Annual worldwide salt production has increased over the past century from 10 million to over 200 million tons. Today, nearly 100 nations have salt-producing facilities, ranging from primitive solar evaporation to advanced multistage evaporation in salt refineries. Besides table salt, other applications include water softeners, manufacturing, agriculture, swimming pools, meat cures, and the highway industry.
With expertise in heating and cooling powder and bulk solids, Bulkflow Technologies Inc has been approached by producers of salt products about using the Bulkflow Heat Exchanger to more uniformly cool salt in some cases or warm salt for stronger pellets in others.
Increasing Production Throughput
A table salt producer was using the solution mining method, in which saturated brine is boiled under partial vacuum with steam in enclosed vessels called vacuum pans. The brine is mined from underground rock salt (halite) and then fed to vacuum pans where the salt crystallizes.
Following crystallization, the salt is continuously dried on rotary filters. Gas burners heat air that passes through the cake of moist salt on the filter screen. A blade shaves the salt off the filter. The salt is then sent through a drying process where its temperature is monitored to make sure all the moisture evaporates. The salt is still hot when it leaves the drying operation.
Previously, this facility used an open cascading type of aspirator, which reduced the salt’s temperature with ambient cooling air. But increasing production throughput using this system was not practical since the dust collection system was fully loaded and space limitations restricted expansion with a larger aspirator. The solution was to find a more compact system that could operate within a limited space.
According to Andy Nix, sales engineer for Bulkflow Technologies Inc, the table salt producer knew of a Bulkflow Heat Exchanger in use at a nearby competitor.
“The Bulkflow system had been in operation several years, was effective, and was trouble free,” Nix said. “Based on feedback the company received, it was felt that the Bulkflow system was the perfect solution.”
Bulkflow Heat Exchanger Installed
To meet the table salt producer’s requirements, Bulkflow customized a system that was compact, meeting existing space requirements, and used existing infrastructure. Since the plant used wet scrubbers for dust collection, plenty of water was already available and only had to be rerouted to the Bulkflow cooler; no additional energy was required. This made the system particularly attractive to the salt producer.
Bulkflow engineers specified a mass flow screw feeder to control the discharge rate through the cooler and maintain a uniform cross-sectional velocity as the salt flowed through the system. The mass flow discharge device was critical with the Bulkflow system, according to Nix, since it ensures constant and consistent cooling of the salt as well as reliable flow downstream to the packaging facility.
The Heat Exchanger had to be shipped in two sections -- the top, or plate bank, and the bottom, or transfer hopper – because of space restrictions in delivering the system to the required location: the unit had to be installed in an area with only 10 feet of headroom.
How the System Works
The Bulkflow Heat Exchanger combines technologies related to indirect heat transfer and the mass flow of bulk solids. Unlike traditional fluid bed or rotary drum cooling methods that were also considered by the table salt manufacturer, the Bulkflow system does not require large volumes of air to operate and instead lets gravity do the work to greatly reduce energy consumption.
The Heat Exchanger consists of several hollow, fully welded, water-cooled, closely spaced stainless steel plates. Salt passes slowly by gravity between the vertical heat transfer surfaces formed by the welded plates in mass flow in from 3-4 minutes. Water circulates between the plates’ dimples, cooling the salt to the desired temperature. Equipment is contained in a totally sealed unit to prevent any dirt or contamination from entering the system.
Salt enters the Heat Exchanger at a temperature of over 200ºF at a rate of up to 35 tons/hour. Depending on its inbound temperature, the system has to cool the salt by 15-25% to prevent sticking and discoloring. The cooling takes place within minutes inside the Bulkflow system. The desired cooling temperature is typically between 150-170ºF.
Cooling the table salt down to this temperature is necessary so it can be sprayed with an iodine solution that helps prevent goiter. The iodine solution has to be applied at a temperature below 170ºF to prevent it and flow-enhancing additives from caramelizing or discoloring.
For even cooling, the salt moves at a uniform rate over the cooler. A discharge feeder regulates the product flow, quality, and temperature. Adjusting the speed to the screw feeder regulates the flow rate. After additives are mixed in, the salt moves to the packaging facility.
“The Bulkflow system is totally enclosed to provide better control for a food grade product versus the previously used system that was open and vulnerable to contamination,” noted Nix. “A consistent cooling temperature is being maintained and has resulted in a 50% increase in product flow for enhanced productivity. Maintenance costs have been minimal and operating costs low.”
Other benefits include zero emissions, lower installed capital cost, modular construction for easy expansion, and a compact design that fits into tight areas. Overall, since installing the Bulkflow system, the table salt manufacturer has had a consistent cooling temperature and a 50% increase in product flow for enhanced productivity.