Chemical Assignment Solutions PDF

Preview

Summary

Chemical Assignment Solutions...

Description

ENGINEERING 1 – Chemical Engineering Home assignment – solutions

1. If using only the information contained in the video, one can draw the following block flow diagram (there may be various ways of representing the link between the power plant, the refrigeration unit and the rest of the plant – as long as they are clearly presented and make sense, they are acceptable)1: Combustion Gases Natural Gas Power plant

Air Natural Gas

Gravity settlers

Liquids (water and “others”)

Sulphur and CO2 removal

CO2 and H2S

Drying

H2O

Propane and butane removal

Propane Butane

Electrical power

Refrigerant Mercury removal

Cooling and liquefaction

Mercury LNG

2. We are told in the video that aqueous solvents are used for removing CO2 and H2S, hence these gases could both be removed in an absorption tower (a “scrubber”) by counter-current contacting with the solvent, as seen in one of the lectures.

3. An energy balance around the refrigeration unit is necessary here, helped by a diagram:

Electrical energy, 1.1 MJ / kg LNG

Heat removed, 0.9 MJ / kg LNG Purified natural gas, 20-24 oC and 12 atm

1

Refrigeration unit

Refrigeration unit

‘Waste’ heat generated, ??? MJ / kg LNG

Refrigerant

Cooling and liquefaction

LNG, – 155 oC and 1 atm

For info: there are various ways that this diagram can be improved, for example “other” liquid will likely be hydrocarbons; Cooling and liquefaction will likely produce not just LNG but also a mixture of inert gases, like nitrogen or even helium. Putting a more descriptive name on some of the unit operations might also be attempted, e.g. a distillation to remove propane and butane?

ENGINEERING 1 – Chemical Engineering Home assignment – solutions Please note how we are not fussed as to whether the incoming gas is at 24 oC (the temperature the gas is at before cooling and liquefaction) or 20 oC (the feed temperature for which we are given a cooling requirement of 0.9 MJ/kg for producing LNG): when compared to a final temperature of – 155 oC, these two temperatures look very nearly the same. An energy balance around the refrigeration unit then gives a straightforward answer: Over one cycle of the refrigerant,

Energy in = Energy out,

with

Energy in = Electrical Energy + Heat removed (per kg LNG)

and

Energy out = Waste heat generated

Hence, Energy out = Electrical Energy + Heat removed (per kg LNG) = 1. 1 + 0.9 = 2.0 MJ/kg LNG.

4. The power plant has an efficiency of 24%, hence for delivering 1.1 MJ/kg LNG it requires a chemical energy of 1.1 / 0.24 = 4.6 MJ/kg LNG, which will have to come from additional natural gas (or perhaps propane and butane by-product?). From an energy balance, the difference between chemical input and electrical (i.e. directly useful) output is the waste heat, i.e. Waste heat generated by the power plant = 4.6 – 1.1 = 3.5 MJ/kg LNG

5. In question 2, any solvent that has captured CO2 or H2S (or both) in the absorption tower must be regenerated for reuse (i.e. the absorbed gases must be removed to recover the solvent in a purified form suitable for reuse in the absorption tower). Typically this is done by heating the solvent in a second tower, with the effect of liberating (‘desorbing’) the gases. The heat by-product from the combustion of natural gas or by-products in the power station will certainly be at high enough temperature. Regarding the waste heat from the refrigeration unit, the pressurized refrigerant vapour leaving the compressor is at 99oC which might also be hot enough for use there.

6. Given the calorific value of the natural gas at 45 MJ/kg and the energy consumption of the power plant (4.6 MJ/kg LNG), and also assuming that there are no other major consumption of energy (e.g. boilers for raising steam that burn more fuel), we can deduce that the consumption of natural gas for powering the plant is 4.6 [MJ / kg LNG] / 45 [MJ / kg natural gas] = 0.102 kg natural gas / kg LNG. Next, we assume that natural gas is mostly methane in order to evaluate the CO2 emissions associated with its burning2. The balanced chemical equation for complete combustion is then CH4 + 2O2 → CO2 + 2H2O

2

Possibly quite a rough assumption if it is raw gas that we are considering!

ENGINEERING 1 – Chemical Engineering Home assignment – solutions

It indicates that for every mole of methane burnt, one mole of carbon dioxide is produced. When considering molar masses, this means that 44 kg of CO2 is produced for every 16 kg methane, i.e. 44/16 = 2.75 kg CO2 / kg natural gas. Hence the figure for CO2 production = 2.75 x 0.102 = 0.28 kg CO2 / kg LNG....