CITRUS PEEL DRYERS AND WASTE HEAT EVAPORATORS PRINCIPLES OF OPERATION AND TROUBLESHOOTING

In a citrus processing plant the feed mill is the end of the line. What is left over ends up in the feed mill. Peel, rejected fruit, rejected concentrate, clean-up waste, cold pressed oil waste, excess water, and sometimes sticks and leaves.

But it is certainly to our industry’s credit that we turn our waste into a useable and profitable products. The principle products are dried citrus peel (or pellets), d-limonene, and citrus molasses.

A good operating feed mill may determine a profitable or unprofitable processing plant.

GENERAL FEED MILL DESIGN

The peel is generally transported to a separate area, the feed mill. The peel is dumped into a peel storage bin. After the peel bin the peel is chopped and lime is added. The mixture is run thru a pug mill for reaction time. Next the peel is pressed in screw type presses (to reduce the moisture content). The press cake is then fed into a rotary drum dryer. The dried peel is usually pelletized and cooled and aged before shipping to the customer.

The press liquor from the presses is concentrated in a molasses evaporator (usually a waste heat evaporator). This molasses can be sold separately or added back to the peel before and/or after the presses.

Distilled peel oil (d-limonene) is recovered from the press liquor in the evaporation process and sold as a separate product.

The objective is to have a smooth and dependable feed mill with low cost production.

PEEL
We need fresh peel with a minimum amount of added water. Do not store peel. It only gets worse with age. It ferments. It gets soft. It gets slimy. The older it is the harder it is to press and to dry. Run your juice room steady. Start up and run. Run with your peel bin almost empty. Arrange your peel system for last in – first out. This means you process the freshest peel first. What about the old peel? Don’t get behind. And if you do get behind run it on a “space available” basis.

LIMING
Good liming is important. Too much lime makes the peel too hard and can jam the presses. Too much lime scales up the waste heat evaporator. Too little lime causes poor pressing and difficult drying. The presses can’t get a good “grip” on the peel. Spent caustic from evaporator cleaning and juice room cleaning can be used to replace up to 50% of the lime.

PUG MILL
The pug mill is to give reaction time. Insufficient reaction time causes inadequate diffusion thus a need for more lime (which scales the waste heat evaporator). 10 to 15 minutes of reaction is normal.

PRESS LIQUOR
Press liquor is the liquid released from the pressing operation. It is concentrated in the waste heat evaporator then generally added back to the peel. The press liquor feed to the waste heat evaporator should be last in/ first out. If you run out of press liquor, run waste water based on its strength. Always feed the freshest press liquor to the evaporator. You should have a surge tank set up for this. Fermentation of the press liquor tanks is often a problem. Stored press liquor or rotten press liquor should be run on a “space available basis”.

PRESSES
Presses need to be in good operating condition. The presses currently available need a well drained peel feed (they don’t press slop very well). Pressing is dependent on liming, reaction time, amount of water added, molasses added, and peel condition. As a general rule, the goal is reduce the press cake moisture to a minimum. 62% can be achieved with good pressing and enough evaporation capacity. (You may not have enough dryer heat for the needed evaporation capacity to handle the press liquor. In this case you would need to run a higher press cake moisture in order balance the
dryer and evaporator loads.)

Add as much molasses before the presses as you can. This allows for some diffusion of solids (helps lower the press cake moisture).

Why not just press harder? Spaghetti, the peel is squeezed thru the press screens like spaghetti. Therefore if you can raise the internal Brix of the peel you can reduce the press cake moisture thru diffusion, still pressing with the same force.

Fresh peel allows for better pressing.

HAMMER MILLS
The purpose of the hammer mill is to reduce the peel size and give a consistent size. The chopped peel allows for better mixing and reaction.

DRYING
The standard dryer used is a design of Ralph Cook. It is a direct contact rotary dryer. The gas or oil fired furnace heats the air. The air heats the peel. The moisture in and on the peel “boils” off. The heat is controlled to give the desired moisture content coming out of the dryer.

It is desired to run a stoichiometric mixture of air and fuel (no excess combustion air). this gives a high dew point temperature (185 degrees F) which is necessary for good waste heat evaporator capacity. Limit air leakage by having the seals in good condition. Good trunnion alignment helps keep the dryer drum in its proper position, thus helping the seals.

There is a large recirculation of vapor from the suction box to the furnace. This helps reduce the temperature of the air which first contacts the peel and provides the transport of the peel thru the dryer drum.

It is important for the peel not to be exposed to the radiation of the burning fuel. If the peel were to “see” the flame, the light itself burns the peel. This is why the dryer furnaces are in a “U” shape.

The dryer provides for good air/peel contact. The first part of the dryer drum is the preconditioning section. The peel is push away from the entrance. The peel is brought up to temperature and the loose water is evaporated. The second two thirds of the dryer drum contains a honeycomb of baffles which tumble the peel thru the air steam multiple times.

Good dust separation is very important. If dust is recirculated thru the furnace it will be burned up. If it goes to the waste heat evaporator, it may go up the stack, be collected in the black water, or collect in the vapor passages. The dryer recirculation fans have dust take off ducts, these ducts can stop up. There is an external (internal separators on the newer dryers) dust separator, this need to be operating properly.

DRYER CONTROL
Controlling the dryer temperature is difficult. It is a very large system with long time (45 minutes) cycles. The closer to full capacity you run the harder it is too overcome upsets. Steady feed with steady characteristics make the control easier. No drastic changes. The dryer is a plug flow machine. There is very little mixing on a length ways basis. This means if you have a change in the input you don’t see much evidence of that change until it works its way all the way thru the length
of the dryer. Therefore it is important to run steady, no major upsets or changes. Good fresh peel. Good pressing. And good juice room operation.

So how do you control a major upset or change. You need a control system which acts quickly based on the slope of the temperature output. In other words the dryer controls start reacting at the very first indication of a raise or drop in temperature.
With good control you can run closer to the actual desired moisture. No over drying to prevent wet peel. That allows you the sell the water in the peel, there is less dust to be lost, and less fuel is needed.

WASTE HEAT EVAPORATOR (WHE)
In order to utilize the exhaust from the dryer as a source of heat, you have to have a dryer which produces a high dew point. The Cook dryer was developed to facilitate the waste heat evaporator. Remember the dryer and the WHE are a “team”.

In general, it is desired to achieve maximum water removal (maximum evaporation) in the waste heat evaporator (not necessarily the maximum Brix). Every pound of water removed in the waste heat evaporator is a pound of water which does not have to be removed in the dryer.

Keep the flow rates up. This provides coverage of the bundles thus improving the evaporation.

SCRUBBER SYSTEM (BLACK WATER)
The black water is the condensation of the vapor from the dryer. The “black” is the dust which was burned in the dryer. Good dust collection reduces the “strength” of the black water. The dust separation needs to be working well in the dryer.

Be sure the scrubber water system (black water system) is in good operating condition. The sprays act as a scrubbing system knocking out the dust from the vapor system. If they are not working well the air bodies can become clogged with dust.

It is desired to run the WHE fan at the proper speed. Too fast pulls cold air into the WHE. Too Slow lets hot vapor escape unutilized.

AIR LEAKS VACUUM LEAKS
The WHE operates under vacuum. Air leaks interfere with and can overload the air removal system. Thus the tube bundles can become air locked reducing evaporation and interfering with d-limonene collection.

CLEANING
The tube of the evaporator can become blocked with lime scale. As the build up increases the capacity of the evaporator decreases. It is necessary to do a good cleaning job in order to keep the best performance. A mixture of soda ash and caustic should be used for cleaning (10 parts caustic and 1 part soda ash). The organics will dissolve in caustic and the calcium citrate can be loosen by the soda ash. If you have serve problems with lime scale, increase the soda ash content of the cleaning solution. To remove lime scale already formed, circulate a soda ash solution over the
weekend, changing the solution when it becomes too muddy.

WHE questions 2/25/2005
Mayt’ Lee
Tropicana Products, Inc
Ft. Pierce, Florida 34981

Question #1: What does the barometric do?
The barometric is more properly called the barometric condenser. It condenses the vapor from the last effect of the evaporator. The condensing is accomplished by direct contact between the cooling water and the last effect vapor. This is done under vacuum. Non condensables (such as air) must be removed in order to maintain the vacuum.

Question #2: How much should (the valve?) be closed or opened?
The waste heat evaporators Cook Machinery has built have been designed to run with the valve wide open. If the pump is not properly sized you may be getting to much water. The barometric tail water temperature should run 100 to 110°F.

Question #3: How is it used to control operation?
This not a normal adjustment point.

Question #4: If I loose vacuum, where do I check first, second, third, etc. to bring vacuum back up?
First: Low boiler steam pressure, if the boiler pressure drops too low then the vacuum jets stop working.
Second: Air leaks: open pipes such as vent lines, pump seals, cracked welds, leaking sight glasses.
Third: Excess air coming in by being sucked in with the barometric water, cracks on the barometric tail water line. Leaks thru check valves, steam traps, steam valves and etc.
Fourth: Fermented Press liquor giving excess CO2 and excess alcohol.
Fifth: Equipment problems. Worn out steam jets, drain lines which are stopped up thus running liquid thru the jet(s). Trash in the steam jet nozzle.
Sixth: Cooling tower problems. Cooling tower not in good operating condition.

Question #5: If molasses Brix is high and I want to reduce Brix or maintain, what should I do? (A lot of operators crack the vacuum line to reduce it, so we need to talk about controlling with the barometric.)
We do not recommend you crack open the vacuum line. To decrease the Brix, increase the incoming feed rate. If this is not possible add some low Brix waste water or water. You also may need to tighten up the press to give more press liquor.

Question #6: How do I know if I am getting flow through all stages?
The sight glasses should be flowing. This is indicates you have flow in the corresponding stage.

Question #7: What do I do when press liquor is in the condensate?
Press liquor in the condensate generally indicates press liquor carry over through a separator into the vapor side of a tube bundle. The corresponding sight glass will probably be completely full. This may clear up as the Brix thru out the evaporator increases. Or you may need to reduce the in feed flow rate. Or there maybe something interfering with the pumping out of the separator.

Question #8: What does the cooling tower do?
The cooling tower cools the barometric cooling water.

Question #9: How does the cooling tower get press liquor in it, what did I do wrong?
See the answer for question #7. The press liquor in the last effect is carried over into the barometric, thus into the barometric water. Breaking vacuum could help cause this by reducing the evaporation.

Question #10: Why is the condensate milky color instead of clear?
Most likely the milky color is caused by d-limonene in the water. This can be due to fermented press liquor (lots of alcohol interferes with oil collection). Also venting problems can lead to oil in the condensate.

March 2, 2005
Tropicana Products Inc.
Mayt’ Lee cc: David Tegreene
Ft. Pierce, Florida

Dear Mayt’

Let me write down some of what I learned from the waste heat evaporator’s operators.
Problem #1. They are not able to increase the feed rate to the evaporator in order to lower the Brix.
Possible causes and possible solutions:
A) The evaporator is dirty. Lime scale in the preheater could be restricting the flow. Solution, clean by circulating caustic and soda ash. Blast out stopped up tubes.
B) Pumps are worn. (Sand wears out the pump internals). Solution, repair or replace.
C) Pumps capacities are too small. Solution, increase impeller diameter and motor horsepower as required. Use VFD’s to increase the pump speeds. Buy pumps with large capacity.
Problem #2. Carry over of press liquor into the condensate or barometric water.
Possible causes and possible solution:
A) Drain from Separator(s) stopped up. Solution: Clear out the blockage.
B) Evaporator is dirty. Solution, clean with a mixture of caustic and soda ash.
C) Press liquor is backing up in the separator due to the stage pump not keeping up. Solution,increase the pumping or decrease the feed rate.
D). Opening the vacuum break line. Solution, do not do this. By introducing air into the evaporator you reduce the evaporation thus giving more press liquor which may back up and carry over.
Problem #3. Fermentation in large press liquor tanks.
Possible solution:
A) When you replace or repair your press liquor tanks this summer add a surge tank for feeding the fresh press liquor directly to the evaporators.

Best regards,
John Cook

Comments

  1. Dear John,

    Quick question. If you have one citrus line (12 FMC extractors) producing an average of 250-300tons of peal a day during season, is that enough peal to make this investment/process economical?

    Thank you in advance,

    Khaled

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