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MEMBRANE UNIT Presented by - Mostafa Ghoniem

mostafaghonem98@gmail.com

11/18/2013

INTRODUCTION

In the next few slides we will review the idea of the membranes

showing how it works and the whole configuration of the unit.

Installation of the membrane banks and the most important

precautions will be presented.

Maintenance of the membrane unit also will be shown .

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CONTENTS

- Introduction

- Review

- Membrane construction

- Membrane unit configuration

- Maintenance

- Installation

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WHAT IS A MEMBRANE?!

Membranes are thin semipermeable barriers that selectively

separate some components from others.

This may appear like a filtration process when in fact it isn’t.

In filters the unfiltered substance is disposed or even trapped in the

filter material.

In this process the membrane selects one component to go through

it and the other is trapped outside the membrane in a property

called “selectivity”.

Selection happens because one gas goes through the membrane

faster than the other.

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WHAT IS A MEMBRANE

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Component A

Component B Membrane layer

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MAIN DEFINITIONS

Permeation rate or flux is the rate at which a gaseous component can diffuse through the membrane medium to the low-pressure side.

Permeability : the ease of gas to dissolve and diffuse through the membrane

Selectivity refers to the ratio of permeation rates of a fast gas (carbon dioxide) and a slow gas (methane or other hydrocarbon). The selectivity of carbon dioxide to methane determines the efficiency of the separation, and thus the hydrocarbon recovery

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MAIN DEFINITIONS

For better understanding for the term “selectivity” we need to

understand the Mechanism of membrane separation.

The main phenomenon are solution and diffusion

Gas molecules solutes in the membrane material due to high

pressure

The solute molecules diffuses in the membrane material due to

difference in concentration on both sides of the membrane 11/18/2013

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Component A

Component B Membrane layer

High pressure Low pressure

Flux direction

Membrane side

Feed side Permeate side

MECHANISM OF

OPERATION 1/3

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MECHANISM OF OPERATION

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The different components of the mixture have different solubility in the polymer of the membrane and each gas has a different permeability in the porous medium of the membrane.

Using these characteristics one component can permeate faster than the other component .

First, molecules of the gas dissolve in the non-porous layer.

Then, it diffuses through the porous medium of the membrane because of the pressure difference between the sides of the membrane..

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MECHANISM OF OPERATION

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Both components dissolve and go through the membrane but with different rates

Components with higher solubility and higher diffusivity called fast components and they form the higher concentration in the permeate stream.

Due to the differences in the ability of the components of the mixture to dissolve and diffuse in the polymer, separation happens.

The penetrating stream is called the permeate and the rejected component is called the residual.

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CONSTRUCTION OF MEMBRANE

Membrane simply constructs of a layer of certain type of polymers “Cellulose acetate” in our case.

This layer is an ultra thin layer with thickness of few microns to enhance the separation efficiency.

This ultra thin layer is so weak and can be fractured under high pressure .

This layer is supported with other layer of thick polymer with much larger pore diameter.

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CONSTRUCTION OF MEMBRANE ELEMENT

The single element consists of several layers of membrane sheets ,

each membrane layer is separated by 2 other layers called feed

spacer and permeate spacer.

Gas is feed to the feed spacer , the membrane layer does the

separation , and the permeate spacer carries permeate gas to

permeate side

These layers are wrapped in a spiral form and connected to a

perforated tube that collects all permeate gas. 11/18/2013

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Feed gas

Feed spacer

Membrane layer

Permeate spacer

Permeate gas

Residual gas

Component A

Component B

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CONSTRUCTION OF MEMBRANE ELEMENT

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GOVERNING EQUATIONS Beginning with Fick’s law for gas diffusion flux of the gas is related to the concentration gradient.

Where D= Fick’s constant, Diffusivity coefficient.

C is the concentration , x distance , N flux of certain component

Integrating the equation,

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This equation is valid for only one substance, and in our case there is two substances.

Using Henry law for concentration

Where Pi = partial pressure of component (i) in the mixture, S= solubility of component (i) in the medium.

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Then the equation becomes

As N= flux (volume flow rate / Area) then we can write the equation in the formPif = partial pressure on the feed side , Pip = partial pressure of the permeate side

Where A is the surface area of the polymer subjected to the mixture.

If the previous equation is solved for A , then the membrane size is known.

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COMMERCIALLY AVAILABLE MEMBRANE MATERIAL A comparison is held to choose the most suitable membrane material that can be used achieving the low economical cost and best separation performance

The term representing the performance is called selectivity of a membrane

, where is the selectivity of membrane y,x is the mole fractions on the

permeate and feed side respectively of components a,b

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MEMBRANE UNIT CONFIGURATION

Turbo Expande

r PTU

HX PTU

Residual gas to sales

Permeate to flare

Permeate gas to 2nd stage

Residual gas recycled

Compressor is used to raise the pressure of the stream to the required value which ensures the required value of flux

Pre-treatment Unit.

The purpose of this unit is to remove any liquids or condensates before the membrane.The presence of liquids or condensates will damage membrane elements.

Pressure = 62 barTemperature = 57 C

P=0.89 bar T=0.42 C

5.91 MMSCFD

11.51 MMSCFD

PRE-TREATMENT UNIT

COLLESANCE FILTER CATALYST GUARD BED Coalescence

Filter

To remove

Bulk Liquids

and

Condensates.

Presence of

liquids will

damage the

membrane

polymer

Catalyst

guard bed

Using

activated

carbon

powder to

absorb

heavy

hydrocarbon

vapors

Particle

filter

To trap any

fine

particles

that may

have

escaped

from the

catalyst

PRE-TREATMENT UNIT

As shown in the schematic a PTU is necessary before each

membrane stage.

There are 3 PTU units before the first stage , 2 in service working in

parallel to handle the total flow rate which estimated with 125

MMSCFD, and one PTU in a standby mode to serve when one of the

other 2 is down for maintenance or for damage

There is only one PTU before the second stage because the flow

rate is low and estimated with 16 MMSCFD.

PRE-TREATMENT UNIT

The Question is what if this only PTU is down for any reason, how to deal with stream??!

The answer is , the risk to feed the membrane untreated gas won’t be taken and the stream will go directly the flare.

EACH STAGE CONFIGURATIONThe single stage is called “skid”.

Each stage consists of “banks”.

Each bank consists of tubes.

Each tube consists of membrane elements.

The first stage consists of 6 banks * 7 tubes * 12 elements.

For the second stage 4 banks* 4 tubes * 10 elements.

And here are some footage to clarify the configuration.

MEMBRANE MAINTENANCE

As any other equipment in the process it needs maintenance.

Maintenance is done by for the membrane elements and other attachments of the configuration like the PTU.

For the Elements , Samples taken from the stream after and before the skids determines if there is any problem with the membrane.

If there is a problem like deviation in pressure or temperature. Other samples are taken after each bank.

The suspected bank is isolated and then elements are extracted and visually inspected if there is no obvious defects in the membrane structure it then inserted into “Nitrogen pressure vessels”

Displaced feed spacers due to high feed flow

Evidence of dust presence Damaged Epoxy

NITROGEN PRESSURE VESSELS The idea of the nitrogen pressure vessels is that Nitrogen is permeable through the membrane material at much lower pressures than CO2.

The element is inserted into vessel filled with Nitrogen at pressure of 7-8 bars and the Nitrogen flux is measured and compared with values in the membrane manual.

If there is match with values in the manual then the element is OK.

If there is a mismatch with the values even it’s replaced with a new element or it’s place is changed.

NITROGEN VESSELS CONFIGURATION

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For the PTUs it’s better to follow this schedule for maintenance.

For the first and second stage PTUs

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Equipment Activity Frequency

Filter coalescer Replacement of filter elements.Requires a full shutdown.

Every 6 months or when differential pressure across filter coalesce reaches 10 psi, whichever occurs first.

Guard Bed Replace activated carbon.Requires a full shutdown

Every 6 months or when differential pressure over guard bed reaches 10 psi, whichever occurs first.

Particle Filter Replace filter elements.Requires a full shutdown

Every 6 months or when differential pressure over particle filter reaches 10 psi, whichever occurs first.

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PRECAUTIONS ON MAINTENANCE !!! Before performing any maintenance activity on the membrane system it must be

fully depressurized, purged with nitrogen to remove flammable and/or toxic

materials, and positively isolated from other system components with the use of

isolation valves arrangements.

Failure to follow these instructions could result in injury or death. Persons performing

maintenance on the system must follow all applicable safety procedures.

Modifications and repairs to the system must meet all applicable codes and

standards.

Elements , casings and tubes should be protected against moisture and humidity.

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INSTALLATION AND ASSEMBLY

Installation of membrane elements is a very precise process

and should be done carefully in order not to damage

elements and here some steps of the assembly process as

found in the UOP user’s manual .

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STEP1

Collect all necessary items to connect the Separex elements. (Check if all items are delivered before starting the loading activities)

The old aluminum clamps should not be used, as they have been known to fail.

STEP2

After removing the flange of the membrane tube, collect all items, which need to be re-used.

(Re-used parts need to be cleaned very carefully). New O-rings should always be used. Some

gases soften or “blister” the elastomer.

STEP3

Remove all damaged / contaminated Separex elements out of the Membrane tube, and clean

the internal surfaces of the Membrane tubes very carefully. The tubes need to be cleaned in

some cases, such as the oil contamination at Esso. Membrane tube can be cleaned with

methanol on cloth

STEP3If elements are to be reused, particulate matter may be removed by blowing dry nitrogen through one end of the element. Oil can be wiped off the epoxy casing, but if there is oil present, it will likely coat the membrane. Storing the elements in a vertical position for several days will often get some of it off. An oil-coated element should be reused only in case of an emergency, for the oil will work its way out and contaminate anything downstream.

CONTAMINATION

STEP4

Put grease (silicone vacuum grease) on

both the Separex connections on the

module and the sealing ring, which

needs to be put between both Separex

connectors of the coupled elements. The

iron gasket of the TaperLok should be

greased also.

STEP5 The first Separex element that will be

inserted into the module needs to be

foreseen with a ‘blind ring’. Take the

‘blindring’, put grease on the seals and put it

against the Separex element connector. The

blindring needs to be fixed with the fixation

component as shown in the picture on the

right. After closing the fixation component

put the safety pin into the appropriate holes

STEP 6

Insert the first elements in that way so the open connector is at the

open end of the Membrane tube. Take the second element (do not

forget to grease the connecting surfaces!) and connect both elements to

each other using the open connector seal rings. Do not forget the safety

pin after closing the fixation component.

When entering and connecting elements, they need to be supported in

order to avoid stressing the element already inserted in the tube. The

latter should be inserted so that the plastic “spider” is fully in the tube

before connecting another element or the permeate tube. That will

avoid cracking the shell and/or creasing the membranes.

STEP 6

STEP 7

Clean the flange and all

contact surfaces very carefully

and put the conical gasket ring

into the Membrane tube.

(contact surface between

gasket ring and module needs

to be cleaned as good as

possible)

STEP 7

STEP 8

Connect the flange connector of the

Membrane tube to the Separex element

connector of the last element (do not forget

the safety pin) Push the complete element

assembly into the module as far as possible

and close the module (torque up of all

flange bolting)

STEP 8 Connecting the permeate tube and TaperLok cover is the time of greatest hazard to both the

elements and the workers. A recommended procedure is to use two pieces of threaded rod,

about 0.7 m long, with the same threads as the bolts (or studs, in the case of the older

systems). These rods are screwed into two of the upper holes (using nuts in the case of the

older design) after the last membrane element is installed. These rods will support the covers

while the connection is made and will prevent damage to both elements and workers. If

threaded rod is not available, cut the head off two bolts and weld them to suitable pipe or

rods. Finally, lubricate the bolts or studs with anti-seize and tighten the bolts or nuts in a

“star” pattern to get a good closure without excessive torque. Be sure to tighten before

attaching the permeate header; once it is attached it can stress the connection and make

torque readings unreliable.

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The End

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