Digester

Digester

Digester & screw press

Digester are vessels in which the stripped fruit stripped and heated to be brought to a suitable condition for pressing. The digester has a vertical rotating shaft with stirring arms are attached on it. There are 9 digester in this mill and 6 or less are running usually. The capacity of the digester is 3.7 tonnes. After the fruit are stripped from the thresher, it will fall into the bottom cross conveyor and being transported to bucket elevator. The bucket eleveator then lift up the fruits to the top cross conveyor. In the top cross conveyor the fruits will be transport to each digester. The fruit will go to the first digester, when the first digester opening is fully covered with fruits, the fruits from the conveyor will bypass the first digester and move to the next digester and so on. By this, the number of digester used in the process must be able to match with the capacity of the thresher so that all of the digester will be full all the time. The digestion time which is also the retention time of the fruit in the digester is 15 minute. 

Function:

1. Breaking open the oil bearing cell in the mesocarp

2. loosening mesocrap from nut

3. Mash and heat the fruit to facilitate the pressing process

The digestion time which is also the retention time of the fruit in the digester is 15 minute. When the first batch of fruits flow into the digester, the fruit will be left in the digester of 15 minute and then only the closing chute to be opened to allow the fruit to move to the screw press machine.

Condition for digester to optimize the performance

1. Digester must be kept more than three quarter full. This is to provide enough load on the fruits to facilitate the loosening of mesocarp from the nut. If the digester is less than 3 quater full, some fruit will not be digested properly because of the inconsistent holding time nad the stirring effect will be affected.

2. Temperautre kept at 85-90 degree celcius. If the temperature is too low, the oil bearing cell would not be break open as much as possible and this will lead to oil losses. And it will affect the press performance on the pressing machine and cause more nut breakage as sufficent heat will increase the elasticity of the nut. However, too high temperature will cause the oxidation and hence affect the bleachability of the oil produced.

There are a few ways to know whether the digester is full. First, movement the hanging metal on the digester indicate the digester is full. The hanging metal is connected to a bar that located at the top of the digester. When the digester is full and the movement of the fruit by the stirring effect will cause the bar and the hanging metal to move. The another way is by reading the ammeter reading of the digester. The digester is full when the ampere goes up to 35A and the needle shaking at the level. This show the the motor is having maximum rated load. The another way to determine the fullness of the digester is to look at the top of the digester to see whether there are steam coming out from the digester. If there are no steam coming out from the top, this means that the fruit are fully covered the digester and trapped the steam from coming out.

Stirring arms

The stirring arms must long enough to prevent the building up of dry material on the wall of the digester. There are 5 pair of stirring arm and each pair made up of a long arm and short arm. And there is an expeller arm to push the fruit out of the closing chute at the bottom of the digester. The number of arm should be consider to produce optimum result. Too many stirring arm will cause overdigestion and cause the bottom of the digester to flood with the oil.

Function of stirrring arms:

- Create movement for the fruit to prevent the fruit to lump up at one part.

- To push the fruit downward to create pressure on the fruit surface

- Crush the fruit to so that the fruit can be pressed better later

Drainage valve

The purpose of the drainage valve at the bottom of the digester is to drain away the oil as it is formed. The presence of oil in the digester may lower the efficiency of digestion because the friction between the fruit is reduced in the presence of oil. This will cause some of the mesocarp still attach to the nut and hence reduce the effectiveness of the press. When the mesocarp are attaching to the nut, there will be less fibre filling up the space in between the nut and provide less cushion. So by draining the oil, it can reduce the kernel breakge nut increase the efficiency of the screw press. However, too much drainage will cause clogging in the chute because it dried up the digester.

Figure 3: Drainage pipe (right) and closing chute pipe (left)

Return conveyor

When all the digester are full, the fruit from top cross conveyor will go to recycle conveyor to be flow back to the bottom cross conveyor. When the fruits are flowing through the recycle conveyor, the fruit will hit the gate and light up the red light. When the red light is lighted up, the autofeeder will be stopped automatically. Then the crane operation also will stop immedietely. This is to prevent the fruit from overloading the conveyor and the bucket elevator. The operator will then manually on the autofeeder after few minutes.

Figure 4: Fruits overflow from return conveyor due to the shaft pin dislocated.

Screw press

The press machine functions to press palm oil out from the oil palm fruits that have been digested in the digester. The screw press are locating under the digester. Inside the press machine, there is a screw press to move the fruit outward, a press cage to hold the fruit and the hydraulic press to press on the fruits. The load of the pressure automatically run and the load is set by amp on the press machine motor. For example, the amp is set at 39 for action, 40 for holding and 41 for release. When the ampere reading of the hydraulic press is 39A, the hydraulic press will move inward to press on the fibre, when it reach 40A, it will hold and not moving until the ampere reach 41, it will release the press. The setting of the load of the press machine needed to be in optimized level so that maximum amount of the oil can be pressed out with the minimise kernel breakage.

The press machine number 6 and 7 is using a different system for the press. The press will only be pressing and not releasing. Once the ampere reading drop, it will press in and hold. This is more efficient in terms of oil pressing because the pressure on the fibre is constant.

Figure 5 internal view of screw press    

Dilution water

 There is a pipe locating above the press which is use to flow the hot water to the cage in the press. The function of flowing hot water into the press is to allow the oil pressed from the fibre to flow smoothly.

Too much water will affect the performance of cake breaker conveyor and cause more load on the conveyor. The water content of the nut also will be higher and cause the nut to stay longer in the nut silo. This lower the efficiency of the kernel extraction. The water need to control at which the water content of the oil in gutter maintain around 8%.

Discussion (lab test on oil content of the fibre from press machine)

For sample 1 and 2, it is taken from the same digester. Sample one is taking when the drainage pipe is closed and sample 2 is when the drainage is opened. From the result, we can clearly see the oil content percentage for both sample 1 and 2 is different by 2%. This show that when the drainage is closed, more oil losses in the fibre will happen.

Sample 3 is taken from digester 3 with the drainage closed. Sample 4 is taken from digester 7 with drainage closed and the condition of digester is not full. The oil content percentage is more on sample 4. 1% more compare to sample 3. This show that when the digester is not full, there will be more oil losses in the fibre. This is due to the inadequate load to for the seperating mesocarp and the nut of the fruit in digester. There are also other factor affecting the result such press performances and digester temperature.

Another comparison can be made on sample 2 and 3. Sample 2 has more 1% of oil content than sample 3. Both of the digester are full and drainage are closed. This also show that the performances if press 2 is better than press 3. However, the digester condition also can affect the result.

All these factors are affecting the oil losses amount in the fibre such as press performances and digester temperature and digester fullness and has to be take in consideration to produce the best result and performances of the machines.