7/31/2019 Final Paper Ethanol http://slidepdf.com/reader/full/final-paper-ethanol 1/12 Sugar and Ethanol Plant Process Control System Luis R. Salazar Santos [email protected]Guillermo Parra Romero [email protected]TISCA S.R.L. Asunción, Paraguay KEYWORDS Ethanol, Process Technology, SISO PID controllers, MIMO control systems, Foundation Fieldbus. ABSTRACT The development of renewable sources of energy, especially in reducing dependency on imported petroleum products, is a major issue for industry today. Ethanol use as an alternative and clean fuel has been rising in the recent years in light of the continuously increasing cost of crude oil. Ethanol can be blended with gasoline (up to 25%), or can be used as hydrated ethanol in “flex” cars, which can be fueled with gasoline as well. This paper is devoted to presenting a complete process control system for a sugar and ethanol plants, using sugar cane as a raw material. A complete description of the production stages and areas and the importance of the industrial automation in all of them are covered in this work, for the production of organic, white and crude sugar and ethanol. This work shows that excellent results can be achieved by properly complementing state-of-the-art process technology and control systems, networking the different areas to optimize every stage in the process. The design and tuning of traditional and advanced controllers are analyzed and the results compared for different performance criteria. Special attention is given to processes that exhibit difficult dynamics, such as time delay. INTRODUCTION Sugar cane is the most common source of raw material for the sugar and alcohol industry worldwide because the plant is cultivated in tropical and numerous sub-tropical zones. It generates its own fuel during its industrial processing and has a big power conversion per area unit. It is an excellent source of food and renewable clean energy as an alternative to fossil fuels. The crop period generally lasts six months and the productivity ranges from 60 to 120 tons per hectare. Distributed with permission of author(s) by ISA 2006 Presented at the ISA EXPO 2006, 17-19 October 2006, Reliant Center Houston, Texas ; http://www.isa.org
Ethanol, Process Technology, SISO PID controllers, MIMO control systems,
Foundation Fieldbus.
ABSTRACT
The development of renewable sources of energy, especially in reducing dependency on imported
petroleum products, is a major issue for industry today. Ethanol use as an alternative and clean fuel has
been rising in the recent years in light of the continuously increasing cost of crude oil. Ethanol can be blended with gasoline (up to 25%), or can be used as hydrated ethanol in “flex” cars, which can be fueled
with gasoline as well.
This paper is devoted to presenting a complete process control system for a sugar and ethanol plants,
using sugar cane as a raw material. A complete description of the production stages and areas and theimportance of the industrial automation in all of them are covered in this work, for the production of
organic, white and crude sugar and ethanol. This work shows that excellent results can be achieved by
properly complementing state-of-the-art process technology and control systems, networking thedifferent areas to optimize every stage in the process.
The design and tuning of traditional and advanced controllers are analyzed and the results compared for
different performance criteria. Special attention is given to processes that exhibit difficult dynamics, such
as time delay.
INTRODUCTION
Sugar cane is the most common source of raw material for the sugar and alcohol industry worldwide because the plant is cultivated in tropical and numerous sub-tropical zones. It generates its own fuelduring its industrial processing and has a big power conversion per area unit. It is an excellent source of
food and renewable clean energy as an alternative to fossil fuels. The crop period generally lasts six
months and the productivity ranges from 60 to 120 tons per hectare.
Distributed with permission of author(s) by ISA 2006Presented at the ISA EXPO 2006, 17-19 October 2006, Reliant Center Houston, Texas ; http://www.isa.org
Raw material has a main role in the sugar production because the actual sugar factory is on the farming
field. Industrial processing just extracts what is inside the cane in the most efficient way by incorporating
equipment and technologies capable of achieving maximum saccharose extraction and alcohol production out of the molasses and juices sent to the distillation area. Among the equipment and
technologies are instrumentation and control systems intended to improve power efficiency, increase
production efficiency and improve final product quality in compliance with industry standards.
The efficiency, expressed as tons of sugar or gallons of alcohol per tons of cane, depends on the wholesystem: the crop system, the cut and transport system and finally, the industrial system, where the
complete production chain work is finished.
PROCESS CONTROL SYSTEM TECHNOLOGIES
Nowadays sugar and alcohol plants are designed to work interconnectedly for making use of the bagasse
produced in the cane crushing as fuel for both plants. They have the capability of controlling the juiceflow quantity among the plants in accordance with the production requirements.
SUGAR PRODUCTION
The term sugar generically refers in this paper to diverse types and qualities of sugar that can be
produced with different processes. In the sugar industry the products saccharose content is not directly
measured, instead, the term POL, which, stands for polarization, is used as an equivalent to saccharose.
A typical sugar factory has the following production process stages:
CANE RECEPTION, PREPARATION AND CRUSHING
Cane is received from farms prior to be weighed and inspected. Cane suppliers and transporters areautomatically identified and registered. Cane is loaded into the feeding tables and transported using
synchronized conveyor belts through cane knives and schredders. These equipment are responsible for
opening cane cells to facilitate the juice extraction at the crushing plant, normally composed of four to
Distributed with permission of author(s) by ISA 2006Presented at the ISA EXPO 2006, 17-19 October 2006, Reliant Center Houston, Texas ; http://www.isa.org
six mill tandems. These tandems can be powered by electrical motors, steam turbines, etc, depending on
the factory design. Mills are designed to extract as much juice and POL as possible with the smallest
possible power consumption. Mixed screened juice and bagasse to be fueled at boilers are the outflow products of this area.
The control system of this area is based on a hybrid PLC, with digital and analog I/O. An HMIsupervisory system is dedicated to the operation of this area. For the cane level control of the Donelly
chute, capacitive sensors are used. All control loops are SISO (add definition) and based on a simple PID.
Field instrument communication technology is 4-20 mA + HART.
Fig 1 – P&I D Cane Preparation and Crushing
The main results obtained with automation are a consistent, stable and scheduled feed; an increase of
juice extraction; obstruction interlocking; a decrease of POL losses in bagasse; a decrease in productiontime losses for shutdowns and breakings; a decrease of saccharose inversion losses; better imbibition and
bagasse humidity; steam and power economy; a more secure and easier operation; better maintenance
and a reduction in direct labor.
JUICE TREATMENT
The mixed and screened juice from the previous area still contains impurities, which must be separated.The main operations in this area are mixed juice sulphitation (for direct white sugar), sulphated juice
alkalization, sulphated and lime juice heating, separation of juice into decanters and mud filters. The
Distributed with permission of author(s) by ISA 2006Presented at the ISA EXPO 2006, 17-19 October 2006, Reliant Center Houston, Texas ; http://www.isa.org
outflow of this area is clear and bright juice ready for the evaporation stage and press-filter cakes that can
be used as fertilizer for farming operations.
Field communication technology recommended for this area is Foundation Fieldbus. All control loops
are simple PID. An HMI supervisory system is dedicated to the operation of this area
Fig 2 – P&I D Juice Treatment
The main results with automation are maintaining SO2 content (in ppm) inside accepted parameters,
ensuring juice pH stability, achieving optimum settle temperature, providing greater mud extraction in
decanters, decreasing retention time in decanters, improving color, decreasing inversion losses, providing better filter recovery (lower cake POL), achieving a higher quality condensate, increasing ease of
operation and achieving truly continuous processing.
SYRUP EVAPORATION
The clarified juice is first heated prior to entering the evaporation process. The evaporation is theenergetic center of the sugar factory, because here 70 to 75 % of water is extracted from the juice,
through simple and multiple-effect evaporation. The inflow is clarified juice with 14.5 to 16 Brix degreeand the outflow is syrup with 62 to 65 Brix degree; a low concentration syrup requires more energy and
implies more processing time in the crystallization.
Distributed with permission of author(s) by ISA 2006Presented at the ISA EXPO 2006, 17-19 October 2006, Reliant Center Houston, Texas ; http://www.isa.org
continuous operation guarantee, increased stability, safety and ease of operations, better quality and
higher quantity of condensed products and optimization of process variables as a function of the final
product quality, measured as syrup Brix degree.
CRYSTALLIZATION AREA
In conventional sugar mills continuous operation ends and batch operation begins here. Syrup coming
from evaporation is sometimes treated and sometimes not, depending on sugar type. Evaporation in this
stage is much slower than before, because it takes place in vacuum pans, which are single-effectevaporator for high density sugary products and grains in suspension. Here is where sugar grain is
formed, grows and reaches its commercial size, depending on sugar type.
There are various cooking schemes as a function of type of operation, type of final product and type of
raw material. The most widely known cooking scheme is with three cooked mass, which is well adaptedto deal with mixed juice purity above 80, and with normal viscosity levels. There are several alternates as
a function of sugar type and quality. For instance it is possible to dissolve all the sugar of second mass,
obtaining high purity syrup which is mixed with rich molasses coming from first mass centrifugation andvirgin syrup, to produce high quality commercial sugar with the A mass.
Sugar produced on vacuum pans is conducted to centrifuges where it is separated from mother molasses
to obtain the wet sugar.
This process stage involves much technological equipment and operations. Therefore Foundation
Fieldbus is recommended as the field instruments communication protocol. Hybrid PLC can deal with
continuous PID control and sequential control. HMI software is dedicated to the operation of this area.
The main results with automation are a decrease in cooking time from 10 to 20 % (because the quantity
of sugary product is a function of the actual super-saturation), increase in the productive capacity of thecrystallization area (due to the decrease in each cooking lapse time); uniformity and repeatability of every cooking independently of the operators with uniform grain of desired size for each cooking type;
standardization of cuts; steam, water and power economy since vacuum pans demand just enough steam
to maintain a given super-saturation.
Automation also minimizes false grain and conglomerate grain formation because the sugary product and
steam feeding and the temperature are functions of desired super-saturation, which allows quick development of crystals on a super-saturation zone where those undesirable effects cannot occur. As a
consequence, centrifuge capacities are increased.
Automation also helps to better exhaust cooked mass, to quantify in-progress materials on theequipments, to operate valves remotely, to protect against overfill on tanks and crystallizers, to control
desired Brix degrees values of molasses and magma dilutions, to better design a control and operation
strategy, to provide operation and personnel safety, to produce sugar color in compliance with desired parameters, to standardize the size and color of crystals, to improve crystal efficiency in each cooking
and to obtain better quality parameters on the final product, to obtain more crystals per cooking.
Distributed with permission of author(s) by ISA 2006Presented at the ISA EXPO 2006, 17-19 October 2006, Reliant Center Houston, Texas ; http://www.isa.org
Fig 4 – P&I D Sugar Factory – Cooking – Centrifuge Process
DRYING PROCESS
Sugar coming from centrifuges is dried until the humidity reaches specified values regarding the type of sugar being processed. From the dryers, dry sugar goes through the screening process to separate
oversized grains; then the sugar goes into silos to be weighed and packaged.
STEAM AND POWER GENERATION
This area extends to steam generation, electric energy generation and steam use and distribution. A sugar mill with a distillation unit, if well designed and properly operated, is energy self-sufficient. The plant
can generate all the energy it needs and even can have bagasse surplus for emergency situations or for
delivery of electric energy to the public system.
Bagasse comes out of the crushing tandems with a humidity between 48 and 52 %. It is then conducted
directly to boilers, or stored in a bagasse yard. Currently, the working pressure of boilers has been
increased from 21 bars to 42 and 64 bars and the generation capacity has been increased up to 150 to 200TVH. Steam temperature has been increased as well to make good use of the bagasse available energy.
Efficiency in boilers has been increased through an increment on recovering surfaces.
Based on the necessary operation mode and control characteristics this paper recommends the use of stand-alone digital PID controllers, with built-in auto-manual stations for each control loop, with frontal
visualization and parameter change capabilities. PLC systems can perform sequential control for start-up
Distributed with permission of author(s) by ISA 2006Presented at the ISA EXPO 2006, 17-19 October 2006, Reliant Center Houston, Texas ; http://www.isa.org
and shut down and motor interlocking. The recommended field instrument communication technology is
4-20 mA + HART. An HMI software is dedicated to the operation of this area.
Fig 5 – P&I D Boiler
The main results with automation are higher energy efficiency of each boiler and of the area in general;supervision and control over the complete steam generation process, increase in total generation capacity,
water and steam economy, safety and protection of boilers, ease of operation, start-up and total load
modulation, burner control and fuel savings, bagasse conveyor belt protection, steam in compliance withquality parameters; auxiliary equipment protection, better maintenance and centralization of operations.
ALCOHOL PRODUCTION
Generally, the term alcohol is referred to the chemical substance denominated ethylic alcohol or ethanol.
Alcohol is composed of carbon (52.2%), oxygen (34.8%) and hydrogen (13.0%).
FERMENTATION PROCESS
Fermentation can be continuous or accomplished in a batch process and is conducted on cane mixed
juice, final molasses and water, or the combination of these products. The alcoholic fermentation processconsists of the transformation of sugars into alcohol and CO2. Based on yeast kinetic, we can identify
four time-phases in the fermentation process: 1) initial phase, 2) exponential phase, 3)stationary phaseand 4) declination phase. The process representative curve is named “Culture Growing Curve”. It is
possible to reduce and even eliminate these phases.
Distributed with permission of author(s) by ISA 2006Presented at the ISA EXPO 2006, 17-19 October 2006, Reliant Center Houston, Texas ; http://www.isa.org
Yeast for the distillation process should present industrial characteristics, such as good productivity,
alcohol tolerance, etc. Industrial yeast is a mesophilus microorganism whose ideal growth temperature is
in the 28 to 35 degree Celsius range.
The most common fermentation process is known as Melle-Boinot, with excellent efficiency, about 92%
Gay-Lussac. After all vats complete their usable volume, the yeast multiplies and the wine centrifugation(continuous and sequential) process begins on separator centrifuges, where the wine is split into two
fractions: wine without yeast (90%) and yeast (10%). The wine goes to the distillation unit and the yeast
goes to the pre-fermentation vats.
Fig 6 – P&I D Fermentation Process
DISTILLATION PROCESS
The main products are hydrated and anhydrous alcohol. To produce the latter, several types of
dehydrators, such as cycle-hexane, ethylene glycol or molecular screens can be used.
High alcohol wine (beer) enters distillation column A1 and descends through column A exhausting itsalcohol content to become weak alcohol wine that leaves column A at its bottom. Rich alcohol vapor
with 45° GL leaves column A to column B where it ascends to the top increasing its alcohol content.
After being condensed, the alcohol leaves column B with 96 ° GL as hydrated alcohol.
Distributed with permission of author(s) by ISA 2006Presented at the ISA EXPO 2006, 17-19 October 2006, Reliant Center Houston, Texas ; http://www.isa.org
The C column, L evaporator, dehydrator decanter column and P column are required to dehydrate
alcohol produced at column B. From the column C bottom, anhydrous alcohol with 99.5 ° GL is released
and it is pumped to storage tanks through the J cooler.
Fig 7 – P&I D – Distillation Process
The main controlled variables are tray A18 temperature, column A pressure, hydrated alcoholconcentration, column C pressure, column C bottom level and column P pressure. Temperature of
columns, heat exchangers, condensers, etc. must be monitored on the HMI software.
For fermentation and distillation processes we recommend hybrid control using PLC for continuous PIDand logical control. Recommended field instrument communication technology is 4-20 mA + HART. An
HMI software is dedicated for the operation of fermentation and distillation processes.
CRYSTALLIZATION PROCESS ON A VACUUM PAN
This is one of the most complex processes on a sugar mill, therefore, simulations and special analysis are performed on it in this paper. Sugar crystallization occurs by the evaporation of the water content of asugar solution inside a vacuum pan specially designed to manage dense and viscous material.
Crystallization occurs only when the solution is supersaturated, i.e. the solution must have more solid
components than the available water at one specific temperature.
Distributed with permission of author(s) by ISA 2006Presented at the ISA EXPO 2006, 17-19 October 2006, Reliant Center Houston, Texas ; http://www.isa.org
The following block diagram corresponds to the concentration for seeding phase:
Next tables show traditional PID and more advance controllers tuning parameters for different performance criteria [1]. Results conclude that better responses are obtained in this process with a time-
delay compensator controller. PID controllers are sluggish because of the large time delay.
Next table shows tuning controller parameters based upon requiring a closed-loop trajectory which is a
first-order-plus-time-delay with a time constant τr.
Distributed with permission of author(s) by ISA 2006Presented at the ISA EXPO 2006, 17-19 October 2006, Reliant Center Houston, Texas ; http://www.isa.org
Additional First Order Filter Parameter (τ*) 1.1279 0.6727 0.6727
Controlled variable is plotted for a set point unit step change using the calculated tuning parameters:
Next pictures show actual plots of the controlled variables for manual and automatic operations:
REFERENCES
[1] Babatunde A. O., Harmon R. W. (1994). Process dynamics, modeling, and control. New York:
Oxford University Press.
[2] Chen James, Chou Chi Chung (1993 ). Sugar Cane Handbook.New York : John Wiley & Sons, Inc.
[3] Salazar Santos Luis R., Lattaro P. E. (1998).Fabricación de Azúcar Utilizando Tecnología Fieldbus.ISA
Sugar 98. Sertaozinho, Brazil.
[4] Pedroza Puertas R. (1975). Fabricación de Azúcar Crudo de Caña.La Habana: Editorial Científico Técnica.
Distributed with permission of author(s) by ISA 2006Presented at the ISA EXPO 2006, 17-19 October 2006, Reliant Center Houston, Texas ; http://www.isa.org
