api separators article solutions

8/10/2019 API Separators Article Solutions

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API Separators - The Workhorse of Refinery Wastewater Treatment

SystemsProper design and selection of support equipment, and understanding issues affecting

API separators are very important to the efficiency of this key piece of refinery wastewatertreatment equipment. The following article discusses all of these issues.Compiled From

Introduction

TheAPI Separatoris normally the first, and arguably most important, wastewater treatment step in most petroleum

refineries. or years, refineries have attempted to use other technologies or treatment scenarios as an alternative

to the API separator. !ut most refineries ultimately select, or return to, the API separator as the technology of

choice for their wastewater treatment primary oil"solids separation step.

The primary function of a properly designed API separator is to remove gross quantities of oil and suspended

solids from refinery wastewater prior to subsequent downstream wastewater treatment processes # normally asecond oil"water separator polishing step and some form of advanced treatment for removal of dissolved organic

compounds $typically biological treatment, though other treatment technologies have been used%.

The API separator was developed over &' years ago in a (oint effort by The American Petroleum Institute $API% and

Siemens )ater Technologies *nvire+ Products$then e+ -hain !elt%. The first API separator was provided in /00

to Atlantic efining1s Philadelphia refinery, and since then, hundreds of refineries around the world have installed

API separators in their wastewater treatment plants.

How it Works

The API separator is a gravity separation device that works on the principle of Stokes 2aw, which defines the rise

velocity of an oil particle based on its density and si3e. Typically, the difference between the specific gravity of oil to

be separated and water is much closer than the specific gravity of the suspended solids and water. Therefore, the

design of the API separator is based on the difference in the specific gravity of the oil to be separated and the

wastewater. If this design criterion is followed, the ma(ority of suspended solids will settle in the unit. 4nce the oil

and suspended solids are removed from the wastewater in the API separator, the middle phase, water, is then sent

on for further treatment in most refinery wastewater treatment plants.
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esi!n Standards

Through the original work done by the American Petroleum Institute and us in the /0's, and numerous

subsequent improvements, the design standards for the API separator have been well documented and can be

found in the current edition of API Specification 56.

Some of the most important design criteria developed for API separators include7

"en!th to width ratio#A minimum length to width ratio of 87 is recommended for all API separator

designs to keep operating conditions as close to plug flow as possible, minimi3ing the potential for short

circuiting.

epth to width ratio#A minimum depth to width ratio of '.0 to '.8 is recommended so that separation

units are not e+cessively deep, minimi3ing the amount of time it takes for oil particles to rise to the surface.

$a%imum channel width and depth# The ma+imum API separator channel width is 6' ft9 ma+imum

depth is : ft.

Hori&ontal 'elocity# ;aintaining a hori3ontal velocity of no more than 0.' ft"sec has been shown to

minimi3e turbulence and its effect on interfering with the separation of oil from wastewater.

Inlet distri(ution# To minimi3e the effect of high wastewater inlet velocities into the API separator, and

possible short circuiting associated with these high velocities, reaction (et baffles are recommended to diffuse

influent flows across the width and depth of the API separator.

)il particle si&e# ;a(ority of oil particles in most refinery wastewaters are 8' micron in si3e or larger.

Therefore, the design standards for API separators were developed for the removal of oil particles of this si3e.

Particles smaller than 8' micron will normally e+it an API separator and will need to be removed by

downstream treatment processes, unless allowances are made in the si3ing of the API separator to remove

these smaller particles.

esi!n Features

In addition to the previously mentioned


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Incorporating a single scraper"skimmer system in the tank to remove settled solids and floating oil to

maintain optimum capacity in the separator. The scraper"skimmer system usually consists of a four shaft, chain

and flight collector system that conveys settled solids to a sludge hopper at the inlet end of the unit, and floating

oil to a skimmer at the effluent end of the unit.

Providing some method to remove accumulated oil from the surface of the API separator. This is usually

a rotating oil skimmer pipe, but may also include an oil roll skimmer. *lectronic or manual monitoring of the oil

level in the API separator tankage may also be incorporated into the design. ;ethods to continuously or intermittently remove accumulated solids $sludge% from the API separator,

usually including some type of sludge pumping system.

A method to transfer $pump% wastewater to the separator if gravity flow is not possible $as with above=

grade API separator tanks%.

)astewater inlet distribution system"dispersion system.

Installing air=tight covers for >4-"vapor containment, which usually includes some type of inert gas

blanketing system for safety.

?sing above=ground steel tanks to contain potentially ha3ardous wastes and wastewaters, as shown in

igure 6.

$yth 's# Fact

The ma(or fallacies of API separator performance are7 that they will always remove a certain percentage of oil and

TSS in a wastewater9 or that they should always achieve a certain effluent quality, independent of influent oil and

TSS concentration.

In reality, igure 0 shows the typical performance of an API separator in a petroleum refinery and illustrates how a

properly designed API separator should operate.


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A well=designed API separator should achieve effluent oil and TSS concentrations of 8' to 6'' mg"l, independent

of influent concentration, to ensure protection and proper operation of downstream treatment processes.

As the graph shows, effluent quality from this API separator is fairly consistent, even if the influent quality is highly

variable.

Selectin! Proper Support *+uipment is ,ey

)hile the design of the API separator seems simple enough, we know that doing a te+tbook design for an API

separator and actually operating an API separator are two different things.


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Slud!e Pumpin!Remo'al Systems

Arguably the most important support equipment in API separator design, sludge pumping"removal systems remove

accumulated oily sludge from the separator. If not removed, internal chain and flight collector components will

likely fail from being overloaded by e+cessive sludge volumes. In fact, lack of effective sludge removal is the most

common reason API separators do not operate properly. )hen designing a sludge removal system, consider the

following7

API separator sludge is heavy, viscous and sticky, and can quickly bridge in sludge hoppers, plugging

sludge withdrawal points. luidi3e and break up compacted sludge by using no33les and water or steam in the

sludge hoppers.

?se clean=out and flushing connections on the sludge withdrawal piping to keep piping from plugging.

Positive displacement, diaphragm pumps have been used successfully in separator sludge pumping

applications. -entrifugal trash pumps have also shown some success here.

2ocate sludge pumps close to, and at the same elevation as, sludge hoppers to provide flooded suction

to the pumps and minimi3e sludge suction piping, which is prone to plugging.

Slud!e Collector Systems

-hain and flight collector systems installed in API separators serve two purposes7

To skim floating oil to a common collection point

To scrape settled oil solids to a common withdrawal point

If floating oil and settled solids are allowed to collect and accumulate in the separator, its effective volume will

decrease, affecting oil and solids removal efficiency which will be noted as increased oil and suspended solids

concentrations in the effluent from the separator. This can adversely affect downstream treatment processes.

.)C.apor Control Systems

In most cases, API separators in petroleum refineries or petrochemical plants need to be provided with one of two

types of covers for >4- containment"control7

loating covers@float on the surface of the separator

i+ed covers@set above the surface of the separator

-onsider the following for proper cover selection7

4il skimming efficiency"interference with oil skimming

*ase=of=access to, and maintenance of, collector components

Safe operation

-apital and operating cost

egulatory compliance


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;aintenance requirements

!oth fi+ed and floating cover systems have been used often and successfully. ote that floating covers can

interfere with oil skimming devices that e+tend above the water surface at the effluent end of the separators. i+ed

covers must be used over these components even if the remaining portion of the separator uses floating covers. In

general, fi+ed covers are more commonly used on new API separators and floating covers are generally used

when e+isting separators must be covered for >4- control.

Interestingly, covering API separators has significantly impacted the process of oil removal from separators. It is

difficult, if not impossible, to see into a covered separator to determine oil levels and skimming needs, resulting in

oil not being removed often enough, causing oil carryover to downstream treatment processes, or oil being

skimmed too often, resulting in significant amounts of water being skimmed with the oil. )hile windows in the

covers have helped, more successful solutions have included the installation of electronic probes in the separator

covers to monitor oil concentration at various depths in the units, or the installation of sample taps on the side of

the units $for above=grade separators% to manually monitor oil depth.

)il CollectionRemo'al Systems

4il collection is key to API separator design. emoving oil from the separator will prevent accumulation and

entrance into downstream treatment processes, but removal also allows for collection, recovery and reprocessing

of the oil.

Typically, oil skimmer pipes are used for oil removal. These slotted pipes e+tend partially into the API separator at

the effluent end. -hain and flight collectors skim oil, which has accumulated on the surface of the separator, from

the influent end to the effluent end of the unit. The skimmer pipe is rotated to begin skimming the oil.

To improve the quality of the skimmed oil $i.e., making sure it does not contain too much water%, many API

separators also use an oil roll skimmer in addition to the skimmer pipe. The oil roll skimmer is a drum that e+tends

across the width of the separator, normally of metallic construction, and partially submerged in the surface of the

wastewater. The oil roll skimmer contains an e+ternal drive, which rotates the drum. As the drum rotates, free oil

adheres to the specially prepared surface of the drum, and a doctor blade removes the accumulated oil from the

surface of the skimmer as it rotates.

The oil then flows into a collection trough and out of the unit. 4il collected by an oil roll skimmer is usually /8

percent pure oil and is easily reprocessed. Bowever, the capacity of oil roll skimmers is limited. ?psets resulting in

large amounts of oil entering sewer systems can quickly overload oil roll skimmers, which is why an oil skimmer


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pipe is always used as a backup oil removal device. 4il roll skimmers will also not remove floating debris and

paraffins, which may be present.

!eyond the systems and components that comprise the design of an API separator, there are issues to be aware

of regarding process operations in petroleum refineries and petrochemical plants which can affect the operation

and effectiveness of API separators, including changes in crude oils, the presence of emulsions and spent caustic

being sent to the separator.

)perational Issues Can Affect Separator *fficiency

;any API separators being used today were designed and installed 6' to 5' years ago, or more. At that time,

many refineries processed much lighter crude oils than they do today. 4bviously, the separator was designed

accordingly. As crude slates have become heavier, the oil contained in the wastewater entering API separators

have also become heavier and closer to the specific gravity of water. Therefore, it takes longer for the oil to

separate by gravity from the wastewater and the efficiency of the separator may have decreased, sometimes

significantly.

To offset this, some refineries today have taken ma(or steps to reduce water usage at their facilities, which has

reduced wastewater flow rate, thereby allowing more time in API separators for heavier oil particles to separate.

Bowever, designs of units installed many years ago should be reviewed to ensure adequate protection of

downstream treatment equipment.

)il *mulsions

4ne of the biggest problems facing e+isting API separators in petroleum refineries is the presence of oil emulsions

in the wastewater. API separators are gravity separation devices, typically designed to remove free oil particles

larger than 8' micron in si3e. Anything that increases the amount, or percentage, of sub=8' micron oil particles

$which includes formation of oil emulsions% can significantly impact the efficiency of API separators. Two common

operational issues can impact the si3e of oil particles, or cause oil emulsions # desalter brine water and spent

caustic.

esalter /rine Water

;any refinery desalters were originally designed to process lighter crude oils. As these lighter crude oil slatesbecome scarce, more refineries are processing heavier crude oils. This has resulted, in many cases, in poorer

separation of water from oil in the desalter and formation of oil emulsions. In many petroleum refineries, the

desalter brine water now contains significant amounts of oil and oil emulsions, which can quickly overwhelm API

separators $and other oil"water separation equipment% in wastewater treatment plants.

In these cases, it may be worthwhile to provide a dedicated oil"water separation step for (ust the desalter brine

water to break any oil emulsions and remove the ma(ority of free oil before the brine water is sent to sewer. This


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can prevent overloading the wastewater treatment plant API separators with oil and consequently affecting

downstream treatment processes.

Spent Caustic

Spent caustic, while not an issue for everyone, has presented problems in the past, particularly when it is sent to

sewer systems in front of oil"water separators. Spent caustic can raise the pB of the wastewater, causingemulsification of oil in the wastewater especially when there is turbulence present, such as by pumping. In order to

avoid these situations, spent caustic should be added to the wastewater downstream of oil"water separation

equipment or should be disposed of by some other method such as separate on=site treatment or off=site disposal.

Summary

The separator is one of the most important wastewater treatment steps in refineries and petrochemical plants.

Proper design and selection of support equipment are crucial to proper operation, and special plant circumstances

can affect operation as well.

)e hope this article provides a good starting point for improving e+isting API separator operation, as well as

making sure that new API separators will meet performance e+pectations.

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