Spray Dryers & the Koshering Process

May 2, 2006

Spray Drying is an effective mode of drying due to simultaneous mass and heat transfer. Spray dryers are used by the industry in chemical and food processing to manufacture dried eggs, powdered milk, animal feed, cake mixes, citrus juices, coffee, corn syrup, cream, creamers, fish and meat concentrates, infant formulas, potatoes, shortening, starch derivatives, tea, tomato puree, yeast, and yogurt. The substances are homogenous in nature and the powders are usually spherical in shape and approximately 10-140 microns.

The spray drying process begins as preheated solution and is pumped to an atomizer. The atomizer increases the surface area of the solution by creating a fine mist. The mist is then sprayed into a chamber of air heated to a temperature above the vaporization temperature of the solution’s solvent. As the mist contacts the hot air, the solvent (typically water) vaporizes. The rate of solvent vaporization is dependent upon the solution flow rate, temperature of solution, flow rate of air, temperature of air, size of the mist droplets, and total solid concentration. The vaporized solvent and dried particles are then removed from the main chamber. A cyclone separates the entrained particles from the humid air. The dry particles are forced to the bottom of the cyclone separator and the air is expelled to the atmosphere or goes to a scrubber.

The drying kinetics as represented by a mathematical model is the core of a dryer-wide computational fluid dynamics model. As a drop of solution is sprayed through a hot gas, heat transfer occurs through the transfer of heat from the gas to the particle. This method of heat transfer is commonly referred to as convection. As the particle passes through the hot gas, vaporization occurs. The energy associated with this term is latent heat. The latent heat of vaporization is the specific enthalpy1. change associated with a phase change (i.e. liquid to gas).

Koshering Concerns And Procedures

The kosher status of spray dryers and their correlating equipment is particularly important when assessing food production areas where sprayed dried food chemicals and minerals are used. For example, an inherently kosher product that was dried on a non-kosher spray dryer or its accompanying equipment cannot be certified kosher. Additionally, production plants often desire to alternate the designation of their product from dairy or non-kosher to kosher pareve or kosher dairy. Thus, the integrity of the koshering process is intimately related to the koshering of spray dryers.

There are over 20,000 drying facilities in the United States alone. These can be divided into two groups: those dedicated spray drying facilities committed to a specific product, and toll drying facilities that contract to spray dry food products of any variety. There are some products reaching these facilities that may be non-kosher by nature or non-kosher because they contain some form of non-kosher ingredient.

Spray dryers in this field come in many varieties, such as tall tower dryers, box dryers (both flat bottomed and coned shaped), rotary dryers, vacuum dryers and many other systems that are still in development. As some varieties of the spray dryer are designed to be product specific (groups of products), the design and the characteristic of the heat transfer occurring between the air and the solution to be dried changes often. Therefore, we cannot design a standardized protocol for the koshering of all spray dryers.

These design changes are made due to the viscosity of the solution sprayed into the equipment, the viscosity of the solvent and the desired final particle formation. The dryer operates on high temperature and the particles are removed of a majority of their humidity via their first contact with the airflow, whether the airflow is static or spiraling. Additionally, the particles land on the walls of the dryer, its ductwork, cyclones, bag house, etc. and the product is actually “baked” so to speak, on the surface. An ideal method of koshering this equipment would be to raise the temperature of the spray dryer to 450°F + surface temperature. Unfortunately, a spray dryer that can realistically endure temperatures of this magnitude does not exist.

Another method that theoretically could be used is to fill the body of the main chamber, the ductwork, and the bag house with boiling water. However, this method is impractical, as the equipment is not designed to withstand the pressure and the heat velocity of the volume of water necessary for the koshering process. Therefore, the goal is to kosher the equipment in a manner that will not cause any damage to the equipment while simultaneously achieving the goal of rendering the equipment kosher or kosher non-dairy as needed by the company.

Fortunately, in the past few years methods have developed to achieve this goal. For example, in a dairy company where milk arrives with a very low solid content in ratio to water, the company would like to incorporate a nondairy and/or kosher dairy drying system. In this scenario, the rabbi needs to establish the status of the steam system. Thus, the Rabbi will review the equipment being used in the process of koshering, as well as the heat sources (i.e. steam) and its kosher status.2.

This company is using an evaporator to strip the excess water from the milk to a ratio of 40-60 percent solids/water. The water (also known as cow water) is collected and fed into the boiler system from one or two stages of the evaporator in order to create new steam. The water collected from the evaporating system might be nonkosher if the evaporating system was used previously for the evaporation of certain whey powders of non-kosher origin. Thus, the steam is either nonkosher or kosher dairy.

What this means in terms of the plants ability to incorporate a kosher dairy or kosher pareve drying system into their operations, is the necessity to drain and replace the water supply of the boilers. This must be achieved in a manner that is within the guidelines of kosher laws without creating financial burden to the plant or damage to the boiler and steam systems.

In a different scenario, an industry that creates a solution on site by blending a group of raw materials with water or another liquid as a solvent, into a sprayable solution, wants to render their system kosher dairy or kosher pareve. Here the rabbi must examine the plant kitchen were the solution was developed and assess which equipment was used and in what manner. The rabbi must always thoroughly examine the rest of the spray drying facility. He will take into account the existence of fluid bed dryers and whether they are a part of the main chamber or freestanding units.

The rabbi will examine the agglomerating system to assess whether it is a freestanding unit with its own heat source or whether it has been incorporated into the atomizing system on top of the main chamber. All homogenizers, the power pump, product pumps, the pressure line into the chamber, and the return lines, the atomizing system and most importantly the CIP (Cleaning In Place) system will be noted. He will then develop SOP’s (Standard Operating Procedures) for the most effective method of koshering the facility.

We must note that every koshering process of a spray drying facility will start by a meticulous cleaning of all equipment and a 24-hour down time to follow. This is the precursor to any koshering process performed.

As such, the main facility for koshering will normally be the CIP kitchen.We must ensure a high volume of boiling water can be used for the koshering process and the systems ability to deliver this boiling water in a large enough line with enough pressure for the koshering. The atomizer and the spray nozzles can be koshered separate from the dryer as for the most part small equipment can be connected through the CIP system and looped to have water circulating into them with a measured return temperature that exceeds 200°F. However, the main chamber, the cyclones, the arms, and the ductwork are more difficult to engulf in the boiling water that is necessary for koshering.

If we spray water at 211°F at the entry into the chamber the water will cool off as it cascades on the walls of the chamber and will exit at about 150°F.This is because every gram of water that is sprayed on the walls will extract 500 BTU’s as it moves downwards.The accumulated BTU force is “looking” for the closest escape rout out of the main, and in most cases, the heat will escape to the cyclone’s arms, and be sucked out towards the exhaust to the atmosphere. The water which cascades down by gravity will be therefore cooled off, and as a result, we will not have the necessary exit temperature of 200°F at any given time.

Therefore, we have to design a program that will specifically address the main chamber and its characteristics suitably. The preferred way to accomplish this would be to enter the system with the highest temperature of water possible, by boosting the temperature with a steam injector as close as possible to the entry into the chamber.The water will then circulate at the highest volume possible to the kitchen or the CIP kitchen.There the water can be re-heated and returned to the chamber. The same applies to the cyclones and the ductwork.

Modern equipment has spray heads, some of which are retractable and can ensure a high volume of water reaching many areas of the chambers and arms, thus shortening the duration of the koshering process dramatically. This will also result in conserving the amount of water and energy used. Spray dryers equipped with bag houses where the fine powders of the drying are trapped and recirculated into the system, as an agglomerating media, need to be koshered separately. This can be achieved through the CIP system.

Our target is to have a high volume of water spray the equipment thoroughly, at a minimum of 210°F.The water temperature will be measured at the exit or return at the lowest point of the equipment, or at the return point to the CIP system. The exit temperature must not be lower than 200°F.

As you can see, it is incumbent upon us to respond consistently to the unique needs of the various spray dryers, the associated equipment, and the substances being dried with innovative and scientifically sound koshering methods.That is, as mentioned above, there are no uniform methods that will suit all the needs of these various systems. It is only through the collaborative efforts of the Rabbi, with the facility’s engineering team, management, and employees, that the unique koshering needs of each spray drying facility can be met.

1. The thermodynamic function of a system, equivalent to the sum of the internal energy of the system plus the product of its volume multiplied by the pressure exerted on it by its surroundings.
2. Keep in mind though that this will change in reference to the specific industry in which the spray drying facility operates.


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