Assignment 1 (Mass Balances)

Part 1.A

26 Marks

In the first assignment (Mass Balances) you will conduct "hand-calculation" of feedstock preparation

and methanol synthesis route. Biomass (sugarcane bagasse) is gasified in a novel Reactive Flash

Volatilisation (RFV) process to produce synthesis gas, along with biochar and water-soluble tar as by-

products. In a pilot plant RFV process, pulverised biomass is feed to catalytic gasifier at a rate of 10.0

kg/h, along with 4.2 L(STP)/h of O₂ and 6.4 kg/h of steam as the gasifying agents. The biomass and

oxygen are supplied at room temperature, whereas the steam is fed at 150°C and the gasifier operates

at 800°C. The steam is supplied in excess; therefore, the unreacted water vapour exiting the gasifier

with the synthesis gas is condensed at room temperature before the synthesis gas is sent for further

processing. The condensed water can be assumed to contain all of the water-soluble tar produced in

the gasifier. The biochar is periodically drawn from the gasifier and sent for analysis. Table 1 provides

the ultimate and proximate analysis of sugarcane bagasse, whereas Table 2 shows the raw data for

synthesis gas flow rate (wet basis) and composition (nitrogen and water free basis) measured at the

outlet of the condenser. Table 3 provides the ultimate analysis of char and tar collected from the

gasifier and condenser, respectively.

1. Conduct an atom species balance for C, N, H and O for the duration of the given measurement

and estimate the average carbon conversion efficiency (percentage of carbon conversion into

gas phase).

(18 Marks)

2. Report the errors in mass balance analysis and reflect upon the reasons for those errors. What

could be the reasons behind mass balances not closing properly?

(8 Marks)

Table 1 - Proximate and Ultimate Analysis of Sugarcane Bagasse

Ultimate Analysis

(% db)

Carbon (C)

Hydrogen (H)

Nitrogen (N)

Sulphur (S)

Oxygen (0)"

Proximate Analysis

Moisture

Ash

Volatile Matter

Fixed Carbon

* Calculated by difference.

46.40

6.19

0.44

0.05

44.42

(% as received, db except moisture)

10.0

2.5

79.16

18.13/nSample Yield

name (kg/h)

Biochar

Tar

0.262

1.552

N

0.472

0.794

Composition (wt% )

H

0.419

7.458

с

50.833

0.243

S

N.D.

N.D.

Part 1.B

The synthesis gas produced from the RFV process is pretreated to make the feed for methanol

synthesis process. The pretreated synthesis gas feed enters the methanol synthesis plant at a flow

rate of 100.0 kmol/h and contains 1.0% methane (inert) with the remainder being a stoichiometric

mixture of CO and H₂. This stream is mixed with a recycle stream to produce a stream which is fed to

the reactor. The product of the reactor is at equilibrium conversion based on reaction (2) and enters

a condenser which condenses some of the methanol. The overhead gas product contains 5.0 mol%

methane, 5.0% methanol, 30.0% CO and the remaining H₂. 5.0% of the overhead gas is purged to

26 Marks

prevent build-up of the methane and the rest is recycled. The liquid product from the condenser

contains pure methanol.

A. Draw and clearly label the block flow diagram.

Calculate the following-

B. The molar flow rate of the purge stream.

C. The molar flow rate of the liquid methanol stream.

D. The equilibrium constant K based on the mole fractions.

E. The composition of the recycle stream on a mass basis.

F. The overall and single pass fractional conversion of CO in the reactor.

(4 Marks)

(3 Marks)

(3 Marks)

(10 Marks)

(3 Marks)

(3 Marks)