Answer the questions:

1. What two major types of high pressure processing of food products do you know?

2. What three main parts does equipment for high pressure batch processing consist of?

3. What kinds of pressure vessels do you know? What is the difference between them?

4. What techniques can be used to apply a tangential compression on the inner core of pressure vessels? What are their peculiarities?

5. Why is the vessel surrounded by a yoke?

6. What does the position of the vessel in the yoke depend on?

Hydraulics.

The pressure vessel is pressurized by means of a pressure medium, which should be a suitable liquid for food application processes. Typically, water mixed with a small percentage of soluble oil for pump lubrication is used. For reasons of food safety, the pressure medium must be nontoxic and hence no mineral oils should be used. The liquid is pressurized using hydraulic pumps, which have a typical power of 100 to 200 kW for production-scale units. After the vessel is loaded with a batch of products and mounted in its yoke, it is filled with pressure medium using a low-pressure, high-volume pump. During this first step, air may be removed through a de-aeration valve. In the second step, the system is pressurized to its operating pressure, either through internal or external compression.

I n the case of internal compression, the system is pressurized by a piston, driven at its larger diameter by a low-pressure pump. The operating pressure in the vessel equals the pressure at the low-pressure side multiplied by the ratio between the surfaces of the piston. The advantage of internal compression is that it achieves fast pressurization rates. Its main disadvantage is that it requires a dynamic high pressure seal between piston and inner core. Another disadvantage is that it is a single-shot operation; in case of leakage, pressure compensation is not possible.

Fig.17. The ACB Pressure System-Alstom Hyperbar.

Pressurization by means of external compression takes place through an external high pressure intensifier that pumps liquid into the vessel under high pressure. This intensifier operates through the same principle of pressure multiplication as the plunger described above. The main advantage of this technique is that static seals can be used. In practice, for operating pressures up to 600 MPa, external pressurization is common. For higher pressures, internal pressurization is more efficient than external pressurization. Because of the rapidly increasing viscosity and relatively low-diameter tubing, pressurization through an external pump is a very slow process at high pressures. Furthermore, all external components in contact with high pressure (intensifier, valves, and tubing) must be well designed to ensure an effective number of cycles.

For operational efficiency, it is recommended to couple two smaller pressure pumps with half the capacity needed instead of using one large pump. If one of the pumps is out of operation because of malfunction or maintenance, the system can still be operated.

For all high pressure processing equipment, seals are the main bottleneck. In all designs, either static or dynamic seals have to be applied. Dynamic seals are especially sensitive to wear and tear and should be replaced after a relatively short number of cycles, depending on pressure. The external intensifier pump is preferable, because the dynamic seal is placed in the external pump on a small diameter. Small-diameter seals have a longer lifetime than large-diameter seals and are easy to replace. In 1998, ABB advised purchasers of their internal intensifier units to replace seals monthly when operating at 400 MPa, and weekly when operating at 600 MPa. Thus, seals may be the main factor in maintenance costs. Recent findings suggest that new materials will do a better job and reduce maintenance costs significantly.