Tissue Digestion - The Tissue Digestion Process

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> Composition of Animal Tissues
> Effects of Alkaline Hydrolysis
> Results of the WR2 Process
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> Control and Safety Systems

Control and Safety Systems

There are multiple levels of control and safety on a BioSAFE/WR2 Tissue Digestor. All Tissue Digestors are built with pressure vessels ASME certified for 100 psi. At 300˚F (~150˚C), the Tissue Digestor operates at approximately 60 psig.

The control system will measure, record, and respond to temperature and pressure information. Contact BioSAFE Engineering if you are interested in recording cycle data and using the data acquisition system.

The unit is equipped with a temperature sensor (thermocouple) in the bottom drain line of the vessel, with additional thermocouples in the piping of the circulation system just beyond the pump (loop temperature), and on the effluent drain line. A thermocouple also monitors the pump bearing housing.

The primary controller maintains the system at 300˚F (~150˚C) by regulating the opening of an automatic valve on the steam supply system. Once the vessel reaches operating temperature this valve opens and closes as needed.

The second level temperature control is set at approximately 15˚F (~8.3˚C) above normal operating temperature and will turn off the steam control valve. If the temperature exceeds the operating temperature by greater than 15˚F (~8.3˚C), this valve closes and remains closed until the unit has cooled to below operating temperature, at which time the normal control system resumes operation.

The system is also programmed to use temperature differences between vessel temperature and loop temperature to determine whether the circulating system is operating correctly. The readings can be used to cause either a second level or third level steam shut-off (as described above under Temperature). The system will stop the cycle time if during the cycle the temperature drops below 300˚F.

The final temperature-regulated action is the addition of cooling co-flush to the effluent as it is being released to the drains. A thermocouple in the effluent line measures the temperature of the effluent leaving the vessel, and controls the flow of cold water that is added directly to the effluent to keep the temperature below a level recommended for release to drain. This functions during the heat-up portion of the cycle to prevent steam condensate (which is channeled to the drain) from overheating the drain, and during draining to keep temperatures within legal drain limits.

The vessel is equipped with a pressure sensor and a pressure relief valve, as well as with a unique emergency pressure release pathway. Although pressure is a direct function of the temperature at which the vessel is operating, separate safety controls based on pressure readings have been built in to increase the number of levels of redundant safeties.

The first level pressure control is contained in a pressure sensor that puts out a 4-20mA signal based on the pressure reading. This signal is fed to the controller and provides the information for continuous recording of the vessel pressure during the operating cycle.

The first level of pressure is set to 80 psig. It may be determined by one of BioSAFE's engineers that this set value needs to be changed. If this pressure is reached, the controller will close the steam valve until the pressure drops below first level pressure and then will continue to operate normally.

The second level pressure control is the pressure relief valve. This is set to release at the rated pressure of the vessel (100 psig). The unique feature of the second level safety pressure control of the BioSAFE/WR2 Tissue Digestor is that the pressure relief valve does not release uncontrollably into the room in which the Digestor is located. Rather, the pressure relief valve releases either under the skid where it can be easily cleaned, out through the roof, or to another location determined by the customer. This yields a discharge in a more controlled fashion allowing anyone in the room to leave until the discharge is complete (a few seconds until the pop-off goes back below 100 psig). Copious amounts of steam will liberate and fill the room, but will dissipate quickly after the discharge ceases.

In the event of a pop-off discharge, clean the area by hosing it down with water, diluting and washing the hydrolysate down the drain. The cause of a release could be run-away steam due to a faulty steam valve or regulator. We suggest that for this Tissue Digestor, 90-120 psig is the maximum steam required for rapid heat-up and optimum performance. If only lower pressure steam is available, contact BioSAFE Engineering to increase the cycle time corresponding to the lower steam pressure available to obtain equal digestion results.