HTC Reactors

In this section three types of main reactors that are used normally in chemical applications are summarized in Table 7 with some brief characterizations at section 2.4.2.1. Further, two more small-scale reactors which used for the hydrothermal carbonization of biomass reviewed in pervious works at the Institute of Chemical and Eenergy Engineering, University of Natural Resources and Life Sciences, Vienna (BOKU), are explained. More details of this last table are available in index 1.

2.4.1 Batch Reactor and Continuous Reactor

Design of Batch Reactor, Continuous Stirred Tank Reactor (CSTR) reactor, and Plug Flow Reactor (PFR) was depend on the mode of operation (Nanda, 2008). Some of their characters, principles and their area of applications are shown in table 7. Some more examples of reactors in different studies are mentioned as well in the following table 8.

Table 7. Comparison of Chemical Reactors (Nanda, 2008)

Type of Reactor Principle of work Advantages Limitations Area of application

Batch Reactor

All reactants are added at the commencement and the product withdrawn at the completion of the reaction. They are conducted in tanks attached with impellers, gas bubbles or pumps.

Suitable for small scale production

• Suitable for processes where a range of different products or grades is to be produced in the same equipment

• Suitable for reactions requiring long reaction times

• Suitable for reactions with superior selectivity

Not suitable for large batch sizes

• It is a closed system in which once the reactants are added in the reactor, they will come out as products only after the completion of the reaction

Batch processes are used in chemical (inks, dyes, polymers) and food industry

Continuous Stirred Tank Reactor

(CSTR)

One or more fluid reagents are introduced into a tank reactor equipped with an impeller while the reactor effluent is recovered. A stepped up concentration gradient exists

• Highly flexible device

• By products may be removed in between the reaction

• It is economically beneficial to operate several CSTRs in series or in parallel.

• Reaction can be carried out in horizontal as well as vertical reactors

• More complex and expensive than tubular units

• All calculations performed with CSTRs assume perfect mixing

• At steady state, the flow rate in must equal the flow rate out, otherwise the tank will overflow or go empty

Chemical industry especially

involving liquid/gas reactions

Plug Flow Reactor (PFR)

One or more fluid reagents are pumped through a pipe or tube.

These are characterized by continuous gradients of concentration in the direction of flow

• Higher efficiency than a CSTR of the same volume

• PFRs may have several pipes or tubes in parallel

• Both horizontal and vertical operations are common

• They can be jacketed

• Reagents may be introduced at locations even other than inlet

Not economical for small batches

The tubular reactor is specially suited to cases needing

considerable heat transfer, where high pressures and very high or very low

temperatures occur

Table 8. example of some HTC reactors in different studies

Reactor Study Reference

Two-chamber reactor HTC of Lignocellulosic

biomass Reza, 2011

Batch reactor

HTC of biomass for energy and crop

production

Reza, 2014

160-ml stainless steel tubular reactor

HTC of Municipal

Waste Stream N. D. Berge, 2011 Grenolmatik ZHAW HTC of bio-waste/fecal

sludge Z. Robbiani, 2013 Autoclave ZHAW HTC of bio-waste/fecal

sludge Z. Robbiani, 2013 Diving Bottle HTC of bio-waste/fecal

sludge Z. Robbiani, 2013 TFC engineering, Buchs HTC of bio-waste/fecal

sludge Z. Robbiani, 2013 AVA-CO2 HTC of bio-waste/fecal

sludge Z. Robbiani, 2013 Umwelt Campus

Birkenfeld

HTC of bio-waste/fecal

sludge Z. Robbiani, 2013 Cube of Destiny HTC of bio-waste/fecal

sludge Z. Robbiani, 2013 Agrokraft HTC of bio-waste/fecal

sludge Z. Robbiani, 2013 40-ml stainless steel

reactor (Optical cell)

Set up and process

control F. Haiböck, 2015

1.7-l self-built reactor

Study of Parameter variation during

biomass HTC

C. Preinknoll, 2013

1 L lab scale reactor (Hanwoul Eng. CO., LTD., Gunpo, Korea)

HTC of Cellulose D. Kim et al., 2015

Batch and continuous

reactor Waste sludge materials M. Child, 2014

2.4.2 Reactors at BOKU University 2.4.2.1 Optical Cell

So far hydrothermal carbonization process has been operated with an optical cell, which is available in University of Natural Resources and Life Sciences (BOKU) in Vienna. This optical cell has been introduced in a bachelor thesis under the title of “Set up and process control of an optical cell used for hydrothermal carbonization” (F. Haiböck, 2015). The general idea of building up this optical cell was to allow the experimenter to have eyesight inside the cell through a sight glass so as to monitor the physical changes during chemical processes of hydrothermal carbonization. The optical cell is an aluminum cube in size of 151*151*118 mm with copper pipes (figure 7) and a stainless steel cylinder-like container with 44ml volume capacity (figure 8) located inside the cube and fixed with screws. Tubes, screws and the glasses used in the cell have to stand temperature up to 200°C and 200 bar pressure (F. Haiböck, 2015).

Figure 7. Aluminum cube with copper pipes Figure Figure 8. Stainless steel cylinder-like container

(F. Haiböck, 2015)

In order to heat up the cell a Haake N6 temperature controller is used connected to a container with a pump that helps streaming the hot non-hazardous mineral oil through the flexible tubes to the optical cell cube (F. Haiböck, 2015). After doing a couple of pre-tests in

May 2016, tubes show leakage problems. Therefore, it was decided to heat up the cell electrically.

A hardware combined with an open source software program called Arduino (figure 9) recorded time, temperature data every 10 seconds during the HTC process. A wire connector from the hardware attached to the body of the cell, transferred heat data to the software that has installed on a computer.

Figure 9. Arduino hardware (F. Haiböck, 2015)

2.4.2.2 Self-built Reactor

The reactor was built by stainless steel material in a cylindrical shape. The reactor had about 1.5 L capacities. Reactor could have sealed with 12 screws and a washer. During each test the reactor is placed in a metal box isolated with wools all around it, plugged into the power and attached temperature and pressure sensors to the sensors coming from a power machine, which is manufactured by B&R company (www.br-automation.com). The power system programed to read temperature and pressure from reactor, and the desired temperature is also set on the system to control the heat of the reactor. Heating power safety can be set to a desirable percentage. And also to prevent over shooting, there is another temperature option to be set to max desired temp. Therefore, at this point the heating power is reduced 10%

automatically for threshold safety. More safety options, operated information and data saving are presented at 3.2.1.1 section.

3. Materials and Methodology