Salva

Question 1

What is two-phase flow, flow patterns?

Answer 1

Two-phase flow involves the flow of a mixture of two phases: gas, solid or liquid. Flow patterns are the ways in which the phases are distributed over the cross section of the pipe or duct. Dependent on the flow of each component, the physical properties of phases e.g viscosity, surface tension, density, and the geometry and dimensions of the pipe or vessel.

Question 2

To consider

• Drag force
• Gravitational force

Answer 2

• Drag force:
  Transfer of energy from the faster to the slower phase
  Because of gravity, density will be v important so lighter phase will move faster in vertical flow -> slip between phases
• Gravitational force:
  In vertical flow, we will have axial symmetry
  In horizontal flow, gravity acts perpendicular to the direction of flow so no axial symmetry.

Question 3

List of vertical flow patterns

Answer 3

Bubbly, slug (plug), churn, wispy annular and annular, mist

bubbly -> mist, increase of flow of gas from left to right

Question 4

What happens when bubbles larger than 30% of diameter of pipe?

Answer 4

They start to coalesce, transitions occur due to bubble coalescence (bubbly to slug) and film flooding (when suddenly liquid goes down) (slug to churn)

Question 5

Characteristics of:

• slug or plug
• churn
• wispy annular
• annular

Answer 5

• slug or plug:
  bubbles characterised by spherical nodes, and occupy almost entire cross section of the pipe
• churn:
  you have oscillations in the liquid (going up and down), irregular shaped portions of gas and liquid
• wispy annular:
  has portions of liquid around sides of pipe and also some in the middle
  annular:
  less oscillations, all of the liquid touching the walls

Question 6

Difference in vertical DOWNWARD co-current flow, to UPWARDS

when does vertical counter current occur?

Answer 6

In vertical downward co-current, similar patterns are obtained but the pattern is very sensitive to the upstream distributor. Drag force and gravitational force are in the same direction. 
Counter current (gas up, liquid down) is difficult to obtain. Most typically is annular flow, only happens w low gas velocities, if higher velocity then it will change the liquid direction.

Question 7

List horizontal flow patterns

Answer 7

bubbly, plug, stratified, wavy, slug, annular, spray or dispersed
lowest flow of gas -> highest flow of gas

Question 8

Characteristics of

• plug
• stratified
• wavy
• slug
• annular

CONSTANT LIQUID FLOW RATES

Answer 8

• plug:
  bubbles are at the top
• Stratified:
  all the bubbles have coalesced, completed separation of has and liquid (g at top)
• wavy:
  more instability
• slug:
  eventually waves will touch the top of the pipe - crest of waves touching the top
• annular:
  liquid touching walls with layer thicker at bottom due to gravity

Question 9

Two phase flow in bend-coils

Answer 9

A bend can induce coalescence to form slug flow. In general, a bend will act as a phase separator. In condition of annular flow, situation is not as bad as intuitively expected:

• liquid continues to bubble as a film on inside surface of the bend
• momentum of gas phase is high

Question 10

Single stage bubble column reactor applications and important parameters to know

Answer 10

Applications: absorption, catalytic slurry reactors, bioreactors, coal liquefaction
Important parameters: 
- residence time
- bubble size -> interfacial area
- rate of mass transfer

Question 11

Gas bubbles in fermenters

Answer 11

• Small bubbles «1mm diameter can become a nuisance as they are slow to rise.
• d < 2 -3 mm, surface tension effects dominate behaviour of the bubble surface. They behave as rigid spheres with immobile surfaces and low internal gas circulation. kL will decrease w decreasing bubble diameter due to surface tension effects.
• > 3mm diameter, bubbles are able to wobble, this increases kL

Question 12

Example of kLa use, aerobic fermentation

Answer 12

• cells in aerobic culture take up oxygen from the liquid
• oxygen solubility approx 10 ppm at ambient T and P
• at high cell densities, cell growth may become limited by availability of oxygen in the liquid phase
• when cells are dispersed and well mixed, the major resistance to oxygen transfer is the liquid film surrounding the gas bubbles

Question 13

Taylor flow

Answer 13

Two phase flow characteristics of capillaries are different from flow in larger channels, capillary forces are significant in monolith channels:
0.5 mm < dchannel < 5mm
Surface tension forces (which are stronger than gravity) tend to render the flow characteristics independent of channel orientation.
Taylor flow is characterised by elongated bullet-shaped has bubbles that almost fill the entire cross section of a channel section, separated by liquid slugs. A thin liquid film separates these bubbles from the wall and also connects the two successive liquid slugs separated by the gas bubble.
The liquid slugs are free from gas entrainment. There is a recirculating flow in the liquid slugs, which improves heat and mass transfer. The separation of liquid slugs reduces axial dispersion. Very important for microreactor applications.

Question 14

Taylor flow practical example:
benzaldehyde -> benzylalcohol -> toluene + H2O
Monolithic catalysts compared to a trickle-bed reactor in mass-transfer limited reactions

Answer 14

• Higher productivity:
  Taylor flow internal circulations and shape of bubbles etc (characteristics) allowed conversion as improves both heat and mass transfer (both solid-liquid and liquid-gas)
• Higher selectivity:
  Residence time better controlled (through length of reactor) so can stop before toluene is produced. Gas and liquid have sharp residence time distributions, also gets rid of non-selective stagnant zones, thin catalyst wall due to monolith diameter limitations so reactant doesn’t stay in too long.

Question 15

What is cavitation?

Answer 15

The pressure driven process of vapour bubble formation and collapse in a liquid

Question 16

What is NPSHa

Answer 16

The excess of pressure head over the vapour pressure of the pumped liquid at the suction nozzle. Greater than zero -> no cavitation

Question 17

What are pumps/compressors

Answer 17

Equipment used to deliver fluids from one location to another.
Pumps for liquids, compressors for gases

Question 18

Classification of pumps

Answer 18

a) positive displacement
- rotary e.g gear, peristaltic
- reciprocating e.g diaphragm, piston/plunger
b) kinetic
- centrifugal
- peripheral
- special

Question 19

Positive displacement pumps

Answer 19

Operate by forcing a fixed volume from the inlet ti the discharge zone of the pump. Flow depends on the velocity of the cavity in the pump. They can give very high pressure. If outlet is closed, they continue increasing pressure so safety valves are needed. Pulsating flow over time.

Question 20

Kinetic, centrifugal pump

Answer 20

Centrifugal force is applied to a fluid to produce kinetic energy. Then, kinetic energy is converted to pressure by reducing the fluid velocity.
Discharge is free of pulsation
Works over a high range of pressures and capacities
Discharge pressure is a function of the fluid density
Low cost

Question 21

What is an oil field?

Answer 21

Hydrocarbons that have accumulated in the pores of rocks

Question 22

Primary oil recovery

Answer 22

Once a well has been drilled, gravity, as well as the pressure in the reservoir, force the oil into the wellbore. From here, the oil is brought to the surface through mechanical means, like a pump jack. Primary phase of oil recovery continues until the pressure inside the well is no longer enough to produce oil in quantities that make it financially worthwhile.

• first stage, 10% of the available reservoir is recovered
• pressure in reservoir > hydrostatic pressure
• If natural pressure difference is too small, pumping will be required
• Gas and water may also appear in fluids recovered from wells

Question 23

Secondary recovery

Answer 23

Injection wells added through which gas or water is injected, increasing pressure in reservoir and moving oil to the surface so can collect more water.
In combination with the primary step, about 15-60% of the available oil in the reservoir is recovered although industry average is 35%/
Injection:
- maintains reservoir pressure
- displaces the oil

Question 24

What are oil recovery % factors?

Answer 24

% pore scale displacement x % sweep x % drainage x % commercial cut-off

Question 25

Video summary

Answer 25

Natural production=amount of oil recovered when a reservoir is first drilled. Reservoirs connected to surfaces via aquifers. Water is more dense than oil so exerts a greater pressure so when first put into production, pressure of reservoir is greater than at the base of the oil well so fluids are naturally pushed to the surface. Over time, reservoir pressure drops until it is the same as the pressure at the bottom of the well so well stops flowing.

Question 26

What causes oil recovery to be so low?

Answer 26

• Pore scale displacement is a measure of how much oil has been pushed out from any of the rock accessed by injected fluids
• sweep: how much reservoir rock has been reached by injected fluid
• Drainage: the extent to which the wells can access all the separate segments of the reservoir
• Commercial cut-off: indicates the limit of economic production

Question 27

What limits pore scale displacement?

Answer 27

• Water wet rock: when rock attracts water and repels oil so a thin film of water coats the sand grain surfaces, oil found in centre of pores. When water is injected, the thin water film expands, replacing oil pushed to the producer. As they expand, they meet up and form blockages, shutting off the thin spaces between the sand grains so oil pathways shut off, oil trapped, remaining oil = residual oil.
• Oil wet rock: rock attracts oil, repels water, thin film of oil coats sand grains. Oil film expands as oil is produced until it finally breaks and oil flow stops.
  Because of variety of rock surfaces, most reservoirs show a combination of oil wet and water wet. Pore scale displacement is controlled by the distribution of wettability and pore grain structure which can vary dramatically within the reservoir.

Question 28

What limits sweep?

Answer 28

Flow factors:
1. Vertical sweep: layering effect
Permeability of rock is not uniform, water flows through the more permeable layers, takes path of least resistance, much of subsequent water injected continues to flow through the same layer which has already been swept of oil.
2. Vertical sweep: slumping effect
Due to water being heavier than oil, tends to slump down under it and breakthrough to the production well at the base.
3. Areal sweep: shortest path
Injected water tends to take shortest path to the production well. Once it reaches the production well, subsequent injected water tends to follow established path, less inclination to spread out, only slowly expands sideways to sweep more rock.
4. Areal sweep: old river beds
Higher permeability channels in the rock such as old river beds where water will naturally follow, in preference to sweeping the rest of the reservoir.
%vol total sweep = %vol vertical sweep x %vol areal sweep

Question 29

Gas injection

Answer 29

Gas produced with the oil from a reservoir can be injected. Gas much lighter than water or oil so tends to gravitate upwards, and so better able to reach upper layers of reservoir but poorer at reaching lower layers. Gas being less viscous than water can be more affective than water in areas of lower permeability. In reservoirs with no extensive barriers to vertical flow, and where there is sufficient supply, gas can be used in a process called gravity drainage. Gas injected in higher levels where it stays. With continuous injection, the gas layer starts expanding downwards as a stable front due to its density difference with the oil. Some of the oil is pushed to the producers, whilst oil behind the gas front continues to drain down. In many cases, the residual oil from gasflood is similar to that of water so gas alone cannot make much diff to 35%.

Question 30

Polymer flooding

Answer 30

The addition of a polymer to the injected water increases the sweeping efficiency (of a secondary recovery method). At the macroscopic scales, the viscosity of the water is increased, improving the displacement/sweep of oil.

Question 31

Surfactant flooding

Answer 31

This helps to recover oil trapped by capillary forces at the microscopic scale. Interfacial properties are altered. Also, the miscibility of oil in the displacing fluid is increased.

Question 32

Alkaline chemical flooding

Answer 32

Interactions are complex e.g
lowering the oil-water interfacial tension, formation of oil-water emulsions.
Examples of chemical used: NaOH, KOH, Na2CO3.

Question 33

Miscible gas flooding

Answer 33

Injection of nitrogen, natural gas, or flue gas (12%Co2, 88% N2) into the well. The light to intermediate components in the crude oil are transferred to the injected fluid. It is necessary to install a separation plant to recycle the produced gas back into the well. Been successfully used since the 60s.

Question 34

Carbon dioxide flooding

Answer 34

Because of the solubility of CO2 in crude oil this leads to:
- oil sweeping from 10 - 40%
- viscosity reduction of oil
mass transfer of light and medium components occurs between oil and CO2 bulk gas.
Operating problems:
- formation of carbonic acids -> corrosion of injection pipes/tubes
- In the production well, formation of deposits, CaSO4
- cost of CO2 recovery from produced gas is high
- CO2 + oil -> solid asphaltanes may form -> problems of permeability and plugging in production tubing

Question 35

Hot fluid injection

Answer 35

Hot water - not widely used
Steam injection - widely used
Recovery by:
- ‘steam drive’: in which oil is swept by steam from the injection well to the production well
- ‘cyclic injection’; injection, soaking (<5 days), and production = huff and puff method. Allows using the same well for both injection and production. Very effective in first cycles.

Question 36

Fire flooding

Answer 36

Needs an igniters. Thermal recovery technique. Injection of an oxidising gas (air or oxygen enriched air) to generate heat by burning a portion of resident oil. No air in reservoir so can easily control the fire. Reactions occur in the gaseous, liquid and at the surface of the solid phase. A variety of combustion and partial oxidation reactions occur.

Question 37

Fire flooding reaction

Answer 37

• O2, temp above 350:
  complete combustion produces CO2 + H20
  Incomplete combustion produces HC + CO + CO2 + H2O
• O2, 200 < T < 350:
  carboxylic acids, aldehydes, ketones, alcohols, hydro-peroxides produced + water
• no O2, pyrolysis occurs:
  cracking reactions -> alkanes and alkenes
  dehydrogenation of alkenes -> bitumen -> coke
  condensation reactions -> high molecular weight + aromatic species

Question 38

Dry-forward combustion

Answer 38

The combustion front moves in the same direction as the injected air, and towards the production well.

Question 39

Dry-forward combustion zones

Answer 39

zone 1:
oil has already been displaced from this zone, the hot porous matrix pre-heats the air stream injected into the well, the temperature increases in the matrix as the combustion zone is approached.
zone 2:
combustion reactions occur, and the coke deposited in the porous structure is also oxidised
zone 3:
hot gases from zone 2 displace the oil and increase its mobility. The lighter components are vaporised. As a result of the high T and low O2, cracking reactions also occur, resulting in the formation of coke in the porous structure.
zone 4:
the hot gases from the combustion zone vaporise some of the light/medium hydrocarbons in the oil and the higher T facilitates flow of oil. At these temperatures some of the water in the vapour starts to condense in the porous structure.

Question 40

Wet forward combustion

Answer 40

Water and air are simultaneously (or alternately) injected

Question 41

Wet forward zones

Answer 41

zone 1:
contains little or no hydrocarbon as the zone has already been swept by the combustion front. Approx 50% of the pore volume is occupied by injected air. The temperature of the water increases and it is vaporised.
zone 2:
the water is in the vapour phase and the pores contain a mixture of air and steam
zone 3:
partial combustion/oxidation occurs in this zone, and oxygen is consumed. As a result of the high T and low O2, cracking reactions also occur, resulting in the formation of coke in the porous structure.
zone 4:
the hot gases from the combustion zone vaporise some of the light/medium hydrocarbons in the oil and the higher T facilitates flow of oil. At these temperatures some of the water in the vapour starts to condense in the porous structure.
zone 5:
temperatures are lower in this zone. as the pores already contain oil, and water is also condensed in these pores, this creates a restriction in flow (high back-pressure). The formation gradually approaches the initial conditions encountered in this reservoir.

Question 42

Reverse combustion

Answer 42

The combustion front moves in the opposite direction to the flow of the injected air. The ignition is initiated at the production well. Less used.

Question 43

Reverse combustion zones

Answer 43

zone 1:
The porous matrix contains oil and some water, and this is swept with air. Depending on the temperature and pressure, partial oxidation reactions may occur in this zone.
zone 2:
heat conduction through the porous matrix from the hot combustion zone helps to vaporise the oil and increase its mobility
zone 3:
partial combustion/oxidation occurs in this zone, and oxygen is consumed. because of the elevated temperatures at the combustion front, and low oxygen levels, hydrocarbon cracking reactions occur and coke is deposited in the porous structure.
zone 4:
as oxygen levels are too low to oxidise the residual carbon, this remains present in the porous matrix. As the temp decreases w distance from the combustion zone, condensation of the heavier oil fraction occurs.

Question 44

Fracking

Answer 44

Used when no/low permeability.
Underlying structure of the rock is modified so not tertiary oil recovery, instead is ‘oil production intensification’.
Consists of pumping large amounts of water, sand and additives at high pressure into subterranean rock formations to fracture rock and free up oil and natural gas trapped there. Uses explosions.
Produces ‘unconventional hydrocarbons’ (oil or gas) from non-conventional reservoirs (i.e not the porous and permeable sandstones and limestones which comprise ‘conventional reservoirs’).
Targeted rocks are shale, fine grained sands, and coal beds.
Shales ordinarily have insufficient permeability to allow significant fluid flow to a production well, thats why they’re not commercial sources of gas.
Typical volume ratios are: 90% water, 9.5% sand and 0.5% chemical additives.
Chemical additives are added to dissolve minerals, reduce friction, prevent corrosion, thicken the fluid (to transport the sand), clean our debris, prevent clay from swelling, kill bacteria
Sand is needed to keep the fractures open
Additive examples: HCl, petroleum distillates, ammonium persulfate, CaCl2, boric acid, citric acid, borate salts etc..

Question 45

Fracking advantages

Answer 45

High potential to provide fuel has (optimistic expectations: enough fuel to supply the UK for 70 years). Natural gas-fired power plants produce half the CO2 of coal plants and 25% of UK’s electricity still comes from coal. Lowers dependence on countries that export gas. Estimated that it has given gas security to US and Canada for 100 years.

Question 46

Cons of fracking

Answer 46

High amounts of water need, many opportunities for spillages. Water supplies for humans have been contaminated (due to inadequate treatment of waste or due to aquifer contamination). Earthquakes of low intensity have been detected (e.g Blackpool 2011). Air pollutants risk. Methane leaks (3% of gas lost to atmosphere), methane causes 21x more greenhouse effect than CO2. Very large energy consumption.Drill holes very quickly exhausted, necessary to drill fracking holes much more frequently than for classical natural gas wells.
Banned in France, Bulgaria, some regions of Spain, Germany, Switzerland, Australia banes the use of BTEX (benzene derived) chemicals. In UK regulated by the dept. for business, energy and industrial strategy. Requirement to have triple casing.