UHT milk processing in an aseptic plant
Steam Infusion System - Milk Processing
A truck with a large tanker of capacity of 5,000-10,000 gallons goes from farm to farm pumping cows milk from the farmers storage tanks into the tanker until capacity is reached. The milk is then dropped off at a dairy plant where other large tankers also bring their loads in from other dairy farms. All this milk is stored in raw milk storage silos with capacities greater than 50,000 gallons, and once all quality control testing has been deemed acceptable, the milk is ready to process. For the sake of simplicity I'll review how the UHT system works without going into detail about the separation process of skim and cream. We'll treat this as milk processing straight from raw storage through the units of a steam infusion type system.
From the storage silo milk is pumped to a balance tank. From the balance tank, another pump moves the milk to a plate heat exchanger inlet. After passing through a regeneration loop the product then moves through a set of heat transfer plates. The product ends up at the plate heat exchanger outlet. From here the product flows to a steam infusion chamber. The milk is then pumped through a holding tube. The product then goes through a flash chamber. The milk is pumped out of this unit and moves onto a homogenizer. After homogenization, the milk passes through a regenerative cooling loop, and a 2nd set of heat transfer plates. The milk then flows into a holding tank, which filling machines are tied into.
The balance tank
The balance tank is a vessel that is open to the atmosphere, and has a rather small volumetric capacity. The purpose of the balance tank is to provide the processing system inlet pump a constant pressure so there is no risk of cavitation. Also, the use of a balance tank will eliminate some product aeration. How is a constant fluid level maintained in this vessel? Depending on the engineer's design this can be accomplished a number of ways. I'll review one approach, and describe another.
Product flows in through a valve positioned above the top of the balance tank, and out through a hole in the bottom of the vessel moving towards the processing system inlet pump. The balance tank inlet valve should be pneumatically controlled to allow product into the balance tank in such a way that the balance tank level remains constant. In order for this to happen the balance tank must be equipped with a level sensor, which can detect the fluid level. If this sensor is tied into the computer controlling the system (PLC - programmable logic controller), changes in the fluid level dropping below the set-point will send a signal to the PLC that will tell the balance tank inlet valve to open via pneumatic pressure. Fluid level changes that exceed the set-point will send a signal to the PLC that will force the balance tank inlet valve to close and reduce flow. This is simple on-off control. While this control mechanism exists in production facilities - more popular schemes utilize PID algorithms to control the level in the balance tank by opening the valve only as much as needed.
Mass transfer in the balance tank can be modeled from a derivation of Newton's 2nd law - Bernoulii's equation. Simply stated: an increase in the velocity of an inviscid, incompressible fluid occurs with a decrease in pressure.
Plate heat exchanger
Regeneration loop and plate heat exchanger
Plate heat exchangers are an excellent way of heating or cooling a product rapidly. I'll review the regeneration loop briefly. It is a method of using a plate heat exchanger to warm the raw product stream with the hot sterile product stream on the other side of the plates - this is called product-product regeneration. Alternatively, a regeneration loop can be a heat transfer medium that gets warmed up as is cools the hot sterile stream, and cooled back down as it warms up the cold raw product stream.
The next section of plates is referred to as the pre-heater. In this section of plates the cold raw product that has just come from the regeneration section of plates needs to be warmed up. Often times, a shell and tube heat exchanger is heating (with steam) hot water as the heat transfer medium on the other side of the plates. Control of the temperature coming out of the preheater is of great importance for the process. This can be accomplished by monitoring the temperature of the preheater outlet, and for temperatures that vary from the set-point - a pneumatic signal can be sent to the steam valve input to the shell and tube heat exchanger. If the temperature is too low, the steam valve will open which will increase the hot water temperature and allow more heat to be transferred across the plates to the raw product.
Modelling energy and mass transfer in plate heat exchangers can be highly complex for dynamic systems with disturbances. Many books have been written about this topic, and every set of plate heat exchangers must be evaluated independently for the process it is to be used in.
A basic energy balance will show:
V1 x density1 x Cp1 x ΔT1 (product) = V2 x density2 x Cp2 x ΔT2 (media)
Temperature profile is looked at with a LMTD (logarithmic mean temperature difference).
Steam Infusion Chamber
The steam infusion chamber is a remarkable piece of equipment. In this vessel milk mixes with steam and reaches sterilization temperature at the exit stream. High pressure steam feeds the vessel and the milk "rains" down vertically through the steam thereby maximizing heat transfer from the steam to the milk by directly allowing the two streams to mix. Modelling this unit requires a great deal of expertise and knowledge of complex thermodynamic principles.
For control purposes - the key parameters to consider are: fluid level in the infusion chamber, infuser inlet and outlet temperature, product flow rate, and chamber temperature. The chamber temperature is controlled directly by the steam valve feeding the infuser. The infuser inlet temperature is an important parameter for controlling the flash chamber. This will be reviewed in the flash chamber capsule.
The holding tube is a section of pipe that must be designed in such a way that assures the process will destroy bacteria in the product. This is done by modelling the flow characteristics, residence time, and minimum allowed temperature at the outlet of the tube. Milk processed this way in a high flow process will typically be around 290F, 60GPM for 2 seconds for the entire length of the the hold tube. I'd strongly recommend further reading on the regulatory aspects of this piece of equipment.
Product inlet to the flash chamber is still very hot as it is coming straight from the holding tube. The goal of this unit is to remove the steam that mixed with the milk in the steam infuser. This product inlet stream of milk/steam is separated into two streams in this vessel. The steam is drawn out of the top of the vacuum vessel to a cooled condenser where is is returned to a liquid form. The product stream exists the bottom of the vessel. The temperature of product exiting the chamber is controlled to match the infuser inlet temperature. This can be controlled by adjusting the flash chamber fluid level or controlling the vacuum. Much like the steam infuser this unit is difficult to model, as it is very complex.
Homogenizer and final cooling
To avoid the issue of milk separating out - homogenizers are employed in nearly every milk processing operation. Fat globules are reduced in size and evenly dispersed in this unit. This is done by way a large motor driving pistons to force milk through small orifices. Pressures of 2,000-4,000psi impart great forces on the individual fat globules in the milk. After the milk exists the homogenizer it is now homogenized. From there it goes to cool in the regeneration loop and a final cooling press.