Manufacturing-Engineering

Transcription

1 Manufacturing-Engineering THE FRENCH SCREW PRESS ON CUSH-CUSH-A STEP TOWARD MAXIMIZING MILLING CAPACITY W. Bradley Kimbrough St. Mary Sugar Cooperative, Inc. Jeanerette, Louisiana ABSTRACT Operators of conventional mill tandems have long realized that separate processing of raw juice screenings, commonly called cush-cush, would result in higher grinding rates and more efficient operations. The French continuous screw press can be used to process cush-cush and is a step toward maximizing the grinding capacity of an existing mill tandem. INTRODUCTION Development of the continuous screw press for the sugarcane industry was started in 1956 at the Lula Sugar Factory in Louisiana. A small machine from the French Oil Mill Machinery Co. was used on an experimental basis to dewater cush-cush from a conventional mill tandem. In subsequent years this same experimental press was used in Florida and finally found its way to the. Pioneer Mill Company in Hawaii, where it was the predecessor of the first commercial screw press for the sugar industry, a French Model ]-88, in The 1st commercial screw press for processing cush-cush was installed at the Bryant, Florida Mill of US Sugar Corporation in and the 2nd was at Talisman, Florida, during the crop. Each of these machines handles about 5 tons of dry cush-cush fiber /hr, dewatering from about 80-85% moisture to 60-63% moisture and reducing sucrose percent fiber by more than 80%. At each mill, the pressed cush-cush is returned to the latter stages of the milling tandem, and increased grinding is possible or improved extraction for a given grinding rate. The 3rd commercial cush-cush installation took place during the season at St. Mary Sugar in Louisiana and consists of 2 presses in series with fresh water maceration between stages. The aim of this system is to increase grinding rate by separately processing cush-cush, discharging pressed fiber into the final bagasse carrier leading to the furnaces, and replacing it on the tandem with an equivalent percentage of fresh cane. The 1st season of operation indicated that an increase in grinding of 12-13% can be realized and that modifications in the maceration system will make it possible to reduce sucrose in cush-cush to an acceptably low level. PROCEDURE Juices extracted by milling contain particles of bagasse that must be removed prior to clarification. The most common method employed is to flow the juice from different stages of the mill tandem onto a drag conveyor with a 1378

2 W. BRADLEY KIMBROUGH 1379 coarse screen bottom. The juice drains through the screen, and the bagasse is picked up by scrapers on a continuous chain and is usually returned behind the 1st mill. The screened bagasse is commonly called cush-cush and usually amounts to 10-20% of the total fiber ground. While the method of handling cush-cush described previously seems quite simple, there are many inherent problems. Some of the most obvious are: a) returning sucrose to early stages of the mill and, b) the recycling of some 10-20% of the total fiber being ground. In addition, the cush-cush returned is made up of fines or bagacillo and causes rolls to polish, often resulting in mill slip. Mill operators have long realized that separate processing of cush-cush by' some practical means would result in higher grinding rates and more efficient operations. The French continuous screw press can be used to process cush-cush and is a step toward maximizing the capacity of an existing mill tandem. Development of the French continuous screw press in the sugarcane industry began at Lula Sugar Factory in, Louisiana in 1956 as a project to separately. dewater cush-cush, In subsequent years, this same experiniental press was employed at the Okeelanta Factory in Florida and then at 3 mills in Hawaii, being used finally at Pioneer Mill Company as the dewatering station for a 25 ton of cane/hr Silver diffuser, where it was the forerunner of a commercial size screw press in The 1st commercial screw press for processing cush-cush was installed at the Bryant Mill of US Sugar Corporation in Florida late in the season. This machine had not been adapted to handle cane fiber and required modification for the season. On December 12, 1971, a Model E-99 cush-cush press was placed in operation at Bryant. The E-99 press is a 2-stage press with provisions for injecting fresh maceration. In the single machine, the cush-cush is pressed, macerated, and pressed a 2nd time, resulting in better sucrose extraction. The E-99 press was installed at the same elevation as the operating floor of the mills and could be operated or not by merely opening or closing a slide gate in the cush-cush drag. Pressed cush-cush fiber was returned to the mill ahead of the 4th milling unit. Cush-cush entered the screw press with 8-9% sucrose and about 80% moisture. Normally, dilute juice of about 3.5% brix was used for injection with cushcush, leaving the press at about 5% sucrose and 63% moisture. The use of fresh maceration water for injection instead of the dilute juice would have led to a greater reduction in final' sucrose. Actual reduction in sucrose percent fiber with with respect to cush-cush exceeded 80%. Operations with the E-99 press at Bryant on some cane varieties resulted in increased grinding rates of about 15%. The press used about 150 hp to handle 50 tons cush-cushjhr. The 2nd commercial screw press for processing cush-cush was installed at the Talisman Sugar Factory in Florida during the latter part of the season. The press installed at Talisman was more specifically designed for processing cane fiber than the press installed at Bryant during the season. From the initial operation, it proved itself capable of handling all the cush-cush from the 8000 ton/day mill, about 5 tons dry fiber [txx, delivering a dewatered cushcush of about 60% moisture containing 4-5% sucrose. During the lst season, the

3 1380 MANUFACTURING-ENGINEERING pressed cush-cush was added ahead of mill no. 3 and in subsequent years ahead of the last mill, no. 5. According to Talisman operating personnel, separately processing cushcush with the press made it possible to apply remaceration 1 stage earlier in the tandem by pumping juice from the 3rd milling unit ahead of the 2nd milling unit. Previously, cush-cush with its rich juice had been added ahead of the 2nd milling unit. This additional maceration stage made it possible to leach out more sucrose from the bagasse blanket, thus increasing sucrose extraction from the whole milling tandem. Another effect was a decrease in top-roll flotationon the mills ahead of the point where cush-cush was returned after being pressed. These mills were reduced in speed by 15% to get back to normal flotation as indicated by Fisher and Porter top-roll flotation recorders. The 3rd commercial screw press for processing cush-cush was installed a~ St. Mary Co-Op in Louisiana for the 1970 season. St. Mary and French began.a 2-crop program to further develop the use ofa French press for processing cushcush. The goals set forth at the outset of the program were to press all cushcush from the mill and send the fiber directly to the furnaces with a moisture and pol content comparable to that of the mill bagasse and, most importantly, to increase the grinding rate by an amount equivalent to the cush-cush removed from the mill. The existing mill tandem at St. Mary, consisting of a 36 X 48 in. 2-roll crusher and 6 28 X 48 in. mills, was being operated at rates averaging over 3000 tons of cane/day on 12-14% fiber cane. This rate was already well above the manufacturer's rated capacity of 1750 tons of cane/day. The cush-cush processing system at St. Mary is a 2-stage system consisting of a light-duty press followed by a final heavy-duty press. This equipmentwas installed parallel to the mill at the same elevation as the mill walkway, where the mill foreman station is located.. After start-up of the press drives, the system was operated by the mill foreman. His primary duties were to take the system on and off the line and shut down the system during mill stops or in the case of equipment malfunction. Taking the system on and off the line is a simple matter and causes no lost time to the mill. A slide door operated by an air cylinder was installed in the bottom of the existing cush-cush drag, and the control valve for the cylinder was located near the mill foreman's station. With the slide door closed, cush-cush was returned to the 2nd knives to be recycled through the mill. Opening the door allowed cush-cush to enter the press system. The cush-cush flow and the counter-current maceration scheme are illustrated in Fig. 1. Cush-cush was handled with the conventional drag conveyor, where juice was screened through the bottom. In the drag conveyor, the last juice to flood the cush-cush was the richest juice, that from the crusher, 1st and 2nd mills. In order to displace the rich juice and enter the press system with lower sucrose, 1st press juice and 2nd press juice were returned to the cush-cush drag for maceration. After the 2-stage juice maceration, cush-cush was conveyed to the Ist press. The l st press was used to remove excess juice and lower moisture to the 65-70% range so that maceration water could be more effective. Maceration water was applied in the intermediate conveyor in approximately the same proportion as that used on the mill. The Ist press discharged cush-cush into the intermediate conveyor, where

4 w. BRADLEY KIMBROUGH 1381 No.1. p~ SS CUSH- CUSH To Sf:C.Or-..\O k.lj\ye"s----"'" [J SLIDE DOOR SEc::OND \'.NIVES ~ '-- G"VSl4-CUSH,0 ~ PfO: S5 '5YS,SV\ Cf<U5HE"f{ r-- W...TER MAC"Ri'TIOṆ I ~ HO. 2 PRE:55 JU\C ND.1. No.1 PI<E 55 MIl-I.. ~ J'UIC E: I"\\LL r[t'lo,?- No. :5 till LL No. Z. PR.ESS /U 4... [ No.4- MILL \ No.5 [ MILL n Fig PRESS 6AGA55c \. I~O. 6 ~ Cush-cush flow system. CUSH-CU5H DRi'G CONVEYOR TO FURNA<.E.s maceration water was applied as described above and the cush-cush was conveyed to the 2nd press. The cush-cush bagasse from the 2'1:Id press was discharged to a conveyor that added the bagasse to the mill bagasse going to the furnaces. The light press, shown in Fig. 2, is a Model M-55 with overall dimensions of 2 ft, 8 in. high, 2 ft, 8 in. wide and 10 ftlong. It is driven by a compact hydraulic motor which is powered by a 125 hp electric motor driving a hydraulic pump. The unit is equipped with a load control to automatically adjust rpm for variations in feed rate.. The final press, shown in Fig. 3, is a Model L-77 with overall dimensions of 4 ft,9 in. high, 3 ft, 2 in. wide, and 11 ft long. On the discharge end,.a hydraulic load cone and bracket extend 8 ft, and on the feed end there is a large gearbox that is 3 ft X 5 ft and 6 ft high. This press is also equipped with a load.control system to handle variations in feed rate. The drive is a 500 hp steam turbine. The operating principle of both presses is the same. The material is fed to an enclosed screw that moves the material through successively smaller volume sections, causing a compression of the material as it advances. Liquid drainage is through the cage that encloses the screw. The primary purpose of the press installation was to increase mill grind- C MilL ~

5 1382 MANUFACTURING-ENGINEERING Fig. 2. Model M-55 press. Fig. 3. Model L-77 press.

6 w. BRADLEY KIMBROUGH 1383 ing capacity, To insure that the 1970 grinding capacity without the press was representative for comparative purposes, the grinding capacity of 3 previous crops was also determined. The criterion for this determination follows: I) The maximum 5-day consecutive run fat each crop was determined; a 5-day run was chosen so that reduced rates for. evaporator cleaning would not be included. 2) The grinding rate for each 5 consecutive days of run is an average of the tons of cane ground/grinding hr, rather than tons of cane ground/crop day. This excludes mill lost time. These capacity determinations, along with mill performance data, are presented in Table 1. The 1970 average of 140 tons of cane[ux for 5 con- Table 1. Maximum 5 consecutive-day-run and mill performance. Bagasse Equivalent Moisture Cane Pol Year 'Tons/hr tons/day Pol (%) (%) % fiber extract 1970 (12/3-12/7) (12/1-12/5) (1l/25-11/29) (12/12-12/16) secutive days is in line with the averages of the 3 previous years and should serve well as a measure of the mill grinding capacity without the press installation. The maximum 5 consecutive days of run with the press system in opera~ tion is compared to the maximum run without the press system in Table 2. Dur- Table 2. Comparison of maximum run with press system and without press system. Bagasse Cane Equivalent Moisture Pol Year Tons/Iir tons/day Pol (%) (%) Pol (%) Fiber (%) extract 1970 (12/3-12/7) (12/8-12/12) ing the run with the press system, the grinding rate increased from 140 tons of cane/hr to 158 tons of cane/hr, or 12.85%. During this run, lost time on the press system was 6%; therefore, there would have been a greater increase had there been no lost time. On tons of cane/day, this is an increase from 3360 tons to 3792 tons, or 432 tons. It is interesting to note the differences in the mill performance during the 2 runs. However, more of the comparative runs are needed to draw firm conclusions. Some of the difference in pol extraction might be accounted for by the increase in pol percent cane during the run with the press system and the reduction in cane percent fiber. It should be noted that since fiber percent cane is a calculated figure, there is some question as to its validity. While the run with the press system proved the principle of increasing grinding capacity by removal of cush-cush, press performance left room for

7 1384 MANUFACTURING-ENGINEERING improvement. The data for the press system operation during the run are presented in Table 3. Some of the high pol in press bagasse can be attributed to Table 3. Press system performance data for maximum run. 1st press 2nd press Pol (%) Moisture (%) Pol (%) Moisture (%) the ineffectiveness of the juice maceration prior to the cush-cush entering the 1st press. Improvements for the 1971 crop, which are still in the planning stages, will add an additional maceration stage, possibly 2. Also it is planned to bring moisture in l st press bagasse down to the 60-65% range. One maceration stage could be added by rerouting the crusher, 1st, and 2nd mill juice canal up-stream from the 3rd mill canal. Another might be added by using all excess mill maceration juice that would ordinarily overflow to the dilute juice for cush-cush maceration. These improvements, along with more effective application of the press juice maceration, should reduce the amount of pol entering the press system. The last 12 days of the grinding season were without any press downtime, and data for this run are presented in Table 4. A marked improvement can Table 4. Press performance data for final run. 1st press 2nd press Pol (%) Moisture (%) Pol (%) Moisture (%) be noted. Unfortunately, due to excessive mud which impaired bagasse burning and clarification, the mill speed had to be reduced for all but 2 days of this last run. Even with mill speed reduced, the average grinding rate for the 12-day run was 144 tons of canej'hr, which is an equivalent rate of 3456 tons of cane/day. DISCUSSION Some general comments on the operations with the screw press system should be in order at this point. The most significant bonus from use of the system was improved bagasse burning. This could be attributed to the low moisture and high temperature of the press bagasse. These qualities actually helped mill bagasse-burn. Also there was better drainage at the crusher as evidenced by the less frequent flow of juice over the top roll. Another benefit was that removal of cush-cush during muddy conditions caused less mud to be recycled to the mill and reduced slip and wear on the mill. On 2 occasions, cane was taken from the 2nd set ofknives and processed by the press system. The cane was prepared 'prior to being pressed by 2 sets of knives only, and the same maceration scheme as that used on cush-cush was used.

8 W. BRADLEY K.IMBRQUGH 1385 The results presented below indicate that with some improvements, primarily in maceration, cane could be processed in this manner. Date Cane processed by French press system Pol (%) Moisture (%) Since the press system installed at St. Mary has capacity for handling more than the amount of cush-cush produced, the results of the tests above indicate that it might be possible to divert some cane from the mill in addition to the cush-cush and do satisfactory work. If this could be accomplished, a capacity increase in excess of the 12.85% achieved during the 1970 crop would be realized. St. Mary operation of the press system for part of only 1 season does not supply enough information to evaluate maintenance requirements; however, the higher sand content of Louisiana soil as compared to the organic muck of Florida will probably result in something higher than the 2A cents/ton experienced in Florida. It should be kept in mind though, that the St. Mary cush-cush system comprises an independent and complete extraction system; so, it cannot be compared directly maintenance-wise with a single screw press stage. At each of the 3 factories where cush-cush is being pressed rather than being returned to an early stage of the mill tandem, many beneficial results have been achieved. For those mills wishing to maximize an existing mill tandem in order to increase production, the French continuous screw press as a practical means of processing cush-cush deserves consideration.