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In years past, when people would ask us about weighing the pros and cons of continuous processing using a vibrating fluid-bed dryer versus drying in a batch process, the conversation largely centered on improving inconsistent product quality and on saving time in cleaning between batches.

The benefits of continuous processing were clear: The continuous drying process yields a consistently homogeneous product that is more likely to meet target specifications at discharge than any batch drying process simply by virtue of eliminating batch-to-batch inconsistencies. By operating nonstop, the continuous drying process also eliminates the cleaning and setup steps required in between every batch. Yet, still, some processors preferred batch processing because it better suited their process and production conditions.

Today, these conversations sound a little different. Product quality remains paramount, but more processors are interested in achieving the desired level of quality without requiring a staff of trained operators. Automating the drying process offers appealing protection against the challenges of hiring and retaining operators with the skill to produce an identical recipe every day — and the willingness to simply show up and do the job.

At a time when many companies are enjoying record spikes in demand, the inability to fully staff processing and production lines often limits companies to a single shift when demand requires two or even three shifts. Factors affecting the equation include items such as the cost of recruitment, wage increases and strengthened benefits packages to make these positions more attractive.

Drying in a continuous process such as this vibrating fluid-bed dryer requires less staff than drying in batches. Photos credit: Witte Co. (Click on the image to enlarge.)

At the same time, processors must address new safety concerns and potential regulatory updates as well as train and upskill new hires. Further, the personnel costs involved in running a batch process recur and must continuously increase to retain staff. On balance, the one-time investment in continuous drying to automate the process becomes more attractive.

Consider the labor requirements of a typical batch drying process. Whether a fluid-bed dryer, tray dryer or another type, once the material is mixed, blended, reacted and ready for drying, in a batch process, a worker typically needs to physically carry it or manage the transfer to a hopper or conveyor to load the dryer. Then, one worker manages the drying process while another worker cleans the upstream equipment in preparation for the next batch.

In a walk-in tray dryer, for example, each of several dozen trays must be loaded manually and then removed before transferring the dried material to equipment downstream or into storage. Though some batch dryer manufacturers have enhanced their equipment with elements of automation, the upgrades are intended primarily as a convenience rather than to allow operation without a large staff of skilled workers.

Contrast the labor requirements of a typical continuous drying process. Whether a vibratory fluid-bed dryer, rotary dryer, drum dryer or belt dryer, once the material is ready for drying, it is automatically transferred from the mixer, blender, reactor or other equipment by conveyor to a feeder. Depending on the material’s moisture content, the feeder automatically meters the material into the dryer at a preset rate. The material moves from infeed to discharge in a first-in, first-out process before continuing directly downstream to the filling or packaging line.

Operator involvement is limited. For instance, an operator may monitor and adjust the process on-site or remotely via the web, or they may pull a sample for testing as desired. Overall, however, the operation occurs without requiring workers to lift heavy sacks of powder, stop the process for cleaning or risk contact with the material (contact that could cause product contamination). The drying process may continue to operate shift after shift regardless of worker attendance.

Continuous processing links drying to upstream processes such as mixing and to downstream processes such as filling without stopping the operation. A vibrating dewatering screen is shown feeding a vibrating fluid-bed dryer Photos credit: Witte Co. (Click on the image to enlarge.)

Impact on Efficiency

The heat transfer efficiency of a drying system differs from one type of dryer to the next based on a number of factors. These include:

• The amount of particle surface area in contact with the heat source.
• The amount of heat generated that is applied to moisture removal versus lost in the process.
• The effects of ambient air temperature and humidity.

These factors remain constant whether in a batch or continuous drying process. What changes in a continuous drying process is not heat-transfer efficiency: it is energy efficiency.

As Newton famously demonstrated, it takes more energy to get started than to keep a process that is already in motion going. In a continuous process, the dryer may operate for several months at a steady-state condition before being shut down and restarted.

In a batch process, however, the dryer is typically shut down, cooled, cleaned, inspected and then restarted as standard procedure for every batch. This may happen several times in a single shift. Each restart requires the burner or other heat source to draw relatively large amounts of energy to ramp up to the proper temperature from ambient conditions. Similarly, starting, stopping and restarting air circulation fans, motors, vibrators, conveyors and other powered equipment causes transient energy losses and also allows heat in the process to be squandered as waste. In many cases, equipment upstream and downstream also needs to be shut down while the drying process is offline. The speed and efficiency of these shutdown and startup processes depend on the skill and ability of staff, which adds to the challenge of controlling costs and maintaining consistent product quality.

The inside of a vibrating fluid-bed dryer is shown with the cover raised to ease access. This view reveals autoweirs that adjust retention time to promote the first-in, first-out material flow of a continuous drying process. Photos credit: Witte Co. (Click on the image to enlarge.)

Quality Control

Instrumentation — to measure temperature, moisture content and other parameters — enables operators to test the condition of the material in the process and verify whether it meets the targeted drying specifications. Sampling and quality testing may be performed in both batch and continuous drying systems in much the same way, taking the same amount of time.

Many modern continuous dryers, however, automate this process. Built-in temperature and moisture meters continuously monitor the material at the inlet and outlet. These controls can trigger an alarm automatically if the material meets preset conditions. This allows adjustments before quality targets can become compromised. Some online sensor-based systems can even self-adjust the dryer to accommodate changing conditions at the inlet such as a surge of material or a higher moisture content than expected. The range of likely conditions and automatic adjustments may be programmed before or during installation to ensure consistently high quality drying with or without worker involvement.

The most common source of inefficiency and quality control failures in a process is often human error. For that reason, the idea that the process and product may be improved — and operating costs reduced — while resolving the current shortage of skilled (and available) labor offers quite an incentive to review existing processes.

In conducting such a review, the first step is to determine the actual costs of operating the existing drying process. This evaluation should consider the costs related to energy consumption, maintenance and replacement parts, and the labor costs required to operate the dryer. Accounting for downtime for cleaning and the occasional batch discarded as waste due to error can help to arrive at a realistic cost assessment.

This cost assessment then can be compared to a cost assessment based on switching to a continuous drying process. In many cases, the enthusiasm over the projected cost reductions may be dampened by the initial investment required to design and manufacture the drying equipment for a continuous process, along with the companion feeding and unloading equipment, plus the modern control system. In some cases, it is less costly to build an entirely new processing line based on continuous principles than to upgrade the drying step alone or to convert each step to continuous.

By considering the payback period, an accurate return on investment may be calculated. In my experience, the payback period on a continuous drying system is often met in one to three years. Beyond the payback period, all of the cost reductions from the switch are realized every year into the future.