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Cambi AS
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PROCESS DESCRIPTION
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509000
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MA
001
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Degremont SA
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TABLE OF CONTENTS
TABLE OF CONTENTS ..................................................................................................................................... 2
1
INTRODUCTION......................................................................................................................................... 3
2
THERMAL HYDROLYSIS ........................................................................................................................... 4
2.1
Pre heating in Pulper ............................................................................................................................. 4
2.2
Thermal Hydrolysis in Reactor .............................................................................................................. 4
2.3
Flash tank and Post-treatment .............................................................................................................. 5
2.4
Odour Management .............................................................................................................................. 6
2.5
Vacuum break system ........................................................................................................................... 7
2.6
Safety .................................................................................................................................................... 7
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1 INTRODUCTION
The main concept for the Cambi thermal hydrolysis process is a stepwise heating and cooling of sludge
through pressure and temperature control. The process consists of three steps:
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Sludge heating in the pulper / pre-heater tank
Thermal hydrolysis in the reactor
Pressure let down in the flash tank
During the hydrolysis process, cells and cell clusters are broken open and resolved and therefore become
more easily accessible for digestion. This results in a higher potential for the conversion of volatile matter
and stabilization, especially for biological sludge. Dissolved organic solids are more digestible than
suspended solids (cell walls, etc.). The dissolution of cellular organic material increases with degree of
hydrolysis.
The process description should be read in conjunction with the Process Flow Diagram (PFD) for the THP
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2 THERMAL HYDROLYSIS
Each train in the thermal hydrolysis plant consist of one pulper, 2 pulper circulation pumps (duty/standby) 2
reactor feed pumps (Duty//Standby), 3 reactors in parallel (extendable to four), one flash tank, 2 mixing
chamber feed pumps (Duty/ Standby), one foul gas cooler, access platform and necessary pipes, valves,
instruments and a process control system. The two trains will have a common foul gas skid, with
duty/standby foul gas compressors.
Thermal hydrolysis of sludge takes place between dewatering of raw sludge and the digesters. The Cambi
thermal hydrolysis process for sludge treatment is designed to achieve the following benefits:
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Secure pathogen free sludge (Grade A)
Minimize digester volume
Enhance the conversion rate of organic material into biogas and green energy.
Increase the dewaterability of sludge after digestion
Reduce the quantity of stabilized sludge for disposal, and produce a product suitable for beneficial use.
2.1 Pre heating in Pulper
Upstream hydrolysis, the raw sludge is dewatered to ~16 % DS. Dewatered sludge is fed from predewatering centrifuges (by others), via sludge silos to the pulper/preheater of each train. In the pulper the
sludge is pre heated by the injection of recycled steam from the reactors and the flash tank. The pulper
provides the necessary buffer capacity for recovery of energy from reactors and flash tank. The sludge is
homogenized by circulation via pumps. Pre-heated sludge is then pumped into the reactor where hydrolysis
at high pressure and temperature takes place.
Sludge from the silos is continuously pumped to the pulpers where it is pre-heated. The increase in
temperature decreases the viscosity of the sludge, enabling it to be kept in suspension through the use of
circulation pumps. The pulper is enclosed and pressurised to assist odour control and enhance energy
transfer requirements.
2.2 Thermal Hydrolysis in Reactor
From the pulper the pre-heated sludge is pumped into the reactor. Steam is then injected directly into the
reactor until the required operating pressure and temperature is reached. The reactors are constructed for an
operating pressure of 11 barg which means a maximum operation temperature of 180C. Normal operation
conditions are 165°C (6 barg)
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The standard time cycle for the thermal hydrolysis process in one reactor with a process temperature of 150180°C is:
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15 minutes for filling the sludge into the reactor
15 minutes for steam injection from the steam boiler
30 minutes retention time
15 minutes to allow the steam to flash into the pulper
15 minutes to flash the sludge to the flash tank
The reactor is a batch process. The total time for one reactor cycle will be optimised during start-up with
respect to energy balance. Multiple reactors operate on a staggered basis, creating the effect of a
continuous flow between the pre-dewatering and the anaerobic digestion system.
When the hydrolysis process is complete, a pressure release valve at the top of the reactor is opened
gradually and the pressure is reduced from the operation pressure to approximately 3 barg (133C). This
steam is released into the pulper for pre heating.
The pressure vessels are equipped with safety valves and all necessary instrumentation.
2.3 Flash tank and Post-treatment
After hydrolysis, sludge is passed into the flash tank, where the pressure and temperature of the hydrolyzed
sludge is decreased by flashing steam back to the pulper. The flash tank gives the required buffer capacity to
release the energy contained in the sludge and for the down stream pumps.
The main purpose of the flash tank is to release steam contained in the sludge. The main outlet valve in the
bottom of the reactor is opened and the sludge is flashed to the flash tank by the pressure difference in the
reactor and the flash tank. Surplus steam goes to the pulper.
The sludge has a temperature of approx. +102/105°C after de-pressurizing.
Between the flash tank and the digesters the sludge will have to be diluted from 13-14% dry solids to 9% dry
solids. Dilution is required for several reasons:
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To decrease the temperature of the sludge in order to protect the mixing chamber feed pumps stators
To decrease the viscosity of the sludge
To avoid high concentrations of ammonium in the digesters.
The sludge is pumped from the flash tank to the mixing chamber with the mixing chamber feed pumps. There
are two feed pumps installed for each line, one duty and one standby.
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2.4 Odour Management
The vapour from the pulper contains gases with a bad odour which have to be controlled before outlet to the
atmosphere.
Cambi’s odour management philosophy for high intensity odours from the hydrolysis process is:
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Contain it
Do not dilute it
Treat it continuously
Duty / assist system
To avoid leakage in any part of the system, the following precautions are taken:
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The vessels are 100% leak proof pressure vessels (certified pressure vessels)
All sludge valves are slide gate valves with enclosed slide to prevent drip
All valves in the foul gas system are leakage proof
All pipes are welded with inert gas backing
All connections are flanged
The odour from hydrolysis process consists mainly of mercaptans, amines and H2S. All flash steam from the
process ends in the pulper. To secure the outlet of the non-condensable gases, a controlled flow is released
from this vessel. The gases released from the pulper first pass a water-cooled foul gas cooler. All condensed
liquid is fed back in to the pulper.
The foul gases are compressed to the necessary pressure using closed circuit ejectors, and injected into the
sludge pipe to the digesters. The ejectors and their circulation systems are duty/standby to secure reliability.
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2.5 Vacuum break system
At normal conditions, there is a small excess pressure in the pulper, but to prevent vacuum in this vessel and
to protect the circulation and reactor feed pumps from cavitation, it is equipped with a vacuum break system.
This system is also connected to the flashtank to prevent vacuum in this vessel and to protect the digester
feed pumps from vacuum as well.
The vacuum break system on top of the pulper vessel includes two non-return valves. Cambi’s experience is
that there might be some very small leakage of foul gas from these valves. To avoid leakage to the
surroundings these valves are connected to the overall odour treatment of the plant.
The overall odour treatment plant continuously draws air from the surroundings through the two swan necks.
This ensures that all minor leakages of foul gas will follow the air to the overall air treatment at the plant.
2.6 Safety
All the vessels in the Cambi system are protected with pressure safety valves. Between the vessels and
pressure safety valves, rupture disks are installed to prevent release of gas in case of a leaking safety valve.
The pulper and flash tank also have vacuum break valves. Any leaks here due to dirt on seats will release
odour. Proper maintenance is critical. We recommend that all these points are connected to the odour
treatment system on site. This odour extraction line must not be made of plastics.
All Vessels and hot pipes are insulated to maintain a surface temperature below 60 ºC.
This document and its content is the confidential and exclusive property of Cambi AS,
And is not to be reproduced or used in any matter whatsoever without Cambi's written permission.