DESC:FIELD OF THE INVENTION
The present invention pertains to a method to improve hydrocarbon (HC) vapors’ recovery efficiency in vapor recovery unit (VRU) for petroleum storage terminals. More specifically, the present invention pertain to a method to improve recovery efficiency of hydrocarbons in vapor recovery unit by bypassing a controlled portion of the feed vapor stream from an adsorber vessel upstream to directly into an absorber column when the adsorber vessel undergoing regeneration is under deep vacuum conditions or “critical” vacuum level.

BACKGROUND OF THE INVENTION
Conventionally, Vapor Recovery Units (VRUs) are installed at the marketing terminals to collect and recover the fugitive gasoline emissions resulting from loading of tankers at the gantry and keep the resultant emission level below 5g/m3 as per the current statutory requirements.

The VRU employs activated carbon based “adsorber vessels” that operate on batch mode with alternating adsorption and desorption cycles through a vacuum swing process. Post desorption, the hydrocarbon (HC) is recovered in an “absorption column” by counter current exchange with liquid gasoline. Few such methods and apparatus described in prior arts are as follows:

3155/MUM/2012 covers the process of vapor recovery for reducing emission levels, said process comprising steps of adsorption in a suitably arranged adsorption layer by an adsorbent material regeneration of said adsorbed layer by a pump at a regeneration layer, recycling adsorbed vapors into an absorber column and mixing said vapors with dispensing fuel.

341/CHENP/2011 covers the gasoline vapor recovery apparatus in which vertical movement of heat generated from each device contained in a frame is suppressed is provided. A gasoline vapor recovery apparatus according to the present invention is characterized in that in a frame, a condensation tower and an adsorption/desorption tower are arranged at a position above a gasoline vapor pump and a desorption pump.

1871/KOLNP/2011 covers the fuel vapor management system for recovering fuel vapors displaced from vehicle fuel tanks, or the like, during the filling thereof from a bulk storage tank is disclosed that has excellent emission control and pressure management. A flow splitter is disclosed that utilizes positive pressure and locates downstream of a vacuum pump. The flow splitter is used to direct a portion of the gasoline vapors and air returned from the vehicle to an adsorbent canister and the remaining portion to the UST, in such a proportion that a selected UST vacuum may be achieved. The flow splitter may be included as a part of the vapor management system that relies upon a rapidly purging canister system and may be regenerated between vehicle refuelings, while minimizing canister volume requirements and stabilizing negative UST pressures. The disclosed system may be installed at the dispenser or centrally located.

WO2018/236935 describes an evaporative emission control canister system that includes: one or more canisters comprising at least one vent-side particulate adsorbent volume comprising a particulate adsorbent having microscopic pores with a diameter of less than about 100 nm; macroscopic pores having a diameter of about 100 - 100,000 nm; and a ratio of a volume of the macroscopic pores to a volume of the microscopic pores that is greater than about 150%, and having a retentivity of about 1.0 g/dL or less. The system may further include a high butane working capacity adsorbent. The disclosure also describes a method for reducing emissions in an evaporative emission control system.

However, in conventional VRU designs, during high vacuum levels at the adsorber bed (absolute pressure < 50 mbar); here referred to as “critical” vacuum level, a low vapor stream flow condition develops at the vacuum pump discharge line. As absorber columns are designed to operate efficiently at rated flow rate of the vacuum pump, this decrease in vapour stream flow causes significant reduction in the absorption i.e., HC vapor recovery in the absorber column.

Hence it is evident that in the conventional vapor recovery unit, the absorption of hydrocarbon suffers from low input vapor stream flow rate to the absorber column towards the end of the desorption process (at deep vacuum conditions) thereby reducing the HC vapor recovery. Therefore, there is a requirement of a method to overcome the said limitation in the conventional VRU process.

OBJECTIVES OF THE INVENTION
It is a primary objective of the invention to provide a method to improve recovery efficiency in vapor recovery unit for tanker loading facilities.

It is another objective of the present invention to provide a method for bypassing a controlled portion of the feed stream from the main adsorber vessel upstream, when the adsorber vessel undergoing regeneration is under deep vacuum conditions providing an enriched vapor with optimum feed rate into the absorber column alongside the low flow stream (from vacuum pump discharge).

It is further objective of the present invention to provide a method with dual advantage of a) enhanced absorption in the absorber column with improved recovery and b) reduced loading of HC vapors on the adsorber vessel enhancing the cyclic life of activated carbon bed.

SUMMARY OF THE INVENTION
This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the invention. This summary is neither intended to identify key or essential inventive concepts of the invention and nor is it intended to determine the scope of the invention.

The present invention provides a method for improving recovery efficiency of a vapor recovery unit (VRU) of petroleum storage terminals, said method comprising:
i. loading an activated carbon in an adsorber vessel assembly to form an adsorber bed, wherein the adsorber vessel assembly comprises adsorber vessel (V2a) and an adsorber vessel (V2b);
ii. directing a feed stream (F1) comprising a mixture of hydrocarbon vapors and air into the adsorber vessel (V2a) containing an adsorber bed (B2a) and allowing the hydrocarbons to get adsorbed over the adsorber bed (B2a); and releasing a hydrocarbon free air (A1) into the atmosphere;
iii. diverting the feed stream (F1) from adsorber vessel (V2a) to the adsorber vessel (V2b) upon the saturation of adsorber vessel (V2a);
iv. regenerating adsorber vessel (V2a) by desorbing the hydrocarbon vapors from adsorber bed (B2a) using a vacuum pump (C001), while the feed stream (F1) is diverted to the adsorber vessel (V2b);
v. introducing a purge air stream (A2), by operating the ON-OFF valve (PIC1), from top into the adsorber vessel (V2a) towards the end of regeneration in step iv) to sweep away hydrocarbon vapors desorbed from the adsorber bed (B2a);
vi. sending the hydrocarbon vapors desorbed from the adsorber bed (B2a) through a vacuum pump discharge stream (DS1) into an absorber column/scrubber (V1) having an inlet point (IP1) for the vacuum pump discharge stream;
vii. recovering the hydrocarbon vapors obtained from the vacuum pump discharge stream (DS1) using an absorbent liquid in the absorber column/scrubber (V1) to obtain a condensed hydrocarbon stream (CHS1) from bottom of the absorber vessel (V1), wherein the absorbent liquid is liquid gasoline stream (CHS2) supplied from top of the absorber column/Scrubber (V1) through an adsorbent liquid tank (AL1) by a centrifugal pump, wherein a hydrocarbon vapor stream (SF1) obtained from top of the absorber column is mixed with feed stream (F1) to obtain the combined feed stream (SF2) which is directed to the bottom of the adsorber vessel (V2b) which is under adsorption process;
viii. channelizing a bypass stream (F2), by operating the ON-OFF valve (PIC2), to the absorber column/ scrubber (V1) by bypassing the adsorber column (V2a) upstream, through a vapor inlet point (IP2) positioned adjacent to the inlet point (IP1) for the vacuum pump discharge stream (DS1), when the pressure in the adsorber bed (B2a) reaches a critical level during regeneration of adsorber vessel (V2a); and
ix. collecting a condensed hydrocarbon stream (CHS1) from step (vii) into a sump (S1) at the bottom of absorber column/scrubber (V1); directing it to mix with a liquid gasoline stream (CS1) routed out after cooling the vacuum pump (C001) to form a combined absorbent liquid return stream (CHS3) and routing it to the absorbent liquid tank (AL1) by a centrifugal pump.

BRIEF DESCRIPTION OF THE DRAWINGS:
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

Figure 1 depicts a general process flow diagram (PFD) for the process.
Figure 2 depicts the vacuum pump characteristic curve.

DETAILED DESCRIPTION OF THE INVENTION
The present invention pertains to a method of bypassing a portion of the feed stream from the main “adsorber vessel” upstream, while the other “adsorber vessel” (that is being regenerated) is under deep vacuum conditions and diverting the bypass feed stream directly into the “absorber column” alongside the low flow stream (from vacuum pump). The method works in a dual way a) enhances the absorption in the absorber column thereby improve the recovery and b) reduces loading of HC vapors on the “adsorber vessel” thereby enhancing the cyclic life of activated charcoal bed during the period when the adsorber bed is under deep vacuum conditions.
In the proposed invention, a portion of the feed stream is diverted to the absorber column when the pressure in the adsorber bed falls below 50 mbar (absolute) i.e., “critical” vacuum level. It improves HC vapor recovery significantly and also reduces the HC load on adsorber bed.

For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments in the specific language to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated process, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. The composition, methods, and examples provided herein are illustrative only and not intended to be limiting.

The articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

The term “some” as used herein is defined as “none, or one, or more than one, or all”. Accordingly, the terms “none”, “one”, “more than one”, “more than one, but not all” or “all” would all fall under the definition of “some”. The term “some embodiments” may refer to no embodiments or to one embodiment or to several embodiments or to all embodiments. Accordingly, the term “some embodiments” is defined as meaning “no embodiment, or one embodiment, or more than one embodiment, or all embodiments”.

More specifically, any terms used herein such as but not limited to “includes”, “comprises”, “has”, “consists” and grammatical variants thereof is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. The specification will be understood to also include embodiments which have the transitional phrase “consisting of” or “consisting essentially of” in place of the transitional phrase “comprising”. The transitional phrase “consisting of” excludes any element, step, or ingredient not specified in the claim, except for impurities associated therewith. The transitional phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps “and those that do not materially affect the basic and novel characteristic(s)” of the claimed invention.

Whether or not a certain feature or element was limited to being used only once, either way it may still be referred to as “one or more features” or “one or more elements” or “at least one feature” or “at least one element”. Furthermore, the use of the terms “one or more” or “at least one” feature or element do NOT preclude there being none of that feature or element, unless otherwise specified by limiting language such as “there NEEDS to be one or more” or “one or more element is REQUIRED”.

Use of the phrases and/or terms such as but not limited to “a first embodiment”, “a further embodiment”, “an alternate embodiment”, “one embodiment”, “an embodiment”, “multiple embodiments”, “some embodiments”, “other embodiments”, “further embodiment”, “furthermore embodiment”, “additional embodiment” or variants thereof do NOT necessarily refer to the same embodiments. Unless otherwise specified, one or more particular features and/or elements described in connection with one or more embodiments may be found in one embodiment, or may be found in more than one embodiment, or may be found in all embodiments, or may be found in no embodiments. Although one or more features and/or elements may be described herein in the context of only a single embodiment, or alternatively in the context of more than one embodiment, or further alternatively in the context of all embodiments, the features and/or elements may instead be provided separately or in any appropriate combination or not at all. Conversely, any features and/or elements described in the context of separate embodiments may alternatively be realized as existing together in the context of a single embodiment.

The terminology and structure employed herein is for describing, teaching, and illuminating some embodiments and their specific features and elements and does not limit, restrict, or reduce the spirit and scope of the invention.

According to the main embodiment, the present invention provides a method to improve the recovery efficiency in a vapor recovery unit (VRU); said method comprises the steps of:
i. a four-step vacuum swing adsorption (VSA) cycle; and
ii. a vapor absorption by mixing with absorbent liquid (Gasoline).

In another detailed embodiment of the present invention, the four-step vacuum swing adsorption (VSA) cycle further comprises of:
i. an adsorption;
ii. a counter-current evacuation;
iii. a counter-current purge; and
iv. a co-current pressurization with feed.

In another detailed embodiment of the present invention, the adsorption comprises the steps of:
i. loading activated carbon of calculated amount and known Butane working capacity (and activity) in two separate vessels;
ii. directing a feed stream comprising hydrocarbon and air into one of the vessels while the other is kept isolated;
iii. allowing the hydrocarbon of the feed stream to get adsorbed on the activated carbon up to a dynamically calculated duration (before activated carbon is saturated or ‘breakthrough’ occurs) allowing a HC-free air to be emitted into atmosphere; and
iv. diverting the feed stream to other adsorber vessel when the above duration is reached and the former adsorber vessel is isolated and enters into ‘regeneration’ mode; consisting of counter-current evacuation and counter-current purge (by atmospheric air).

In another detailed embodiment of the present invention, the counter-current evacuation comprises removing the hydrocarbon molecules adsorbed on the activated carbon by decreasing the bed pressure from atmospheric pressure i.e., about 1013 mbar to near perfect vacuum i.e., about 25 mbar (absolute), using a vacuum pump, wherein more is the vacuum applied, more HC molecules are disengaged from activated carbon surfaces.

In another detailed embodiment of the present invention, the counter-current purge comprises sending atmospheric air from top of the activated carbon bed for a limited time to accelerates desorption and recovery of hydrocarbon molecules adsorbed on the activated carbon bed to obtain a polished activated carbon bed after the removal of adsorbed HC molecules.

In another detailed embodiment of the present invention, the co-current pressurization with feed comprises re-pressurising the adsorber vessel to atmospheric pressure slowly for next adsorption cycle.

In another detailed embodiment of the present invention, the vapor absorption by mixing with absorbent liquid comprises mixing the HC molecules recovered from the VSA cycle in an absorber column/scrubber comprising a specially designed random packing, wherein liquid gasoline is sprayed from the top; and collecting a condense hydrocarbon stream from bottom of the absorber column/scrubber as ‘recovered gasoline’.

The present invention provides a method for improving recovery efficiency of a vapor recovery unit (VRU) of petroleum storage terminals, said method comprising:
i. loading an activated carbon in an adsorber vessel assembly to form an adsorber bed, wherein the adsorber vessel assembly comprises adsorber vessel (V2a) and an adsorber vessel (V2b);
ii. directing a feed stream (F1) comprising a mixture of hydrocarbon vapors and air into the adsorber vessel (V2a) containing an adsorber bed (B2a) and allowing the hydrocarbons to get adsorbed over the adsorber bed (B2a); and releasing a hydrocarbon free air (A1) into the atmosphere;
iii. diverting the feed stream (F1) from adsorber vessel (V2a) to the adsorber vessel (V2b) upon the saturation of adsorber vessel (V2a);
iv. regenerating adsorber vessel (V2a) by desorbing the hydrocarbon vapors from adsorber bed (B2a) using a vacuum pump (C001), while the feed stream (F1) is diverted to the adsorber vessel (V2b);
v. introducing a purge air stream (A2), by operating the ON-OFF valve (PIC1), from top into the adsorber vessel (V2a) towards the end of regeneration in step iv) to sweep away hydrocarbon vapors desorbed from the adsorber bed (B2a);
vi. sending the hydrocarbon vapors desorbed from the adsorber bed (B2a) through a vacuum pump discharge stream (DS1) into an absorber column/scrubber (V1) having an inlet point (IP1) for the vacuum pump discharge stream;
vii. recovering the hydrocarbon vapors obtained from the vacuum pump discharge stream (DS1) using an absorbent liquid in the absorber column/scrubber (V1) to obtain a condensed hydrocarbon stream (CHS1) from bottom of the absorber vessel (V1), wherein the absorbent liquid is liquid gasoline stream (CHS2) supplied from top of the absorber column/Scrubber (V1) through an adsorbent liquid tank (AL1) by a centrifugal pump, wherein a hydrocarbon vapor stream (SF1) obtained from top of the absorber column is mixed with feed stream (F1) to obtain the combined feed stream (SF2) which is directed to the bottom of the adsorber vessel (V2b) which is under adsorption process;
viii. channelizing a bypass stream (F2), by operating the ON-OFF valve (PIC2), to the absorber column/ scrubber (V1) by bypassing the adsorber column (V2a) upstream, through a vapor inlet point (IP2) positioned adjacent to the inlet point (IP1) for the vacuum pump discharge stream (DS1), when the pressure in the adsorber bed (B2a) reaches a critical level during regeneration of adsorber vessel (V2a); and
ix. collecting a condensed hydrocarbon stream (CHS1) from step (vii) into a sump (S1) at the bottom of absorber column/scrubber (V1); directing it to mix with a liquid gasoline stream (CS1) routed out after cooling the vacuum pump (C001) to form a combined absorbent liquid return stream (CHS3) and routing it to the absorbent liquid tank (AL1) by a centrifugal pump.

In an embodiment of the present invention, the adsorber vessel (V2a) and the adsorber vessel (V2b) are loaded with the activated carbon in a range of 1000 to 5000 Kg each.

In an embodiment of the present invention, the activated carbon is a mineral based activated carbon having a butane capacity in the range of 10 to 15 %, wherein the mineral based activated carbon is in the form of a cylindrical pellet having a diameter of 3 to 5 mm and length of 4 to 8 mm.

In an embodiment of the present invention, the feed stream (F1) has a temperature in a range of about 0 ? to 50 ? and has an operating pressure in a range of 990 to 1100 mbar (absolute).

In an embodiment of the present invention, the feed stream (F1) has an average flow rate in the range of 100 to 1000 m3/hr.

In an embodiment of the present invention, the vacuum pump (C001) is dry screw vacuum pump comprises a vacuum pump casing jacket, a vacuum pump injection port and screw rotors.

In an embodiment of the present invention, the vacuum pump (C001) is cooled by directing the liquid gasoline stream (CS1) to the vacuum pump casing jacket for cooling; and directing a liquid gasoline stream (CS2) to vacuum pump injection port for mixing with vacuum pump suction stream (SS1) inside the pump for cooling of screw rotors.

In an embodiment of the present invention, the adsorber vessel (V2a) is regenerated at a pressure in a range of 1100 mbar (absolute) to 20 mbar (absolute) created by the vacuum pump, the critical level is pressure less than 50 mbar (absolute), and wherein a purge air (A2) is introduced at critical level in the adsorber vessel (V2a) and continued till the end of the regeneration process.

In an embodiment of the present invention, the feed stream (F1) has the hydrocarbon vapors and air in a molar ratio of 10:90 to 40:60, the hydrocarbon vapors comprises hydrocarbons of C2 to C9 and an alcohol containing group, the hydrocarbon vapors comprises volatile organic compounds (VOC), and wherein the absorbent liquid is gasoline.

In an embodiment of the present invention, the bypass stream (F2) is a controlled flow rate of the feed stream (F1); the flow rate of the bypass stream (F2) in the range of 100 m3/hr to 1000 m3/hr is controlled through a flow control valve (FIC1).

In an embodiment of the present invention, the adsorber vessel (V2a) and adsorber vessel (V2b) are equipped with sensor to detect pressure drop across the adsorber bed and temperature at different levels of adsorber bed height.

In an embodiment of the present invention, the absorber column/scrubber (V1) has an internal diameter in the range of 300 to 1500 mm and has a height in the range of 1000 to 4000 mm, equipped with a demister adjusted to work at a pressure drop less than 0.5 mbar for stream velocities up to 2 m/s.

In an embodiment of the present invention, the absorber column/scrubber (V1) is filled with metallic very special packing (VSP) having diameter in the range of 10 to 50 mm, specific surface area in the range of 100 to 300 m2/ m3 and a void space in the range of 70% to 99%.

EXAMPLES:
The present disclosure with reference to the accompanying examples describes the present invention. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. It is understood that the examples are provided for the purpose of illustrating the invention only and are not intended to limit the scope of the invention in any way.

EXAMPLE 1:
Process Stream Details:
The process stream was a mixture of hydrocarbon vapors and air generated at the gasoline tanker truck (TT) loading bays of marketing terminal. The above stream was generated at each loading points and directed to the vapor recovery unit (VRU) inlet through a common header pipeline.

The HC:Air ratio in the stream was 30:70 (molar). The stream composition was derived from the detailed hydrocarbon analysis (DHA) carried out on Gasoline sample taken from field. The results of DHA were used to model the stream in the simulation. The stream properties are listed below in Table 1.

Table 1: Stream Properties
S. No. Parameters/ Properties Value
1 Minimum Flow Rate (m3/hr) 186
2 Design Average Flow Rate (m3/hr) 283 (@ 26 ? and 1010 mbar absolute)
3 Peak Flow Rate (m3/hr) 1584
4 Operating Pressure Range (mbar (absolute)) 990 to 1050
5 Temperature Range (?) 20 to 40
6 Composition C3 to C7, Oxygen and Nitrogen
7 Molecular Weight 42
8 Vapor Density @ 26 ? 1.71
9 HC mass flow rate (kg/ m3/hr) 0.925

Two ‘adsorber’ vessels were loaded with 2667 kgs of activated carbon. Each adsorber vessel designed for the system to operate continuously. These mineral based activated carbon particles were in the form of cylindrical pellets of suitable size to ensure minimal pressure drop across the bed and optimum surface area for adsorption. Mineral based carbon eliminates the risk of self-ignition associated with wood based Carbon. Their Butane working capacity was around 10%. Thus, they can absorb a minimum of 266.7 kgs of hydrocarbon (HC) vapors before saturation/ breakthrough.

One carbon bed captured all the hydrocarbon vapors fraction from the vapor inlet, allowing clean air to exit to atmosphere thus maintaining VOC emission levels below limit (5g/m3). When the Software in the PLC sensed that activated carbon bed is saturated with HC vapor (cumulative mass of HC adsorbed in the vessel calculated empirically = flow rate x HC concentration in feed (w/v) x time), the inlet vapor flow from the loading bays was switched to the second activated carbon bed. While the latter was adsorbing, the former was regenerated using a deep vacuum generated by a vacuum pump (Dry screw type). The vacuum pump could generate vacuum level of 20 mbar (absolute) in the Adsorber vessels in less than 2 minutes. However, the bed was not subjected to vacuum level below 25 mbar (absolute) to avoid ‘dusting’ of carbon pellets and to reduce power consumption.

Throughout the regeneration process all HC vapors evacuated from the carbon bed were pumped by the vacuum pump into an absorber column/scrubber. In the absorber column arranged vertically, the vapors were made to mix with liquid gasoline stream sprayed from the top. The liquid gasoline stream is circulated from an absorbent liquid tank (AL1) through a centrifugal pump. The absorber column having internal diameter of 500 mm and height 1300 mm was designed for a flow rate of 1060 cum/hr (rated flow of vacuum pump) and was filled with specialized packing (make: VFF, model VSP 25) that increased contact surface area and promoted condensation during mixing. It was equipped with a demister to prevent carry away of gasoline droplets to the carbon bed. The recovered gasoline flowed into the lower portion of the absorber column/scrubber where it was stabilized and pumped back to the absorbent liquid tank (AL1).

By measuring the hydrocarbon concentration in the vapor stream exiting the top of the carbon beds (through a gas detector) it is ensured that the hydrocarbon concentration in the exit air stream was less than 5g/cum, as per statutory guidelines. The difference in the level of gasoline in the absorbent liquid tank (AL1) is calculated from the readings of level indicator on the tank to determine the gasoline recovery.

The process flow diagrams have been shown below in Figure 1.

SIMULATION DETAILS:
The simulation study of above process was carried out on Aspen HYSYS software to generate mass balance sheet and process parameters and the key variables were validated from actual process conditions in the field.

Simulation studies of the Absorber column revealed the mass flow rate of vapors (entering the Absorber column V1) to be around 815 kg/h (for the Absorber column internal diameter = 0.5 m) for efficient absorption, although absorption is ensured at all flow rates. Thus, a vacuum pump having rated volume flow rate of 1060 m3/hr was selected. At the selected volume flow rate, density (or pressure) and HC concentration decide the ‘mass flow rate.’ Thus, the characteristic curve of the selected pump was collected and studied. The same is reproduced in Figure 2.

It was noted that dry screw vacuum pump ensures almost constant flow rate throughout its operation, except below ‘critical’ pressure i.e., around 50 mbar (absolute) as highlighted in Figure 2. As vacuum level increases, more and more HC moles would be desorbed from the bed. With little air left, the concentration of HC would increase in the stream.

Volume flow and thus mass flow rate drops at absolute pressures below 50 mbar i.e., ‘critical’ pressure but optimization of absorption is not evident in existing literature on VRUs.

A modification in conventional VRU apparatus has been carried out to optimize HC recovery even during the operation below the ‘critical’ pressure. During high vacuum levels at the adsorber bed (absolute pressure < 50 mbar), a portion of the incoming feed stream was diverted (bypass) to the absorber column to make up the deficiency in flow rate coming through vacuum pump discharge as shown in Figure 1. The comparison of the ordinary VRU process without feed stream bypass system (consider ON-OFF valve (PIC2) closed) and our VRU process with feed stream bypass system has been made below:

Existing VRU process (no feed stream bypass)
1) The vapor recovery rate calculated as the difference in the flow rates of supply and return liquid gasoline:
gasoline supply rate (stream CHS2) = 5040 kg/hr
gasoline return rate (stream CHS1) = 5083 kg/hr,
Thus, recovery rate = 5083 – 5040 = 43 kg/hr …..(i)

2) There in no decrease in the HC load on the adsorber bed ‘B2a’ as the combined feed steam has the flow rate of 484.5 kg/hr (= 251.11 kg/hr HC + 233.39 kg/hr Air) that is in fact greater than the feed (F1) flow rate of 483 kg/hr (= 251.0 kg/hr HC + 232.0 kg/hr Air).

VRU process of the present invention (with feed stream bypass):
1) The vapor recovery rate calculated as the difference in the flow rates of supply and return liquid gasoline:
gasoline supply rate (stream CHS2) = 5040 kg/hr
gasoline return rate (stream CHS1) = 5124 kg/hr,
Thus, recovery rate = 5124 – 5040 = 84 kg/hr …..(ii)

2) There is a decrease in the HC load on the adsorber bed (B2a) as the combined feed stream (SF2) has the flow rate of 444.1 (= 211.7 kg/hr HC + 232.4 kg/hr Air) kg/hr as compared to the feed (F1) flow rate of 483 kg/hr (= 251.0 kg/hr HC + 232.0 kg/hr Air). It can be shown that the HC flow rate has been decreased by 16% from that of the feed stream.

The bypass stream (F2), having flow rate of 483 kg/hr (volume flow rate about 283 m3/hr) or more, when made to enter the absorber column during the period when the pressure in the adsorber bed falls below 50 mbar (absolute), the HC recovery is increased by up to 95% and HC load on adsorber bed is decreased by 16%.

This scheme makes the Vapor Recovery Unit efficient throughout the entire regeneration process including during the ‘critical’ operation in the absorber column. Besides, it reduces the HC load on the carbon bed as the feed stream is diverted to the absorber column before entering the adsorber bed. This increases the bed saturation time and thus reduces the no. of regeneration cycles required during a day that ultimately reduces operational energy cost.
The results are tabulated below Table2:
Table 2
Critical Zone results
Liq. gasoline return (kg/hr) Liq. gasoline supply (kg/hr) Recovery (kg/hr) Recovery improvement
(%) Feed HC
(kg/hr) Adsorber bed HC
(kg/hr) Reduction in HC load on adsorber bed
(%)
without bypass 5083 5040 43 NA 251 251.11 0
with bypass 5124 5040 84 95.35 251 211.7 16

,CLAIMS:1. A method for improving recovery efficiency of a vapor recovery unit (VRU) of petroleum storage terminals, said method comprising:
i. loading an activated carbon in an adsorber vessel assembly to form an adsorber bed, wherein the adsorber vessel assembly comprises adsorber vessel (V2a) and an adsorber vessel (V2b);
ii. directing a feed stream (F1) comprising a mixture of hydrocarbon vapors and air into the adsorber vessel (V2a) containing an adsorber bed (B2a) and allowing the hydrocarbons to get adsorbed over the adsorber bed (B2a); and releasing a hydrocarbon free air (A1) into the atmosphere;
iii. diverting the feed stream (F1) from adsorber vessel (V2a) to the adsorber vessel (V2b) upon the saturation of adsorber vessel (V2a);
iv. regenerating adsorber vessel (V2a) by desorbing the hydrocarbon vapors from adsorber bed (B2a) using a vacuum pump (C001), while the feed stream (F1) is diverted to the adsorber vessel (V2b);
v. introducing a purge air stream (A2), by operating the ON-OFF valve (PIC1), from top into the adsorber vessel (V2a) towards the end of regeneration in step iv) to sweep away hydrocarbon vapors desorbed from the adsorber bed (B2a);
vi. sending the hydrocarbon vapors desorbed from the adsorber bed (B2a) through a vacuum pump discharge stream (DS1) into an absorber column/scrubber (V1) having an inlet point (IP1) for the vacuum pump discharge stream;
vii. recovering the hydrocarbon vapors obtained from the vacuum pump discharge stream (DS1) using an absorbent liquid in the absorber column/scrubber (V1) to obtain a condensed hydrocarbon stream (CHS1) from bottom of the absorber vessel (V1), wherein the absorbent liquid is liquid gasoline stream (CHS2) supplied from top of the absorber column/Scrubber (V1) through an adsorbent liquid tank (AL1) by a centrifugal pump, wherein a hydrocarbon vapor stream (SF1) obtained from top of the absorber column is mixed with feed stream (F1) to obtain the combined feed stream (SF2) which is directed to the bottom of the adsorber vessel (V2b) which is under adsorption process;
viii. channelizing a bypass stream (F2), by operating the ON-OFF valve (PIC2), to the absorber column/ scrubber (V1) by bypassing the adsorber column (V2a) upstream, through a vapor inlet point (IP2) positioned adjacent to the inlet point (IP1) for the vacuum pump discharge stream (DS1), when the pressure in the adsorber bed (B2a) reaches a critical level during regeneration of adsorber vessel (V2a); and
ix. collecting a condensed hydrocarbon stream (CHS1) from step (vii) into a sump (S1) at the bottom of absorber column/scrubber (V1); directing it to mix with a liquid gasoline stream (CS1) routed out after cooling the vacuum pump (C001) to form a combined absorbent liquid return stream (CHS3) and routing it to the absorbent liquid tank (AL1) by a centrifugal pump.

2. The method as claimed in claim 1, wherein the adsorber vessel (V2a) and the adsorber vessel (V2b) are loaded with the activated carbon in a range of 1000 to 5000 Kg each.

3. The method as claimed in claim 1, wherein the activated carbon is a mineral based activated carbon having a butane capacity in the range of 10 to 15%, wherein the mineral based activated carbon is in the form of a cylindrical pellet having a diameter of 3 to 5 mm and length of 4 to 8 mm.

4. The method as claimed in claim 1, wherein the feed stream (F1) has a temperature in a range of 0 ? to 50 ? and has an operating pressure in a range of 990 to 1100 mbar (absolute).

5. The method as claimed in claim 4, wherein the feed stream (F1) has an average flow rate in the range of 100 to 1000 m3/hr.

6. The method as claimed in claim 1, wherein the vacuum pump (C001) is dry screw vacuum pump comprises a vacuum pump casing jacket, a vacuum pump injection port and screw rotors.

7. The method as claimed in 6, wherein the vacuum pump (C001) is cooled by directing the liquid gasoline stream (CS1) to the vacuum pump casing jacket for cooling; and directing a liquid gasoline stream (CS2) to vacuum pump injection port for mixing with vacuum pump suction stream (SS1) inside the pump for cooling of screw rotors.

8. The method as claimed in claim 1, wherein the adsorber vessel (V2a) is regenerated at a pressure in a range of 1100 mbar (absolute) to 20 mbar (absolute) created by the vacuum pump, the critical level is a pressure less than 50 mbar (absolute), and wherein a purge air (A2) is introduced at critical level in the adsorber vessel (V2a) and continued till the end of the regeneration process.

9. The method as claimed in claim 1, wherein the feed stream (1) has the hydrocarbon vapors and air in a molar ratio of 10:90 to 40:60, the hydrocarbon vapors comprises hydrocarbons of C2 to C9 and an alcohol containing group, the hydrocarbon vapors comprises volatile organic compounds (VOC), and wherein the absorbent liquid is gasoline.

10. The method as claimed in claim 1, wherein the bypass stream (F2) is a controlled flow rate of the feed stream (F1); the flow rate of the bypass stream (F2) in the range of 100 m3/hr to 1000 m3/hr is controlled through a flow control valve (FIC1).

11. The method as claimed in claim 1, wherein the adsorber vessel (V2a) and adsorber vessel (V2b) are equipped with sensor to detect pressure drop across the adsorber bed and temperature at different levels of adsorber bed height.

12. The method as claimed in claim 1, wherein the absorber column/scrubber (V1) has an internal diameter in the range of 300 to 1500 mm and has a height in the range of 1000 to 4000 mm, equipped with a demister adjusted to work at a pressure drop < 0.5 mbar for a stream velocity of up to 2 m/s.

13. The method as claimed in claim 1, the absorber column/scrubber (V1) is filled with metallic very special packing (VSP) having diameter in the range of 10 to 50 mm, specific surface area in the range of 100 to 300 m2/m3 and a void space in the range of 70% to 99%.