Level measurements of corn steep tanks - Emerson Global · After steeping, the corn kernel is transferred through a milling process that is made up of three separate mill systems

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Level measurements of corn steep tanks in corn wet milling

White PaperLevel Measurements of Corn Steep Tanks in Corn Wet Milling

Level measurements of corn steep tanksin corn wet milling

Corn millingCorn is processed by one of two methods: corn dry milling and corn wet milling.

The corn dry milling is typically used in plants where the primary end products are ethanol and a byproduct known as Dried Distillers Grain (typically a cattle feed).

Figure 1. Corn Wet Milling

The corn wet milling process is used to produce a variety of products, which are taken from different components of the corn kernel. Thus, the corn kernel must be broken down to allow separation. To do this, the dried corn is steeped in a mild sulfuric acid and water solution to soften and bloat the kernels, making them easier to break open. After steeping, the corn kernel is transferred through a milling process that is made up of three separate mill systems. The goal is to separate the kernel into its components: fiber, germ, gluten, and starch. After the milling process, the components are transferred to the refinery area where they are purified. After this stage, the starch is typically used to make such things as specialty starches used within the paper industry, food-grade starches, dextrose, high fructose corn syrup (HFCS), and alcohols in both fuel and potable grades. The fiber, gluten and germ are typically dried and sold as byproducts.

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Application: steep tank

Figure 2. Corn Steep Tank

A corn steep tank can be equipped with non-contacting radar, vibrating fork switch, and DP level.

Steep tanks are large vertical tanks with conical bottoms. The tank size varies with the mill, but most are about 45 feet high. Some of the larger mills may have tanks of 60 feet or more. For level instrumentation, they are typically equipped with a DP level transmitter for initial water levels, a top-down instrument for overall level, and a high-level shutdown switch.

Within the steeping process, previously mentioned, the corn may steep in this solution for up to 36 hours. The total amount of time involved varies based on the corn and tank size. Water is often recirculated through the vessel to enhance contact with the corn kernels.

After the corn is steeped, the water is drained. The remaining corn is transferred to the first grind mill tank. From there it will be feed into the first grind mill process.


Level measurement of steep tanksThe level measurement of the steep tanks includes several steps. For the initial fill, a small amount of water is added to the heel of the tank. This level measurement is traditionally controlled by a DP level. Once the water has reached a predetermined height, the tank is then filled with dry corn. Steep water is added and recirculated through the steep while controlled at a level to fully submerge the corn.

Figure 3. Fill Steps Sequence of Steep Tanks

The overall level (liquid level) is measured and controlled with a top-down device such as a non-contacting radar. The challenge for the top-down device is that it must be able to measure the level of the dry corn as it fills and then measure the overall level. The dry corn requires a device that can measure solids and handle the dusty vapor space. The same unit must then measure the liquid water which often has some foam layers induced from corn debris and chemical reactions, plus surface turbulence caused by the water circulation. Subsequently, the level instrument must be flexible and able to handle two very different measurement challenges and reliably track the water level as recirculation occurs.

As an extra measure of safety, a switch is installed as a high-level shutdown. It, too, must able to detect overfill from water or corn.

1. Water is added to a predetermined height and measured with DP level transmitters.

2. Corn is added to its maximum fill level and measured with NCR. The high level switch guards against overfill.

3. Water is added to cover the corn and is measured with NCR.

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Figure 4. Water Circulation

Water is recirculated to enhance contact with the corn. The water needs to be maintained at a constant level to ensure corn is always covered. The radar level measurement is used to control the recirculation pump.

Figure 5. Rosemount 5408 Non-contacting Radar

The process seal antenna can handle any dust buildup from the corn and any splashing from the water sequence.


Antenna selection and configuration parametersThe Rosemount 5408 Non-contacting Radar has successfully met the challenges of steep tanks. It can measure the dry corn level as the tank is filled and then switch to the water level measurement for the overall level and control of the circulation system. It can be configured for solids measurement but can easily measure the liquid level as it covers the top of the corn. Other non-contacting radars often require a lot of reconfiguration to optimize their capabilities for both conditions.

To optimize the non-contacting radar for the steep tank, a process seal antenna should be used. This antenna will handle any dust buildup from the dry corn and will tolerate any splashing during the water fill and recirculation sequences.

To set up the device to handle both the dry corn filling and the liquid sequences, the device should be configured as a solids measurement unit. This will provide low threshold settings and echo detection capabilities for those conditions (dry corn, fast filling, and emptying) where the signal amplitude is typically low such as 500 to 700 mV. The unit will still be able to measure the higher amplitude echo of 1200 to 2300 mV, which is common for the turbulent water environment.

Tank filling with dry corn. Amplitude varies 500 to 700 mV.

Full, recirculating level.Signal amplitude ranges1200 to 2300 mV.

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Since the tank has a cone bottom, the bottom offset tank settings for the lower area should be implemented for when the echo is lost in the lower, inclined area of the tank. The unit will then show an empty tank level.

SummaryCorn steep tanks are an important first step in the corn wet-milling process. Obtaining good measurements of the steep tank process is vital to ensuring kernels are ready for the rest of the milling process.

Nearing empty; signal amplitude 735 mV

Empty tank, no echo

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Level measurements of corn steep tanks in corn wet millingCorn millingApplication: steep tankLevel measurement of steep tanksAntenna selection and configuration parametersSummary

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Level measurements of corn steep tanks - Emerson Global · After steeping, the corn kernel is transferred through a milling process that is made up of three separate mill systems