Outside an industrial unit on the outskirts of Lorquí in south-east Spain, a large industrial structure has slowly risen over the last year. In fact, the 16m unit is the largest evaporation unit ever constructed by HRS Heat Exchangers and will soon be dismantled for transport and installation at its operating site. In this piece, Matt Hale, International Sales & Marketing Director at HRS explains the purpose of the unit, and how the company has tackled this unique challenge.
All text and images courtesy of HRS Heat Exchangers
The combined evaporator and crystalliser, which will have a total capacity of nine tonnes per hour once operational, has a footprint of 135 m2 and is designed to treat concentrated waste brine produced by an on-site reverse osmosis plant resulting in the production of distillate – which is returned as process water – and crystallised waste. As such the HRS evaporator unit will form part of an upgraded zero liquid discharge (ZLD) system for treating the client’s challenging waste stream and meeting local environmental regulations.
Zero liquid discharge
Historically, hazardous wastes have been treated by a number of different physical, thermal, chemical and biological methods, including precipitation, high temperature incineration and even burial in specialist secure sites. However, many of these treatment methods have significant disadvantages and often fail to protect local environments from the pollution and damage caused by hazardous wastes.
In general terms, zero liquid discharge (ZLD) is a liquid waste stream treatment which involves transforming liquid waste streams from industrial plants into clean water, which can be reused in the process, together with the minimum quantity unt of solid residues, which often include valuable by-products that can be sold or reused. The effective design of any ZLD system is dependent on the correct analysis of the waste stream, making it essential to have an accurate analysis of composition, flow rates, chemistry, etc. Without this, any designed solution will fail to deliver the required results.
ZLD systems often begin with a pre-treatment phase (in this case reverse osmosis) followed by an evaporation phase to remove most of the water, and a further concentration or crystallisation phase to produce the final solid residue. Heat exchangers play a crucial role in reducing the running costs of a ZLD system utilising surplus and regenerated heat and minimising the amount of fresh energy required by the process.
New HRS evaporator system
The new system, which has been manufactured for an un-named Spanish client operating in the heavy industry sector, has a total weight of 54 tonnes and covers a total area (across all four operating levels) of 420 m2. In total it includes 19 tanks, 15 heat exchangers of different sizes, and some 100 different lines of piping of various diameters.
The system includes a pre-heater, degasser, evaporator and crystallisation sections, together with a final condenser. In order to meet the challenging specification, including the highly corrosive nature of the sodium chloride-rich brine, which is produced by the client’s reverse osmosis system, the HRS system incorporates a number of non-standard features. Firstly, the heat exchangers have been constructed from titanium, rather than HRS’s standard construction materials of AISI 304 and AISI 316L stainless steel. Secondly, rather than using a traditional five-effect forced recirculation evaporator, this system uses a five-stage multistage flash evaporator comprising the following elements:
• Main recirculating pump
• Five sequential heat exchanger sections
• Five sequential flash tanks
• Final condenser
The benefits of this flash evaporator over forced recirculation are that in only requires a single recirculation pump, instead of one pump for each effect, and that the pump is installed at the lowest temperature level in the loop, meaning less risk of cavitation and fewer issues with corrosion – a important consideration given the nature of the brine being handled. Finally, this design means that a lower overall recirculation rate is required, meaning less energy is required for the pumping process, improving overall process energy efficiency.
Once the brine has passed through the five-stage evaporator, the now concentrated brine is sent to a crystalliser unit, which removes the last of the distillate and produces 700 kg per hour of humid crystals (at approximately 70% dry matter). The crystalliser unit itself
comprises a two-stage flash evaporator with:
• Main recirculating pump
• Two sequential heat exchanger sections
• Flash tank without crystal separation
• Flash tank with crystal separation
• Final condenser (the system actually uses the same condenser as the evaporator section
Once the crystals have been formed, the crystal ‘slurry’ is transferred to the client’s mechanical separator, where the humid crystals are extracted and any remaining saturated brine solution is returned to the crystalliser. As a result of this final crystallisation stage, no liquid from this process or waste stream will be discharged from the client’s site, making this a true ZLD effluent treatment solution.
Production, installation and commissioning
From winning the order in early 2022, it has taken 15 months for the multi-disciplinary HRS team, led by Technical Sales Engineer Tomas Stiebe, to design the evaporator and crystalliser and construct it at the manufacturing hub in Murcia. The unit is currently undergoing final checks and testing and will then be partially dismantled for delivery to the client. It is expected to be delivered to site in May and will then be commissioned by HRS engineering staff later in the year. Once fully operational it will process nine tonnes of brine effluent per hour, producing 700 kg of humid crystals and returning all distillate water to the existing client processes. This system is the largest evaporator produced by HRS to date and builds on previous ZLD systems that the company has delivered to clients in Europe and further afield.
About HRS Heat Exchangers
Located in the UK, HRS Heat Exchangers is part of the EIL Group (Exchanger Industries Limited) which operates at the forefront of thermal technology. HRS offers innovative heat transfer solutions worldwide across a diverse range of industries.
With more than 40 years’ experience in the waste treatment and environmental sectors, specialising in the design and manufacture of an extensive range of turnkey systems and components, incorporating our corrugated tubular and scraped surface heat exchanger technology, HRS products are compliant with global design and industry standards.