Cotton Gin Dust Abatement

Author: John Shumbera

Suggested Courses: Agricultural Engineering, Environmental Engineering

Level:  Sophomore, Junior & Senior


I. Narrative

Cotton gins produce a medically harmless, but nuisance dust in their operations.  Cotton is grown in the United States in a belt across the southeastern and southwestern states. Harvesting of this cotton occurs over a three month period in the fall of each year.  What is harvested is cotton lint attached to cotton seed.  There are two types of harvesting methods, a spindle picker and a stripper harvester.  Stripper harvesting is very economical with specific varieties of cotton plant, but is not suitable for all varieties.  One difference is that the stripper harvester retrieves cotton tufts from the plant with significant quantities of leaf and stalk matter.  Both types of cotton are presented to the gin where removal of the lint from the seed is accomplished on modern machinery derived from the cotton gin invented by Eli Whitney.   A secondary function of the gin is partial removal of dust, leaf and stalk matter to the extent possible without damaging the fiber.  This process  involves the transportation of cotton lint along with the foreign matter in an air stream at about 3400 feet per minute (fpm).   The cotton seed is separated from the lint at the gin stand.  The cotton lint is separated from the remaining foreign matter downstream.  It is here that a problem exists.  The cotton is emptied into the bale press for compaction and shipment.   The dust and other matter has to be collected.  Because of the fineness of the remaining matter, no economically feasible system presently available can trap 100% of the dust.  When this dust escapes from the gin it falls out in the neighborhood on houses, cars, and the like.  This dust is not medically harmful, but it is a nuisance to those neighbors.  The Environmental Protection Agency (EPA) has responded to complaints from the neighbors with a regulation on the amount of nuisance dust that can be released from a gin.  You are an engineer working for John's Gin and Warehouse in middle Texas.  The task you have been assigned is to help the gin meet the new regulations set by the EPA. 

 

II. Data

Contact with the regulatory commission gives you an allowable nuisance emission rate of 2.24 lbs. of particulate per bale.  A bale is nominally 500 lbs. of cotton.   The ginner has said that the gin operates at 14 bales per hour.  Past experience with the gin stands shows that at this production rate, the set of gin stands require 100,000 ft³ per minute of air flow.  It is this air stream that must be cleaned to regulatory standards.  Average waste for the two different harvesting methods are:

Spindle Picker	Stripper Harvester
203 lb. per bale	740 lb. per bale

The devices available to you for separation of dust from the air stream are a cyclone system or a bag house.  In this application, the cyclone system would consist of a preseparator and 14 cyclone units.  Each cyclone is designed for the particular gin where the system is to be used.  Cyclones have a tapered conical shape (the tip is pointed downward) and require an entry air velocity of 3400 linear feet per minute.  The efficiency of a cyclone is fixed by the particle sizes of the contaminants in the air stream.   Efficiency, h, is a function of P₁, the smallest particle size, and P₂, the average particle size, in mm.  The equation is as follows.

This efficiency represents waste exiting from the cyclone over the waste fed to the cyclone.

 

III. Numerical Problems

1) Calculate the entry diameter of each cyclone unit.

2) For a smallest particle size of 8.56 mm and an average particle size of 62 mm, what is the cyclone efficiency?

3) What is the amount of waste exiting from the cyclone for spindle picker feed cotton and for stripper harvester feed cotton?

IV. Narrative (continued)

The preseparator added in line to this system has the effect of reducing the quantity of waste fed to the cyclones.  For spindle picked cotton the reduction in waste is a factor of 2.5 and for stripper harvested cotton the reduction is 7.25.  Preseparators cost $0.30/cfm and cyclones cost $1.00/cfm. 

As the other alternative to this system, the ginner could install a bag (filter) house.   Essentially, this system has the air flowing into a sealed chamber whose only exit ports are holes in the floor from which fabric filter bags, perhaps twenty or thirty, are suspended.  Mechanisms knock down dust collected on the bag walls regularly.   The cost of installing a bag house with the associated machinery is $8.00/cfm.   The bag house manufacturer informs you that the bag system will filter dust emissions to 1.0 lb. per bale for spindle picked cotton and to 1.2 lb. per bale for stripper harvested cotton.

 

V. Problems

4) Calculate the emission levels (lb./bale) for the cyclone system with a preseparator installed.

5) What is the cost of a cyclone system with preseparator for this gin?

6) What is the cost of a bag house system for the gin?

 

V.Narrative (continued)

Discussion with the gin owner reveals that there is only a small financial margin in a ginning operation.  Therefore, it is very important that costs for the new equipment be kept to a minimum.  A gin's profits for a year are on the order of magnitude of $100,000.  The gin owner just heard of a gin across the state who was fined $100,000 by the EPA for consistent violation of the dust emission regulation.

The engineer knows that any nuisance emission causes problems for the gin's neighbors.   His situation is that he has his employer to consider with the need to be as cost effective in capital expenditures as possible, he has the regulatory requirements to meet, and he has the public interest in mind with respect to the nuisance aspects of even small dust discharges.

 

VI. Ethical Problems

7) Which design should you recommend to the gin owner?

8) Where do your primary responsibilities lie as an engineer?

9) If you were told that continuing complaints to the EPA by persons affected by the nuisance dust was already bringing hearings on a new stricter standard and that a new standard will likely follow, will that change your decision?

 

VII. Solutions

1)The total volume flow rate of air, Q, is 100,000 cfm.  The volume per cyclone is 100,000 / 14 = 7143 cfm.  Assuming uniform velocity distribution across the entry diameter, the entry cross sectional area, A, and velocity, V are related simply by the formula:

Thus, A = 7143 / 3400 = 2.1 ft², the diameter is:

2) Solving the equation given for efficiency, P₁ / P₂ = 8.56 / 62 = 0.138.  This squared is 0.019.  The efficiency is 1 / 1.019 = 98%.

3) Cyclone efficiency yields 2% waste exiting from the system.  The waste values per bale for the two harvesting methods is:

0.02 X 203 lb. / bale = 4.06 lb. / bale (spindle)
0.02 X 740 lb. / bale = 14.8 lb. /bale ( stripper)

4) Reducing in quantity of dust fed reduces the quantity of dust exiting the system.   The assumption is that the cyclone efficiency is constant over total dust volume.   This is generally not true, at least in the extremes.  The given values for dust reduction by the preseparator yields the following results:

5) The preseparator component of cost is $0.30, and the cyclone component is $1.00 per cfm of air processed.  The total cfm of process air is 100,000 cfm.  The total cost is $130,000.

6) The bag house system costs $8.00 per cfm giving a total cost of $800,000 for the installation.

7) The gin owner may not be able to stay in business and purchase the bag house.   Nevertheless, the bag house is the clear choice when the ethical consideration of the effect of dust on the gin's neighbors is taken into consideration.  Your approach is to see first if there is an alternative that may achieve the dust level of a bag house at lower cost.  One such consideration might be that of having two preseparators before the cyclone.

8) Your responsibilities are divided between what you owe to your employer and what you owe to the public and its well being.  Your task when facing ethical choices is to work diligently to see that an engineering solution may resolve an impasse, but if not then to present the sides involved the information they need and to assist them with a mutually agreeable and responsible compromise.

9) The engineer will not make the decision to purchase a bag house.  That is done by the gin owner.  Nevertheless, the engineer has the responsibility to press the case strongly for the solution which gives immediate and near term relief from the problem.   Then the engineer must present the case for a long-term solution.  In fact, the engineer may have a responsibility to look not just to the next generation of regulations, but to the generation after that.   Here the question becomes one of not making a choice between the two alternatives, but finding yet another and better alternative.

 

VIII. Additional Ethical Problems and Solutions

1. List 2 conflicting obligations faced by the engineer.
    Hint: Refer to your code of ethics.

While you have an obligation to protect the health and welfare of the public, engineers also have an obligation to be loyal and faithful agents to their employers.  Here the issue revolves around the obligations to the neighbors to the gin the obligations to the gin owner.  One can argue that the dust is harmless,since it does not produce any danger to the public.  The engineer then must provide the best dust abatement equipment that he can while working under the economic constraints of the ginner.   One can also argue that in this situation that the dust does create enough of a nuisance that the neighbors should not be subjected to it.  In this case, the engineer should do everything possible to protect the public, regardless what the ginner wants.

2. Based on your previous calculations and the data provided, show how the two options (the bag house and the cyclone) meet each obligation.

Both designs meet the obligation to protect the public.  However, the bag filter seems to be a better system than a cyclone by itself.  But, can be seen from the calculations, adding a preseparator to the cyclone, is the better choice.
Both designs fail to meet the obligation to the employer.  The bag house is clearly to expensive for the ginner to purchase. The cyclone design would be the best choice to recommend.  It is both economically feasible for the ginner and it reduces the dust admissions to a minimum.

3.  A creative-middle-way solution is an alternative solution to a problem with conflicting issues.  This solution usually will be able to meet both obligations in conflict better that the original solution(s).  From your calculations and the data presented in the case, list an option that would be a creative-middle-way solution for the engineer. 

Here the students can come up with several ideas, including newer technology and designs. The obvious creative-middle-way solution would be to use the cyclone design with more than one preseparator.

4.  Of the three choices the engineer has (the bag house, cyclone, and the creative-middle-way solution) which one is the best? Why?

Here the creative-middle-way solution should be the best choice, although it doesn't have to be.  This choice will better meet the conflicting issues in the case.   Engineers should strive to meet the conflicting issues in all situations that they face, though this is not always possible.  In any event, engineers should always act in a professional way to ensure that they are upholding the ethics and integrity of their profession.