Extraction module

4.1.1 Description of module

4.1.1.1 Introduction

Sugarcane needs to be processed in order for the valuable component (sucrose) to be extracted. Cane preparation refers to the process of taking whole stick cane and turning it into a fine mulch. This process is done in two parts: Firstly, the cane stalks are roughly broken up by knives; secondly the cane is shredded into a fine mulch. Juice (a mixture of sucrose and water) can now easily be extracted from the fibrous matter (Rein, 2007, page 79).

4.1.1.2 Cane preparation

Cane arrives at sugar mills (in sticks of about 1 metre length) and is offloaded onto feeder or spiller tables. The cane is then conveyed to one or two sets of cane knives. Steam is used to power a turbine which drives the knives. The cane is roughly broken up by the action of the knives.

 The cane knives are modelled by a duplicator block in Aspen Plus®_{. Only a single stream} leaves the duplicator block (i.e. the inlet stream is copied to the outlet stream). The block is shown because of its corresponding turbine.

The chopped cane is conveyed to a set of hammers which are called shredders. The hammers expose the juice bearing cells by smashing the cane (Rein, 2007, page 79 and 86).

 The shredders are modelled in the same way as the cane knives.

 High pressure steam demands in the cane knives and shredders are related to the cane throughput (described in Appendix B.1.1 and B.1.3).

4.1.1.3 Juice extraction

A diffuser is a solid-liquid separator in which the sucrose is leached out of the cane through a washing process. The cane bed is moved along the length of the diffuser by a conveyor system. The floor of the diffuser has perforations in order for liquid to pass through.

 The diffuser is modelled by a perfectly mixed tank with separation coefficients.

 The separation coefficient for water is determined by a calculator block (described in Appendix B.1.5)

This is a rough approximation of the actual 12-stage counter-current diffuser which exists in real sugar mills. Each stage of the diffuser involves the pumping of juice collected in that stage to the previous stage. This is poured onto the cane bed and increases in sucrose content as it percolates through (Schmidt and Wise, 1956).

The fibre is conveyed all the way through the diffuser and exits after being pressed down by a roller. This exit stream is called megasse.

Steam is injected into the diffuser in order to maintain a high temperature to increase sucrose recovery (Rein, 2007, page 150). Heat also minimizes bacterial action (Ravnö and Purchase, 2005).

 Direct steam injection to the diffuser is proportional to the cane throughput (described in Appendix B.1.7). The steam used to maintain a high temperature in the diffuser is a portion of the steam produced in the first effect evaporator (vapour bleed V1).

Hot water, called imbibition, is added in a counter-current fashion. This washing process helps to displace the juice from the fibre (leaching). The megasse and juice streams leave at opposite ends of the diffuser.

 Imbibition flow rate is proportional to the flow rate of fibre in the megasse stream (described in Appendix B.1.4).

Some of the juice, called scalding juice, is heated by condensing steam. It is then recycled into the diffuser. The scalding juice is poured onto the cane feed to the diffuser in order to raise the temperature of the cane quickly. The heat increases the permeability of unbroken juice cells (Rein, 2007, page 150).

 The scalding juice heater is modelled by a shell-and-tube heat exchanger.

 A portion of the steam produced in the second effect evaporator (Vapour bleed V2) is sent to the scalding juice heater in proportion to the flow rate of cane.

The residence time of the cane in the diffuser is around one hour.

 Heat losses in the diffuser are accounted for by a cooler block placed on the draft juice stream.

The rest of the juice which is called draft juice, is sent to the mixed juice tank for further processing. In practice, the megasse and draft juice leave the diffuser at different temperatures.

The imbibition is pumped backwards through 12 stages losing heat. The scalding juice heater helps to increase the temperature in the front end. However, the draft juice leaves at a temperature of about 60 °C and the megasse leaves at about 64.5 °C.

 To account for the different temperatures, a heater and cooler block where placed after the diffuser on the megasse and draft juice streams, respectively.

 A calculator block handles the heat transfer from the draft juice to the megasse (described in Appendix B.1.8).

4.1.1.4 Megasse dewatering

The megasse is saturated with dilute juice which is pressed out in a series of dewatering mills.

 The dewatering mills are modelled by a separator with split coefficients.

 High pressure steam demand to the dewatering mill is proportional to the flow rate of fibre in the megasse stream.

The dilute juice which is removed from the fibre is called press water. This water, which is recovered from the bottom of the mills, still contains some sucrose and is recycled back to the diffuser.

The removal of moisture from the megasse increases the calorific value of the biomass and thus it burns better in the boilers. The ‘dry’ megasse is now called bagasse and still contains approximately 50 % moisture (Starzak, 2015).

 A calculator block manipulates the split coefficient in order to maintain a specified bagasse moisture content (described in Appendix B.1.9).

Most of the bagasse is sent to the boiler. A small portion (about 1%) is sent to the clarification module where it is used as a filter aid in the mud filters.

 Heat losses in the dewatering mills are accounted for by a cooler block placed on the press water stream directly after the mills.

4.1.1.5 Press water recycle

The press water from the dewatering mills is pumped into a temporary holdup tank in order to increase its temperature before putting it back into the diffuser. This tank is kept at a constant temperature by directly injecting steam.

 The tank is modelled by a mixer in Aspen Plus®_.

 A calculator block determines the amount of steam which is required to maintain the temperature in the tank (described in Appendix B.1.10). Vapour bleed V1 (a portion of the steam produced in the first effect evaporator) is sent to the press water tank. The flow rate of steam is manipulated by the specified temperature of the hot press water.

4.1.1.6 Extraction module mechanical drives

The cane knives, shredders and dewatering mill mechanical drives are modelled by turbines in Aspen Plus®. These turbines are driven by high pressure steam. The boilers supply steam at 31 bar absolute (bara) from the boilers (Starzak, 2016a). The exhaust steam which exits the turbines is at 2 bara (Starzak, 2016a). This exhaust steam is sent to the evaporation module.

 Calculator blocks manipulate the flow rate of steam to the cane knives turbine and shredder turbine based on the flow rate of cane (described in Appendix B.1.1 and B.1.3).  A calculator block manipulates the flow rate of steam to the dewatering mills turbine

based on the flow rate of fibre in the megasse stream (described in Appendix B.1.2).  A calculator block determines the amount of steam needed by the extraction module

mechanical drives by summing the requirements of the three turbines (described in Appendix B.1.3).

4.1.2 Flowsheet

IN connectors, eg. CANE(IN), show where streams are coming from an external source into a flowsheet/hierarchy. OUT connectors, eg. DJ(OUT), show where streams leave the flowsheet/hierarchy.