Zarina Khan

Senior Editor

26 November 2018