Salt, sweat and tears

Indirect cooling helps producer increase production by 50% 

Author: Jill Caskey

Salt continues to be big business within our global economy. While production volumes have fluctuated over the past decade, an estimated 290 million tonnes were still produced in 2021. By comparison, just 195 million tonnes were produced 20 years ago.  

And while salt continues to be commonly recognized for its role as a popular food seasoning, most of its demand comes from industrial applications that range from pharmaceuticals to pulp and paper. In fact, that demand in some areas is increasing. For example, alternative energy is offering a promising niche for salt with the development of sodium chloride-run batteries. 

Whether table or industrial salt, producers have an opportunity to turn their attention toward improving their operations – namely the holy grail of increased production, improved product quality and operational reliability. One might add sustainability to that list as stakeholders hold industries like salt accountable to the environmental impact of their operations. 

The cooling stage during salt production has always played an essential role in ensuring a quality finished product. Yet the technology at this stage now has an opportunity to check off many additional boxes – as was the case where Solex Thermal Science helped a producer increase production by 50 per cent. (More on that to come later. But first …) 

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Salt production 

There are several methods for obtaining salts, including what’s commonly called “solution mining.” Water is injected into underground salt deposits, and the resulting brine is extracted and transported to an onsite plant for processing. There, it enters either an evaporating plant where dry salt is made, or a chemical processing plant where chlor-alkali is made. 

At the plant level, salt needs to go through some form of heat exchange before it becomes a finished product. In the case of most solution mining operations, that process starts with the brine that’s mined from the underground rock salt being fed to enclosed vessels and boiled under partial vacuum with steam. Once crystallized, the salt then dries on rotary filters. Gas burners heat the air that passes through the cake of moist salt on the filter screen.  

After a blade shaves the salt off the filter, the salt goes through a drying process where the temperature is monitored to ensure all moisture evaporates. The salt typically leaves the drying operation around 93°C (199°F), and then must be cooled to between 66°C and 76°C (151°F and 168°F). 

That’s where moving bed heat exchangers (MBHEs) come in. 

Solex and salt cooling 

Solex Thermal Science worked with a U.S.-based producer of table salt to help increase production at its existing plant with a more efficient cooling solution. 

The producer had been using an open cascading-type aspirator in which ambient cool air was being used to reduce the temperature of the salt. However, the producer was unable to increase throughputs because the dust-collection system with the aspirator was already fully loaded, and there wasn’t enough space to install a larger aspirator and the accompanying air-handling equipment. 

(Note: An additional drawback to this type of direct-contact cooling system is that once the air contacts the product, it increases the risk of contamination and moisture content changes). 

Solex offered a more effective and compact solution in the form of a MBHE that uses a proprietary vertical plate design to indirectly cool the salt via conduction (instead of convection). 

Within this tower-like unit, the product enters an inlet hopper and then slowly passes between a parallel series of heat exchanger plates that contain a counter-current flow of water or other heat transfer fluids. 

Heat transfers from the salt to the heat transfer fluid via a steel plate wall. The product then cools to its target temperatures as, pulled by gravity, it slowly and uniformly moves downwards, controlled by a discharge device. Complex thermal modelling calculations guarantees precise discharge temperature control and the product temperature to storage and transport is optimal. 

In this installation, the salt entered the heat exchanger at more than 93°C (199°F) at a rate of 35 tonnes per hour, and needed to be cooled by 15 to 25 per cent to prevent sticking and discolouring. 

Solex engineers specified a mass flow cone to control the discharge rate, a requirement for maintaining a uniform cross-sectional velocity. This was critical since it ensures consistent cooling of the salt as well as reliable flow downstream to the packaging facility. 

In addition, Solex applied small amounts of purge air in the heat exchanger at a target dew point – which is below the temperature of the fluid-cooled plates. This was crucial to prevent condensation from forming and to avoid caking prior to packaging, storage and transport.

An important point is that air was not used as the cooling medium. Less than 1,000 Nm3/h or 600 SCFM of purge air was needed at a specific location in the cooler.

Benefits 

There were several benefits to installing a MBHE with vertical plate technology. Most importantly, the facility was able to realize a 50 per cent increase in production within and achieve enhanced productivity within the limited space available. 

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Want to learn more about Solex’s heat exchange solutions? Visit our Technology page for videos, applications and more. 

Ready to talk specifics? Contact a Solex team member today.