HYDROCARBON VAPOR RECOVERY UNIT: AN OVERVIEW J. J. Luthan Department of Mechanical Engineering UPN “Veteran” Jakarta Jl. RS Fatmawati PondokLabu, Jakarta Selatan 12450 Telp. 021 7656971 Abstract Pada umumnya tangki penyimpanan atau penimbunan di Indonesia dibuat dengan konstruksi atap tetap (fixed roof). Konstruksi atap tetap ini juga dipergunakan pada tangki-tangki penimbunan produk hidrokarbon dengan volatilitas tinggi seperti bensin.Volatilitas ini pada gilirannya mengakibatkan evaporasi produk hidrokarbon yang menyumbang pada kehilangan Komoditas berharga. Kehilangan pada tangki timbun atap tetap dapat dikategorikan sebagaikehilangan kerja (working loss) dan kehilangan pernafasan (breathing loss).Dalam rangka meminimisasi kehilangan-kehilangan ini, beberapa cara dapat diterapkan untuk menangkap uap evaporasi atau untuk menekan penguapan tersebut. Salah satucara yang dapat dipergunakan ialah denganmemasang alat Unit Penangkap Uap (Vapor Recovery Unitatau VRU). Tulisan ini membahas sejumlah proses VRU yang dapat diaplikasikan dalam mencapai objektif penangkapan uap hidrokarbon yang terevaporasi atau terhadap fluida-fluida lain yang mudah menguap. It is widely known that for some unknown reasons many of storage tanks for hydrocarbon products in Indonesia are constructed using fixed-roof. These fixed-roof storage tanks are also used to store high-volatile hydrocarbon products such as motor gasoline. Because of this volatility, evaporation of hydrocarbon products will take place that accounts for product losses. Losses in fixed-roof storage tanks may be categorized as working and breathing losses. In order to minimize the losses, several means may be employed to capture the evaporation or as much as possible to suppress the evaporation. One way of capturing the evaporated products is by installing Vapor Recovery Unit (VRU). This paper outlines several VRU processes that may be applied to meet with the objective of capturing hydrocarbon evaporation or the evaporations of other volatile fluids. Key Words: volatile hydrocarbon compounds, evaporations in storage tank, recovering hydrocarbon vapor

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INTRODUCTION Raw materials or finished products in the form of liquid or gas are usually kept in storage tanks for further processing as outlined by McKetta (1992). Whilst the material in the form of gas has reached its stable phase in that the entropy is at maximum, materials in the form of liquid may still undergo some phase change by becoming gas. The transformation process from liquid to gas phase will be quite easy for high volatile materials such as some hydrocarbon products and alcohols. If the evaporating materials are of some values, it is certain that the evaporation is undesirable as it contributes to the depletion of the material if measures to prevent it from freely taking place are not devised. For some unknown reasons many of storage tanks for hydrocarbon products in Indonesia are constructed using fixed-roof. These fixed-roof storage tanks are also used to store highvolatile hydrocarbon products such as motor gasoline. Because of the high volatility, evaporation of hydrocarbon products will take place that accounts for significant product losses. Losses in fixed-roof storage tanks may be categorized as working and breathing losses. In order to minimize the losses, several means may be employed to capture the evaporation or as much as possible to suppress the evaporation. One way of capturing the evaporated products is by installing Vapor Recovery Unit (VRU). This paper outlines several VRU processes that may be applied to meet with the objective of capturing hydrocarbon evaporation or any other volatile fluids. VRU can also be used in gas gathering terminal to prepare fuel gas for powering gas-engine generating sets. The reason is because of the restrictive nature of gas engine fuel composition (usually measured in Methane Number) while the gas obtained from oil fields has wide range of variations in composition and, hence, many times the gas’ Methane Number falls outside the range can be accepted by the gas engine. If this happens, especially when the gas’ heavy fractions are in abundance, the combustion temperature would be very high making operating temperature exceedingly high. Abnormally high temperature would result in lower engine performance in terms of heat utilization, increase maintenance efforts, and a number of other operating problems. Thus in order to obtain a smooth operation, the hydrocarbon heavy fractions must be stripped off from the gas fuel and this can be accomplished using, among others, VRU.

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THEORETICAL BACKGROUND If focus is switched to the losses taking place in a fixed-roof storage tank then working loss is caused by the movements (up and down) of the liquid surface due to product loading and unloading in such a way that these movements displace the product vapor accumulated above the liquid level (this resembles the working of piston in a cylinder). Working loss is by far the dominant loss in this type of storage tanks. Breathing loss is that loss which takes place when the liquid level is stationary whereby evaporation is lost to the surroundings through roof vents. The amount of this loss is lower in comparison with working loss but it cannot be ignored. API publication 2518 (1991) deals with evaporative loss from fixed-roof tanks. The basis to estimate the potential amount of hydrocarbon evaporation is the simple fact that when liquid enters a tank a quantity of vapor equivalent to the volume entering liquid is displaced. When the vapor has achieved equilibrium with the liquid, the quantity of displaced vapor may be expressed as a percentage of the entering liquid as

(1)

The ratio of

(the vapor pressure divided by total pressure) gives the equilibrium hydro-

carbon composition of the vapor. The density ratio simply reflects the fact that the vapor has a liquid equivalent in either mass or volume given by this ratio. In practice if a tank is moving up and down very often the vapor above the liquid surface may not have time to reach equilibrium. The use of tank mixers and the effects of temperature will also play a part in this equilibrium process. The API procedure to estimate for this loss attempts to account for these influences on the overall quantity of working loss. To establish appreciation, take for example Plumpang Oil Terminal, which according to Tribun News, 1st January 2011 edition, delivered 15,517 kiloliter fuel oil daily consisted of (only high-volatile commodities) 10,797 kiloliters gasoline, 1,267 kilolitersPertamax and 195 kilolitersPertamax Plus. These numbers can be treated as throughput volume being transacted by Plumpang daily and hence might have to be replaced by the same amount each day to maintain sufficient fuel oil stocks. A typical vapor pressure for gasoline would be 0.35 bar against a storage pressure of 1 bar with typical gas density around 3.0 kg/m3 while that of the liquid phase usually around 750 kg/m3. Evaporation loss when equilibrium between liquid and its vapor is reached may, therefore, be expected to be around 17.2 kiloliters per day if no means 3

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to suppress this evaporation is installed. This is a huge quantity in terms of monetary value and air pollution caused by the evaporation. In numbers, the aggregate amount of fuel oil loss is equivalent to 77.4 millions IDR/day or 28.2 billions IDR/annum. The exact figure will certainly depend upon the extent to which the displaced vapor has attained saturation condition. Another installation that might benefit from the use of VRU is flares usually found in oil refineries or gas terminals. Internationally with the hot issue of Global Warming and Climate Change, it has become a bad practice to flare gases. The API procedure quotes working loss as (2) where : Vapor molecular weight : Liquid vapor pressure : Annual net throughput : Turnover factor : Product factor The basis of the calculation is clear. The first three parameters represent the loss under full saturation conditions in which their values might be estimated using the methods presented by Reid et al (1977). The turnover factor reflects the degree to which saturation has been achieved; whilst the product factor reflects the fact that crude oil appears to generate vapor at a slower rate than products. To estimate the amount working loss, the following information is needed: 1. The stock vapor molecular weight. 2. The stock vapor pressure (or the stock Reid vapor pressure). 3. The stock annual net throughput (associated with increasing the stock liquid level). 4. The stock turnover rate. 5. The stock type.

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Breathing or Standing Storage loss depends on the conditions around the tank and in particular on the temperature and wind conditions. API proposes the following expression to estimate the amount of breathing loss (3) where : Vapor space volume : Stock vapor density : Vapor space expansion factor : Vented vapor saturation factor The minimum information below is needed to estimate the amount of standing storage loss: 1. The tank diameter. 2. The tank shell height. 3. The tank roof type (cone roof or dome roof). 4. The tank outside paint color. 5. The tank location. 6. The stock type. 7. The stock liquid bulk temperature. 8. The stock vapor pressure (or the stock Reid vapor pressure). Improved estimates of the standing storage loss can be obtained through the knowledge of some or all of the following additional information: 1. The tank cone roof slope or dome roof radius. 2. The breather vents pressure and vacuum settings. 3. The daily average ambient temperature range. 4. The daily total solar insolation on a horizontal surface. 5. The atmospheric pressure. 6. The molecular weight of the stock vapor. 7. The stock liquid surface temperature. As before the saturation factor depends upon the degree of movement in the tank. Temperature effects and the p/v (pressure/vacuum) valve settings largely govern the expansion factor. For the example cited the standing storage loss is around 25% of the total working loss. Again 5

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given the degree of uncertainty associated with the calculation, fixed percentages of working loss may be used to estimate the loss. In view of the facts that hydrocarbon evaporation such as that from product storage tanks in terminals is very undesirable, the vapor must be recovered for at least three main reasons. They are: 1. To reduce product losses and hence to reduce financial losses from such unrestricted evaporations. 2. To reduce air pollution due to hydrocarbon evaporations from storage tanks. 3. To reduce the degree of safety risks because hydrocarbons vapor is highly flammable and hence to improve working environment in terminals. To meet the objective of capturing hydrocarbon evaporations or otherwise, the so-called Vapor Recovery Unit (VRU) may be used to great advantages. Basically, VRU technology is divided into three main processes that are Membrane, Absorption and Adsorption processes. This paper outlines each VRU process based on intensive communications with Mohri (2008) from Cosmo Engineering Co., Ltd. as a sister company of Cosmo Oil Co., Ltd. that is one of the major oil companies in Japan that owns four refineries and hence faces the problem of products evaporations. In general, the three hydrocarbon vapor recovery methods above follow the same working sequence, which is, capturing hydrocarbon vapor, releasing the captured vapor from the capturing media and returning the vapor that has been dissolved in solvent (usually the product from which the vapor evaporates) to storage tanks. The only difference lies in the media used to catch the vapor with the consequence that effectiveness and capability of each method also differs in addition to the difference in operating parameters, consumables, and types and numbers of supporting facilities. Membrane VRU Features Membranes find many applications to separate or purify gases. As an example, membrane process is used to recover off-gas in oil refineries. The capacity to perform separation for 6

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membranes lies on the membrane’s permeability and the speed with which the gas mixture pass through the membrane in which performance is mainly depending on diffusivity, size of gas molecules and permeating affinity between the gas the membrane material. Therefore, the membrane must be specifically designed and fabricated for the target gas so that the gas may penetrate the membrane quite easily. In relation with the use of this method in VRU applications, there are only a few companies adopting the method due to the rare commercial experience. The materials used for membrane are usually polyamide, combination of polyamide and silica-based materials or cellulose acetate. Process using membrane has a number of advantages such as acceptable performance, relative ease in maintenance, and good operating safety. However, if this process is aimed at high performance application in which the content of hydrocarbon vapor in vented mixture is very low, the process must be made in multi stages or must be made using the combination of membrane and adsorption processes. Process Flow The schematic diagram depicting the process flow can be seen in Fig. 1. Gas mixture that comprised of evaporated hydrocarbon and air coming out from fuel facilities is sucked using blower or compressor at pressure ranging from 10 to 490 kPa through vapor line and directed to VRU. Because membrane process is less flexible in handling the rate of liquid feed flow, it is frequently that other collecting tank is installed to obtain a stable flow. The flowing pressurized gas mixture is passed through the filter separator that is equipped with falling liquid and then it is sent to membrane unit. Hydrocarbon vapor and air are separated in this unit by creating vacuum environment on the side of the permeating surface of the membrane. As is explained before, multi-stage membrane process must be utilized to reach the objective of reducing hydrocarbon vapor content in the VRU’s emission to the level of 1%. The hydrocarbon vapor is then sucked using vacuum pump from the membrane unit and transported to the Recovery Column. Inside the Recovery Column, hydrocarbon vapor resulting from flashing process due to the imposed vacuum is absorbed by liquid feed that is taken from the liquid from which the hydrocarbon vapor is originated, that is, if the hydrocarbon vapor is originally from motor gasoline then the liquid feed will be motor gasoline. The liquid feed that has absorbed the hydro-

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carbon vapor is pumped back to the storage tank. Part of the hydrocarbon vapor that cannot be recovered by the liquid feed flows back to filter separator to follow the next cycle of recovery. Absorption VRU Features The followings are the main features of Absorption VRU: 

A relatively matured process with plenty of commercial applications;



More than 90% hydrocarbon vapor may be captured;



Ease of operation at low pressure and ambient temperature;



Operations and construction costs are low.

Process Flow Schematic diagram giving the sequence of process for this type of VRU is presented in Fig. 2. Gas mixture of hydrocarbon vapor and air flows through vapor line to the middle section of Absorber Column. While the hydrocarbon vapor flows upward due to buoyancy in that the density of vapor is relatively low, the vapor comes into contact with absorbing solvent that flows from the top of the column to the bottom. During this counter-flow contact, the solvent absorbs most of the vapor. The gas mixture that has been cleaned from hydrocarbon vapor is then vented to the surroundings. The solvent that is now rich with absorbed hydrocarbon vapor is collected at the bottom of Absorber Column. This rich solution, with the help of a pump, flows to Regenerator Column in which the rich solution is stripped from hydrocarbon vapor. The process of stripping the rich solution is conducted by creating vacuum inside the Regenerator using a vacuum pump. The pressure inside the Regenerator is maintained in the range of 5 to 25 mm Hg. Because of the vacuum environment, the hydrocarbon within the solution vaporizes (flashing process) so that separation can be achieved. The solvent used is usually suitable liquid that will perform well to absorb the hydrocarbon vapor. The “purified” or regenerated solvent flows back to the Absorber Column to perform the next cycle or recovery.

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The resulting hydrocarbon vapor obtained from the flashing process inside the Regenerator Column flows to the middle section of the Contactor Column. Inside this column, once again, a counter-flow contact between the vapor that flows upward and the liquid feed that flows from the top section of the column to the bottom occurs. In general the liquid feed is the same hydrocarbon from which the evaporation originated. The vapor that has been absorbed by the liquid feed is then pumped to the storage tank while the hydrocarbon vapor left unabsorbed by the liquid feed is returned to the Absorber Column to follow the next absorption process inside the column. Solvent Solvents to be used in absorption process of hydrocarbon vapor are chosen based on several considerations. If the vapor to be absorbed is not hydrocarbon but single-component products such as those found in petrochemical plants, the solvents is selected based on the characteristics of vapor to be absorbed. Generally, VRU is used to recover motor gasoline vapor because it is easily evaporated and the flammable nature of the vapor holds great potential for accidence. Hence the usual solvents to be used in Absorption VRU have the following properties: 

Type of solvent: oil product



Specific gravity: 0.839



Flash point: 140C

Adsorption VRU Features The followings are the main features of Adsorption VRU: 

“Complete” recovery (may be more than 99%) of hydrocarbon vapor with certain adsorbent(s);



Very stable and fully automatic operations;



Long adsorbent life time (can be more than 8 years);



Simple equipment configuration and low construction costs.

Process Flow

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The process flow diagram for this type of VRU is given in Fig. 3. By using blower or compressor, the mixture of hydrocarbon vapor and air flows through the vapor line to the Adsorber Tower. Usually a signal sent by an observing device connected to the filling pump will activate the Adsorption process. In this process, the adsorbent (activated silica) will “catch” or adsorb the hydrocarbon vapor flowing to that tower. The resulting gas after adsorption process that is now almost totally cleaned from hydrocarbon vapor is vented to the surroundings. Two Adsorber Tower are synchronized to work alternately in such a way that they can change mode from Adsorption to Desorption mode automatically. Whilst adsorption process is taking place in either one of the columns, the other column performs desorption process, i.e., releasing hydrocarbon vapor that has been adsorbed during the previous cycle). Desorption process is done under vacuum such that the hydrocarbon vapor contained within the adsorbent is released due to flashing. The vacuum environment inside the tower performing desorption process is maintained at the minimum level of 30 mm Hg. During adsorption process, a certain amount of heat is generated (exothermic process) whilst desorption process needs heat (endothermic process). To balance the need and excess heat due to different thermal processes, VRUs are equipped with a certain cooling/heating water circuit that holds the role of transferring heat from one process to the other. The vapor being obtained from desorption process is then transferred to Recovery Tower in which the vapor is then contacted counter-currently with the liquid phase from which the vapor was evaporated. This contacting process forces the liquid feed to absorb the hydrocarbon vapor. The absorbed vapor is then pumped to the storage tank while the unabsorbed portion of the vapor is returned to Adsorber Columns to follow the next cycle of adsorption and desorption. Adsorbent An important consideration in choosing the adsorbent is that the material shall not easily or holds the potential to cause fire hazards because it is stored in an environment that is rich with hydrocarbon vapor and oxygen (from the air) mixture. In view of this, the state-of-the-art adsorbent recently introduced is activated silica in contrast to activated carbon that was widely 10

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used in the past. Activated silica is not easily burnt unlike activated carbon that is categorized as organic matter. Conclusions Hydrocarbon vapor emitted from fuel storage and handling facilities because of evaporation may be recovered to capture the valuable product(s) and hence reduce economic loss, preserve the environment by reducing the pollutant and to improve working environment by reducing significantly the flammability potential. To meet with these objectives, Vapor Recovery Unit (VRU) may be utilized to great advantages. Based on the process utilized, VRUs are classified as Membrane, Absorption and Adsorption. The features of each process may be summarized as follows. Table 1. Summary of VRU technologies for recovering motor gasoline Remarks

Membrane

Absorption

Adsorption

Method of separation

Multi-stage

Using absor-

Using adsor-

(hydrocarbon vapor vs.

membrane

bent (liquid

bent (solid)

air)

solvent)

Footstep area

Medium

Medium

Smallest

Motor gasoline recovery

Able

Able

Able

Performance

Good

Better

Best

Reasonable

High

Highest

Highest

Intermediate

Lowest

Easy

Easier

Easiest

50

65

40

Reliable

Reliable

Most reliable

Recovery effectiveness Investment cost Maintenance Utility (electricity, kWh) Operations 

These figures are for 500 Nm3/hour VRU. Utility needed to operate a

VRU is predominantly electricity.

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References 1.

API (American Petroleum Institute), 1991,Manual of Petroleum Measurement Standards Chapter 19—Evaporative Loss Measurement. Section 1—Evaporative Loss from FixedRoof Tanks, API Publication 2518, 2nd edition, Washington, D.C. 20005, Oct. 1991.

2.

McKetta, John J. (editor), 1992, Petroleum Processing Handbook, Marcel Dekker, 1992.

3.

Mohri, Tadami, General Manager of Business Development, Cosmo Engineering Co., Ltd., Tokyo, Japan. Private communications, 2008.

4.

Reid, Robert C.; Prausnitz, John M. and Sherwood, Thomas K., 1977,The Properties of Gases and Liquids, 3rd. ed., McGraw-Hill Book, Inc.

5.

Tribun News, January 1, 2011, DepoPlumpangKeluarkanMinyak 15.517 kl per Hari.

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Tanker Loading

LOADING AREA (by Trucks and Tankers)

PIS

Lorry Truck Loading

PIC

H : Unit starts L : Unit stops

No. 12 (RG)

No. 1 (GP-1)

No. 2 (GP-1)

No. 3 (RG)

B - 201 Blower

MU - 201 Membrane Unit

VP - 201 A/B VP - 202 A/B VP -203 A/B Vacuum Pump

Gas Vent

Fig. 1. Vapor Recovery Unit based on membrane process

FIL - 201 Filter Separator

HCI

MEMBRANE PROCESS

T - 201 Recovery Column

LIS

P - 202 Gasoline Recovery Pump

P - 201 Gasoline Feed Pump

Tanker Loading

LOADING AREA (by Trucks and Tankers)

PIS

Lorry Truck Loading

PIC

H : Unit starts L : Unit stops

No. 12 (RG)

No. 1 (GP-1)

No. 2 (GP-1)

No. 3 (RG)

B - 201 Blower

LC

Koleksi Perpustakaan UPN "Veteran" Jakarta VP - 01 A/B Vacuum Pump

E - 01 Cooler

cw

Fig. 2. Vapor Recovery Unit based on absorption process

P - 01 Circulation Pump

C - 02 Regenerator

VP - 01B

VP - 01A

ABSORPTION PROCESS

P - 02 Circulation Pump C - 01 Absorber

t neV sa G

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C - 03 Gasoline Contactor

LC

P - 03 Gasoline Recovery Pump

P - 04 Gasoline Feed Pump

Tanker Loading

LOADING AREA (by Trucks and Tankers)

PIS

Lorry Truck Loading

PIC

H : Unit starts L : Unit stops

No. 12 (RG)

No. 1 (GP-1)

No. 2 (GP-1)

No. 3 (RG)

B - 201 Blower

P - 01 Circulation Water Pump

C - 01B Adsorption Tower

B

Koleksi Perpustakaan UPN "Veteran" Jakarta VP - 01 A/B Vacuum Pump

Fig. 3. Vapor Recovery Unit based on adsorption process

In this figure, Tower A tackles Adsorption while Tower B is performing Desorption

C - 01A Adsorption Tower

A

Water Circulation

Two Operational Modes (Adsorption and Desorption) are alternately switched between Tower A and B.

ADSORPTION PROCESS t neV sa G

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E - 01 Cooler (Air cooled)

P - 02 Gasoline Recovery Pump

C - 02 Recovery Tower

LC

P - 03 Gasoline Feed Pump