hydrocarbon vapor recovery unit: an...

Koleksi Perpustakaan UPN "Veteran" Jakarta

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HYDROCARBON VAPOR RECOVERY UNIT: AN OVERVIEW

J. J. LuthanDepartment of Mechanical Engineering UPN “Veteran” Jakarta

Jl. RS Fatmawati PondokLabu, Jakarta Selatan 12450Telp. 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 pe-

nimbunan 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 kehi-

langan-kehilangan ini, beberapa cara dapat diterapkan untuk menangkap uap evaporasi atau

untuk menekan penguapan tersebut. Salah satucara yang dapat dipergunakan ialah dengan-

memasang 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 vola-

tility, 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 or-

der 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 evapo-

rations of other volatile fluids.

Key Words: volatile hydrocarbon compounds, evaporations in storage tank, recovering hy-

drocarbon 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 hydrocar-

bon 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 high-

volatile hydrocarbon products such as motor gasoline. Because of the high volatility, evapora-

tion 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 or-

der 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 frac-

tions must be stripped off from the gas fuel and this can be accomplished using, among oth-

ers, 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 un-

loading 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 ig-

nored. 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 dis-

placed. 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 equili-

brium. The use of tank mixers and the effects of temperature will also play a part in this equi-

librium process. The API procedure to estimate for this loss attempts to account for these in-

fluences on the overall quantity of working loss.

To establish appreciation, take for example Plumpang Oil Terminal, which according to Tri-

bun 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 kiloli-

tersPertamax 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 main-

tain 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


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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 cer-

tainly 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 refi-

neries 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 particu-

lar on the temperature and wind conditions. API proposes the following expression to esti-

mate 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. Tempera-

ture 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


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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 Va-

por 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 cap-

turing 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


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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 applica-

tions, there are only a few companies adopting the method due to the rare commercial expe-

rience. 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 ad-

vantages 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 sepa-

rated 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 hy-

drocarbon vapor is originated, that is, if the hydrocarbon vapor is originally from motor gaso-

line 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 in-

side 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 characteris-

tics 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 acci-

dence. 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: 140C

Adsorption VRU

Features

The followings are the main features of Adsorption VRU:

“Complete” recovery (may be more than 99%) of hydrocarbon vapor with certain ad-

sorbent(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 com-

pressor, the mixture of hydrocarbon vapor and air flows through the vapor line to the Adsor-

ber 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 surround-

ings.

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., releas-

ing 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 re-

leased 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 va-

por 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 desorp-

tion.

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 ad-

sorbent recently introduced is activated silica in contrast to activated carbon that was widely


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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, pre-

serve the environment by reducing the pollutant and to improve working environment by re-

ducing significantly the flammability potential. To meet with these objectives, Vapor Recov-

ery 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

(hydrocarbon vapor vs.

air)

Multi-stage

membrane

Using absor-

bent (liquid

solvent)

Using adsor-

bent (solid)

Footstep area Medium Medium Smallest

Motor gasoline recovery Able Able Able

Performance Good Better Best

Recovery effectiveness Reasonable High Highest

Investment cost Highest Intermediate Lowest

Maintenance Easy Easier Easiest

Utility (electricity,

kWh)

50 65 40

Operations Reliable Reliable Most reliable

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 Fixed-

Roof 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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hydrocarbon vapor recovery unit: an...

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