WO2013030618A1

WO2013030618A1 - System and method for optimum operation of a sugarcane milling unit

Info

Publication number: WO2013030618A1
Application number: PCT/IB2011/003122
Authority: WO
Inventors: Babji Buddhi Srinivasa; Arun Kumar M; Nandkishor Kubal
Original assignee: Abb Research Ltd
Priority date: 2011-08-29
Filing date: 2011-12-22
Publication date: 2013-03-07

Abstract

Described herein is a method for operating a sugarcane milling unit. The method includes providing a working range for at least one operating variable for a plurality of mills of the sugarcane milling unit, estimating bagasse pol produced and amount of imbibition water consumed corresponding to a first plurality of set points for the at least one operating variable followed by selecting one or more first set points corresponding to minimum estimated bagasse pol produced and amount of imbibition water consumed for the at least one operating variable as use set points, and operating the sugarcane milling unit at the use set points. In another aspect the invention provides a sugarcane milling unit that includes an optimizing unit to implement the method described herein.

Description

SYSTEM AND METHOD FOR OPTIMUM OPERATION OF A SUGARCANE MILLING

UNIT

TECHNICAL FIELD

[0001] The invention relates generally to optimized operation of a sugarcane milling unit of the sugar plant and more specifically to improved and efficient extraction of sugarcane juice from cane chips.

BACKGROUND

[0002] Sugarcane generally comprises two fundamental constituents: fibre and sugarcane juice, also referred to herein as juice. The juice comprises the important ingredient sugar. A sugar plant is involved in the extraction of juice from the sugarcane. A sugar plant typically includes a cane preparation unit or section, where sugarcane from stock/trucks are transferred to a series of cane carriers, where they are subjected to cutting and shredding, following which the cane is carried to a first mill chute for cane processing. The cane preparation unit is used to increase the cane density by cutting it into smaller pieces referred to as cane chips and tearing the cane chips into shreds with no extraction of juice. The cane chips are then fed into the sugarcane milling unit or section.

[0003] The primary objective of sugarcane milling unit is to extract as much sugar bearing juice as possible from the cane passed by cane preparation section. The received cane is passed through a series of crushing mills called milling train as a whole. The mills squeeze or crush the cane to separate the juice from its fibrous part called bagasse. To help extraction of juice, some of the produced juice or water is sprayed on the bagasse between mills, this process of adding juice/water to soften bagasse is called imbibition.

[0004] In a typical sugarcane milling unit, the objective is to maximize juice extraction while avoiding overloading of mills. The most common disturbances affecting the processes include cane quality and cane feed rate. These problems are generally overcome by manipulating the chute flap and speed of the mill and to add appropriate amount of imbibition water or juice. Overloading of mills usually occurs due to increased roller lifts.

The manipulated variables to avoid problems due to cane quality and cane feed rate include speed of the mill and chute flap. This may be achieved manually or using an automatic control system. Such manipulation of variables generally ensures smooth operation of the milling unit. The amount of imbibition water is further varied depending on the fiber rate and is issued as set-point to a flow controller. The imbibition water amount and addition flow (or flow rate) may also depend on the amount of bagasse pol for the bagasse from the final mill in the mill train.

[0005] After extraction of the sugarcane juice from the cane chips, the juice is subjected to a concentration step, which involves removal of substantial amount of water from the juice. For this step, it is desirable to maintain the amounts of water to be as low as possible, so that the energy consumption during this step is minimized.

[0006] This process is being used for individual mill, and the smoothness of operation is effective for each mill. However in a sugarcane milling unit comprising plurality of mills, the process is not as smooth, which results in inefficient operation of the entire sugarcane milling unit, and consequently, the sugar plant itself. With the increasing demand for sugar in the global market, sugar industries are looking forward to novel ways to maximize their productivity. Therefore, in order to optimize the operation of the sugarcane milling unit and reduce the operating costs of the sugar plants, there exists a need to maximize production by minimizing losses through bagasse pol, and to reduce costs by minimizing energy consumption.

BRIEF DESCRIPTION

[0007] In one aspect, the invention provides a sugarcane milling system comprising at least one sugarcane mill configured for milling cane chips based on at least one operating variable, wherein the sugarcane milling unit comprises a chute with a chute flap, plurality of rollers configured to crush and transport cane chips, conveyor belt, and an imbibition pipe; and an optimizing unit for generating use set points for each of the sugarcane mill based on minimum bagasse pol produced and amount of imbibition water for the at least one operating variable for the sugarcane milling unit.

[0008] In another aspect, the invention provides a method for operating a sugarcane milling unit. The method includes providing a working range for at least one operating variable for a plurality of mills of the sugarcane milling unit, estimating bagasse pol produced and amount of imbibition water corresponding to a first plurality of set points for the at least one operating variable, selecting one or more first set points corresponding to minimum bagasse pol produced and amount of imbibition water for the at least one operating variable as use set points, and operating the sugarcane milling unit at the use set points.

DRAWINGS

[0009] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

[0010] FIG. 1 is an exemplary schematic of a sugarcane mill;

[001 1] FIG. 2 an exemplary schematic of a sugarcane milling unit;

[0012] FIG. 3 is a block diagrammatic representation of an exemplary sugarcane milling system of the invention; and

[0013] FIG. 4 is a flowchart representation for an exemplary method for operating a sugarcane milling unit.

DETAILED DESCRIPTION

[0014] As used herein and in the claims, the singular forms "a," "an," and "the" include the plural reference unless the context clearly indicates otherwise.

[0015] As used herein, the phrase "bagasse pol" means the sucrose content of bagasse.

Bagasse is the residue obtained after crushing cane in a mill. Sucrose is the main chemical component of sugar. Pol is generally determined by a polarization method known to one skilled in the art, and is expressed as a percentage by mass.

[0016] As used herein, the term "fluid" means water or sugarcane juice. The sugarcane juice is the liquid juice comprising sugar in an aqueous solution that is extracted from the cane chips. The fluid is used to wet the cane chips to facilitate the milling process.

[0017] An exemplary schematic of a typical sugarcane mill is shown in Fig. 1, wherein the numeral 10 is used to represent the solitary sugarcane mill. The sugarcane mill comprises a chute 12 to deliver the cane chips. The speed and extent of cane chips being fed may be modulated using a chute flap 14 having a suitable opening size and shape. The cane chips are then fed into a plurality of rollers 16 in a certain direction. The rollers are positioned in such a way that the distance between them is less than the width of the cane chips being fed, so that when the cane chips pass through the rollers, they get crushed and thus extracting sugarcane juice. The extracted sugarcane juice is then collected into a suitable receptacle 18. The receptacle may further comprise additional units such as a mesh to filter, and the like. The cane chips may be simultaneously subjected to imbibitions through a nozzle 20, wherein the fluid for imbibitions is fed from a suitable container 22. Manner of feeding the imbibitions fluid is known in the art, and may include pumping the fluid or using gravity to allow fluid to flow, and so on. The imbibitions render the cane chips softer thus facilitating crushing, and improves the efficiency of extraction of sugarcane juice from the fibrous part of cane chips.

[0018] Fig. 2 shows a schematic of an exemplary sugarcane milling unit generally represented by numeral 24, wherein the sugarcane milling unit comprises a plurality of mills 26. Each mill 26 in the sugarcane milling unit 24 is linked to the next mill through a carrier 28. The cane chips being fed into the first mill is allowed to pass through the roller 16 to crush them sufficiently to extract the sugarcane juice. Subsequently, the at least partially crushed cane chips are transported through the carrier 28 to the next mill wherein the cane chips are subjected to similar treatment. As shown herein, the cane chips are subjected to imbibitions by the sugarcane juice from the subsequent mill extraction. Other exemplary techniques of imbibitions are known in the art and are contemplated to be within the scope of the invention.

[0019] When the cane chips arrive at the last mill, they are subjected to imbibitions using water made available from a suitable source, such as a storage tank 30 shown in Fig. 2. After being processed at the last mill, the remaining portion is substantially the fibrous part of sugarcane. This is transported as bagasse for further processing, such as use as a fuel. The bagasse is analyzed for precent (%) fiber and/or bagasse pol using on-line analyzer or laboratory analysis.

[0020] Fig. 3 is a block diagrammatic representation of a sugarcane milling system of the invention that comprises at least one sugarcane mill 26. The sugarcane milling system comprises an optimizing unit of the invention 32 for generating use set points for each of the plurality of carriers based on minimum bagasse pol produced and amount of imbibition water for the at least one operating variable for the sugarcane milling unit. The at least one operating variable as referred herein includes level and amount of cane chips in chute, torque of rollers, conveyor belt speed, amount of imbibitions water to be added, amount of imbibitions sugarcane juice to be added, and combinations thereof. In an exemplary implementation the mill roll torque and imbibitions water flow have been used as operating variables, and the objective function has been designed to minimize the bagasse pol and imbibitions water and obtain the torque set points for mill rolls and flow set point for the imbibitions water that lead to an optimized solution resulting in optimum operation of the sugarcane milling unit.. It will be understood by those skilled in the art that the use sets are also generated for all of these operating variables or a group of these operating variables in different embodiments, or to only one operating variable.

[0021] To obtain the use set points, the optimizing unit 32 is configured for receiving a working range for the chosen one or more operating variables as an input. The working range may be provided by an operator or may be predefined inputs for the optimizing unit. The optimizing unit estimates the future bagasse pol produced and future imbibitions water consumed corresponding to a first plurality of set points in the working range of the chosen one or more operating variables. The optimizing unit then selects one or more first set points corresponding to minimum estimated bagasse pol produced and imbibitions water consumed for the at least one operating variable as use set points. The use set points are then applied to the plurality of mills for the chosen one or more operating variables. In certain embodiments where more than one operating variable is considered, the working range for each of the operating variables serves as an input for the optimizing unit. The reference numeral 34 indicates the communication and control instruction flow between the optimizing unit 32 and the different mills 26. The communication flow includes the inputs in the form of torque values and imbibitions water flow rate, and control instructions include the use set points for each mill.

[0022] The optimizing unit 32 may also be pre-programmed or manually set for a sampling time to receive the use set points and other operating variables at periodic intervals during the operation of the sugarcane milling unit 24 to estimate the minimum bagasse pol produced and imbibitions water consumed and correspondingly generate the new use set points. In some implementations the optimizing unit may dynamically monitor the current one or more operating variables, which would also include the use set points, and periodically or continuously estimate the bagasse pol produced and imbibitions water consumed for current operating variables, and generate second set points corresponding to the minimum estimated bagasse pol produced and imbibitions water consumed. In case the second set points are different than the first set points, then the optimizing unit 32 selects the second set points as the new use points and applies them for the sugarcane milling unit. In case if both first and second set points are same then no change is made for set points for the sugarcane milling unit. It will be appreciated by those skilled in the art, that the optimizing unit may be a distinct component or may be integrated as or with an existing controller for the sugarcane milling unit or a central control unit for the sugarcane plant.

[0023] The sugarcane milling system 24 may further include a display unit 36 for displaying the use set points and the bagasse pol produced and imbibitions water consumed by each sugarcane milling unit at the use set points. The display unit may be in the form of a graphical user interface. Further the display unit may be integrated with the optimizing unit 32 in one exemplary implementation. The data for the use set points over a period of time and the corresponding bagasse pol produced and imbibitions water consumed may be stored in an appropriate storage medium for archival and retrieval purposes and for the purpose of generating reports and further analysis of such data.

[0024] The optimizing unit realizes the objective of minimizing the bagasse pol produced and imbibitions water consumed in the sugarcane milling unit together with maintaining uniform feed and avoidance of overloading of chutes and/or each individual mill and ensuring smooth acceleration for the different mills. This is achieved by using a model based optimal control framework for the sugarcane milling unit as explained below:

Min∑bp(k +j) + W∑d_m+l (k + j)

subject to

where

Torque^

Torque₂

x2

X =

Torque,

wf

bagasse pol after last mill; Np = prediction horizon, Nc = control horizon, ■ imbibition water flow

[0025] It is assumed that the milling train has m mills each driven by an electric motor. The first term of the objective function minimizes bagasse pol at the output of the last mill. The second term corresponds to minimizing the amount of imbibition water used. This term is essential because, by adding more imbibition water bagasse pol will come down; however, it will render the subsequent evaporator operation highly inefficient through the consumption of more steam. W is a weighting term to establish trade-off between sucrose loss in bagasse and loading of evaporators. The state-space model of the system shown herein predicts the bagasse pol after last mill; wherein X is the state vector whose dimension n depends upon the order of the system and time-delays. It is worth mentioning that the model will capture fibre rate (defined as instantaneous mass rate of fibre) as one of its states since it is a significant variable in determining the amount of imbibition water to be added. The workable range for the decision variables, such as torque range for each crushing mill due to physical limitation is appropriately translated into constraints on the decision variables d. The model is used to predict any increase or decrease in bagasse pol in the future time interval Np, called as prediction horizon and the solution of the optimization problem gives torque set-points to individual crushing mill controllers and flow set-point to imbibitions water flow controller for the future time interval Nc, called as Control Horizon. Only the 1^st set of torque set-points and imbibition water flow setpoint are implemented and the procedure is repeated at the next time interval. [0026] Another aspect of the invention is a method for operating a sugarcane milling unit as represented in the flowchart 38 of FIG. 4. The method includes at step 40 providing a working range for at least one operating variable for a plurality of mills of a sugarcane milling unit. The at least one operating variable as referred herein includes level and amount of cane chips in chute, torque of rollers, conveyor belt speed, amount of imbibition water to be added, amount of imbibitions sugarcane juice to be added, and combinations thereof.

[0027] The method then involves at step 42 estimating bagasse pol produced and amount of imbibition water consumed corresponding to a first plurality of set points for the at least one operating variable. Step 44 of the method of the invention comprises generating one or more first set points corresponding to minimum bagasse pol production and amount of imbibition water consumption for the at least one operating variable as use set points. Then, the method of the invention includes step 46 of operating the sugarcane milling unit at the use set points.

[0028] The method may further include a step for monitoring the at least one operating variable for bagasse pol produced and imbibitions water consumed at the use set points at distinct time intervals, and subsequently re-estimating bagasse pol produced and imbibitions water consumed corresponding to a second plurality of set points. Then, it is checked if the second plurality of set points provides a more optimized operating solution. Subsequently, one or more second set points may be selected as new use set points corresponding to minimum bagasse pol produced and imbibitions water consumed based on the above step. Then, the sugarcane milling unit is operated at the new use set points. The first set points and the second set points are generated by a model for sugarcane milling unit as described herein. As explained earlier, in one example the one or more second set points are same as one or more first set points, and no change is required in the operation of the sugarcane milling unit.

[0029] Thus the optimizing unit and the underlying method may be implemented as a tool integrated with a system for operating the sugar plant to provide an optimized control solution for operating the sugarcane milling unit that achieves optimal performance and thus improves efficiency of extraction and reduces operating costs for the sugar plant.

[0030] While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

We Claim:

1. A sugarcane milling system comprising: at least one sugarcane mill configured for milling cane chips based on at least one operating variable, wherein the sugarcane milling unit comprises a chute with a chute flap, plurality of rollers configured to crush and transport cane chips, conveyor belt, and an imbibition pipe; and an optimizing unit for generating use set points for each of the sugarcane mill based on minimum bagasse pol produced and amount of imbibition water for the at least one operating variable for the sugarcane milling unit.

2. The sugarcane milling system of claim 1 wherein the at least one operating variable includes level and amount of cane chips in chute, torque of rollers, speed of conveyor belt, amount of imbibition water, amount of imbibition sugarcane juice, or combinations thereof..

3. The sugarcane milling system of claim 1 wherein the generating use set points for the at least one operating variable is done periodically or on a real-time basis.

4. The sugarcane milling system of claim 1 wherein the minimum bagasse pol produced and amount of imbibition water is estimated using a model for the sugarcane milling unit.

5. A method for operating a sugarcane milling unit, the method comprising: providing a working range for at least one operating variable for a plurality of mills of a sugarcane milling unit; estimating bagasse pol produced and amount of imbibition water corresponding to a first plurality of set points for the at least one operating variable; generating one or more first set points corresponding to minimum bagasse pol production and amount of imbibition water for the at least one operating variable as use set points; and operating the sugarcane milling unit at the use set points.

6. The method of claim 5 wherein the at least one operating variable includes level and amount of cane chips in chute, torque of rollers, speed of conveyor belt, amount of imbibition water, amount of imbibition sugarcane juice, or combinations thereof.

7. The method of claim 5 further comprising: monitoring the at least one operating variable for bagasse pol produced and amount of imbibition water at the use set points at distinct time intervals; re-estimating bagasse pol produced and amount of imbibition water corresponding to a second plurality of set points; generating one or more second set points corresponding to a minimum bagasse pol produced at the one or more first set points or the second plurality of set points as new use set points; and operating the sugarcane milling unit at the new use set points.

8. The method of claim 7 wherein the one or more second set points are same as one or more first set points.

9. The method of claim 5 wherein the plurality of first set points are generated by a model for sugarcane milling unit.

10. The method of claim 7 wherein the plurality of second set points are generated by a model for sugarcane milling unit.