The present invention relates to the field of pellet mills, and more particularly to an automated and efficient control of pellet mills.
Pellet mills and the process of producing pellet material are well known in the art. Pellet mills are generally known for pelletizing raw materials, foodstuffs, feedstuffs, wood, and biofuels. The pelletizing process results in the transformation of a solid powdery or pasty material into hard pellets or granules which are easier to handle for a consumer that unpelletized materials.
While simple from an overview perspective, pellet mills present unique design challenges. The lengthy transit time across the conditioner, along with the change in material properties make traditional PID controls inadequate for fast responding control of a pellet mill.
Using advanced control theories, the present invention implements an estimator to monitor the conditioner input variables of steam, material density, feed rates, and estimates the material departure temperature and moisture content while accounting for transit time.
These and other attributes of the invention will become more clear upon a thorough study of the following description of the best mode for carrying out the invention, particularly when reviewed in conjunction with the drawings wherein:
As can be seen by references to the drawings and particularly to
For the purpose of control, the pellet mill manufacturing “flow” is divided into the following groups: feed source 100, pellet mill 200, discharge path 300, and supervisory control 400 as shown in
The feed source 100 is relatively simplistic, consisting of a source bin with low level indicators, with the ability to have multiple source bins, augers, surge hoppers, etc. Feed source 100 consists of everything up to, but not including, the feeder screw to the pelleter.
As illustrated in
The discharge path 300 entails everything from the cooler to the final pellet destination, this can include coaters, screeners, crimpers, etc.
Supervisory Control 400 provides comprehensive interface between the safety interlocks 401 and automatic control modes 402 for the entire mill.
Supervisory control 400 is represented by the state diagram of
The pellet mill entails the heart of the control system, and is where the most complex control algorithms are used. The material flows as illustrated in
Connected to a VFD, the feeder 201 provides the primary control point for feed entering the system. By monitoring VFD speed, motor current and the source bin low level status, an accurate estimate of the material entering the system can be calculated.
The conditioner 203 provides a continuous, but slow, mixing of feed and steam to increase feed temperature and moisture content. The travel time for feed to go from one end of the mixer to the other end can be as short as 10 seconds to as long as 90 seconds.
The pelleter 205 includes a set of rollers and die to produce pellets by pressing the feed through the holes in the die.
The pellets get passed over a continuous flow of air through the cooler 206 to bring the pellets down to a manageable temperature range for storage and handling.
Given target pelleter load (in amps) and pelleter throughout (tons/hours) the feeder controller 221 automatically regulates the feed rate to the pelleting system. Its parameters allow it to accurately calculate throughout in ft3 and pounds. Two modes of automatic operation are included with automatic switchover, as illustrated in
In the event of a HOLD signal, the feeder will be stopped. Upon release of HOLD, ramp-up will again be entered, with the minimum set as starting point. Conditioner, pelleter or cooler NOT running constitutes a HOLD signal, along with user entered HOLD state.
The feeder will only run in the FILLING, RUNNING or CLEANOUT steps (see
In the event of a mill motor over-amp condition, the feeder equipment stops. The system an optionally auto restart equipment after the mill motor amps have dropped below the high threshold.
The feeder motor amps will be monitored along with source bin level inputs to properly detect empty conditions.
To target mill motor amps, the pelleter controller 225 will feed a “Feed Rate/Motor Amps” parameter. This parameter is used for feeder speed target calculations.
RAMP HOLD—the conditioner can signal the feeder to NOT ramp up any further conditioner temperatures are not within acceptable ranges.
The feeder controller actions are summarized in Table II.
The conditioner controller 223 receives an estimated feed rate from the feeder 201. Given a transit time parameter, the conditioner is broken into several slices represents a section of the conditioner 201.
Each slice period, material is moved through the conditioner slices AND calculations of temperature rise based on steam application rates are performed. At the end of the transit time, the estimated temperature and actual temperature are compared and used to update estimation constants.
Each Slice period, the following steps are preformed:
The conditioner controller 223 also regulates the steam in the system. When two-thirds of the conditioner is in a loaded state, steam will be enabled. When fully loaded, steam PID will be enabled.
There are 2 PID's in the conditioner controller 223.
The primary job of the pelleter controller 225 is to feed the FeedRatePerMotorAmp variable back up to the feeder controller 221. This is calculated by taking the estimated feed rate from the conditioner 203 and dividing the motor amps. Before being fed up to the feeder controller 221, this value goes through an averaging filter.
The pelleter 205 also passes along the feed rate to the cooler 206 for cooler integration and checks.
Motor over amperage conditions are monitored for and signaled to the feeder for feed rate compensation.
In the event a motor stops running, or extreme, 150% overcurrent for extended period of 10 seconds the conditioner, steam and feeder will be immediately stopped.
The cooler controller 226 controls blower, airlock, separator and cooler discharge. On system shutdown, the cyclone will enter clean out mode, where the airlock continues to run, but the blower is turned off.
If the cooler controller 226 detects that the material in motion, conditioner material, pellet material and cooler material, exceeds capacity or if there is a cooler 206, blower, and airlock 207 failure, the feeder will be placed into HOLD status.
Based on material in motion and level of grain in the cooler, it regulates cooler discharge to maximize cooling of pellets, and maintains an estimate of material in the cooler 206, and material that has passes through the cooler 206.
As illustrated in
The first control system is the mash product flow loop which is limited by the lesser of the RATE mode or MOTOR lode. The system will ramp up product flow rate at programmable fixed rate. in the event of motor overloads, the RATE will be decreased for the duration of the over load. In MOTOR LOAD mode, the percent of rated motor amps is the target, i.e. 95% motor load, of the PID loop.
The second control system is the thermal steam regulation loop. If uses either an estimated or actual temperature output of the conditioner to regulate the steam valve and maintain desired temperature output. An estimator is provided to increased loop response time. The estimator monitors the actual temperature output and the predicted and adjusts its calibration in an ongoing fashion.
The third control system regulates operational liquids for injection into the conditioner. Each liquid gets and application rate, starting condition includes temperature, material flow rate and additional delayed start. Liquid application rates, lbs/ton, are given and the actual pump speeds will change when the feeder rate changes.
The first control system, the mash control loop, is shown separately in the diagram of
The Controller implements both a RAMP and PID controller. In RAMP mode, pellet mill over ramp conditions cause immediate back off of the ramp. The controller drives the feeder VFD which turns feeder at variable speeds to control the feed rate of material into the conditions. Feeder Amps and Power factor are monitored and used to detect out of product conditions and verify product is being delivered.
When the feeder has detected product flow, it will provide the mash flow rate to the temperature and liquid controls, allowing them to have immediate response to throughput changes. The feeder will also totalize the flow of materials through it to provide a running display of tons pelleted, and tons remaining for targeted orders.
The conditioner estimator also models the flow of material through the conditioner and after the program transit time, provides the pellet mill an estimate of the material delivery rate, temperature and moisture content.
The pellet mill motor amps are monitored for sudden unexpected changes to motor amps as well as over-amp conditions. Motor overamps will initiate a lowering of feeder rates. Sudden, unexpected, changes in motor current will initiate anti-plug and anti-roll recovery procedures.
Using the estimator discharge rate and mill amps, a value is derived of amps vs tons/hour. This value is also used for rate correction and to limit the rate to no more than that which would be product maximum amps.
The second control system, the thermal loop, is shown separately in the diagram of
The thermal control loop implements 2 modes:
1) Estimator Mode which utilize the estimator to provide the maximum, minimum and current predicted mash discharge temperatures. This provides for much faster response to system changes. The feed forward of the feed rate changes is accounted for inside the estimator.
2) Actual Mode which utilizes actual, vice estimated, conditioner discharge temperature. As the estimator output is not used, the feed forward should be implemented. Delay for the feed forward should be approximately one-half the conditioner transit time.
The estimator will run regardless of mode. When running in manual or Actual mode, only the auto-corrector inside the estimator will be actually be running.
The third control system, the liquid controller, is shown in the diagram of
Multiple configurations of liquids may be added to the conditioner. The liquid controller implements a simple open loop controller and may drive either regulator valve or a variable speed pump. Liquids may be measured by either volumetric or mass flow systems. In the absence of a measuring device, liquids will be quasi-measured based on command speed vs maximum speed.
The liquid controller is fed mash throughout and corrects liquid delivery rate to match mash flow. Liquid start point can be based on conditioner output temperature, mush throughput, percentage of target flow rate, or any combination of the 3 parameters. Additionally, the controller supports a timed delay as well, i.e. Start 20 seconds AFTER the conditioner outlet temperature has reached 180 degrees AND throughput is at least 50% of target flow rate.
A additional control system, the cooler control loop, show in the diagram of
Cooler control operates in either a Maximum cooling mode or Targeted Mode. Targeted mode set points can be either moisture, temperature or a combination of both. The opening/closing of the cooler discharge gate is controlled via a slow speed pulse width modulation. The open vs closed times are determined by the PWM output.
Sensors for Low, High, and High-High are provided. The system increases the open time of the PWM when the cooler has a high level. High-High level initiates an error condition, causing the Pellet Mill to decrease the feed rate.
The High-High sensor should be placed at a point where the cooler still has room to hold all material in the conditioner and pellet mill.
The following EXAMPLES use a pellet mill with the following characteristics:
Also, the following Examples use a controller having:
Given a reasonably accurate calculation of BTU/sec being delivered through the steam valve, the volumetric mash rate, incoming mash temperature, mash density and conditioner dwell time, one can reasonably estimate the expected temperature departing the conditioner at a given time.
Reference to
Given the equation for Specific heat
we can determine conditioner output temperature as:
We can also correct for errors in Cp by readjusting Cp to the calculated value determined by:
The error between formula specific heat (Cp_formula) and the current calculated Cp_observed, is fed into a proportional control loop to correct for disturbances between Cp_formula and Cp_observed. At the end of a pellet run, the Cp_observed will then be used to update the system data tables to have a better starting point when the same formula is ran again. The result of this control loop is Cp_current which is used by the system as the current specific heat.
BTU (vpos)(t), Psteam(t)) is used to determine the heat energy flow into the conditioner. This will generally be determined by either a linear or quadratic equation based on valve flow characteristics. The valve characteristics are statistically determined by observation of several mash runs at differing mass rate and target temperatures.
As a (rough) starting point, we an estimate a valve's full open heat capacity using m=1.61 C, PSteam2 to determine steam lbs/hour and approximating the steam heat capacity per pound as 1180 BTU/lb3 (enthalpy of saturated steam).
This gives a BTU/second as approximately Hsteam=0.528×Vpos×Cv×Psteam.
The steam control loop utilizes two modes of operation:
Closed Loop—Normal operating mode, closed loop operation. Steam valve position adjustments are made by calculating the heat requirements needed to affect the desired temperature change, for the current mass flow rate and specific heat of mash. The thermal errors are processed by a PID function. An inverse of the BTU function is used to derive the new position based on updated target heat energy flows. The PID function runs at a clock rate equal to the conditioner loop time.
If the steam valve is a linear 7.9 Cv valve with a constant 90 PSIA, then the linear coefficient would be 374. If we are in Closed Loop mode with the following parameters:
= Feeder Speed × Vol/Hz = 0.127 ft
= V
D
= 4.44 lbs/s (6 Ton/hour)
= T
− T
= 60° F.
= BTU(V
, P
) = 130 BTU/s
C
= M
- M
= H
+ C
M
T
T
Feeder Speed
indicates data missing or illegible when filed
Open Loop—During this warm-up phase operation, the system operates as an open loop system, utilizing mash feed rate (lbs/sec), density and specific heat, along with the required temperature rise to calculate the heat requirements for the mash. The steam valve is determined by the heat requirements and steam valve characterization. When conditioner outlet temperature is below a system programmable threshold, a configurable amount of “boost” steam may be applied to accelerate temperature rise. When within a configurable threshold, the system switches to Warm Loop mode.
Same system parameters and configuration.
= Feeder Speed × Vol/Hz = 0.127 ft
= V
D
= 4.44 lbs/s (6 Ton/hour)
K
K
= H
+ H
(T
⇐ [T
- T
]) =
(130° F. ⇐ [145° F. - 15° F.])
to C
, make proportional
.
indicates data missing or illegible when filed
Typically, feed rate ramping is done in a linear fashion. This is adequate for fast responding systems where the effect on the change of feed rate is near immediate. In the pellet mill system, the effectes of the feed rate changes can take up to a minute for the full effect to be seen in the way of amps draen by the pellet mill. This presents a problem when nearing the target mill amps setpoint, as we will most likely overshoot out target if operating on a linear time ramping scale.
A common solution to prevent this is to have 2 ramping values, a fast ramp and a slow ramp, where the determination of ramp speed is determined by how off the amps are from the target ramps.
A better solution is to approach the target from the curved approach. The closer we get to the target rate, the slower the ramp runs. This is implemented using the following approach.
1) Calculate the linear ramp rate, based on target on target time for the ramp.
2) Calculate the percent error on amps
E
amps
=A
target
−A
actual
3) Utilize the error to decrease the ramp rate as we get closer to target amps.
The HIGH-HIGH sensor should be placed at a point where the cooler still has room to hold all material in the conditioner and pellet mill.
Although only an exemplary embodiment of the invention has been described in the detail above, those skilled in the art will readily appreciate that many modifications are possible without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims.
This application claims priority from U.S. Provisional patent Application, Ser. No. 62/406,629 filed Oct. 11, 2016, entitled Automated Pellet Mill Controller, which is incorporated herein by reference in its entirety.
Number | Date | Country | |
---|---|---|---|
62406629 | Oct 2016 | US |