Tutorial - Page 16

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Sub-problem 4.1: Observations on k_La in function of liquid viscosity for a liquid mass flow rate of 5 kg/m²s.

1) Start a new project by selecting Start new query .

2) Apply the following problem specifications, namely the Experimental technique: Physical absorption, Liquid viscosity: values between 2 and 30 cP and Liquid mass flow rate: 5 kg/m²s.

First, select the Mass transfer tab and check the box related to k_La_LG, then Execute. It will extract 882 data sets containing k_La values.

Comment: Notice that a large number of fields in the Mass transfer tab disappears after executing. When no corresponding values or information are found in the restricted database (882 data sets), the fields are hidden from view (i.e. there is no interfacial area values associated with the k_La values). It avoids selecting fields which would inevitably result in an empty Search results.

Check the Experimental technique box in the Mass transfer tab and select Physical absorption from the restricted list and Execute. Consequently, it is found that 4 authors combined for 326 data sets have measured liquid-side mass transfer coefficients (k_La) associated to a physical absorption process.

In the Liquid tab, check the Liquid viscosity box and change the corresponding units from Pa.s to cP. Click on the corresponding Value field . The new dialog window denotes the minimum-to-maximum range for the 326 data sets (0.868 - 23.4 cP). To fulfill the problem specifications, enter 2 cP and 30 cP in the appropriate boxes. Press OK, then Execute. A total of 25 data sets from Iliuta (1996) are extracted.

The last specification involves the Liquid mass flow rate in the Operating conditions tab. Select the Value field

for this parameter. The following window dialog notifies that the liquid mass flow rate for the 25 data sets varies between 4.99 and 7 kg/m²s. Write 5 +/- 0% in the corresponding boxes. Press OK.

Comment: Even though 5 kg/m²s is enclosed within the min-max boundaries, there is no liquid mass flow rates in the restricted database (25 data sets) that were recorded exactly as 5.00 kg/m²s. The Warning message (Search result empty with this entry) informs the user that an empty search results would be obtained with the actual entry and demands the user to re-specify it.

Consequently, write 5 +/- 5% to include the min value (4.99 kg/m²s). Press OK, then Execute. The final Search results list contains 15 lines.

3) Select the Plot button . Choose Gas mass flow rate (Operating conditions: Gas mass flow rate) in the Horizontal axis panel and Experimental (Mass transfer: k_La_LG: Experimental) in the Vertical axis panel.

4) In the Advanced tab, select Liquid viscosity (Liquid: Liquid viscosity) as the discriminate value. Press OK.

5) Select the Discriminate series button and choose the Value discrimination option. Write 100 in the Number of classes box to create 100 equally divided series between 3.18 and 23.4 cP. Press OK. The diagram is effectively divided into 13 series. However, the observation of a persisting trend remains difficult. Instead, the liquid viscosity should be divided in 3 equally spaced series since 3 definite liquid viscosity clusters are covered in the legend (1: 3.18-3.6 cP; 2: 8.91-10.8 cP; 3: 18.7-23.4 cP). Return to the Discriminate series dialog window and change 13 for 4 (By trial and error, 4 equally divided series gives a better diagram).

CONCLUSION #1: Based on the latter diagram, it becomes clear that greater liquid viscosity up to 25 cP increases invariably the liquid-side mass transfer coefficient (k_La). For example, at G = 0.035 kg/m²s, k_La increases with liquid viscosity at a proportion close to 0.0008 s^-1/cP and lowers at G = 0.185 kg/m²s with a proportion of ca. 0.0006 s^-1/cP. The next step would be to verify that the 15 data sets showcased in the diagram are really representative of the liquid viscosity influence on k_La and do not involve any other factors which were not defined in the query. These factors could be different packing type, packing material or liquid distributor for example.

6) To verify this matter, select the Discriminate series button and choose the Code discrimination option. Select first the Packing type option from the combo list and press OK. It shows that all 15 experiments were carried out with a spherical packing. Secondly, select the Packing material option and press OK. A single packing material, namely SiO₂/Al₂O₃/Cr₂O₃ catalyst material was used.

7) To view the general aspects surrounding liquid distribution, select Liquid distributor from the Code discrimination option in the Discriminate series dialog window. A perforated plate was used in Iliuta's work (1996). To determine how the liquid viscosity variation was done by Iliuta, the liquid composition must be examined. Select Liquid composition from the Code discrimination option in the Discriminate series dialog window.

CONCLUSION #2: A single packing shape & material combination and liquid distributor for the 15 analyzed data sets in Conclusion #1 were depicted from the last 4 diagrams. It acknowledges the fact that all other parameters which were not taken into account in the query do not influence the general trend discussed in Conclusion #1. Moreover, different carboxymethyl cellulose salt (CMC) concentrations were employed to increase the liquid viscosity as presented in the last figure.

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