CRS Recycle Digester 2.0

Purpose and Description

The CRS Recycle Digester is a compact digester system with one or two (two being the standard, often referred to as a twin digester) recirculation digesters for increased throughput. It is designed for charges of around 2 kg OD per digester. It is a compact and versatile system for producing typical amounts of pulp in laboratory scale under precise conditions.

The Recycle Digester is primarily designed for conventional batch cooking, but can be used for other purposes, such as simulating pre-hydrolysis. More advanced processes will require a real pilot digester – please inquire for customized systems. The twin digester layout is usually not particularly expandable, if a more expandable unit is desired the single digester is a better choice. The single digester has a lower price in the basic version.

The Recycle Digester features functionality such as steaming (high pressure regulation is an option), automatic pressure control, mid-process sampling, flow metering, high capacity heating, and variable flow control – all through the advanced PC system where complex cooking recipes can be designed.

Other features include very good temperature measurement and control, rapid cool-downs and heat-ups and excellent flow distribution over the chips. Cranes and lifts assist the operators in handling the vessel lids and internal chip baskets.

Valves that are operated during the cooks are pneumatically controlled. Valves for cleaning, discharging etcetera are manual.

The CRS Recycle Digester is the CRS basic system for pulping, but options and expansions can make it more complex, more or less turning it into a real pilot that can run cooks like superbatch.

A twin version of the CRS Recycle Digester

A twin version of the CRS Recycle Digester

Technical Overview

Design pressure: 30 barg

Operational pressure: 0 – 20 barg

Design temperature: 230 C

Operational temperature: app. 30 – 190 C

Materials: 316L equivalent on all process parts, 304 equivalent on supporting structures.

Vessel sizes: App 17 L without basket, 15 L packable size with basket, per digester

Heating: 10 kW for each loop

Flow: 4 – 8 Lpm (recipe controlled)

Sampling: Hot and pressurized vessels can be sampled any time

Steaming: Atmospheric or high pressure, automatic and recipe controlled

Recycle Digester a schematic overview


More technical information

The temperature is controlled with an electrical heater and a water cooler on the digester loops, both dimensioned to achieve rapid heating and rapid cooling. The digester vessels are designed to produce even flows of liquor over the chip beds and the temperature sensors are positioned to really measure the correct temperatures of the in- and out-going liquors – getting these measurements even slightly wrong can have a big impact on data quality.

The packable volume inside the digester baskets is around 15 L. Depending on chip type the charge can range from around 2 kg to nearly 3 kg OD. The chip baskets simplify charging and discharging, and ergonometric considerations have been taken into account when designing handling of the lids and the baskets. The lids are lifted with a crane and motor and the baskets are winched up from the digesters.

A digester charged with chips, but not yet with chemicals

A digester charged with chips, but not yet with chemicals


H- and P-factors are calculated on both the digester top and bottom temperatures, the operator can select which one to use as a stopping point for cooking. The chip bed top temperature can be controlled quite accurately, both during ramps and stable phases.

The flow is top to bottom. The volume of the loop has been minimized to allow lower liquor to wood ratios. The flow is variable and can be set quite high for a system like this. The flow is also monitored with a transmitter and reported to system continuously.

The steam connection allows automated atmospheric or high pressure steaming of the chips – this process can be run against a P factor target if desired.

Charging of liquors is done directly into the vessels, pouring from a canister or similar – there is also an inlet through a quick fit where liquor can be pumped into the digesters. The system is easy to clean between experiments so that carry-over and residue can be minimized.

The system is highly automated and requires little operator attention while in operation. Data acquisition and logging is done automatically. Only samples are taken manually.

The digesters have independent pressure controls, with automated ventilation and nitrogen inlet. Pressure profiles can be set in the recipes as a varying min and max pressure profile over time.

The digester is controlled from a PC, with an HMI system developed in a standard industrial framework, Wonderware Intouch or similar. The program is designed with the ambition to be easy to use, offer broad functionality and flexibility, and also to respect safety.

Double screen HMI program for a recycle digester with two vessels (twin type)

Double screen HMI program for a recycle digester with two vessels (twin type)

A recipe program allows recipes to be built at any time before the experiment is run. The recipe program can be installed on other computers as well, though unless a server solution is also installed by CRS any new recipes will have to be copied manually to the control PC.

Recipes can be started on each digester, independent of the other. The recipes allow profiles of temperatures, pressures and flows, to be built over time, with pre-programmed rapid cool-downs triggered on H- or P-factors or a set time.

The system will continuously log temperatures, pressures, H- and P-factors and other control attributes to either a database or CSV based files (which can be viewed in a spreadsheet program like MS Excel).

On a lower level the machine is controlled from a PLC inside the electrical cabinet.


Scope of delivery & options

The standard scope of delivery includes the following:

  1. Complete recycle digester.
  2. Control computer with all necessary programs installed. Network cables and switch.
  3. Required licenses for hardware and software.
  4. CE labels.
  5. Documentation
  6. Connection points for required supplies.

The options follow below. Some additional customization or options are always possible, please inquire.

Option 1:          Start-up & training – one week on site by one engineer from CRS, assisting with practical issues for the installation as well as doing the start-up and training of operators. The customer will need to provide the necessary resources for bringing the machine to the correct location, as well as connecting it to the supplies, but CRS can supervise where needed.

If this option is not ordered the start-up has to be done by customer. However, in such a case CRS will be happy to receive the customer in our work-shop for a one-day free training and walk-through of the machine.

Option 2:          Spare part package – a selection of spare parts aimed at giving the least possible downtime in case parts fail. Spare parts can of course be order individually at a later stage if needed.


Installation requirements & practicalities

The system has some necessary requirements that need to be fulfilled in order to run it. These have to be prepared by the customer before any installation is to take place.

  1. Power connection – the requirement is a 3-phase 400 VAC 50 Hz connection at no less than 50A capacity. Other setups, like other voltages or 60 Hz, are possible but please check with CRS first.
  2. Steam connection – the system can be run without a steam connection (of course that would limit the steaming though).
  3. Pressurized air connection – the requirement is 6 – 10 bar, water and oil free.
  4. Nitrogen connection – the requirement is at least 20 bar.
  5. Water connection – a water pressure of at least 3 bar is recommended, otherwise the coolers will not work optimally. Tap water quality is preferred.
  6. Drain connection – the drain will receive liquors at up to 100 C. The vent is connected to the drain inside the machine. Beware of smells and toxic gases.

Additionally there are some less critical but nonetheless important aspects to consider.

  1. Safety – the system should be used in accordance with what the risk analysis states. Please make sure that the requirements stated in that document can be met with the chosen lab setup.

The machine weighs around 1,000 kg. It will be supported by machine feet (included with the delivery). The outer measurements of the machine and the recommended floor space are shown below.


To transport the machine into a room the door needs a width of at least 1050 and a height of at least 2050.