1.0 Evaluation of Refinery Gases
Various refinery gases are generated in the process of oil refining. These vary, depending upon the type of crude oil being distilled, such as Reduced Crude distillation, or Naphtha cracker. For the purposes of this case study the description and range of refinery gases are those gases produced from refining Crude Oil and Reduced Crude only.
Crude oil is the base product for the distillation of light and heavy Naphtha, Kerosene, light or heavy Diesel, Liquefied Petroleum Gas, Reduced Crude and other products. At this stage normal pressure distillation process (topping) is used. In the preliminary stage, refinery gases are driven off. To avoid creating too much hydrocarbon with high concentrations of carbonated substances, a vacuum distillation process at a medium working temperature is used for stripping out fuel oil, asphalt distillate, different grade and different gravity immediate products for further rectification. If Naphtha, steam and Hydrogen are mixed together and then transferred into the Pt-A1203 catalyst reactor and internal-heater for catalytic reforming and isomerization, the mixture would then to be conducted to the condenser for condensation. At that point it is pumped to a stabilizer to obtain high octal number stabilized platformate (gasoline).
Refinery gases recovered from the process would normally be conducted to the next rectification processor for further processing. This would include, absorbent rectification process, ultra low temperature rectification process to extract immediate products such as Propane, Propylene, Butene and Butane and so on. By means of further separation, propane could be used as the main material for extracting Acetylene and Butene. Similarly, Butane could be used as the main material for extracting Butadiene or some immediate products. Alternatively, the mixture of Propane and Butane could be combined with methyl mercaptan or amyl mercaptan as liquefied petroleum gas (LPG) for sale. Propylene could then be used as the intermediate product for forming Polypropylene, PAN, and as many as 10 other petrochemical materials.
In normal circumstances, refinery gases would represent approximately 16.5% by weight relative to the total feedstock of a refinery. The target total volume of refinery gas recovery is expected to be in the order of 80%. This is relative to the total of recoverable gases, and the balance of 20%, which is 4.1% of the total feedstock by weight, is usually de-composed and then mixed in with other gases for transporting to the flare gases torch, for flaring (burning).
The recoverable refinery gases are as follows:
- Hydrogen: 0.1%
- Methane: 1.3%
- Ethane: 1.9%
- Ethylene: 0.6%
- Propane & Propylene*: 5%
- Butane & Butene*: 4%
Other types of gases such as Hydrogen Sulphide (H2S), Pentane.etc: amount to (on average) 3.6%
*Propane, Propylene, Butane & Butene are the high value products and are the target for recovery by using this process due to the economic viability.
Absorbent rectification process is the accepted efficient way to recover the target material noted above. The main process utilises an absorbent column, in which gasoline is used as the absorption media to absorb Propane.
The re-boiler is then used to provide heating energy to split Alkyl-C2 and Alkyl-C3, to avoid fogging & evaporation of gasoline distillate from the re- boiler. Sponge oil would normally be sprayed from the top of the column, and oil at the bottom of the column is then recovered.
Alkyl-C3 & C4 are passed through the de-Butane column for separation. At this point the Alkyl-C3 and C4 are distilled from the top of the column, and gasoline recovered from the bottom.
A mixture of Alkyl-C3 and C4 is then sent to the de-propane separator in order to get Propane & Propylene. Butane & Butene should be recovered from the bottom of the de-Propane column. In the process, condensers confabulated downstream of the de-propane and de-butene column are required. They are used for the removal of internal (sensible and latent) heat of superheated steam of Alkyl-C3 and C4, so that Propane and Propylene, Butene and Butane can be condensed.
During normal operation, the system should be capable of recovering between 70% and 90% Propane , Propylene, Butene and Butane by means of the absorbent rectification process. The other oil gases such as HsS and Di. Oleofin, etc. are treated as flare gases and are sent to the torch for burn off with other flare gases from other sources. As an alternative the HsS can be transported to the de-sulfide column, at which point sulfide is extracted by means of the de-sulfide process.
Due to the fact that the H/C ratio of Propane is 0.222, and Butane is 0.268, are all high carbon content Alkyl. These generate black smoke when burned off without pretreatment. For this reason, to eliminate this problem during burning in the flare stack, the torch is built to a height of 200 feet (60 meters) or more. This helps to disperse the burnt waste of flare gases to help reduce the environmental impact on the immediate areas.
2.0 Improvement of recovery rate of refinery gases
In section 1 on above it is noted that condensers are the main items of equipment used in the process of recovery of the mixture of superheated steam of Alkyl- C3 & C4 from the de-Butane column as well as Propane & propylene from the de-propane column and Butane & Butene. At this point it is possible to remove the latent and sensible heat of superheated steam of Alkyl- C3 & C4. The heat exchange rate or heat conductivity efficiency is the key-point at this stage. Improving the heat conductivity efficiency is absolutely necessary to increase the recovery rate of refinery gases.
It is important to verify that it is economically and technically possible to improve the heat conductivity efficiency to the condensers before a decision is made to carry out any work. The verification can be made based on the following information:
2.2.1 Heat-Conducting Law
In the first instance, check out the efficiency of the de-propane & de-butane process based on the following:
a. It is known that when petroleum fractions are superheated, its sensible heat is much greater than latent heat. Therefore sensible heat can be conducted first, which will be an advantage when Alkyl- C3 & C4 is superheated as part of the ongoing process.
b. Condensation speed of superheated steam of Alkyl- C3 & C4 will be affected by the temperature of the heat conductivity interface. So, if the temperature of the heat contact interface is lower than the condensate temperature of the product, it will give much more heat-conductivity for heat transfer in the condenser. The condensation speed of the superheated steam of Alkyl- C3 & C4 would be increased, then again, condensation speed may well be eliminated.
c. In the event of the temperature of the discharge point of the condenser being higher or very close to the saturated steam temperature of the products (superheated steam of Alkyl- C3 & C4), condensation phenomenon would not occur and no condensate will come out at the heat conductivity interface of the condenser.
The well-known condensation phenomenon is explained by the following formula:
Q = hA(Th-Tw)
Q = Overall heat transfer rate
A = Contact surface area between superheated steam of the products and the condenser
Th = Temperature of the superheated steam of the products
Tw= Temperature of the heat conductivity interface of the condenser
h = Average heat conductivity coefficient
Using the formula, it is assumed that the average heat conductivity coefficient (h) will be a constant factor and will not result in any direct affect in this case but other factors will. Therefore consider the following:
1. Heat conductivity is improved and better efficiency is achieved when the contact surface area between the superheated steam of the products and the condenser (A) is as big as possible.
2. Heat conductivity efficiency is directly proportional and again would be greatly improved if the temperature of the superheated steam of the products (Th) is as high as possible, or temperature of the heat conducting interface of the condenser (Tw) is as low as possible.
2.2.2 The Newton's Cooling Law
Newton's Cooling Law can be used to double check and determine whether the above simulation is correct
Newton's Cooling Law basically explains Heat Flux (heat conductivity rate), and the theory is to proove that heat flux should depend on the surface area of the plan plate and differential temperature, which is between the product and the plate. Newton's Cooling Law is set out below:
Q /A ¥ ?T or Q/A =h ?T =(Tw-T¥)
In this case, the symbol 'h' stands for ' heat convection coefficient', or so called ' film convection coefficient', seen as a constant factor in the formula. 'A' stands for the contact surface of the plan plate, and '?T' or 'Tw-T¥' refers to the differential temperature between the product and the plate. Therefore if 'A' is set at a certain amount, it can be seen that total Heat Flux would be greater when ?T is increased at the same time, whereas, when ?T is decreased, total Heat Flux could be eliminated in the mean time.
Given this explanation, preliminary conclusions can be made which in turn can be converted into working principles with regard to the recovery of the refinery gases as follows:
- In the refining operation, try to channel Alkyl- C3 & C4 to superheated steam format, so that Alkyl- C3 & C4 could be separated easily from the mixture of Alkyl- C3 & C4 and Gasoline.
- Try to maintain heat-conduct-interface-temperature of superheated steam as low as possible, so that recovery rate and the recovery speed of Propane & Propylene, Butane and Butene would be greater.
Try to keep the critical temperature of the products as high as possible in the de-propane and de-butane column in the process, but then again, try to maintain the temperature of the heat exchange media , that is the water coming to the condenser as low as possible, so that a low threshold temperature is achieved at the contact point of the condensers as well, in order to obtain a better recovery efficiency at this point.
Why is it that the refineries do not do this, even though it is possible to improve the heat conductivity efficiency? It is because the normal temperature of cooling water coming to the condenser is extremely high, creating a much lower deferential temperature to the inlet portion of the condenser than that which would be required, therefore not being able to provide a good enough heat conducting differential to the heat contact interface between the contact surface of the condenser and the products (superheated steam of Alkyl- C3 & C4). This will then result in a poor recovery rate of the target products (Propane and Propylene, Butane and Butene).
2.2.3 Estimated quantities of recoverable Propane & Propylene, Butane & Butene Generally, the anticipated average recovery rate of Propane & Propylene, Butane & Butene would be around 9%, and total volume of these recovered refinery gases would be around 80% by comparison to the total volume recoverable, that is to say, quantities of Propane & Propylene, Butane & Butene not recovered in the normal process would be as follows:
(Recoverable quantities of) Propane & Propylene, Butane & Butene = 9%/0.8 - 9% = 2.25%
This equates to a total loss of 2.25% of Propane & Propylene, Butane & Butene lost in the process. Take an oil refinery with a feedstock rate of 50,000 bbl for example, Propane & Propylene, Butane & Butene not recovered would be:
200 X 0.9 X 50,000 X 0.5 X 0.0225 = 101,250 kg /day
'200' is the total volume of crude oil per barrel, which is 200 litres; '0.9' is the specific gravity of crude oil;'50,000' refers to feedstock rate - 50,000 bbl per day, '0.5' is the mean specific gravity of the combined gases and 0.0225 refers to further quantities of recoverable Propane & Propylene, Butane & Butene from the process.
The quantity of available Propane & Propylene, Butane & Butene could be as high as 101,250kg per day. Propylene is a valuable petrochemical material and is used for forming different type of plastic material. Propane can be used for extracting Acetylene. Butane is the base material for forming Butene through the dehydrogenation process and then for forming Butadiene. Butadiene is the base material for producing Styrene and Butadiene Rubber for different applications. Recognising the difficulties in being able to check the value of each of these products, they are grouped together collectively as LPG, for easy valuation. It is assumed that the spot-market price of LPG is USD 0.20 per kilogram (USD 0.20 / kg), the value of those recoverable Propane & Propylene, Butane & Butene can be calculated as follows:
USD 0.20 / kg X 101,250 kg = USD 20,250, USD 567,000 per month (28 days) or USD 6,804,000 per year. (12 months)
As previously discussed, to help increase recovery efficiency either keep the temperature of the coolant water to be used as the heat transfer media of dephlegmation of Alkkyl - C3 and C4 as low as possible in the heat exchanger, or increase the volume of coolant water. Therefore either of the following solutions could be used:
Increase the total volume of coolant water into the condenser in order to remove much more internal heat of superheated steam of Alkyl-C3 and C4, so that rectification of Alkyl-C3 and C4 is cooled down rapidly and completely in a given period of time.
Try to decrease the temperature of the coolant water to the condenser (not increase the volume of water) in order to get much more heat conducting surface area. This should then lower the critical temperature on the contact interface between superheated steam of Alkyl-C3 and C4 and the heat contact surface of the condenser. The condensates (Propane & Propylene, Butane & Butene) could then be obtained rapidly and completely.
3.2 Analysis of the possibilities
Technically, Plan A would be possible, but impractical due to huge requirements of water resources that would be needed. The refinery would have to invest vast sums of money to build a big pumping station, water tanks, and related facilities. In addition to capital cost, running cost would be also be very high. Therefore, using a torch to burn off flare gases would be a much cheaper option. That is why Plan A is very rarely put into operation by the oil refineries.
As to Plan B, in general, potential equipment used for cooling down coolant water temperature would possibly include Helical Screw Refrigeration Compressors or Lithium Refrigeration systems. Consider an oil refinery with a feedstock rate of 50,000 bbls per day as an example. The total volume of coolant water to be used for the purpose of recovering Propane & Propylene, Butane and Butene would be hundreds of cubic meters per hour, and ' total heat conductivity rate' would be hundreds of Kcal per hour as well. Assume that specific gravity of superheated steam of Alkyl-C3 and C4 is 1.55 as an example, then the total volume of Alkyl-C3 and C4 to be recovered in the process from de-Propane and de-Butane process can be calculated as follows:
200 X 50,000 X 0.09/0.8/1.55 = 725,806 liter /day, or 30,242 liter /hour.
'200' is the total volume of crude oil per barrel, '50,000' refers to feedstock rate,'0.09/0.8 ' refers to total volume of superheated steam of Alkyl-C3 and C4, 1.55 refers to the specific gravity of superheated steam of Alkyl-C3 and C4.
As previously discussed, in order to condense superheated steam of Alkyl-C3 and C4, a heat-conductive interface temperature of superheated steam should be maintained which is lower than the saturated steam temperature of Alkyl-C3 and C4. In this case, it is assumed that the outgoing temperature at the top of the de-Propane & de-Butane column is 130 °C, and the required condensing temperature is 45°C. The incoming temperature of the coolant water is 20°C and outgoing temperature from the discharge point of the condenser is 30°C, The differential temperature to condense the total volume of Alkyl-C3 and C4 is then calculated to be as follows:
? qm=(45-20)-(30-20) / ln (45-20) / (30-20)
It is known that the heat conductivity coefficient of organic steam would be up to 400 Btu/(hr)(ft2)(°F), or up to 1,952 Kcal / (hr) (m2) (°C).Therefore, the total heat conducting rate to be removed for cooling down Alkyl-C3 and C4 from 130°C down to 45°C in this case is calculated as:
Q/A=1,952 Kcal/hr (m2) (°C) X (30-20) (°C) = 19,520 Kcal / hr
Therefore, total heat conducting rate should be removed for a quantity of 30,242 liter/hr (Alkyl-C3 and C4) would be:
Q/A=19,520 X 30,242 = 590,323,840 Kcal / hr (m2)
If water is used as the intermediate media for the condensers, given that the heat conducting coefficient of water at 20°C is approx. 0.62, and heat conducting coefficient of Alkyl-C3 and C4 is about 0.04, when the Gradient of Temperature Field is available, the heat conducting coefficient between the coolant water and superheated steam of Alkyl-C3 and C4 is 0.62/0.04 = 15. This means that the heat conducting rate of coolant water would be 15 times in comparison to superheated steam of Alkyl-C3 and C4 in the same period of time, therefore, total heat conducting rate of the coolant water required will be:
590,323,840 Kcal /hr (m2) / 15 = 39,354,923 Kcal / hr (m2)
If a helical screw compressor is used as the main refrigerator in the system, and the assumption is made that the power factor of the helical screw compressor is 1 KW: 2,400 Kcal/hr, power demand would then be 39,354,923 Kcal / hr / 2,400 KW/hr = 16,357 KW in total.. If it also assumed that cost of the power is USD. 0.09 per kw-hr, then the running cost for the helical screw compressor would be USD 0.09 X 16,357 = USD. 1,472 per hour or USD. 1,472 X 24 = USD 35,358 per day. When compared to the value of un-recovered refinery gases in USD 17,212; this idea is commercially impractical. In addition consideration must be made for the capital cost for the system.
Because of this and other factors, it is unlikely that a refinery will use helical screw compressor technology in the coolant system for this purpose. This leads to the consideration of other alternatives.
3.3 Alternative solution
As shown on above, it would be impractical to adapt either Plan A or plan B using a helical screw compressor. Consideration should be given to alternative technologies, which can provide the best and most cost effective way of converting flare gases to usable products. The selected method would have to include:
- Reducing the cooling water temperature with low capital cost equipment
- Providing low running and maintenance cost for this equipment
By incorporating these features the system chosen should provide reasonably quick capital cost recovery & ongoing financial, as well as environmental benefits. Note the following key factors for consideration:
- Installation of the equipment should not have any affect on the normal process
- Capital cost will be acceptable from the beginning
- Re-payment shall be greater than maintenance cost and running cost - all operating costs
- Facility shall be reliable and easy to maintain
- Utility consumption should be as low as possible and should be available from the refinery
- If a solution covers all the conditions listed above, serious consideration should be made to invest in it
4.0 The Wap-Flaro Technology
Waste Heat Powered Flare Gas Recovery System for oil refineries (hereinafter called Wap-Flaro) uses low working pressure waste steam from the refinery (around 2~4 kg/cm2) as power. It incorporates clusters of jet ejector and condensers as refrigeration unit, it cools down the normal temperature of cooling water from approximately 20~25°C down to 1~10°C, then pumps this low temperature cooling water to the heat exchangers or condensers. This heat exchanges with the products Alkyl-C3 and C4, which has been evaporated from the de-Propane or de-Butane column. The heat conductivity rate can be increased in order to condense Propane & Propylene, Butane & Butene as effectively as possible.
4.2 Design Concept
The following design concepts have been adapted in the Wap-Flaro process:
- It uses low-pressure waste steam (min. 1 kg/cm2) as main power
- It can increase recovery rate of refinery gases up to 100%
- It suppresses air pollution problem from torch emissions
- It incorporates a fully automatic control system
- Mechanical life of the Wap-Flaro system is more than 20 years
As can be seen from the flow chart of Wap-Flaro, the main power of the process is waste steam from the refinery, although there is a continuous electric power demand for some miscellaneous/peripheries including pumps, fan, and ancillary coolant column. However, they are relatively low when considered against the repayment obtained from the process.